MA23.03 BAP1 Loss Induces Genome Instability Through BRCA1-Dependent and Independent Mechanisms in Mesothelioma

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BRCA1 associated protein 1 (BAP1) is a tumor suppressor that is the most frequently mutated in the majority of mesotheliomas. We have previously reported that loss of BRCA1 expression in mesothelioma is a common event, and mediates resistance to spindle checkpoint activator vinorelbine, a drug with relevance to treatment of mesothelioma. However, the loss of BRCA1 is unknown in mesothelioma. The aim of this study is to determine the functional relationship between BAP1 and BRCA1 and examine their role in vinorelbine resistance in mesothelioma cells. We conducted functional genetic analysis of BAP1 and BRCA1 in two MPM cell lines, MSTO and H2452, the latter carrying an inactivating A95D mutation in the UCH domain of BAP1. BAP1 knockdown was achieved by siRNA transfection, while BRCA1 knockdown was achieved by doxycycline induction of an integrated shRNA. Patient samples were processed for BAP1 and BRCA1 immunohistochemistry from MEDUSA cohort. Loss of BAP1 expression led to reduced expression of BRCA1, whereas knockdown of BRCA1 did not affect BAP1 expression. Treatment with the proteasome inhibitor, MG132, restored BRCA1 expression in the absence of BAP1 indicating that BAP1 contributes to post-translational stabilization of BRCA1 protein and the stabilization of BRCA1 by BAP1 is independent of its de-ubiquitination activity. Knockdown of BAP1 induced SAC deficiency and vinorelbine resistance concurrent with reduced expression of BRCA1 and the SAC component, MAD2L1. We also identified a positive and significant correlation between BAP1 and BRCA1 expressionin patient samples. Our data demonstrates that BAP1 regulates BRCA1 expression through regulating its protein stability. Our findings suggest that BAP1 inactivation dysregulates the spindle assembly checkpoint via BRCA1 dependent and independent mechanisms, conferring resistance to vinorelbine. As such BAP1 may have potential as a predictive biomarker for spindle poisons, to underpin chemotherapy stratification. A hypothesis that would be tested in a multicentre randomised phase II VIM trial.