Cellular pathways that repair chromosomal double-strand breaks (DSBs) have pivotal roles in cell growth, development and cancer. These DSB repair pathways have been the target of intensive investigation, but one pathway — alternative end joining (a-EJ) — has long resisted elucidation. In this Review, we highlight recent progress in our understanding of a-EJ, especially the assignment of DNA polymerase theta (Polθ) as the predominant mediator of a-EJ in most eukaryotes, and discuss a potential molecular mechanism by which Polθ-mediated end joining (TMEJ) occurs. We address possible cellular functions of TMEJ in resolving DSBs that are refractory to repair by non-homologous end joining (NHEJ), DSBs generated following replication fork collapse and DSBs present owing to stalling of repair by homologous recombination. We also discuss how these context-dependent cellular roles explain how TMEJ can both protect against and cause genome instability, and the emerging potential of Polθ as a therapeutic target in cancer.

修复染色体双链断裂 (DSB) 的细胞途径在细胞生长、发育和癌症中发挥着关键作用。这些 DSB 修复途径一直是深入研究的目标，但其中一种途径——替代末端连接 (a-EJ)——长期以来一直未能得到阐明。在这篇综述中，我们重点介绍了对 a-EJ 理解的最新进展，特别是 DNA 聚合酶 theta (Polθ) 作为大多数真核生物中 a-EJ 的主要介体，并讨论了 Polθ 介导的终止的潜在分子机制。发生加入（TMEJ）。我们探讨了 TMEJ 在解决难以通过非同源末端连接 (NHEJ) 修复的 DSB、复制叉崩溃后生成的 DSB 以及由于同源重组修复停滞而出现的 DSB 时可能存在的细胞功能。我们还讨论了这些上下文相关的细胞作用如何解释 TMEJ 如何既可以防止基因组不稳定，又可以导致基因组不稳定，以及 Polθ 作为癌症治疗靶点的新兴潜力。