Protein phosphatase PP6 is required for homology-directed repair of DNA double-strand breaks.

DNA double-strand breaks (DSBs) are among the most lethal lesions associated with genome stability which, when destabilized, predisposes organs to cancers. DSBs are primarily fixed either with little fidelity by non-homologous end joining (NHEJ) repair or with high fidelity by homology-directed repair (HDR). The phosphorylated form of H2AX on serine 139 (γ-H2AX) is a marker of DSBs. In this study, we explored if the protein phosphatase PP6 is involved in DSB repair by depletion of its expression in human cancer cell lines, and determined PP6 expression in human breast cancer tissues by immunohistochemistry staining. We found that bacterially-produced PP6c (the catalytic subunit of PP6)-containing heterotrimeric combinations exhibit phosphatase activity against γ-H2AX in the in vitro phosphatase assays. Depletion of PP6c or PP6R2 led to persistent high levels of γ-H2AX after DNA damage and a defective HDR. Chromatin immunoprecipitation assays demonstrated that PP6c was recruited to the region adjacent to the DSB sites. Expression of PP6c, PP6R2, and PP6R3 in human breast tumors was significantly lower than those in benign breast diseases. Taken together, our results suggest that γ-H2AX is a physiological substrate of PP6, and PP6 is required for HDR and its expression may harbor a protective role during the development of breast cancer.