Functional differences between wild-type and mutant-type BRCA1-associated protein 1 tumor suppressor against malignant mesothelioma cells

Malignant mesothelioma (MM) shows inactivation of the BRCA1-associated protein 1 (BAP1) gene. In this study, we found BAP1 mutations in 5 (26%) of the 19 cell lines that we established from Japanese MM patients, and examined functional differences between the WT and mutant BAP1. First, we studied the subcellular localization of BAP1, demonstrating that the WT primarily resides in the nucleus and that the mutant BAP1 is found in the cytoplasm of the cells. Transduction of the WT BAP1 vector into MM cells with homozygous deletion at the BAP1 3' side resulted in both inhibition of cell proliferation and anchorage-independent cell growth, whereas BAP1 mutants of a missense or C-terminal truncated form showed impaired growth inhibitory effects. Next, we studied how BAP1 is involved in MM cell survival after irradiation (IR), which causes DNA damage. After IR, we found that both WT and mutant BAP1 were similarly phosphorylated and phospho-BAP1 localized mainly in the nucleus. Interestingly, BRCA1 proteins were decreased in the MM cells with BAP1 deletion, and transduction of the mutants as well as WT BAP1 increased BRCA1 proteins, suggesting that BAP1 may promote DNA repair partly through stabilizing BRCA1. Furthermore, using the MM cells with BAP1 deletion, we found that WT BAP1, and even a missense mutant, conferred a higher survival rate after IR compared to the control vector. Our results suggested that, whereas WT BAP1 suppresses MM cell proliferation and restores cell survival after IR damage, some mutant BAP1 may also moderately retain these functions.