Abstract A035-PR003: EWS::FLI1 Expression in Human Embryonic Mesenchymal Stem Cells Leads to Transcriptional Reprograming, Defective DNA Damage Repair and Ewing Sarcoma

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. We have generated MSCs from experimental teratomas out of human embryonic stem cells. EWS::FLI1 expression in these human embryonic mesenchymal stem cells (heMSCs) results in the acquisition of an ES transcriptome, with the oncogene not preferentially binding to promoters, but to intronic -with >10 CA dinucleotides- and intergenic microsatellites with GGAA repeats. In heMSCs, EWS::FLI1 directly regulates BRCA1 expression, even though EWS::FLI1-expressing cells show defects in DNA damage repair. Xenografting of EWS::FLI1-transduced heMSCs resulted 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. Citation Format: Inmaculada Hernandez, Irene cuervas, Estela prada, Angel Raya, Jaume Mora. EWS::FLI1 Expression in Human Embryonic Mesenchymal Stem Cells Leads to Transcriptional Reprograming, Defective DNA Damage Repair and Ewing Sarcoma [abstract]. In: Proceedings of the AACR Special Conference in Cancer Research: Discovery and Innovation in Pediatric Cancer— From Biology to Breakthrough Therapies; 2025 Sep 25-28; Boston, MA. Philadelphia (PA): AACR; Cancer Res 2025;85(18_Suppl_2):Abstract nr A035-PR003.