Specificity and sensitivity of INI-1 labeling in epithelioid sarcoma. Loss of INI1 expression as a frequent immunohistochemical event in synovial sarcoma.

INI1 antigen is a product of the INI-1/SMARCB1 gene localized on chromosome 22q. It is well known that INI1 gene inactivation or loss of INI1 antigen expression is observed in epithelioid sarcomas; however, there are only few reports concerning specificity and sensitivity of immunohistochemical INI1 labeling as a marker of this neoplasm. That is why we decided to test 99 soft tissue sarcomas for the presence of the INI1 gene product. More specifically, the analyzed group consisted of 33 synovial sarcomas, 14 fibrosarcomas, 8 desmoid tumors, 8 DFSPs, 5 MPNSTs, 9 epithelioid sarcomas, 11 Ewing sarcomas/PNETs, 9 rhabdomyosarcomas and 2 clear cell sarcomas. Additionally, 7 malignant melanomas and 9 adenocarcinomas were included into the study. Positive staining with an antibody against the INI-1 gene product was observed in all studied cases of MPNST, Ewing sarcoma/PNET, rhabdomyosarcoma, malignant melanoma, clear cell sarcoma, and adenocarcinoma. On the contrary, none of 9 epithelioid sarcomas was labeled. The loss of INI1 expression was also detected in 7 (21.2%) synovial sarcomas, confirmed cytogenetically or by FISH. Considering the lack of reaction with INI-1 antibody as a diagnostic test for epithelioid sarcoma we estimated that its sensitivity reached 100% and specificity - 83.5% (p < 0.0001).