Sensitivity of mesothelioma cells to PARP inhibitors is not dependent on BAP1 but is enhanced by temozolomide in cells with high-Schlafen 11and low-MGMT expression

Purpose: BRCA1 associated protein-1 (BAP1), a nuclear deubiquitinase thought to be involved in DNA double-strand break repair is frequently mutated in mesothelioma. Because poly-(ADP-ribose) polymerase inhibitors (PARPIs) induce synthetic lethality in BRCA1/2 mutant cancers, we evaluated whether BAP1 inactivating mutations confer sensitivity to PARPIs in mesothelioma and if combination therapy with temozolomide (TMZ) is beneficial. Methods: Ten patient-derived mesothelioma cell-lines were generated and characterized for BAP1 mutation status, protein expression, nuclear localization and sensitivity to the PARPIs olaparib and talazoparib alone or in combination with TMZ. BAP1 deubiquitinase (DUB) activity was evaluated by ubiquitin-AMC assay. BAP1-knockout (KO) mesothelioma cell-lines were generated by CRISPR/Cas9. Because Schlafen 11 (SLFN11) and O6-methylguanine-DNA methyltransferase (MGMT) also drive response to TMZ and PARPIs, we tested their expression and relationship with drug response. Results: BAP1 mutations and/or copy-number alterations were present in all ten cell lines. However, four cell lines exhibited intact DUB activity and two had nuclear BAP1 localization. Half-maximal inhibitory concentrations of olaparib and talazoparib ranged from 4.8 µM to >50 µM and 0.039 µM to >5 µM, respectively, classifying them into sensitive (two) or resistant (seven) cells, independent of their BAP1 status. Cell lines with BAP1-KO resulted in loss of BAP1 DUB activity but did not increase sensitivity to talazoparib. Response to PARPI tended to be associated with high SLFN11 expression, and combination with temozolomide increased sensitivity of cells with low or no MGMT expression. Conclusions: BAP1 status does not determine sensitivity to PARPIs in patient-derived mesothelioma cell-lines. Combination of PARPI with TMZ may be beneficial for patients whose tumors have high SLFN11 and low or no MGMT expression.

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