NHEJ

Non-homologous end joining (NHEJ) is a major pathway for repairing DNA double-strand breaks (DSBs) in which the broken ends are directly ligated without requiring a homologous template. It operates throughout the cell cycle, but is especially important in G0 and G1 phases when a sister chromatid is not available for homologous recombination (HR). NHEJ is generally fast and versatile, yet it is more error-prone than HR, often introducing small insertions or deletions at the repair junction.

The canonical NHEJ process begins with the binding of the Ku70/Ku80 heterodimer to DNA ends, which protects the ends and recruits the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) to form the DNA-PK complex. End processing is then coordinated, involving nucleases and polymerases such as Artemis, Pol μ, and Pol λ, to make the ends compatible for ligation. The final ligation step is carried out by the Ligase IV–XRCC4–XLF (Cernunnos) complex, which seals the ends. If ends are highly incompatible, resection and more extensive processing may occur, increasing the chance of sequence changes at the junction.

Alternative, or backup, end-joining pathways exist and are often collectively termed alt-NHEJ or microhomology-mediated end joining (MMEJ). These pathways rely on limited microhomology and are typically more error-prone than canonical NHEJ and HR. They can be engaged when core NHEJ factors are deficient or when chromatin context favors two-ended resection.

Defects in NHEJ components can lead to impaired immune system development, radiosensitivity, or genome instability. Key proteins include Ku70/Ku80, DNA-PKcs, Artemis, Ligase IV, XRCC4, and XLF, with Pol μ and Pol λ contributing to end-processing activities.