MDC1 interacts with TOPBP1 to maintain chromosomal stability during mitosis

In mitosis, cells inactivate DNA double-strand break (DSB) repair at the chromatin level. However, some early signalling events still occur, such as recruitment of the scaffold protein MDC1 to phosphorylated histone H2AX at DSBs. Yet it remains unclear whether these events are important for maintaining genome stability during mitosis. Here, we identify a highly conserved protein interaction surface in MDC1 that is phosphorylated by CK2 and recognized by the DNA damage response mediator protein TOPBP1. Disruption of MDC1-TOPBP1 binding causes a specific loss of TOPBP1 recruitment to DSBs in mitotic but not interphase cells, accompanied by mitotic radiosensitivity, increased micronuclei and chromosomal instability. Mechanistically, we find that TOPBP1 forms filaments capable of bridging MDC1 foci in mitosis, indicating that MDC1-TOPBP1 complexes stabilize DSBs until repair is reactivated in the following G1 phase. Thus, we reveal an important, hitherto-unnoticed cooperation between MDC1 and TOPBP1 in maintaining genome stability during cell division.

在细胞有丝分裂过程中，细胞在染色质水平上失活DNA双链断裂（DSB）修复。然而，一些早期的信号事件依然发生，例如支架蛋白MDC1在DSB处的磷酸化组蛋白H2AX上的募集。然而，这些事件是否对于维持有丝分裂期间的基因组稳定性至关重要尚不明确。在本研究中，我们鉴定出MDC1中一个高度保守的蛋白质相互作用表面，该表面被CK2磷酸化并被DNA损伤反应介质蛋白TOPBP1识别。MDC1-TOPBP1结合的破坏导致TOPBP1在有丝分裂而非间期细胞中对DSB的募集特异性丢失，伴随着有丝分裂的辐射敏感性增强、微核增多和染色体不稳定。从机制上，我们发现TOPBP1在有丝分裂中形成能够连接MDC1焦点的丝状结构，这表明MDC1-TOPBP1复合物在有丝分裂中稳定DSB，直至在随后的G1期修复重新激活。因此，我们揭示了MDC1和TOPBP1在维持细胞分裂期间基因组稳定性中的重要作用，这一作用此前尚未引起注意。