TRIM8 modulates the EWS/FLI oncoprotein to promote survival in Ewing sarcoma

Fusion-transcription factors (fusion-TFs) represent a class of driver oncoproteins that are difficult to therapeutically target. Recently, protein degradation has emerged as a strategy to target these challenging oncoproteins. The mechanisms that regulate fusion-TF stability, however, are generally unknown. Using CRISPR-Cas9 screening, we discovered tripartite motif-containing 8 (TRIM8) as an E3 ubiquitin ligase that ubiquitinates and degrades EWS/FLI, a driver fusion-TF in Ewing sarcoma. Moreover, we identified TRIM8 as a selective dependency in Ewing sarcoma compared with >700 other cancer cell lines. Mechanistically, TRIM8 knockout led to an increase in EWS/FLI protein levels that was not tolerated. EWS/FLI acts as a neomorphic substrate for TRIM8, defining the selective nature of the dependency. Our results demonstrate that fusion-TF protein stability is tightly regulated and highlight fusion oncoprotein-specific regulators as selective therapeutic targets. This study provides a tractable strategy to therapeutically exploit oncogene overdose in Ewing sarcoma and potentially other fusion-TF-driven cancers. [Display omitted] •The E3 ligase TRIM8 is an exquisitely selective dependency in Ewing sarcoma•TRIM8 regulates EWS/FLI expression but not its wild-type counterparts•K334 is critical for TRIM8-mediated degradation of EWS/FLI•Increased EWS/FLI levels induce cell death in Ewing sarcoma cells Using CRISPR-Cas9 screens, Seong et al. demonstrate that the E3 ligase TRIM8 degrades the EWS/FLI fusion oncoprotein in Ewing sarcoma cells. Knockout of TRIM8 is selectively lethal in Ewing sarcoma compared with >700 non-Ewing cancer models. TRIM8 loss increases EWS/FLI protein levels, which is toxic to Ewing sarcoma cells.