Loss of MST/Hippo Signaling in a Genetically Engineered Mouse Model of Fusion-Positive Rhabdomyosarcoma Accelerates Tumorigenesis

A hallmark of fusion-positive alveolar rhabdomyosarcoma (aRMS) is the presence of a chromosomal translocation encoding the PAX3–FOXO1 fusion oncogene. Primary cell-based modeling experiments have shown that PAX3–FOXO1 is necessary, but not sufficient for aRMS tumorigenesis, indicating additional molecular alterations are required to initiate and sustain tumor growth. Previously, we showed that PAX3–FOXO1-positive aRMS is promoted by dysregulated Hippo pathway signaling, as demonstrated by increased YAP1 expression and decreased MST activity. We hypothesized that ablating MST/Hippo signaling in a genetically engineered mouse model (GEMM) of aRMS would accelerate tumorigenesis. To this end, MST1/2-floxed (Stk3F/F;Stk4F/F) mice were crossed with a previously established aRMS GEMM driven by conditional expression of Pax3:Foxo1 from the endogenous Pax3 locus and conditional loss of Cdkn2a in Myf6 (myogenic factor 6)-expressing cells. Compared with Pax3PF/PF;Cdkn2aF/F;Myf6ICN/+ controls, Stk3F/F;Stk4F/F;Pax3PF/PF;Cdkn2aF/F;Myf6ICN/+ animals displayed accelerated tumorigenesis (P < 0.0001) and increased tumor penetrance (88% vs. 27%). GEMM tumors were histologically consistent with aRMS. GEMM tumor-derived cell lines showed increased proliferation and invasion and decreased senescence and myogenic differentiation. These data suggest that loss of MST/Hippo signaling acts with Pax3:Foxo1 expression and Cdkn2a loss to promote tumorigenesis. The rapid onset and increased penetrance of tumorigenesis in this model provide a powerful tool for interrogating aRMS biology and screening novel therapeutics.Significance: A novel mouse model sheds light on the critical role of Hippo/MST downregulation in PAX3-FOXO1–positive rhabdomyosarcoma tumorigenesis. Cancer Res; 78(19); 5513–20. ©2018 AACR.

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