Cell Cycle Regulation and Cancer

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Chapter 18

• Cell Cycle Regulation and Cancer

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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

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Cancer Rates in US
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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 riskabout 1 of total)
• Alter cellular functions
• DNA repair, cell division , apoptosis, cellular
  differentiation and cell-cell contact/communicatio
  n

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Normal and Cancer Karyotypes

• Chromosome painting
• (a) is a normal cell, (b) is a very messed up
  cancer cell

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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

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Clonal Origin of Tumors

• Tumor arises from a single cell
• Burkitts 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)

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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

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Age and Cancer

• Note log scale for incidence rate

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Multistep ProcessContinued

• Cancers often develop in progressive steps
• From mildly aberrant cells to malignant
• See figure 18-3
• Process called tumorigenesis

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Tumorigenesis of Cervical Cancer
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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

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Genome Instability

• Double minutes (DMs)
• Miniature chromosomes giving many copies of rgion
• Homogeneous staining regions (HSRs)
• Tandem gene duplications

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Chromosomal Translocation in CML
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Xeroderma Pigmentosum

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

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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

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HNPCC Pedigree

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

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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

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Cell Cycle
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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?

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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.)

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Cyclin Levels
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Activation of CDKs
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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

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Control of Apoptosis
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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

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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 cells
  progression from G0 phase and begin division

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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

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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

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Mutant Ras Protein

• Single amino acid changes create N-ras and K-ras
  variants

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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

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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

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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

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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 gt30
  genes required for transition to S phase

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Familial Retinoblastoma
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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

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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

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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)

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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

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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

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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

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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

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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

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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

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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???)