Dr. David Libich, UT Health at San Antonio

Host: Dr. Venditti

Ewing sarcoma is an aggressive bone and soft tissue cancer that primarily affects children and adolescents. Ewing sarcoma arises from a chromosomal translocation that fuses the low-complexity domain (LCD) of EWSR1 with the DNA binding domain (DBD) of an ETS family transcription factor, most commonly FLI1. The resulting fusion protein, EWS-FLI1, not only acts as an aberrant transcription factor but is also thought to associate with RNA-binding proteins in biomolecular condensates via its LCD and dysregulate DNA and RNA processing. In particular, EWS-FLI1 interacts with EWSR1 in vivo and exerts a dominant-negative effect on the normal biological functions of EWSR1. We investigated the biophysical properties of biomolecular condensates formed by EWS-FLI1, EWSR1 and EWSR1 LCD and determined FLI1 DBD enhanced phase separation of EWS LCD, and both EWS-FLI1 and FLI1 DBD colocalized to EWS LCD condensates and catalyzed hardening of these condensates. Three related ETS family DBDs were also found to induce hardening of EWS LCD condensates, indicating that the effect is generalized across the ETS transcription factor family. Colocalization to EWS LCD condensates was found to be dependent upon an interaction between the FLI1 DBD and EWS LCD via the DNA-binding interface of FLI1 DBD, as the presence of DNA inhibited colocalization. Further supporting this, nuclear magnetic resonance spectroscopy determined that ETS DBDs interact with EWS LCD via residues also involved in DNA binding. Our results provide a possible explanation for the dominant-negative effect EWS-FLI1 exerts on EWSR1 functions; however, the implications of the interaction between EWS LCD and ETS DBDs in biomolecular condensates in vivo remain to be determined.