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Malignant pleural mesothelioma (MPM) is a highly lethal cancer of the lining of the chest cavity. To expand our understanding of MPM, we conducted a comprehensive integrated genomic study, including the most detailed analysis of BAP1 alterations to date. We identified histology-independent molecular prognostic subsets, and defined a novel genomic subtype with TP53 and SETDB1 mutations and extensive loss of heterozygosity. We also report strong expression of the immune checkpoint gene VISTA in epithelioid MPM, strikingly higher than in other solid cancers, with implications for the immune response to MPM and for its immunotherapy. Our findings highlight new avenues for further investigation of MPM biology and novel therapeutic options.

Leiomyoma and leiomyosarcoma share morphological features and smooth muscle differentiation, and both arise most frequently within the uterine corpus of middle-aged women. However, they are considered biologically unrelated tumors due to their disparate clinical, cytogenetic, and molecular features. MED12, the mediator complex subunit 12 gene, has been recently implicated as an oncogene in as many as 70% of sporadic uterine leiomyoma. In the present study, we show MED12 hotspot exon 2 mutations in extrauterine leiomyoma (3 of 19 cases) and in leiomyosarcoma (3 of 13 uterine cases). We also show that MED12 mutations are found in both primary and metastatic leiomyosarcoma. Immunoblotting studies demonstrated MED12 protein expression in 100% of leiomyomas (13) Keywords: fibroid; leiomyoma; leiomyosarcoma; MED12; oncogene; smooth muscle tumor; uterus Seventy percent of women in the general population develop uterine leiomyomas, 1 and B25% of women experience substantial symptoms from these tumors, leading to over 200 000 hysterectomies annually in the United States. 1 By contrast, uterine leiomyosarcomas are infrequent, with an estimated incidence of 0.64 per 100 000 women. Nonetheless, uterine leiomyosarcomas are among the most common sarcomas, accounting for 2-5% of tumors of the uterine body. [2][3][4] There is substantial morphological overlap between leiomyoma and low-grade spindle cell leiomyosarcoma, and diagnostic distinction between these entities relies on histopathological criteria, including nuclear atypia, mitotic activity, and tumor cell necrosis. 5 Morphological variants and unusual features can further complicate the diagnosis between leiomyoma versus leiomyosarcoma, as in smooth muscle tumors of uncertain malignant potential that cannot be unequivocally classified as benign or malignant. 6 Recently, somatic mutations affecting MED12 (the mediator complex subunit 12 gene) were discovered in B70% of sporadic uterine leiomyoma. 7,8 Mediator is a modular protein complex of 25 subunits that regulates RNA polymerase II-mediated transcription, thereby orchestrating cell development and survival in cooperation with CDK8. 9,10 MED12 is located on chromosome sub-band Xq13.1 and is composed of 45 exons, although all MED12 mutations thus far described in uterine leiomyoma have affected exon 2 exclusively, with mutation hotspots in codons 36-44.In the present study, we analyzed leiomyomas and leiomyosarcomas of various biological behaviors and anatomical locations to better determine the relevance of MED12 exon 2 mutations across a broad spectrum of smooth muscle neoplasia. In these studies, we also determined whether MED12 expression is dysregulated in smooth muscle neoplasia compared with a normal myometrium control. These studies demonstrate that MED12 mutations are the first known oncogenic mechanism shared by uterine and extrauterine leiomyoma and uterine leiomyosarcoma.