Expanding the clinicopathologic and molecular spectrum of BCOR‐associated sarcomas in adults

AIMS: BCOR gene alteration is a genetic signature of rare subsets of sarcomas. Most BCOR-associated sarcomas thus far reported are in the pediatric population, except for uterine sarcomas. We studied seven cases of BCOR-associated non-uterine sarcomas in adult patients. METHODS AND RESULTS: The patients were four men and three women ranging from 26 to 71 years in age. Three tumors, two of which primarily affected the kidney, showed BCOR-CCNB3. One tumor with a ZC3H7B-BCOR occurred in the chest wall, and a tumor with a novel CIITA-BCOR was found in the sinonasal tract. Two tumors in the lung and breast harbored exon 15 internal tandem duplications of BCOR, a highly unexpected observation in this age group. All seven sarcomas consisted of dense proliferations of uniform round to spindle cells with fine chromatin within vascular stroma. BCOR-CCNB3 sarcomas showed swirling fascicular growth. The tumor with the ZC3H7B-BCOR fusion showed a multinodular growth of spindle cells, and the tumors with the CIITA-BCOR fusion showed palisading of oval cells. Both tumors with BCOR internal tandem duplication demonstrated nested to palisading growth of round cells within sclerotic non-myxoid stroma. All seven sarcomas diffusely expressed BCOR and SATB2 immunohistochemically, with all three BCOR-CCNB3 sarcomas being immunopositive for CCNB3. BCOR alterations were confirmed by RNA sequencing, polymerase chain reaction, Sanger sequencing, and/or fluorescence in situ hybridization. CONCLUSIONS: This study expands the clinicopathologic and molecular spectrum of BCOR-associated sarcomas, and emphasizes the importance of being aware of this entity in the differential diagnosis of adult non-uterine sarcomas.