5th lecture - oncogenes and TSGs

What is a proto-oncogene?

A normal gene that promotes cell growth and survival.

Mutations in proto-oncogenes can convert them into oncogenes, which then promote cancer cell growth and survival (GOF mutation)

What is an oncogene?

An activated proto-oncogene that promotes uncontrolled cell growth and survival.

What is a tumour suppressor gene (TSG)?

A gene that limits cell division, promotes cell death, or helps repair DNA.

What type of mutation converts (“activates“) a proto-oncogene into an oncogene?

Gain-of-function: increased gene or protein activity.

point mutations → amplification → translocations

What type of mutation inactivates a TSG?

Loss-of-function: reduced or lost gene function.

point mutations → deletions → frameshift mutations

What is the key difference between oncogenes and TSGs?

Oncogenes act like a gas pedal; TSGs act like brakes.

What is Ras? What it activates/inactivates?

Ras is oncogene (small GTPase)

•The RAS family consists of KRAS, NRAS and HRAS.

WWhat Ras activates/inactivates?

•RAS normally acts as a molecular switch:

RAS-GDP = inactive

RAS-GTP = active

•RAS switches off by hydrolysing GTP to GDP.

Which pathway does RAS transmit signal?

Active RAS then transmits the signal to downstream pathways, such as:

RAF–MEK–ERK

PI3K–AKT–mTOR

These pathways regulate cell proliferation, survival, growth and differentiation.

What does it mean for RAS to be overactive in cancer?

•In cancer, mutated RAS is unable to efficiently hydrolyse GTP → GDP and switch off so, it remains predominantly GTP-bound and active.

•This causes persistent activation of downstream pathways such as RAF–MEK–ERK and PI3K–AKT–mTOR.

The result is increased cell proliferation, differentiation and survival

For which cancer KRAS and NRAS mutations are tested?

•KRAS and NRAS mutation testing is part of the molecular assessment of metastatic colorectal cancer (mCRC).

How is ABL gene classified?

ONCOGENE

WHat is Philadelphia chromosome?

•Reciprocal translocation between chromosomes 9 and 22: t(9;22).reciprocal translocation between chromosomes 9 and 22: t(9;22).

•The translocation creates the BCR::ABL1 fusion gene, which encodes an abnormal fusion protein

→ causes the ABL1 kinase to remain constitutively active → more proliferation

In which cancer is Philadelphia chromosome present?

The BCR-ABL gene shows up in patients with certain types of leukaemia,

•Imatinib is a tyrosine kinase inhibitor.

•Binds to the ATP-binding pocket of BCR::ABL1.

How can a chromosomal translocation activate an oncogene?

It can create a fusion gene or place a gene under different regulatory elements.

What is Knudson's two-hit hypothesis?

Both alleles of a TSG usually need to be inactivated for complete loss of function.

What are growth factors (GF)?

Growth factors:

Usually, steroids or secreted proteins by cells to stimulate their own cell proliferation or the proliferation of other nearby or distant cells,

What is Growth factor receptors (GFRs)

Growth factor receptors:

•Most oncogenic GFRs are receptor tyrosine kinases (RTKs).

Receptor tyrosine kinases (RTKs): bind to many  growth factors, cytokines, and hormones, and play key role in signalling pathways

What is MDM2?

oncogene; inhibits p53 (degredades it)

•In some cancers, MDM2 is amplified or overexpressed, causing excessive degradation of p53.

What is TP53?

•TP53 is one of the most frequently mutated genes in human cancers.

TP53 = tumor suppressor gene

What is RB1 gene?

TUMOR SUPPRESSOR GENE

•Retinoblastoma is a childhood retinal tumour that most commonly affects children around one to two years of age.

•In sporadic retinoblastoma, both RB1 alleles must be inactivated by somatic events in the same retinal cell.

What is loss of heterozygosity (LOH)?

Loss of heterozygosity, often involving loss of the remaining functional TSG allele.

•RB binds to E2F, a transcription factor that normally activates genes required for entry into S phase.

•As a result, E2F-dependent cell-cycle genes are switched off.

•This prevents the cell from progressing from G1 into S phase.

What is RB in normal cell

•Normal cell: RB binds E2F → E2F target genes are repressed → G1/S transition is controlled.

What do RB do in cancer cells?

•Cancer cell: RB is lost, mutated, deleted, or functionally inactivated.

•Loss of RB function or excessive CDK4/6 activity can result in persistent E2F activity and uncontrolled proliferation.

•E2F is released and becomes active.

What is BRCA1/2

TUMOR SUPPRESSOR GENE

Why does mutated RAS promote cancer?

It fails to efficiently hydrolyse GTP to GDP and remains active, sustaining growth signals.

Which major pathways are activated by RAS?

RAF-MEK-ERK and PI3K-AKT-mTOR.

Why may anti-EGFR therapy fail in RAS-mutant tumours?

RAS remains active downstream of EGFR, so blocking EGFR does not switch off the pathway.

What is the main function of RB1 protein?

RB binds E2F and inhibits activation of genes needed for S phase.

How do CDK4/6 inhibitors affect RB?

They keep RB active, inhibit E2F, and arrest the cell cycle.

How does inherited retinoblastoma differ from sporadic retinoblastoma?

Inherited cases start with one altered RB1 allele in all cells; sporadic cases acquire both hits in tumour cells.

Which of the following best defines an oncogene?

A) An oncogene codes for a cell cycle control protein.

•

B) An oncogene codes for a mutated form of a protein that forms part of a signal transduction pathway.

•

C) An oncogene codes for a protein that prevents the cell from undergoing apoptosis.

•

D) An oncogene is a dominantly expressed mutated gene that gives a cell a growth or survival advantage.

D) An oncogene is a dominantly expressed mutated gene that gives a cell a growth or survival advantage.

2. Which alteration is most characteristic of tumour suppressor gene inactivation?

A)Gain-of-function mutation causing constitutive signalling

B) Gene amplification leading to protein overexpression

C) Loss-of-function mutation or deletion affecting both functional alleles

D) Chromosomal translocation creating an active fusion kinase

C) Loss-of-function mutation or deletion affecting both functional alleles

3. Answer true (T) or false (F) for each of the following statements:

A) Ras codes for an anti-apoptotic protein, which is produced in abnormally large amounts.

B) Ras codes for a GTPase switch protein, which in its mutated form cannot be switched off.

C) Ras codes for a transcription factor, which is produced in abnormally large amounts

D) Ras codes for a truncated form of a growth factor receptor, which is continually active.

A) Ras codes for an anti-apoptotic protein, which is produced in abnormally large amounts. - F

B) Ras codes for a GTPase switch protein, which in its mutated form cannot be switched off. - T

C) Ras codes for a transcription factor, which is produced in abnormally large amounts - F

D) Ras codes for a truncated form of a growth factor receptor, which is continually active. - F

4. Which of the following types of mutations give rise to an oncogene? More than 1 answer can be correct.

B) A point mutation changing just one amino acid in the protein product.

C) A translocation which puts the gene under the control of a strong promoter, producing over-expression.

5. Which of the following correctly matches an oncogene with its typical activating alteration?

A) KRAS — gene deletion

B) HER2 — nonsense mutation

C) BCR::ABL1 — gene amplification

D) MYC — loss of heterozygosity

E) None of the above

E) None of the above

KRAS → activating missense point mutation

HER2 → gene amplification

BCR::ABL1 → chromosomal translocation

MYC → translocation or amplification

SAQ 1. How does p53 regulate cell cycle ? (5 marks)

•P53 is a tumour suppressor gene that controls cell growth, cell proliferation, cell cycle arrest and DNA repair.

•In normal cell p53 binds to mdm2 and undergoes proteosome mediated ubiquitination.

•In stressed cell with damaged DNA by UV light, oxidative stress or radiation the p53 is getting phosphorylated and that prevents its binding with mdm2 and that prevents its degradation, and it becomes active.

•P53 acts as a checkpoint protein during G1-S phase. 

•It also acts depending on p21.

SAQ 2. What does the BCR-ABL fusion gene do?  Explain how Imatinib drug works. (10 marks) 

BCR::ABL1 fusion gene – 5 marks

•Formed by the t(9;22) translocation, creating the Philadelphia chromosome.

•Fuses the BCR gene with ABL1.

•Encodes a constitutively active tyrosine kinase.

•Causes continuous phosphorylation and activation of downstream signalling pathways.

•Promotes abnormal cell proliferation and survival, contributing particularly to CML.

Imatinib – 5 marks

•Imatinib is a tyrosine kinase inhibitor.

•It binds to the ATP-binding site/pocket of BCR::ABL1.

•This prevents ATP-dependent kinase activity.

•BCR::ABL1 can no longer efficiently phosphorylate its target proteins.

•Downstream proliferative and survival signalling is reduced, inhibiting growth of BCR::ABL1-positive leukaemic cells.