Abstract
Ewing sarcoma (ES) is an aggressive bone and soft tissue neoplasm characterized by EWSR::ETS rearrangements whose cellular origin remains unclear. EWS::FLI1 expression in human pediatric mesenchymal stem cells (MSCs) induces a transcriptional response distinct from that of human adult MSCs, but fails to form tumors. Here we show that EWS::FLI1 expression in human embryonic mesenchymal stem cells (heMSCs) results in the acquisition of an ES transcriptome, with the oncogene not preferentially binding to gene promoters, but to intronic and intergenic microsatellites. In heMSCs, EWS::FLI1 directly regulates the expression of the DNA repair protein BRCA1, although cells expressing EWS::FLI1 show DNA damage. Xenografting of EWS::FLI1-transduced heMSCs results in the formation of tumors expressing characteristic ES markers. In summary, we show that EWS::FLI1 enforces an aberrant transcriptome and solely is able to endow transforming capacity when expressed in undifferentiated, early heMSCs.