Abstract 3016: Translocation-dependent activation of WNT signaling is essential in synovial sarcoma cells

Abstract

Abstract Synovial sarcoma is a rare malignant soft tissue tumor affecting mainly adolescents and young adults. The hallmark of synovial sarcoma is the presence of a reciprocal balanced t(X;18) translocation, leading to the fusion of the SS18 gene to either the SSX1, SSX2 or rarely the SSX4 gene, resulting in a chimeric transcriptional modifier. Therapeutic outcome of synovial sarcomas is primarily determined by the efficiency of surgery as a high tendency for local relapse is documented. Standardized chemo- and radiotherapy are further therapeutic options, however, specific targeted therapies are currently not available. Recently, several expression profiling studies in mesenchymal malignancies revealed gene expression signatures indicating WNT signaling activation in synovial sarcomas. This study was performed to examine the functional relevance of WNT signaling in synovial sarcomas and to evaluate if interference with the WNT signaling pathway might represent an option in the development of novel and highly selective drugs in the treatment of synovial sarcoma. To assess the prevalence of WNT signaling activation in a set of 30 synovial sarcoma tumor samples, nuclear staining of beta-catenin was analyzed immunohistochemically. Nuclear beta-catenin signals were observed in a significant subset of these tumors, indicating activation of the WNT signaling pathway. In order to evaluate whether WNT activation is molecularly dependent on the SS18/SSX fusion proteins, tetracycline-inducible systems overexpressing the SS18/SSX fusion proteins were established in T-Rex293 cells. In luciferase reporter assays employing the TOP-/FOPflash system, expression of SS18/SSX proteins effectively activated TCF/beta-catenin mediated transcriptional activity, which was associated with nuclear recruitment of beta-catenin. Five human synovial sarcoma cell lines were subsequently treated with small molecular inhibitors of WNT signalling. In MTT assays, a significant dose-dependent inhibition of cellular growth was observed, which was accompanied by decreased expression of the WNT downstream targets c-Myc and Cyclin D1. In flow cytometric analyses, the growth effects exerted by the inhibitors were shown to be due to a reduction of cellular proliferation combined with an increase of apoptosis. In summary, our data emphasize the pivotal role of WNT signaling in synovial sarcoma and indicate its functional dependence on the characteristic SS18/SSX translocations. Furthermore, our study demonstrates that targeting the WNT signaling pathway provides a specific, molecularly founded therapeutic strategy in the treatment of synovial sarcoma. Additional functional studies in vitro and in vivo are required to further understand the role of WNT signaling and its therapeutic applicability in synovial sarcomas. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 3016. doi:1538-7445.AM2012-3016