Oncogenes and Tumour Suppressor Genes

Question 1

What is cancer critical?

Where are cancers thought to originate?

What does evolution of a tumour depend upon?

What is epigenetic mechanisms?

Answer 1

Cancer critical - refers to all genes whose alteration controbutes to, causing or evolution of, cancer by driving tumourogenesis

Cancers are thought to originate froma single cell that has mutated and then acquired additional abnormalities

Evolution depends upon its microenvironment, provided by the stromal cells

Epigenetic mechanisms: modifications of DNA that switch specific gene expression on/off

Question 2

What are the two types of disrupted genome?

Answer 2

1. Karyotype (chromosomes) at mitosis appear normal - indicating there must be point mutations in individual genes –> failure of repair mechanisms
2. Karyotype severely abnormal with chromosome breaks and re-arrangements (easily visible under microscope)

Question 3

What are the two main classes of cancer critical genes?

Answer 3

Proto-oncogenes: where a gain of function mutation can drive a cell towards cancer - mutant, overactive or over-expressed forms of these genes are called oncogenes

Tumour suppressor genes: where loss of function mutations can contribute towards cancer e.g. TP53

Question 4

Define loss of function:

Gain of function:

Answer 4

Loss of function: generally refers to the tumour suppressor genes, where loss of function leads towards cancer development

Gain of function: generally refers to a situation where gain of function leads towards cancer development e.g. when proto-oncogenes are mutated, overactive or over-expressed (oncogenes)

Question 5

List some disruptions in key pathways which are common to many cancers:

Answer 5

• p53 pathway: genes within the pathway that regulate responses to stress and DNA damage
• retinoblastoma pathway: Rb itself and genes that regulate Rb, involved in initiation of the cell division cycle
• RTK/Ras/P13K pathway: this pathway transmits signals for cell growth and division from outside into the cell

Question 6

What is p53?

Answer 6

p53: A transcription factor
- encoded by the gene TP53
- TP53 mapped within a region of DNA consistently deleted in human tumours
- TP53 one of the most frequently mutated genes in human cancer
- p53 is the protein product of the gene, which is a TF –> controlling gene expression
- induced in response to stress including DNA damage, activated oncogenes, telomere shortening, spindle damage, hypoxia and metabolic stress
- p53 activation can result in cell cycle arrest (G1/G2), senescene or apoptosis

Question 7

How does DNA damage arrest the cell cycle in G1?

Answer 7

• when DNA is damaged, activated protein kinases phosphorylate p53
• mdm2 normally binds to p53 and promotes its ubiquitylation and destruction in proteasomes
• phosphorylation of p53 blocks its binding to mdm2: p53 accumulates to high levels and stimulates transcription of the gene encoding the CKI protein p21
• p21 binds and inactivates G1/S-Cdk and S-Cdk complexes, arresting the cell in G1
• sometimes DNA damage also induces either phosphorylation of mdm2 or a decrease in mdm2 production, which causes an increase in p53

Question 8

How can p53 be lost by targeted degradation?

Answer 8

• HPV encodes a protein that activates enzyme E3
• the enxyme ubiquitinates the tumour suppressor p53 and other proteins that control DNA repair, which are then destroyed
• the activation of this E3 enzyme is observed in more than 90% of cervical carcinomas
• inappropriate marking of key regulatory proteins for destruction can trigger further events leading to tumour formation

Question 9

What is ubiquitin?

How can p53 be lost by gene mutation?

Answer 9

Ubiquitin: a protein TAG that is attached to a protein that targets it for degradation

Gene mutation:

• a monoallelic mutation may confer a dominant-negative function on tumour suppressor gene products
• the mutant protein with changes to its DNA-binding domain, can form hetero-oligomers with wild type p53 but these exhibit impaired DNA association and transcriptional activity resulting in loss of p53 function

Question 10

What is a retinoblastoma?

Answer 10

Retinoblastoma: rare childhood tumour in the neural precursor cells of the retina

• can be hereditary but not always
• deletions at a specific region of chromosome 13 were found in patients with Rb
• this allowed discovery of the Rb gene
• Rb gene encodes the Rb protein, a key universal suppressor of cell cycle progression

Question 11

What are oncogenes and how do they work?

Answer 11

Oncogenes: tumour promotors

• encode proteins that have the ability to cause cellular transformation (morphological changes, loss of contact inhibition, anchorage independent growth)
• oncogenes first identified as viral genes that infect normal cells and transform them into tumour cells
• act in a dominant fashion through over-expression or activating mutations

Question 12

List some types of over-activity:

Answer 12

• deletion or point mutation in coding sequence –> normal amounts of hyperactive protein
• mutation in the regulatory region of the gene –> normal protein overproduced
• gene amplification –> normal protein greatly overproduced
• chromosome re-arrangement, changes to regulatory region –> normal protein overproduced or alters the protein coding region causing transcribed gene –> hyperactive protein

Question 13

Define:

• exome analysis:
• epigenetic changes:
• transcriptome:

Answer 13

• exome analysis: looking for mutations
• epigenetic changes: hereditary changes
• transcriptome: changes to the profile of mRNAs produced by a cell