TEST 4 (CLINICAL GENETICS)

Hemolytic disease of the newborn (HDN)

A condition of immunological incompatibility between mother and fetus that occurs when the mother is Rh2 and the fetus is Rh1.

Memory B cell

A long-lived B cell produced after exposure to an antigen that plays an important role in secondary immunity.

B cell

A type of lymphocyte that matures in the bone marrow and mediates antibody-directed immunity.

T cell

A type of lymphocyte that undergoes maturation in the thymus and mediates cellular immunity

Surrogacy is a controversial form of ART

ART has altered traditional and accepted patterns of reproduction and redefined the meaning of parenthood. In the United States, surrogate motherhood is a reproductive option for infertile couples. In one form of surrogacy, called egg donor surrogacy, a woman is artificially inseminated with the sperm of the infertile couple's male partner and carries the child to term. After the child is born, she surrenders the child to the father. In this case, the surrogate is both the genetic and the gestational mother of the child. In another form of surrogacy, called gestational surrogacy, a couple provides both the egg and the sperm for IVF. The surrogate mother is implanted with the developing embryo and serves as the gestational mother but is genetically unrelated to the child she bears. After the birth of the child, she surrenders the infant to the couple who contracted for her services Laws regarding surrogacy vary from state to state. In some states, all forms of surrogacy are legal; in others, only gestational surrogacy is legal; and in still other states, all forms of surrogacy are illegal.

Cancer

Age is a Leading Risk Factor for Cancer §Some viral genes promote and maintain cancer growth in infected cells §Specific chromosomal changes are associated with certain forms of cancers

Components of the Immune System Are Genetically Controlled

About every 80 minutes, someone in the United States dies while waiting for an organ transplant. At any given time, about 77,000 people are waiting for transplants. Although more Americans are signing pledge cards to become organ donors at death, the demand for organs continues to far outstrip the supply. To address the shortage, research scientists and biotechnology companies are developing an alternative source of organs: animals. Nonhuman primates such as baboons and chimpanzees are poor candidates as organ donors; they are endangered species and harbor viruses that may cause disease in humans (HIV, for instance, originated in nonhuman primates). Most attention is focused on using strains of mini-pigs developed over the last 30 years as potential organ donors. Those pigs have major organs (hearts, livers, kidneys, etc.) that are similar in size to those of adult humans and have a compatible physiology The major stumbling block to xenotransplants (transplants across species) using pigs as organ donors—or using any other animal, for that matter—is rejection by the immune system of the human recipient. To overcome this problem, researchers have used several strategies. They have transferred human genes to pigs so that their organs carry molecularmarkers found on human organs. Other workers have deleted specific pig genes to make their organs look more like human organs to the human immune system. More radical approaches to making pigs and humans compatible for transplants involves altering the immune system of the human recipient so that a transplanted pig organ will be tolerated. To do this, purified bone marrow cells from the donor pig are infused into the human recipient. After modification in this way, the recipient's immune system accepts the donor pig's organ with fewer complications. Transplant trials across species in animal-animal transplants have been successful, making it likely that this method would work in humans. Proponents of xenotransplantation point to the lives that will be saved if pig organs can be used for organ transplants. Opponents point out that there is no evidence that pig organs will work properly in humans and that pig organs may harbor harmful viruses that will be transferred to the human recipients. Others question the ethics of genetically modifying animals with human genes or modifying humans by transplanting parts of the pig's immune system. In addition to its role in organ transplants, the immune system protects against infection. The immune system is controlled by several groups of genes that encode proteins found on the surface of cells and proteins called antibodies that directly attack invading pathogens. Mutations in these genes are responsible for immune system disorders that can range from inconvenient to fatal. In the Genetics in Practice section at the end of this chapter, you can read about a couple who have a child with a serious and often fatal immune system disorder and the challenges they face in deciding whether to have another child.

Newborn screening is universal in the United States.

All states and the District of Columbia require newborns to be screened for a range of genetic disorders. These programs began in the 1960s with screening for phenylketonuria (PKU) and gradually expanded. Most states screen for 3 to 8 disorders, but states that use newer technology can screen for 30 to 50 heritable metabolic disorders.

Gamete intrafallopian transfer (GIFT)

An ART procedure in which gametes are collected and placed into a woman's oviduct for fertilization.

Blood Types Are Determined by Cell-Surface Antigens

Antigens on the surface of blood cells determine compatibility in blood transfusions. There are about 30 known antigens on blood cells, each of which constitutes a blood group, or blood type. For successful transfusions, certain critical antigens of the donor and recipient, if present, must be matched. If transfused red blood cells do not have matching surface antigens, the recipient's immune system will produce antibodies against the non-matching antigen, clumping the transfused cells. The clumped blood cells block circulation in capillaries and other small blood vessels, with severe and often fatal results. In transfusions, two blood groups are of major significance: the ABO system and the Rh blood group.

When the immune system no longer distinguishes between self and nonself, _________ can result

Autoimmunity

A person with A positive blood would not be able to receive _________ blood during a transfusion

B negative

Assisted Reproductive Technologies (ART) Expand Childbearing Options

Because of the increase in infertility over the last few decades, many major medical centers now have fertility clinics and there are independent programs in many cities. These clinics use several methods, grouped under the term assisted reproductive technologies (ART), to help infertile or subfertile individuals and couples have children. ART focuses on three areas: retrieval or donation of gametes, fertilization, and implantation of an embryo. Some of these methods are discussed below.

Gleevec is a treatment for

CML

Tumor-suppressor genes

Genes encoding proteins that suppress cell division.

The use of __________ is associated with an increased risk of transmitting genetic defects, such as those related to infertility, onto male children.

ICSI

Biotechnology makes xenotransplants possible.

In the United States, almost 29,000 organs were transplanted in 2013, but about 77,000 qualified patients are on waiting lists. Each year, almost 4,000 people on waiting lists die before receiving transplants, and another 100,000 die even before they are placed on a waiting list. Although the demand for organ transplants is rising, the number of donated organs is growing very slowly. As outlined at the beginning of this chapter, one way to increase the supply of organs is to use animal donors for transplants. Animal-human transplants (called xenotransplants) have been attempted many times, but with little success. Two important biological problems are related to xenotransplants: (1) complement-mediated rejection and (2) T cell-mediated rejection. In complement rejection, species-specific MHC proteins on the donor organ are detected by the complement system of the recipient. When an animal organ (e.g., from a pig) is transplanted into a human, the pig's MHC proteins are so different that they trigger an immediate and massive immune response known as hyperacute rejection. This reaction, which is mediated by the complement system, usually destroys the transplanted organ within hours. To overcome this rejection, several research groups isolated and cloned human genes that block the complement reaction. Those genes were injected into fertilized pig eggs, and the resulting transgenic pigs carry human-recognition antigens on all their cells Organs from these transgenic pigs should appear as human organs to the recipient's immune system, preventing a hyperacute rejection. Transplants from genetically engineered pigs to monkey hosts have been successful, but the ultimate step will be an organ transplant from a transgenic pig to a human. Even if hyperacute rejection can be suppressed, transplanted pig organs will still face T cell-mediated rejection of the transplant. Because transplants from pig donors to humans occur across species, the tendency toward rejection may be stronger and require the lifelong use of immunosuppressive drugs. Those powerful drugs may be toxic when taken over a period of years or will weaken the immune system, paving the way for continuing rounds of infections. One solution to this problem is to transplant bone marrow from the donor pig to the human recipient. The recipient would then have a pig-human immune system (called a chimeric immune system). This chimeric immune system would recognize the pig organ as self, would not trigger a rejection response, and still retain normal human immunity to fight infectious diseases. As farfetched as this may sound, animal experiments using this approach have been successful in preventing rejection for more than 2 years after transplantation without the use of immunosuppressive drugs. A similar method of bone marrow transplants from donor to recipient is already used in human-to-human heart transplants to increase the chances of successful outcomes , so pig-human transplants would be the next logical step. As recently as 15 years ago, the possibility of animal-human transplants seemed remote, more suited to science fiction than to medical reality. But today, more than 200 people in the United States have already received xenografts of animal cells or tissues. One of the first recipients was Jim Finn (Figure 17.18) who suffers from Parkinson disease, which is caused by the death of certain cells in the brain. Neural cells from a fetal pig brain were injected into his brain, and he had a good recovery. Before treatment, he could not walk, talk, or use his hands. Six months after treatment, he could sit, stand, and walk independently. Today, he is an advocate for using animal cells to treat this condition.

A proto-oncogene can become an oncogene when

It is translocated next to a highly expressed gene

Tumor

Mass of abnormally dividing cells Normal cells exhibit contact inhibition in culture Benign Usually well-defined borders; unable to metastasize Malignant Has ability to metastasize "cancer"

Which of the following is a process whereby cancer cells travel to other sites in the body and establish secondary tumors?

Metastasis

Proto-oncogenes

Normal genes that initiate or maintain cell division and that may become cancer genes (oncogenes) by mutation.

Fertilization normally occurs in the

Oviduct

What technique can be used to detect genetic defects in early stage embryos, prior to implantation?

PGD

Antisense oligonucleotides (AONs)

Short, single-stranded DNA or RNA molecule synthesized to be complementary to a sequence of interest.

Killer T cells

T cells that destroy body cells infected by bacteria. These cells can also attack and kill cancer cells and cells of transplanted organs.

Exon skipping

The directed removal of exons during mRNA processing to restore the reading frame of the mature mRNA to make a shortened, but functional, protein.

Cancer and the Environment

The relationship between the environment and cancer has been studied for more than 50 years. Many environmental agents damage DNA, causing mutations, some of which lead to cancer. Certain viruses, radiation, chemicals, infection, as well as lifestyle choices and behaviors such as diet, sun exposure, and tobacco use are relevant examples. In addition to the external environment, events inside the cell can be mutagenic and carcinogenic. Reactive oxygen species (ROS) generated during metabolism and unrepaired errors that are by-products of DNA replication can be cancer-causing events.

Exon skipping and gene therapy

The pre-mRNA transcribed from most human genes contains regions called introns and exons (see Chapter 9). The introns are spliced out and discarded during mRNA processing, and the exons are joined together to form the mRNA codon sequence that, in the cytoplasm, is translated into the amino acid sequence of a protein. Deletion of one or more of the 79 exons in the gene for dystrophin (MIM 300377) is responsible for over 70% of all cases of Duchenne (DMD) and Becker muscular dystrophy (BMD) (see Chapter 4 for a detailed description of these disorders). The dumbbell-shaped dystrophin protein has a functional region at each end, connected by a central rod. In DMD, exon deletions disrupt the codon reading frames and only shortened fragments of dystrophin are produced. These fragments do not contain the two end regions necessary for normal function. In BMD, the exon mutations do not disrupt the reading frame (see Chapter 11 for a discussion of frameshift mutations) and produce dystrophin with two functional ends but shortened central rods. As a result, DMD is a progressive and fatal disorder, but individuals with BMD have near-normal life spans. Because of this feature of dystrophin structure, researchers developed a way to enlarge DMD deletions in a way that would also restore the reading frame of the mRNA, producing a shortened, but functional, dystrophin. It was hoped that this method, called exon skipping, would convert the DMD phenotype into the BMD phenotype. Specific exons are targeted for skipping during mRNA processing by synthesizing antisense oligonucleotides (AONs) that bind to the exons and block the splicing signals. The AONs are delivered by injections into muscle tissue, and mutant exons are skipped over and removed along with introns during processing. The remaining exons are joined together to produce an mRNA with a restored reading frame (Figure 16.17). Most DMD mutations occur in regions that encode the central rod, mostly in exons 45-53. By making exon-specific AONs, the therapy can be tailored to each patient's mutations Gene therapy has and is being used successfully to treat cancer, cardiovascular disease, and HIV infection . With new vectors and new methods such as exon skipping, the use of gene therapy to treat single and multiple gene disorders is expected to grow significantly and eventually become a standard form of treatment. As of this writing, however, gene therapy is still an experimental procedure performed on only a few carefully selected patients, under strict regulation by governmental agencies. The first sign that this may be changing is the recent approval of a gene therapy drug, Glybera, for treatment of lipoprotein lipase deficiency (MIM 246650). The drug is a modified adeno-associated virus vector carrying the human lipoprotein lipase gene, which is delivered by injection. Other gene therapy drugs such as eteplirsen, an AON for the treatment of DMD, and ProSavin, a viral vector that delivers three genes for dopamine production to treat Parkinson disease, are undergoing clinical trials and may soon enter the marketplace. These and other drugs may soon make gene therapy a standard tool in the treatment of genetic disorders.

Lymphocytes

White blood cells that originate in bone marrow and mediate the immune response.

The technology of transplanting nonhuman organs into humans is known as

Xenotransplantation

Metastasis 2

A process by which cells detach from the primary tumor and move to other sites, forming new malignant tumors in the body

X-linked agammaglobulinemia (XLA)

A rare, X-linked recessive trait characterized by the total absence of immunoglobulins and B cells.

Major histocompatibility complex (MHC)

A set of genes on chromosome 6 that encodes recognition molecules that prevent the immune system from attacking a body's own organs and tissues.

Haplotype

A set of genetic markers located close together on a single chromosome or chromosome region.

Anaphylaxis

A severe allergic response in which histamine is released into the circulatory system.

ABO blood typing allows for safe blood transfusions.

ABO blood types are determined by a gene I (I for isoagglutinin) encoding an enzyme that alters a cell-surface protein. This gene has three alleles, IA, IB, and i O—often written as A, B, and i. The dominant A and B alleles each produce a slightly different version of the enzyme, and the recessive i allele produces no enzyme. People with type A blood have A antigens on their red blood cells and do not produce antibodies against this cell- surface marker. However, people with type A blood do make antibodies against the antigen encoded by the B allele Those with type B blood carry the B antigen on their red cells and make antibodies against the A antigen. If you have type AB blood, both A and B antigens are present on your red blood cells and no antibodies against A and B are made. If you have type O blood, you have neither antigen but do make antibodies against both the A antigen and the B antigen. Because AB individuals carry no antibodies against A or B, they can receive a transfusion of blood of any type. Type O individuals have neither antigen and can donate blood to anyone, even though their plasma contains antibodies against A and B; after transfusion, the concentration of these antibodies is too low to cause problems. When transfusions are made between people with incompatible blood types, several problems arise. Figure 17.13 shows the cascade of reactions that follows when someone with type A blood is transfused with type B blood. Antibodies to the B antigen are present in the blood of the type A recipient. These antibodies bind to the transfused red blood cells, causing them to clump. The clumped cells restrict blood flow in capillaries, reducing oxygen delivery. The breakdown of these clumped red blood cells releases large amounts of hemoglobin into the blood. The hemoglobin forms deposits in the kidneys that block the tubules of the kidney and often cause kidney failure.

Hybrid Genes and Cancer

Changes in the number and structure of chromosomes are a common feature of cancer cells (Figure 12.14). These changes include translocations, deletions, chromosome loss, aneuploidy, duplications, and amplification of certain genes. Cells from solid tumors grown in the laboratory continue to exhibit genomic instability by acquiring new chromosome aberrations as they divide. Although most of these chromosome changes are nonspecific, in other cases, specific chromosomal abnormalities lead to certain forms of cancer.

Plasma cells

Daughter cells of B cells, which synthesize and secrete 2,000 to 20,000 antibody molecules per second into the bloodstream.

Cancer Can be Sporadic or Inherited

Sporadic Caused by the gradual accumulation of mutations in key genes within a single cell At least two mutations are required Accounts for the majority of all cancers Inherited A mutant allele is inherited from one parent The second, normal, allele mutates, causing cancer growth Accounts for <5% of all cancers

Signal transduction

A cellular molecular pathway by which an external signal is converted into a functional response.

Complement system

A chemical defense system that kills microorganisms directly, supplements the inflammatory response, and works with (complements) the immune system

Antibody

A class of proteins produced by B cells that bind to foreign molecules (antigens) and inactivate them.

Acquired immunodeficiency syndrome (AIDS)

A collection of disorders that develop as a result of infection with the human immunodeficiency virus (HIV).

Severe combined immunodeficiency disease (SCID)

A collection of genetic disorders in which affected individuals have no immune response; both the cellmediated and antibody-mediated responses are missing.

Membrane-attack complex (MAC)

A large, cylindrical multiprotein that embeds itself in the plasma membrane of an invading microorganism and creates a pore through which fluids can flow, eventually bursting the microorganism.

Helper T cell

A lymphocyte that stimulates the production of antibodies by B cells when an antigen is present and stimulates division of B cells and cytotoxic T cells.

Are some people more resistant or more susceptible to HIV infection?

About 15 years after the first AIDS case was reported in the United States, researchers discovered a small number of people who engaged in high-risk behavior, such as unprotected sex with HIV-positive partners, but did not become infected with HIV. Shortly thereafter, it was discovered that these individuals carried two copies of a mutant allele of a gene called CC-CKR5 (MIM 601373). Normal alleles of this gene encode a cell-surface protein that signals the immune system when an infection is present. HIV binds to this protein and uses it to infect T4 helper cells. The mutant allele contains a small deletion (32 base pairs) that produces a shorter protein that HIV cannot use to infect T4 cells. As a result, people who are homozygous for this CC-CKR5 mutation are resistant to HIV infection. Population studies show that the mutant allele is present only in Europeans and those of European ancestry. The highest frequency of the mutant CC-CKR5 allele is in northern Europe and the lowest frequency is in Sardinia and may have offered protection against another deadly, unknown infectious disease. Subsequently, researchers discovered that other populations carry mutant alleles of several genes that also confer resistance to HIV infection (MIM 609423). As shown in Table 17.6, HIV infection is highest in sub-Saharan Africa. A 25-year study of this population involving DNA samples from thousands of participants led to the discovery that an allele of the DARC gene (MIM 613665), which encodes a cellsurface protein, protects carriers from malaria but increases susceptibility to HIV infection by 40%. In other words, people who carry the anti-malaria allele of DARC have a greater chance of contracting HIV if exposed to this virus. In addition to increasing risk of HIV infection, this allele interferes with the immune system's ability to fight early stages of HIV infection. About 60% of African Americans carry this DARC allele and are therefore more susceptible to HIV infection than the general population. Studies of other populations have revealed that genes scattered across the human genome influence resistance and susceptibility to HIV infection and the rate of progression to AIDS. Many of these genes are now being studied to find new drugs that can be used in treating HIV infection and AIDS. For example, using information gathered from studying how HIV infects cells using the CC-CKR5 protein and other proteins on the cell surface, a drug, enfuvirtide (Fuzeon), has been approved for treatment of HIV infection. Other drugs in this class are now in clinical trials and should be on the market in a few years.

Overreaction in the immune system causes allergies.

Allergies result when the immune system overreacts to antigens that do not cause an immune response in most people (Figure 17.19). These antigens, called allergens, include a wide range of substances: house dust, pollen, cat dander, certain foods, and even medicines. One of the most serious food allergies is sensitivity to peanuts, which is a growing health concern in the United States (see Exploring Genetics: Peanut Allergies Are Increasing). The first time someone is exposed to an allergen, B cells make IgE antibodies instead of IgG antibodies. In a second exposure, the allergen binds to IgE antibodies made during the first exposure, causing mast cells to release histamine, triggering a systemic inflammatory response that causes fluid accumulation, tissue swelling, and mucus secretion. This reaction can be severe in some individuals. As histamine is released into the circulatory system it may cause a life-threatening decrease in blood pressure and constriction of airways in the lungs. This reaction, is called anaphylaxis or anaphylactic shock. Anaphylaxis causes the bronchial tubes to constrict, restricting airflow into the lungs, making breathing difficult. Irregular heartbeats and cardiac shock can develop and cause death within a few minutes. Children and adults with severe allergies often carry an injectable medicine (epinephrine in an EpiPen) that can be used as soon as an anaphylactic reaction begins. This drug counteracts the immune response and is a lifesaving treatment.

The immune system has a memory function

Ancient writers observed that exposure to certain diseases made people resistant to second infections by the same disease. B and T memory cells produced as a result of the first infection are involved in generating this resistance. When memory cells are present, a second exposure to the same antigen results in an immediate, large-scale production of antibodies and cytotoxic T cells (Figure 17.12). Because of the presence of the memory cells, the second reaction is faster and more massive and lasts longer than the primary immune response. The immune response controlled by memory cells is the reason we can be vaccinated against infectious diseases. A vaccine stimulates the production of memory cells against a disease-causing agent. A vaccine is really a weakened disease-causing pathogen, or an antigen produced by the pathogen. The vaccine can be given orally, nasally, or by injection and provokes a primary immune response and the production of memory cells. Often, a second dose is administered to elicit a secondary response that raises, or "boosts," the number of memory cells (that is why such shots are called booster shots). Vaccines are made from killed or weakened strains (called attenuated strains) of disease-causing agents that stimulate the immune system but do not produce life-threatening symptoms of the disease. Recombinant DNA methods now are used to make antigens used as vaccines against diseases that affect humans and farm animals. A global vaccination program eliminated smallpox in 1972, and a new effort is attempting to eliminate polio by vaccinating children worldwide. Overall, millions of lives have been saved by vaccination, and it remains one of the foundations of public health.

Antibodies are molecular weapons against antigens.

Antibodies secreted by plasma cells are Y-shaped protein molecules that bind to specific antigens in a lock-and-key manner to form an antigen- antibody complex (Figure 17.9). Antibodies belong to a class of proteins known as immunoglobulins (Ig). There are five classes of Igs—abbreviated IgD, IgM, IgG, IgA, and IgE. Each class has a unique structure, size, and function (Table 17.2). Antibody molecules have four polypeptide chains: two identical long polypeptides (H chains) and two identical short polypeptides (L chains). The chains are held together by chemical bonds (see Figure 17.9). Antibody structure is related to its functions: (1) recognize and bind an antigen and (2) inactivate the bound antigen. At one end of each polypeptide chain is an antigen- binding site formed by the ends of the H and L chains. This site recognizes and binds to a specific antigen. Formation of an antigen-antibody complex leads to the destruction of an antigen. Humans can produce billions of different antibody molecules, each of which can bind to a different antigen. Because there are billions of such combinations, it is impossible for each antibody molecule to be encoded directly in the genome; there simply is not enough DNA in the human genome to encode hundreds of millions or billions of antibodies. Synthesis of a vast number of different antibodies is possible as a result of genetic recombination in three clusters of antibody genes. These are the heavy-chain genes (H genes) on chromosome 14 and two clusters of light-chain genes—L genes on chromosome 2 and L genes on chromosome 22. These recombination events take place in B cell nuclei during maturation, producing a unique gene in each B cell that produces one type of antibody. This rearranged gene is stable and is passed on to all daughter B cells. This process of recombination makes it possible to produce billions of possible antibody combinations from only three gene sets.

Cancer-causing mutations of the cell cycle

Cause the cell to bypass checkpoints in the cycle

Stem cells

Cells with two properties: the ability to replicate themselves, and the ability to form a variety of cell types in the body

The technique of _________ is a form of genetic disorder therapy in which a patient's defective protein is artificially provided.

Enzyme replacement therapy

The Adaptive Immune Response Is a Specific Defense Against Infection

If the nonspecific inflammatory response fails to stop an infection, the last line of defense, a powerful system called the adaptive immune response, is called into action. The adaptive immune system generates chemical and cellular responses that neutralize and/or destroy viruses, bacteria, fungi, and cancer cells. The adaptive immune response develops more slowly than the innate response, but it is more effective than the nonspecific defense system and has a memory component that remembers previous encounters with infectious agents (the innate immune system has no memory component). Immunological memory allows a rapid, massive response to a second exposure to a pathogen.

Antigens

Molecules usually carried or produced by viruses, microorganisms, or cells that initiate antibody production.

Autoimmune reactions cause the immune system to attack the body.

One of the most elegant properties of the immune system is its capacity to distinguish self from nonself and destroy what it perceives as nonself. Shortly after birth, the immune system "learns" not to react against the cells of the body. In some disorders, this immune tolerance breaks down, and the immune system attacks and kills cells and tissues in the body. Juvenile diabetes, also known as insulin-dependent diabetes (IDDM; MIM 222100), is an autoimmune disease. Normally, cells in the pancreas produce insulin, a hormone that lowers blood sugar levels. In IDDM, the immune system attacks and kills the insulin-producing cells, causing lifelong diabetes and the need for insulin injections to control blood sugar levels. Other forms of autoimmunity—such as systemic lupus erythematosus (SLE; MIM 152700)—attack blood cells, organelles such as mitochondria, and DNA-binding proteins in the nucleus. Lupus slowly destroys major organ systems, including the kidneys and the heart. Table 17.5 lists some autoimmune diseases

Which combination poses the greatest risk for hemolytic disease of the newborn due to Rh incompatibility

Rh negative mother X Rh positive father

Organ Transplants Must Be Immunologically Matched

Successful organ transplants and skin grafts depend on matches between cell-surface antigens of the donor and the recipient. These antigens are proteins found on all cells in the body and serve as identification tags, helping distinguish self from nonself. These antigens are encoded by the major histocompatibility complex (MHC), a cluster of 140 genes on chromosome 6. A set of 9 genes within the MHC are known as human leucocyte antigen (HLA) genes. Each HLA gene has many different alleles, and the combinations of these alleles are nearly endless. The set of HLA alleles carried on each copy of chromosome 6 is known as a haplotype. Because each of us has two copies of chromosome 6, we each have two HLA haplotypes Because there are so many allele combinations, it is difficult to find two people with exactly the same HLA haplotypes. The exceptions are identical twins, who will have identical HLA allele haplotypes, and siblings, who have a 25% chance of being matched. In the example shown in Figure 17.15, each child receives one haplotype from each parent. As a result, four new haplotype combinations are represented in the children. (Thus, siblings have a one-in-four chance of having the same haplotypes.)

Rh blood types can cause immune reactions between mother and fetus.

The Rh blood group (named for the rhesus monkey, in which it was discovered) includes people who can make the Rh antigen (Rh-positive, Rh1) and those who cannot make the antigen (Rh-negative, Rh2). The Rh blood group is a major concern when the mother is Rh2 and the fetus is Rh1. This incompatibility can result in a condition known as hemolytic disease of the newborn (HDN) (Figure 17.14). If Rh1 blood from the fetus enters the Rh− maternal circulation (this often happens during the birth process), the mother's immune system will produce antibodies against the Rh antigen. During a subsequent pregnancy with an Rh+ fetus, massive amounts of maternal antibodies against the Rh+ antigen cross the placenta in late stages of pregnancy and destroy the fetus's red blood cells, resulting in HDN. To prevent HDN, Rh2 mothers are given an Rh-antibody preparation (RhoGam) during the first pregnancy or after a miscarriage or abortion if the child or fetus is Rh1. The injected Rh antibodies destroy any Rh1 fetal cells that may have entered the mother's circulation. To be effective, this antibody must be administered before the mother's immune system can make antibodies against the fetal Rh antigen.

How does the immune response function?

The adaptive immune response is mediated by white blood cells called lymphocytes. The two main cell types in the immune system are called B cells and T cells. Both cell types are formed by mitotic division from stem cells in bone marrow, and both play important roles in the immune response. Once formed, B cells mature in the bone marrow. As they develop, each B cell becomes genetically programmed to produce large quantities of a unique protein called an antibody. Antibodies are displayed on the surface of the B cell and bind to foreign molecules and microorganisms such as bacterial or fungal cells and toxins in order to inactivate them. Antibodies recognize and bind to molecules called antigens (antibody generators). Most antigens are proteins or proteins combined with polysaccharides, but any molecule, regardless of its source, that can bind to an antibody is an antigen. T cells are formed in bone marrow, and while still immature, migrate to the thymus gland where they become genetically programmed to produce unique cell-surface proteins called T-cell receptors (TCRs). These receptors bind to pathogen proteins present on the surface of cells infected with viruses, bacteria, or intracellular parasites. Mature T cells circulate in the blood and are also found in lymph nodes and the spleen. It is important to remember that each B cell makes only one type of antibody and each T cell makes only one type of receptor. Because there are literally billions of possible antigens, there are billions of possible combinations of antibodies and TCRs. When an antigen binds to a TCR or an antibody on the surface of a T cell or B cell, it stimulates that cell to divide, producing a large population of genetically identical descendants, or clones, all with the same TCR or antibody. This process is called clonal selection Specific molecular markers on cell surfaces also play a role in the immune response. Each cell in the body carries recognition molecules that prevent the immune system from attacking our organs and tissues. These markers are encoded by a set of genes on chromosome 6 called the major histocompatibility complex (MHC). MHC proteins also play a major role in successful organ transplants, as will be described in a later section. The immune system has two interconnected parts: antibody-mediated immunity, regulated by B-cell antibody production, and cell-mediated immunity, controlled by T cells. The two systems are connected by helper T cells. The steps involved in the responses of the two systems are similar: 1. B cells or T cells recognize an antigen. 2. The cells become activated and divide to form a clone of identical cells. 3. The clones of activated cells attack and destroy the invading pathogens, clearing the antigens from the body. 4. Some activated cells form memory cells that circulate through the body, ready to mount a rapid and massive response if the same pathogen invades the body again. Antibody-mediated reactions detect antigens circulating in the blood or body fluids and interact with helper T cells, which signal the B cells that make antibodies against that antigen to divide. Cell-mediated immunity attacks cells of the body infected by viruses or bacteria. T cells also protect against infection by parasites, fungi, and protozoans. One group of T cells also can kill cells of the body if they become cancerous. 1 compares the antibody-mediated and cell-mediated immune reactions

The antibody-mediated immune response involves several stages.

The antibody-mediated immune response has three stages: (1) antigen detection, (2) activation of helper T cells, and (3) division of B cells to form antibody-producing plasma cells A specific immune system cell type controls each of these steps. Let's start with a T cell as it encounters an antigen and follow the stages of antibody production and the resulting immune response. In this example, we'll begin with a type of white blood cell called a dendritic cell, which engulfs and destroys bacteria. Once the bacterial cell is inside, some of the partially digested bacterial proteins bind to dendritic proteins called class II MHC proteins. These protein complexes move to the surface of the dendritic cell, converting it to an antigen-presenting cell (APC). When a T cell with antigen-specific TCRs on its surface encounters a matching antigen on the surface of an APC, the APC activates the T cell, which then divides to form a large clone of cells called helper T cells. The steps in T-cell activation are summarized in In the last stage of the antibody-mediated immune response, a helper T cell activates a B cell carrying the antigen recognized by the T cell. In response to activation, the B cell begins to divide and secrete antibodies. If the B cell binds to bacterial antigen molecules it encounters in the bloodstream, the B cell can be directly activated by a helper T cell. Once the antigen is internalized, pieces of the antigen bind to class II MHC proteins, which move to the surface of the B cell. When a helper T cell meets a B cell displaying the same antigen, they link together, and the T cell secretes a cytokine called interleukin that activates the B cell. The activated B cell divides to form two types of daughter cells. The first type is plasma cells, which synthesize and secrete 2,000 to 20,000 antibody molecules per second into the bloodstream. The steps in B-cell activation are summarized in Figure 17.7. Plasma cells have cytoplasm filled with rough endoplasmic reticulum— an organelle associated with protein synthesis (Figure 17.8). A second cell type, a memory B cell, also forms at this time. Plasma cells live only a few days, but memory cells have a life span of months or even years. Memory cells are part of the immune memory system and are described in a later section.

Inflammatory response

The body's reaction to invading microorganisms, a nonspecific active defense mechanism that the body employs to resist infection.

T cells mediate the cellular immune response

The cellular immune response is mediated by cytotoxic, killer T cells. Cytotoxic T cells find and destroy cells of the body that are infected with a virus, bacteria, or other infectious agents (Figure 17.10). When a cell becomes infected with a virus, viral proteins bound to class I MHC proteins appear on the surface, forming an APC cell. Those foreign antigens are recognized Immunoglobulins (Ig) The five classes of proteins to which antibodies belong. Killer T cells T cells that destroy body cells infected by bacteria. These cells can also attack and kill cancer cells and cells of transpby receptors on the surface of a type of T cell called a CD8+ cell. The activated T cell divides to form a clone of cells, some of which form memory T cells. The cytotoxic T cell attaches to the infected APC cell and secretes a protein, perforin, which punches holes in the plasma membrane of the infected cell. The cytoplasmic contents of the infected cell leak out through the holes, and the infected cell dies and is removed by phagocytes. Cytotoxic T cells also kill cancer cells (Figure 17.11) and transplanted organs if they recognize them as foreign. Table 17.3 summarizes the nonspecific and specific reactions of the immune system

The Complement System Kills Microorganisms

The complement system is a chemical defense mechanism that works with nonspecific responses (inflammation) and specific responses (adaptive immune response) to combat infection. Its name derives from the way it complements the action of the immune system. The complement system consists of 20-30 different proteins synthesized in the liver and secreted into the blood plasma as inactive precursors. Complement proteins are activated by contact with certain molecules on the surface of pathogens and respond by mounting one or more responses. Proteins activated at the site of infection activate other nearby complement proteins, starting a cascade of responses (Figure 17.2). Several components in this pathway form a large, multiprotein complex called the membrane-attack complex (MAC). The MAC embeds itself in the plasma membrane of an invading microorganism, creating a pore (Figure 17.3). Fluid from the blood plasma flows through the pore into the invading cell in response to an osmotic gradient, eventually bursting the cell. In addition to destroying microorganisms directly, some complement proteins guide white blood cells called phagocytes to the site of infection. The phagocytes engulf and destroy the invading cells. Other parts of the complement system aid the immune response by binding to the surface of microorganisms and marking them for destruction.

Genetic disorders can impair the immune system

The first disease of the immune system was described in 1952 by a physician who examined a young boy who had suffered at least 20 serious infections in the preceding 5 years. Blood tests showed that the child had no detectable antibodies anywhere in his body. Other patients with similar problems were soon discovered. All affected individuals were boys who were highly susceptible to bacterial infections. In all cases, either B cells were completely absent or immature and unable to produce antibodies, but there were nearly normal levels of T cells. In other words, antibody-mediated immunity is absent or impaired, but cellular immunity is normal. This heritable disorder, called X-linked agammaglobulinemia (XLA; MIM 300300), usually appears 5 to 6 months after birth, when maternal antibodies disappear and the infant's B-cell population begins to produce antibodies. People with XLA lack mature B cells but do have normal populations of immature B cells, indicating that the defective gene controls some stage of B-cell development. The XLA gene encodes an enzyme that transmits signals that initiate a program of gene expression that triggers B-cell maturation. Understanding the role of this enzyme in B-cell development may permit the use of gene therapy to treat this disorder. A rare genetic disorder of the immune system causes a complete absence of both antibody-mediated and cell-mediated immune responses. This condition is called severe combined immunodeficiency disease (SCID; MIM 102700, 600802, and others). Affected individuals have recurring and severe infections and usually die at an early age from seemingly minor infections. One of the longest known survivors of this condition was David, the "boy in the bubble," who died at 12 years of age after being isolated in a sterile plastic bubble for all but the last 15 days of his life.

Immunoglobulins (Ig)

The five classes of proteins to which antibodies belong.

HIV attacks the immune system

The immunodeficiency disorder currently receiving the most attention is acquired immunodeficiency syndrome (AIDS). AIDS is a collection of disorders that develop after infection with the human immunodeficiency virus (HIV) (Figure 17.21). Worldwide, about 33 million people are infected with HIV (Table 17.6). Once in the body, HIV infects and kills T4 helper cells. As we discussed earlier, these cells act as the "on" switch for the immune response. After HIV invades a T4 cell, it copies its genetic information (carried as an RNA molecule) into a DNA molecule, which is inserted into a chromosome in the infected cell, where it can remain inactive for months or years. Later when the infected T4 cell is called upon to participate in an immune response, the viral genes become active. New viral particles are formed in the cell and bud off the surface of the T cell, rupturing and killing it. The released viruses spread through the body and infect other T4 cells. Over the course of an HIV infection, the number of helper T4 cells gradually decreases, and the body loses its ability to fight infection. Without an active immune system, infected people become ill from many diseases they would otherwise fight off. Eventually, by killing the T4 helper cells, HIV infection disables the immune system, resulting in AIDS. The result is increased susceptibility to infection and increased risk of certain forms of cancer. The eventual outcome is premature death brought about by any of a number of diseases that overwhelm the body and its compromised immune system. HIV is transmitted from infected to uninfected individuals through body fluids, including blood, semen, vaginal secretions, and breast milk. The virus cannot live for more than 1 to 2 hours outside the body and cannot be transmitted by food, water, or casual contact.

Genetic disorders cause inflammatory diseases.

The inner layer of intestinal cells is a physical barrier that prevents bacteria in the digestive system from crossing into the body. Failure of the immune system to monitor or respond to bacteria that somehow cross this barrier results in inflammatory bowel diseases. Inflammatory bowel diseases are genetically complex and involve the interaction of environmental factors with genetically predisposed individuals. Ulcerative colitis (MIM 191390) and Crohn disease (MIM 266600) are two types of inflammatory bowel disease caused by malfunctions in the immune system. Crohn disease occurs with a frequency of 1 in 1,000 individuals—mostly young adults. The frequency of this disorder has increased greatly over the last 50 years, presumably as a result of unknown environmental factors. A genetic predisposition to Crohn disease maps to chromosome 16. The gene for this predisposition has been identified and cloned; the NOD2 gene encodes a receptor found on the surface of certain cells of the immune system. Normally, the receptor detects the presence of signal molecules on the surface of invading bacteria. Once activated, the receptor signals a protein in the cell nucleus to begin the inflammatory response. In Crohn disease, the protein encoded by the mutant allele is defective and causes an abnormal inflammatory response that damages the intestinal wall. The mutant allele of NOD2 confers only a predisposition; unknown environmental factors and other genes are probably involved in this disorder.

Colon Cancer Is a Model for the Development of Cancer

The transition from a normal cell to a cancerous one requires the acquisition of a specific number of mutations in specific genes. In retinoblastoma, two mutational steps are required to convert a normal cell into a cancerous one. In other cases, a half dozen or more mutations are required to initiate the formation of a cancer cell. Colon cancer is one of the latter types. The number and order of genetic changes in colon cancer are a model for defining how a normal cell transforms into a cancer cell. Colon/rectal cancer is one of the most common forms of cancer in the United States and worldwide, more than 1 million cases are diagnosed each year. The development of colon cancer involves genetic and environmental factors such as diet and lifestyle as well as interactions among these factors. Unraveling the genetic causes of colon cancer has resulted in improved screening and targeted therapies for treatment. Most cases of colorectal cancer are sporadic and occur in people with no family history of the disease. However, in about 5% of all cases, an inherited predisposition to colon cancer associated with mutations in specific genes has been identified. Inherited predispositions lead to colon cancer along one of two pathways, both of which are autosomal dominant traits. One form, called familial adenomatous polyposis (FAP; MIM 175100), is coupled with chromosomal instability; the other, connected to failures in DNA repair, is called hereditary nonpolyposis colon cancer (HNPCC; MIM 120435 and 120436). FAP accounts for only about 1% of all cases of colon cancer but has been useful in deriving the main features of the genetic model for colon cancer described next.

Breast cancer risks depend on genotype.

Together, mutations in BRCA1 and BRCA2 account for only 15% to 20% of all cases of breast cancer. Women who carry one mutant allele of BRCA1 or BRCA2 have up to an 85% risk of developing breast cancer by age 70. In contrast, women in the United States who carry two normal alleles of BRCA1 or BRCA2 have about a 12% risk of developing breast cancer by age 90. However, common alleles of other genes have a strong influence on whether a carrier of a BRCA1 mutation will develop breast cancer. Depending on the allele combinations present in the genome, the lowest risk ranges from 28% to 50%, and the highest risk ranges from 81% to 100%. Women carrying a mutant BRCA1 or BRCA2 allele also have an increased risk of developing ovarian cancer. For women with a BRCA1 mutation, the lifetime risk is about 55%, and for women with a BRCA2 mutation, the risk is about 25%. Women who carry normal alleles of BRCA1 or BRCA2 have roughly a 1.8% risk of ovarian cancer. The risk for breast cancer associated with mutant alleles of BRCA1 and BRCA2 extends to men as well as women. Men who inherit a mutant BRCA1 or BRCA2 allele are also at risk of developing breast cancer and several thousand cases of male breast cancer are reported every year. In addition, men carrying these mutations are 3 to 7 times more likely to develop prostate cancer than men carrying normal alleles of these genes.

Successful transplants depend on HLA matching.

Transplanted organs will be attacked by the recipient's immune system if the transplanted HLA antigens do not match those encoded by the recipient's HLA genes. This mismatch generates an immune response that causes rejection of the transplant. Even in cases where the HLA haplotypes are closely matched, drugs are used to suppress any rejection response. Organ donors and recipients are matched by testing for HLA haplotype compatibility. Because there are so many HLA alleles, the best chance for a match is usually between related individuals, with identical twins having a 100% match. The order of preference for organ and tissue donors among relatives is identical twin, sibling, parent, and unrelated donor. Among unrelated donors and recipients, the chances for a successful match are only 1 in 100,000 to 1 in 200,000. Because the frequency of HLA alleles differs widely across ethnic groups, matches across groups are often more difficult. When HLA types are matched, the survival of transplanted organs is dramatically improved. The first successful organ transplant in the United States took place in 1954, when a kidney was transplanted between identical twins (Figure 17.16). The surgeon, Dr. Joseph Murray, was awarded a Nobel Prize for this medical breakthrough.

T-cell receptors (TCRs)

Unique proteins on the surface of T cells that bind to specific proteins on the surface of cells infected with viruses, bacteria, or intracellular parasites.

Disorders of the Immune System

We are able to resist infectious disease because we have an immune system. The development and function of the immune system are genetically controlled, and mutations in these genes can result in abnormal or even absent immune responses. The consequences of these failures can range from mild inconvenience to systemic failure and death. In this section, we briefly catalog some ways in which mutations can cause the immune system to fail.

The use of PGD raises ethical issues

A 2009 movie, My Sister's Keeper, is based on a novel in which PGD is used to select an embryo of the same tissue type as a sibling. The parents intended that this child be a transplant donor for her older sister, who had leukemia. While this scenario is fictional, it does have a real-life counterpart. Jack and Linda Nash's daughter, Molly, was born with Fanconi anemia (MIM 227650), a fatal bone marrow disorder. They used PGD to screen for an embryo that would be a healthy child without Fanconi anemia, but one that would also be a suitable stem-cell donor for Molly. Umbilical cord blood from their PGD-selected son, Adam, was transfused into Molly, who is now free of Fanconi anemia At the time, bioethicists debated whether it was ethical to have a child who was destined to be a donor for a sibling, a practice called having "savior babies." This case was complicated by the fact that the parents planned to have other children and used PGD to screen out embryos with Fanconi anemia. Since then, other couples used IVF and PGD to have babies who were tissue-matched to siblings with leukemia and other diseases. In these cases, the embryos were not screened for genetic disorders— only for alleles that would allow the children produced from the embryos to serve as transplant donors for their siblings. These cases have reignited the debate on whether it is ethical to select for genotypes that have nothing to do with a genetic disorder and whether screening to benefit someone else is acceptable.Advocates of embryo screening to match transplant donors and recipients say that there are no associated ethical issues, but critics wonder if embryo screening for transplant compatibility will eventually lead to screening for the sex of the embryo or designer baby traits. A survey by the Genetics and Public Policy Center at Johns Hopkins University shows that 61% of Americans surveyed approved of using PGD to select an embryo for the benefit of a sibling, but it also revealed that 80% of those surveyed were concerned that reproductive genetic technologies could get out of control. Some countries, including Great Britain, now permit PGD screening for breast and ovarian cancer, two genetic diseases with less than a 100% chance of occurrence. Other nations have laws against using PGD for sex selection or for screening embryos to be donors unless they are also screened to avoid a genetic disorder, but the United States has no such restrictions. Perhaps the most controversial use of PGD is the selection of embryos with conditions that most people would consider disabilities. One survey reported that a small percentage of clinics used PGD to select embryos that would result in deaf children or children with dwarfism. This procedure allows parents to have children who have the same physical attributes they have, but the ethics of this practice are still being debated.

Cancer and Genetics

A link between cancer and genetics, originally proposed by Theodor Boveri in the nineteenth century, is supported by four lines of evidence: 1. A predisposition to more than 50 forms of cancer is inherited to one degree or another. 2. Most chemicals that cause cancer also cause mutations. 3. Some viruses carry genes that promote and maintain the growth of cancer in infected cells. 4. Specific chromosomal changes are found in certain forms of cancer, especially leukemia. The link between cancer and genomic changes has been strengthened by results from the Human Genome Project and a number of cancer genome projects. These projects have identified genes and gene interactions that are important in transforming a normal cell into a cancerous cell. This knowledge is being used to develop individualized and often gene-specific methods of treating cancer. Mutation is a universal feature of all cancers. In the vast majority of cases, these mutations take place in somatic cells, and the mutant alleles are not passed on to offspring. In about 1% of all cases, the cancer-related mutation occurs in germ cells and the mutant allele is passed on to succeeding generations as a predisposition to cancer. Mutations that cause cancer can include single nucleotide substitutions, insertions, deletions, variations in gene copy number, and chromosome rearrangements. As we will see, cancer is a genetic disorder that begins in a single cell. Mutation is the ultimate cause of cancer and because there is a constant background of spontaneous mutations, there will always be a baseline rate of cancer. The environment (ultraviolet light, chemicals, and viruses) and behavior (diet and smoking) also can play a significant role in cancer risks by increasing the rate of mutation.

Retinoblastoma

A malignant tumor of the eye arising in retinoblasts (embryonic retinal cells that disappear at about 2 years of age). Because mature retinal cells do not transform into tumors, this is a tumor that usually occurs only in children.

In vitro fertilization (IVF) is a widely used form of ART.

After the birth of Louise Brown in 1978, in vitro fertilization (IVF) quickly became the method of choice for helping many infertile couples become parents. IVF is one of the most successful methods of ART and has resulted in the birth of millions of children worldwide. For IVF, an egg is collected and placed in a dish. Using sterile technique, sperm are added to the dish and a technician watches the process of fertilization using a microscope. After fertilization, the newly formed zygotes are placed in an incubator (Figure 16.7) for embryonic development to begin before implantation in the uterus of a female partner or a surrogate for development. The gametes used in IVF can come from a couple, from donors, or from a combination of a couple and donors. The embryo can be transferred to the uterus of the female partner or to a surrogate. In one famous case, a child ended up with five parents. This situation began with an infertile couple who wanted to be parents. They used an egg donor and a sperm donor who contributed the gametes, which were combined using IVF. To carry the child, the couple entered into a contract with a surrogate mother, who gave the child to the infertile couple. For some cases of male infertility, many couples now use a variation of IVF called intracytoplasmic sperm injection (ICSI). In this procedure, an egg is injected with a carefully selected single sperm from the male partner. The embryo develops in an incubator before transfer to the uterus of the female partner. ICSI is used mostly in cases where low sperm count or motility problems are present.

Mutant Cancer Alleles Impair DNA Repair Systems and Genome Stability

All forms of cancer share several properties: (1) higher-than-normal rates of mutation, (2) abnormalities of chromosome structure and number, and (3) one or more forms of genomic instability. This instability is seen as progressive chromosomal changes as the cancer develops, including the loss of chromosomes, and duplications, deletions, and other abnormalities. These changes are related to loss of the ability to repair DNA damage in cancer cells. Several forms of cancer associated with DNA repair defects have been identified, including breast cancer and a form of colon cancer.

New therapies for treating cancer

Cancer therapy has traditionally used radiation and chemicals to target and kill all rapidly dividing cells (see The Genetic Revolution: Cancer Stem Cells). Although cancer cells are dividing rapidly, so are other cells in the body, including those in bone marrow (making red blood cells), the intestine (replacing worn-away cells), and many other tissues. All these cells are destroyed or damaged along with cancer cells during radiation treatment or chemotherapy, often with serious side effects for the patient.

Therapy for Genetic Disorders

Although PGD and other methods of genetic testing allow couples to have children who are free of genetic disorders, about 5% of all newborns have a genetic or chromosomal disorder. The idea that some genetic disorders could be treated began in the mid-1960s, when a cell biologist, Christian DeDuve, proposed that symptoms of disorders caused by defective enzymes might be improved by providing the missing enzyme. He suggested that this procedure, called enzyme replacement therapy (ERT), could be used as a therapy for genetic disorders involving missing lysosomal enzymes (see Chapter 2 for a discussion of lysosomes). In the 1970s, the underlying enzyme defect in Type I Gaucher disease, lysosomal disease, was identified, and this disease became a promising candidate for ERT (see Chapter 2 for a description of Gaucher disease). The first attempts involved simply purifying the enzyme and putting it into the body by intravenous transfusion. Other investigators delivered the enzyme sealed in red blood cell membrane fragments or microscopic lipid spheres. The results were encouraging, but the method remained experimental for almost 20 years. In the 1990s, the gene encoding the missing enzyme was isolated and cloned, making it possible to manufacture a recombinant DNA-based product. Advances in enzyme chemistry in combination with biotechnology now make ERT one of the main treatment options for Gaucher disease. However, the procedure requires intravenous treatment every 2 weeks for life at a cost of $150,000-$200,000 per year. While ERT is successful in controlling the symptoms of Gaucher disease, the ultimate goal is to cure genetic diseases, and the most direct way to do this is by transferring a correct copy of a mutant gene into the body. This approach, called gene therapy, involves the delivery of a normal version of a mutant gene to cells in the body. Once in the cell, the expression of this gene will produce a functional protein that restores cellular function and results in a normal phenotype.

Prenatal testing is associated with risks.

Although many genetic disorders and birth defects can be detected with prenatal testing, the technique has some limitations. Prenatal testing poses measurable risks to the mother and the fetus, including infection, hemorrhage, fetal injury, and spontaneous abortion. For example, conventional testing strategies will not always detect the majority of certain defects. Amniocentesis is recommended for all pregnant women 35 years of age and older to test for Down syndrome, because maternal age is the biggest risk factor for having an affected child. Older women have only about 5% to 7% of all children, but they have 20% of all children with Down syndrome. This means that about 80% of all Down syndrome births are to mothers who are not candidates for amniocentesis. It is now recommended that all pregnant women have noninvasive screening for Down syndrome (noninvasive testing is discussed in Chapter 6). Genetic testing on a large scale is not always possible. For disorders such as sickle cell anemia, a single mutation is present in all cases, so testing is efficient and uncovers all cases. In cystic fibrosis (CF), however, over 1,900 different mutations have been identified (see Chapter 11), and testing for all these mutations is impractical. Many mutations are found only in one family, and others are found primarily in one ethnic group or another. Using a panel of 25 of the most common mutations to test for CF results in an accurate diagnosis for some mutations but poor results for others (Table 16.2). Thus, at the moment, CF testing is not widely performed.

Zygote intrafallopian transfer (ZIFT)

An ART procedure in which gametes are collected, fertilization takes place in vitro, and the resulting zygote (fertilized egg) is transferred to a woman's oviduct

Hereditary nonpolyposis colon cancer (HNPCC)

An autosomal dominant trait associated with genomic instability of microsatellite DNA sequences and a form of colon cancer.

Familial adenomatous polyposis (FAP)

An autosomal dominant trait resulting in the development of polyps and benign growths in the colon. Polyps often develop into malignant growths and cause cancer of the colon and/or rectum.

To illustrate how targeted therapy works, we will discuss two drugs: Gleevec (imatinib), for CML; and Herceptin, a monoclonal antibody used to treat certain types of breast cancer

As discussed above, CML is caused by the action of a hybrid BCR-ABL protein that signals the cell to divide continuously. CML cells are the only cells in the body that contain the hybrid protein, offering an opportunity to develop a drug that targets only these cells. Researchers discovered that the hybrid protein folds to form a pocket for binding ATP, a molecular-energy source required for its signaling activity (Figure 12.18). Using that information, researchers designed a drug (Gleevec) that fits into the ATP-binding pocket of the BCR-ABL protein and prevents ATP from entering the pocket. Without ATP to activate the hybrid protein (see Figure 12.18), the BCR-ABL protein does not signal for division, and the cancer cells stop dividing. More than 90% of patients with CML treated with this drug go into remission and show a dramatic reduction in the number of white blood cells carrying the Philadelphia chromosome. Gleevec has also proven effective in treating other forms of cancer, including some gastrointestinal cancers. Herceptin (tratuzumab) is a monoclonal antibody that binds to a receptor protein called HER2 on the surface of invasive breast cancer cells. The HER2 protein transfers signals from the external environment to the nucleus, where new programs of gene expression drive cell growth and division. In breast cancer, the HER2 protein is locked in the "on" position, continuously signaling cells to divide. When Herceptin binds to HER2, signaling is silenced and uncontrolled cell division stops. Binding also signals the immune system to attack and destroy the cancer cells. Other forms of immunotherapy are being used to treat leukemia, lymphoma, and several other forms of cancer. The discovery that epigenetic modification of key genes is important in some cancers, and the fact that epigenetic changes are reversible, has opened the way for the development of a new class of anticancer drugs. These drugs target epigenetic modifications and reactivate genes silenced by methylation or histone modification. Several epigenetic drugs have been approved by the FDA and another 12-15 are in clinical trials. One drug, Vidaza, is used to treat a precursor to leukemia and for acute myeloid leukemia. The drug is an analog of cytidine and is incorporated into DNA during S phase of the cell cycle. Methylation enzymes bind irreversibly to the drug, preventing methylation at promoter regions, reducing the level of methylation in cancer cells. Success in the development of cancer-treatment drugs based on detailed knowledge about the genes involved in cancer and the three-dimensional structure of their gene products and knowledge about epigenetic mechanisms has changed the way anticancer drugs are developed. In the past, drugs were discovered by screening hundreds or thousands of chemicals for their ability to slow or stop the growth of cancer cells. With an understanding of the molecular events linked to cancer, it is now possible to design drugs for treatment of specific cancers, without the side effects of other treatments.

12-2 Cancer Is a Genetic Disease

Cancer is a complex group of diseases that affects many different cells and tissues in the body. Cancers have two properties: (1) uncontrolled cell division and (2) the ability to spread, or metastasize, to other sites in the body. If a cell begins to divide in an uncontrolled way, it may form a benign tumor. This type of tumor is not cancerous and can be removed by routine surgery. However, tumors that acquire the ability to grow continuously and can break away and move to other locations are malignant, or cancerous. Unchecked, the combination of uncontrolled growth and metastasis results in death, making cancer a devastating and feared disease. Improvements in medical care have reduced deaths from infectious disease and allowed people to live longer, but these benefits have also helped make cancer a major cause of illness and death in our society. The risk of many cancers is age related , and because more Americans are living longer, they are at greater risk of developing cancer. About one in three people will be diagnosed with cancer at some time in their life, and about one in four will die from cancer. Each year, more than a million and a half new cases of cancer are identified in the United States and about 500,000 people die of cancer each year (about one death every minute), making it one of the leading causes of death in developed countries. In the United States, over 10 million individuals are receiving medical treatment for cancer in hospitals and medical centers.

Epigenetics and cancer.

As discussed in Chapter 11, epigenetics is a rapidly expanding research field that studies heritable alterations in gene expression caused by mechanisms that do not alter any DNA sequence. Abnormal patterns of DNA methylation are associated with many types of cancer. Lower-than-normal levels of methylation (demethylation or hypomethylation) are commonly observed in some types of cancer. Removal of methyl groups can activate genes involved with cell growth and can increase genomic instability. In other cancers—including retinoblastoma, breast cancer, and colon cancer—methylation inactivates key genes these changes transform normal cells into cancer cells. Imprinting (discussed in Chapter 11) selectively silences either a maternal or paternal allele by methylation. In addition to its role in development, imprinting is also involved in cancer. The NOEY2 gene (MIM 605193) is expressed in normal cells of the breast and ovary and some other cell types. Imprinting inactivates the maternal copy of the gene, and only the paternal copy is expressed. In breast cancer, both copies of the gene are imprinted and silenced, indicating that this gene may be important in controlling cell division, and that silencing this gene may represent one of the steps in converting normal cells into cancerous cells. New methods of analyzing methylation patterns in cancer cells have improved cancer diagnosis. Analysis of methylation patterns is used to distinguish invasive from noninvasive forms of cancer and to diagnose subtypes of cancer. It is hoped that the development of new drugs that can reverse abnormal methylation of specific genes and normalize gene expression in cancer cells will reestablish cell-cycle control and halt tumor growth.

BRCA1 and BRCA2 are DNA repair genes.

BRCA1 and BRCA2 are each involved in DNA repair. The normal alleles of both genes encode large proteins found only in the nucleus. In rapidly dividing cells, expression of BRCA1 and BRCA2 is highest at the G1/S transition and into S phase. The BRCA1 protein is a tumor suppressor that helps maintain genome stability during DNA repair, regulates cell-cycle checkpoints, and performs several other functions. We will focus on the role of BRCA1 in DNA repair. BRCA1 is activated when breaks in DNA strands are detected. Unrepaired breaks increase the rate of cancer-causing mutations in genes located at or near breakpoints. The activated BRCA1 protein stops DNA replication, and then participates in repairing the breaks. The mutant form of BRCA1 does not participate in DNA repair. As a result, mutations gradually accumulate within a cell and the cell becomes cancerous (remember that cancers begin in a single cell).

Mutant DNA repair genes cause a predisposition to breast cancer.

Breast cancer is the most common form of cancer in women in the United States. Each year, more than 200,000 new cases are diagnosed (Figure 12.10). Although most cases of breast cancer are unrelated to heredity, geneticists struggled for years with the question, is there a genetic predisposition to breast cancer? After more than 20 years of work, the answer is clearly yes. Mutations in at least two different genes, BRCA1 and BRCA2, can predispose women to breast cancer and ovarian cancer. Carriers of either of these mutations have a 10-to-30-fold higher risk of breast cancer than the general population. The BRCA1 gene (MIM 113705) is located on chromosome 17 (Figure 12.11); its mutant allele produces a dominantly inherited predisposition to breast cancer. Approximately 85% of women inheriting one mutant BRCA1 allele will acquire a mutation in the other BRCA1 allele and develop breast cancer. A second breast cancer predisposition gene, BRCA2 (MIM 600185), maps to the long arm of chromosome 13 (see Figure 12.11) and may be responsible for the majority of inherited predispositions not caused by BRCA1. Although mutations in BRCA1 and BRCA2 account for two-thirds of inherited predispositions to breast cancer, together they account for only about 15% to 20% of all cases.

Metastasis

Cancer cells break away from their original tissue. The metastasizing cells become attached to the wall of a blood vessel or lymph vessel. They secrete digestive enzymes to create an opening. Then they cross the wall at the breach Cancer cells creep or tumble along inside blood vessels, then leave the bloodstream the same way they got in. They start new tumors in new tissues

Genomics, Epigenetics, and Cancer

Cancer is a genomic disease generated by the accumulation of a number of specific mutations in tumor-suppressor genes and proto-oncogenes. Using strategies that include family studies, DNA sequencing, and genome-wide association studies, researchers are identifying the mutations and genetic modifications in cancer cells with the goal of cataloging all the mutations present in cancers. It is hoped that this knowledge will open the way to the development of new diagnostic tools and drugs for treatment.

Cancer Begins in a Single Cell

Cancers have several characteristics: ■ Cancer begins in a single cell. All the cells in a cancerous tumor are clones directly descended from one cell. ■ A cell becomes cancerous after it accumulates a number of specific mutations over a period of time; most cancers are age-related. ■ Once formed, cancer cells divide continuously. Mutations continue to accumulate, and the cancer may grow more aggressive over time. Cancer cells are invasive and can infiltrate surrounding tissues by breaking down the intercellular matrix as they migrate. In addition, cancer cells can detach from the primary tumor and move to other sites in the body, forming new malignant tumors. This process is called metastasis The ability to invade new tissues results from additional new mutations in cancer cells. In the following sections, we consider the genetic changes that take place within cells that lead to cancer

The use of ART carries risks to parents and children.

Each year, about 1.5% of babies born in the United States are conceived through IVF. In about 40% of all cases, three or more embryos are transferred, and about 20% of all pregnancies from IVF involve multiple births. These births generate health costs of $1 billion because twins and higher multiples are at a threefold higher risk of premature birth (Figure 16.11). About 42% of IVF births are premature, compared to about 13% of births in the general population. To avoid problems associated with multiple births, recent guidelines recommend that only one embryo should be transferred after IVF. IVF risks also include a threefold increase in ectopic pregnancies (a situation in which the fertilized egg implants outside the uterus, and the placenta and embryo begin to develop there). Infants born after ART have an increased risk of low birth weight and often require prolonged hospital care. When ICSI is used in ART, there is an increased risk of transmitting genetic defects to male children. About 13% of infertile males with a low sperm count carry a FIGURE 16.10 Teresa Anderson, a surrogate mother carrying quintuplets for parents Mr. and Mrs. Gonzales. Dave Cruz/Arizona Republic Copyright 2016 Cengage Learning. All Rights small deletion on the Y chromosome. With ICSI, this form of infertility is passed on to their sons. The same is true for some chromosomal abnormalities, such as Klinefelter syndrome. Questions arise as to whether it is ethical to use ICSI to produce sons who will be infertile. There is an increased risk for epigenetic imprinting disorders associated with ART. These disorders include Beckwith-Wiedemann syndrome (BWS; MIM 130650) and Silver-Russell syndrome (SRS; MIM 180860). One epidemiological study showed a tenfold increase in the frequency of BWS and SRS following ART. Most of these cases had abnormal patterns of DNA methylation (see Chapter 11 for a discussion of epigenetics) at several sites in the genome. The results of this study suggest that the abnormal methylation patterns occurred after fertilization and may be associated with events in the ART procedure. There has been a long-standing debate about whether children conceived using ART have increased risks for birth defects. A recent comprehensive study that examined the risk of birth defects (heart circulatory system, skeletal, muscular) associated with different types of ART has shown that there is an increased risk for birth defects associated with ART. The risk of birth defects associated with ART was 8.3%, compared to 5.8% for unassisted conception. The study also showed that risks differ depending on the type of ART used. The overall risk for IVF was 7.2% but was 9.9% for ICSI. So while ART is an effective treatment for infertility, it imposes several types of costs on individuals, families, and on society

Infertility in women has many causes.

Earlier, we listed some components of successful reproduction. Three of these are associated with the female reproductive system: the egg, the oviduct, and the uterus. Problems with any of these components can lead to infertility. Figure 16.2 lists causes of infertility in women. One of the most common problems is hormone imbalance that causes ovulation problems. Release of an egg from the ovary (ovulation) depends on the levels of female hormones and their interactions. Several conditions can prevent or interfere with ovulation: ■ Hormone levels. If estrogen levels are too low, or no estrogen is present, ovulation will not occur. Normally, during a monthly cycle, estrogen levels rise to a peak, trigger ovulation, and stimulate production of another hormone, called luteinizing hormone (LH). Hormonal problems occur in about 50% of all cases in which ovulation does not take place. ■ Ovarian problems. Damage to the ovaries caused by abdominal surgery, inflammation, autoimmune disease, infection, or the presence of ovarian cysts can prevent egg maturation or release. If ovaries have been removed or are not present because of a developmental accident or a genetic disorder, infertility results. ■ Oviduct and uterine problems. Oviduct blockage occurs in about 15% of all cases of female infertility. Chlamydia infection and other sexually transmitted diseases (STDs), viral infections, and several other agents can cause inflammation, scarring, and blockage of the oviduct. Even seemingly unrelated conditions such as appendicitis or a bowel problem called colitis can cause abdominal inflammation that blocks oviducts. A condition called endometriosis, a problem with the inner lining of the uterus, can also cause infertility. If the endometrium is not formed properly during each menstrual cycle, an embryo may not be able to implant, resulting in a miscarriage.

Cancer Develops in Several Steps

Families with high rates of cancer were identified hundreds of years ago, but in most cases no clear-cut pattern of inheritance can be identified, and the cancers are classified as familial. Why is it that some families have a rate of cancer that is much higher than average? Many explanations have been offered, including inheritance, environmental agents, and chance. A small fraction of these families do carry mutant alleles related to cancer, and studies using these families have helped identify a number of genes involved in cancer. In these families, some members inherit a mutant allele that causes a predisposition to cancer The mutant allele is present in the germ cells and all the somatic cells of these individuals in a heterozygous condition. A mutation in the other, normal allele of the gene must occur before the cell can become cancerous. This mutational event is known as loss of heterozygosity (LOH). In most cases, mutations in several other genes in this cell are needed to complete the transformation from a normal cell into a cancerous one. Only a small fraction (less than 5%) of all cancers is associated with an inherited predisposition; most cancers are sporadic and arise by the gradual accumulation of mutations in key genes within a single cell The exact number of mutations needed to cause cancer is highly variable and specific for different forms of cancer, but two mutations may be the minimum number needed. In most cases, cancer-causing mutations accumulate over a period of years; this explains why age is a leading risk factor for many cancers.

Egg retrieval or donation is an option.

For infertile women who can ovulate but have blocked oviducts or related problems, several eggs at a time can be collected after hormone treatment and sorted using a microscope to remove those that are too young or too old for use in fertilization. After IVF, one or more embryos are implanted in the uterus. Extra eggs that will not be fertilized and implanted immediately can be stored in a freezer for later use or donation to other women. The discovery that the age of the egg, not the age of the reproductive system, is a leading factor responsible for age-related infertility makes it possible for women in their late 50s or even into their 60s to become pregnant using IVF and eggs donated by younger women. Shows the dramatic increase in the use of ART by older women in Great Britain over a 10-year period. In 1992, no women over 50 had had IVF treatments, but by 2002, nearly 100 women over the age of 50 had used the procedure, and 24 children were born to those mothers. Pregnancy in older women poses higher risks of diabetes, stroke, high blood pressure, and heart attacks, and these women have a threefold higher risk of having low-birthweight and premature infants. A decision by a postmenopausal woman to have a child is usually made after her health has been carefully evaluated. This discovery also means that younger women can safely postpone childbearing until they are older without having to rely on donated eggs. Women can have their eggs collected and fertilized while they are young and frozen for later use. This form of ART allows younger women to produce embryos at a time when their risks for chromosome abnormalities in the offspring are low. The embryos can be thawed and implanted over a period of years—including after menopause—allowing women to extend their childbearing years.

GIFT and ZIFT are based on IVF

For some couples, IVF is not the only option. In a method known as GIFT, or gamete intrafallopian transfer (oviducts are also known as fallopian tubes), gametes are collected and placed into the woman's oviduct through a small incision (Figure 16.9a). Fertilization takes place in the oviduct, and the woman carries the child to term. This method can be used where infertility is not the result of oviduct blockage. Another option is ZIFT, or zygote intrafallopian transfer (Figure 16.9b). In ZIFT, eggs are collected and fertilized by IVF. The resulting fertilized egg, or zygote, is implanted into the oviduct through a small abdominal incision. ZIFT is used in cases where the woman has ovulation problems, or the man has a low sperm count, and the oviducts are not blocked

Gene therapy showed early promise.

Gene therapy began in 1990, when the human gene for the enzyme adenosine deaminase (ADA) was inserted into a retrovirus and transferred into the white blood cells of a young girl born with a form of severe combined immunodeficiency (SCID; MIM 102700). She had no functional immune system and was prone to infections, many of which could be fatal. The normal ADA gene, which was inserted into her white blood cells, encodes an enzyme that allows cells of the immune system to mature properly. As a result, she now has a functional immune system and is leading a normal life. In the early-to-middle 1990s, gene therapy was used to treat several genetic disorders, including cystic fibrosis and familial hypercholesterolemia. Over a 10-year period, more than 4,000 people underwent gene therapy. Unfortunately, those trials were largely failures, leading to a loss of confidence in gene therapy. Hopes for gene therapy plummeted even further in September 1999, when an 18-year-old patient died during gene therapy to treat ornithine transcarbamylase deficiency (MIM 300461). His death was triggered by a massive immune response to the vector, a modified adenovirus (adenoviruses cause colds and respiratory infections). In 2000, two French children who underwent successful gene therapy for an X-linked form of SCID (MIM 300400) developed leukemia. In those children, the recombinant virus inserted itself into a gene that controls cell division, activating the gene and causing uncontrolled production of white blood cells and the symptoms of leukemia. In 2007, a woman receiving gene therapy for inflammation associated with arthritis died after receiving a second round of therapy. As in the 1999 case, the vector was a modified adenovirus. In the wake of her death, the U.S. Food and Drug Administration (FDA) stopped all gene therapy trials using those vectors until the cause of death could be determined.

There are ethical issues associated with gene therapy.

Gene therapy is carried out using an established set of ethical and medical guidelines. All patients are volunteers, gene transfer is started only after the case has undergone several reviews, and the trials are monitored to protect the patients' interests. Newer guidelines instituted after gene therapy deaths have strengthened these protections and coordinated the role of government agencies that regulate gene therapy. Other ethical concerns have not been resolved, as described next. At present, gene therapy uses somatic cells as targets for transferred genes. This form of gene therapy is called somatic gene therapy. Genes are transferred into somatic cells of the body; the procedure involves only a single target tissue, and only one person is treated (only after obtaining informed consent and permission for the treatment). Two other forms of gene therapy are not yet in use, mainly because the ethical issues surrounding them have not been resolved. One of these is germ-line gene therapy, in which cells that produce eggs and sperm are targets for gene transfer. After germ-line therapy, the transferred gene would be present in all the cells of the individual produced from the genetically altered gamete, including his or her germ cells. As a result, members of future generations will be affected by this gene transfer, without their consent. Do we have the right to genetically modify others without their consent? Can we make this decision for members of future generations? These and other ethical concerns have not been resolved, and germ-line therapy is currently prohibited. The other is enhancement gene therapy, which raises even more ethical concerns. If we discover genes that control a desirable trait such as intelligence or athletic ability, should we use them to enhance someone's intellectual ability or athletic skills? For now, the consensus is that we should not use gene transfer for such purposes. However, the FDA allows the use of growth hormone produced by recombinant DNA technology to enhance the growth of children who have no genetic disorder or disease but are likely to be shorter than average adults. Critics point out that approving transfer of a gene for enhancement is only a short step from the current practice of approving a gene product for enhancement.

Enhancement gene therapy

Gene transfer to enhance traits such as intelligence and athletic ability rather than to treat a genetic disorder.

Germ-line gene therapy

Gene transfer to gametes or the cells that produce them. Transfers a gene to all cells in the next generation, including germ cells.

Somatic gene therapy

Gene transfer to somatic target cells to correct a genetic disorder.

Oncogenes

Genes that induce or continue uncontrolled cell proliferation.

Genetic Counseling Assesses Reproductive Risks

Genetic counseling is a process of communication about the occurrence of or risk for a genetic disorder in a family (Figure 16.20). Counseling involves one or more trained professionals, who help an individual or family understand each of the following: ■ The medical facts, including the diagnosis, progression, management, and any available treatment for a genetic disorder ■ The way heredity contributes to the disorder and the risk of having children with the disorder ■ The alternatives for dealing with the risk of recurrence ■ Ways to adjust to the disorder in an affected family member or to the risk of recurrence Genetic counselors achieve these goals in a nondirective way. They provide the information necessary for individuals and families to make the decisions best suited to them based on their own cultural, religious, and moral beliefs.

Genetic Testing and Screening

Genetic testing is done to identify individuals who have or may carry a genetic disease, those at risk of producing a genetically defective child, and those who may have a genetic susceptibility to environmental agents. Genetic screening is done on populations in which there is a risk for a particular genetic disorder. Genetic testing is most often a matter of choice, whereas genetic screening is often a matter of law. There are several types of testing and screening: ■ Carrier testing done on members of families or ethnic groups with a history of a genetic disorder such as sickle cell anemia or cystic fibrosis Prenatal testing on a fetus for a genetic disorder such as cystic fibrosis ■ Presymptomatic testing, also called predictive testing, for those who will develop adultonset genetic disorders such as Huntington disease and polycystic kidney disease (PCKD) ■ Newborn screening for infants within 48 to 72 hours after birth for a variety of genetically controlled metabolic disorders

The Inflammatory Response Is a General Reaction

If microorganisms such as bacteria penetrate the skin or the epithelial cells lining the respiratory, digestive, and urinary systems, a nonspecific inflammatory response develops (Figure 17.1). For example, in the area around a wound, white blood cells called macrophages detect and bind to molecules on the surface of the invading bacteria. Binding to the bacterial cell activates the macrophage, which then engulfs and destroys the bacteria. Activated macrophages also secrete chemical signal molecules called cytokines. The cytokines, along with histamine secreted by other cells in the area of infection, cause nearby capillaries to dilate, increasing blood flow to the area (that's why the area around a cut or scrape gets red and warm). The heat creates an unfavorable environment for bacterial growth, mobilizes additional white blood cells, and raises the metabolic rate in nearby cells. These reactions promote healing. Additional white blood cells migrate out of the capillaries and flood into the area in response to the chemical signals, engulfing and destroying the invading bacteria. If infection persists, clotting factors in blood plasma trigger a cascade of small blood clots that seal off the injured area, preventing the escape of invading organisms, recruiting more white blood cells to destroy the invading bacteria. Finally, the area is targeted by white blood cells that clean up dead bacteria and dispose of dead cells and debris. This chain of events, beginning with the release of chemical signals and ending with cleanup, is the inflammatory response. The inflammatory response is usually enough to stop the spread of infection. In some cases, however, mutations in genes encoding proteins involved in the inflammatory response can produce clinical symptoms of an inflammatory disease.

Cell-mediated immunity

Immune reaction mediated by T cells directed against body cells that have been infected by viruses or bacteria.

Antibody-mediated immunity

Immune reaction that protects against invading viruses and bacteria using antibodies produced by plasma cells.

Loss of heterozygosity (LOH)

In a cell, the loss of normal function in one allele of a gene where the other allele is already inactivated by mutation.

Infertility in men involves sperm defects.

In males, problems with sperm formation or the structure and function of mature sperm are the main causes of infertility (Figure 16.3). These include: ■ Low sperm count. Too few sperm (less that 20 million per milliliter of ejaculate) is a condition called oligospermia. This condition makes it difficult for the necessary number of sperm to swim up the fallopian tubes and meet the egg. Low sperm count can be caused by many factors, including environmental exposure to chemicals or radiation, use of recreational drugs (including marijuana), mumps and other diseases (such as diabetes), alcohol consumption, or even underwear that is too tight. Other factors including obesity or hormonal imbalances also contribute to low sperm count. The most common cause of low sperm count is a condition called varicocele, or enlarged veins in the testes. These enlarged veins raise the temperature in the testes above the optimum for critical events in sperm formation, leading to infertility. Surgical correction of this condition often leads to fertility. ■ Low sperm motility. If sperm move too slowly through the cervical mucus, they cannot get to the egg while it is in the oviduct for fertilization. Some sperm have malformed or extra tails or do not swim toward the egg in an organized way; others just move too slowly. ■ Genetics plays a role in male infertility. About 7% of infertile men carry microdeletions of the Y chromosome, but the location and size of these deletions are not always correlated with defects in sperm production. ■ No sperm in semen. This condition is known as azoospermia. About 40% of all cases involve blockage of the epididymis or vas deferens, which prevents sperm from moving out of the testes. This can be caused by a urinary tract infection, STDs such as chlamydia or gonorrhea, or a vasectomy. Another condition, called nonobstructive azoospermia, is caused by a lack of sperm production. Often, problems with sperm formation are caused by hormonal imbalances, including use of anabolic steroids, developmental problems such as undescended testes, or injury to the testes.

In summary, FAP-associated colon cancer

In summary, FAP-associated colon cancer requires a series of specific mutations that accumulate over time in a specific sequence within a single cell. Each mutation confers a slight growth advantage to the cell, allowing it to grow and divide to form a polyp, which enlarges and transforms in later stages as it gradually breaks away from cell-cycle controls. Eventually, one cell in the polyp accumulates enough mutations to escape completely from cell-cycle controls to become a malignant tumor. Later, additional mutations accumulate in some tumor cells, which become metastatic and break away to form tumors at remote sites. Genomic techniques have been used to identify other cases in which cancer involves a number of mutations at specific chromosomal sites, often on different chromosomes

In summary, there are at least two paths to colon cancer

In summary, there are at least two paths to colon cancer: one (FAP) that begins with a mutation in the APC gene and the other (HNPCC) that begins with a mutation in a DNA repair gene. Mutation in APC causes formation of hundreds or thousands of polyps. These noncancerous growths progress slowly to cancer by accumulating mutations in other genes. Because there are thousands of polyps, there is a good chance that at least one of them will become cancerous. In HNPCC mutations, polyps accumulate slowly, if at all, forming only a small number of benign tumors. However, mutations in these polyps accumulate at an accelerated rate that is two to three times faster than that in normal cells, making it almost certain that at least one polyp will progress to colon cancer. Mutations in DNA repair genes do not directly lead to cancer but do increase the mutation rate across the genome. Genetic disorders with DNA repair defects (Table 12.5) have chromosome instability and a susceptibility to cancer. The idea that DNA repair defects lead to genomic instability may help explain why many forms of cancer are characterized by genomic instability and aneuploidy.

The Body Has Three Levels of Defense Against Infection

In the course of an average day, we encounter pathogens (disease-causing agents) of many kinds: viruses, bacteria, fungi, and parasites. Fortunately, we possess several levels of defense against infection by pathogens. Each level brings an increasingly aggressive response. Humans have three levels of defense: (1) the skin and the organisms that inhabit it, (2) the innate immune system that uses nonspecific responses such as inflammation, and (3) the adaptive immune system, which mounts specific responses to infection

Infertility is a complex problem

Infertility problems fall into two classes: primary and secondary. Infertile couples who have been trying to have a child for a year or more without success have primary infertility. If a couple has had one child but has trouble conceiving a second, they have secondary infertility; some 3.3 million couples have this condition. The causes are unknown, although age may be a factor in some cases. Our discussion will focus on primary infertility.

Which of the following can be used as a treatment for cancer?

Inhibiting angiogenesis

Intrauterine insemination uses donor sperm.

Insemination with donor sperm, called IUI (intrauterine insemination), was one of the earliest methods of ART and was developed over 50 years ago to overcome problems of male infertility. In its simplest form, if a man is infertile because of low sperm count, low sperm motility, or aspermia, his female partner has sperm from a donor placed in her uterus (Figure 16.5) while she is ovulating, resulting in pregnancy. Donor sperm can be obtained from sperm banks. Donor profiles, including physical characteristics, blood type, health information, and, in some cases, educational level and profession, are provided to help select a donor. Some banks make it possible for women to use the same donor for subsequent pregnancies. Although the number of children born by IUI is unknown, the number of children born through artificial insemination is growing, and groups of donor siblings are being formed. The Donor Sibling Registry tracks these groups and provides information about the half-siblings a child may have. Registries have turned up donor sibling groups of 50 or more children, and in one case, over 150 offspring from one sperm donor have been identified. Some European countries have laws that limit how many children can be fathered by a single donor, but in the United States, there is no limit. This has raised several concerns, including the spread of genetic disorders and the chance of accidental inbreeding. Although most sperm banks in the United States screen potential donors for some common genetic disorders, it is impossible to screen for all disorders. In one case, a donor to a Philadelphia sperm bank had a rare genetic disorder called severe congenital neutropenia (MIM 202700), which affects only about 1 in 5 million individuals. Unfortunately, five children with this disorder were born to four couples who used sperm from this donor.

Knudson's multistep model of cancer

Knudson performed a statistical analysis on cases of retinoblastoma, a tumor of the retina that occurs both as an inherited disease and sporadically. He noted that inherited retinoblastoma occurs at a younger age than the sporadic disease. In addition, the children with inherited retinoblastoma often developed the tumor in both eyes, suggesting an underlying predisposition. Knudson suggested that two "hits" to DNA were necessary to cause the cancer. In the children with inherited retinoblastoma, the first mutation in what later came to be identified as the RB1 gene, was inherited, the second one acquired. In non-inherited retinoblastoma, instead two mutations, or "hits", had to take place before a tumor could develop, explaining the later onset. It was later found that carcinogenesis (the development of cancer) depended both on the mutation of proto-oncogenes (genes that stimulate cell proliferation) and on the deactivation of tumor suppressor genes, which are genes that keep proliferation in check. Knudson's hypothesis refers specifically, however, to the heterozygosity of tumor suppressor genes. A mutation in both alleles is required, as a single functional tumor suppressor gene is usually sufficient. Some tumor suppressor genes have been found to be "dose-dependent" so that inhibition of one copy of the gene (either via genetic or epigenetic modification) may encourage a malignant phenotype, which is termed haploinsufficiency.

HNPCC is caused by DNA repair defects.

Most cancers are caused by mutations in two or more genes that accumulate over time. If one of these mutations is inherited, fewer mutations are required to cause cancer, resulting in a genetic predisposition to cancer. But if the overall mutation rate in cells is low (as we saw in Chapter 11), how do the multiple mutations needed for cancer formation accumulate in a single cell within the lifetime of an individual? Research on a second form of colon cancer has provided a partial answer to this question. Mutations in several genes—including MSH2 (MIM 120435), MLH1 (MIM 120436), and at least five other DNA repair genes—have been identified in hereditary nonpolyposis colon cancer (HNPCC). Mutations in MSH2 and MLH1 account for about 90% of all cases of HNPCC. Anyone who inherits mutant alleles of either MSH2 or MLH1 has a dominantly inherited predisposition to colon cancer that largely bypasses polyp formation. Mutations in either gene destabilize the genome, generating a cascade of mutations in short (2 to 9 nucleotides in length) DNA sequences called microsatellites. These repetitive DNA sequences are present in thousands of copies distributed throughout the genome on many chromosomes. Clusters of microsatellite repeats (usually 10 to 100 copies of the sequence), also called short tandem repeats (STRs), are found every few thousand nucleotides in the human genome. Proteins encoded by MSH2 and MLH1 repair errors made during DNA replication. When these genes are inactivated by mutation, DNA repair is defective, and microsatellite mutation rates increase by at least 100-fold. These mutations include alterations in the number of microsatellite repeats as well as changes in repeat sequence. Cells from HNPCC tumors can carry more than 100,000 microsatellite mutations scattered throughout the genome. This genomic instability, spread across many different chromosomes, promotes mutations in nearby genes (including APC and other genes involved with growth control), eventually leading to colon cancer.

Cancer-Causing Mutations Disrupt Cell-Cycle Regulation

Most of the somatic cells in the body are structurally and functionally specialized and are nondividing. These differentiated cells perform highly specific tasks, such as muscle contraction, light reception, and nerve conduction. Many of these cell types (such as liver or kidney cells) can be stimulated to divide by external signals to replace dead and damaged cells. Other cells, such as those that line the surface of ducts and cavities inside the body (e.g., cells lining the intestines and ducts) or that cover the surface of the body (skin cells), divide more or less continuously throughout an individual's lifetime. These are called epithelial cells and are the source of 80% to 90% of all cancers, including skin breast, prostate, colon, and lung cancers. Cell growth and division are regulated by many genes, whose protein products respond to external signals and control progress through the cell cycle. Cancer cells carry mutant alleles of these regulatory genes and bypass checkpoints in the cell cycle that normally regulate cell division and divide continuously. As a result, studies of the cell cycle are an important part of cancer research. In this section, we outline the steps in the cell cycle, describe how the cycle is regulated, and discuss how mutations and abnormal gene expression lead to cancer.

The RB1 tumor-suppressor gene controls the G1/S checkpoint.

Mutation or loss of activity in the tumor-suppressor gene RB1 (retinoblastoma 1; MIM 180200) is involved in several forms of cancer, including retinal, bone, lung, and bladder cancer. The role of the RB1 gene in regulating the cell cycle was discovered through its effects in retinoblastoma, a childhood cancer that affects the light-sensitive retinal cell layer of the eye. In the familial form of retinoblastoma, individuals who inherit one mutant allele of RB1 have an 85% chance of developing retinoblastoma and other cancers. Those who inherit the mutant allele carry it in all their somatic cells, and loss of heterozygosity by mutation of the normal allele anywhere in the body can result in cancer. Another form, called sporadic retinoblastoma, is extremely rare and no mutant allele of the RB1 gene is inherited. Instead, mutations in both copies of the RB1 gene occur in a single retinal cell and cause retinoblastoma The RB1 gene encodes a protein called pRB, which is always present in the nuclei of all cell types in the body, including retinal cells. pRB is present at all stages of the cell cycle, but its activity is closely regulated during the cycle. pRB is a molecular switch that controls progression through the cell cycle. If pRB is "switched on" during G1, it binds to a transcription factor called E2F and prevents transcription of genes that move the cell through the cycleAs a result, the cell will not pass through the G1/S checkpoint into S phase, but instead will enter the inactive G0 stage. If pRB is "switched off" in G1, it does not bind to E2F, which then activates transcription of genes that move the cell through the G1/S checkpoint into S phase, through G2, and on to mitosis. If both copies of the RB1 gene are mutated or inactivated in a cell, the cell moves continuously through the cell cycle in an unregulated way and forms a cancerous tumor.

Two Classes of Cell-Cycle Regulatory Genes Are Involved in Cancer

Mutations in the genes regulating checkpoints are important in cancer development. Two classes of genes regulate the checkpoints: (1) genes that turn off or decrease the rate of cell division and (2) genes that turn on or increase the rate of cell division. The first class is known as tumor-suppressor genes. Products of these genes normally act at either the G1/S or the G2/M control point to inhibit cell division (Figure 12.6). If these genes are mutated or inactivated, this inhibition is removed and cells pass through the checkpoints and divide in an uncontrolled manner. The second class of checkpoint genes, called proto-oncogenes, encode proteins that start or maintain cell growth and division. When the products encoded by these genes are active, cells grow and divide. When proto-oncogenes and/or their proteins are inactivated, normal cells stop dividing. In cancerous cells, mutant proto-oncogenes are often switched on permanently or overproduce their products. As a result, cells receive constant internal signals to keep dividing, and uncontrolled cell division results. Mutant forms of proto-oncogenes are called oncogenes.

Cancer-Causing Mutations Disrupt Cell Cycle Regulation

Normal differentiated cells regulate cell growth and division Nondividing Muscles, nerves Divide to replace dead or damaged cells Liver, kidney Continuously dividing Epithelial cells Cancer cells carry mutant alleles of genes involved in cell cycle regulation or signal transduction

Clonal evolution

One cell acquires a mutation which is passed to all daughter cells Over time, additional mutations accumulate Genes that are involved with DNA repair or proper chromosome segregation are involved with cancer

Other causes of infertility

Personal habits and environmental factors can affect a couple's chances of having a child. For example, smoking and other uses of tobacco lower sperm count in men and increase the risk of low-weight babies and miscarriages in women. Overall, for each menstrual cycle, smoking by men or women reduces the chances of conceiving by one-third. Women who are significantly underweight or overweight often have trouble becoming pregnant. The relationship between age and infertility has taken on new importance as many people in highly developed countries postpone marriage and/or childbearing until educational or career goals are reached. Unfortunately, this delay means that women are older when they begin to have children and age is associated with increased infertility. In addition, older women have a higher risk of producing eggs with abnormal chromosome number, leading to miscarriages and an increased risk of having a child with a chromosome abnormality such as Down syndrome (see Chapter 6 for a discussion of these risks). It is becoming increasingly difficult for infertile couples to find a child to adopt because of the increasing use of birth control and the use of legal abortion. This has resulted in an increased demand for medical answers to infertility despite the complexity of the problem and the cost of treatment.

Both carrier and prenatal testing are done to screen for genetic disorders.

Prenatal testing can detect genetic disorders and birth defects in the fetus. More than 200 single-gene disorders can be diagnosed by prenatal testing. In most cases, testing is done only when there is a family history or another indication fortesting. If there is a family history for autosomal recessive disorders such as Tay-Sachs disease or sickle cell anemia, the parents are usually tested to determine if they are heterozygous carriers. If both parents are heterozygotes, the fetus has a 25% chance of being affected. In such cases, prenatal testing can determine the genotype of the fetus. For other conditions, such as Down syndrome (trisomy 21), testing of free fetal DNA (see Chapter 6), or karyotyping, is the most direct way to detect an affected fetus. Testing for Down syndrome is usually done because of maternal age, not because there is a family history of genetic disease. Because the risk of Down syndrome increases dramatically with the age of the mother (see Chapter 6), chromosomal analysis of the fetus is recommended for all pregnancies in which the mother is age 35 or older. Samples for prenatal testing can be obtained through amniocentesis or chorionic villus sampling (CVS), both of which are described in Chapter 6. The fluids and cells obtained for testing can be analyzed by several techniques, including karyotyping, biochemistry, and recombinant DNA techniques. Because recombinant DNA technology can be used to analyze the genome directly, it is the most specific and sensitive method of prenatal testing currently available. In addition to prenatal genetic testing, another method called preimplantation genetic diagnosis (PGD) can be used to test for genetic disorders in the earliest stages of embryonic development. In PGD, eggs are fertilized in the laboratory and develop in a culture dish for several days. For testing, one of the six-to-eight embryonic cells (called blastomeres) is removed DNA is extracted from this single cell, amplified by PCR, and tested to determine whether the embryo is homozygous or hemizygous for a genetic disorder. PGD is useful when parents are carriers of autosomal and X-linked disorders that would be fatal to any children born with the disorder (Lesch-Nyhan syndrome or Tay-Sachs disease). The use of PGD and genetic testing allows couples to select embryos without a genetic disorder for implantation. However, in one case, parents with achondroplastic dwarfism (MIM 100800) selected an embryo with that trait. PGD can also be used to select the sex of an embryo before implantation. A related method called polar body biopsy is used to test for genetic disorders even before fertilization takes place. If a woman is a heterozygous carrier for an X-linked recessive disorder (such as muscular dystrophy), during the first meiotic division, the X chromosome with the mutant allele can segregate into the cell destined to be the egg or into the much smaller polar body (see Chapter 7 to review gamete formation). A polar body can be removed by micromanipulation) and tested to see if it carries the normal allele or the mutant allele. If the polar body carries the mutant allele, then the egg must carry the normal allele. Eggs that pass this test can be used for IVF, ensuring that the woman's sons will not be affected with the disorder.

The ras genes are protooncogenes that regulate cell growth and division.

Proto-oncogenes switch on or maintain cell division. These genes are normally switched off or their protein products are inactivated when cell division stops. Mutant alleles of these genes are oncogenes; they cause uncontrolled cell divisions and cancer. What is the difference between a proto-oncogene in a normal cell and its mutant form as an oncogene in a cancer cell? Many differences are possible; (1) single base changes that produce an altered gene product, (2) mutations that cause underproduction or overproduction of the normal gene product, and (3) mutations that increase the number of copies of the normal gene. In fact, all these mutations have been identified in oncogenes or their adjacent regulatory regions. We'll examine an example from a proto-oncogene family that is mutated in more than 40% of all human cancers. The ras proto-oncogene family (MIM 190020) is a group of related genes that encodes signal-transduction proteins. These help transmit signals from outside the cell, through the plasma membrane, and into the nucleus, initiating a cascade of gene expression that stimulates cells to divide. One member of the ras proto-oncogene family encodes a protein of 189 amino acids and is located in the cytoplasm at the border with the plasma membrane. The ras protein cycles between an active ("switched on") state and an inactive ("switched off") state. When a growth factor binds to the cell's plasma membrane, a receptor on the plasma membrane transmits a signal and the ras protein is "switched on." The "switched-on" ras protein transfers growth-promoting signals to other molecules in the cytoplasm and then to the nucleus, where changes in gene transcription moves the cell through the cell cycle and cell division. Once the signal has been transmitted, the ras protein returns to its inactive state and is "switched off." Analysis of mutant ras alleles from many different human tumors shows that single nucleotide changes at one of two sites in the gene are the only difference between the proto-oncogene in normal cells and the mutant oncogene in cancer cells. In all cases, the nucleotide substitutions change a single amino acid in the ras protein at either amino acid 12 or amino acid 61. Changing glycine to valine at position 12 changes the structure of the ras protein and prevents it from folding into an inactive state. As a result, the mutant protein is locked into the active state and is permanently "switched on." It signals for cell growth at all times, even in the absence of an external signal. Cells carrying this mutation escape from growth control and become cancerous. An amino acid change at position 61 has a similar effect.

Checkpoints

Recall from Chapter 2 that cells in the body alternate between two basic states: division and nondivision. The sequence of events between divisions is called the cell cycle, which has two main parts: (1) interphase, the stage between divisions, and (2) mitosis, followed by cytokinesis The cycle is regulated at three main checkpoints: ■ in G1 just before cells enter S (the G1/S transition) ■ at the transition between G2 and M (the G2/M transition) ■ a point in late metaphase of mitosis called the M checkpoint As cells approach the G1/S checkpoint, they will finish G1 and enter the S phase or will enter a nondividing state called G0 (G-zero). Many specialized cells, such as white blood cells, remain in G0 until stimulated by external signals to reenter the cycle and divide. These external signals are processed in the cell by a mechanism known as signal transduction to produce changes in gene expression, leading to cell division. Some cancers have abnormalities in signal-transduction systems and receive signals to divide continuously. At each checkpoint, a combination of external signals and internal regulatory pathways determines whether the cell will proceed to the next stage of the cycle. The G2/M checkpoint ensures that the DNA has been replicated and that any damage to the DNA has been repaired. If not, progress through the cycle is stopped until these events have been completed. At the M checkpoint, attachment of spindle fibers to chromosomes is monitored; this ensures that anaphase separation of chromosomes occurs normally.

Preimplantation genetic diagnosis (PGD)

Removal and genetic analysis of a single cell from a 3- to 5-day-old embryo. Used to select embryos free of genetic disorders for implantation and development.

Some viral infections lead to cancer.

Several viruses that infect human cells—including the papilloma viruses (HPV 16, HPV 18, HPV 31), herpes virus, and Epstein-Barr virus—are associated with cancer. It is estimated that about 15% of all cancers result from viral infection, making it a major cause of cancer. Infections by HPV 16 and HPV 18 can cause cervical cancer. One of the proteins encoded by HPV (the E7 protein) binds to and inactivates the retinoblastoma protein pRB, which regulates the G1/S checkpoint. With pRB inactivated, the infected cell bypasses the checkpoint, completes the cell cycle, and divides. As long as E7 is present, the cell will continue to divide. Other HPV proteins inhibit different tumor-suppressor proteins, maintaining a cellular environment that allows continued division of the infected cells and the replication and dispersal of new virus particles. Two vaccines, Gardasil and Cervarix, were developed to prevent infection by HPV 16, HPV 18, and several other strains. The vaccines contain recombinant DNA- derived HPV coat proteins that stimulate the immune system to make antibodies to HPV. The vaccine is effective only if given before infection and protects against strains that cause about 70% of all cervical cancers.

Nonheritable gene therapy is performed on

Somatic cells

Infertility Is a Common Problem

Successful reproduction depends on a small number of important factors: healthy gametes (egg and sperm), a place for fertilization to occur (oviduct), and a place for the embryo to grow and develop (the uterus). If any of these components are missing or malfunctioning, infertility may result. In the United States, the number of infertile couples has increased over the past 10 years, and at present, about one in six couples are infertile. A number of physical or physiological problems can prevent the production of normal sperm or eggs, inhibit fertilization, or hinder implantation of an embryo in the uterus. In about 40% of all cases, the woman is infertile; in another 40% of cases, the male is infertile; and in the remaining 20% of cases, the cause is unexplained or both partners are infertile. Infertility in women becomes more common with increasing age (Figure 16.1); up to one-third of couples in their late 30s are infertile. As women in the United States have delayed the age at which they have their first child, infertility has become a larger problem. In 1975, only 5% of all births were to women over 30 years of age. By 2011, however, 24% of all births were to women over 30.

What other environmental factors are related to cancer?

The American Cancer Society estimates that 85% of lung cancer cases in men and 75% of cases in women are related to smoking. Smoking produces cancers of the oral cavity, larynx, esophagus, and lungs and is responsible for 30% of all cancer deaths. In smokers, lung cancer can take up to 20 years to develop (Figure 12.19). Most of these cancers have very low survival rates. Lung cancer, for example, has a 5-year survival rate of 13%. Cancer risks associated with tobacco are not limited to smoking; the use of snuff or chewing tobacco carries a 50-fold increased risk of oral cancer. About 1 million new cases of skin cancer are reported in the United States every year, almost all related to ultraviolet (UV) light from the sun or tanning lamps. Skin cancer cases are increasing rapidly in the population, presumably as a result of exposure to ultraviolet light from tanning lamps or an increase in outdoor recreation. Epidemiological surveys show that lightly pigmented people are at much higher risk for skin cancer than heavily pigmented individuals. This supports the idea that genetic characteristics can affect the susceptibility of individuals or subpopulations to environmental agents that cause a specific form of cancer. In the atmosphere, ozone depletion in certain regions of the globe also contributes to increased levels of UV radiation exposure, which in turn is associated with increases in skin cancer frequency

Some chromosome rearrangements cause leukemia

The connection between chromosome aberrations is evident in certain leukemias. In these cancers (which involve the uncontrolled division of white blood cells), specific chromosome changes are well-defined and diagnostic. One of the best-established links between cancer and a chromosomal aberration is a translocation between chromosome 9 and chromosome 22 in chronic myelogenous leukemia. Originally called the Philadelphia chromosome after the city in which it was discovered, this relationship was the first example of a chromosome translocation related to cancer, and for that matter, to any human disease. Other cancers associated with specific chromosome translocations are listed in Table 12.6. In CML, the ABL1 gene (MIM 189980) on chromosome 9 is combined with part of the BCR gene (MIM 151410) from chromosome 22 to form a hybrid gene. The normal allele of the ABL1 gene encodes a protein involved in signal transduction, and the normal BCR allele encodes a protein that switches other proteins "on" or "off" by adding phosphate groups to them. These BCR target proteins control gene expression and cell growth. The translocation produces a hybrid gene with BCR sequences at its beginning and ABL1 sequences at the end (Figure 12.16). This hybrid gene encodes a unique protein that constantly signals the white blood cell to divide, even in the absence of external signals, resulting in CML.

How does genetic counseling work?

The counselor usually begins by taking a detailed family and medical history and constructing a pedigree. Prenatal screening and cytogenetic or biochemical tests can be used along with pedigree analysis to help determine what, if any, risks are present. The counselor uses as much information as possible to establish whether the trait in question is genetically determined and who is at risk. For genetic traits, the counselor constructs a risk-assessment profile for the couple. In this process, the counselor uses all the information available to explain the risk of having a child affected with the condition or the risk that the individual who is being counseled will be affected with the condition. Often, conditions are difficult to assess because they involve polygenic traits or disorders that have high mutation rates (such as neurofibromatosis).

Ethical Issues in Reproductive Technology

The development and use of ART have progressed more rapidly than the social conventions and laws governing their use. In the process, controversy about the moral, ethical, and legal grounds for using these techniques has arisen but has not been resolved. ART is responsible for more than 3.5 million annual births worldwide. Although the benefits of ART have been significant, there are risks associated with the use of these alternative methods of reproduction. Some risks have been well documented, whereas others are still matters for debate and more study. In other cases, the use of ART raises ethical questions (see Exploring Genetics: The Business of Making Babies). We'll discuss some of these risks and questions in the following sections.

Recent successes in gene therapy.

The development of improved viral vectors and widespread success using these vectors and the use of a new technique, exon skipping, has stimulated new interest in gene therapy. In 2008, the pendulum began to swing back in favor of gene therapy when it was used successfully to treat blindness associated with a genetic disorder called Leber congenital amaurosis (LCA; MIM 204000). In these cases, the gene (RPE65) was carried by a modified adenovirus vector, and patients showed improvements in visual function with no adverse effects. Since then, several hundred individuals with LCA have successfully undergone gene therapy. Other successes followed in short order. In 2009, researchers used a modified lentivirus (a type of retrovirus) as a vector to treat two children with adrenoleukodystrophy (MIM 300100), a fatal disorder of lipid metabolism. In this case, the defect is not in an enzyme, but a protein, ABCD1, which is involved in lipid transport. In 2010, gene therapy was used to treat beta-thalassemia major (MIM 613985), a debilitating disorder that requires monthly blood transfusions. Bringing the use of gene therapy full circle, in 2013, five children with ADA-SCID were treated with gene therapy, and all have continued to improve and develop immune systems following treatment. Some of the recent uses of gene therapy to treat genetic disorders are listed in.

FAP causes chromosome instability and colon cancer.

The genetic pathway from the mutant FAP allele to colon cancer has two important features: (1) the accumulation of five to seven mutations in a single cell—fewer mutations result in benign growths or intermediate stages of malignant-tumor formation; (2) a specific sequence of mutational events, meaning that both the number and the order of mutations are important in tumor formation. In FAP-associated colon cancer, individuals inherit a mutation in the APC gene (MIM 611731). All epithelial cells of the inner lining in the large intestine are heterozygous for an APC mutation and can partially escape control of the cell cycle. Some cells divide to form small clusters called polyps Heterozygotes carry a mutant copy of APC in all their cells and, as a result, develop hundreds or thousands of polyps in the colon and rectum Each polyp is a clone, derived from a single cell that has partially escaped cell-cycle control. However, a single APC mutation is not enough to cause cancer—it is only the first step. Colon cancer develops only after mutations in several other tumor-suppressor genes cause the transition from polyp to colon cancer. In the second step mutation of one copy of the K-RAS proto-oncogene in a polyp cell transforms the polyp into an adenoma, a larger noncancerous tumor. These cells now carry two mutations—one in APC and one in K-RAS—but other mutations are still necessary to transform the adenoma cells into a cancer. In subsequent steps, cells must acquire mutations in both alleles of the downstream genes shown in Figure 12.13. These mutations usually occur through deletions in specific chromosome regions. The 18q region contains a number of colon cancer genes, including DCC, DPC4, and JV18-1. Deletion of both alleles of any of these genes leads to the formation of late-stage adenomas with fingerlike outgrowths called villi. In the last step, mutations in both alleles of the p53 gene cause the late-stage adenoma to become cancerous. Mutations in the p53 gene are pivotal in the formation of other cancers, including lung, breast, and brain cancers.

There are two parts to the immune system that protect against infection

The second line of defense is a series of chemical reactions and cellular responses that respond to pathogens that have entered the body. These reactions, which are part of the innate immune system, are nonspecific and work against most pathogens. The nonspecific responses identify, inactivate, and kill pathogens such as bacteria and viruses. If these defenses do not stop the disease-causing agents, the third and most effective defense system is the adaptive immune system. This part of the immune system is specific; it recognizes particular pathogens and responds to neutralize or kill the invader. It has two components: antibody-mediated immunity and cell-mediated immunity. In this chapter, we examine the components of the innate and adaptive immune systems and explore how they are mobilized to respond to an infection. We also consider how the immune system determines blood groups and affects mother-fetus compatibility. The parts of the immune system that play roles in organ transplants and in risk factors for a wide range of diseases will be discussed. Finally, we describe a number of disorders of the immune system, including how HIV/AIDS acts to cripple the immune response of infected individuals.

The skin is not part of the immune system but is the first level of defense

The skin's outer surface is home to bacteria, fungi, and even mites, but they cannot penetrate the protective layers of dead cells or the tightly interlocked cells in the lower layers of skin cells. Epithelial cells that line the internal body cavities and ducts (such as the lungs) are coated with mucus that protects against infection. The mucus in some parts of the body contains an enzyme, lysozyme, that breaks down the cell walls of bacteria, adding another layer of protection.

Gene doping is a controversial form of gene therapy

The use of performance-enhancing drugs has confounded athletic events in recent years, including cycling's Tour de France and the pursuit of the home-run record in U.S. professional baseball. In the Tour de France, cyclists have been suspended for using erythropoietin (EPO), a hormone that increases the production of red blood cells, which increases the oxygen-carrying capacity of the blood (Figure 16.19). EPO and other drugs can be detected by blood tests. Concern over the use of genes instead of gene products to enhance athletic performance began in 2001, when the International Olympic Committee (IOC) Medical Commission met to discuss how gene therapy might affect sports competition. Other agencies, including the World Anti-Doping Agency (WADA), have prohibited gene transfer (gene doping) as a means of enhancing athletic performance. An example of gene doping is the use of Repoxygen, a product that consists of a viral vector into which the human EPO gene has been inserted adjacent to a control element that regulates expression of the gene. Once in the cells of the body, the control element senses low oxygen levels in the blood during strenuous activity and turns on the adjacent EPO gene, increasing the synthesis and release of the hormone erythropoietin. Repoxygen use may be difficult or impossible to detect, although several athletes at the Turin 2006 Olympic Games were suspected of using this form of gene doping. Although agencies such as the IOC and WADA prohibit the use of Repoxygen, others are calling for legalization of gene enhancement, arguing that regulating the use of this gene therapy is more effective than attempting to prevent its use. They also argue that gene doping is only an extension of technology such as artificial nutrition and hydration by intravenous fluids, which is already permitted. More than 20 genes have been associated with athletic performance, so many choices are available for gene doping

Why do people seek genetic counseling?

Typically, individuals or families with a history of a genetic disorder, cancer, birth defect, or developmental disability seek genetic counseling. Women older than 35 years of age and individuals from ethnic groups in which particular genetic conditions occur more frequently are counseled to teach them about their increased risk for genetic or chromosomal disorders and the availability of diagnostic testing. Counseling is especially recommended for the following individuals or families: ■ Women who are pregnant, or are planning to become pregnant, at or after age 35 ■ Couples who already have a child with intellectual disability, an inherited disorder, or a birth defect ■ Couples who would like testing or more information about genetic defects that occur more frequently in their ethnic group ■ Couples who are first cousins or other close blood relatives ■ Individuals who are concerned that their jobs, lifestyle, or medical history may pose a risk to a pregnancy, including exposure to radiation, medications, chemicals, infection, or drugs ■ Women who have had two or more miscarriages or babies who died in infancy ■ Couples whose infant has a genetic disease diagnosed by routine newborn screening ■ Those who have, or are concerned that they might have, an inherited disorder or birth defect ■ Women who have been told that their pregnancies may be at increased risk for complications or birth defects, based on medical tests

Sequencing cancer genomes identifies cancer-associated genes.

With the sequence of the gene-coding regions of the human genome in hand and the development of newer sequencing methods, scientists can now sequence and analyze the protein-coding genes in cancer cells to systematically identify all the mutations present in a cancer cell. One such study sequenced over 13,000 genes in breast and colorectal cancers and found that individual tumors carried an average of 90 mutant genes. This unexpected finding revealed that the number of mutational events that occur during the transition from normal to malignant cells is much larger than previously thought. Second, the work showed significant differences in the types of mutations found in breast and colorectal cancers (Figure 12.17). Third, each tumor analyzed had a unique combination of mutations, and no tumor had more than six mutations in common with any other tumor analyzed, emphasizing the clonal and unique origins of cancer. Genomic studies have greatly expanded the number of genes associated with many forms of cancer. In breast cancer, for example, three classes of genes have been identified: (1) rare high-risk alleles (including BRCA1 and BRCA2), (2) rare moderate-risk alleles, and (3) common but low-risk alleles. In addition to cases of breast cancer caused by the action of a single high-risk gene such as BRCA1, it is now clear that other cases can be caused by mutations in a number of low-risk genes in several possible combinations, with implications for both diagnosis and treatment. The development of high-resolution methods for detecting genome-wide mutations in cancers has led to the creation of the Cancer Genome Atlas (see Exploring Genetics: The Cancer Genome Atlas), an international effort to use new technologies to catalog and understand the genetic changes associated with cancer and to improve the diagnosis and treatment of this disease.