Collaborative roles of γH2AX and the Rad51 paralog Xrcc3 in homologous recombinational repair

One of the earliest events in the signal transduction cascade that initiates a DNA damage checkpoint is the phosphorylation on serine 139 of histone H2AX (γH2AX) at DNA double-strand breaks (DSBs). However, the role of γH2AX in DNA repair is poorly understood. To address this question, we generated chicken DT40 cells carrying a serine to alanine mutation at position 139 of H2AX (H2AX−/S139A) and examined their DNA repair capacity. H2AX−/S139A cells exhibited defective homologous recombinational repair (HR) as manifested by delayed Rad51 focus formation following ionizing radiation (IR) and hypersensitivity to the topoisomerase I inhibitor, camptothecin (CPT), which causes DSBs at replication blockage. Deletion of the Rad51 paralog gene, XRCC3, also delays Rad51 focus formation. To test the interaction of Xrcc3 and γH2AX, we disrupted XRCC3 in H2AX−/S139A cells. XRCC3−/−/H2AX−/S139A mutants were not viable, although this synthetic lethality was reversed by inserting a transgene that conditionally expresses wild-type H2AX. Upon repression of the wild-type H2AX transgene, XRCC3−/−/H2AX−/S139A cells failed to form Rad51 foci and exhibited markedly increased levels of chromosomal aberrations after CPT treatment. These results indicate that H2AX and XRCC3 act in separate arms of a branched pathway to facilitate Rad51 assembly.