UC Irvine

Key PointsMammalian non-homologous DNA end joining (NHEJ) is the primary pathway for the repair of DNA double-strand breaks (DSBs) throughout the cell cycle, including during S and G2 phases.NHEJ relies on the Ku protein to thread onto each broken DNA end. Ku recruits the enzymes and complexes that are needed to trim (nucleases) or to fill in (polymerases) the ends to make them optimally ligatable by the DNA ligase IV complex.The configuration of the DNA ends determines which of several subpathways of NHEJ is able to join the ends. Because NHEJ is flexible and iterative, any of these subpathways can be used but some pathways are more efficient than others for certain DNA ends.When NHEJ is absent owing to a lack of Ku or the DNA ligase complex, alternative end joining (a-EJ) can join the ends using microhomology (usually >4 bp) and there is often some evidence of templated insertions of substantial length (>10 nucleotides). DNA polymerase θ (Pol θ) is of key importance for a-EJ.The single-strand annealing (SSA) pathway requires further end resection by exonuclease 1 (EXO1), Bloom syndrome RecQ-like helicase (BLM) or DNA replication helicase/nuclease 2 (DNA2) to generate the long 3′ single-strand DNA (ssDNA) tails (>20 nucleotides) that are bound by replication protein A (RPA) to prevent the formation of DNA secondary structures. The 3′ ssDNA tails are annealed by RAD52.