A rapid one-generation genetic screen in a drosophila model to capture rhabdomyosarcoma effectors and therapeutic targets

Rhabdomyosarcoma (RMS) is an aggressive childhood malignancy of neoplastic musclelineage precursors that fail to terminally differentiate into syncytial muscle. The most aggressive form of RMS, alveolar-RMS, is driven by misexpression of the PAX-FOXO1 oncoprotein, which is generated by recurrent chromosomal translocations that fuse either the PAX3 or PAX7 gene to FOXO1. The molecular underpinnings of PAX-FOXO12mediated RMS pathogenesis remain unclear, however, and clinical outcomes poor. Here, we report a new approach to dissect RMS, exploiting a highly efficient Drosophila PAX7-FOXO1 model uniquely configured to uncover PAX-FOXO1 RMS genetic effectors in only one generation. With this system, we have performed a comprehensive deletion screen against the Drosophila autosomes and demonstrate that mutation of Mef2, a myogenesis lynchpin in both flies and mammals, dominantly suppresses PAX7-FOXO1 pathogenicity and acts as a PAX7-FOXO1 gene target. Additionally, we reveal that mutation of mastermind, a gene encoding a MEF2 transcriptional coactivator, similarly suppresses PAX7-FOXO1, further pointing toward MEF2 transcriptional activity as a PAX-FOXO1 underpinning. These studies show the utility of the PAX-FOXO1 Drosophila system as a robust one-generation (F1) RMS gene discovery platform and demonstrate how Drosophila transgenic conditional expression models can be configured for the rapid dissection of human disease.