The functional consequences of gamma-H2AX dephosphorylation by Wip1 include inhibition of cell cycle checkpoints and inhibition of DNA repair (Figure 4). Following exposure to the genotoxic agent doxorubicin, cells overexpressing Wip1 progressed into mitosis and exhibited an absence of gamma-H2AX or Mdc1 foci, indicating that overexpression of Wip1 prevents the G2/M cell cycle checkpoint after genotoxic stress through inhibition of gamma-H2AX (8). In addition, overexpression of Wip1 impaired foci formation of many repair factors including Nbs1, Rad50, and 53BP1 (7, 56). Impaired recruitment of DNA repair factors should also reduce DNA repair, and this was shown to be true in cells overexpressing Wip1. As measured by a neutral comet assay, IR-induced DSBs persisted in Wip1-overexpressing cells compared to similarly treated control cells (7). In a similar manner, site-specific DSBs initiated by the homing endonuclease, I-PpoI, exhibited impaired recruitment of γ-H2AX and were inefficiently repaired in control-treated MCF7 cells, which express high levels of Wip1, compared with Wip1 shRNA-treated cells, in which Wip1 was depleted (56). Therefore, accumulation of gamma-H2AX at DSBs is critical for cell cycle arrest and efficient DNA
