Non-homologous end joining

NHEJ - The Quick & Dirty Fix

• Repairs double-strand breaks (DSBs) when no sister chromatid is available (e.g., G1 phase).
• Fast but error-prone, often causing insertions or deletions (indels) at the repair site.
• Does not use a homologous template, unlike homologous recombination.
• 📌 Mnemonic: No Homolog Ever Joins.

⭐ NHEJ is essential for V(D)J recombination, the process that creates diverse antibodies and T-cell receptors. Defects in NHEJ proteins can lead to severe combined immunodeficiency (SCID).

The NHEJ Machinery - A Protein Party

• Ku (Ku70/80): The "first responder." A heterodimer ring that recognizes and binds to broken DNA ends, acting as a scaffold and preventing further degradation.
• DNA-PKcs (DNA-dependent protein kinase, catalytic subunit): The "master regulator." Recruited by Ku, its kinase activity phosphorylates and activates downstream repair proteins like Artemis.
• Artemis: The "DNA surgeon." An endonuclease that trims mismatched or damaged single-stranded overhangs to create clean, ligatable ends.
• DNA Ligase IV / XRCC4 / XLF: The "ligation crew." This complex performs the final sealing of the DNA backbone. XRCC4 and XLF act as stabilizing and stimulating factors for Ligase IV.

⭐ Defects in NHEJ components are a key cause of radiosensitive Severe Combined Immunodeficiency (SCID), as this pathway is crucial for V(D)J recombination in developing B and T cells.

When NHEJ Fails - Clinical Chaos

• Ataxia-Telangiectasia (AT): Autosomal recessive defect in the ATM gene. ATM is a serine/threonine kinase that detects DNA double-strand breaks and activates repair proteins.
  • Clinical Features: Cerebellar ataxia, oculocutaneous telangiectasias (spider angiomas), and severe immunodeficiency.
  • Lab Findings: ↑ serum AFP (alpha-fetoprotein), ↓ IgA, IgG, & IgE levels.
  • Complications: Extreme sensitivity to ionizing radiation. ↑ risk of malignancies, especially lymphoma and leukemia.
• LIG4 Syndrome: A rare condition caused by mutations in DNA Ligase IV. Presents with microcephaly, developmental delay, immunodeficiency, and radiosensitivity.

⭐ High-Yield Pearl: Avoidance of CT scans and radiotherapy is critical in AT patients. Their inability to repair double-strand breaks means that radiation exposure, even at diagnostic levels, can significantly increase cancer risk and tissue damage.

NHEJ vs. HR - Two Paths Diverged

High‑Yield Points - ⚡ Biggest Takeaways

• The predominant pathway for repairing DNA double-strand breaks (DSBs).
• Active throughout the cell cycle, especially in the G1 phase when no sister chromatid is available.
• It is a fast but error-prone mechanism as it does not use a homologous template.
• Frequently causes small insertions or deletions (indels) at the break site, leading to mutations.
• Key enzymes include the Ku70/80 heterodimer and DNA Ligase IV.
• Defects can cause immunodeficiency and predispose to cancer.