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.

Purpose

Recently, a 36-kDa variant of estrogen receptor α (ER-α66), ER-α36, has been identified and cloned. ER-α36 predominantly localizes on the plasma membrane and in the cytoplasm and mediates a membrane-initiated “nongenomic” signaling pathway. Here, we investigate the association between ER-α36 expression and tamoxifen resistance in patients with breast cancer.

Patients and Methods

ER-α36 protein expression in tumors from 896 women (two independent cohorts, 1 and 2) with operable primary breast cancer was assessed using an immunohistochemistry assay.

Results

In the first cohort of 710 consecutive patients, overexpression of ER-α36 was associated with poorer disease-free survival (DFS) and disease-specific survival (DSS) in patients with ER-α66–positive tumors who received tamoxifen treatment (chemotherapy plus tamoxifen or tamoxifen alone, n = 307). In contrast, ER-α36 was not associated with survival in patients with ER-α66–positive tumors who did not receive tamoxifen (chemotherapy alone, n = 129) and in patients with ER-α66–negative tumors whether they received tamoxifen (n = 73) or not (n = 149). In the second cohort of 186 patients who only received tamoxifen as adjuvant therapy, overexpression of ER-α36 was significantly associated with poorer DFS and DSS in 156 ER-α66–positive patients from this cohort, and ER-α36 remained an independent unfavorable factor for both DFS and DSS in these 156 patients by a multivariate analysis (DFS: hazard ratio [HR] = 5.47; 95% CI, 1.81 to 16.51; P =. 003; DSS: HR = 13.97; 95% CI, 1.58 to 123.53; P = .018).

Conclusion

Women with ER-α66–positive tumors that also express high levels of ER-α36 are less likely to benefit from tamoxifen treatment.