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

Pleural malignant mesothelioma (MM) is a therapy-resistant cancer affecting the serosal lining of the thoracic cavity. Mutations/deletions of BAP1, CDKN2A, and NF2 are the most frequent genetic lesions in human MM. We introduced various combinations of these deletions in the pleura of conditional knockout (CKO) mice, focusing on the contribution of Bap1 loss. While homozygous CKO of Bap1, Cdkn2a, or Nf2 alone gave rise to few or no MMs, inactivation of Bap1 cooperated with loss of either Nf2 or Cdkn2a to drive development of MM in ~20% of double-CKO mice, and a high incidence (22/26, 85%) of MMs was observed in Bap1;Nf2;Cdkn2a (triple)-CKO mice. MM onset was rapid in triple-CKO mice, with a median survival of only 12 weeks, and MMs from these mice were consistently high-grade and invasive. Adenoviral-Cre treatment of normal mesothelial cells from Bap1;Nf2;Cdkn2a CKO mice, but not from mice with knockout of one or any two of these genes, resulted in robust spheroid formation in vitro, suggesting that mesothelial cells from Bap1;Nf2;Cdkn2a mice have stem cell-like potential. RNA-seq analysis of MMs 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 Bap1, Nf2, and Cdkn2a results in rapid, aggressive MMs, and that deletion of Bap1 contributes to tumor development, in part, by loss of PRC2-mediated gene repression of tumorigenic target genes and by acquisition of stem-cell potential, suggesting a potential avenue for therapeutic intervention.