Diffusion of activated ATM explains γH2AX and MDC1 spread beyond the DNA damage site

During DNA repair, ATM-induced H2AX histone phosphorylation and MDC1 recruitment spread megabases beyond the damage site. The mechanism underlying this spread remains unclear. To elucidate this step of the DNA damage response, we measured ATM and MDC1 recruitment kinetics at micro-irradiation-induced complex DNA lesions. While MDC1 spreads beyond the damage-induced focus of ATM accumulation, cohesin loader NIPBL and cohesin subunit RAD21 accumulate later than MDC1. Such delayed recruitment suggests that, in complex DNA lesions, loop extrusion cannot alone explain γH2AX and MDC1 spread. To determine if diffusion of activated ATM could account for the observed behavior, we measured the exchange rate and diffusion constants of ATM and MDC1 within damaged and unperturbed chromatin. Based on these measurements, we introduced a quantitative model that describes the dynamic behavior of ATM and subsequent MDC1 spread at complex DNA lesions. Altogether, we propose activated ATM diffusion as an underlying mechanism of MDC1 spread. ### Competing Interest Statement The authors have declared no competing interest.