Human EWS-FLI protein levels and neomorphic functions show a complex, function-specific dose-response relationship in .

Ewing sarcoma (EwS) is a cancer that arises in the bones and soft tissues, typically driven by the Ewing's sarcoma breakpoint region 1-Friend leukemia virus integration 1 (EWS-FLI) oncogene. Implementation of genetically modified animal models of EwS has proved difficult largely owing to EWS-FLI's high toxicity. The EWS-FLI frameshift variant that circumvents toxicity but is still able to perform key oncogenic functions provided the first study model in . However, the quest for lines expressing full-length, unmodified EWS-FLI remained open. Here, we show that EWS-FLI's lower toxicity is owed to reduced protein levels caused by its frameshifted C-terminal peptide, and report new strategies through which we have generated lines that express full-length, unmodified EWS-FLI. Using these lines, we have found that the upregulation of transcription from GGAA-microsatellites (GGAAμSats) presents a positive linear correlation within a wide range of EWS-FLI protein concentrations. In contrast, rather counterintuitively, GGAAμSats-independent transcriptomic dysregulation presents relatively minor differences across the same range, suggesting that GGAAμSat-dependent and -independent transcriptional upregulation present different kinetics of response with regards to changing EWS-FLI protein concentration. Our results underpin the functional relevance of varying EWS-FLI expression levels and provide experimental tools to investigate, in , the effect of the EWS-FLI 'high' and 'low' states that have been reported and are suspected to be important for EwS in humans.