PAX3-FOXO1 is essential for initiation but not for recurrence during rhabdomyosarcoma tumorigenesis

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Authors

• PR Pandey
• SM Hewitt
• MM Miettinen
• FG Barr

Abstract

The PAX3-FOXO1 (P3F) fusion gene is generated by a 2;13 chromosomal translocation in rhabdomyosarcoma (RMS), and is associated with aggressive behavior and poor prognosis. This study utilizes a novel inducible expression system in human myoblasts to dissect the molecular mechanism of P3F in RMS tumorigenesis. An immortalized human myoblast cell line was generated that constitutively expresses MYCN and/or a doxycycline inducible-P3F. Transformation in vitro and tumorigenesis in vivo were assessed by focus formation and xenograft experiments, respectively. Myogenic differentiation was assessed by detecting muscle-specific protein expression. P3F-transduced myoblasts treated with doxycycline demonstrated a time- and dose-dependent increase in P3F expression. Though myoblasts expressing P3F or MYCN alone did not induce transformation, combined P3F and MYCN expression resulted in myoblast transformation. Furthermore, combined P3F and MYCN expression inhibited differentiation. Intramuscular injection of myoblasts with MYCN and P3F resulted in rapid RMS tumor formation. Myoblasts with MYCN expression alone did not form any tumors while P3F resulted in tumors after a much longer latency period. Down-regulation of P3F expression by doxycycline withdrawal resulted in tumor regression associated with cell death and myogenic differentiation. The regressed tumors slowly grew back in the absence of doxycycline demonstrating a P3F-independent oncogenic mechanism for recurrence. In summary, P3F collaborated with MYCN in the initial stage of tumorigenesis to promote dysregulated cell proliferation and inhibit differentiation. Though most cells in the initial tumor were dependent on the fusion protein, recurrent tumors formed in which the fusion protein was not required to maintain the tumorigenic phenotype.

Scientific Focus Area: Cancer Biology

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