Non-homologous DNA end joining and alternative pathways to double-strand break repair

• Published in: Nature reviews. Molecular cell biology Vol. 18; no. 8; pp. 495 - 506
• Main Authors: Chang, Howard H. Y., Pannunzio, Nicholas R., Adachi, Noritaka, Lieber, Michael R.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 01.08.2017; Nature Publishing Group
• Subjects: 631/208/211; 631/337/1427/2122; 631/337/1427/2191; Ablation; Animals; Biochemistry; Cancer Research; Cell Biology; Cell cycle; Configurations; Deoxyribonucleic acid; Developmental Biology; DNA; DNA Breaks, Double-Stranded; DNA damage; DNA End-Joining Repair - genetics; DNA End-Joining Repair - physiology; DNA repair; DNA Repair - genetics; DNA Repair - physiology; Double-strand break repair; Genetic recombination; Homology; Humans; Ionizing radiation; Life Sciences; Lymphocytes; Mammalian cells; Mutation; Non-homologous end joining; Physiological aspects; Proteins; Repair; review-article; Stem Cells
• ISSN: 1471-0072; 1471-0080; 1471-0080
• DOI: 10.1038/nrm.2017.48

Key Points Mammalian 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. In mammalian cells, DNA double-strand breaks (DSBs) are repaired predominantly by the non-homologous end joining (NHEJ) pathway, which includes subpathways that can repair different DNA-end configurations. Furthermore, the repair of some DNA-end configurations can be shunted to the auxiliary pathways of alternative end joining (a-EJ) or single-strand annealing (SSA). DNA double-strand breaks (DSBs) are the most dangerous type of DNA damage because they can result in the loss of large chromosomal regions. In all mammalian cells, DSBs that occur throughout the cell cycle are repaired predominantly by the non-homologous DNA end joining (NHEJ) pathway. Defects in NHEJ result in sensitivity to ionizing radiation and the ablation of lymphocytes. The NHEJ pathway utilizes proteins that recognize, resect, polymerize and ligate the DNA ends in a flexible manner. This flexibility permits NHEJ to function on a wide range of DNA-end configurations, with the resulting repaired DNA junctions often containing mutations. In this Review, we discuss the most recent findings regarding the relative involvement of the different NHEJ proteins in the repair of various DNA-end configurations. We also discuss the shunting of DNA-end repair to the auxiliary pathways of alternative end joining (a-EJ) or single-strand annealing (SSA) and the relevance of these different pathways to human disease.