12 - Tumour Suppressor Genes Flashcards
Hybrid cells
- Genetically unstable and frequently shed chromosomes from either parent
- When they shed the mutated chromosome from the malignant parent cell, a normal phenotype was maintained.
- When chromosomes from the normal parent were shed, then a malignant phenotype was acquired.
Retinoblastoma
- Rare childhood cancer affecting retina
- Sporadic (unilateral) and hereditary/familial (bilateral)
- Hyperphosphorylation of pRB at the G1/S transition relieves pRB’s inhibition of E2F and allows cell cycle progression.
Alfred George Knudson
- Made the Knudson/two hit hypothesis
- Explains the effects of mutation on carcinogenesis
Hereditary retinoblastoma
- First mutation in 1 of 2 alleles is inherited and is present in all daughter cells.
- The loss of the remaining normal/wild type allele may occur during embryonic development or later, resulting in multiple tumors, sometimes in areas other than the retina.
- First hit = inherited mutation.
- Second hit = somatic mutation
Sporadic (non-hereditary) retinoblastoma
- Mutations to both alleles must occur in a single cell, after conception, resulting in tumors only in the eye where they occurred
- Both hits are somatic
‘Two hits’
Inactivated both functional copies of pRB, leading to retinoblastoma
Explanations for second hit
Loss of heterozygosity (LOH)
Multiple mechanisms for loss of function
- Two independent mutations within same cell
- Mitotic recombination
- Gene conversion
- Chromosomal nondisjunction and LOH
- Promoter methylation
Mitotic recombination
- Exchange of genetic information during G2 phase via crossing over, and subsequent chromatid separation can result in a pair of daughter cells that both retain heterozygosity at the Rb locus
- Alternatively, can cause two daughter cells that have undergone LOH at the Rb locus, one of which is homozygous mutant at the Rb locus while the other is homozygous wild type at this locus.
Gene conversion
- DNA polymerase starts using a DNA strand on one chromosome as a template for a new daughter strand.
- Partway through, the polymerase may switch to the homologous chromosome as a new template.
- It can then switch back to the original strand to continue replication.
- This process can transfer a mutant allele (e.g. Rb) from one chromosome to its homolog, replacing the wild-type allele there.
Chromosomal nondisjunction
- Loss of heterozygosity (LOH) can occur due to mis-segregation of chromosomes during mitosis.
- Nondisjunction happens when one daughter cell receives both chromatids of a chromosome instead of one.
- This leads to triploidy for that chromosome in one daughter cell, which may be disadvantageous.
- To revert to a diploid state, the cell may shed the extra chromosome.
- This shedding is random, potentially resulting in homozygosity for that chromosome and loss of the other allele
How to identify TSGs
Restriction fragment length polymorphisms (RFLP) and localisation of tumour suppressor genes
Important mechanism for inactivating TSGs
Promotor methylation
Promotor methylation
If CpG methylation occurs in the vicinity of a gene promoter, it can cause repression of transcription of the associated gene, and conversely, when methyl groups are removed, transcription of this gene is often de-repressed.
Mutant TSGs
- Recessive at the cellular level
- Therefore, inactivation of both alleles (Knudson’s two-hit hypothesis) is required for mutant phenotype
