Single Stranded DNA Repair Mechanisms

Nucleotide Excision Repair (NER)

• Purpose: Repairs pyrimidine dimers caused by UVA/UVB radiation.
  • Pyrimidine dimers often occur between two thymines (thymidine dimers).
  • These dimers create bulky, helix-deforming lesions.
  • If left unrepaired, these lesions can lead to mutations and melanoma.
• Timing: Occurs primarily during the G1 phase of the cell cycle.
  • Important to fix mutations before S phase (DNA replication) to prevent propagation.
• Steps:
  1. Endonuclease Complex:
     • Removes damaged bases (e.g., thymidine dimers).
  2. DNA Polymerase:
     • Adds new bases to replace the removed, damaged bases.
  3. DNA Ligase:
     • Reseals the DNA chain, linking the new bases to the old chain.
• Defect: Xeroderma Pigmentosum
  • Autosomal recessive defect in nucleotide excision repair.
  • Inability to repair thymidine dimers caused by sunlight exposure (UVA/UVB rays).
  • Symptoms:
    • Extreme sunlight intolerance (severe sunburns, blistering).
    • Higher predilection for skin cancer, including melanoma.
    • Actinic keratoses (precancerous lesions) at a young age.
    • Ulcerations of the cornea, leading to filmy, inflamed eyes.

Base Excision Repair (BER)

• Timing: Occurs throughout the entire cell cycle (not cell cycle specific).

• Purpose: Repairs damage from alkylation, deamination, and oxidation.

  • Example: Repairs deamination, such as cytosine converting to uracil.

• Mechanism: Uses the mnemonic "GEL PLEASE" to remember the steps.

  1. Glycosylase:

     • Enzyme excises the incorrect base (e.g., uracil) and creates an apurinic/apyrimidinic site (AP site).

  2. AP Endonuclease:

     • Cleaves the 5' end of the phosphodiester backbone at the AP site.

  3. Lyase:

     • Cleaves the 3' end of the phosphodiester backbone at the AP site ('Lyase cleaves less').

  4. DNA Polymerase {\beta}

  5. DNA Ligase:

     • Links the nucleotides together, sealing the strand.

• Defect: Familial Adenomatous Polyposis (FAP)

  • Usually an autosomal dominant defect of the APC gene (relevant for Step 1 exams).
  • When FAP is the result of a defect in basic excision repair, it is an autosomal recessive defect in the glycosylase enzyme.
  • Pathology: Thousands of polyps in the colon, leading to a high risk of colon cancer at a young age.
  • Treatment: Prophylactic colectomy is often performed to prevent colon cancer.

Mismatch Repair (MMR)

• Timing: Occurs at the S phase checkpoint.
• Purpose: Repairs mismatched bases inserted during DNA replication (e.g., G-T, A-C).
  • Uses MutS, MutH, and MutL enzymes.
• Steps:
  1. Recognizing Mismatch
     • MSH proteins (e.g. MutS) identify the mismatched base in the newly synthesized daughter strand
  2. Endonuclease:
     • Breaks the strand containing the mismatched base.
  3. Exonuclease:
     • Removes the incorrect bases that were cut out in the previous step.
  4. DNA Polymerase:
     • Adds new, correct bases to the strand.
  5. DNA Ligase:
     • Reseals the strand.
• Defect: Lynch Syndrome (Hereditary Non-Polyposis Colorectal Cancer - HNPCC)
  • Autosomal dominant mutation in mismatch repair genes (e.g., MLH1, MSH2).
  • Leads to microsatellite instability (high mutation rate in repeating dinucleotide regions).
  • Pathology:
    • Colorectal cancer (but with fewer polyps than FAP).
    • Increased risk of endometrial and ovarian cancer.
    • Possible skin cancer.