Monosomy 22 and partial loss of INI1 expression in a biphasic synovial sarcoma with an Ewing sarcoma-like poorly differentiated component: Report of a case.

Poorly differentiated synovial sarcoma (PDSS) is a less common subtype of synovial sarcoma (SS) associated with a poor prognosis. We present a case of a SS with a poorly differentiated component that resembles Ewing sarcoma (ES). Initial immunohistochemical staining revealed a characteristic and strong expression of transducin-like enhancer of split 1 (TLE1) and weak to absent expression of integrase integrator 1 (INI1) staining. Stainings for keratin and epithelial membrane antigen (EMA) were negative in the tumoral lesion. Fluorescence In Situ Hybridization (FISH) analysis showed a rearrangement of the synaptotagmin (SYT) gene, confirming the diagnosis of SS. FISH analysis for the EWS RNA-binding protein 1 (EWSR1) gene revealed monoallelic loss of EWSR1. This finding was confirmed by an array comparative genomic hybridization (aCGH), showing complete loss of chromosome 22. Based on literature review, showing only a handful of cases of cytogenetically studied SS with loss of chromosome 22, this is probably a rare event in SS. Therefore, we assume that monoallelic loss of chromosome 22 cannot fully elaborate the underlying mechanism of the INI1 staining pattern in all SS, but it could account for the weak to absent INI1 staining in at least some cases.