13. Mechanisms of mutation in DNA & 16. Origins of other structural abnormalities Flashcards
What are the 3 main mechanisms of mutation?
- DNA damage - due to endogenous and exogenous agents
- Deficiency in DNA replication - variants escape 3’-5’ exonuclease proof-reading enzyme
- Defects in DNA repair - closely tied to cell cycle. Failure at checkpoint can lead to accumulation of DNA damage
What is the role of BER?
Corrects damage from oxidation, deamination, alkylation
Protects against ageing, neurodegeneration, cancer - e.g. MUTYH
Give an example of a gene involved in BER
MUTYH - DNA glycosylase that recognises and removes damaged base, fixed by short (1 base) or long (>2 bases) patch pathway enzymes
What is the role of NER?
Removes pyrimidine dimers / ICLs caused by UV radiation
Give an example of a gene involved in NER
Mutations in NER protein complex cause Cockayne syndrome and xeroderma pigmentosa
What is the role of MMR?
MMR proteins repair mismatched bases incorporated during DNA replication
Give an example of a gene involved in MMR
MSH2-MSH6 or MSH2-MSH3 heterodimers recruit MLH1-PMS2 heterodimer - mismatch excised, gap filled by DNA pol, strand sealed by DNA ligase
What does defective MMR cause?
Decrease in apoptosis, increase in cell survival –> selective growth advantage to cell –> cancer
How is MMR deficiency often observed?
Microsatellite instability caused by replication slippage - alternation in length of tandem repeats due to lack of MMR
Why are DSBs significant?
By what two methods are they repaired?
Can lead to translocations and creation of fusion gene
Homologous recombination repair (HRR) and Non-homologous repair (NHR)
How does homologous recombination repair work?
Repairs DSBs using sister chromatid during G2 phase (after chromosome duplication)
Single strand of sister chromatid acts as template for repair
Facilitated by RAD51C, BRCA1/2, NBS, BLM
What are the different types of non-homologous repair?
- Non-replicative mechanisms:
a) Non-homologous end joining (NHEJ)
b) breakage-fusion-bridge cycle
c) microhomology-mediated end joining (MMEJ) - Replicative mechanisms
a) Fork stalling and template switching (FoSTes)
b) Microhomology-mediated break induced replication (MMBIR)
What are the 3 main types of DNA repair?
- Direct reversal of DNA damage by enzyme, e.g. MGMT
- Excision repair - recognition of damaged site
- DSB repair
What are the 3 excision repair mechanisms?
- Base excision repair
- Nucleotide excision repair
- Mismatch repair
How do chromosomal rearrangements occur?
Most recurrent rearrangements by NAHR between LCRs
Non-recurrent and those associated with translocations in leukaemia due to incorrect repair by DNA repair mechanisms
How does NHEJ work?
No homology required, leads to 1-4bp deletion/insertion
Ku70/80 proteins recognise DSB and form scaffold to hold ends together
Artemis enzyme trims overhanging ends, DNA pol fills gap, DNA ligase joins ends
What is breakage-fusion-bridge cycle?
What does it have a major role in?
Fusion of chromosome due to telomere erosion + unrepaired DSB –> creates dicentric chromosome
During anaphase, centromeres separated –> breakage of chromosome
Cycle only stops when chromosome acquires a telomere
Repeated cycles causes rearrangements
Major role in amplification in cancer
How does FoSTeS work?
Fork stalling caused by formation of secondary structures
3’ end of lagging strand disengages from template and anneals to ssDNA in nearby replication fork where synthesis re-starts - causes dels/dups
What is MMBIR?
How does it work?
Microhomology-mediated break-induced replication
Associated with restart of collapsed replication fork, initiated by ssDNA at DSB - 3’ end anneals to any ss homologous template - break repaired during replication
What type of mutation does NHEJ often cause?
Translocations in cancer
How can DSBs be repaired?
- NHEJ
- MMEJ
- HRR
How does MMEJ work?
Associated with deletions/insertions that flank DSB
Require shot region of homology either side of DSB
What are the three main non-replicative, non-homologous repair mechanisms?
- NHEJ
- MMEJ
- Breakage-fusion-bridge cycle
What are the two main replicative, non-homologous repair mechanisms?
- FoSTes
- MMBIR
What is the most common recurrent translocation & how does it occur?
What risk is associated with the translocation?
t(11;22)(q23.3;q11.2) - mediated by palindromic AT-rich repeats susceptible to DSBs
NHEJ between the 2 regions forms the translocation
Balanced carriers are normal, risk of Emanuel syndrome in offspring due to der(22)
