Institute of Medical Radiation Biology, University of Duisburg-Essen Medical School, Essen, Germany.
Nucleic Acids Res. 2020 Feb 28;48(4):1905-1924. doi: 10.1093/nar/gkz1167.
In vertebrates, genomic DNA double-strand breaks (DSBs) are removed by non-homologous end-joining processes: classical non-homologous end-joining (c-NHEJ) and alternative end-joining (alt-EJ); or by homology-dependent processes: gene-conversion (GC) and single-strand annealing (SSA). Surprisingly, these repair pathways are not real alternative options restoring genome integrity with equal efficiency, but show instead striking differences in speed, accuracy and cell-cycle-phase dependence. As a consequence, engagement of one pathway may be associated with processing-risks for the genome absent from another pathway. Characterization of engagement-parameters and their consequences is, therefore, essential for understanding effects on the genome of DSB-inducing agents, such as ionizing-radiation (IR). Here, by addressing pathway selection in G2-phase, we discover regulatory confinements in GC with consequences for SSA- and c-NHEJ-engagement. We show pronounced suppression of GC with increasing DSB-load that is not due to RAD51 availability and which is delimited but not defined by 53BP1 and RAD52. Strikingly, at low DSB-loads, GC repairs ∼50% of DSBs, whereas at high DSB-loads its contribution is undetectable. Notably, with increasing DSB-load and the associated suppression of GC, SSA gains ground, while alt-EJ is suppressed. These observations explain earlier, apparently contradictory results and advance our understanding of logic and mechanisms underpinning the wiring between DSB repair pathways.
在脊椎动物中，基因组 DNA 双链断裂 (DSBs) 通过非同源末端连接过程被修复：经典的非同源末端连接 (c-NHEJ) 和替代末端连接 (alt-EJ)；或通过同源依赖过程：基因转换 (GC) 和单链退火 (SSA)。令人惊讶的是，这些修复途径并不是真正的替代选择，它们以相同的效率恢复基因组完整性，而是在速度、准确性和细胞周期依赖性方面表现出显著差异。因此，一种途径的参与可能与另一种途径不存在的基因组处理风险有关。因此，对参与参数及其后果的特征描述对于理解 DSB 诱导剂（如电离辐射 (IR)）对基因组的影响至关重要。在这里，通过在 G2 期解决途径选择问题，我们发现 GC 中的调控限制，对 SSA 和 c-NHEJ 的参与具有后果。我们发现，随着 DSB 负荷的增加，GC 的抑制作用明显，这不是由于 RAD51 的可用性引起的，而是受到 53BP1 和 RAD52 的限制而不是定义。引人注目的是，在低 DSB 负荷下，GC 修复了约 50%的 DSB，而在高 DSB 负荷下，其贡献则无法检测到。值得注意的是，随着 DSB 负荷的增加和 GC 的抑制，SSA 占据了优势，而 alt-EJ 则被抑制。这些观察结果解释了早期似乎相互矛盾的结果，并提高了我们对 DSB 修复途径之间逻辑和机制的理解。
