An ATM-MYBL2-CDC7 axis regulates replication initiation and prevents replication stress in pluripotent stem cells

Replication stress is a major source of genetic lesions in cycling cells. In somatic cells, replication stress is mainly prevented by the ATR cell cycle checkpoint signalling pathway. However, cell cycle regulation in embryonic stem cells (ESCs) is substantially different. The transcription factor MYBL2, which is involved in cell cycle regulation, is between hundred to thousand-fold more highly expressed in ESCs compared to somatic cells. Through the generation of MYBL2 mutant ESCs (), here we show that MYBL2 prevents replication stress in ESCs by acting not only in the ATR pathway, but suprisingly by acting in the same pathway as the checkpoint kinase ATM. Both loss of MYBL2, and inhibition of ATM in wild type ESC, results in replication stress phenotypes including elevated origin firing, increased replication factory activation, replication fork slowing and replication fork collapse. Mechanistically, the activity of CDK1 is elevated in ESCs, and inhibition of replication initiation through targeting CDC7 rescues replication stress induced by loss of MYBL2 and ATM inhibition. Overall, this study proposes that not only ATR but also an ATM-MYBL2-CDC7 axis is required for control of DNA replication initiation to prevent replication stress in pluripotent stem cells.