PAX3–FOXO1 promotes aRMS tumor growth via induction of RASSF4 and inhibition of Hippo signaling.

  • Major finding: PAX3–FOXO1 promotes aRMS tumor growth via induction of RASSF4 and inhibition of Hippo signaling.

  • Mechanism: MST1 inhibition by RASSF4 and YAP upregulation stimulate proliferation and suppress senescence.

  • Impact: Targeting of RASSF4 or other regulators of the Hippo pathway may limit aRMS growth.

Expression of the paired box 3 (PAX3)–forkhead box O1 (FOXO1) oncogenic fusion gene is common in the alveolar subtype of rhabdomyosarcoma (aRMS) and is associated with poor prognosis. Recent studies have shown that the PAX3–FOXO1 protein facilitates escape from cellular senescence; however, the molecular mechanisms by which this fusion transcription factor promotes aRMS tumorigenesis remain poorly understood. Crose and colleagues found that expression of RAS association domain family member 4 (RASSF4) was upregulated in PAX3–FOXO1-positive aRMS cell lines and tumors. RASSF4 was directly induced by PAX3–FOXO1 via regulation of a 5′ enhancer region, and elevated expression of RASSF4 was correlated with decreased patient survival. Depletion of RASSF4 triggered cellular senescence and cell-cycle arrest in PAX3–FOXO1-expressing aRMS cells and impaired aRMS xenograft tumor growth, supporting a role for RASSF4 in promoting PAX3–FOXO1-driven tumorigenesis. RASSF4-mediated stimulation of cell proliferation and inhibition of senescence were dependent in part on its interaction with mammalian sterile 20-like kinase 1 (MST1, also known as STK4), a tumor-suppressive Hippo pathway kinase, and subsequent inhibition of downstream MST1 signaling through MOB1 in aRMS cells, suggesting that PAX3–FOXO1/RASSF4 suppresses Hippo signaling in aRMS. In addition, the transcriptional coactivator Yes-associated protein 1 (YAP1), which is negatively regulated by the Hippo kinase cascade and induces the expression of proproliferative and prosurvival genes, was upregulated in both aRMS cell lines and tumors compared with normal muscle samples. Although additional studies are necessary to determine the connection between RASSF4 and YAP1 regulation, these results identify important roles for RASSF4 and dysregulation of Hippo signaling in PAX3–FOXO1-driven aRMS and suggest that targeting of the Hippo pathway may be therapeutically beneficial in patients with this disease.

Crose LE, Galindo KA, Kephart JG, Chen C, Fitamant J, Bardeesy N, et al. Alveolar rhabdomyosarcoma-associated PAX3-FOXO1 promotes tumorigenesis via Hippo pathway suppression. J Clin Invest 2014;124:285–96.

©2014 American Association for Cancer Research.
2014