DNA damage and repair Flashcards
How does DNA get damaged - endogenous sources
- replicative errors - incorrect base insertion, insertion, deletions
- oxidative damage by free radicals
- spontaneous alteration in DNA
- akylating agents (malondialdehyde )
How does DNA get damaged - exogenous sources
- UV
- pollution
- carcinogens smoking
- radiotherapy - x-ray
- chemotherapy - alkylating agents, cisplatin, mitomycin C
What sort of changes occur?
- intrastrand crosslink - two bases on opposite strands become covalently attached
- strand break
- pyrimidine dimers - intrastrand crosslink
- base change
- bass loss
- interstrand cross-link
- base modification
How are the errors dealt with?
- direct reversals - multiple specific pathways
- nucleotide excision repair (NER)
- base excision repair (BER)
- mismatch repair
- homologous replication (HR)
- break induced replication (BIR)
Describe direct reversals
- reverses chemical process which has occurred
- eg reverses methylation - by removing methyl group
Describe NER
- error is removed as a stretch of nucleotides
- enzymes recognise damage and make cuts in damaged area, whole area is removed and gap is filled using polymerase
Describe BER
- only the affected base is removed
- DNA glycosylase makes excision and AP endonuclease removes base
- filled with DNA polymerase and DNA ligase
Describe mismatch repair
- similar to NER, but not removing a modified base just removing base in wrong position
- protein group involved = Mut protein (MutS, MutH, MutL)
Describe strand invasion
- a DNA strand from a broken double helix invades another double helix
Describe HR - double strand break repair (DSBR)
- only occurs in S phase when homologous chromosomes are present, so late S/G2
Describe HR - strand displacement strand annealing SDSA
- preferred in mitosis - late S/G2
- many proteins - MPH1, SRS2
Describe HR - single strand annealing SSA
- only occurs when there are adjacent repeats, always lose one of the repeats
- many proteins - Rad52, Rad1, Rad10
Describe HR - break induced replication BIR
- only occurs when there is only 1 end with homology eg during DNA replication ahead of the fork or outside of S phase
- also some role at telomeres if telomere is absent
- can be used for replication restart at collapsed replication forks
Describe HR - micro homology mediated end joining
- occurs through S phase
- main proteins - Ku, DNA-PK,
Describe non-homologous end joining NHEJ
- double strand breaks DNA are repaired by randomly fusing them usually occurs early S before replication
- find two DNA ends, protein brings ends together
- DNA ligase joins them
- proteins involved - Ku, DNA-PK
What is trans lesion synthesis
- occurs during DNA replication
- if error is encountered two ways to solve it
- synthesis past lesion by translesion polymerases (creates many errors)
- template switching = other option - leading strand synthesis up to lesion, lagging strand synthesises past the lesion
- holt - formation of structures (chicken foot structure, or recombination structure)
- replicate leading strand from lagging strand
What is ICL repair (intrastrand cross link)
- combination of mechanisms
- proteins - FANC group, Rad51
What are the signalling/caretaker proteins?
- involved in checkpoints
- p53
- BRCA1
- ATR
- ATM
- ATRIP
How is the role of p53 decided?
- post translational modification leads to specific function
- eg. acetylation at K120 = apoptosis
How is BRCA1 activated?
- forming protein complexes
- several proteins interact with BRCA1 to form complex
- A, B and C complex involved in DNA repair and cell cycle checkpoint
Describe ATM/ATR
- ATM mainly involved in DNA damage
- ATR mainly involved in DNA replication
- both are kinases
- activated by autophosphorylation
- once activated they phosphorylate many proteins and cause either apoptosis, repair, cell-cycle arrest or chromatin remodelling
What are the consequences of repair pathway failure?
- point mutations = altered gene expression
- DNA breaks ss or ds = interference replication, chromosome rearrangements
- failure to stimulate senescence
- failure to stimulate apoptosis
What is the significance of repair genes in cancer
- p53 = multiple
- BRCA1/2 = breast, ovarian
- ATM = leukemia
- MRE11 = breast
- BLM - leukemia
What repair proteins are involved in treatment of cancer?
- prognostic indicators - status of several repair genes correlated to poor prognosis
- drug targets - if cells cannot repair their DNA they are often driven to cell death
- combination therapy - targeting repair proteins sensitises cells for other forms of treatment - allows better targeting and lower doses of radiation/chemo
