Distinct Roles of Chromatin-Associated Proteins MDC1 and 53BP1 in Mammalian Double-Strand Break Repair

Phosphorylated histone H2AX (“γ-H2AX”) recruits MDC1, 53BP1, and BRCA1 to chromatin near a double-strand break (DSB) and facilitates efficient repair of the break. It is unclear to what extent γ-H2AX-associated proteins act in concert and to what extent their functions within γ-H2AX chromatin are distinct. We addressed this question by comparing the mechanisms of action of MDC1 and 53BP1 in DSB repair (DSBR). We find that MDC1 functions primarily in homologous recombination/sister chromatid recombination, in a manner strictly dependent upon its ability to interact with γ-H2AX but, unexpectedly, not requiring recruitment of 53BP1 or BRCA1 to γ-H2AX chromatin. In contrast, 53BP1 functions in XRCC4-dependent nonhomologous end-joining, likely mediated by its interaction with dimethylated lysine 20 of histone H4 but, surprisingly, independent of H2AX. These results suggest a specialized adaptation of the “histone code” in which distinct histone tail-protein interactions promote engagement of distinct DSBR pathways.