Figure 2.

(See previous page). Mechanisms of non-homologous end joining. (A). Canonical NHEJ (C-NHEJ). C-NHEJ is initiated by binding of the Ku80-Ku70 heterodimer to the DSB, which recruits the DNA-PK catalytic subunit, DNA-PKcs. Several proteins, including Artemis, the polynucleotide kinase (PNK) and members of the polymerase X family, process the DNA ends to make them competent for the subsequent ligation steps. Finally, ligase IV, in association with Xrcc4 and Cernunos/Xlf, seals the double-strand ends. (B) Alternative end-joining (A-EJ). In the absence of Ku70/Ku80, the DNA ends are resected in a reaction favored by the nuclease activity of Mre11 and CtIP. Note that Parp1 is involved in A-EJ initiation. The resulting ssDNA reveals complementary microhomologies (2–4 nt or more) that can be annealed; gap filling completes the end joining. Finally, Xrcc1 and ligase III complete the A-EJ process. Notably, A-EJ is always associated with deletions at the junctions and frequently, but not systematically, involves microhomologies that are distant from the DSB. (C) The 2-step DSB repair pathway choice model. After signaling of the DSB by ATM and MRN, (I) binding of Ku80/Ku70 protects the DSB ends from resection, routing DSB repair toward the conservative C-NHEJ pathway. (II) The nuclease activity of Mre11 and CtIP favor ssDNA resection, which can then initiate the HR or A-EJ pathway. A short ssDNA resection is sufficient for A-EJ but not for HR. A-EJ is an exclusively non-conservative mutagenic process.