Live-Cell Tracking of γ-H2AX Kinetics Reveals the Distinct Modes of ATM and DNA-PK in Immediate Response to DNA Damage.

DNA double-strand break (DSB) is 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, a DNA-PK subunit, diffused freely in the nucleus without DNA damage, whereas ATM repeatedly bound to and dissociated from chromatin. The accumulation of ATM at damage sites were regulated by a histone H4K16 acetyltransferase, but its accumulation was not necessarily reflected in γ-H2AX level. These results suggest distinct actions of ATM and DNA-PK that plays a primary role in immediate γ-H2AX accumulation.