SS18-SSX-dependent YAP/TAZ Signaling in Synovial Sarcoma

Purpose: Synovial sarcoma is a soft tissue malignancy characterized by a reciprocal t(X;18) translocation. The chimeric SS18-SSX fusion protein acts as a transcriptional dysregulator representing the major driver of the disease; however, the signaling pathways activated by SS18-SSX remain to be elucidated in order to define innovative therapeutic strategies. Experimental Design: Immunohistochemical evaluation of the Hippo signaling pathway effectors YAP/TAZ was performed in a large cohort of synovial sarcoma tissue specimens. SS18‑SSX dependency and biological function of the YAP/TAZ Hippo signaling cascade were analyzed in five synovial sarcoma cell lines and a mesenchymal stem cell model in vitro . YAP/TAZ‑TEAD‑mediated transcriptional activity was modulated by RNAi‑mediated knockdown and the small molecule inhibitor verteporfin.The effects of verteporfin were finally tested in vivo in synovial sarcoma cell line-based avian chorioallantoic membrane and murine xenograft models and a patient-derived xenograft. Results: A significant subset of synovial sarcoma showed nuclear positivity for YAP/TAZ and their transcriptional targetsFOXM1 and PLK1. In synovial sarcoma cells, RNAi‑mediated knockdown of SS18-SSX led to significant reduction of YAP/TAZ-TEAD transcriptional activity. Conversely, SS18-SSX overexpression in SCP-1 cells induced aberrant YAP/TAZ‑dependent signals, mechanistically mediated by an IGF-II/IGF-IR loop leading to dysregulation of the Hippo effectors LATS1 and MOB1. Modulation of YAP/TAZ‑TEAD-mediated transcriptional activity by RNAi or verteporfin treatment resulted in significant growth inhibitory effects in vitro and in vivo. Conclusions: Our preclinical study identifies an elementary role of SS18‑SSX‑driven YAP/TAZ signals, highlights the complex network of oncogenic signaling pathways in synovial sarcomapathogenesis and provides evidence for innovative therapeutic approaches.