EWS‐FLI1 regulates a transcriptional program in cooperation with Foxq1 in mouse Ewing sarcoma

EWS‐FLI1 constitutes an oncogenic transcription factor that plays key roles in Ewing sarcoma development and maintenance. We have recently succeeded in generating an ex vivo mouse model for Ewing sarcoma by introducing EWS‐FLI1 into embryonic osteochondrogenic progenitors. The model well recapitulates the biological characteristics, small round cell morphology, and gene expression profiles of human Ewing sarcoma. Here, we clarified the global DNA binding properties of EWS‐FLI1 in mouse Ewing sarcoma. GGAA microsatellites were found to serve as binding sites of EWS‐FLI1 albeit with less frequency than that in human Ewing sarcoma; moreover, genomic distribution was not conserved between human and mouse. Nevertheless, EWS‐FLI1 binding sites within GGAA microsatellites were frequently associated with the histone H3K27Ac enhancer mark, suggesting that EWS‐FLI1 could affect global gene expression by binding its target sites. In particular, the Fox transcription factor binding motif was frequently observed within EWS‐FLI1 peaks and Foxq1 was identified as the cooperative partner that interacts with the EWS portion of EWS‐FLI1. Trib1 and Nrg1 were demonstrated as target genes that are co‐regulated by EWS‐FLI1 and Foxq1, and are important for cell proliferation and survival of Ewing sarcoma. Collectively, our findings present novel aspects of EWS‐FLI1 function as well as the importance of GGAA microsatellites.