Inactivation of <i>Bap1</i> Cooperates with Losses of <i>Nf2</i> and <i>Cdkn2a</i> to Drive the Development of Pleural Malignant Mesothelioma in Conditional Mouse Models

Abstract

Pleural malignant mesothelioma is a therapy-resistant cancer affecting the serosal lining of the thoracic cavity. Mutations/deletions of <i>BAP1, CDKN2A</i>, and <i>NF2</i> are the most frequent genetic lesions in human malignant mesothelioma. We introduced various combinations of these deletions in the pleura of conditional knockout (CKO) mice, focusing on the contribution of <i>Bap1</i> loss. While homozygous CKO of <i>Bap1, Cdkn2a</i>, or <i>Nf2</i> alone gave rise to few or no malignant mesotheliomas, inactivation of <i>Bap1</i> cooperated with loss of either <i>Nf2</i> or <i>Cdkn2a</i> to drive development of malignant mesothelioma in approximately 20% of double-CKO mice, and a high incidence (22/26, 85%) of malignant mesotheliomas was observed in <i>Bap1;Nf2;Cdkn2a</i> (triple)-CKO mice. Malignant mesothelioma onset was rapid in triple-CKO mice, with a median survival of only 12 weeks, and malignant mesotheliomas from these mice were consistently high-grade and invasive. Adenoviral-Cre treatment of normal mesothelial cells from <i>Bap1;Nf2;Cdkn2a</i> CKO mice, but not from mice with knockout of one or any two of these genes, resulted in robust spheroid formation <i>in vitro</i>, suggesting that mesothelial cells from <i>Bap1;Nf2;Cdkn2a</i> mice have stem cell-like potential. RNA-seq analysis of malignant mesotheliomas from triple-CKO mice revealed enrichment of genes transcriptionally regulated by the polycomb repressive complex 2 (PRC2) and others previously implicated in known Bap1-related cellular processes. These data demonstrate that somatic inactivation of <i>Bap1, Nf2</i>, and <i>Cdkn2a</i> results in rapid, aggressive malignant mesotheliomas, and that deletion of <i>Bap1</i> contributes to tumor development, in part, by loss of PRC2-mediated repression of tumorigenic target genes and by acquisition of stem cell potential, suggesting a potential avenue for therapeutic intervention. SIGNIFICANCE: Combinatorial deletions of <i>Bap1, Nf2</i>, and <i>Cdkn2a</i> result in aggressive mesotheliomas, with <i>Bap1</i> loss contributing to tumorigenesis by circumventing PRC2-mediated repression of oncogenic target genes.