Biochem FINAL Cancer
Recombinant HPV Vaccine
Almost every person will acquire an HPV infection at some time in his/her life. 75% of cervical cancer is caused by HPV 16 and 18. 90% of genital warts caused by HPV6 and 11. Other HPVs responsible for vulvar, vaginal and anal cancer. HPV vaccines licensed for use in the United States: • 4vHPV (quadrivalent): HPV 6, 11, 16 and 18 • example: Gardasil® • 9vHPV (9-valent): HPV 6, 11, 16, 18, 31, 33, 45, 52, 58 • example: Gardasil 9®
Multidrug treatment :
Better to simultaneously treat with multiple drugs
Normal, healthy cells: sugar meta
Glucose>pyruvate via glycolysis: Some pyruvate is converted to lactate, but most is directed to the TCA cycle via acetyl-CoA. The TCA cycle generates NADH, which donates electrons to the mitochondrial electron transport chain so that oxidative phosphorylation can progress. Because O2 is required as the final electron acceptor to completely oxidize the glucose, O2 is essential for this process.
A hallmark of cancer cells
Gross chromosomal abnormalities
Li-Fraumeni Syndrome (LFS)
Li-Fraumeni syndrome is an autosomal dominant disorder that greatly increases the risk of developing multiple types of cancer The primary cause is the inherited germ-line mutations of TP53. Diagnosed based on clinical criteria and/or genetic testing No standard treatment or cure
The oncogenic form of MYC is
NOT mutated in human tumors.
Tumor
a mass of abnormally proliferating cells • can be solid or dispersed • can be benign or malignant
Carcinomas t
arise from epithelial cells that are covering layers over external and internal body surfaces. ~90% of all human malignant tumors are carcinomas. subtypes of carcinoma include: lung, prostate, breast, skin, colon
Metastasis
describes the ability of cancer cells to journey through the lymphatic and circulatory system invading vascular basement membrane and extracellular matrix of normal tissues elsewhere in the body. This process proceeds in an orderly and predictable manner, sometimes termed the 'metastatic cascade' • Metastasis is the most deadly and least understood aspect of cancer. • Metastasis is responsible for 90% of cancer related deaths; after spreading, it is often impossible to eradicate tumors.
• Retinoblastoma occurs in two forms,
familial (hereditary) and sporadic • sporadic -- occurs in children with no family history of the disease • familial (hereditary) -- occurs in children with a positive family history for the disease.
The three-stages tumor development in human cancers:
initiation, promotion, progression
(ii) Gene amplification
multiple copies of the proto-oncogene resulting in gene overexpression ex MYC
Leukemias t
originate in blood-forming tissue such as the lymphatic system and bone marrow refers to cancers in which large numbers of malignant blood cells proliferate in the bloodstream
Sarcomas t
originate in supporting tissues such as connective tissues/muscle cells (rare in humans; ~1% of total malignant cancers) blood vessels, adipose, muscle, bone
Lymphomas t
originate in the cells of the immune system refers to a solid mass of white blood cells
Normal XP
pyrimidine dimers (adjacent T-T, C-C or C-T pairs) formed upon sunlight exposure are repaired by a particular DNA repair mechanism before it causes problems.
• Example: abemaciclib
• class of drugs as kinase inhibitors • inhibitor of CDK4/6
Tyrosine kinase inhibitors for cancer treatment: Imatinib (Gleevec®)
Gleevec binds to the active site of Bcr-Abl, preventing it from phosphorylating its target proteins.Result: preventing cell proliferation
Gene arrangement/ Translocation
MYC: • Proto-oncogene is under control of a strong promoter or enhancer. • Protein is overproduced CML: Proto-oncogene (or a portion of it) is fused with another gene. Fusion protein is either overproduced or hyperactive.
(i) Point mutation
Mutation in coding region causes production of hyperactive protein. e.g., RAS Mutation in promoter causes excessive expression.
Tumor Initiators and Tumor Promoters
Need initiator, then promoter with frequency to develop tumor
(iii) Gene Arrangement:(b) Translocation of a gene to create a fusion protein • Example: BCR-ABL fusion in chronic myelogenous leukemia (CML)
Normally, ABL gene on Ch9; BCR on Ch22 Reciprocal exchange occurs; Fusion of BCR and ABL is created; Fusion mRNA is produced thus fusion protein is produced. Unlike the proto-oncogene form of ABL which is dependent on growth signaling for its tyrosine kinase activity, the chimeric protein Bcr-Abl possesses a constitutively active tyrosine kinase The fusion protein is oncogenic!
Oncogenes and signal transduction cascades
Oncogenes can cause normal mitogenic signaling pathways to become corrupted, such that the cell is able to cross the restriction point in the absence of a mitogen. Three points of receptor-tyrosine kinase (RTK) signaling pathway that can become overactive as a result of mutations: (1) RTK itself (2) Ras GTPase (3) transcription factor Myc An increase in signaling in any of these 3 cases will ultimately increase the levels of Cyclin D and allow the cell to cross the restriction point.
Targeted Therapies in Cancer
Targeted therapies based on the molecular basis of malignancy: Targeted therapies are often cytostatic (they block tumor cell proliferation) whereas standard chemotherapy agents are cytotoxic (they kill tumor cells).
The human papillomavirus genome encodes two proteins, E6 and E7, that interfere with normal DNA- damage control mechanisms:
The E6 protein binds to p53 and promotes destruction of p53 thus inhibiting the ability of p53 to halt the cell cycle and its induction of apoptosis The E7 protein binds to Rb and disrupts its ability to halt cells at the restriction point. E2F is liberated so E2F can stimulate transcription of S- phase genes, inducing cell division.
(iii) Gene Arrangement:(a) Amplification of Myc protein due to chromosomal rearrangement
The MYC gene also behaves as an oncogene when a chromosomal rearrangement occurs such that transcription of the normal MYC gene falls under the control of a very active gene enhancer. Result: over-production of the Myc protein Example: Burkitt's lymphoma
Example: Burkitt's lymphoma
Burkitt's lymphoma is a solid tumor of B lymphocytes that the immune system uses to make antibodies; lymphoma cells build up in the lymph nodes, causing swelling The disease is associated with genetic changes, involving the MYC gene and immunoglobulin genes In ~90% of the cases of Burkitt's lymphoma, MYC, on chromosome 8, is translocated to chromosome 14, to a region of highly active gene transcription : t(8;14) with translocation, MYC is now under the control of a strong gene enhancer. Result: over-production of Myc protein This is a reciprocal translocation: translocation between two non- homologous chromosomes, with no gain nor loss of chromosomal materials
Benign vs. Malignant Tumors
Normally, epithelial cells are kept from migrating due to their tight adhesion to one another and due to the underlying proteinaceous basal lamina which acts as a barrier. When a tumor has broken through the basal lamina and invaded the • Cancer cells secrete matrix metalloproteinases (MMPs), and MMPs can digest through the basal lamina to remodel the extra-cellular matrix (ECM). This remodeling is to stimulate cancer cell growth.
Who is at risk for cancer?
Older population Individuals with a positive family history of cancer (10-20% of cancer is due to inheriting alleles that predispose individuals to cancer) Men (~60%) Individuals in developed countries (due to environmental factors)
Cell-cycle based Treatment
Some chemotherapy drugs are designed based on components of cell cycle machinery. Most agents are specific for S phase of the cell cycle. These agents are structurally similar to normal purine or pyrimidine precursors, either by inhibiting their synthesis or by competing with them in DNA synthesis. Most effective against rapidly growing cell populations.
Current targeted therapies include:
(1) Immunotherapies using antibodies against tumor cells (2) Signal transduction inhibitors, e.g., inhibition of cancer- promoting proteins (3) Angiogenesis inhibitors to preventing the growth of blood vessels that nourish the tumor
Q: How do proto-oncogenes become oncogenes? Mutation occurs through one of these mechanisms:
(i) Point mutationleads to altered product (ii) Gene amplificationresults in gene overexpression (iii) Gene arrangement/ Translocationcreating a fusion gene with novel properties
Metastasis is a multi-step process:
A somatic mutation with considerable survival and growth advantages Growth Factors support the expansion of mutation- bearing clones Intravasation (vascular invasion) Migration (Fewer than 1 in 1000 cells will survive to form metastases) Adhesion to endothelium Extravasation (out of a blood vessel) Induction of angiogenesis Growth in metastatic location
In XP patients,
DNA damage is not repaired. As more abnormalities form in DNA, cells malfunction and eventually become cancerous or cell death would occur. • Individuals with XP are extremely intolerant of UV rays. • XP patients are 1000 times more at risk of developing skin cancer; skin cancers develop in children at the average age 8, compared to the average age of 50 for non-XP adults.
One mutated copy of MSH2 or MLH1 is inherited:
With a second hit to MSH2 or MLH1, the repair mechanism that involves Msh2/Mlh1 will no longer work. Thus, that cell will not repair mutations, which can be passed on to subsequent generations of cells. • Because DNA repair is compromised, 90% of individuals with a known HNPCC mutation will develop cancer. • Progression from an adenoma to a carcinoma takes place in only 2-3 years compared to the usual 8-10 years seen in non-HNPCC patients.
Healthy cells under hypoxic condition:
cells can redirect the pyruvate generated by glycolysis away from mitochondrial oxidative phosphorylation by generating lactate (anaerobic glycolysis) This results in minimal ATP production when compared to oxidative phosphorylation.
Oncogenes and tumor suppressor genes
Since oncogene mutations confer a new function to the mutated protein, only one copy of a mutated gene is necessary to affect the growth or survival of the cell (i.e. oncogenes are gain-of-function mutations). Tumor suppressor genes require two inactivating mutations to promote cancer. • Loss-of-function can contribute to cancer
Mutations in NF1 gene
produces a nonfunctional version of neurofibrominthis causes Ras to remain in an active state results in uncontrolled, stimulated cell division. • As a result, in neuronal tissues, the growth of neurofibromas (tumors) can grow around Schwann cells and also along nerves throughout the body.
In familial retinoblastoma
the first "hit" occurs at the embryonic level, as the child inherits one defective copy of RB from a parent. All of the child's somatic cells, including the cells of the retina, will contain one defective copy of RB > this explains the autosomal dominant inheritance pattern. (i.e., one copy of the altered gene in each cell is sufficient to increase the risk of developing cancer, but both copies must be mutated to manifest the disease) • Patients with inherited mutations are more likely to get cancer (10,000X); more likely to get cancer at a young age; more likely to get second chancer of a different type • Retinoblastoma exhibits 90% penetrance, so not all individuals with a familial RB mutation will be affected.
In sporadic retinoblastoma
the first mutation of RB occurs spontaneously in a single, somatic cell, giving rise to a small subpopulation of somatic cells that each carry a single RB mutation. Retinoblastoma can result only when one of these cells sustains a second mutation in the Rb gene
Angiogenesis
the formation of new blood vessels from pre-existing vasculature. This is a part of normal biological process. This process involves the migration, growth, and differentiation of endothelial cells. Angiogenesis provides O2 and nutrients.
Secretion of MMPs causes remodeling of ECM. Changes include:
• Epithelial-type cancer cells undergo epithelial-mesenchymal transition to permit motility by decreasing amount of cell adhesion molecules, leading to decreased cell adhesion. • Growth promoting factors released from matrix enhance tumor cell proliferation. • Changes in the oligosaccharide chains of cell surface glycoproteins causes structural re-organization of receptors and other molecules. These changes allow cancer cells to "invade"(direct extension and infiltration into the surrounding tissues) • Invasion will progress to intravasation as tumor cells penetrate the lymphatic or vascular circulation.
Targeted Therapy: (1) Immunotherapy
• Example: Trastuzumab (HerceptinTM) for metastatic HER2-positivebreast cancer • 25%-30% of breast cancer cells overexpress HER2 genes and HER2 receptors • Mechanism of action: Trastuzumab binds to extracellular domain of the HER2 receptor that has intrinsic tyrosine kinase activity and blocks downstream signaling pathways. • result: it decreases the number of cells in the S- phase.
Radiation sources
• Examples such as UV rays, X-rays, g-rays • attributed to the distortion in DNA structure (UVB) • cause double-strand DNA breaks (X-rays and g-rays)
Familial Adenomatous Polyposis (FAP)
• FAP is characterized by cancer of colon and rectum• FAP is due to heritable mutation in the APC (adenomatous polyposis coli) gene. • When familial adenomatous polyposis results from mutations in the APC gene, it is inherited in an autosomal dominant pattern.• (FYI: there are other mutations that will result in FAP when inherited.) • The patients with FAP can develop thousands of colorectal polyps (adenomas) as early as in their teenage years. Unless the entire colon is removed, it is virtually certain these polyps will become malignant. • Average age at which an individual develops colon cancer is ~39 years.
• MSH2 and MLH1 belong to the tumor suppressor gene class.
• HNPCC is due to mutations in the MSH2 and MLH1 genes that are involved in DNA repair mechanism. • HNPCC is a disorder inherited in an autosomal dominant pattern. • It is a familial cancer syndrome that represents 3-5% of all colon cancer cases. It usually presents with a few to less than 50 polyps.
Autosomal Dominant Inherited Syndromes
• In hereditary cancer syndromes, individuals start life with a germline mutation in one of the alleles linked to cancer susceptibility, but it is balanced by a normal counterpart (heterozygous). • These individuals are predisposed to cancer because all of their cells have already sustained the 'first hit' to cancer-linked genes. • Tumor suppressor genes are commonly affected: p16, p21, Rb What is dominant is the predisposition to developing cancer. • If the critically needed normal suppressor gene that balances this germline mutation is lost at some time during an individual's life, a condition called 'loss of heterozygosity' (LOH) occurs. It is an 'allelic loss' event.
Paclitaxel (Taxol)
• Isolated from the bark of the Pacific yew tree • Taxol disrupts the equilibrium between free tubulin and microtubules by favoring the assembly, rather than disassembly > this causes both the stabilization of microtubules and the formation of abnormal bundles of microtubules, still disrupting mitosis.
MYC as a proto-oncogene:
• MYC proto-oncogene is a transcription factor that binds to DNA to stimulate the transcription of ~15% of known (growth-related) genes.
Cancer Development - Clonal Expansion
• Most cancers are thought to originate from a single cell that has undergone a series of somatic mutations. • one cell with a single mutation gives rise to a subpopulation of mutated cells. Over time, each of these undergoes additional mutations to progress towards a malignant phenotype. • The process of a single progenitor cell giving rise to a subpopulation of genetically identical cells is known as clonal expansion. • The cancer cells are evolved through process of clonal expansion.
3. Genetic factors: Mutations
• Mutations in genes that encode factors that regulate cell growth can promote cancer; Mutations in DNA damage repair pathways are common in cancer cells. • Mutations can be spontaneous or inherited. • Mutations affect two types of genes : -Proto-oncogenes and Tumor suppressor genes
Tumor promoters
• not mutagenic themselves; stimulate cell proliferation of initiated cells • repeated exposure to areas treated with tumor initiator will cause cancer ("group2" of the diagram below) • examples: chronic inflammation, infection, hormones, alcohol, tobacco#
Tumor suppressor gene TP53 (gene that encodes p53)
• p53 is constitutively expressed• Under healthy conditions, there is very little p53 protein. • Normally, upon detecting DNA damage, p53 is phosphorylated and p53 is activated. Either, p53 acts as a transcription factor to activate expression of p21, which inhibits cyclin/CDK to halt the cell cycle, then activates DNA repair; OR p53 triggers apoptosis if damage can't be repaired • If p53 is inactivated, uncontrolled cell proliferation will give a rise to tumor. • >50% of human tumors have a mutation or deletion(s) in the TP53 gene.
• Inactivated Rb:
• usually, due to genetic mutation and viral infection • Inactiated Rb cannot bind E2F. This results in E2F being able to activate transcription of S-phase genes, and DNA synthesis is 'on' continuously.
2. Tumor Viruses
• ~15% of human cancers may be caused by viruses. • Genetic material of viruses (RNA or DNA) is incorporated into the genome of the host cell and affect the cell division in the host cell. DNA Virus - Human papillomavirus - Cervical cancer
Knudsen's "Two-Hit" Hypothesis
In 1971, Alfred Knudsen proposed the two-hit model of Rb: the mutation of two copies of RB are required to give rise to a retinoblastoma.
initiation
In the initiation phase, the cell receives a mutation that gives it the potential for abnormal growth (genetic alteration).
progression
In the progression phase, the cell has sustained enough mutations that it is no longer dependent on external factors to proliferate, and can continue to proliferate on its own. New mutations can be acquired.
promotion
In the promotion phase, the initiated cell is stimulated to grow by external factors (growth factors, inflammation, etc.).The cell cycle checkpoints are turned off temporarily to allow the initiating mutations to expand.
Warburg effect
In tumors (and other proliferating or developing cells), the rate of glucose uptake dramatically increases and lactate is produced, even in the presence of oxygen and fully functioning mitochondria, i.e., cancer cells tend to convert most glucose to lactate (via pyruvate) even in the presence of oxygen. This phenomenon is known as the Warburg Effect, named after Dr. Otto Warburg.It is termed as "aerobic glycolysis". Many of the intermediates generated during glycolysis are diverted toward biosynthetic uses for synthesis of the proteins, nucleic acids, and lipids required for tumor growth.
6 Properties of Cancer Cells at the Cellular Level
Self-sufficiency in growth signals Limitless replicative potential and loss of contact inhibition Genetic instability Tissue invasion and metastasis Sustained angiogenesis Evading apoptosis
• Example: Methotrexate (MTX)
• a structural analog of folic acid • Mode of action: By competitively inhibiting DHFR, MTX leads to a decline of THF levels and compromises DNA synthesis.
• NF1 encodes for neurofibromin:
• neurofibromin functions as a GTPase activating protein (GAP), that facilitates the deactivation of Ras by removing the phosphate group.
Tumor initiators
• mutagenic / damage DNA • single application does not cause cancer but does cause latent genetic damage. • examples: chemical (tobacco#, herbicides, pesticides), environmental (radiation, UV light).
The Circulatory System and Metastasis
Tumor cells are larger than RBCs and do not have the same plasticity. Once in the bloodstream, they can become lodged in the first downstream capillary bed they encounter. They can then extravasate into the surrounding tissue. 1 For primary tumor cells arising from most locations, tumor cells are carried to the capillary bed of the lungs via the heart. 2 Tumor cells of the gastrointestinal tract are instead carried first to the capillary bed of the liver. The liver is a common site of metastasis for stomach and colon cancer. Some tumor cells display a preference for microenvironments that are not in accordance with blood flow. For example, breast and prostate cancer typically metastasize to bone.
3b. Mutations can inactivate Tumor Suppressor Genes
When TSGs are mutated, TSGs are inactivated. As a result, uncontrolled cell growth may occur. The inactivation leads to the development of tumor cells.• Mutated TSGs results in loss of the normal function of the protein product. Thus, these mutations are termed as "loss-of-function mutation." • In several cases, tumor suppressor proteins inhibit the same cell regulatory pathways that are stimulated by the products of oncogenes. • Examples of tumor suppressor genes are: Rb, TP53, APC, BRCA1/2, INK4, NF1,
Apoptosis
an intracellular death program that occurs in a controlled way for the body to get rid of cells that are (i) no longer needed OR (ii) cells which have received too much genetic damage to be successfully repaired. • Apoptosis is a normal part of development to eliminate unwanted cells from within a tissue. • "Too little" apoptosis results in hyperplasia, • "Too much" apoptosis results in tissue atrophy • Two apoptotic pathways:• Intrinsic pathway: mitochondrial mediated• Extrinsic pathway: death receptor mediated
Mitotic inhibitors
are effective during the M-phase of the cell cycle. • These drugs bind to the microtubules and inhibit the reorganization of the microtubule network. So, mitosis is arrested in metaphase, inhibiting cell division, eventually leading to cell death.
Cancer intro
• A term for diseases in which abnormal cells divide without control to form a malignant tumor and can invade nearby tissues. • The aberrant growth pattern results from mutations in genes that regulate proliferation, differentiation and survival of cells in a multicellular organism. • Cancer is a genetic disease, whether it is hereditary or spontaneous. • Cancer results from accumulation of somatic mutations clustered in one cell that are passed onto other cells.
TSG
• A tumor suppressor gene (TSG) codes for the protein that normally inhibit tumor development. They are negative regulators of abnormal cell proliferation. • Normal functions of TSGs include: • preventing the cell from dividing without a growth signal; • slowing or halting the cell cycle upon DNA damage, and/or • inducing a death response in irrevocably damaged cells.
Proto-oncogenes if mutated > Oncogenes
• Altered proteins • Results in altered expression level (usually high), activate cell proliferation, cell transformation. • Oncogenes are found in most cancers. • Oncogenes will function when they should not in the absence of normal activating events. • Conversion by mutations in the DNA results in the gain of a new function; hence these mutations are termed "gain-of-function mutations"
Targeted Therapy: (3) Inhibiting Angiogenesis
• As a tumor grows in size, it stimulates angiogenesis • An angiogenesis inhibitor denies the tumor access to nutrients and oxygen needed to grow. • Example: Bevacizumab (Avastin®) • Inhibitor of VEGF receptor • Binds to VEGF extracellularly to prevent interaction with VEGF receptors, thereby inhibiting its activities.
(5) Sustained Angiogenesis
• As tumors grow, they eventually reach a point in their size where the inner cells of the tumor mass become oxygen-starved. • This creates additional regions of hypoxia. • Hypoxia causes elevated levels of hypoxia-inducible factor-1 (HIF-1). • HIF-1 increases levels of vascular endothelial growth factor (VEGF). • New blood vessels are produced in response to hypoxia ("Hypoxia- induced angiogenesis").
Familial breast cancer: BRCA1 and BRCA2 mutations
• Breast Cancer susceptibility genes 1 and 2 (BRCA1/2) are essential for the accurate repair of double-stranded DNA breaks. • Belong to the tumor suppressors gene class • Mutations in BRCA1 and BRCA2 are associated with familial breast and ovarian cancers. • Women who have inherited certain mutations in these genes have a high risk of developing breast cancer, ovarian cancer, and several other types of cancer during their lifetimes. Family history is used to determine the increased risk.
(2a) Limitless replicative potential: cancer
• Cancer cells continue to divide indefinitely (i.e., immortal"). • 'limitless replicative potential'
(1) Self-sufficiency in growth signals: cancer
• Cancer cells grow in the absence of stimulatory growth signals (~10% of that is required for normal cells to grow.) • Cancer cells can produce a growth factor to which it and neighboring cells respond, resulting in continuous stimulation of cell proliferation. (Autocrine growth stimulation)
1. Chemical and Environmental Factors
• Carcinogens and mutagens damage DNA and cause mutations. • Carcinogens are thought to interact covalently with DNA, forming a wide variety of adducts • ~80% of human cancers are due to exposure to carcinogens.
(3) Genetic instability normal
• Cell cycle checkpoints are enabled. • When the chromosome content of a normal cell becomes disturbed, a programmed-cell-death pathway ("apoptosis") is activated. • Ultimately, cells maintain their diploid chromosomal complement asthey grow and divide. • Undergoes normal differentiation.
(3) Genetic instability cancer
• Cell cycle checkpoints are disabled. (Ex: Mutations in cyclins, CDKs, RAS, MYC, RB and p53 have been reported.) • Cells are unable to repair DNA damage and unable to correct replication errors. • With a deranged chromosome content, an apoptotic response is not elicited. • Defects in mitotic checkpoint result in highly aberrant chromosome complements (known as "aneuploidy" or "polylploidy"). • Cancer cells do NOT undergo differentiation.
Chemical carcinogens ex
• naturally occurring chemicals, including free radicals from normal metabolism • occupational exposure such as: arsenic, asbestos, benzene • alcohol and tobacco Tobacco is referred to as a complete carcinogen because it can act as both an initiator and tumor promoter.
Neurofibromatosis type 1
• Neurofibromatosis type 1 is a condition characterized by changes in skin pigmentation and the growth of tumors along nerves in the skin, brain, and other parts of the body. • Neurofibromatosis type 1 is considered to have an autosomal dominant* pattern, caused by mutation and/or inactivation of both copies of the NF1 gene, a tumor suppressor gene that encodes the protein neurofibromin. • note: NF1 can also be result of a de novo mutation • Mutations in the NF1 gene lead to the production of a nonfunctional version of neurofibromin that cannot regulate cell growth and division. As a result, tumors such as neurofibromas can form along nerves throughout the body. • under the skin, near the spinal cord, along nerves, etc. • NF1 is 100% penetrant with variable expressivity thus showing high degree of variability in the symptoms. Some common symptoms are: neurofibromas, skin hyperpigmentation ("Café-au-lait" spots), and Lisch nodules (colored bumps on the iris)
(2b) Loss of contact inhibition: normal
• Normal cells cease proliferation when they contact surrounding cells in the dish. This is described as exhibiting 'contact inhibition.'
(1) Self-sufficiency in growth signals: normal
• Normal cells depend on growth factors (mitogens) such as epidermal growth factor (EGF) and insulin-like growth factor (IGF).
(2a) Limitless replicative potential: normal
• Normal cells exhibit a limited capacity for cell division. • After a finite number of mitotic divisions, they enter a non- replicative state that they cannot return from (i.e., senescence, not G0).
3c. Inherited genetic mutations or alleles: predisposition Example: Retinoblastoma develops due to mutation of Rb
• Normal function of Rb: functions as a tumor suppressor • controls the restriction checkpoint in G1 • Mutation of Rb gives a rise to retinoblastoma. • Retinoblastoma is identifiable when a camera flash or intense light cause a white glow in the eye, caused by a reflection of the tumor.
Proto-oncogenes
• Proto-oncogenes encode "normal" proteins. • They play roles in the normal growth control pathway to promote cell survival. • Can be growth factors, GF receptors, cell cycle control proteins, transcription factors.
Ras as a proto-oncogene
• Ras is a GTPase and encodes an intracellular signal transducer (G-protein) • Ras as a proto-oncogene, i.e., Ras with normal function: • When a receptor tyrosine kinase is activated by a ligand, activated Ras switches from a GDP-bound state (resting) to an active GTP-bound state (active).
• Normal functions of Rb:
• Rb must be phosphorylated for cells to progress through G1 to S phase; • When unphosphorylated Rb is bound to E2F, cells remain in G1 phase.
Mutations in RTK
• Receptor tyrosine kinases (RTKs) are activated only when a growth factor binds to the extracellular portion of the receptor (Figure A). • The truncated tyrosine kinase receptors do not need a ligand to dimerize; they are able to phosphorylate their cytoplasmic domains and continuously send a growth signal into the cell (Figure B). • The amplification of receptors results in too many normal receptors at the cell surface which can bind to even small concentrations of ligand to signal cell growth this results in enhanced signaling when ligand binds (Figure C). • example: over-expression of human epidermal growth-factor receptor, Her2R, on breast cancer cells
Gene Arrangement:(b) Translocation of a gene to create a fusion protein - Example: Philadelphia chromosome
• Reciprocal translocation found in 95% cases of chronic myelogenous leukemia (CML). • between chromosomes 9 and 22, t(9;22)the changed chromosome 22 is called the "Philadelphia chromosome". • Involves juxtaposition of BCR and ABL genes thus creating the BCR/ABL fusion gene. • BCR gene: involved in signaling pathway as a GTPase • located on chromosome 22 • ABL gene: a tyrosine kinase• located on chromosome 9
MYC as an oncogene:
• The MYC gene behaves as an oncogene when an abnormally large amount of Myc protein is synthesized in the cell. • Due to a DNA replication error, increased copies of the MYC gene (up to 150 copies per cell) are incorporated into chromosomes, resulting in an elevation of MYC mRNA transcripts.
Apoptosis Cascade
• This signal cascade is activated by proteins called caspases. • Caspases are synthesized as inactive precursors called procaspases. • Procaspases are activated by proteolytic cleavage. • Upon DNA damage, activated caspases are involved in releasing cytochrome C and mitochondrial proteins from the intermembrane space of mitochondria into the cytosol. • Other actions of activated caspases include: cutting off contact with surrounding cells, shutting down cellular metabolism, triggering signals for phagocytosis
Evading apoptosis in cancer cells: Intrinsic pathway in cancer cells:
• Tumorigenesis is promoted by gain-of-function mutation in genes that suppress cell death or loss-of-function of genes that promote apoptosis. Damaged cells evade apoptosis; Mutated cells will continue through the cell cycle
(2) Inhibiting the activity of cancer-promoting proteins
• Tyrosine kinase inhibitors (TKIs) are small molecules that block the ATP binding site of the TK enzyme. • Example: Imatinib (example: Gleevec ®) for treatment for chronic myelogenous leukemia (CML) • The growth and/or survival of tumor cells is dependent on the continued activity/expression of one or more deviant proteins. • If one of the activities of these proteins can be selectively blocked, the population of malignant cells can be destroyed.
(2b) Loss of contact inhibition: cancer
• Under the same conditions, cancer cells continue to grow, piling on top of one another to form clumps. Cancer cells have show loss of contact inhibition.
RAS as an oncogene: GGC(gly) > GTC(val)
• With a single point mutation in the coding region, Ras cannot be inactivated, and this causes GTP hydrolysis to be inhibited. • Result: The signal is constantly ON. Cell continues to generate its own growth signal, even when the receptor is not activated by ligand. • Mutated Ras is an oncogene. • In ~1/3 of human tumors, RAS becomes mutated.
Xeroderma Pigmentosum (XP)
• XP is caused by mutations in genes that are involved in repairing damaged DNA upon UV radiation. • This condition is inherited in an autosomal recessive pattern (i.e, both parents are carriers)