DNA damage-induced G₂-M checkpoint activation by histone H2AX and 53BP1

Activation of the ataxia telangiectasia mutated (ATM) kinase triggers diverse cellular responses to ionizing radiation (IR), including the initiation of cell cycle check-points¹. Histone H2AX, p53 binding-protein 1 (53BP1) and Chk2 are targets of ATM-mediated phosphorylation^2-5, but little is known about their roles in signalling the presence of DNA damage. Here, we show that mice lacking either H2AX or 53BP1, but not Chk2, manifest a G2-M check-point defect close to that observed in ATM^-/- cells after exposure to low, but not high, doses of IR. Moreover, H2AX regulates the ability of 53BP1 to efficiently accumulate into IR-induced foci. We propose that at threshold levels of DNA damage, H2AX-mediated concentration of 53BP1 at double-strand breaks is essential for the amplification of signals that might otherwise be insufficient to prevent entry of damaged cells into mitosis.