Mullerian adenosarcoma: clinicopathologic and molecular characterization highlighting recurrent BAP1 loss and distinctive features of high-grade tumors

Abstract Mullerian adenosarcoma is an uncommon mesenchymal tumor of the gynecologic tract, usually of uterine origin. Tumors are generally low-grade and associated with good prognosis, whereas high-grade adenosarcomas are rare and less well studied. Herein, we sought to characterize the molecular features of 27 adenosarcomas (primary uterine, n = 19, cervical, n = 3, ovarian, n = 4, peritoneal, n = 1), enriched for high-grade tumors (n = 17) subjected to targeted panel sequencing. Recurrent genetic alterations in adenosarcomas included TP53 mutations (n = 4, 15%), restricted to high-grade cases, BAP1 homozygous deletions (n = 4, 15%), DICER1 mutations (n = 4, 15%), ARID1A mutations (n = 3), TERT promoter mutations (n = 2) and amplification (n = 1), ATRX frameshift mutation/homozygous deletions (n = 3), MDM2 (n = 2), CDK4 (n = 2) and CCNE1 (n = 2) amplifications, as well as alterations involving members of the PI3K (PTEN, n = 3; PIK3CA, n = 4; AKT1, n = 2) and MAPK (KRAS, n = 4, BRAF, n = 2) signaling pathways. One tumor harbored an ESR1-NCOA3 fusion and another had an MLH1 homozygous deletion, associated with loss of MLH1 and PMS2 protein expression. The fraction of genome altered was significantly higher in high-grade compared to low-grade adenosarcomas (P = 0.001). Somatic ATRX frameshift mutations were found in two patients with low-grade adenosarcoma with high-grade recurrences and one case of high-grade adenosarcoma with an adjacent low-grade component. Immunohistochemical analysis for BAP1 revealed loss of nuclear expression in 6/24 (25%) cases, including all 4 tumors with BAP1 deletions. Notably, out of 196 mesenchymal neoplasms of gynecologic origin, BAP1 homozygous deletion was only found in adenosarcomas (4/27,15% adenosarcomas vs 0/169, 0% other mesenchymal neoplasms, P = 0.0003). This study demonstrates that high-grade adenosarcomas are heterogeneous at the molecular level and are characterized by genomic instability and TP53 mutations; ATRX loss may be involved in high-grade transformation of low-grade adenosarcoma; and BAP1 inactivation appears to be a specific pathogenic driver in a subset of adenosarcomas.