Rapid and transient recruitment of DNMT1 to DNA double-strand breaks is mediated by its interaction with multiple components of the DNA damage response machinery.

DNA methylation is an epigenetic mark critical for regulating transcription, chromatin structure and genome stability. Although many studies have shed light on how methylation impacts transcription and interfaces with the histone code, far less is known about how it regulates genome stability. We and others have shown that DNA methyltransferase 1 (DNMT1), the maintenance methyltransferase, contributes to the cellular response to DNA damage, yet DNMT1's exact role in this process remains unclear. DNA damage, particularly in the form of double-strand breaks (DSBs), poses a major threat to genome integrity. Cells therefore possess a potent system to respond to and repair DSBs, or to initiate cell death. In the current study, we used a near-infrared laser microirradiation system to directly study the link between DNMT1 and DSBs. Our results demonstrate that DNMT1 is rapidly but transiently recruited to DSBs. DNMT1 recruitment is dependent on its ability to interact with both PCNA and the ATR effector kinase CHK1, but is independent of its catalytic activity. In addition, we show for the first time that DNMT1 interacts with the 9-1-1 PCNA-like sliding clamp and that this interaction also contributes to DNMT1 localization to DNA DSBs. Finally, we demonstrate that DNMT1 modulates the rate of DSB repair and is essential for suppressing abnormal activation of the DNA damage response in the absence of exogenous damage. Taken together, our studies provide compelling additional evidence for DNMT1 acting as a regulator of genome integrity and as an early responder to DNA DSBs.