oncogens and tumours suppressor genes(lecture1)

8/3/2019 Oncogens and Tumours Suppressor Genes(Lecture1)

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Oncogenes & TumourSuppressor Genes

Dr Usama ALAlami


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Viruses, Cancer & Oncogenes

DNA viruses (SV40) and RNA viruses (retroviruses) are capable

of transforming cells they infect into cancerous ones.

Researchers discovered SRC gene in Rous sarcoma virus

Rous sarcoma virus causes myeloma in chickens

However, the SRC gene was not of viral origin (where did it comefrom?)

The gene is brought under the control of the viral promoter aberrant expression


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The original cellular gene is the PROTO-ONDOGENE (Potentialto cause tumour id abnormally activated)

Previous mechanism in humans = not evident

Mechanism in humans = InsertionalMutagenesis

DNA copy of virus inserts into cellular genome near proto-oncogene abnormal activation of proto-oncogene

tumoregenisis.


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Viruses Associated With HumanCancers

Virus Associated Tumour

DNA

Epstein-Barr Burkitts LymphomaNasopharyngeal

carcinoma

Papilloma virus Cervical cancer

RNA

HIV-1 Kaposis Sarcoma

HTLV-2 Hairy cell leukaemia


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Cellular Oncogenes

Mutated forms of a normal cellular proto-oncogene, theypromote cell growth thus tumourformation

Chromosomal abnormalities associated with tumours are nowknown to involve oncogenes

e.g. CML (t9,22) and Burkitts lymphoma (t8,14)

In both examples, the breakpoint for translocation coincided withthe location of an oncogene (ABL on chromosome 9 and MYC onchromosome 8)


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Double minute chromosomal abnormalities in breast cancerassociated with MYC oncogene amplification

60-100 cellular proto-oncogenes identified

Functions Of Cellular Proto-oncogenes

Proto-oncogenes involved in basically all functions of growth,proliferation and differentiation

Mutations in proto-oncogenes result in abnormal activation ofcell growth in the ABSENCE of external stimuli (malignanttransformation)


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Functions Of Proto-oncogenes

Gene expression

Signal transduction

Binding to cell surface receptor

External stimuli


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[1] Growth Factors

Growth factors exist as polypeptides or steroid hormonesReact with receptors stimulate or inhibit cell growth

Enhance cell growth by:

@ G0-G1 advance (EGF, PDGF, FGF)@ Progress through G1 (IGF)

In contrast, TGB- inhibits advance through G1 phase of the cellcycle


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Abbreviations

EGF: Epidermal Growth Factor

PDGF: Platelet Derived Growth Factor

FGF: Fibroblast Growth Factor

IGF: Insulin-Like Growth Factor


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Growth Factor Action Models

PARACRINE

Stimulate secretion by cell1

Paracrine secretions by cell 2

Stimulate cell 2

Growth factor

Cell 1

AUTOCRINE

Stmulate cell 1

Growth factor

Cell 1

Growth Factor Action Models


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[2] Growth Factor Receptors

Second function for proto-oncogenes is encoding growth factorreceptors (GFR)

Link information from extracellular environment to differentintracellular signalling pathways

e.g. Steroid receptors, haemopoiesis growth factor receptors andtransmembrane receptor tyrosine kinases

When growth factors bind to transmembrane receptor kinase activate receptor transmit signal to the nucleus


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[3] Signal Transduction

Several proteins capable of transacting with tyrosine kinaseExamples include ABL and SRC

RAS = best studied signal transducer

RAS = H, K and N RAS subtypes

RAS GTPase activity signal transduction


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[4] Transcription Factors

Act on DNA to control gene expressionThis is final site of action for messages sent sent from growthfactors

Also final level at which control of growth and proliferation

ultimately operates

e.g. FOS, JUN and MYC

MYC = C, N and L MYC

MYC has carboxyl terminal DNA-binding domain

MYC dimerizes with MAX

Binding to DNA occurs at the CACGTG sequence


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Mechanics Of Oncogene Activation

Insertional mutagenesis (already mentioned)

When MYC is translocated from chromosome 8 to 14 (site of Ig) inBurkitts lymphoma, constitutive expression of MYC results. Thisresults in continuous cell proliferation

No single mechanism for activation of oncogenes

e.g. RAS activated by point mutations and amplifications


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Mechanism Of Oncogene Activation

Method Of Activation Oncogene

Amplification MYC, ERBB2

Insertional mutagenesis INT1, INT2

Translocation MYC

Structural alteration ABL, KRAS


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Detection Of Mutation

@ RT-PCR

@ Direct sequencing

@ Southern blotting


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Tumour Suppressor Genes (TSG)

Genes involved in control of abnormal cellproliferation

Also known as ortho-genes, onco-suppressor genes and anti-oncogenes

The Retinoblastoma Model

Retinoblastoma = childhood cancer occurring in two forms +affects the retina

40% of cases are hereditary

Two hot hypothesis


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The Retinoblastoma Model: Continued .

Inherit one mutation in the germ line (FIRST HIT)

First hit = phenotypically harmless

(SECOND HIT)= occurs in retinal cell = tumour develops

Mutation analysis studies revealed deletion of the retinoblastomagene (RB) on chromosome 13 (second hit) + assumed mutation infirst copy on chromosome 13 (first hit)

RB gene encodes RB protein with DNA-binding activity

Viral proteins bind to RB and inactivate it allow uncontrolled celldivision to occur


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Phosphorylation Of Retinoblastoma

Active

Dephosphorylated

Inactive

Phosphorylated

RB


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Hereditary Cancers

{1} Wilms Tumour

Rare childhood renal tumour

Deletion of 11p13

Associated with genito-urinary abnormalities + mental retardation11p13 is the site for WT1 gene

WT1 binds to early growth response gene 1 (EGR1) andsuppresses it suppress growth TSG

IGF2 also implicated in Wilms tumour


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{2} Familial Adenomatous Polyposis

Polyps in colon + rectum mainly

Turns malignant

Translocated Adenomatous Polyposis Coli (APC)gene = TSG onchromosome 5q21


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P53: The Guardian Of The Genome

p53 located on chromosome 17p

Identified ten years ago because of involvement of chromosome17 in a wide variety of cancers

p53 functions as cell cycle checkpoint protein

It works alongside genes that have growth-suppressing activities

It functions in the G1 phase of the cell cycle

p53 also induces apoptosis (programmed cell death)

p53 prevents or repairs gene amplification

p53 Interacts with DNA replication factor (RPA) and inhibits

its binding to ssDNA Inhibit DNA replication and growth


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P53 as transcriptional Transactivator

p53 inhibits cell growth by promoting transcription of certain genes

The p53-activated genes encode proteins with growthsuppressing activities

1993, a gene called WAF1 was shown to be induced by p53

WAF1 encodes p21 protein

Overexpression of p21 protein growth suppression

WAF1 identical to CIP1


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Inactivation Of p53

Mouse double minute 2 (mdm2) cellular oncogene encodes p90

protein

p90 interacts with amino terminal of p53 inactivation

Interaction Between Oncogenes And TSGIn the Colorectal Cancer Model

Colorectal cancer has several hereditary formsWell-defined pattern of progression from adenomas to carcinomas


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Colorectal Cancer Model

Metastases

Carcinoma

Late adenoma

Intermediate adenoma

Early adenoma

Hyperproliferative epithelium

Normal epithelium5q mutation orloss APC gene

12p mutationKRAS

18q loss DCC

17p loss p53

Other alterations

oncogens and tumours suppressor genes(lecture1)

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