L13 - Tumour Suppressor Genes Flashcards
When and how were tumour suppressor genes identified?
1970s-1980s - experimental evidence of a second growth controlling gene - suppress cell proliferation
Tumour suppressor genes involved in cancer when inactivated or lost
Do viral oncogenes have a dominant or recessive effect?
Dominant
Infection of a normal cells results in?
Cell transformation to a cancer cell
Happens despite continued expression of opposite cellular genes that usually mediate normal cell proliferation
Cancerous cell growth - dominant phenotype
Normal cell growth – recessive phenotype
How was it discovered that non-viral oncogenes have a dominant effect?
Cell fusion technique –> comparison of 2 alternative alleles and specified phenotypes when both alleles forced to coexist
- Dominant allele wins
How are two different phenotypes grown together?
Fusing agent is Sendai virus or PEG
The hybrid cell formed is a heterokaryon
What are the two possible outcomes when a cancer cell is fused with a normal cell?
Hybrid cell
Not tumorigenic
- Cancer alleles are recessive - correct pathway
Tumorigenic
- Cancer alleles are dominant
Normal cells carry tumour suppresser genes that constrain their proliferation - inactivated during tumour development
What were the arguments for and against tumour suppressor genes?
For
- Easier to lose a TSG by mutation than activating an oncogene
Against
- 2 copies of TSG must be lost –> 2 genetic alterations complex and unlikely in short period of time
Can’t be addressed with fusion experiment - looked at retinoblastoma
What was looked at to understand the genetic reasons for the recessive phenotype of cancer cells?
Retinoblastoma
Tumour of the retina, arising in photoreceptor precursors
1:20,000 children
Diagnosed from birth up to the age of 6 to 8 years
What are the two forms of retinoblastoma?
Sporadic form
- Children from family with no history of retinoblastoma
- Develop a single tumour in one eye - unilateral
Familial form
- Children with parent who suffered from retinoblastoma
- Develop multiple foci of tumours in both eyes - bilateral
– Elevated susceptibility of developing other tumours
What was Knudson’s 1 hit/2 hits hypothesis used for?
1971 – studied kinetics with which retinal tumours appeared in children affected by retinoblastoma
What were the results of Knudson’s 1 hit/2 hits hypothesis for retinoblastoma?
Rate of appearance of familial tumours - 1 single random event
Rate of appearance of sporadic tumours - 2 random events
Retinoblastoma is caused by a mutation in what gene?
Rb gene
What is needed to cause retinoblastoma in children with wildtype Rb from parents?
2 successive alterations in retinal cell lineage required to inactivate the 2 Rb copies
What is needed to cause retinoblastoma in children with mutant Rb from family?
Mutation of the other copy in enough to drive retinoblastoma
How does sporadic retinoblastoma occur?
Probability of both mutations occurring one after the other is 10-12 per cell generation
More likely to occur due to mitotic recombination
How can mitotic recombination cause retinoblastoma?
Chromosomal arm carrying WT Rb allele replaced with arm carrying mutant allele
Frequency of 10-5 - 10-4 per cell generation - easier than mutational inactivation
Loss of heterozygosity
- Heterozygous phenotype –> homozygous phenotype
What are the two possible ways tumour suppressor genes can stop cancer development?
Direct suppression of cell proliferation in response to growth-inhibitory and differentiation-inducing factors
Components of cellular machinery that inhibit proliferation in response to metabolic imbalance and DNA damage
What are two of the most studied tumour suppressor genes?
Rb and p53
Role in control of the cell cycle disrupted in most human tumours
What are the 3 examples of how tumour suppressor genes function in a diversity of ways?
NF1: negative regulator of Ras signalling
APC: negative regulator of b-catenin signalling
pVHL: negative regulator of HIF-1 transcription factors
What is neurofibromatosis and what is it caused by?
Caused by LOH of NF1
Familial cancer syndrome
Benign tumours in PNS – some progress to malignant neurofibrosarcomas
What is NF1?
Is a Ras GTPase activating protein
What do mutations in NF1 cause?
Create a protein with 1000 fold decrease in GTPase stimulating activity
What happens to NF1 if a cell is stimulated with growth factors?
NF1 degraded - enable Ras signalling to proceed
After 60-90min NF1 levels return to normal - shut down Ras signalling
What happens when NF1 is lost in a cell?
Ras remains activated for longer
Loss of NF1 mimics hyperactivation of Ras observed in presence of mutant ras oncogenes
What is APC and what does it facilitate?
Adenomatous Polyposis Coli
Facilitates the loss of cells from colonic crypts
What cancers is APC found in?
95% of colon cancers - sporadic
1% inheritable adenomatous polyposis coli – familial
What is inheritable adenomatous polyposis coli?
Susceptibility to develop colon polyps which are prone to develop into carcinomas
Where are stem cells normally found in a colonic crypt?
Stem cells are found at bottom of intestinal crypts
- Some progeny stay behind to maintain stem cells
- Most progeny dispatched upward and out of crypt toward epithelium
What is the defence mechanism found within the colonic crypt?
Out-migration and death takes 3 to 4 days
If cells get mutations, they die within days without damaging intestine
What do APC mutations that drive cancer do?
Block the out-migration of cells from the crypt
B-catenin controls this process - B-catenin levels controlled by Wnt signalling
Mutated cells accumulate in the crypt rather than being lost
How does APC control B-Catenin?
APC negatively controls B-catenin levels in the cytosol
Where is APC expressed within the colonic crypt?
APC is not expressed in the cells at the bottom of the crypt
- B-catenin can accumulate and move in the nucleus
- As cells move upward APC expression increases
What does inactivation of APC do?
B-catenin cytosolic accumulation and nuclear translocation
Transcription of growth-promoting genes - myc
90% of sporadic carcinomas caused by B-catenin accumulation
Wild-type mice colonic crypts
Have small defined crypts in small intestine with little nuclear β-catenin at bottom of crypt
What is the phenotype of APC -/- mice?
Show crypt-progenitor cell phenotype with increased crypt size and nuclear β-catenin
What is Von Hippen Lindau syndrome?
Hereditary predisposition to the development of a variety of tumours
- Clear cell kidney carcinomas
- Pheochromocytomas - adrenal gland cell tumours
- Hemangioblastomas - blood vessel tumours in CNS and retina
What does VHL code for?
pVHL protein
What is VHL?
Tumour suppressor gene
VHL gene inactivated in 70% of sporadic kidney carcinomas
What is the function of pVHL?
Promote destruction of HIF-1a
Enables cells to survive hypoxia and acquire access to oxygen supply
What are the target genes of pVHL?
Genes that lead to:
Angiogenesis
Erythropoiesis
Glycolysis and glucose uptake
What are the two molecular mechanisms of inactivation of pVHL?
Mutant alleles of VHL - pVHL protein undetectable in cancer cells
Point mutations in amino acid residues in hydrophobic pocket recognising hydroxyproline residues in HIF-1a
What does inactivation of pVHL lead to?
Constitutive activation of HIF-1 transcription factors
- Leads to growth promoting genes stimulating proliferation - tumour
