Tumour suppressor genes Flashcards
Are viral oncogenes dominant or recessive? (2)
- Dominant
- The normal cell features are recessive
Is the cancer phenotype dominant or recessive?
Recessive
What is the cell fusion technique? (3)
- Technique to determine whether a phenotype is dominant or recessive
- Fuse 2 cells of different phenotypes with a fusing agent to form a hybrid cell
- The phenotype of the hybrid cell will be the dominant phenotype
What is the fusing agent used in the cell fusion technique?
Sendai virus or PEG
What is the hybrid cell called in the cell fusion technique?
Heterokaryon
What happens when you fuse a cancer cell with a normal cell using the cell fusion technique? (3)
- Inject hybrid cell into a mouse = no tumour is formed
- This means that the cancer phenotype is recessive
- Indicates that normal cells carry tumour suppressor genes
What is the argument in favour of tumour suppressor genes?
It is easier to lose the function of a tumour suppressor gene than acquire the specific mutation required to activate an oncogene
What is the argument against tumour suppressor genes?
2 copies of the tumour suppressor gene must be lost to completely lose the function which seems improbable in a short period of time
What is retinoblastoma?
Tumour in the retina originating from retinoblasts which usually stop growing during embryogenesis and differentiate
What are retinoblasts?
Precursor of photoreceptor cells
What are the 2 forms of retinoblastoma?
- Sporadic
- Familial
What is the sporadic form of retinoblastoma? (2)
- Occurs in children with no family history of retinoblastoma
- Causes a single tumour in one eye (unilateral retinoblastoma)
What is the familial form of retinoblastoma? (2)
- Occurs in children with a parent who suffered from retinoblastoma
- Causes multiple foci of tumours in both eyes (bilateral retinoblastoma) and increased risk of developing other tumours
Is retinoblastoma dominant or recessive?
Recessive
What is Knudson’s 1 hit/2 hits hypothesis for retinoblastoma? (3)
- Rate of diagnosis of bilateral retinoblastoma is consistent with a 1 hit model
- Unilateral retinoblastoma is consistent with a 2 hit model
- A single event is required to acquire bilateral retinoblastoma but 2 events are required to acquire unilateral retinoblastoma
What is the 1 hit model?
A single random event is required to acquire the disease
What is the 2 hit model?
2 random and independent events are required to acquire the disease
Which form of retinoblastoma is more severe?
Bilateral (familial)
What causes retinoblastoma?
Loss of function of Rb gene
How does familial (bilateral) retinoblastoma develop? (3)
- Child inherits one wildtype Rb and one non-functional Rb
- Only one ‘hit’ is required to take out the wildtype Rb allele, resulting in no functional Rb
- One mutation is enough to drive bilateral retinoblastoma
How does sporadic (unilateral) retinoblastoma develop? (3)
- Child inherits 2 wildtype copies of Rb
- A first mutation doesn’t cause disease because there is still one functional allele
- Second random mutation results in no functional Rb and unilateral retinoblastoma
What kind of gene is Rb?
Tumour suppressor gene
What is the probability of 2 random mutations knocking out both copies of the Rb allele in sporadic retinoblastoma?
Very very low
What is a more likely cause of sporadic retinoblastoma than 2 random mutations? (3)
- Mitotic recombination
- Causes loss of heterozygosity without any further mutations
- This process is more frequent than random mutations
What is mitotic recombination in retinoblastoma? (4)
- Cell starts heterozygous for non-functional Rb
- DNA is replicated
- Recombination occurs which transfers a copy of mutant Rb onto the normal chromosome so now both chromosomes carry a mutant Rb allele
- Can result in a loss of heterozygosity in the daughter cells where a daughter cell is left with no functional Rb
What are the 2 main mechanisms of stopping cancer development?
- Direct suppression of cell proliferation in response to growth-inhibitory and differentiation-inducing factors
- Cellular machinery which inhibit proliferation in response to metabolic imbalance and DNA damage
What are the first 2 tumour suppressor genes that were extensively studied?
- Rb
- p53
What kind of molecule is NF1? (2)
- RasGAP
- GTPase activating protein (GAP) which induces GTP hydrolysis by Ras, causing Ras inactivation
What is caused by loss of function of NF1? (2)
- Neurofibromatosis
- Familial cancer syndrome which causes development of benign tumours in the PNS which can become malignant
How does NF1 work normally? (2)
- Normally, growth factors cause NF1 degradation and Ras signalling can proceed
- Then NF1 levels increase to induce GTP hydrolysis by Ras and Ras inactivation
What happens when NF1 is mutated? (2)
- Causes non-functional NF1 so can’t inactivate Ras
- Results in hyperactivation of Ras
Why can’t Ras inhibitors be used in the case of NF1 null cells? (4)
- Ras inhibitors only work for oncogenic forms of Ras
- NF1 null cells contain normal Ras that is hyperactive due to lack of inhibition
- Can’t inhibit normal Ras because it is needed in normal cells
- Need to target downstream signalling of Ras
How could you target NF1 null cells? (3)
- AKT and mTOR pathway is downstream of Ras and is overactive in the absence of NF1
- This is reversed by the inhibition of mTOR with rapamycin
- Rapamycin treatment reduces proliferation in NF1 null cells
What is APC?
Adenomatous Polyposis Coli gene
What causes heritable colon cancer? (2)
- Loss of function of APC gene causes the development of benign polyps which can become cancerous
- Loss of APC is the first step in carcinogenesis followed by oncogenic mutations of Ras and mutations/loss of p53
How does Wnt signalling regulate intestinal crypts? (4)
- Stem cells are at the bottom of the intestinal crypts in close proximity to stromal cells which secrete wnt signalling
- Wnt signalling stimulates proliferation of stem cells and progenitor cells in the bottom of the crypt
- Proliferating cells migrate upwards to the top of the crypt, lose wnt signalling which causes differentiation
- Differentiated cells undergo apoptosis and are shed within 3-4 days
Why do intestinal cells migrate and shed? (2)
- Intestinal cells in contact with the contents of the intestines can be easily damaged
- Defensive mechanism to remove potentially mutated cells before they can give rise to cancer
What kind of mutation causes colon cancer?
Mutations that block cell migration out of the crypt so they continue to proliferate and aren’t shed
How does APC inactivation cause colon cancer? (4)
- APC is not present at the bottom of the crypt, expression is increased further up
- APC normally binds to β-catenin and mediates β-catenin degradation
- APC inactivation causes β-catenin to accumulate in the cytosol and nuclear translocation
- Results in excessive expression of growth-promoting genes causing colon cancer
How does Wnt signalling cause intestinal cell proliferation? (3)
- Wnt signalling at the bottom of the crypt increases β-catenin levels in the cytosol
- β-catenin translocates into the nucleus and causes transcription of growth-promoting genes including myc
- Causes cell proliferation and cells migrate out of the crypt
What is seen in APC negative intestinal crypts? (2)
- Crypts are enlarged (indicative of increased growth)
- Increased nuclear β-catenin levels in the whole crypt rather than just at the bottom of the crypt
What is the result of a loss of APC?
Increased nuclear β-catenin levels
What is β-catenin?
Transcription factor
What is VHL syndrome? (2)
- Von Hippen-Lindau syndrome
- Causes a hereditary predisposition to tumour development in the kidney, adrenal gland and blood vessels in the CNS and retina
What causes VHL syndrome? (2)
- Germ line mutations in the tumour suppressor gene VHL
- VHL also inactivated in ~70% sporadic kidney carcinomas
What does the VHL gene code for?
pVHL protein
What is the function of normal pVHL? (2)
- Normoxia: HIF-1α is hydroxylated by proline hydroxylase, pVHL binds to hydroxylated HIF-1α and causes ubiquitination and degradation of HIF-1α
- Hypoxia: no activity of proline hydroxylase so no degradation of HIF-1α, HIF-1α accumulates and promotes transcription of target genes
What are the target genes of HIF-1α during hypoxia? (3)
Genes involved in:
- Angiogenesis
- Erythropoiesis
- Glycolysis and glucose uptake
What happens with pVHL in tumours? (3)
- pVHL completely lost or mutations in the recognition site for hydroxylated HIF-1α
- This means that HIF-1α is not degraded and is constitutively active in tumour cells regardless of oxygen levels
- Causes excessive expression of growth promoting genes
True or false: cells in culture can be transformed by RSV infection
True
True or false: proto-oncogenes were the first oncogenes to be discovered
False (viral oncogenes)
True or false: multiple genes are responsible for RSV-induced transformation
False
True or false: H-ras is activated by a single nucleotide change
True
True or false: the cancer phenotype is always dominant
False
True or false: LOH is observed in the vicinity of a TSG
True
True or false: APC is overexpressed in colorectal cancer
False
True or false: VHL is inactivated in several cancers including kidney carcinoma
True
What are the 2 main types of cancer-controlling genes?
- Proto-oncogenes
- Tumour suppressor genes
What does NF1 regulate?
Negative regulator of Ras signalling
What does APC regulate?
Negative regulator of β-catenin signalling
What does pVHL regulate?
Negative regulator of HIF-1α transcription factors
