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

Abstract

Summary 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.

Pioneer factors in development and cancer

“…Evidence suggests that core circuitry established by CEBPα is suppressed at the chromatin level by AML-ETO fusions. Interestingly, CEBPα deletion is toxic in AML, suggesting that maintaining the right balance of pioneer activity is essential, as has been evidenced in sarcomas ( Seong et al., 2021 ) ( Figure 3 ). Expression of the CBFb-MYH11 chimera during early development restricts myeloid lineage specification ( Castilla et al., 1996 ).…”

Neomorphic DNA-binding enables tumor-specific therapeutic gene expression in fusion-addicted childhood sarcoma

“…Although EF1 and P3F1 would represent ideal therapeutic targets, oncogenic fusion transcription factors typically lack enzymatic activity and are as such often considered to be undruggable 56 . Importantly, accumulating evidence in EwS suggests that low levels of EF1 expression (aka activity) may rather foster metastasis than acting tumor suppressive, possibly rendering approaches that aim at therapeutically lowering the fusion oncogenes activity futile [57][58][59][60] . In stark contrast, our approach aims at exploiting the neomorphic DNA-binding properties of chimeric fusion-oncoproteins for tumor-specific therapeutic gene expression.…”

HOXD13 is a direct EWS-FLI1 target and moderates fusion-dependent transcriptional states

“…For example, while local tumor growth is reliant on continued expression of the fusion, migratory and metastatic properties of EwS cells rely on acquisition of an EWS-FLI1-low state (51, 55, 58, 59). In addition, too much EWS-FLI1 activity is toxic and leads to cell death (57). These observations have led to the premise that EwS cells adhere to the “Goldilocks principle”: they require a dose of oncogene that is “just right” (57).…”

“…In addition, too much EWS-FLI1 activity is toxic and leads to cell death (57). These observations have led to the premise that EwS cells adhere to the “Goldilocks principle”: they require a dose of oncogene that is “just right” (57). Our current studies have for the first time identified HOXD13 as a key tumor cell-autonomous factor that contributes to maintaining the “just right” level of EWS-FLI1 oncogene activity.…”

“…Recent studies have shown that successful propagation and metastatic progression of EwS tumors depends on maintaining precise levels of EWS-FLI1 expression and transcriptional activity (57). Moreover, the critical level for tumor growth and progression is dynamic and likely differs at different stages of tumor evolution.…”