DNA damage and repair Flashcards
spontaneous deamination of DNA
at 37 degC, spontaneous deamination of C, A and G bases in DNA
C deaminates to form U
A to hypoxanthine
G to xanthine
What does C deaminate to?
U
What does A deaminate to?
hypoxanthaine
What does G deaminate to?
xanthine
What happens during spontaneous depurination?
cleavage of the glycosol bond connecting purines to the backbone
leaves the backbone of DNA intact
What are depurinated sites called?
abasic (lacking a base)
or
AP sites
What are APsites?
depurinated sites
What forms from hydroxyl radical and G?
8-oxoguanine
What does 8-oxoguanine do?
mutagenic
it pairs with A not C in DNA replication
What can replication stress give rise to?
DNA singlestrand breaks SSB
What can SSBs give rise to?
souble strand breaks DSB
2 types of mutations
- point mutations - single AA change
- frameshift mutation - insertion/deletion of bases
types of point mutations
- silent - has no effect on AA sequence
- missense - new AA substitution
- nonsense - stop codon for an AA
types of frameshift mutations
deletion - change in AA dequence
insertion - change in AA sequence
-> frameshift - multiple changes in AA
-> non-frameshift - insertion of single AA
what does gene amplification produce
multiple copies of the gene
what does gene deletion result in
reduced/loss of protein function
hemizygous - loss of 1 copy
homozygous - loss of both copies
example of gene amplification
HER2 amplificaiton in breast cancer
example of gene deletion
PTEN in prostate cancer
What can DNA damage be?
- misincorporation of a single base
- chemical modification of bases
- chemical cross-links between the 2 strands of the double helix
- breaks in 1/both of the phosphodiester backbonoes
What thing can correct DNA mismatches during replication?
DNA polymerase
what can happen at exonuclease
the polynucleotide chain can leave polymerase site, go to active site of exonuclease
here, nucleotides can be removed, removing any incorrect bases
5 DNA repair mechanisms
- BER: base excision repair
- NER: nucleotide excision repair
- MMR: mis-match repair
- NHEJ: non-homologous end joining
- HR: homologous recombination
How does BER work?
BER used to correct damaged DNA bases or single strand DNA breaks
DNA glycolase excises faulty base
2nd strand used as a template
DNA polymerase fills in DNA gaps
DNA ligase seals the DNA
no defects to BER are inherited
important protein in BER
PARP
NER
acts on variety of helix-distorting DNA lesions
important in the response to adduct forming chemo agents (Pt based)
removal of damage such as pyrimidine dimers - UV light
important protein in NER
XPA
XPC
inherited disorders from NER
XP - xeroderma pigmentosum
autosomal recessive inherited condition
mutations in XP genes - XPA, XPC
sensitivity to sunlight - inc skin cancers
neurological impairment
problems with xeroderma pigmentosum
sensitivity to sunlight
neurological impairment
similarities in BER and NER
DNA polymerase fills gap
DNA ligase seals nick
MMR - mismatch repair
catches damage not repaired by BER/NER
removes mis-paired nucleotides that result from replication errors
consist of at least 2 proteins
- 1 to detect the mismatch
- other for recruiting endonuclease that cleaves the newly synthesised DNA
repair of DNA adducts from Pt chemo agents
important protein in MMR
MLH1
NHEJ - non homologous end joining
double strand breaks critical - cell must repair asap
ATM halts cell cycle - checkpoint
rapid repair mechanism
repairs 85% of DNA damage from ionising radiation
operates at ALL stages of cell cycle
BUT IS ERROR PRONE
important protein in NHEJ
ATM
inherited condition with NHEJ
ATM - ataxia telangiectasia mutated gene
autosomal recessive inherited condition
neurological condition
ataxia = lack of order, poor coordination
telangiectasia - dilated/spider blood vessels (eye)
mutations in ATM
severe sensitivity to ionising radiation
20% incidence in developing cancers
immunodeficiency - higher levels of infection
mean age of death = 25yrs (1/3 cancer, 1/3 chronic lung disease)
HRR - homologous recombination repair
ERROR FREE
only works in S and G2 phases of cell cycle
needs sister chromstids present
ATM
BRCA1, BRCA2
What inheritence are BRCA mutations?
autosomal dominant inherited condition
synthetic lethality
- normal cells can rely on BRCA2 and PARP for DNA repair
- BRCA BC are addicted to PARP (BER)
- BRCA BC are sensitive to PARP inhibition
-> give PARP inhibitors for BC
