BISC305 CHAPTER 16

breast cancer

-1 in 8 women are stricken by breast cancer in the US. -5-10% are due to inheritance of a gene that predisposes them to it (= minor, but important factor). -BRCA1 and BRCA2 genes were identified as the mutated genes in the majority of these INHERITED predispositions to breast cancer. -These mutations also predispose women to ovarian cancer. -BRCA1 and BRAC2 are genes found to be mutated in inherited, but not sporadic breast cancer. -Precise BRCA gene function is still unclear.

brain tumor types

-120+ types -most medical institutions use the WHO (world health organization) classification system -WHO classifies brain tumor by cell origin and how the cells behave, from least aggressive (benign) to most aggressive (malignant) -some tumor types are assigned a grade, ranging from Grade I (least malignant) to Grave V, which signifies rate of growth and invasion -there is variation in grading system -from 1980-2000 there was an increase in nearly 50% in the estimated # of new cases of malignant brain/central nervous system tumors?

tumor suppressor genes

-Both copies must be mutated to cause loss of growth control. -Mutations are loss-of-function.

cancer stem cells

-CSCs are thought to be able to arise from DNA mutations that occur in: 1) differentiated cells 2) neural stem cells 3) committed progenitor cells -CSCs self-renew, but mostly give rise to heterogeneous cell types within a GBM -CSCs must be present and motile for metastasis. CSCs are resistant to treatments.

"genetics" of cancer

-Cancer afflicts 1 out of 3 people (common disease), but transformation of cells to being cancerous is a rare event. -Gene mutations that result in cancer occur in only about 1/3 of the human population during an entire lifetime. -The human body contains trillions of cells. Billions of cells divide every day. Must have multiple gene mutations to become a transformed cell. -Cancer is MONOCLONAL: events that cause a cell to become cancerous generally happen in a single cell in an individual, that then expands into a tumor. -MALIGNANT transformation requires multiple gene alterations to have occurred in the same cell. -There is a progression of gene alterations: -mutation occurs that makes cell less responsive to growth control -cell becomes more motile -more mutations accumulate in cancer cell that results in even less control

Cancer cell karyotype showing highly abnormal chromosomes

-Cancer cells are often aneuploid, with highly aberrant chromosomes: extra ones, missing ones, rearranged ones -Normal diploid cells have 22 pairs of autosomes and 2 sex chromosomes. -Above breast cancer cell line is aneuploid. Chromosomes should be single color. Multicolored chromosomes have large translocations of DNA. -These abnormalities would have triggered apoptosis in normal cells at cell cycle checkpoints. Cancer cells have overcome these checkpoints.

Hereditary vs. sporadic cancer

-Cancer is a common disease, so most families will have some members who have had cancer. -Cancer that is not due to inherited gene changes is called "sporadic cancer." It is believed that most—perhaps 90%—of all cancers are sporadic.

Retinoblastoma

-Cancer of the retinal cells in the eye. Mutation in the RB gene. -2 patterns exist: high frequency in children in some families and low frequency in adults. 1)HIGH RISK VERSION: occurs in 90% of childhood family members, therefore a dominant trait. Also at high risk for developing sarcomas. -One RB mutation is inherited, but must develop mutation in other copy to develop cancer. -10% never develop mutation and retinoblastoma. 2)SPORADIC ADULT VERSION: is low frequency in older general population. -Mutations in both gene copies occurs randomly. -RB gene is also commonly mutated in breast, lung, and prostrate cancer. -Reintroduction of normal RB gene back into transformed cells in vitro causes loss of transformed phenotype. *2 normal RB copies: no cancer. -Must have 2 independent mutations. -Takes a long time, so only in older adults.

Rb

-Certain mutations in the RB gene cause a loss of the ability of pRb protein to bind to E2F and repress transcription. -This causes cells to cycle continuously → tumor formation. ->40 proteins bind to pRb, which suggests that it also carries out other unknown functions. -Some DNA tumor viruses like SV40 and adenoviruses encode proteins (e.g. large T-antigen) that bind to pRb, which blocks pRb's ability to bind to E2F, which causes continuous cycling.

proposed origin or malignant tumors

-Common feature is that cells acquiring mutations must be a dividing population. -Cancer can arise by mutations in cells in multiple different stages along the differentiation pathway.

DNA damage checkpoint (recall)

-DNA damage in G2 activates ATR -which activates checkpoint kinase Chk1 -which phosphorylates Cdc25 -which causes an adaptor protein to bind to it to keep it in the cytoplasm -which prevents it from dephosphorylating Cdk1 -which causes cell cycle arrest in G2. -DNA damage in G1 recruits sensor MRN complex -which activates ATM -which activates Chk2, that stabilizes p53 transcription factor -which causes expression of p21 -which inhibits G1-Cdk, which causes cell cycle arrest in G1.

DNA damage checkpoints

-DNA damage in G2 activates ATR -which activates checkpoint kinase Chk1 -which phosphorylates Cdc25 -which causes an adaptor protein to bind to it to keep it in the cytoplasm -which prevents it from dephosphorylating Cdk1 -which causes cell cycle arrest in G2. -DNA damage in G1 recruits sensor MRN complex -which activates ATM -which activates Chk2, that stabilizes p53 transcription factor -which causes expression of p21 -which inhibits G1-Cdk, which causes cell cycle arrest in G1.

Culturing Cancer Cells

-From primary tumor- tumor can be removed, dissociated into single cells, and cultured. This can lead to the establishment of cancer cell lines. -Transformation of normal cells- cells can be transformed by using carcinogenic chemicals, radiation, or tumor-causing viruses. These cells will grow indefinitely, like naturally occurring tumor cells. Injection of these cells into an animal will often form tumors. This can also lead to the establishment of cancer cell lines, but their properties may be different from cell lines made from naturally occurring primary tumors.

human glioblastoma multiforme

-GBM is derived from stem/progenitor cells or glial cells/ GBM is an example of a highly motile cancer in brain, which makes it lethal because of local invasiveness -5 year survival rate (3.4%)

IRSs activate signaling pathways

-MAPK cascade is very important. -Ras MAP kinase pathway causes transcription of genes involved in proliferation (cyclin D1). -Cyclin D1 is a protein required for progression through the G1 phase of the cell cycle. -Cancer can be caused by mutations in: Ras, Raf, c-fos, c-jun, RTKs (EGFR, PDGFR). -Mutations in Ras found in about 30% of human cancers. Mutations prevent hydrolysis of GTP to GDP, so always activates Raf and MAP kinase cascade. Mutations in GAPs can also keep Ras active (no GTPase activity).

tumor suppressor genes

-Most act as negative regulators of cell proliferation. About 2 dozen tumor suppressor genes: -Transcription factors: APC, BRCA1, p53, WT1 -Cell cycle regulators: RB, p16 -Signaling regulators: NF1, PTEN Some mutations widespread, others specific

oncogenes

-Mutation of 1 of 2 copies of an oncogene may be sufficient to cause a loss of growth control. -Mutations are gain-of-function. -Oncogenes lead to: genetic instability increased proliferation rescue from apoptosis increased motility and metastasis

Mechanisms of action of carcinogens

-Mutations shown in DNA sequence of p53 tumor suppressor gene. -p53 is mutated in about 50% of cancers. -Chemical agents or UV irradiation cause distinct nucleotide substitutions. -Normal metabolic processes can also lead to deamination and nucleotide substitution.

Effects of serum deprivation on normal vs. cancer cells

-Normal cells depend on growth factors like EGF and insulin to divide in culture, which is supplied by serum. -Stimulate MAPK pathway. -Cells can deplete the media of growth factors or become confluent and stop dividing. -Cancer cells do not need these stimulatory factors to divide in culture

Growth properties of normal and cancerous cells

-Normal cells grown in culture grow as a monolayer until they reach confluency. Then, they become contact-inhibited and stop dividing. -Cancer cells do not stop growing and form clumps or foci. They are not contact-inhibited.

Metastasis to Another Organ

-Primary tumor must grow and cancer cells must detach and invade (intravasate) blood vessels or lymph vessels. Cells must successfully travel through blood or lymph to distant site. -Cells must exit (extravasate) vessel and invade into new tissue or organ, like brain. -Cells must grow to form secondary tumor.

proto-oncogenes to oncogenes

-Proto-oncogenes are proteins that have various functions in cells, whose activities are normally tightly regulated. -They can be converted to oncogenes by several mechanisms: 1) Proto-oncogene can be mutated to cause altered protein structure or function. 2) Mutations in the regulatory region or gene duplications can cause increased expression. 3) A chromosomal rearrangement can increase expression or cause abnormal fusion genes. 4) Retroviral insertion next to gene can cause increased or unregulated expression.

Role of pRb protein in controlling the cell cycle

-RB gene encodes the pRb protein, which regulates the cell cycle. -Regulates G1 to S phase transition. -Time of activation of DNA This transition commits the cell to go through cell cycle and divide. -polymerase, cyclin E, and histones. -pRb normally interacts with E2F transcription factor to repress transcription of multiple genes needed for G1 to S transition. -Activation of cyclin-dependent kinase (Cdk) at the end of G1 causes phosphorylation of pRb and dissociation from E2F. -E2F activates transcription. -Many proteins needed for G1 to S phase transition are expressed (e.g. cyclin E and DNA polymerase). -G1 to S transition occurs.

oncogenes

-Remember, oncogenes are derived from proto-oncogenes, which have functions in normal cells. -About 100 oncogenes have been identified, but only about a dozen play a role in human carcinogenesis. -***RAS GTPase is the oncogene mutated most often in human cancers. -Other oncogenes encode growth factors, growth factor receptors, protein kinases, transcription factors, and apoptosis regulators. -Some are cancer-specific, indicating the importance of certain pathways in those cell types. proteins encoded by oncogenes 1) growth factors 2) growth factor receptors 3) protein kinases 4) cell cycle regulators 5) apoptosis regulators 6) transcription factors *Appears that all levels of signaling pathways that control the cell cycle contain proto-oncogenes that are subject to activation. Proteins involved in mitosis, tissue invasion, and metastasis are not shown, but could be.

Premalignant polyps in colon

-The inherited disorder familial adenomatous polyposis coli (FAP) results in hundreds or thousands of premalignant polyps (adenomas). -These people inherit a mutation in the APC gene, and the sporadic mutation of the other copy results in polyps. -APC mutations are also found in 80% of sporadic colon tumors. -APC is a tumor suppressor protein that inhibits transcription of genes (e.g., MYC) that stimulate cell proliferation. Action is complex.

brain tumor types (cont)

-acoustic neuroma -astrocytoma -Grade I--pilocytic astrocytoma (30% survival) -0-20yrs -neuroaxis, optic pathway, years to decades -Grade II-- low grade astrocytoma -30-40 years, cerebral hemispheres, pons and brainstem in children, 5 years -Grade III-- anaplastic astrocytoma -early 40s, cerebral hemispheres, 2-5 years -Grade IV --glioblastoma (GBM) (3.4% survival) -mid 50's and 60's, cerebral hemispheres, 14 months

treatment for malignant gliomas

-almost incurable via surgery -if you remove as much of the tumor as possible, chemotherapy or radiation is more likely to work -typical radiation is 5 days a week for a month

2006 deaths and leading causes

-cancer 559,888 (23% of all deaths) -lung cancer (28.3%) -lung cancer is leading killer in both men and women in US -lung cancer causes more deaths than the next 3 most common cancers combined (lymphoid, colon, breast) -smoking causes (90%) of lung cancer deaths

Genes that control carcinogenesis

-carcinogenesis is the initiation of cancer formation -Many genes that are altered in carcinogenesis can be placed in 2 broad categories: 1) TUMOR SUPPRESSOR GENES- encode proteins that restrain cell growth and prevent cells from becoming malignant. Mutations can cause loss of growth control. Both copies of these genes must be mutated to result in loss of their function. 2) ONCOGENES- encode proteins that promote loss of growth control and promote transformation. They arise by mutation or activation of normal cellular proto-oncogenes, which are involved in normal cell function. Only 1 copy of these genes needs to be mutated or activated to gain uncontrolled function. *Most tumors contain alterations in both oncogenes and tumor suppressor genes. Cells with active oncogenes still do not become transformed if they have normal tumor suppressor genes. As many as 7 different genes must be mutated in colon cancer to become fully malignant.

pathways of GBM development

-cell of origin ---> GBM (PRIMARY) -cell or origin ---> low grade glioma ---> anaplastic glimoa ---> glioblastoma (SECONDARY, not metastic)

p53

-p53 is a tumor suppressor gene, perhaps the most important. -Mutations occur in breast, brain, and leukemia cancers. -Individuals with a single inherited p53 mutation have Li-Fraumeni syndrome, which predisposes them to high incidences of the above cancers. -2nd sporadic p53 mutation causes cancer (similar to RB). ->50% of all human cancers have mutations in both copies of p53. ->1,000 different mutations in p53 are known. -Elimination of p53 function is an important step in the progression of many cancer cells toward the fully malignant state. -p53 levels rise in cells with damaged DNA, causes transcription of p21, which inhibits Cdk to remain in G1 phase until DNA is repaired (Fig. 14.9). -High levels of p53 can also activate apoptosis by upregulating Bax gene -Most common mutations occur near where p53 interacts with DNA.

incidence and mortality

-prostate cancer incidence is very high, but mortality is low (14%) -lung cancer incidence is high, with high mortality (86%) -pancreatic and brain cancer incidence is low, but very deadly (90%)

gene expression profiling

-~350 genes have a causal role in development of cancers. -Monitoring gene expression in cells is an important tool for diagnosing different cancers. -DNA microarrays are made by placing thousands of spots of DNA from known genes on a slide. -cDNAs are made from tumor cells, fluorescently labeled, and hybridized to slide. -Colors different depending on expression levels. -This type of analysis can be used to detect differences in gene expression between different types of tumors, which can aid classification as opposed to simple pathological appearance. -It can tell you which cell signaling pathways have been turned on or off. -It can also provide a list of genes to look at as targets for therapeutic drugs.

DNA damage, repair, and apoptosis

1) DNA damage can cause double strand breaks. 2a) Breaks normally repaired by complexes with BRCA1 and BRCA2 proteins. 2b) Mutations in these proteins can cause failed repair. 3a) Failed repair activates p53 system. 3b) p53 is normally inhibited by MDM2 protein by causing p53 degradation. 4a) Activated p53 can cause expression of p21 and cell cycle arrest. 4b) Or, it can cause expression of and/or binding to Bax and apoptosis.

Angiogenesis is important for tumor growth

1) Transformed cell proliferates into small tumor. Will stay small if it remains without blood vessels. 2) Tumor produces angiogenic factors (e.g., VEGF) that attract blood vessels to grow into it. 3) Tumor becomes vascularized and is now capable of unlimited growth and metastasis to distant sites.

Causes of Cancer

DNA mutagens- alter the DNA genome. ---Chemical carcinogens- in soot, in cigarette smoke, in grilled meat, benzene, etc. ---Ionizing radiation (skin cancer). Tumor viruses- only cause a small fraction. Mostly increase risk of developing cancer. ---DNA: polyoma virus, SV40, adenovirus, herpes-like viruses. ---RNA: retroviruses carry oncogenes like v-src, or they insert next to proto-oncogene in genome and activate it

Use of DNA microarrays for determining treatment

The earlier that a cancer is discovered, the more likely a person can be cured???? -Not true for all cancers, because some release metastatic cells early on.

changing cancer incidence of Japanese after moving to Hawaii

This shows the importance of environmental factors in causing different cancers

Model of p53 function

a) Cell division normally does not involve p53. Levels very low and unstable. b) DNA damage increases p53 level by increased stability, causing G1 arrest (via p21) or apoptosis (via increased expression of or binding to Bax). c) Mutated p53 (both copies) results in no arrest if DNA is damaged, and mutated cell either divides into tumor, or fails to divide and dies. *Cancer cells with p53 mutations respond to radiation and chemotherapeutic agents poorly because they cannot be triggered into apoptosis by p53 pathway or halt cell cycle to repair DNA.