Genome Stability and Tumourigenesis Group, Department of Oncology, Oxford Institute for Radiation Oncology, University of Oxford, Oxford OX3 7DQ, UK.
Department of Molecular Biology, University of Geneva, 1205 Geneva, Switzerland.
Mol Cell. 2022 Sep 15;82(18):3382-3397.e7. doi: 10.1016/j.molcel.2022.07.011. Epub 2022 Aug 23.
Aberrant replication causes cells lacking BRCA2 to enter mitosis with under-replicated DNA, which activates a repair mechanism known as mitotic DNA synthesis (MiDAS). Here, we identify genome-wide the sites where MiDAS reactions occur when BRCA2 is abrogated. High-resolution profiling revealed that these sites are different from MiDAS at aphidicolin-induced common fragile sites in that they map to genomic regions replicating in the early S-phase, which are close to early-firing replication origins, are highly transcribed, and display R-loop-forming potential. Both transcription inhibition in early S-phase and RNaseH1 overexpression reduced MiDAS in BRCA2-deficient cells, indicating that transcription-replication conflicts (TRCs) and R-loops are the source of MiDAS. Importantly, the MiDAS sites identified in BRCA2-deficient cells also represent hotspots for genomic rearrangements in BRCA2-mutated breast tumors. Thus, our work provides a mechanism for how tumor-predisposing BRCA2 inactivation links transcription-induced DNA damage with mitotic DNA repair to fuel the genomic instability characteristic of cancer cells.
异常复制导致缺乏 BRCA2 的细胞进入有丝分裂，此时 DNA 复制不足，这会激活一种称为有丝分裂 DNA 合成（MiDAS）的修复机制。在这里，我们确定了 BRCA2 缺失时 MiDAS 反应发生的全基因组位点。高分辨率分析显示，这些位点与阿非迪霉素诱导的常见脆弱位点的 MiDAS 不同，因为它们位于早期 S 期复制的基因组区域，靠近早期启动的复制起点，转录水平高，并显示出 R 环形成的潜力。早期 S 期的转录抑制和 RNaseH1 的过表达均降低了 BRCA2 缺陷细胞中的 MiDAS，表明转录-复制冲突（TRCs）和 R 环是 MiDAS 的来源。重要的是，在 BRCA2 缺陷细胞中鉴定的 MiDAS 位点也是 BRCA2 突变型乳腺癌中基因组重排的热点。因此，我们的工作为肿瘤易感性的 BRCA2 失活如何将转录诱导的 DNA 损伤与有丝分裂 DNA 修复联系起来，为癌细胞的基因组不稳定性提供了一种机制。
