Monoubiquitination of H2AX protein regulates DNA damage response signaling

Double strand breaks (DSBs) are the most deleterious of the DNA lesions that initiate genomic instability and promote tumorigenesis. Cells have evolved a complex protein network to detect, signal, and repair DSBs. In mammalian cells, a key component in this network is H2AX, which becomes rapidly phosphorylated at Ser139 (γ-H2AX) at DSBs. Here we show that monoubiquitination of H2AX mediated by the RNF2-BMI1 complex is critical for the efficient formation of γ-H2AX and functions as a proximal regulator in DDR (DNA damage response). RNF2-BMI1 interacts with H2AX in a DNA damage-dependent manner and is required for monoubiquitination of H2AX at Lys119/Lys120. As a functional consequence, we show that the H2AX K120R mutant abolishes H2AX monoubiquitination, impairs the recruitment of p-ATM (Ser1981) to DSBs, and thereby reduces the formation of γ-H2AX and the recruitment of MDC1 to DNA damage sites. These data suggest that monoubiquitination of H2AX plays a critical role in initiating DNA damage signaling. Consistent with these observations, impairment of RNF2-BMI1 function by siRNA knockdown or overexpression of the ligase-dead RNF2 mutant all leads to significant defects both in accumulation of γ-H2AX, p-ATM, and MDC1 at DSBs and in activation of NBS1 and CHK2. Additionally, the regulatory effect of RNF2-BMI1 on γ-H2AX formation is dependent on ATM. Lacking their ability to properly activate the DNA damage signaling pathway, RNF2-BMI1 complex-depleted cells exhibit impaired DNA repair and increased sensitivity to ionizing radiation. Together, our findings demonstrate a distinct monoubiquitination-dependent mechanism that is required for H2AX phosphorylation and the initiation of DDR.