In-Depth Notes on DNA Damage and Repair Mechanisms

Learning Outcomes

• Understand DNA Repair Mechanisms:
  • Describe various environmental and endogenous DNA damaging agents and their role in cancer development.
  • Explain how these agents mutate DNA, leading to specific types of mutations.
  • Outline five main DNA damage repair mechanisms.
  • Discuss how mutations in components of DNA repair pathways can predispose to cancer.
  • Comprehend synthetic lethality in treating cancers with DNA repair defects.

DNA Damage Agents

• Environmental and Endogenous Agents:
  • Reactive Oxygen Species (ROS):
  • Include hydrogen peroxide and hydroxyl radicals; produced during cellular respiration or inflammation.
  • Can oxidate DNA bases, notably guanine, leading to templates like 7,8-dihydro-8-oxo-2-deoxyguanosine, which can mimic thymine.
  • Cytosine Deamination: Converts cytosine (including 5-methylcytosine) to uracil, recognized as thymine, often leading to mutations in antibodies.

DNA Damage Repair Mechanisms

• 1. Mismatch Repair (MMR):
  • Components: MSH2/MSH6 (MutS) complex scans for errors.
  • Action: Excises mismatched bases, MLH1 recruits DNA polymerase to repair the excised region, followed by ligation.
  • Defects in MMR:
  • Mutations in MLH1, MSH2, MSH6, PMS2 linked to hereditary non-polyposis colorectal cancer (HNPCC or Lynch Syndrome).
• 2. Base Excision Repair (BER):
  • Function: Repairs small lesions that cause minor distortions.
  • Process: Specific glycosylase detects and flips out damaged bases. AP endonuclease cuts the abasic site, and DNA polymerase fills gaps, sealed by DNA ligase.
  • MUTYH Glycosylase Defect:
  • Associated with G:C → T:A transversions in colorectal cancer.
• 3. Nucleotide Excision Repair (NER):
  • Types: Global genome NER and transcription-coupled NER.
  • Recognition: XPC-Rad23B complex identifies DNA damage to initiate repair.
  • Process: Helicases unwind damaged DNA, endonucleases excise the damaged region, and repair occurs by DNA polymerases and ligases.
  • Defect Example: Xeroderma Pigmentosa (XP), predisposing to an increased risk of skin cancer.
• 4. Non-homologous End Joining (NHEJ):
  • Characteristics: Quick, error-prone repair process for double strand breaks (DSBs).
  • Mechanism: Ku complexes recognize breaks; DNA-PK phosphorylates Artemis while XRCC4 facilitates DNA ligase sealing.
• 5. Homologous Recombination (HR):
  • Usage: More accurate repair used during late S and G2 phases of the cell cycle when sister chromatids are available.
  • Involvement of MRN complex: Clean up ends and recruit DDR proteins for repair activation.

Double Strand Breaks (DSBs)

• Causes:
  • Ionizing radiation, some chemicals, DNA replication errors.
• Repair Strategies:
  • NHEJ for quick fixes or HR for a more accurate restoration.

Synthetic Lethality in Cancer Treatment

• Concept: Two genes are considered synthetic lethal when loss of either does not affect cell viability, but loss of both leads to cell death.
• Target in Therapies:
  • PARP inhibitors exploit this concept by targeting cancers with BRCA1/2 mutations.
  • Inhibition of PARP increases DSBs, leading to cancer cell demise due to the lack of HR for repair.