Repair Mechanisms & DDR Checkpoint Flashcards
What are the four mechanisms of gaining the genetic instability phenotype?
MIN - microsatellite instability; CIN - chromosomal instability; Chromosome translations arising from eroded telomeres; Copy number variations
What is MIN?
Microsatellite instablity, a small subtle base change/point mutation.
What is CIN?
Chromosomal instability, where entire chromosomes are lost or gained, giving a much larger scale change affecting thousands of genes.
How can point mutations (MIN) occur?
1) Enivornment carcinogens such as tobacco, UV. 2) Intrinsic metabolic processes that produce ROS. 3) Error in DNA replication
Types of DNA damage
1) Double or single stranded breaks in the sugar phosphate backbone. 2) Loss of individual bases. 3) Chemical modification of bases, such as chemical crosslinking or pyrimidine dimers
What are the four mechanisms of DNA repair?
1) Mismatch repair (MMR). 2) Nucleotide Excision Repair (NER). 3) Base Excision Repair (BER). 4) Double Strand Break Repair (DSB) by either i) homologous replication or ii) non-homologous end joining
Why are DNA repair pathways a key vunerability in cancer?
Cancer cells lose these pathways and they are therefore a major route to genetic instability
Describe mismatch repair
MSH1 & MLH1 scan newly replicated DNA until it comes across DNA with nicks in the backbone, which it removes and replaces.
Describe nucleotide excision repair
Proteins scan along and recognise DNA damage, and cleave either side of the damage, then fill it in with DNA polymerases (delta, epsilon)
Describe base excision repair
BER is a simpler version of NER, the DNA is scanned by proteins and a single base in excised and refilled.
Describe DSB repair by homologous recombination
Homologous recombination involves repairing the DSB via the homologous chromosome. This will only really work after replication, so S & G2. Although it repairs the DSB, it gives a loss of heterozygosity; it could make you homozygous for a mutation carried on the non-damaged chromosome
Describe DSB repair by non-homologous end joining
Non homolgous end joining sticks the DSB back together but relies on them remaining in close proximity to each other and no loss of nucelotides from the breaks. Error prone
What is the result of mutations in repair pathways through inheritance? Give examples.
Inherited mutations in DNA repair pathways lead to the individual having a syndrome which predisposes them to cancer. 1) Hereditary non-polyposis colorectal cancer, MMR enzymes; 2) Xeroderma Pigmentosum, NER enzymes
What are the two possible pathways to take when DNA damage/stalled replication is detected?
1) Stop the cells (quiescence) and stimulate repair; 2) Senescence/apoptosis
What proteins stop the cell cycle to allow for repair in response to DNA damage?
DNA damage checkpoint proteins
What are ATM/ATR?
PI3-K like enzmes which sense DSBs
What are Chk2/Chk1?
Checkpoint kinases activated by both ATM & ATR
What is the role of Cdc25?
Rate limiting phosphatase which activates cell cycle kinases
p53 is a transcription factor which activates??
The transcription of p21
Describe the relationship between p53 & MDM2
p53 is usually synthesised and then maintained at a low concentration by the ubiquitin ligase MDM2. When p53 or MDM2 is phosphorylated this prevents their interaction.
Replicative stresses stimulate what pathway?
ATR
Mutations in the DNA damage checkpoint which are inherited result in familial cancers such as?
Familial breast and ovarian cancer (BRCA1/2); Li Fraumeni Syndrome (p53, Chk2)
How do cancer treatments target DNA damage checkpoint?
Using DNA damaging agents; alkylating agents, radiotherapy, chemotherapy. Works best in cells with damaged repair pathways which leads straight to senescence/apoptosis.
Describe the usefulness of PARP inhibitors?
PARP is an essential enzyme in BER, and when this is inhibited in combination with DNA damaging agents causes accumulation of DNA damage and death of the tumour cells
