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

Abstract

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.

Mesothelioma families without inheritance of a BAP1 predisposing mutation

“…Loss of nuclear BAP1 and presence of cytoplasmic staining in epithelial cells is in line with our identified mutation in the UCH domain, which promotes BAP1 cytoplasmic retention [39]. Instead, the retaining of BAP1 nuclear staining in a prevailing fraction of neoplastic spindle cells ( Figure 2C and Table 2) is in line with a lower allelic frequency of mutant alleles in sarcomatoid MM DNA compared to that of the epithelial DNA, that might be due to an heterogeneous population of mutated and wild type sarcomatoid cells.…”

Novel ATP‐competitive Akt inhibitor afuresertib suppresses the proliferation of malignant pleural mesothelioma cells

“… The race and histological subtypes of cell lines used in this study are indicated by referring to Hakiri et al. . …”

BAP1 immunohistochemistry and p16 FISH results in combination provide higher confidence in malignant pleural mesothelioma diagnosis: ROC analysis of the two tests

“…Cell blocks derived from five mesothelioma and a non‐neoplastic mesothelial cell lines were also evaluated for BAP1 immunostaining (Table ). Two mesothelioma cell lines with known BAP1 gene alteration, ACC‐MESO‐4 (C.2172delc : P.y724x) and NCI‐H2452 (C.284c > a : P.a95d) showed loss of BAP1 protein expression (positive cell proportions: both 0.0 %), while expression of BAP1 protein was preserved in three mesothelioma cell lines with wild type BAP1 gene, ACC‐MESO‐1, NCI‐H2052 and MSTO‐211H (97.1 %, 96.5 % and 98.9 %, respectively). A non‐neoplastic mesothelial cell line MeT‐5A also showed preserved expression of BAP1 (95.6 %).…”

“…BAP1 mutations and immunohistochemistry in malignant mesothelioma and non-neoplastic mesothelial cell lines.Bott M, et al 22 and ‡ Hakiri S, et al29 IHC, immunohistochemistry; WT, wild type.…”