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

Abstract

A hallmark of fusion-positive alveolar rhabdomyosarcoma (aRMS) is the presence of a chromosomal translocation encoding the <i>PAX3-FOXO1</i> fusion oncogene. Primary cell-based modeling experiments have shown that <i>PAX3-FOXO1</i> is necessary, but not sufficient for aRMS tumorigenesis, indicating additional molecular alterations are required to initiate and sustain tumor growth. Previously, we showed that <i>PAX3-FOXO1</i>-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 (<i>Stk3^F/F;Stk4^F/F</i> ) mice were crossed with a previously established aRMS GEMM driven by conditional expression of <i>Pax3:Foxo1</i> from the endogenous <i>Pax3</i> locus and conditional loss of <i>Cdkn2a</i> in <i>Myf6</i> (myogenic factor 6)-expressing cells. Compared with <i>Pax3^PF/PF;Cdkn2a^F/F;Myf6^ICN/+</i> controls, <i>Stk3^F/F;Stk4^F/F;Pax3^PF/PF;Cdkn2a^F/F;Myf6^ICN/+</i> animals displayed accelerated tumorigenesis (<i>P</i> < 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 <i>Pax3:Foxo1</i> expression and <i>Cdkn2a</i> 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.<b>Significance:</b> A novel mouse model sheds light on the critical role of Hippo/MST downregulation in PAX3-FOXO1-positive rhabdomyosarcoma tumorigenesis. <i>Cancer Res; 78(19); 5513-20. ©2018 AACR</i>.