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Cell Cycle Regulation and Cancer. Cancer Second leading cause of disease in Western Countries 1 million new cases per year in U.S. –500,000 per year die.

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Presentation on theme: "Cell Cycle Regulation and Cancer. Cancer Second leading cause of disease in Western Countries 1 million new cases per year in U.S. –500,000 per year die."— Presentation transcript:

1 Cell Cycle Regulation and Cancer

2 Cancer Second leading cause of disease in Western Countries 1 million new cases per year in U.S. –500,000 per year die War “declared” on cancer approximately 30 years ago Slowly treatments are changing/improving based upon better genetic understanding of the varieties

3 Cancer Rates in US

4 Cancer is a Genetic Disease Genome alterations –One nucleotide to large-scale chromosome rearrangements, amplifications and deletions –Mostly in somatic cells (unless associated with inherited risk—about 1% of total) –Alter cellular functions DNA repair, cell division, apoptosis, cellular differentiation and cell-cell contact/communication

5 Normal and Cancer Karyotypes Chromosome painting (a) is a normal cell, (b) is a “very messed up” cancer cell

6 What is Cancer? Large number of complex diseases Behave differently depending upon cell type from which originate –Age on onset, invasiveness, response to treatment Common general properties –Abnormal cell growth/division (cell proliferation) If only this is a benign tumor When grow in culture without contact inhibition are referred to as transformed –Spread to other regions of body (metastasis) Malignant tumors

7 Clonal Origin of Tumors Tumor arises from a single cell Burkitt’s lymphoma –Translocation involving chromosome 8 (myc) and either chromosomes 2, 14, or 22 (near an immunoglobulin gene –All cells from a patient have breakpoints at exactly the same points as shown by DNA sequence analysis –Cancer cells in tumors of females all use same X chromosome (same one in Barr body)

8 Multistep Process Cancer requires mutation of multiple genes Age relationship with cancer consistent with this –If one mutation caused cancer then rate would be constant independent of age It increases dramatically with age… Delay between carcinogen exposure and onset –5-8 year delay between carcinogen exposure (Hiroshima and Nagasaki) and onset of leukemia –15 year delay between tuberculosis X-ray treatment and onset of breast cancer

9 Age and Cancer Note log scale for incidence rate

10 Multistep Process…Continued Cancers often develop in progressive steps –From mildly aberrant cells to malignant –See figure 18-3 –Process called tumorigenesis

11 Tumorigenesis of Cervical Cancer

12 Properties of Cancer Cells Genetic instability –Mutator phenotype –Duplicating, losing and translocating chromosomes or portions of them common Chronic myelogenous leukemia (CML) –Chromosome 9/chromosome 22 translocation –BCR gene fused to ABL (protein kinase) –Mutant signal transduction protein stimulates cells constantly to proliferate

13 Genome Instability Double minutes (DMs) –Miniature chromosomes giving many copies of rgion Homogeneous staining regions (HSRs) –Tandem gene duplications

14 Chromosomal Translocation in CML

15 Xeroderma Pigmentosum Failure to remove pyrimidine dimers from DNA –Excision repair defect Patients often develop skin cancer and must stay out of sunlight

16 HFNPCC Hereditary nonpolyposis colorectal cancer Higher than normal rates of colon (first noted) but also elevated rates of ovary, uterine and kidney cancers 1/200 persons, autosomal dominant Eight genes associated and four involve mismatch repair systems

17 HNPCC Pedigree Colon, Stomach endometiral, pancreatic, bladder Orange also other cancers, multiple slashes unknown cause of death

18 Defects in Cell Cycle Regulation Cell cycle G1, S, G2, M phases Progression through cycle is regulated and specific blocks or checkpoints exist Nondividing cell (quiescent) is in an extended G1 phase called G0 –Cancer cells never enter G0

19 Cell Cycle

20 Cell Cycle Checkpoints G1/S –Monitors cell size and for DNA damage G2/M –Replication complete, DNA damage? M –Spindle fibers connected, etc.? G0 –Does body require more of my type of cell?

21 Regulators of Cell Cycle Cyclins and cyclin-dependent kinases (CDKs) Cyclins synthesized and destroyed in a precise pattern –A cyclin bind to a specific CDKs, activating it Other proteins phosphorylated/activated CDK4/cyclinD activate transcription factors for genes such as DNA polymerase delta and DNA ligase CDK1/cyclinB trigger events of early mitosis (chromosome condensation, nuclear membrane breakdown, etc.)

22 Cyclin Levels

23 Activation of CDKs

24 Apoptosis Programmed cell death, cell suicide Pathway should be activated if “something goes wrong” –Especially involving DNA/chromosome damage Involves proteases called caspases Regulated by Bcl2 and BAX –BAX homodimer promotes apoptosis, Bcl2 homodimer blocks apoptosis –Some cancer cells overproduce Bcl2 and are resistant to some chemotherapies and radiation treatment Proteins involved in cell cycle checkpoints regulate pathway

25 Control of Apoptosis

26 Functions of Cancer Causing Genes/Alleles Many disrupt control of cell cycle Oncogenes –Proto-oncogenes Normal genes that if mutated may act to make a cell cancerous Recessive, cancer causing forms active and stimulates cell division C-oncogenes and v-oncogenes Tumor suppressors –Genes whose products act to regulate cell cycle –Loss of gene product function contributes to cancer process –Recessive, commonly involved with inherited risk About 200 proto-oncogenes and tumor suppressor genes


28 Oncogenes/Proto-oncogenes Cyclin D1 and Cyclin E are proto-oncogenes –Often amplified or over expressed due to other mutations (e.g. translocation) in many cancers cyclinD1 allows for DNA replication (S phase) Over expression seems to contribute to cell’s progression from G0 phase and begin division

29 ras Proto-oncogenes Involved in signal transduction pathway –As are many proto-oncogene products ras family genes mutated in 40% of all cancers Involved in signal transduction pathway from growth factor receptor to nucleus –G protein –Mutant form lacks GTPase activity and remains active See figure 18-11

30 Ras Pathway Growth factor binds receptor Receptor exchanges GTP for GDP on Ras –Ras activated Ras  Raf  Mek  Map Kinase  transcription factors  genes turned on

31 Mutant Ras Protein Single amino acid changes create N-ras and K-ras variants

32 p53 Tumor Suppressor Gene Mutated (inactivated) in more than 50% of all cancers p53 regulates (activates or represses) transcription of more than 50 different genes p53 regulated by Mdm2 (prevents the phosphorylations and acetylations that activate inactive p53) Activated p53 levels rise rapidly if DNA is damaged or repair intermediates accumulate

33 P53 Function Activated p53 acts as transcription factor to turn on genes that –arrest the cell cycle so DNA can be repaired Initiates synthesis of p21, which inhibits CDK4/cyuclinD1 complex, blocking entry into S phase Genes expressed which retard rate of DNA replication Other products block G2/M progression –Initiate apoptosis if DNA cannot be readily repaired Turns on Bax gene, represses Bcl2 gene Bax homodimers activate process of cell destruction Cancer cells lacking p53 do not initiate pathway even if DNA/cellular damage is great

34 RB1 Tumor Suppressor Gene Retinoblastoma 1 gene Involved in breast, bone, lung, bladder and retinal cancers (among others) Inheriting one mutated (inactivated) copy of gene increases chances of retinoblastoma formation from 1/14,000-20,000 to 85% (plus increases other cancer rates) –Loss of second copy in a cell eliminates function –Normal cells unlikely to lose both good copies

35 pRB Function Tumor suppressor protein that controls the G1/S checkpoint Found in nucleus and activity regulated by level of phosphorylation (by CDK4/cyclinD1 complex) –Nonphosphorylated version binds to TFs such as E2F, inactivating them –Free E2F and the other regulators turn on >30 genes required for transition to S phase

36 Familial Retinoblastoma

37 Inherited Predisposition for Cancer About 1-2% of cancer has an inherited or familial component –50 different forms known at present Inherited in Mendelian fashion but most all genes/alleles are recessive –Second copy must be mutated in a somatic cell Called loss of heterozygosity (and loss of function) Loss of second copy in germ line lethal RB1 and APC (lost in FAP, familial adenomatous polyposis) are examples of such genes

38 Multistep Development of Colon Cancer APC loss causes cells to partially escape cell cycle regulation, DCC seems to be involved in cell adhesion and differentiation

39 Transforming Viruses Viruses discovered to cause cancer in animals –Acute transforming viruses Commonly but not always retroviruses –Rous sarcoma virus (RSV) discovered by Francis Peyton Rous discovered in 1910 as a causative agent of chicken sarcomas (solid tumors of muscle, bone or fat) Many years later shown to be retrovirus Nobel Prize in 1966 (link of viruses to cancer)

40 Retroviruses ssRNA chromosome Chromosome copied to DNA by reverse transcriptase upon entry into cell DNA integrated into host cell chromosome –Provirus Provirus has strong promoter elements in U5 and U3 terminal sequences –U5 expresses gag, pol and env Oncogenic when –Integrate near proto-oncogene and cause inappropriate or over expression –Bring v-onc as part of viral chromosome

41 Retroviruses Many transforming retroviruses are defective in the sense that one or more of gal/pol/env have been deleted to make room for the v-onc

42 Viral Oncogenes Most v-onc genes have normal cellular counterparts –If simply mutated to the oncogenic form and not in a virus are called c-onc

43 Human Cancer-Associated Viruses To date no acute transforming retroviruses have been discovered in humans –Viruses can contribute to but not be the sole cause of human cancer –However, up to 15% of all cancers have a viral association Papillomaviruses HPV 16 and 18, hepatitis B virus, Epstein-Barr virus, Human T-cell leukemia virus are examples of cancer-associated viruses

44 Human Viruses Associated With Cancer Non-retroviral varieties Many of these v-onc genes act to stimulate the cell cycle (viruses needs host replication apparatus to multiply

45 V-onc Gene Product Action Some v-onc gene products have their transforming effect by binding and thereby “taking out” certain tumor suppressor gene products –Cell division required to provide replication apparatus for virus –Bad, but does open some interesting treatment possibilities…

46 Environmental Agents and Cancer Natural and man-made carcinogens –Chemicals, radiation, chronic infections 30% of cancer deaths associated with cigarettes –Seems to preferentially mutate proto-oncogene and tumor suppressor genes Red meat consumption –How cooked? Alcohol-based inflammation of the liver Aflatoxin (mold on peanuts) UV light or ionizing radiation –Radon gas (up to 50% of radiation exposure???)

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