10. Oncogenes and Tumour Suppression Genes

Question 1

Define proto-oncogene and oncogene

Answer 1

• Proto-­‐oncogenes code for essential proteins involved in maintenance of cell growth, division and differentiation
• Mutation can convert a proto-­‐oncogene into an oncogene, whose protein product no longer responds to control influences
  
  • Oncogenes can be aberrantly expressed, over-­‐expressed or aberrantly active E.g. Myc, Ras, ERB, Sis
  • A proto-­‐oncogene can be converted to an oncogene by a single mutation

Question 2

Explain the relationship between proto-oncogenes and oncogenes.

Answer 2

Oncogene Activation

The normal proto-oncogene can be activated in 4 ways:

1. Mutation in the coding sequence.
   
   • Point mutation or deletion.
2. Gene amplification.​
   
   • Having multiple copies of a gene will lead to overproduction of the gene product
   • A protein may block the DNA polymerase so the polymerase repeatedly backs up to go over the area a few times creating many identical genes.
3. Chromosomal translocation.
   
   • Chimeric genes.
   • Chimeric genes are genes that are formed by combinations of portions of one or more coding sequences to produce new genes (e.g. the swapping of tips of chromosomes)
   • This can be a problem if one of the pieces of translocated DNA is a promoter, leading to upregulation of the other gene portion (this occurs in Burkitt’s Lymphoma)
   • This can also be a problem if the fusion gene formed produces an abnormal protein (e.g. Philadelphia chromosomes in CML)
4. Insertional mutagenesis.
   
   • Viral infections – some viruses insert their genome into our DNA and usually this isn’t a problem as much of our DNA does not code but if it’s in a coding region, this could be cancer.

Question 3

Give examples of oncogene activation

Answer 3

The Philadelphia Chromosome – CML:

This is an example of INSERTIONAL MUTAGENESIS.

• This is the Philadelphia Chromosome that is formed by the translocation of chromosome segments from chromosomes 9 + 22
• The two key areas that are translocated are:

1. ABL -­‐ chromosome 9
2. BCR -­‐ chromosome 22

• The resulting BCR-­‐ABL fusion gene leads to the development of cancer

Question 4

Recall how oncogenes disrupt cellular pathways and explain, with examples, how mutation of a proto-oncogene can disrupt normal cell division.

Answer 4

Signal Transduction Pathways

• Proteins that are involved in signal transduction pathways are potentially critical gene targets (proto-­‐oncogenes)
• Activation of proto-­‐oncogenes to oncogenes can disrupt normal activity

It can lead to downstream activation of signalling pathways such that they no longer respond to the upstream stimuli

Mutant Ras has aberrant activity

• Normally, upon binding GTP, Ras becomes active and activates the kinase cascade leading to the production of gene regulatory proteins
• Dephosphorylation of the GTP to GDP to switch Ras off
• Mutant Ras will fail to dephosphorylate GTP meaning that the GTP persists so Ras remains active
• The Ras pathway is part of a much more complex signalling cascade called the mitogen-­‐activated protein kinase cascade (MAPK)
• Summary of Ras Signalling:

Question 5

Define tumour suppressor gene.

Answer 5

Tumour Suppressor Genes

• TSGs encode proteins whose function is to regulate cellular proliferation and maintain cell integrity – e.g. pRb.
• Typically proteins whose function is to regulate cellular proliferation and maintain cell integrity (e.g. retinoblastoma)
• Each cell has two copies of each tumour suppressor gene
• Mutation or deletion of just one gene copy is usually insufficient to promote cancer
• Mutation or loss of both copies means loss of control

Question 6

Explain how tumour suppressor genes have been discovered through heritable malignancies.

Answer 6

Inherited Cancer Susceptibility

This has led to the discovery of tumour suppressor genes. Features include:

• Family history of related cancers
• Unusually early onset
• Bilateral tumours in paired organs
• Synchronous or successive tumours
• Tumours in different organ systems in same individual
• Mutation inherited through the germline

Example of Inherited Cancer: Retinoblastoma

• It is a malignant cancer of developing retinal cells
• Sporadic disease usually involves one eye
• Hereditary causes can be unilateral or bilateral and multifocal
• It is caused by mutation of the RB1 tumour suppressor gene on the chromosome 13q14
• RB1 encodes a nuclear protein that is involved in regulation of the cell cycle

Question 7

Outline TSG functional classes

Answer 7

Functional Classes of Tumour Suppressor Genes

• Regulate cell proliferation
• Maintain cellular integrity
• Regulate cell growth
• Regulate the cell cycle
• Nuclear transcription factors
• DNA repair proteins
• Cell adhesion molecules
• Cell death regulators

These genes suppress the neoplastic phenotype of a cell VS oncogenes that enable the neoplastic phenotype.

TSG mutation examples.

• P53 – ubiquitous, it has so many roles
• P16 – relatively new
• MLH – quite a driver certainly in colon cancer

Question 8

Summarise the roles of p53 in cellular decision making.

Answer 8

p53 -­‐ The Guardian of the Genome

p53 will be switched on by a range of damage induced kinases (Mapk and Atr) Once activated it causes the transcription of p21 - stops cell cycle progression induces a bunch of DNA oriteubs tgat will attemot ti cirrect any errors - p53 will stay phosphoryalted

• P53 may be a TSG but mutation of a SINGLE copy is enough to cause dysregulation of activity and cancer.
  
  • Mutants of p53 act in a dominant matter.
• When p53 is bound to MDM2 it is inactive
• When the cell comes under stress, p53 dissociates from MDM2 and becomes active by forming a p53 tetramer which then has cellular effects

P53 is important in a wide range of activities, including regulation of p53 target genes and protein-­‐protein interactions

P53 can be activated by many different types of cellular stresses

Although p53 is a tumour suppressor gene, mutants of p53 act in a DOMINANT manner and mutation of a single copy is sufficient to get dysregulation of activity

Phosphorylation of p53 destabilises it so that it isn’t degraded so quickly and can exert its effects

Phosphorylation is triggered by cellular stress

Question 9

Gene mutations: recall the way in which successive gene mutations are thought to lead to clinical cancer.

Answer 9

APC TSG deletion -> Familial adenomatous polyposis coli.

• Due to deletion in 5q21 resulting in loss of APC gene (TSG) -> loss of APC Tumour Suppressor Gene
• It is involved in cell adhesion and cell signalling
• People with the APC mutation develop multiple benign adenomatous polyps in the colon
• There is a 90% risk of developing colon cancer

APC participates in the WNT signalling pathway.

• APC helps control activity of b-catenin and thereby prevents uncontrolled cell growth.
• Mutation of APC is a frequent event in any colon cancer.

Cancer can be triggered by:

• Oncogene + TSG.
• Proto-oncogene + defective TSG.
• Oncogene + defective TSG.

Cancer is achieved via multiple mutations such as in colo-rectal cancer.

• Apc – hyperproliferation.
• K-ras – adenoma.
• p53 – carcinoma.
• Metastasis.