🧩 Introduction: Why DNA repair matters

• DNA is constantly under attack—from both inside (reactive oxygen species from metabolism, spontaneous base changes) and outside (UV ☀️, ionizing radiation, pollutants, chemotherapy drugs).
• Mammalian cells face ~100,000 lesions per day 😱 .
• If left unrepaired, this leads to:
  • Mutations → cancer 🧨
  • Cell death/atrophy → neurodegeneration 🧠
  • Aging ⏳
• To survive, cells evolved multiple DNA repair pathways, which differ in activity depending on whether cells are dividing (cycling) or non-dividing (post-mitotic) .

🛠 DNA Repair Pathways

1. O6-methylguanine-DNA methyltransferase (MGMT)

• Fixes O6-methylguanine (O6-meG), a highly mutagenic lesion from alkylating agents.
• MGMT works like a “suicide enzyme”—it transfers the methyl group from DNA to itself, then inactivates 💀.
• Important for brain cells, as loss of MGMT makes neurons hypersensitive to alkylating damage .

2. Nucleotide Excision Repair (NER)

Repairs bulky lesions that distort DNA helix (e.g. UV-induced thymine dimers, cisplatin adducts).

• Two main subpathways:
  • Global Genome NER (GG-NER): patrols the whole genome for distortions 🕵️.
  • Transcription-Coupled NER (TC-NER): repairs lesions that stall RNA polymerase II during transcription 🎤 .
• Diseases linked to faulty NER:
  • Xeroderma pigmentosum (XP) ☀️: extreme UV sensitivity, skin cancer risk.
  • Cockayne syndrome (CS) 🧒: growth/developmental defects, neurodegeneration.
  • Trichothiodystrophy (TTD) 💇: brittle hair, developmental issues.
• NER is vital in both dividing & non-dividing cells, especially neurons .

3. Base Excision Repair (BER)

Repairs small, non-bulky base lesions like oxidized, deaminated, or alkylated bases. Steps:

1. DNA glycosylase removes bad base 🦷.
2. AP endonuclease cuts the backbone ✂️.
3. DNA polymerase fills the gap 🧱.
4. DNA ligase seals the nick 🔒 .

• Handles 8-oxoG, uracil in DNA, thymine glycol, etc.
• Works in all stages of cell cycle, so essential for both proliferating and non-dividing cells.
• Related diseases: MUTYH-associated polyposis (MAP), linked to BER defects .

3a. DNA Single-Strand Break Repair (SSBR)

• Closely tied to BER.
• Specialized proteins:
  • TDP1 → fixes topoisomerase I “stuck” ends.
  • Aprataxin (APTX) → removes abnormal 5’ ends.
  • PNKP → processes DNA ends with missing phosphate groups.
• Defects → neurological disorders like ataxia with oculomotor apraxia (AOA1) and SCAN1 (spinocerebellar ataxia with neuropathy) .

4. Mismatch Repair (MMR)

• Corrects replication errors (mismatched bases, insertion/deletion loops).
• Prevents microsatellite instability.
• Mutations in MMR genes (like MLH1, MSH2) → hereditary non-polyposis colorectal cancer (HNPCC) 🧬.
• Mainly important in dividing cells.

5. Double-Strand Break Repair (DSBR)

Double-strand breaks are lethal ⚡. Two main repair strategies:

1. Homologous Recombination (HR)
   • Uses sister chromatid as template (error-free).
   • Active in S/G2 phases of dividing cells.
2. Non-Homologous End Joining (NHEJ)
   • Directly ligates broken ends (error-prone 🩹).
   • Works in both dividing and non-dividing cells (especially neurons, immune system V(D)J recombination).
   • Key players: DNA-PKcs, Ku70/80, XRCC4, DNA ligase IV .

Defects → cancer, immunodeficiency, and neurodegeneration.

🧠 Dividing vs. Non-Dividing Cells

• Dividing cells (stem cells, glia, epithelial cells):
  • More reliance on MMR and HR, since replication errors and chromosomal breaks are common.
• Non-dividing cells (neurons, muscle cells):
  • Depend heavily on BER, NER (esp. TC-NER), and NHEJ.
  • Defects often → neurological diseases (ataxias, premature aging, neurodegeneration).

🔋 Mitochondrial DNA Repair

• Mitochondria have their own genome but limited repair.
• Many nuclear DNA repair proteins (like CS proteins, TDP1, APTX, PNKP) also work in mitochondria to protect them from ROS damage ⚡.
• Mitochondrial dysfunction is now linked to DNA repair–related disorders .

🎯 Key Takeaways

1. DNA damage is constant—cells must repair to survive.
2. Different pathways fix different types of lesions.
3. Dividing vs. non-dividing cells rely on different “repair toolkits.”
4. Many human diseases (cancer, aging, neurodegeneration, immunodeficiency) are directly tied to defects in repair.
5. Mitochondrial repair is an emerging key player.