Husni Maqboul, M'D

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Neoplasia III Oncogenes Cancer

• Husni Maqboul, M.D

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Oncogenes Historical Considerations-I

• 1911-Peyton Rous discovers a filterable agent
  in extracts of chicken tumors that can induce new
  tumors when injected into otherwise normal
  chickens. (Nobel Prize)
• Filterable substance is later identified as a
  virus that had an RNA genome instead of DNA and
  is thus termed a retrovirus.
• Retroviruses have three basic genes Group
  Antigen Gene (GAG), a special Polymerase (POL),
  and viral envelop proteins (ENV).

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Oncogenes Historical Considerations-II

• 1970-Temin and Baltimore discovered that the POL
  enzyme can reverse-transcribe or direct DNA
  synthesis from an RNA template (Nobel Prize).
• Two types of the retroviruses identified
  transforming and non-transforming.
  1970-Isolation of an additional genetic element
  termed SRC from transforming Rous sarcoma viruses
  that is directly responsible for causing cancer
  (an oncogene).

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Oncogenes Historical Considerations-III

• 1976-Varmus and Bishop discovered that normal
  cells contain a homolog of the oncogene v-src.
  Thus, all normal cells contain potential cancer
  causing genes (Nobel Prize). All v-onc genes
  have normal cellular homologs.
• 1983-Hanafusa elucidated the mechanism by which
  normal cellular oncogenes are acquired by
  retroviruses.

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Oncogenes Historical Considerations-IV

• 1982-Weinburg transforms mouse fibroblasts with
  human DNA derived from bladder carcinoma cells.
  Those cells that become transformed have a human
  gene homologous to v-ras. First demonstration
  that an oncogene can cause human cancer.
• Over 50 oncogenes and proto-oncogenes have been
  identified by studying oncogenic viruses and
  transforming genes from human and animal tumors.

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Oncogenes are normal cellular genes that are
involved in growth control. Cancer results
when these genes become dysregulated such that
they are inappropriately activated.
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Molecular Basis Of Cancer
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Tumor Cell Physiology

• Self-sufficiency in growth signals
• Signal transduction cascade
• Cell cycle regulation
• Insensitivity to growth inhibitors
• Evasion of apoptosis
• Immortality
• Sustained angiogenesis
• Invasion and metastases

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Signal transduction via II messeng.
Transient limited activ of GF recep.
Binding to specific receptor
Entry of the cell to the cell cycle
Induction of nuclear factors initiating transcript
ion
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Types of Normal Cellular Genes that can be
involved in carcinogenesis

• Growth Factors PDGF (sis)
• Glioblastomas? PDGF
• Sarcomas ? TGF-a
• Mutated RAS oncogene? overexpression of GF genes
• Growth Factor Receptors
• ERBB1-EGF receptor- Overexpression SCC of the
  lung
• ERBB1-HER2 receptor- amplification in some breast
  carcinomas ! treatment with anti HER2 antibody

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Types of Normal Cellular Genes that can be
involved in carcinogenesis

• Signal Transducing Proteins (RAS ABL)
• RAS mutations the most common in human cancers
• GTPase mutations in familial neurofibromatosis I
• ABL gene on chromosome 9 (TK cytoplasmic and
  proapoptotic nuclear activity )
• In leukemia ( BCR-ABL hybrid gene) is retained in
  cytoplasm, with inappropriate TK activity, and
  lost proapoptic properties

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Types of Normal Cellular Genes that can be
involved in carcinogenesis

• Nuclear Transcription Proteins
• MYC ,MYB, JUN, FOS, REL genes
• Division signal ? MYC protein ? binds to DNA?
  CDKs
• t(814) in Burkitts lmphoma? dysregulation of MYC
  protein? sustained proliferation
• MYC is amplified in other tumors such as in
  neuroblastoma (N-MYC) and small cell carcinoma of
  the lung (L-MYC)

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Cell Cycle, Cyclins and Cyclin-Dependent Kinases
(CDKs)
G1 - S Checkpoint
D/CDK4,CDK6 and E/CDK2 phosphor.Rb gene
Progrss from S to G2 by Cyclin A CDK2 , CDK1
Early in G2 B Cyclins CDK1
Cyclin E Late in G1
CDKs Constitively synthesized in inactive form
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CDKs, activation and inhibition
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Types of Normal Cellular Genes that can be
involved in carcinogenesis

• Cyclins and Cyclin-Dependent Kinases
• Activity is regulated by two families of CDKs
  inhibitors
• P21(CDKN1A), p27 and p57 have broad action
• p15, p16(CDKN2A), p18 and p19 inhibit D/CDK4 and
  D/CDK6
• Overexpression of cyclin D in breast, esophagus
  and liver cancers, and in some lymphomas
• Amplification of CDK4 gene in melanoma, sarcomas
  and glioblastoma

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Types of Normal Cellular Genes that can be
involved in carcinogenesis

• Insensitivity to growth inhibitory signals
• RB gene mutation
• Genes controlling RB phosphorylation
• TGF-ß
• stimulates CDKI p15
• inhibits transcription of CDK2 , CDK4 ,cyclins A
  and E
• Mutations seen in cancers of pancreas colon,
  stomach, and endometrium

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Activation of Oncogenes

• Point Mutations
• The RAS gene is an oncogene that becomes
  activated by a point mutation, c-fms in AML
• Chromosomal Translocations and inversions
• Translocation of chromosome 9 and 22 in CML
  creating a fusion gene that produces an activated
  tyrosine kinase.

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Activation of Oncogenes

• Gene Amplification
• Specific oncogenes such as N-myc and C-neu are
  amplified in neuroblastoma and breast cancer
  respectively.
• Deletions.
• Loss of cancer suppressor genes

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• RAS Point mutation
• 90 of pancreatic and cholangiocarcinoma
• 50 of colon, endometrial and thyroid Ca
• 30 of lung Ca myeloid leukemia

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Chromosomal Translocations
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Common Cytogenetic Abnormalities in Hematopoietic
Neoplasms
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Gene Amplification

• Reduplication and manifold amplification of their
  DNA sequences
• Overexpression of their products
• Patterns
• Double minutes ( multiple small chromosome like
  structures
• Homogenous staining regions (HSR)
• N-myc in Neuroblasoma , c-erb B2 in breast Ca

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Gene Amplification
N-myc Nuclear Regulat protein
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Tumor Suppressor Genes (Misnomer )

• A class of genes that normally suppress cell
  proliferation. Examples are TP53 and RB.
• Mutations that inactivate the tumor suppressor
  gene products can release cells from growth
  suppression and lead to hyperproliferation.
• Both alleles of the tumor suppressor gene must be
  inactivated by mutation for hyperproliferation to
  occur.

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Nuclear transcription cell cycle regulators

• RB gene ( Cancer suppressor )
• Brakes the advancement of cells from G1 to S
  phase
• pRB ( gene product ) regulates the function of
  transcription factor E2F
• Can be lost by deletion of 13q14
• Retinoblastoma, and to a lesser extent
  osteosarcoma, breast, bladder and small cell
  carcinoma of the lung

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• TGF mutation
• Pancreatic and
• Colonic Ca

Melanoma Panc. Ca Glioblastoma NSCCL
RB gene product Active Hypophos
Inactivated by some viral proteins e.g HPV
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Nuclear transcription cell cycle regulators

• TP53 gene
• Tumor suppressor located on 17p13.1
• Molecular Policeman preventing propagation of
  genetically damaged cells
• Has a short half life of 20 minutes
• Over 50 of human cancers contain mutated gene
• Li-Fraumeni syndrome Single mutated allele have
  25-fold greater chance to develop cancer, most
  commonly sarcomas, breast ca ,leukemia,

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TP-53 Guardian of the genome

• Activated by DNA damage and hypoxia leading to
  apoptosis ( tumor cells with mutant TP-53
  tolerate hypoxia better )
• Arrests cell cycle at G1 (transcription of p21)
  and induces DNA repair genes (GADD45)
• If DNA repair cannot be achieved, activates other
  genes (BAX) or antagonizes others (IGF. BCL2 )
  leading to apoptosis

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TP-53 Guardian of the genome

• TP53 damaged by
• Inactivating mutations in somatic cells
• Inherited mutation Li Fraumeni
• Inactivated by DNA viruses, HPV, HBV, EBV
• p73 Newly discovered relative of TP-53

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Evasion of Apoptosis
Reduced in HCC
Inactivated by FLIP proteins
Overexpression of BCL2 in lymphomas
Loss of APAF-1 in melanomas
Upregulation of caspase inhibitors in MALT
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Genes Regulating Apoptosis

• Apoptosis Inhibitors
• BCL-2 in 85 of follicular B-cell lymphoma
  (t1418) leading to juxtaposition with active Ig
  locus
• located on the outer mitochondrial membrane,
  blocks the exit of cytochrome-c that activates
  apoptosis proteolytic enzymes
• BCL-XL
• c-myc triggers proliferation, BCL-2 prevents
  death

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Genes Regulating Apoptosis

• Apoptosis Stimulators
• BAX, bcl-xS, BAD,BID
• Form channels allowing cytochrom-c to escape from
  the mitochondria
• TP53 upregulates BAX, codes for CD95 expression
  on the surface

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Nuclear transcription cell cycle regulators

• BRCA-1 (17q12.21) BRCA-2 (13q12.13)
• Tumor suppressors associated with breast and
  other cancers ( ovary , prostate, colon)
• Mutated genes account for 80 of familial breast
  cancer cases
• ? Involved in DNA repair
• Negative regulation of cell cycle by activating
  CDK inhibitor p21

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Regulation of Signal Transduction

• Tumor suppression by down-regulation of
  growth-promoting signals
• APC (Adenomatous Polyposis Coli ) gene on 5q21
• Familial adenomatous polyposis
• 70-80 of non-familial colorectal carcinoma
• APC protein causes degradation of ß catenin, that
  enters the nucleus and up-regulates proliferation
• NF-1 (Neurofibromatosis type I)
• Product -Neurofibromin- regulates RAS protein
  transduction ( inactivates RAS )

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WNT factor
Binds to Cadherin for cell adhesiveness
Prevents Catenin degradation
Activates cell proliferation
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APC favors Catenin destruction
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Transcription of D1 and MYC
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Regulation of Signal Transduction

• Cell Surface Receptors (for growth inhibitors)
• Receptor of TGF-beta leads to synthesis of
  cyclin-dependant Kinas (CDK) inhibitor
• 15 of colonic Ca
• Cadherins - Glycoprotiens that glue epithelial
  cells
• Loss leads to loose cohesiveness that facilitates
  MTS
• DCC ( Deleted in Colonic Carcinoma ) gene on
  18q21
• Cell surface molecule involved in cell-cell and
  cell-matrix interaction ( proliferation
  differentiation)

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Other Tumor Suppressors

• NF-2 gene
• VHL ( Von Hippel - Lindau )
• PTEN ( Phosphate and Tensin homolog )
• WT-1 ( Wilms Tumor ) on 11p13

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Genes that Regulate DNA Repair(Caretakers)

• HNPCC ( Hereditary Nonpolyposis Colon Cancer)
• 2 - 4 of all colonic cancers
• Defect in mismatch repair ( Spell Checkers )
• Cells have (replication error ) phenotype and
  microsatellite instability
• 50 of cases mutation in hMSH2 on 2p16
• 30 of cases mutation in hMLH1 on 3p21

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Genes that Regulate DNA Repair

• Xeroderma Pigmentosum
• UV causes DNA damage by cross linking of
  pyrimidine residues
• This damage is repaired normally by nucleotide
  excision repair system

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Genes that Regulate DNA Repair

• Autosomal recessive disorders charecterized by
  hypersensitivity to ionizing radiation
• Ataxia Teleangioectasia
• AT protein acts as a sensor of DNA damage and
  activates TP53 gene
• Fanconi Anemia
• Bloom syndrome
• BRCA-1, BRCA2

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Major Classes of Ca-Associated Genes