Increased ionizing radiation sensitivity and genomic instability in the absence of histone H2AX

In mammalian cells, DNA double-strand breaks (DSBs) cause rapid phosphorylation of the H2AX core histone variant (to form γ-H2AX) in megabase chromatin domains flanking sites of DNA damage. To investigate the role of H2AX in mammalian cells, we generated H2AX-deficient (H2AX(Δ)(/)(Δ)) mouse embryonic stem (ES) cells. H2AX(Δ)(/)(Δ) ES cells are viable. However, they are highly sensitive to ionizing radiation (IR) and exhibit elevated levels of spontaneous and IR-induced genomic instability. Notably, H2AX is not required for NHEJ per se because H2AX(Δ)(/)(Δ) ES cells support normal levels and fidelity of V(D)J recombination in transient assays and also support lymphocyte development in vivo. However, H2AX(Δ)(/)(Δ) ES cells exhibit altered IR-induced BRCA1 focus formation. Our findings indicate that H2AX function is essential for mammalian DNA repair and genomic stability.