Impaired Wnt Signaling in Embryonal Rhabdomyosarcoma Cells from p53/c-fos Double Mutant Mice

Rhabdomyosarcoma is a primitive neoplasm with a poorly understood etiology that exhibits features of fetal skeletal muscle. It represents the most frequent malignant soft tissue sarcoma affecting the pediatric population and is often treated very aggressively. Embryonal rhabdomyosarcoma (ERMS) and alveolar rhabdomyosarcoma constitute the two major subtypes and exhibit different molecular features. We investigated one potential molecular basis for ERMS by using cells derived from tumors produced in p53−/−/c-fos−/− mice. This model closely recapitulates the timing, location, molecular markers, and histology seen in human ERMS. A combined chromatin immunoprecipitation/promoter microarray approach was used to identify promoters bound by the c-Jun-containing AP-1 complex in the tumor-derived cells that lacked c-Fos. Identification of the Wnt2 gene and its overexpression in ERMS cells was confirmed in human rhabdomyosarcoma cell lines and prompted further analysis of the Wnt signaling pathway. Contrary to our expectations, the canonical Wnt/β-catenin signaling pathway was down-regulated in ERMS cells compared with normal myoblasts, and activating this pathway promoted myogenic differentiation. Furthermore, the identification of both survivin and sfrp2 through promoter and expression analyses suggested that increased resistance to apoptosis was associated with the inhibition of the Wnt signaling pathway. These results suggest that altered AP-1 activity that leads to the down-regulation of the Wnt pathway may contribute to the inhibition of myogenic differentiation and resistance to apoptosis in ERMS cases.