DNA Damage Response Is Suppressed by the High Cyclin-dependent Kinase 1 Activity in Mitotic Mammalian Cells*

DNA damage response (DDR) is vital for genomic stability, and its deficiency is linked to tumorigenesis. Extensive studies in interphase (G1-S-G2) mammalian cells have revealed the mechanisms of DDR in great detail; however, how mitotic cells respond to DNA damage remains less defined. We report here that a full DDR is suppressed in mitotic mammalian cells until telophase/cytokinesis. Although early DDR markers such as the phosphorylations of ataxia telangiectasia mutated (ATM) and histone H2A.x (H2AX) can be readily detected, the ionizing radiation-induced foci (IRIF) formation of late DDR markers such as breast cancer type 1 susceptibility protein (BRCA1) and p53-binding protein 1 (53BP1) are absent until the telophase/cytokinesis stage. We further showed that the IR-induced ubiquitination cascade around DNA damage sites did not occur in mitotic cells, which explains, at least in part, why BRCA1 and 53BP1 cannot be recruited to the damaged sites. These observations indicate that DDR is suppressed in mitotic cells after the step of γH2AX formation. Not surprisingly, we found that the absence of a full DDR in mitotic cells was associated with the high cyclin-dependent kinase 1 (CDK1) activities. More 53BP1 IRIF could be detected when the irradiated mitotic cells were treated with a CDK1 inhibitor. Further, the activation of CDK5 in interphase cells impedes the formation of 53BP1 IRIF. Together, these results suggest that the DDR is suppressed by the high CDK1 activity in mitotic mammalian cells.

DNA损伤应答（DDR）对基因组稳定性至关重要，其缺陷与肿瘤发生相关。在间期（G1 - S - G2）哺乳动物细胞中进行的大量研究已经非常详细地揭示了DDR的机制；然而，有丝分裂细胞如何对DNA损伤作出反应仍不太明确。我们在此报告，在有丝分裂的哺乳动物细胞中，完整的DDR受到抑制，直至末期/胞质分裂。尽管早期DDR标志物，如共济失调毛细血管扩张突变（ATM）和组蛋白H2A.x（H2AX）的磷酸化很容易被检测到，但晚期DDR标志物，如乳腺癌1型易感蛋白（BRCA1）和p53结合蛋白1（53BP1）的电离辐射诱导焦点（IRIF）形成在末期/胞质分裂阶段之前是不存在的。我们进一步表明，在有丝分裂细胞中，DNA损伤位点周围的电离辐射诱导的泛素化级联反应不会发生，这至少在一定程度上解释了为什么BRCA1和53BP1不能被招募到损伤位点。这些观察结果表明，在γH2AX形成步骤之后，有丝分裂细胞中的DDR受到抑制。不出所料，我们发现有丝分裂细胞中完整DDR的缺失与细胞周期蛋白依赖性激酶1（CDK1）的高活性有关。当用CDK1抑制剂处理受辐射的有丝分裂细胞时，可以检测到更多的53BP1 IRIF。此外，间期细胞中CDK5的激活会阻碍53BP1 IRIF的形成。总之，这些结果表明，在有丝分裂的哺乳动物细胞中，DDR被高CDK1活性所抑制。