Lecture 3 - Tumour Suppressor Genes Flashcards
What is a tumour suppressor gene?
Genes which normally function to restrict growth
Induce cell cycle arrest to induce apoptosis of damaged cells
Main allelic difference between oncogenes and tumour suppressor genes
- oncogenes are dominant so only need mutation in one allele for a constitutively active protein
- tumour suppressor genes are recessive so need mutations/loss of copies in both alleles for cancer to occur
What kind of mutations are common in patients carrying tumour suppressor gene mutations?
Germline/inherited mutations
Therefore only one sporadic mutation required in lifetime to have 2 recessive mutations
What kind of mutations usually occur in tumour suppressor genes and how do these lead to nonfunctional proteins?
- Point mutations or deletions which result in no protein or a protein with altered functions
e.g. early stop codons causing non-functional truncated proteins
What are the 6 main classes of tumour suppressor genes?
- Growth/development suppressors
- Cell cycle checkpoint proteins
- Cell cycle inhibitors
- Inducers of apoptosis
- DNA repair enzymes
- Developmental pathways
Example of a tumour suppressor gene in the growth/development suppressors class
TGF Beta acts as a break on cell signalling through the recruitment of SMAD proteins
A mutation of what gene can be inherited leading to increased risk of colon cancer?
APC gene
What occurs in APC to make you more susceptible to colon cancer?
- Loss of function of APC gene which can accelerate process of formation of precancerous intestinal polyps
What hereditary predisposition is often associated with breast cancer and inherited?
BRCA1
What is the normal function of BRCA1/2?
DNA repair enzymes
Involved in homologous recombination and double strand break repair
What can a mutation in BRCA cause?
mutations lead to defective recombination which destabilises the genome
May cause chromosomal rearrangements
If BRCA is in every cell type, why do we usually see it associated with breast and ovarian cancer?
- May be because reactive oxygen species (ROS) which are produced in the menstrual cycle cause DNA damage and are therefore dependent on the BRCA to repair the damage each month
- Loss of the enzymes can lead to accumulation of mutation
What inhibitor can be used to target BRCA deficient cells?
Poly ADP Ribose Polymerase inhibitors (PARP)
What would happen if you block PARP pathway in normal cells?
Normal cells have a BRCA mediated repair pathway, therefore blocking PARP will mean they can still repair their DNA and survive
What would happen if you blocked PARP pathway in BRCA deficient tumour cells?
Block PARP and also have deficient BRCA so the tumour cells can’t repair through the PARP or BRCA pathway
Tumour gets more and more mutations and damaged and dies
What is the name for the area of treatment using PARP inhibitors in BRCA deficient tumours?
Selective lethality
What happens to tumour suppressor genes to cause the cancer?
Loss of function
What was the first tumour suppressor gene to be identfied?
Retinoblastoma
What causes loss of function of Retinoblastoma and what does this lead to?
Point mutation or truncation
- Leads to development of tumours in the retina
What is pRb and what is its normal function in the cell?
- Retinoblastoma protein
- Normally it binds and sequesters E2F transcription factors, acting as a break on the cell cycle
What does loss of function in pRb mean for the function of the protein in the cell and the cell fate?
There is no functional protein, so cell cycle can continue without extra checks
Virus example and how it is involved with pRb
The E7 subunit of HPV can bind and inhibit pRb
What happens if there is no functional pRb in the context of the cell cycle?
E2F is free and active all the time to drive the cell into S phase
What is P53 and what kind of molecule is it in its functional state?
A transcriptional regulator, the guardian of the genome
Tetrameric (4 molecule come together)
How does P53 work as a tumour suppressor?
Prevents tumour development by checking integrity of the DNA.
If DNA damaged can upregulate repair mechanisms
Upregulates apoptotic enzymes or repair enzymes
In resting cells what is the role of P53
Upregulates antioxidant genes to counteract any ROS
P53 state in normal cells
Present at low levels bound to MDM2
What do stress signals cause to happen to P53?
Signals inhibit MDM2 which frees up P53 to act as a transcription factor, binding to damaged DNA and upregulating DNA repair enzymes
Example of stress factors which will trigger activation and upregulation of P53
DNA damage
Oncogenes
Hypoxia
Loss of trophins
Examples of responses of P53 in response to stress
Cell cycle arrest
Differentiation
DNA repair
Apoptosis
Senescence
How does P53 trigger apoptosis
Upregulating apoptotic proteins Bad and Bax
Examples of P53 mutations which cause LUNG cancer
Benzo(a) pyrine in cigarettes generates mutagens
Causes G>T transversions in DNA
Examples of P53 mutations in LIVER cancer
Aflatoxin leads to G>T transversion in P53
Brief structure of P53
- N terminal domain contains MDM2 binding site
- Middle section DNA binding domain
- C terminal contains tetramerization domain where 4 P53 molecules bind to each other
How does the HPV effect P53?
E6 subunit of HPV binds to P53, preventing it from binding to the DNA
What is different about P53 as a tumour suppressor compared to others?
Only need one mutated copy of the P53 gene to predispose the cancer
Doesn’t follow the recessive rule
Why does the recessive rule not follow for P53?
- due to the tetrameric nature of P53, if one protein has a mutation the 4 molecule complex cannot form
- the mutated protein is more stable than the wild type protein so it hangs around for longer and chance of being in the tetramer is higher
What is the issue with using chemotherapy agents to treat cancers with P53 mutations?
- Chemotherapy induces DNA damage in cancer cells so they die
- relys on the apoptopic pathway to therefore kill off the damaged cells
- need P53 to do this
- therefore all WT cells die and P53 mutant cells survive
P53 status in cervical cancer and how this contributes to cervical cancer
Rarely mutated, it is instead bound by the HPV E6 which mediates the degradation of the P53 so it cannot carry out function
What can mean you cannot fight HPV off and are more susceptible to cervical cancer?
P53 polymorphisms
Examples of alternative therapeutic approaches to P53?
Advexin - adenoviral delivery of WT P53 to outcompete the mutated P53 proteins
CDB3/ PRIMA-1 stabilises mutant p53 and restores transcriptional function
Nutlin - MDM2 inhibitors
Pifithrin –suppresses endogenous p53 in normal tissue to reduce susceptibility to chemotherapy/ radiation induced apoptosis
Outline how Onyx 015 works as a synthetic lethality approach to treat mutated P53 cancers
- Can replicate in cells by inactivating p53 (E1B) and pRb (E1A) to drive the cell cycle and drive viral load
- The Onyx version is deficient in the protein which inactivates P53
- In WT cells with P53 the virus activates the P53 pathway causing cell cycle arrest and blocking viral replication
- In cancer cells with no functional P53, there is nothing to cause cell cycle arrest so the virus replicates and the E1A inactivates pRb which drives cell through cell cycle and virus causes cell lysis of the cancer cell
Does LOF make cancer cells more resistant or more reactive to cancer therapies?
More resistant
What is a hereditary predisposition?
Inheriting a germline mutation in one allele
What are common examples of hereditary predispositions?
APC - precancerous intestinal polyps, increased risk of colon cancer
BRCA1- 60% probability of inheriting breast/ovarian cancer
Possibly lung also
