Endogenous degron alleles enable tunable control of EWSR1-FLI1 oncoprotein expression

Pediatric malignancies frequently harbor chromosomal translocations that induce expression of fusion oncoproteins. The EWSR1-FLI1 fusion oncoprotein acts as a neomorphic transcription factor and is the dominant genetic driver of Ewing's sarcoma. However, interrogation of the mechanisms by which EWSR1-FLI1 drives tumorigenesis has been limited by a lack of model systems to precisely and selectively control its expression in patient-derived cell lines and xenografts. Here, we report the generation of a panel of patient-derived EWS cell lines in which inducible protein degrons have been engineered into the endogenous EWSR1-FLI1 locus. These systems enable precise control of EWSR1-FLI1 expression and the timing of oncoprotein depletion. We establish that complete suppression of EWSR1-FLI1 leads to a reversible cell cycle arrest at the G1-S phase transition, and we identify a core set of transcripts downstream of EWSR1-FLI1 across multiple cell lines and degron systems. Additionally, depletion of EWSR1-FLI1 using this system potently suppresses tumor growth in xenograft models validating efforts to directly target EWSR1-FLI1 in Ewing's sarcoma. Overall design: CUT&RUN analysis of FLI in a human Ewing Sarcoma cell line (A673). EWS-FLI was either untagged or endogenously tagged with an AID degron. Degron lines were either treated with DMSO or IAA before FLI CUT&RUN.