What is the cell of origin for synovial sarcoma?
The histology of this tumor may be biphasic, in which epithelial cells arranged in clusters, tubules, and acini are interspersed in a spindle-cell stroma; or monophasic, in which either the epithelial or spindle cells predominate. Other morphologic types are recognized, however, including mixed and hemangiopericytic. Behavioral features include calcifying, ossifying, and poorly differentiated types. Though this tumor is called synovial sarcoma, ultrastructural and immunohistochemical studies have implicated an epithelial origin. The majority (80%–90%) of synovial sarcomas are characterized by a translocation which fuses the SS18 gene on chromosome 18 to either SSX1, SSX2, or SSX4 on chromosome X. The SS18-SSX fusion gene product competes with the normal endogenous SS18 protein causing the formation of an altered chromatin remodeling complex which lacks a specific tumor suppressor. This altered protein complex then binds the Sox2 locus, reverses repression, resulting in activation of the transcription factor, Sox2, which promotes self-renewal of stem cells without inhibition. Detection of the translocation in tumor cells using a SS18 probe is important for confirming the diagnosis and determining the prognosis of a synovial sarcoma.
