Abstract 1939: Loss of MST/Hippo signaling promotes tumorigenesis in a genetically engineered mouse model of fusion-positive alveolar rhabdomyosarcoma

Abstract

Abstract While improvement in survival for pediatric cancer patients over the last 40 years has been encouraging, certain cancer types evade cure. One such example is fusion-positive alveolar rhabdomyosarcoma (aRMS), a pediatric soft tissue sarcoma of mesenchymal origin with skeletal muscle features and a 5-year survival rate of <50%. A hallmark of this aggressive malignancy is the t{2;13} translocation fusion gene PAX3-FOXO1 (PF). Primary cell-based modeling experiments in our laboratory and others have shown that PF is necessary but not sufficient for aRMS tumorigenesis, indicating additional molecular alterations are required to initiate and sustain tumor growth. Previously we showed that PF-positive aRMS tumorigenesis relies on alterations in Hippo pathway signaling, including upregulation of the YAP transcriptional co-activator, and downregulation of the MST/Hippo kinase, which occurs when the scaffold protein RASSF4 (itself a PF transcriptional target) binds and inhibits MST1/Hippo. We hypothesized that ablating MST/Hippo signaling in an existing genetically engineered mouse model (GEMM) of aRMS would accelerate tumorigenesis and provide insight into the role of this pathway in aRMS. To assess the role of MST/Hippo signaling in aRMS, MST1/2-floxed (Stk3F/F;Stk4F/F) mice were crossed with an established MSTWT aRMS GEMM driven by conditional expression of Pax3-Foxo1 from the endogenous Pax3 locus and conditional loss of Cdkn2a in Myf6-expressing cells. Statistical analysis revealed that compared to MSTWT aRMS control animals, MSTNull have significantly accelerated tumorigenesis (median survival 112 vs. 224 days, p< 0.0001) and increased tumor penetrance (76% vs. 27%). MSTNull animals developed tumors disproportionately in the head and neck as compared to control, and incurred multiple tumors per animal. Tumors were analyzed via immunohistochemistry for MyoD and myogenin, markers of RMS, as well as Myf5, an early marker of myogenesis. Tumors in both cohorts were for positive for aRMS markers. Tumor-derived cell lines were used for in vitro cell-based assays and molecular interrogation. We have identified the MST/Hippo signaling axis as an important tumor suppressor mechanism in aRMS. The rapid onset and increased penetrance of tumorigenesis in this GEMM provides a powerful tool for interrogating aRMS biology and screening novel therapeutics. Citation Format: Kristianne Oristian, Lisa E. Crose, Rex Bentley, Nina Kuprasertkul, David G. Kirsch, Corinne Linardic. Loss of MST/Hippo signaling promotes tumorigenesis in a genetically engineered mouse model of fusion-positive alveolar rhabdomyosarcoma [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 1939. doi:10.1158/1538-7445.AM2017-1939