Lecture 4: Tumour Suppressor Genes Flashcards
What is the Knudson’s Two Hit Hypothesis
How is a tumour suppressor genes ‘function lost’?
Functionally recessive - therefore both copies of the gene need to undergo a deactivating mutation.
How can tumour suppressor genes be lost?
- Mutation
- Deletion
- Loss of heterozygosity
- Methylation
- Up-reg of oncogenic miRNAs
- Haploinsufficiency
What sort of genes can be tumour suppressors?
- Signalling: Smad4, DCC, APC
- Transcription: WT-1, p53
- Gene expression: VHL
- Cell cycle control: pRb, p53
- Cell adhesion: E-cadherin
- Cytoskeletal: NF-2
- DNA damage and repair: p53, ATM, BRCA1/2
8.miRNAs: let-7
What is NF1 and what is its relevance?
GTPase-activating protein (GAP) for Ras provokes Ras to activate intrinsic GAP activity.
NF1 expressed everywhere, especially PNS and CNS
Upon GF stimulation, NF1 degraded, Ras signalling proceeds. After 60-90 mins, NF1 levels back to normal, shuts down Ras signalling
What is PTEN and what is its relevance?
PTEN is responsible for turning PIP3 back to PIP2.
Mutated NF1 and PTEN affect linked pathways leading to proliferation and survival.
What is APC and what is its relevance?
Familial adenomatous polyposis (FAP).
Inherited susceptibility to dev adenomatous polyps in colon (1/8000).
non-malignant, but can dev into carcinomas.
In absence of Wnt (but presence of APC), beta-catenin levels are low, TLE represses TCF mediated gene expression.
In presence of Wnt (or absence of APC) beta-catenin is not degraded, TLE repression released, stem cell phenotype remains
What is pRB?
pRB = p105^RB1
Two related pRB proteins - p107 & p130
Connects cell cycle clock with transcriptional control mechanisms, mediating progression through the G1 phase
Broad transcriptional effects of pRB are mediated by its inhibition of factors such as E2F
Phosphorylation of pRB essential for progression through cell cycle. Unphosphorylated = G0, Hypo-phosphorylated = entry to G1, hyper-phosphorylated at end of G1.
After M phase PP1 removes phosphate groups
pRB phosphorylated by cyclin/CDK complexes during G1
What is the relevance of E2Fs to pRB?
E2F1, 2, 3a, 3b, 4, 5, 6, 7
Bind either DP1 or 2 subunits
E2F-DP complexes recognise and bind to TTTCCCGC TF’s
Hypo-phos pRB associates with E2F, blocking activity, and recruits HDAC.
Phosphorylation of pRB causes dissociation of pRB/E2F
E2F-DP induces transcription by attracting proteins that remodel chromatin
what is P53
Upon cellular damage, p53 halts cell cycle and allows repair or death signals
homotetramer
Mutant p53 often a dominant negative protein
What is the LOSS of NF1 the same as?
Gain of oncogenic gene Ras
What does E2F do and when is it activated?
E2F aids DNA synthesis (S-phase), it is activated via the phosphorylation of pRB, which releases E2F when it is hyperphosphorylated
Unphosphorylated/hypo phosphorylated pRB stops DNA synthesis. Without pRB, E2F is able to send the cell into a DNA synthesis state
