Molecular mechanisms of the muscle differentiation blockade in Rhabdomyosarcoma

Background: Rhabdomyosarcomas (RMS) are common pediatric soft tissue tumors, showing skeletal muscle features, unsatisfying responses to current therapies and poor outcome that reflects the limited knowledge of RMS’ pathogenesis. Since WNT signaling impacts skeletal muscle development, our preliminary expression study addressed this pathway in RMS cell lines, revealing overexpression of WNT5A (i.e. a ligand that activates the non-canonical branch of the WNT pathway) in the less aggressive RME. Therefore, the current study aimed to assess the significance of WNT5A in RMS. Methods: We analyzed WNT5A as well as different WNT pathway receptors and coreceptors expression in different RMS cell lines, performed functional studies (proliferation, migration, invasion and sphere formation assays) of various representative RMS cell lines following stable WNT5A knock down (KD) and overexpression (OE) in vitro and established a cell line based RMS xenograft model to validate the impact of WNT5A on RMS proliferation in vivo. Results: We found that WNT5A KD increases migration and self-renewal capacity of RMS cell lines and suppresses muscle differentiation markers in addition to inducing proliferation and growth of ERMS both in vitro and in vivo. These effect were associated with an upregulation of active β-catenin and related target genes, and with inhibition of non-canonical WNT signaling pathways. WNT5A OE shows opposite effects. Conclusion: Our results suggest that WNT5A has tumor suppressor effect in RMS by activating the non-canonical WNT pathway and inhibiting the canonical WNT pathway which indicates that WNT5A could be a promising diagnostic and therapeutic target.