DNA replication stress, a feature of human cancers, often leads to instability at specific genomic loci, such as the common fragile sites (CFSs). Cells experiencing DNA replication stress may also exhibit mitotic DNA synthesis (MiDAS). To understand the physiological function of MiDAS and its relationship to CFSs, we mapped, at high resolution, the genomic sites of MiDAS in cells treated with the DNA polymerase inhibitor aphidicolin. Sites of MiDAS were evident as well-defined peaks that were largely conserved between cell lines and encompassed all known CFSs. The MiDAS peaks mapped within large, transcribed, origin-poor genomic regions. In cells that had been treated with aphidicolin, these regions remained unreplicated even in late S phase; MiDAS then served to complete their replication after the cells entered mitosis. Interestingly, leading and lagging strand synthesis were uncoupled in MiDAS, consistent with MiDAS being a form of break-induced replication, a repair mechanism for collapsed DNA replication forks. Our results provide a better understanding of the mechanisms leading to genomic instability at CFSs and in cancer cells.

DNA 复制应激是人类癌症的一个特征，通常会导致特定基因组位点的不稳定，例如常见脆弱位点 (CFS)。经历 DNA 复制应激的细胞也可能表现出有丝分裂 DNA 合成 (MiDAS)。为了了解 MiDAS 的生理功能及其与 CFS 的关系，我们以高分辨率绘制了用 DNA 聚合酶抑制剂 aphidicolin 处理的细胞中 MiDAS 的基因组位点。 MiDAS 的位点是明显的，具有明确的峰，这些峰在细胞系之间基本上是保守的，并且涵盖了所有已知的 CFS。 MiDAS 峰映射在大的、转录的、来源贫乏的基因组区域内。在用阿菲迪霉素处理过的细胞中，这些区域即使在 S 期晚期也仍然没有复制。 MiDAS 在细胞进入有丝分裂后完成复制。有趣的是，前导链和滞后链合成在 MiDAS 中解偶联，这与 MiDAS 是断裂诱导复制的一种形式（断裂诱导复制的一种修复机制）一致。我们的结果让我们更好地了解导致 CFS 和癌细胞基因组不稳定的机制。