DNA repair disorders 2

Question 1

XP variant form

Answer 1

20% of XP patients
Cells are highly UV mutable
NER proficient
Delayed replication of damaged DNA
Defective in DNA pol n (eta)
DNA pol n replicates CPDs (past T-T CPDs error free)

Question 2

XPV and skin cancer

Answer 2

Most UV damage removed by NER
Remaining damage (mostly CPDs) cannot be replicated by pol delta/epsilon
XP-V cells lack pol n so no mechanism to replicate CPDs
Accumulate mutations at di-pyrimidic sites in TSGs
Alternative translesion polymerases

Question 3

UV signature mutations in XPV

Answer 3

Absence of pol n leads to alternative TLS. Mis-insertion opposite damage
DNA pol zeta extends beyond addition of first base (error free)
Replication (DNA pol delta/e) leads to C to A or T to T transversion

Question 4

Cockayne syndrome

Answer 4

Autosomal recessive
Severe developmental disorder
Low birthweight, growth failure
Deficient neurological development
No UV-induced skin cancer

Question 5

Cellular features of CS

Answer 5

Cells sensitive to killing by UV/ other damaging agents
Normal incision at pyrimidine dimers and repair synthesis
Deficient recovery of semi-conservative DNA synthesis
Defective recovery of RNA synthesis
Defective TC-repair

Question 6

GGR

Answer 6

Global genome repair
Slow repair of damage throughout the genome
Damage recognition by XPC/E recruits repair machinery
Specificity dependent on XPC/E
Prevents mutations

Question 7

Transcription coupled repair

Answer 7

Fast repair of damage in transcribed strand of active genes
Stalling of RNA pol II at damage recruits machinery
Broad specificity
Functions mainly to prevent cell death

Question 8

CSA/B

Answer 8

Transcription-repair coupling proteins
Mutated in 90% of CS patients
CSA protein interacts with RNA pol II, TFIIH and CSB
CSB (SWI/SNF protein) associates with RNA pol II, TFIIH and PARP

Question 9

CS and lack of skin cancer predisposition

Answer 9

CS-A and CS-B cells are defective in TC-repair
RNA pol II stalls at damage. Signals for apoptosis pathway
Global genome repair normal (proliferating tissue)
No mutation accumulation as GGR repairs damage in proliferating tissue

Question 10

Developmental disorder in CS

Answer 10

Partly explained by defect in TCR
Neural tissue is vulnerable to endogenous ROS damage
TCR normally upregulated in neural tissue
CS cells are deficient in TCR of oxidative DNA damage
Blockage of RNA pol leads to apoptosis

Question 11

XP and neurological abnormalities

Answer 11

20 to 30% of XP patients
XP-A,B,D,F,G defective in TCR and GGR
Neural cells generate ROS which damage DNA. Damage persists and leads to apoptosis

Question 12

Lynch syndrome /HNPCC

Answer 12

Hereditary non-polyposis colorectal carcinoma
2-3% of al colorectal cancers
Predisposition is inherited as autosomal dominant
Increased risk of other cancers

Question 13

HNPCC cell features

Answer 13

Microsatellite instability (MSI)
Usually stable within one individual
100-1000 fold increase in mutation rates in HNPCC cells than normal cells
Mutations in MMR genes (mismatch repair) result in instability

Question 14

MMR protein complexes

Answer 14

5 MutS homologues (hMSH2-6)
4 MutL (hMLH1-4)
90% of HNPCC mutations in hMSH2 or hMLH1
MutSalpha recognises mismatch and binds ATP. Recruits MutL-alpha.
Locates exonuclease/polymerase complex
MutS-beta recognises loop resulting from replication slippage

Question 15

DNA MMR genes

Answer 15

Two mutations required for MSI
1st in germ-line (inherited predisposition)
Normal MS stability until loss of wt allele or epigenetic changes
Failure to repair insertions/deletions (normally by MMR) leads to mutations
Signal transduction, apoptosis, transcription regulation, DNA repair genes contain MS

Question 16

TGFB-RII

Answer 16

Negative growth regulation of colonic epithelial cells

Inactivation (MSI) is a key step in colorectal carcinogenesis