Live-cell tracking of γ-H2AX kinetics reveals the distinct modes of ATM and DNA-PK in the immediate response to DNA damage.

DNA double-strand breaks (DSBs) are a serious form of DNA damage that can cause genetic mutation. On the induction of DSBs, histone H2AX becomes phosphorylated by kinases, including ataxia telangiectasia-mutated (ATM), ataxia telangiectasia and Rad3-related (ATR) and DNA-dependent protein kinase (DNA-PK). Phosphorylated H2AX (γ-H2AX) can be a platform to recruit DNA repair machinery. Here, we analyzed the immediate early kinetics of γ-H2AX upon laser-induced DNA damage in ATM-proficient and -deficient living cells by using fluorescently labeled antigen-binding fragments specific for γ-H2AX. The accumulation kinetics of γ-H2AX were similar in both ATM-proficient and -deficient cells. However, γ-H2AX accumulation was delayed when the cells were treated with a DNA-PK inhibitor, suggesting that DNA-PK rapidly phosphorylates H2AX at DSB sites. Ku80 (also known as XRCC5), a DNA-PK subunit, diffuses freely in the nucleus without DNA damage, whereas ATM repeatedly binds to and dissociates from chromatin. The accumulation of ATM at damage sites was regulated by the histone H4K16 acetyltransferase MOF (also known as KAT8 in mammals), but its accumulation was not necessarily reflected in the γ-H2AX level. These results suggest distinct actions of ATM and DNA-PK in immediate γ-H2AX accumulation.