DNA Repair

Question 1

MMR

Answer 1

• Target base substitution & 1-4 NT insert/deletion mismatches from replication errors not damage
• Distinguishing between new/old strand: Prok = methylation; Euk = Okazaki nicks

Enzymes:

• 1. Recog by MSH 2/6 for mismatch or MSH 2/3 for insert/deletion
• 2. MLH1/PMS2 nick around MM
• 3. helicase & exonuclease cleave MM
• 4. polymerase & ligase

Clincial:

• Predisposes to cancer = Mut homolog mut
• Lynch syndrome 100-1Kx more prone to further mutations
• • most mutation MSH2 or MLH1

Question 2

BER (SP/LP)

Answer 2

Repair DNA base modifications, eg. de/amination, depurination, oxidation, alkylation

Repair of single NT (SP)

Repair 2-10 NT (LP)

Enzymes:

• 1. Recog by glycosylase, cleaves dmg base
• 2. AP endonuclease cleaves backbone
• 3. deoxyribose phosphate lyase removes 5’ sugar-P residue
• 4. polymerase, RecQ & ligase

Clinical:

• adenomatous colorectal polyposis syndrome or FAP & ↑↑CC risk
• • biallelic mut in glycosylase MYH gene Werner’s syndrome
• • Mut in RecQ helicase WRN

Question 3

BER (SSBR)

Answer 3

Repair of DNA discontinuities in 1 strand
eg. loss of NT or damaged 5’/3’ termini; from oxidative damage

Enzymes/Steps

• 1. Recognition by PARP-1
• 2. Recruit XRCC1, scaffold for recruiting
• 3. Enzymes/APTX restore 3’OH/5’P ends
• 4. polymerase & ligase

Clinical:

• Ataxia Ocularmotor Apraxia
• • Mut in APTX (hydrolase/transferase 5’ SSB end processor)

Question 4

TC-NER

Answer 4

Transition Coupled

Repair large change in structure of double helix Eg. bulky lesions, pyrimidine dimers, from radiation/chemicals

***deamination = C to U ***depurination = removal of A/G

Enzymes:

• 1. Stalled RNA pol II recog by CSA/B
• 2. RNA pol II displacement
• 3. Helicase TFIIH (XPA/B/D)
• 4. Excinuclease (endonuc) XPF/ERCC1 & XPG excise 5’→3’ damage
• 5. polymerase & ligase

Clinical:

• Cockayne Syndrome - Mut in CSA/B
• TC recog only = neuro symptoms but no cancer
• Chemo drug – introduce bulky adducts & tumor cells deficient in NER v sensitive

Question 5

GG-NER

Answer 5

Global: Non- transcription coupled

Repair large change in structure of double helix Eg. bulky lesions, pyrimidine dimers, from radiation/chemicals

***deamination = C to U ***depurination = removal of A/G
Melanoma without repair, lung cancer from carcinogens binding to G res, cause frameshift

Enzymes/Steps:

• 1. Recog by XPE/C
• 2. Helicase TFIIH (XPA/B/D)
• 3. Excinuclease (endonuc) XPF/ERCC1 & XPG excise 5’→3’
• 4. polymerase & ligase

Clincial:

• Xeroderma Pigmentosa
  
  • • Mut in XP proteins C/E/D/A affects damage recog or helicase
  • Cancer but no neuro (XPC/E; XP A/D have neuro)

Question 6

DSBR (HR)

Answer 6

• Repair DNA lesions from stress & agents (Eg. ionizing radiation, oxidizing agents, topoisomerase inhibs, metabolic products)
• Repair using sister chromatid, requires S/G2 phase
• HR non mutagenic

Enzymes:

• 1. Recog/resection by MRN complex/ATM
• 2. RAD51 recombinase to find/align homolog; RAD51 reg by BRCA1/BRCA2
• 3. nuclease & helicases to make ss nick
• 4. RAD51 promote ATP-dep ss invasion 5. polymerase, RecQ & resolvase

Clinical:

• ↑risk hereditary breast/ovarian cancer
• • Mut in BRCA1 or 2
• • some chemo drugs induce DSBs Ataxia Telangiectasia
• -autosomal recessive mut in ATM

Question 7

DSBR (NHEJ)

Answer 7

• Repair DNA lesions from stress & agents (Eg. ionizing radiation, oxidizing agents, topoisomerase inhibs, metabolic products)
• * NHEG error prone
• NH rejoining of remaining ends, occurs in any phase

Enzymes:

• 1. Recog by Ku70/Ku80 nucleases, align ends; recruit DNA-PKcs/ATM
• 2. DNA-PKcs nucleases remove frayed ends
• 3. polymerase, RecQ & ligase

Clinical:

• Werner’s syndrome
  
  • • Mut in RecQ helicase WRN
  • AT (ATM involved in pre-repair end processing)