Molecular Biology of Cancer - Molecular Biology of Cancer - Tumor suppressor genesTumor suppressor genes

Dr.CSBR.Prasad, M.D.,

Failure of growth inhibition is one of the fundamental alterations in the

process of carcinogenesis

• Oncogenes drive the proliferation of cells, the products of tumor suppressor genes apply brakes to cell proliferation

• The tumor suppressor proteins form a network of checkpoints that prevent uncontrolled growth

• Eg: RB and p53– They are part of a regulatory network that recognizes

genotoxic stress from any source, and responds by shutting down proliferation

Tumor suppressor gene

Oncogene

The tumor suppressor

proteins form a

network of checkpoints that prevent uncontrolled

growth

Oncogene expression in an otherwise normal cell leads to …….

Expression of an oncogene in an otherwise completely normal cell leads to quiescence, or to permanent cell cycle arrest (oncogene-induced senescence), rather than uncontrolled proliferation

In this class we will deal with…• Tumor suppressor genes, their products, and • Mechanisms by which loss of their function

contributes to unregulated cell growth• The protein products of tumor suppressor genes:– Transcription factors– Cell cycle inhibitors– Signal transduction molecules– Cell surface receptors, and – Regulators of cellular responses to DNA damage

Selected Tumor Suppressor Genes Involved in Human Neoplasms

Tumor suppressor genes

• Physiologically they regulate cell growth• It is a misnomer• Failure of growth inhibition is key event in

carcinogenesis• Loss of function of these genes - tumor• The proteins that apply break to cell

proliferation are the products of these genes

Tumor suppressor genes

• Genes act by coding for growth controlling molecules or growth factors

• Two pathways– Control over mitosis– Control over biochemical processes in the cell that

govern growth and differentiation i.e., gene expression

Protein products of tumor suppressor genes

• Cell cycle control• Regulation of apoptosis• Activities of cell survival and growth

Protein products of tumor suppressor genes-function

• Transcription factors• Cell cycle inhibitors• Signal transduction factors• Cell surface receptors• Regulators of cellular response to DNA

damage

Tumor suppressor genes

• RB gene / retinoblastoma gene• p53 gene• APC gene/adenomatous polyposis coli gene• B cell lymphoma gene/bcl 2 gene• WT 1 gene/Wilms tumor gene• NF1/NF2,Neurofibromatosis gene

Selected Tumor Suppressor Genes Involved in Human Neoplasms

RB gene- two hit hypothesis

• Located on 13q14• Both normal alleles must be inactivated (2- hits)– First hit: Familial cases-born with one defective copy of

gene– Second hit: The second intact copy undergoes somatic

mutation

• Sporadic cases-both normal RB alleles are lost by somatic mutation in one of the retinoblasts

RB Gene

• Familial RB show increased risk for osteosarcoma & soft tissue sarcomas

• RB locus is seen in adenocarcinomas of breast, small cell ca lung & bladder ca

• Alterations in “RB pathway” involving INK4a, CDK’s, RB proteins are present in cancer cells

LOH- Loss of heterozygosity

• Child carrying inheritent mutant RB allele in all somatic cells is perfectly normal, except for increased risk for RB

• Child is heterozygous for the RB gene, which does not affect cell behavior

• Cancer develops when cell becomes homozygous for mutant allele

LOH- Loss of heterozygosity

• The cell loses heterozygosity for normal RB gene (i.e., loss of heterozygosity)

Pathogenesis of Retinoblastoma

Pathogenesis of Retinoblastoma

Pathogenesis of Retinoblastoma

RB protein

• Nuclear phosphoprotein, regulates cell cycle• Active hypophosphorylated state in quiescent

cells• Inactive hyperphophorylated state in G1/S cell

cycle transition

RB gene

• Regulates advancement of cells from G1/S phase of cell cycle

• With RB mutation- transcription factor regulation is lost- persistent cell cycling

• TGF b is a growth inhibiting cytokine that upregulates CDK inhibitors, preventing hyperphosphorylation

p53

p53 gene

• Normal function- prevent propogation of genetically damaged cells

• When DNA is damaged-p53 upregulation-transcription of genes that arrest cell cycle and repair DNA

• Cell cycle arrest is mediated by p53 dependent transcription CDK inhibitor p21

• If DNA cannot be repaired, p53 induces apoptosis

Li-fraumeni syndrome

• High risk of developing carcinoma by inactivation of 2 nd normal allele of somatic cells

• Leukemia, sarcoma, breast cancer, brain tumor

• Homozygous loss of p53-DNA damage goes Unrepaired-many mutant genes-cancer

Li-fraumeni syndrome

APC / ß catenin

APC gene/ ß catenine pathway

• Develop thousands of adenomatous polyps• APC protein binds and regulates the

degradation of b catenine levels in cytoplasm• Absence of APC protein-b catenine levels

increase- translocate to nucleus-up regulate cell proliferation

• APC is a negative regulator of b catenine

Multiple adenomas

TGFß pathway

• Up regulate growth inhibitory genes

• Colon cancers, gastric ca in HNPCC

Mutated TGF ß• receptors prevent growth restraining effects

TGFß pathway

NF1 gene

• Regulates signal transduction by RAS pathway• Homozygous loss impairs conversion of active

RAS to inactive RAS• Germ line inheritance of one mutant allele

predipose to multiple NF• Loss of 2 nd NF gene - progression to

malignancy

RAS pathway

WT -1 gene

• WT-1 protein transcriptional activator of genes involved in renal and gonadal differentiation

• Tumorigenic function - role in differentiation of genitourinary tissues

• Wilms’ tumor of kidney

Evasion of Apoptosis

BCL-2, p53, MYC

• Genes inhibiting apoptosis –BCL2,BCL-XL • Genes promoting apoptosis –BAD, BAX, BID• t(14;18) in follicular lymphoma – over expression of

BCL2

Defects in DNA repair

• Mismatch repair (HNPCC syndrome)• Nucleotide excision repair (Xero. Pigmentosa)• Recombination repair (Ataxia telangiectasia,

Fanconi’s anemia & Bloom syndrome)

• Mutations not repaired in defects – Genomic instability syndrome – Cancer

Self sufficiency in growth signalsInsensitivity to growth inhibitory signalsEvasion of apoptosisDefects in DNA repairLimitless replicative potentialSustained angiogenesisAbility to invade and metastasis

Bloom’s syndrome, Xeroderma pigmentosum

Ataxia telangiectasia

HNPCCHereditary nonpolyposis colon ca syndrome

• Born with one defective copy of one of several DNA repair genes involved in mismatch repair (MSH2 & MLH1)

• Loss of normal spell checker function• Microsatellite repeats• Variation in microsatellite – instability• Hall mark of mismatch repair defects

Xeroderma pigmentosum

• UV light induces mutagenic - pyrimidine dimers– defect of nucleotide excision repair

• Develop skin cancer

BRCA-1 and BRCA-2

• Involved in repair of double stranded DNA breaks by homologous recombination

• Familial breast cancers, ovarian ca, melanoma, pancreatic ca

Defects in DNA repair

Self sufficiency in growth signalsInsensitivity to growth inhibitory signalsEvasion of apoptosisDefects in DNA repairLimitless replicative potentialSustained angiogenesisAbility to invade and metastasis

Limitless Replicative Potential

Limitless replicative potentialteleromerase

• Telomerase not active in somatic cells• Cellular telomerase progressively shorten with

each cell cycle - replicative senescence• Cancer cells reactivate telomerase• 90 % of human tumors show telomerase

activity

E N D