Oncogenes and tumour Suppressants

Question 1

State the 6 Hallmarks of Cancer

Answer 1

1. Disregard of signals to stop proliferating
2. Disregard of signals to differentiate
3. Capacity for sustained proliferation
4. Evasion of apoptosis
5. Ability to invade
6. Ability to promote angiogenesis

Soon to be added:

1. Avoiding Immune destruction
2. Deregulates cellular energetics
3. Genome instability and mutation
4. Tumour-promoting inflammation

Question 2

Which are the steps in the Cell Cycle that ensure genetic fidelity?

Answer 2

• G0 - the cell is in a quiescent phase - it is not replicating
• G1 - the cell makes sure that it has enough nutrients, nucleotides etc. to replicate
• At the end of G1, the cell has a checkpoint where you get growth arrest to ensure the genetic fidelity of the cell
• Specific proteins accumulate/are destroyed during the cycle e.g. cyclins, Cdks, Cdk inhibitors
• Permanent activation of a cyclin can drive a cell through a checkpoint

Question 3

Describe Proto-oncogenes, their expression and their mutation

Answer 3

• Proto-oncogenes code for essential proteins involved in maintenance of cell growth, division and differentiation
• It changes the activation of the protein and it can therefore not work properly
• 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 4

How can Oncogene Activation occur?

Answer 4

1. 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.
2. Mutation in the coding sequence
   
   • Point mutation or deletion
3. Gene amplification is the production of multiple gene copies

• It can occur due to problems with a polymerase protein
• 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.
  
  • Having multiple copies of a gene will lead to overproduction of the gene product

1. 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)
   • This is known as Chromosomal translocation

It can affect the binding of a protein and it can not be controlled anymore

Question 5

Give an Example of Chromosomal Translocation in Cancer

Answer 5

• This is the Philadelphia Chromosome that is formed by the translocation of chromosome segments from chromosomes 9 + 22
• This is an example of INSERTIONAL MUTAGENESIS
• The two key areas that are translocated are:
  
  • ABL - chromosome 9
  • BCR - chromosome 22
• The resulting BCR-ABL fusion gene leads to the development of cancer
• Prognostic indicator for leukaemia
• It encodes a tyrosine kinase receptor that does not switch off and thus drives uncontrolled proliferation

Question 6

Describe Oncogene activation

Answer 6

• 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
• Examples:
  
  • Tyrosine kinase receptors EC – met, neu.
  • Tyrosine kinase receptors IC – src, ret.
  • Transcription factors – myc, fos, jun.
  • GPCR g-proteins – ras, gip-2.
  • Kinases – raf, pim-1.
    *

Question 7

Describe Ras activity and how it can be mutated, what are its results?

Answer 7

• 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, ability to not depend on a growth signal and cannot signal (colorectal cancer)
• The Ras pathway is part of a much more complex signalling cascade called the mitogen-activated protein kinase cascade (MAPK)
• g-coupled protein receptors

Question 8

Give a summary of Ras Signalling:

Answer 8

Binding of the ligand to the receptors, then you get phosphorylation, then mek, erk, which will have an effect on the transcription factor on the cells DNA

Question 9

Oncogenes and Human Tumours

Answer 9

• Ras codes for a family of proteins such as Ki-Ras and Ha-Ras, which are membrane bound GTPases that are important in the stimulation of cell proliferation
• Problem is that cancer is multiple diseases
• Different location of the mutation will have a different effect on the toumour that is created, drugs for the sae mutations might not work in all places

Question 10

State the function of Tumour Suppressor Genes, how could things go wrong?

Answer 10

• Typically proteins whose function is to regulate cellular proliferation and maintain cell integrity (e.g. retinoblastoma)
• Regulate and maintain integrity (stop anything bad from happening), each cell has two copies and mutation or deletion of one of the genes is insufficient to promote cancer
• 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 11

Which are the features of Inherited Cancer Susceptibility?

Answer 11

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 (of the same type pf cancer or the same cancers that run in the family)
• Tumours in different organ systems in same individual
• Mutation inherited through the germline (braca’s mutation)

Question 12

Give an example of Example of Inherited Cancer

Answer 12

• Inherited pre-disposition
• 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, cell cycle control genes
• RB1 encodes a nuclear protein that is involved in regulation of the cell cycle
• Treatment is to remove the eye

Question 13

What are the different Functional Classes of Tumour Suppressor Genes?

Answer 13

1. Regulate cell proliferation
2. Maintain cellular integrity
3. Regulate cell growth
4. Regulate the cell cycle
5. Nuclear transcription factors
6. DNA repair proteins
7. Cell adhesion molecules
8. Cell death regulators
9. OVERALL: THEY SUPPRESS THE NEOPLASTIC PHENOTYPE

Question 14

State some examples of Tumour Suppressor Genes and Human Tumours

Answer 14

P53 is really difficult to target, it would not produce a viable offspring if they were inherited

The fact that they cause syndromes tells you that they cause a lot

Question 15

What type of gene is p53, how can it be mutated? - The Guardian of the Genome

Answer 15

• When p53 is bound to MDM2 it is inactive
• It is the gradian of the genome, some of them are non-driving mutations (that the arose as the result of the initial mutation) together with driving mutations (also what effect you’ll have to normal tissues)
• 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
• Early malignancies and rare tumours
• Phosphorylation is triggered by cellular stress

Question 16

Describe how the APC Tumour Suppressor Gene works - Familial Adenomatous Polyposis Coli (APC)

Answer 16

• Due to deletion in 5q21 resulting in loss of APC gene (TSG)
• It is involved in cell adhesion and cell signalling, dysregulation of signallinf and dysregulation of cell protection
• 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 to alter transcription and growth
• APC protein helps control the activity of Beta-catenin - thereby preventing uncontrolled growth
• Mutation of APC is a frequent event in colon cancer

Question 17

Give a simplistic Overview of the Route to Cancer

Answer 17

Question 18

The Development of Colorectal Cancer

Answer 18

• The APC gene in the normal epithelium gets mutated and becomes inactive (loss of TSG) leading to hyperproliferation of the epithelium
• DNA hypomethylation (an epigenetic change) combined with a K-ras mutation (oncogene) will make the polyps develop into adenomas (low risk vs high risk adenoma)
• The mutation of p53 (the guardian of the genome) will result in the development of carcinoma
• The carcinoma can then go on to metastasise
• Both tumour suppressor genes and oncogenes are involved in the development of colorectal 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.

Question 19

State the differences between Oncogenes and Tumour Suppressor Genes

Answer 19