lecture 4 - tumour suppressor genes

Question 1

What is the definition of a TSG and what kind of mutation is required to lead to cancer?

Answer 1

Genes which normally function to restrict growth and induce cell cycle arrest and apoptosis of damaged cells
recessive, loss of canton mutations (unlike oncogenes, which are dominant)
point mutations and deletions result in no protein or a protein with altered functions

Question 2

What are the main classes of TSGs?

Answer 2

growth/development suppressors (e.g. TGFB), cell cycle checkpoint proteins, cell cycle inhibitors, inducers of apoptosis, DNA repair enzymes and developmental pathways

Question 3

What is the function of BRCA1/2 and what happens when it is mutated?

Answer 3

Involved in homologous recombination and double-strand break repair
mutations lead to defective recombination, destabilising the genome
tumours particularly form in breasts/ovaries as they are particularly dependent on BRCA enzymes to repair damage caused by ROS in the menstrual cycle

Question 4

What other class of enzymes are BRCA linked with and how can they be used to kill cancer cells?

Answer 4

PARP enzymes - inhibitors of these can be used to target BRCA deficient cells
by treating BRCA-deficient tumours with PARP inhibitors, you can block both pathways and cause the cells to die because they have such damaged DNA
synthetic fatality

Question 5

What was the first TSG discovered and what happens when it is mutated? What is its normal function?

Answer 5

Retinoblastoma gene - loss of pRb can be caused by point mutation or truncation
leads to development of tumours in the retina
pRb normally acts as a break in the cell cycle - if there is no pRb, the cell can continue without extra checks and growth factors
It normally binds to and sequesters E2F, so loss of function means it cannot carry out this function
E2F would be free to signal and drive the cell into S phase constantly

Question 6

What factors are stressors for p53 and what are the cellular responses that result? How does p53 work?

Answer 6

p53 is a tetrameric transcriptional regulator that prevents up regulation of antioxidant genes
if DNA is damaged, it up regulates repair enzymes and apoptotic enzymes
it is found in complex with MDM2
stress signals (DNA damage, oncogenes, hypoxia) inhibit MDM2 and allow activation of p53
responses - cell cycle arrest, apoptosis (up regulating BAK and BAX), DNA repair

Question 7

What p53 mutations can occur and where? hint: binding site

Answer 7

in lung cancer, mutagens target the gene sequence of p53 - causes G>T transcersions
in liver cancer, fungal metabolites can lead to G>T transversion
In the middle of the sequence is the DNA-binding domain - this is the binding site for HPV (E7 subunit)
because of its tetrameric nature, only need 1 mutation - the whole complex is inactivated if just one subunit has a mutation

Question 8

What are some therapeutic approaches for dysfunctional p53?

Answer 8

Advexin - adenovirus delivery of WT p53 - cell still has mutant p53, but the WT is able to outcompete it
in WT cells with p53, this activates the p53 pathway and cell cycle arrest occurs
in cancer cells that do not have functional p53, there is nothing to cause cell cycle arrest - cell goes through cell cycle and lots of copies of the virus are produced, and the virus causes lysis of the cancer cells
MDM2 inhibitors, such as nutlins, can be used

Question 9

extra reading: how can PARP inhibitors be used?

Answer 9

PARP1 and 2 play a role in base excision repair - PARP inhibitors trap PARP on DNA and cause inhibition
inhibition of PARP sensitises tumour cells to cytotoxic agents such as alkylators and topoisomerase I inhibitors
can be used in combo with chemo - veliparib and chemo combined was shown to increase overall response rate when tested on patients with solid tumours