MA12.03 PARP Inhibitor Sensitivity Does Not Depend on BAP1 but Is Enhanced by Temozolomide in MGMT Deficient Human Mesothelioma Cells

Abstract

BRCA1 associated protein 1 (BAP1), a nuclear deubiquitinase involved in DNA double-strand (DSB) break repair by homologous recombination (HR), is frequently mutated in mesotheliomas. Because poly (ADP-ribose) polymerase inhibitors (PARPIs) target PARP1 and PARP2 and induce synthetic lethality in BRCA1/2 mutant cancers deficient in HR, we evaluated whether BAP1 inactivating mutations confer sensitivity to PARPIs in mesotheliomas. Ten patient-derived mesothelioma cell lines were generated and characterized for BAP1 mutation status, protein expression and function. Cellular sensitivity to two clinical PARPIs, olaparib and talazoparib were tested as single agents, and in combination with temozolomide. BAP1-deleted mesothelioma cellular models were generated by CRISPR/Cas9 and assessed for sensitivity to PARPIs. Because Schlafen 11 (SLFN11) and O6-methylguanine methyltransferase (MGMT) also drive response to temozolomide and PARPIs, we tested their expression and relationship with drug response. BAP1 inactivating mutations were present in eight of ten cell lines, with two harboring homozygous deletion. Cell lines exhibiting BAP1 expression also showed deubiquitinase activity (DUB). IC50 of olaparib and talazoparib plot classified them into sensitive or resistant population irrespective of BAP1 status (Figure 1). Although BAP1 knockout led to the loss of DUB activity, it did not increase the sensitivity of the cell ines to PARPI. Interestingly, cellular sensitivity to PARPI was increased by temozolomide in MGMT-negative and SLFN11-positive cell lines (Table 1).Table 1Summary reflecting combination study between talazoparib and temozolomide in different cell lines having varying MGMT and SLFN11 expression status.View Large Image Figure ViewerDownload Hi-res image Download (PPT) BAP1 status does not determine cellular sensitivity to PARPIs in patient-derived mesothelioma cell lines. In MGMT-deficient and SLFN11-positive cells, combination of PARPI and temozolomide is synergistic.