gamma-H2AX is a critical component of the DDR and is important in maintaining genomic stability (53, 54). In the DDR, gamma-H2AX facilitates the recruitment of repair factors such as Mdc1, Rad51, and NBS1 to the sites of double strand breaks (DSBs), which is necessary for efficient repair (55). These repair factors, including gamma-H2AX, form foci when viewed with immunofluorescence microscopy that are removed once the repair is complete and DDR signaling is silenced. True to its role as a stress signaling silencer, Wip1 plays an important role in reversing gamma-H2AX levels through dephosphorylation. Three research groups (7, 56, 57), including ours, have shown that gamma-H2AX is a target of Wip1. Wip1 was found in the chromatin-bound subcellular fraction, and immunofluorescence analysis showed that Wip1 co-localized with foci formed by the DNA repair factors gamma-H2AX and Mdc1 (8). In vitro phosphatase assays showed that Wip1 dephosphorylated gamma-H2AX phosphopetides, and co-immunoprecipitation assays showed that Wip1 physically interacts with H2AX. Furthermore, deletion of Wip1 enhanced gamma-H2AX levels after genotoxic stress, whereas overexpression of Wip1 reduced gamma-H2AX levels, which was ATM-independent (7, 8, 56).
