Rhabdomyosarcoma (RMS) is the most common pediatric
soft tissue sarcoma and outcomes have stagnated, highlighting a need for novel
therapies. Genomic analysis of RMS has revealed that alterations in the
receptor tyrosine kinase (RTK)/RAS/PI3K axis are common and that FGFR4 is frequently mutated or overexpressed. Although FGFR4 is a potentially druggable
receptor tyrosine kinase, its functions in RMS are undefined. This study tested FGFR4-activating mutations and overexpression for the ability to generate RMS in mice. Murine
tumor models were subsequently used to discover potential therapeutic targets and to test a dual PI3K/mTOR inhibitor in a preclinical setting. Specifically, we provide the first mechanistic evidence of differential potency in the most common human RMS mutations, V550E or N535K, compared to FGFR4wt overexpression as murine myoblasts expressing FGFR4V550E undergo higher rates of cellular transformation, engraftment into mice, and rapidly form
sarcomas that highly resemble human RMS. Murine
tumor cells overexpressing FGFR4V550E were tested in an in vitro dose-response drug screen along with human RMS cell lines. Compounds were grouped by target class, and potency was determined using average percentage of area under the dose-response curve (AUC). RMS cells were highly sensitive to PI3K/
mTOR inhibitors, in particular,
GSK2126458 (
omipalisib) was a potent inhibitor of FGFR4V550E
tumor-derived cell and human RMS cell viability. FGFR4V550E-overexpressing myoblasts and
tumor cells had low nanomolar
GSK2126458 EC50 values. Mass cytometry using mouse and human RMS cell lines validated
GSK2126458 specificity at single-cell resolution, decreasing the abundance of phosphorylated Akt as well as decreasing phosphorylation of the downstream mTOR effectors 4ebp1,
Eif4e, and S6. Moreover, PI3K/mTOR inhibition also robustly decreased the growth of RMS
tumors in vivo. Thus, by developing a preclinical platform for testing novel
therapies, we identified PI3K/mTOR inhibition as a promising new
therapy for this devastating pediatric
cancer.