Tumor Suppressor Gene & Proto-Oncogene

8/3/2019 Tumor Suppressor Gene & Proto-Oncogene

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They won the Noble

Prize in 1989 for

proving that viruses

contain a cancer-causing gene derived

from the genome ofthe organism they

infect.

Specifically, they

showed that chicken

Rous Sarcoma Virus(RSV) carried an

oncogene called v-

src and this gene wasan intronless version of

a normal chickengene called c-src.


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Cellular functions related to growth andproliferation.

The proteins encoded by them function as Growth factor/ receptor

Signal transducers

Transcription factors

Cell cycle component

Mutation converts them into constitutivelyactive oncogene/oncoprotein.


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Ras geneMyc gene

Myc-IgH

FusionBcr-AblFusion


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Oncogene causes cancer by affecting:

1. Cell Proliferation: (example;Ras, Raf, EGF)

2. Cell differentiation (example,PML/RAR that inhibitsthe differentiation of promyelocyte to granulocyte which

will maintain the cell in its active proliferate state)

3. Cell Survival (example;Pl-3/AKTwhich will activate

BCL-2 inhibit Apoptosis & Maintain cell survival.)


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1. Growth Factor (example, Epithelium growth factor EGF ,

and platelet derived growth factor PDGF)

2. Growth Factor Receptor (Example; PDGFR)3. Signal transudation (example; Ras, Raf, & MEK)

4. Transcription Factor (example; Jun, Fos, Elk-1 & myc)


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Growth factore.g. SIS oncogene (PDGF)- low-grade

astrocytomas and osteosarcomas


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Growth factor receptor RET is a receptor forGCDNF mutation causesMEN 2A

and 2B FLT3 AML

PDGFR in CMML ERB B1- 80% ofsquamous cellcarcinomas of thelung,in

50% ormore of high-gradeastrocytomas calledglioblastomas,in80% to100% of head and neck tumors,and less commonly, in carcinomas of the urinary bladderand the gastrointestinal tract.

the ERB B2 gene ( HER 2/Neu) is amplified in approximately25% of breast cancers and in human adenocarcinomasarising within the ovary, lung, stomach, and salivaryglands.

a mutation in c-KIT, (also known as steel factor)in GIST


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G proteins (signal transduction) - RAS

Point mutation of RAS familygenesis thesinglemost commonabnormality ofdominantoncogenes in human tumors

reduce the GTPase activity of the RAS proteins. The mutations generally involve codons 12, 59, or

61 of HRAS,KRAS,and NRAS.

Colonand pancreatic tumors - KRAS, bladdertumors have HRASmutations,and hematopoietic

tumors - NRASmutations. RAS is also involved in regulation of the cell cycle

RAS proteins can indirectly regulate the levels ofcyclins by activating the MAP kinase pathwayand the AP-1 transcription factor.


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RAF and MAP Kinase are also involved in

oncogenesis.

BRAF, a member of RAF - 60% ofmelanomas and in more than 80% of

benign nevi


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CHROMOSOMALTRANSLOCATION/

REARRANG

EM

ENT

C-ABL TK

BCR-ABLIN CML

POINT MUTATION OFNEGATIVE REGULATORY

DOMAIN

JAK 2

STAT INPCV/ET/PMF


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Nuclear transcription factors implicated asoncogenes e.g. MYC,MYB,JUN,FOS and

REL. MYC

transcription factor that can act in concert toreprogram somatic cell into pleuripotent cell.

C-MYC in burkitts, some cases of breast, colonand lung ca

N-MYC Neuroblastoma

L-MYC-Small cell ca of lung


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Cyclins

Cyclin D in mantle cell lymphoma t(11;14).

Cyclin E in BRCA 1 mutated breast ca

CDK

AmplificationCDK4 gene melanoma,sarcoma and GBM


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Tumor suppressor genes apply brakes

to cell proliferation.

These gene helps in regulating the cellgrowth and differentiation at various

stages of cell cycle.


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Two main cell cycle checkpoints - G1/S transition andG2/M transition.

To function properly, cell cycle checkpoints requiresensors of DNA damage, signal transducers, and effector

molecules. The sensors and transducers of DNA damage seem to be similar

for the G1/S and G2/M checkpoints. They include, as sensors,proteins of the RAD family and ataxia telangiectasia mutated(ATM) and as transducers, the CHK kinase families.

In the G1/S checkpoint, cell cycle arrest is mostly mediatedthrough p53, which induces the cell cycle inhibitor p21.

Arrest of the cell cycle by the G2/M checkpoint involves bothp53-dependent and p53-independent mechanisms.Defects incell cycle checkpoint components are a major cause of geneticinstability in cancer cells.


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CIP/KIP family: p21, p27 (CDKN2A-C) Block the cell cycle by binding to cyclin-CDK

complexes;

p21 is induced by the tumor suppressor p53; p27 responds to growth suppressors such as TGF-

.

INK4/ARF family (CDKN1A-D) p16/INK4a binds to cyclin DCDK4 and promotes

the inhibitory effects of RB; p14/ARF increases p53 levels by inhibiting

MDM2 activity

Mutations causes pancreatic ca, familialmelanoma and sq cc of esophagus.


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The genes normal function is to regulatecell division. Both alleles need to be

mutated or removed in order to lose thegene activity.

The first mutation may be inherited orsomatic.

The second mutation will often be a grossevent leading to loss of heterozygosity inthe surrounding area.


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RB, the first, and prototypic, tumorsuppressor gene discovered.

Approximately 60% of retinoblastomas aresporadic, and the remaining are familial,with the predisposition to develop thetumor being transmitted as an autosomaldominant trait.

Patients with familial retinoblastoma arealso at greatly increased risk of developingosteosarcoma and other soft-tissuesarcomas.


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A child carrying an inherited mutant RB allele in all somaticcells is perfectly normal (except for the increased risk ofdeveloping cancer).

Heterozygosity for the RB gene does not affect cell

behavior and cancerdevelops when thecell becomeshomozygous forthemutant allele,a condition knownasLOH, forloss of heterozygosity).

Similarly,one or more genes on the short arm ofchromosome 11 play a role in the formation of Wilms' tumor,hepatoblastoma, and rhabdomyosarcoma.

The von Hippel-Lindau (VHL) gene is a tumor suppressorgene that causes familial clear cell renal carcinomas and isalso involved in sporadic forms of the same tumor.

Consistent andnonrandom LOH has providedimportantclues to thelocation ofseveral tumorsuppressorgenes.


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Thep53 gene is located on chromosome 17p13.1 .

is the most common target for genetic alteration in

human tumors. critical gatekeeper against the formation of

cancer/Molecular policeman.

center of a large network of signals that sensecellular stress, such as DNA damage, shortenedtelomeres, and hypoxia

inheritance of one mutant allele - Li-Fraumeni syndrome, have a 25-fold greater chanceof developing a malignant tumor by age 50 than thegeneral population.

80% of point mutations present in human cancers -DNA-binding domain of the protein


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p53 thwarts neoplastic transformation by

three interlocking mechanisms:

activation of temporary cell cycle arrest(quiescence),

induction of permanent cell cycle arrest

(senescence), triggering of programmed cell death

(apoptosis).


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Chk1 and Chk2 kinases are serine/threonine kinases that are activated by the ATM and ATRkinases in response to DNA damage.

Ashwell S , Zabludoff S Clin Cancer Res 2008;14:4032-4037


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p53 links cell damage with DNA repair, cellcycle arrest, and apoptosis.

Hence it is considered guardian of genome.

The new members of p53 family are p63 -differentiation of stratified squamous

epithelia p73 - strong pro-apoptotic effects after DNA

damage induced by chemotheraputic agents Mutations in Three musketeers is seen in the so-

called basal subset of breast cancers and poorprognosis.


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Adenomatous polyposis coli(APC)genes a class of tumor suppressors

whose main function is to down-regulate growth-promotingsignals.

Germ-line mutations at the APC(5q21) loci are associated withfamilial adenomatous polyposis.

a component of the WNTsignaling pathway,which has amajor role in controlling cellfate, adhesion, and cell polarityduring embryonic development

down-regulate -catenin.

Dysregulation of the APC/-catenin pathway causes Colonic carcinoma HCC Hepatoblastoma


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TGF- potent inhibitor of proliferation. Dimerization of the receptor

upon ligand binding leads toactivation of the kinase andphosphorylation of receptorSMADs (R-SMADs).

R-SMADs can enter the nucleus,bind to SMAD-4, and activatetranscription of genes, includingthe CDKIs p21 and p15/INK4b.

leads to repression of c-MYC,CDK2, CDK4, and cyclins A andE.

TGF- type II receptor are seen in

cancers of the colon, stomach,and endometrium

SMAD 4 mutations causespancreatic and coloniccarcinoma.


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PTEN (Phosphatase and tensinhomologue)

a membrane-associatedphosphatase

chromosome 10q23 that is

mutated in Cow

densyndrome, an autosomaldominant disorder marked byfrequent benign growths, suchas tumors of the skinappendages, and anincreased incidence ofepithelial cancers, particularly

of the breast , endometrium,and thyroid. a tumor suppressor by serving

as a brake on the pro-survival/pro-growth PI3K/AKTpathway.


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NF1 gene

develop numerous benignneurofibromas and opticnerve gliomas as a result ofinactivation of the secondcopy of the gene and is calledneurofibromatosis type 1

Neurofibromin, the proteinproduct of the NF1 gene,contains a GTPase-activatingdomain,which regulatessignal transduction throughRAS proteins.

Neurofibromin facilitatesconversion of RAS from anactive to an inactive state.

With loss of neurofibrominfunction, RAS is trapped in anactive, signal-emitting state


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NF2 gene predispose to the development of

neurofibromatosis type 2,benignbilateral schwannomas of the acousticnerve.

somatic mutations affecting both allelesof NF2 have also been found in sporadicmeningiomas and ependymomas.

The product of the NF2 gene, calledneurofibromin 2 or merlin, shows a great deal of homology with

the red cell membrane cytoskeletalprotein 4.1

is related to the ERM (ezrin, radixin, andmoesin) family of membranecytoskeleton-associated proteins.

is a key member of the Salvador-Warts-Hippo (SWH) tumor suppressor pathway,originally described in Drosophila.

controls organ size during developmentby modulating cell growth, proliferation,and apoptosis.

Many human homologues of genes inthe SWH pathway have been implicatedin human cancers


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Von Hippel-Lindau (VHL) gene - chromosome 3p hereditary renal cell cancers,

pheochromocytomas,hemangioblastomas of the centralnervous system, retinal angiomas, andrenal cysts.

Mutations of the VHL gene have alsobeen noted in sporadic renal cellcancers .

The VHL protein is part of a ubiquitinligase complex. In the presence of oxygen, HIF1 is

hydroxylated and binds to the VHLprotein, leading to ubiquitination andproteasomal degradation.

in hypoxic environments the reaction

cannot occur, and HIF1 escapesrecognition by VHL and subsequentdegradation.

HIF1 can then translocate to thenucleus and turn on many genes, suchas the growth/angiogenic factorsvascular endothelial growth factor(VEGF) and PDGF.


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PTCH1 and PTCH2

are tumor suppressor genes thatencode a cell membrane protein(PATCHED), which functions as areceptor for a family of proteinscalled Hedgehog.

The Hedgehog/PATCHED pathwayregulates several genes, includingTGF- and PDGFRA and PDGFRB.

Mutations in PTCH are related toGorlin syndrome, an inheritedcondition also known as nevoidbasal cell carcinoma syndrome .

PTCH mutations are present in 20%

to 50% of sporadic cases of basalcell carcinoma.

About one half of such mutationsare of the type caused by UVexposure.


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The WT1 gene located on chromosome 11p13, is associated with

the development of Wilms' tumor, a pediatric kidney

WT1 protein is a transcriptional activator of genesinvolved in renal and gonadal differentiation. Itregulates the mesenchymal-to-epithelial transitionthat occurs in kidney development.

a variety of adult cancers, including leukemias and

breast carcinomas, have also been show

n tooverexpress WT1.

Another Wilms' gene, WT2, located on 11p15, isassociated with the Beckwith-Wiedemannsyndrome


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p53 in cases of NSCLC


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FLOURESCENT IN SITUHYBRIDISATION for bcr-abl

transcript


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A

C

BA; Normal bronchial

ept, Fhit +

B; Fhit mod +

C, Fhit

POLYMERASE CHAIN REACTION


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TARGE

TED

THERAPY


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Metastases

Carcinoma

Late adenoma

Intermediate adenoma

Early adenoma

Hyperproliferative epithelium

Normal epithelium5q mutation or

loss APC gene

12p mutation

KRAS

18q loss DCC

17p loss p53

Other alterations


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Tumor Suppressor Gene & Proto-Oncogene