Significance
The oncogenic fusion protein, EWS::FLI1, responsible for more than 85% of Ewing sarcoma tumors, combines the transactivation domain from EWS and the DNA-binding domain (DBD) from Friend leukemia integration 1 (FLI1). The fusion impacts the function of wild-type EWS and drives oncogenesis via aberrant transcriptional and splicing changes as well as defects in the DNA-damage response. Both components of the fusion are required for oncogenesis, suggesting a synergistic function between the domains. Here, the authors describe the structural underpinnings of EWS::FLI1’s effect on EWS, mediated by the FLI1 DBD, that enhances the phase separation propensity of EWS and drives aberrant changes to the physical properties of condensates.
Abstract
Ewing sarcoma (EwS) is an aggressive cancer of bone and soft tissue that predominantly affects children and young adults. A chromosomal translocation joins the low-complexity domain (LCD) of the RNA-binding protein EWS (EWSLCD) with the DNA-binding domain of Friend leukemia integration 1 (FLI1DBD), creating EWS::FLI1, a potent fusion oncoprotein essential for EwS development and responsible for over 85% of EwS tumors. EWS::FLI1 forms biomolecular condensates in vivo and promotes tumorigenesis through the mediation of aberrant transcriptional changes and by interfering with the normal functions of nucleic acid-binding proteins like EWS through a dominant-negative mechanism. In particular, the expression of EWS::FLI1 in EwS directly interferes with the biological functions of EWS leading to alternate splicing events and defects in DNA-damage repair pathways. Though the EWSLCD is capable of phase separation, here we report a direct interaction between FLI1DBD and EWSLCD that enhances condensate formation and alters the physical properties of the condensate. This effect was conserved for three related E-twenty-six transformation-specific (ETS) DNA-binding domains (DBDs) while DNA binding blocked the interaction with EWSLCD and inhibited EWS::FLI1 condensate formation. NMR spectroscopy and mutagenesis studies confirmed that ETS DBDs transiently interact with EWSLCD via the ETS DBDs “wings.” Together these results revealed that ETS DBDs, particularly FLI1DBD, enhance EWSLCD condensate formation and rigidity, supporting a model in which electrostatic and structural interactions drive condensate dynamics with implications for EWS::FLI1-mediated transcriptional regulation in EwS.