The Drosophila FET ortholog Cabeza is an essential cofactor for ETV4-mediated activation of GGAA microsatellite neoenhancers

Abstract

The conversion of transcriptionally silent GGAA microsatellites (GGAAµSats) into functional enhancers by FET::ETS oncogenic fusions is a hallmark of Ewing sarcoma. However, emerging evidence implicates nonfused, full-length oncogenic E26 transformation-specific (ETS) transcription factors in activating these repeats in other malignancies. Evaluating the in vivo transcriptional requirements of various human ETS factors in Drosophila, we found that human ETS variant transcription factor 4 (ETV4) associates with and robustly activates GGAAµSats in a tissue-specific manner. This activation is strongly inhibited by the human ETS repressor ETS variant transcription factor 6 (ETV6). Taking advantage of low genetic redundancy in Drosophila, we identified Cabeza (Caz), the single fly FUS/EWSR1/TAF15 family (FET) ortholog, as a necessary cofactor for ETV4-mediated transcription at GGAAµSats. Conversely, EWS::FLI1-mediated transcriptional activation of GGAAµSats is entirely independent of endogenous Caz, highlighting the distinct mechanics of covalent tethering vs noncovalent physical complexes. Collectively, our findings provide in vivo evidence that nonfused ETS factors cooperate with endogenous FET proteins to drive transcription from silent GGAA repeats, mechanistically validating this regulatory transformation known to operate as an oncogenic mechanism beyond Ewing sarcoma.