Reversing the oncogenic roles of misdirected chromatin remodeling: Mechanistic insights into the SS18-SSX fusion protein in synovial sarcoma.

Abstract

10515 Background: Synovial sarcoma (SS) accounts for ~10% of soft-tissue malignancies and is generally resistant to chemotherapy-based approaches, underscoring the need for a mechanistic understanding of its pathogenesis and the development of disease-specific biologic agents. The hallmark molecular feature is a precise and uniform translocation, t(X;18), which results in the fusion of exactly 78 amino acids of SSX to the SS18 C-terminus. Because the SS18-SSX genetic lesion is observed in 100% of cases, it is likely the driving oncogenic event in these tumors; however, the molecular basis for its role in oncogenesis is undefined. Methods: We performed an affinity purification-/mass spectrometry-based analysis of endogenous mSWI/SNF (BAF) chromatin remodeling complexes in several primary cell types. Using these data in combination with protein biochemical methods, we discovered that SS18 is a dedicated, non-exchangeable subunit of these complexes with a binding affinity comparable to that of ribosomal subunits. Subsequent biochemical and functional investigations were performed to assess the oncogenic consequences of addition of 78aa of SSX to the SS18 subunit in SS. Results: We demonstrate that the SS18-SSX fusion incorporates into BAF complexes, evicting both wild-type (WT) SS18 and the tumor suppressor subunit, hSNF5 (BAF47), known to be biallelically inactivated in pediatric malignant rhabdoid tumors (MRTs). The altered complex binds the Sox2 locus, reversing polycomb-mediated repression and activating Sox2. Sox2, a pro-pluripotency transcription factor, is uniformly expressed in SS tumors and is essential for proliferation. Remarkably, increasing the concentration of WT SS18 leads to reassembly of WT complexes, retargeting of BAF complexes, returned polycomb-mediated repression at the Sox2 locus and cessation of SS cell proliferation. This mechanism of transformation depends on a region of only two amino acids of SSX, and hence provides a strong foundation for therapeutic intervention. Conclusions: These studies provide a novel oncogenic mechanism for SS tumors and inform strategies for therapeutic development in this intractable cancer.