ONCOGENES

Presented by, Tina K.J. 2nd Sem MSc Biotechnology CUSAT

CANCER

Characterized by uncontrolled cell proliferation

Arises from irreversible genetic damage to cell’s DNA, block in normal process of differentiation, or block in apoptosis

Term ‘cancer’ : Hippocrates (400 BC)

Observation: Veins radiating from breast cancer resembled legs of crab, hence karkinoma in Greek & cancer in Latin

Oncogene : an altered form (allele) of a normal cellular gene (proto-oncogene).

Operationally defined as a regulatory gene with dominant transforming properties.

Tumor suppressor genes: recessive genes that restrain cell proliferation.

First discovered through their association with specific retroviruses [ v-oncogene ] from the Avian Sarcoma Virus and was called src.

What Kinds of Genes are Mutated in Cancer?

Oncogenes : positively regulate the cell cycle

(move it forward) - dominant mutations in proto-

oncogenes cause excess cell proliferation

Tumor Suppressors : negatively regulate the cell

cycle - recessive mutations in these genes

also cause excess cell proliferation, but they

generally need homozygosity for phenotypic

expression

Oncogene causes cancer by affecting:

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

2. Cell differentiation (example, PML/RAR that inhibits the differentiation of promyelocyte to granulocyte which will maintain the cell in its active proliferate state)

3. Cell Survival (example; Pl-3/AKT which will activate BCL-2 inhibit Apoptosis & maintain cell survival.

CELL CYCLE

Regulators of Cell Cycle

Oncogenes/Proto-oncogenes

Cyclin D1 and Cyclin E are proto-oncogenes

Often amplified or over expressed due to other mutations (e.g. translocation) in many cancers

Cyclin D1 allows for DNA replication (S phase)

Over expression seems to contribute to cell’s progression from G0 phase and begin division

APOPTOSIS

Involves proteases called caspases

Regulated by Bcl2 and BAX

BAX homodimer promotes apoptosis, Bcl2 homodimer blocks apoptosis

Some cancer cells overproduce Bcl2 & are resistant to some chemotherapies & radiation treatment

Proteins involved in cell cycle checkpoints regulate pathway

Control of Apoptosis

Hypotheses of the Origin of Neoplasia

1. Oncogenes and Tumor Suppresor Genes

2. Viral Oncogene Hypothesis

3. Epigenetic Hypothesis

4. Failure of Immune Surveillance

Origin of Neoplasia – two general types

Monoclonal

Initial neoplastic change affects a single cell

Field origin

Carcinogen acts on large number of cells producing field of potentially neoplastic cells

Two-hit Hypothesis

1 ) Oncogenes and Tumor Suppresor Genes

Proto-oncogenes

2) Viral Oncogene Hypothesis

RNA Retrovirus – produces DNA provirus

DNA provirus containing viral oncogene (v-onc) is introduced, or

DNA provirus without v-onc is inserted adjacent to c-onc in host cell DNA

RNA viruses is thought to have acquired v-onc sequence by recombinant mechanism from animal cells

DNA virus

Do not contain viral oncogenes

Act by blocking suppressor gene products

Examples – HPV, EBV,HBV

3) Epigenetic Hypothesis

Changes in the regulation of gene expression rather than in the genetic apparatus

Pattern of gene expressions responsible for tissue differentiation (ie. epigenetic mechanism) are thought to be heritable

4) Failure of Immune Surveillance

Concepts:

Neoplastic changes frequently occur in cells

Altered DNA result in production of neoantigens & tumor-associated antigens

Immune response (cytotoxic) to neoantigens as foreign antigens

Neoplastic cells escaping recognition and destruction become clinical cancers

Types Of Oncogenes

Two main types :

Viral oncogene: gene from the retrovirus itself

Non-Viral oncogene (Cellular oncogene): genes derived from the genes of the host cell that are in an inactive form usually. Occasionally if the gene incorporates with the viral genome will form a highly oncogenic virus.

Proto-oncogenes: are the form of cellular genes that inactive normally but can incorporate with the viral genome to produce a highly oncogenic virus

Funtions Of Proto-oncogenes

SIGNAL TRANSDUCTION PATHWAY AS A SOURCE OF PROTO-ONCOGENES

Growth factor binds receptorReceptor exchanges GTP for GDP on Ras

Ras activatedRasRafMekMap Kinasetranscription factors genes turned on

Ras Pathway

Myc is also a target of the RAS Pathway

V-onc’s Are Mutated Proto-oncogenes

V-erbB Expresses a Truncated EGF-Receptor Which Is ALWAYS on, regardless of [EGF]

Receptor Tyrosine Kinases under normal & abnormal conditions

Tumour Suppressor Genes

Tumour Suppressor genes: are genes that act to inhibit cell proliferation and tumour development.

If Tumor Suppresor Gene was

Mutated Inactivated

It will lead to cell transformation

OR

Mutation of the tumour suppressor gene will cause cancer.

Example; deletion of Rb gene will cause retinoblastoma. The development of retinoblastoma can be either:

Hereditary: a defective copy of Rb gene is inherited from the affected parents.

Nonhereditary: in which 2 normal Rb genes are inherited and develop mutation during life.

Retinoblastoma is developed if 2 somatic mutations inactivate both copies of Rb in the same cell.

Inactivation of Tumour suppressor gene will cause

cancer!!!

If the Rb gene interact with DNA tumour virus (SV40) it will induce cell transformation.

SV40

Functions of Tumour Suppressor gene1. Antagonize the action of

oncogene. (ex.PTEN which converts PIPIII to PIPII because PIPIII will activate Pl-3/AKT which will activate BCL-2 that will inhibit apoptosis and induce cell transformation)

PIPII PIPIIIPTEN

AKT

BCL-2

Inhibit apoptosis & induce cell transformation

PI-3

2. Transcription factors

Repressor transcription factors: example; WT1 is a repressor that appears to suppress transcription factor ( Insulin like growth factor) which will contribute in the development of tumour.

Activator transcription factors: example; SMAD family that are activated by TGF-β, leading to inhibition of cell proliferation.

3. Regulate cell cycle :

Rb gene: that inhibits the cell cycle in the G1 phase decrease cell proliferation.

INK-4 gene: that produces P16 that inhibits cdk4/cyclin D action ( to phosphorylate Rb gene to inactivate it’s action)

P53: that produces P21 that has the same action of P16 in inhibiting the action of cdk4/cyclin D

Regulation of cell cycle

Rb Rb

PP16

Cell Cycle Blocked Cell Cycle Proceeds

Rb inactive

Cdk4/cyclin D

G1

M G2

SS

G1

M G2

4. Induce apoptosis:

P53 release will increase Bax form holes in the mitochondria release cytochrom c activate apoptosis

p53

Critical roles:

Prevents mutations and repairs DNA

Cell cycle arrest in G1

3 main functions in core domain:

Sequence specific transactivator in conjuntion with the N-

terminal transactivation domain

Recognize non-B forms of DNA

Remove nucleotides from ends of DNA by 3’-5’ exonuclease

activity

Also responsible for the activation of several proteins

involved in apoptosis.

QuickTime™ and aTIFF (Uncompressed) decompressor

are needed to see this picture.

Structure of p53

MDM2

An E3 ubiquitin ligase

Inactivates p53

Binds specifically to the N-terminus and inhibits transactivation function

Also activates p53 to proteasomal degradation

p16

Another tumor suppressor gene

Function in DNA damage prevention and repair

Like p53, mutation of this aids in tumor progression

Prone to methylation

Methylated state of the protein causes chromosome instability, and increases mutation rates

MECHANISMS OF CONVERSION OF PROTO-ONCOGENE INTO ONCOGENE

1) Point Mutation

2)Gene Amplification

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How Cellular Oncogenes Arise

3)Chromosomal Translocation

4)Local DNA Rearrangements

5)Insertional Mutagenesis

Occurs during viral DNA integration

Eg: Avian leukosis virus- integrate within c-myc oncogene

Exon 1-unknown function

Exon 2 & 3 encode MYC protein

ALV integrates between exon 1 & 2

Familial cancer syndromes involving DNA repair enzymes

Nucleotide excision repair (NER) genes: Xeroderma pigmentosum

. In NER-deficient cells non-repaired dimers

lead to missense mutations during DNA

replication

Activating oncogene mutations

Inactivating tumor suppressor gene mutations

Hereditary Non-Polyposis Colon Carcinoma

Autosomal dominant inheritance

Penetrance ~80%

Genes belong to DNA mismatch repair (MMR) family

Tumor site in proximal colon predominates

Extracolonic cancers: endometrium, ovary, stomach, urinary tract, small bowel, bile ducts, sebaceous skin tumors

HNPCC Results From Failure of Mismatch Repair (MMR) Genes

Base pair mismatch

Normal DNA repair

Defective DNA repair (MMR+)

T CT A C

A G C T G

T C G A C

A G C T G

T CT A C

A G C T G A G A T G

T C T A C

Mismatch Repair Failure Leads to Microsatellite Instability (MSI)

Normal

Microsatellite instability

Addition of nucleotide repeats

Epigenetic control of cancer genes

Epigenetics:

Mechanisms of gene expression control that can be passed from one cell to its offspring, that are not reflected in changes in DNA sequence

Examples:

DNA methylation

Histone modification

Noncoding RNAs

DNA methylation

Some growth suppressing proteins are found to be absent from cancers, but the promoter and coding region are intact

Examples:

p16

RASSF1/NORE1

Local regions of DNA, usually in gene promoters (CpG rich regions) maintain C-methylation during DNA replication

DNA methyltransferase

DNA Methylation

DNA methylation can be detected by DNA sequencing after 5Me-dC deamidation to dU (using bisulfite)

Reversal of DNA methylation (e.g. restoring expression of tumor suppressor genes) is being attempted using 5-aza Cytidine

Histone acetylation

Generally, histone acetylation is associated with transcriptionally active genes

Histone acetylation is effected by Histone acetyl transferase (HAT) and deacetylation by HDACs

HDAC inhibitors make you smarter too

microRNAsRapidly emerging field

Certain, but complex mechanisms of gene expression control

Some miRNAs (e.g. miR15) have associations with cancer

Steroid hormone receptors in cancers

Cancers arising in hormonally-responsive tissues often retain a hormone-responsive proliferation drive

“normal”

increased hormone sensitivity

Hormone receptors tend not to be mutated as oncogenes in early tumor progression

Anti-hormone therapy is however effective in treating hormonally-responsive tumors

Anti-estrogen therapy in breast cancer

Anti-androgen therapy in prostate cancer

Abnormal levels of hormones may predispose to cancer due to increased cell replication

Breast cancer, endometrial cancer

Hormone drive may be endogenous or exogenous

Hormone

Mutation acquisition,Tumor progression

Normal cell

Hormone-stimulatedcell proliferation

Hormonally-responsivecancer

Antihormonaltherapy

PREVENTING CANCER

DO’S

DON’Ts