Project Summary: Synovial Sarcoma (SS) is driven by the SS18-SSX oncofusion, and SS18-SSX is the only reoccurring mutation in SS. SS normally metastasizes, resulting in a 15-year overall survival rate of less than 50%. This presents a particular problem as 1/3 of the patients that are diagnosed are under the age of 30. Polychemotherapy has a modest and variable effect on patients, immunotherapy activity is unremarkable, and there are currently no targeted therapy options to combat SS. SS18-SSX remains undruggable despite clearly being the driving event in these cancers. Thus, SS requires entirely new therapeutic approaches. In light of this, we tried to identify potential novel therapies by assessment of full genome short-interfering (si)-RNA screen data deposited into the DepMap database and subsequent cell culture experiments with SS cell lines and patient-derived xenograft cell cultures. Through these efforts, we have identified a clinically actionable synthetic lethality with SSX-SS18 in SS, namely disruption of the post-translational modification, SUMOylation. We find SS18-SSX activates the SUMOylation, and disruption of this pathway with the in-clinic SUMOylation inhibitor, TAK-981, disrupts SS18- SSX function, induces DNA damage and shrinks SS tumors in mice. Specific Aims Specific Aim 1: Test a diverse set of synovial sarcoma mouse models for efficacy and safety of SUMOylation inhibition Specific Aim 2: Investigate the relationship between SS18-SSX and the SUMOylated proteome in synovial sarcoma Study Design: We will further characterize the sensitivity of TAK-981 (alone and with chemotherapy and BRD9 degrader) in SS cell culture models including patient-derived organoids, and in vivo, in orthotopic patient-derived xenograft (PDX) models and genetic mouse models of SS. Through a series of proteomic and biochemical experiments, we will further categorize the mechanism of SS18-SSX-dependent toxicity stemming from disruption of the SUMOylation pathway. We will further examine the role of SS18-SSX in activating the SUMOylation pathway and the mechanism of action of TAK-981 in SS, which involves disrupting the SS18-SSX- ncBAF transcriptome. In all, we will attempt to gather the preclinical evidence supporting the translation of SUMOylation inhibitors to treat SS.