Loss of INI1 expression is characteristic of both conventional and proximal-type epithelioid sarcoma.

INI1 (hSNF5/SMARCB1), a member of the SWI/SNF chromatin remodeling complex located on chromosome 22q11.2, is deleted or/or mutated in strictly defined malignant rhabdoid tumors (MRT) of infancy. Recent studies suggest that some epithelioid sarcomas (ES) also show inactivation of INI1. However, very few cases of ES have been studied, and INI1 expression in other epithelioid malignant neoplasms has not been examined systematically. The purpose of this study was to evaluate the immunohistochemical expression of INI1 in ES compared with histologic mimics. We evaluated 350 tumors: 136 ES, including 64 conventional ("distal") ES, 64 proximal-type ES, and 8 with hybrid features of conventional and proximal-type ES; 54 metastatic carcinomas (22 from lung, 6 breast, 6 stomach, 5 colorectum, 5 kidney, 5 prostate, 5 pancreas); 12 metastatic testicular embryonal carcinomas; 20 metastatic melanomas; 20 epithelioid mesotheliomas; 20 epithelioid angiosarcomas; 10 epithelioid hemangioendotheliomas; 24 epithelioid malignant peripheral nerve sheath tumors (MPNST); 22 myoepithelial carcinomas of soft tissue; 7 anaplastic large cell lymphomas; 5 histiocytic sarcomas; and 10 control MRT of infancy (4 brain, 3 liver, 2 soft tissue, 1 kidney). Immunohistochemistry was performed following pressure cooker heat-induced epitope retrieval using monoclonal antibody BAF47 (BD Biosciences). In total, 127 of 136 (93%) ES cases showed complete absence of INI1 expression, including 58 (91%) conventional ES, 61 (95%) proximal-type ES, and all 8 (100%) hybrid ES. Of the non-ES cases, 12 (50%) epithelioid MPNST also showed loss of INI1, as did 2 (9%) myoepithelial carcinomas and all control MRT cases. INI1 expression was intact in all other tumor types examined. In conclusion, similar to MRT of infancy, loss of INI1 expression is characteristic of both conventional and proximal-type ES, being detected in >90% of cases. Moreover, 50% epithelioid MPNST and occasional myoepithelial carcinomas are also negative for INI1. Immunostaining for INI1 can be used to confirm the diagnosis of ES in the appropriate context. Loss of INI1 expression may also be helpful to distinguish epithelioid MPNST from metastatic melanoma in a subset of cases.