Role of Myc family proteins in transcriptional regulation of growth and oncogenic transformation in fusion-positive rhabdomyosarcoma

Fusion-positive rhabdomyosarcoma (FP-RMS) is driven by a PAX3::FOXO1 (P3F) or PAX7::FOXO1 fusion gene. After finding that MYCN is required for P3F-induced oncogenic transformation in a human myoblast model of FP-RMS, we further investigated the role of Myc family proteins in FP-RMS. Expression studies revealed that myoblast models and a subset of FP-RMS lines have predominant MYCN or MYC expression whereas other FP-RMS lines have both high MYCN and MYC expression. In myoblast models, MYCN was required for optimal P3F binding to and high-level activation of FGF8, a P3F target that is necessary and sufficient for oncogenic transformation. MYCN or MYC knockdown suppressed transformation in myoblast and FP-RMS lines with dominant MYCN or MYC expression, respectively. In FP-RMS lines with high MYCN and MYC expression, there was partial loss of transformation when one gene was targeted and complete loss when both genes were targeted. Despite the loss of oncogenic activity in lines with knockdown of a dominant Myc family member, FGF8 expression was not decreased. Transcriptomic analyses revealed that P3F target genes were not affected by MYCN or MYC knockdown, and instead a group of Myc-specific targets was down-regulated. Collectively, these results indicate that MYCN or MYC is functionally dominant in myoblast models and FP-RMS lines with dominant expression of one Myc family protein whereas MYCN and MYC are functionally redundant in FP-RMS cell lines with high expression of both. These Myc family proteins contribute to oncogenic properties by facilitating P3F activation of FGF8 and increasing expression of Myc-specific targets.