Abstract 1808: Loss of BAP1/BRCA1 counteracts spindle assembly checkpoint activation by the anti-mesothelin antibody drug-conjugate anetumab ravtansine

Abstract

Abstract Mesothelioma is a rapidly lethal cancer with limited treatment options. Spindle poisons have demonstrated clinical usefulness in mesothelioma, however, predictive biomarkers are lacking. Anetumab ravtansine (ARav) is an antibody drug-conjugate (ADC) comprising a mesothelin targeting antibody coupled to the payload, DM4, a microtubule destabilizer. We have recently identified BRCA1 as a direct regulator of the spindle assembly checkpoint (SAC), which is activated by spindle poisons and mediates mitotic arrest. BAP1 is one of the most frequently inactivated tumor suppressors in mesothelioma and it interacts with and regulates expression of BRCA1. We therefore hypothesized that BAP1 inactivation may confer resistance to ARav, highlighting a potential opportunity to refine personalization with this ADC. Three primary cell lines were derived from mesotheliomas resected during pleurectomy decortication expressed mesothelin (MSLN) and further characterized via whole exome sequencing. Two of the three primary cell lines have BAP1 mutation (R417 frameshift (fs); E631fs) and show reduced BRCA1/MAD2 protein expression. MSLN expression was found to be independent of BAP1 inactivation status or BRCA1 expression level. BAP1 mutant primary cell lines exhibited resistance to ARav by overriding the mitotic arrest with continued cell cycle progression, indicating a defective SAC whereas the cell line with wt BAP1 underwent mitotic arrest followed by cell death. Increase in mitotic index was also observed in primary cell line with wt BAP1 following ARav treatment without any significant change in BAP1 mutant cell lines. Resistance to ARav in BAP1 deleted cell line NCI-H226 was reversed by stable expression of wildtype BAP1 in NCI-H226 indicating critical regulation of SAC by BAP1. Vinorelbine resistant cell line RVR with a defective SAC also exhibited resistance to ARav compared to the parental REN with no change in MSLN expression suggesting that an active SAC is required for prolonged mitotic arrest and subsequent cell death. Wildtype BAP1 and MSLN is critical for efficacy of ARav. Inactivation of BAP1 correlates with loss of BRCA1 expression and dysregulation of the SAC. Citation Format: Anita Singh, Jin-li Luo, Sara Busacca, Alan Dawson, Krishna Kalyan Kolluri, Samuel Janes, Andrew Fry, Anette Sommer, Dean Fennell. Loss of BAP1/BRCA1 counteracts spindle assembly checkpoint activation by the anti-mesothelin antibody drug-conjugate anetumab ravtansine [abstract]. In: Proceedings of the Annual Meeting of the American Association for Cancer Research 2020; 2020 Apr 27-28 and Jun 22-24. Philadelphia (PA): AACR; Cancer Res 2020;80(16 Suppl):Abstract nr 1808.