Expression of TLE-1 and CD99 in Carcinoma: Pitfalls in Diagnosis of Synovial Sarcoma

The characteristic immunoprofile for the diagnosis of synovial sarcoma, a neoplasm of unclear tissue origin, is expression of transducer-like enhancer of split 1 (TLE-1), CD99, partial expression of cytokeratin, and epithelial membrane antigen by immunohistochemistry (IHC). Diagnostic dilemma or misdiagnosis can occur due to overlap in IHC and morphology with carcinomas, and particularly poorly differentiated and metastatic tumors. The frequency of TLE-1 and CD99 expression in carcinomas by IHC has not been previously assessed. We evaluated TLE-1 and CD99 expression in various carcinomas and evaluated the expression of the SS18 (SYT) gene rearrangement (a characteristic biomarker for synovial sarcoma) in tumors with TLE-1 and/or CD99 expression. Immunostains of TLE-1 and CD99 were performed in 100 various carcinomas. Seven of the 98 cases (7%) of carcinomas showed TLE-1 expression, including 1 each of prostate adenocarcinoma (ADCA), esophageal ADCA, basal cell carcinoma, adrenocortical carcinoma, endometrial ADCA, ovarian serous carcinoma, and small cell carcinoma. Twenty-one of the 100 cases (21%) of carcinomas demonstrated CD99 expression, including 6 prostate ADCA, 3 esophageal ADCA, 5 squamous cell carcinomas, 2 hepatocellular carcinomas, 1 each for endometrial ADCA, renal cell carcinoma, urothelial cell carcinoma, neuroendocrine carcinoma, and mucoepidermoid carcinoma. An esophageal ADCA was positive for both TLE-1 and CD99. None of the carcinomas with positive TLE-1 (n=7) or CD99 (n=21) by IHC showed SS18 gene rearrangement by fluorescent in situ hybridization. TLE-1 and CD99 expression were identified in 7% and 21% of carcinomas, respectively. This is a potential pitfall in the IHC interpretation for diagnosis of synovial sarcoma. SS18 gene rearrangement by fluorescent in situ hybridization is helpful for the diagnostically challenging cases, either for confirmation or exclusion of synovial sarcoma.