The SS18-SSX Oncoprotein Hijacks KDM2B-PRC1.1 to Drive Synovial Sarcoma

Synovial sarcoma is an aggressive cancer invariably associated with a chromosomal translocation involving genes encoding the SWI-SNF complex component SS18 and an SSX (SSX1 or SSX2) transcriptional repressor. Using functional genomics, we identify KDM2B, a histone demethylase and component of a non-canonical polycomb repressive complex 1 (PRC1.1), as selectively required for sustaining synovial sarcoma cell transformation. SS18-SSX1 physically interacts with PRC1.1 and co-associates with SWI/SNF and KDM2B complexes on unmethylated CpG islands. Via KDM2B, SS18-SSX1 binds and aberrantly activates expression of developmentally regulated genes otherwise targets of polycomb-mediated repression, which is restored upon KDM2B depletion, leading to irreversible mesenchymal differentiation. Thus, SS18-SSX1 deregulates developmental programs to drive transformation by hijacking a transcriptional repressive complex to aberrantly activate gene expression.

Identification of cytotoxic agents disrupting synovial sarcoma oncoprotein interactions by proximity ligation assay

Conventional cytotoxic therapies for synovial sarcoma provide limited benefit. Drugs specifically targeting the product of its driver translocation are currently unavailable, in part because the SS18-SSX oncoprotein functions via aberrant interactions within multiprotein complexes. Proximity ligation assay is a recently-developed method that assesses protein-protein interactions in situ. Here we report use of the proximity ligation assay to confirm the oncogenic association of SS18-SSX with its co-factor TLE1 in multiple human synovial sarcoma cell lines and in surgically-excised human tumor tissue. SS18-SSX/TLE1 interactions are disrupted by class I HDAC inhibitors and novel small molecule inhibitors. This assay can be applied in a high-throughput format for drug discovery in fusion-oncoprotein associated cancers where key effector partners are known.

Death by HDAC Inhibition in Synovial Sarcoma Cells

Conventional cytotoxic therapies for synovial sarcoma provide limited benefit, and no drugs specifically targeting the causative SS18-SSX fusion oncoprotein are currently available. Histone deacetylase (HDAC) inhibition has been shown in previous studies to disrupt the synovial sarcoma oncoprotein complex, resulting in apoptosis. To understand the molecular effects of HDAC inhibition, RNA-seq transcriptome analysis was undertaken in six human synovial sarcoma cell lines. HDAC inhibition induced pathways of cell-cycle arrest, neuronal differentiation, and response to oxygen-containing species, effects also observed in other cancers treated with this class of drugs. More specific to synovial sarcoma, polycomb group targets were reactivated, including tumor suppressor CDKN2A, and proapoptotic transcriptional patterns were induced. Functional analyses revealed that ROS-mediated FOXO activation and proapoptotic factors BIK, BIM, and BMF were important to apoptosis induction following HDAC inhibition in synovial sarcoma. HDAC inhibitor pathway activation results in apoptosis and decreased tumor burden following a 7-day quisinostat treatment in the Pten^fl/fl;hSS2 mouse model of synovial sarcoma. This study provides mechanistic support for a particular susceptibility of synovial sarcoma to HDAC inhibition as a means of clinical treatment. Mol Cancer Ther; 16(12); 2656-67. ©2017 AACR.

HDAC and Proteasome Inhibitors Synergize to Activate Pro-Apoptotic Factors in Synovial Sarcoma

Conventional cytotoxic therapies for synovial sarcoma provide limited benefit, and no drugs specifically targeting its driving SS18-SSX fusion oncoprotein are currently available. Patients remain at high risk for early and late metastasis. A high-throughput drug screen consisting of over 900 tool compounds and epigenetic modifiers, representing over 100 drug classes, was undertaken in a panel of synovial sarcoma cell lines to uncover novel sensitizing agents and targetable pathways. Top scoring drug categories were found to be HDAC inhibitors and proteasomal targeting agents. We find that the HDAC inhibitor quisinostat disrupts the SS18-SSX driving protein complex, thereby reestablishing expression of EGR1 and CDKN2A tumor suppressors. In combination with proteasome inhibition, HDAC inhibitors synergize to decrease cell viability and elicit apoptosis. Quisinostat inhibits aggresome formation in response to proteasome inhibition, and combination treatment leads to elevated endoplasmic reticulum stress, activation of pro-apoptotic effector proteins BIM and BIK, phosphorylation of BCL-2, increased levels of reactive oxygen species, and suppression of tumor growth in a murine model of synovial sarcoma. This study identifies and provides mechanistic support for a particular susceptibility of synovial sarcoma to the combination of quisinostat and proteasome inhibition.