Repair Mechanisms & DDR Checkpoint

Question 1

What are the four mechanisms of gaining the genetic instability phenotype?

Answer 1

MIN - microsatellite instability; CIN - chromosomal instability; Chromosome translations arising from eroded telomeres; Copy number variations

Question 2

What is MIN?

Answer 2

Microsatellite instablity, a small subtle base change/point mutation.

Question 3

What is CIN?

Answer 3

Chromosomal instability, where entire chromosomes are lost or gained, giving a much larger scale change affecting thousands of genes.

Question 4

How can point mutations (MIN) occur?

Answer 4

1) Enivornment carcinogens such as tobacco, UV. 2) Intrinsic metabolic processes that produce ROS. 3) Error in DNA replication

Question 5

Types of DNA damage

Answer 5

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

Question 6

What are the four mechanisms of DNA repair?

Answer 6

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

Question 7

Why are DNA repair pathways a key vunerability in cancer?

Answer 7

Cancer cells lose these pathways and they are therefore a major route to genetic instability

Question 8

Describe mismatch repair

Answer 8

MSH1 & MLH1 scan newly replicated DNA until it comes across DNA with nicks in the backbone, which it removes and replaces.

Question 9

Describe nucleotide excision repair

Answer 9

Proteins scan along and recognise DNA damage, and cleave either side of the damage, then fill it in with DNA polymerases (delta, epsilon)

Question 10

Describe base excision repair

Answer 10

BER is a simpler version of NER, the DNA is scanned by proteins and a single base in excised and refilled.

Question 11

Describe DSB repair by homologous recombination

Answer 11

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

Question 12

Describe DSB repair by non-homologous end joining

Answer 12

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

Question 13

What is the result of mutations in repair pathways through inheritance? Give examples.

Answer 13

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

Question 14

What are the two possible pathways to take when DNA damage/stalled replication is detected?

Answer 14

1) Stop the cells (quiescence) and stimulate repair; 2) Senescence/apoptosis

Question 15

What proteins stop the cell cycle to allow for repair in response to DNA damage?

Answer 15

DNA damage checkpoint proteins

Question 16

What are ATM/ATR?

Answer 16

PI3-K like enzmes which sense DSBs

Question 17

What are Chk2/Chk1?

Answer 17

Checkpoint kinases activated by both ATM & ATR

Question 18

What is the role of Cdc25?

Answer 18

Rate limiting phosphatase which activates cell cycle kinases

Question 19

p53 is a transcription factor which activates??

Answer 19

The transcription of p21

Question 20

Describe the relationship between p53 & MDM2

Answer 20

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.

Question 21

Replicative stresses stimulate what pathway?

Answer 21

ATR

Question 22

Mutations in the DNA damage checkpoint which are inherited result in familial cancers such as?

Answer 22

Familial breast and ovarian cancer (BRCA1/2); Li Fraumeni Syndrome (p53, Chk2)

Question 23

How do cancer treatments target DNA damage checkpoint?

Answer 23

Using DNA damaging agents; alkylating agents, radiotherapy, chemotherapy. Works best in cells with damaged repair pathways which leads straight to senescence/apoptosis.

Question 24

Describe the usefulness of PARP inhibitors?

Answer 24

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