THE MOLECULAR BASIS OF CANCER

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THE MOLECULAR BASIS OF CANCER
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THE MOLECULAR BASIS OF CANCER

• Nonlethal genetic damage lies at the heart of
  carcinogenesis.
• Number of hypothesis proposed and rejected grow
  more rapidly than the growth of highly malignant
  tumors
• Knowledge on oncogenic pathway is doubling every
  3 years

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THE MOLECULAR BASIS OF CANCER

• The genetic hypothesis of cancer implies that a
  tumor mass results from the clonal expansion of a
  single progenitor cell that has incurred the
  genetic damage.
• Clonality of tumors.

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THE MOLECULAR BASIS OF CANCER

• Carcinogenesis is a multistep process at both the
  phenotypic and the genetic levels.

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• Primary or environment (75)

Alterations to the human genome First
hit Second hit
Cancer genome
Normal genome
TIME
Carcinogenesis is a multistep event with somatic
mutations
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• dysplasia sequence in cervical carcinoma

Normal tissue (epithelium)
Dysplastic epithelium
CIN (I-IV) Intraepithelial lesion
CIS Carcinoma in situ (no penetration of BM)
Invasive Carcinoma (BM destroyed)
Loss of polarity/maturation pattern Increased
number of mitotic figure Variation of cell size
/shape (pleomorfism) Large
nuclei/cytoplasmic ratio Hyperchromatic
nuclei Primitivity of nuclear chromatin
(altered differentiation)
Rarely curable
Curable
Curable
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Dysplasia neoplasia sequence in colonic carcinoma
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Dysplasia neoplasia sequence in colonic carcinoma
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THE MOLECULAR BASIS OF CANCER

• Tumor progression results from the accumulation
  of genetic lesions involving
• 1- growth-regulatory genes
• 2- genes that regulate angiogenesis
• 3- genes that regulate invasion and metastases
  
• Cancer cells also must bypass the normal process
  of DNA repair and aging that limits cell division

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THE MOLECULAR BASIS OF CANCERGrowth-regulatory
genes

• Three classes of normal regulatory genes
• growth-promoting protooncogenes
• growth-inhibiting cancer suppressor genes
  (antioncogenes)
• genes that regulate programmed cell death
  (apoptosis)
• These are the principal targets of genetic
  damage.

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THE MOLECULAR BASIS OF CANCER

• A fourth category of genes
• genes regulate repair of damaged DNA

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THE MOLECULAR BASIS OF CANCER

• Six acquired capabilities of cancer cells
• 1. Self-sufficiency in growth signals.
• 2. Insensitivity to growth-inhibitory signals.
• 3. Evasion of apoptosis.
• 4. Limitless replicative potential (e.g. over-
• coming cellular senescence).
• 5. Sustained angiogenesis.
• 6. Ability to invade and metastasize.
• When genes that normally sense and repair DNA
  damage are lost (enabler genes), the resultant
  genomic instability favors mutations in genes
  that regulate the six acquired capabilities of
  cancer cells.

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THE MOLECULAR BASIS OF CANCER
SELF SUFFICIENCY IN GROWTH SIGNALS

• Genes that promote autonomous cell growth in
  cancer cells are called oncogenes.
• Derived by mutations in protooncogenes.
• Oncoproteins control the sequence of events that
  characterize normal cell proliferation.

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THE MOLECULAR BASIS OF CANCER

• Oncogenes and oncoproteins
• Growth factors
• Growth factors receptors
• Intracellular signaling transducing proteins
• Nuclear transcription factors
• Entry of the cell in cell cycle

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Events that characterize normal cell
proliferation.

• The binding of a growth factor to its specific
  receptor in the cell membrane.
• Transient and limit activation of the growth
  factor receptor, which in turn activates several
  signal-transducing proteins on the inner leaflet
  of the plasma membrane.
• Transmission of the transduced signal across the
  cytosol to the nucleus via second messengers.
• Induction and activation of nuclear regulatory
  factors that initiate DNA transcription.
• Entry and progression of the cell into the cell
  cycle.

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GROWTH FACTORS (GF)

• Paracrine and autocrine
• Cancer cells acquire self-sufficiency in growth
  signals by acquiring the ability to synthesize GF
• Examples PDGF in
  glioblastoma
• TGF-alpha in sarcomas
• Mutation of GF genes or the products of other
  genes e.g. RAS cause
• Overexpression of GF genes

GF
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GROWTH FACTOR RECEPTORS
Mutation and overexpression of GROWTH FACTOR
RECEPTORS found in many tumors e.g. EGF receptor
family ERBB1, 80 of SCC of Lung ERBB2 (HER2),
25-30 of breast CA, adenocarcinoma of lung,
ovary and salivary glands
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SIGNAL-TRANSDUCING PROTEINS
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Principle of signalling



Common mechanism e.g. RAS and ABL genes
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Signaling completed successfully - transcription
proceeds
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RAS gene
RAS-RAF cascade
RAS gene mutation found in 30 of all human
tumorse.g. carcinomas of colon, pancreas
thyroid
GTPase activating protein (GAPs) NF-1 mutation in
familial neurofibromatosis type 1
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MAP kinase

• Mitogen-activated protein kinase (mitogens are
  hormones that cause mitosis)
• Activated by a kinase cascade
• MAP kinase is phosphorylated and activated by MEK
  (MAP kinase)
• MEK (MAP kinase) is phosphorylated and activated
  by Raf
• Raf is activated by Ras

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SIGNAL-TRANSDUCING PROTEINS

• ABL gene
• Found in 90 of chronic myeloid leukemia
• BCR- ABL hybrid has potent tyrosine kinase
  activity that activates cell growth by several
  pathways including
• RAS-RAF cascade
• Normal ABL protein localizes in nucleus to
  promote apoptosis
• Drug STI 571 act in both ways

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NUCLEAR TRANSCRIPTION FACTORS
MYC gene bind to DNA and activate transcription
of several growth related genes e.g. CDKs C-MYC
gene is amplified in CA breast, colon lungs N-
MYC gene is amplified in neuroblastomas L-MYC
gene is amplified in small cell CA
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NUCLEAR TRANSCRIPTION FACTORS

• MYC gene is dysregulated in 90 of Burkitt
  lymphoma

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NUCLEAR TRANSCRIPTION FACTORS
N- MYC gene is amplified in neuroblastomas
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CYCLINS AND CYCLIN-DEPENDENT KINASES
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Cell cycle

A
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Cell cycle checkpoints
CDK6/cyclinD/CDK4 cyclin2/CDK2 complex
Rb Rbpp
1
Release of E2F TF
1a
Transcription begins
2a
Completion of S
2
Spindle checkpoint
p53 protein binds p21 (block inhibition)
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CYCLINS AND CYCLIN-DEPENDENT KINASES

• Cyclin D gene is overexpressed in cancers of
  breast, esophagus, liver and some types of
  lymphoma
• Amplification of CDK4 gene occur in melanoma,
  sarcoma and glioblastoma
• At least one of the four key regulators of cell
  cycle is mutated in most human cancers (INK4a,
  cyclin D, CDK4, RB)

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INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS
Cancer suppressor genes

• The products of tumor suppressor genes apply
  brakes to cell proliferation whereas oncogenes
  encode proteins that promote cell growth
• Disruption of cancer suppressor genes renders
  cells refractory to growth inhibition.
• e.g. Retinoblastoma (RB gene), TP53, TGF-?, APC
• Two hit hypothesis.

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INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS
Cancer suppressor genes

• Two hit hypothesis.

• Heterozygous.
• Homozygous.
• Lose of heterozygosity.
• Recessive cancer genes

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Cancer suppressor genes

• Prevent cell proliferation by two complementary
  mechanisms
• 1- Cause dividing cells to go into G0
• 2- Cell enter a postmitotic, differentiated
  pool and lose its replicative potential.

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Cancer suppressor genes

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Cancer suppressor genes

• Retinoblastoma gene ( RB gene)
• Two mutations (hits) are required to produce
  retinoblastoma
• These involve the RB gene, located on chromosome
  13q14.
• The RB gene product is a DNA-blinding protein
  that is expressed in every cell type examined.
• Found in other neoplasms e.g. breast cancer,
  small cell cancer of the lung, and bladder
  cancer.

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INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS
RB gene

• Sporadic cases 60
• -2 hits are required.
• Familial cases 40
• -one hit is rerquired.
• -Patients with familial retinoblastoma also are
  at greatly increased risk of developing
  osteosarcomas, breast carcinoma, small cell
  carcinoma of lung, some soft tissue sarcomas and
  some brain tumor.

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RB GENE AND CELL CYCLE
INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS

• RB serves as a brake in the advancement of cells
  from G1 to the S phase of the cell type.
• Quiescent cells (in G0 to G1) contain the active
  hypophosphorlyated form of RB.
• RB prevents cell replication by binding and
  possibly sequestering, the E2F family of
  transcription factors.

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RB GENE AND CELL CYCLE
RB a brake
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TO CYCLE OR NOT TO CYCLE A CRITICAL DECISION IN
CANCER
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INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS
RB gene

• Loss of normal cell cycle control is central to
  malignant transformation
• at least one of the four key regulators of cell
  cycle is mutated in most human cancers (INK4a,
  cyclin D, CDK4, RB)

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INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS
RB gene

• Transforming proteins of several oncogenic animal
  and human DNA viruses seem to act by neutralizing
  the growth-inhibitory activities of RB.
• e.g. SV40
• polyomavirus large T antigens
• adenoviruses EIA protein
• human papillomavirus (HPV) E7 protein
• bind to the hypophosphorylated form of RB
  (functionally deleted).

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Cancer suppressor genes TRANSFORMING GROWTH
FACTOR ? PATHWAY
INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS

• TGF-? a member of a family of dimeric growth
  factors that includes bone morphogenetic
  proteins.
• It is a potent inhibitor of proliferation in
  epithelium, Ec and hematopoietic cells
• At least one component of the TGF-? pathway is
  mutated in 100 of pancreatic cancers and 83 of
  colon cancers.

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Receptor activation occurs when TGF-b mediates
the association of two type I and two type II
transforming growth factor- (TGF-b )
receptors. TGF-b arrest cell in G1 phase by
stimulating CDK1 and by inhibitjng the
transcription of CDK2, CDK4, cyclin A E.
TGF-b signaling pathway 
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Cancer suppressor genes ADENOMATOUS POLYPOSIS
COLI-?-CATENIN PATHWAY
INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS

• APC gene (chromosome 5) has antiproliferative
  effects
• ?-catenin is a protein with many
  functionsintercellular adhesion and can
  translocate to nucleus, activating cell
  proliferation
• This leads to transcription of growth-promoting
  genes such as cyclin D1 and MYC.
• In malignant cells APC gene is lost, ?-catenin
  degradation is prevented, and the WNT signaling
  response is continually activated

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The WNT signalling pathway
proliferation
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Cancer suppressor genes TP53 GENE GUARDIAN OF
THE GENOME
INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS

• One of the most commonly mutated genes in human
  cancers.
• TP53 can exert antiproliferative effects and
  regulates apoptosis.
• TP53 assists in DNA repair by causing G1 arrest
  and inducing DNA repair genes. A cell with
  damaged DNA that cannot be repaired is detected
  by TP53 to undergo apoptosis.

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INSENSITIVITY TO GROWTH-INHIBITORY
SIGNALS Cancer suppressor genes

• TP53
• With homozygous loss of TP53, DNA damage goes
  unrepaired, mutations become fixed in dividing
  cells and the cell become malignant

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Cancer suppressor genes TP53 GENE
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TP53
INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS

• More than 70 of human cancers have a defect in
  this gene and the remaining have defects in genes
  up-stream or down-stream of TP53.
• Homozygous loss of the TP53 gene is found in
  virtually every type of cancer, including
  carcinomas of the lung, colon and breast, the
  three leading causes of cancer deaths.

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TP53
INSENSITIVITY TO GROWTH-INHIBITORY SIGNALS

• Li-Fraumeni syndrome
• inherited loss of TP53-
• 25 fold greater chance to develop cancer
• e.g. sarcomas, breast cancer, leukemia, brain
  tumors and carcinomas of the adrenal cortex
• Normal TP53 can be rendered nonfunctional by
  certain DNA viruses
• oncogenic HPVs
• hepatitis B virus (HBV)
• possibly Epstein-Barr virus (EBV)