Ewing sarcoma usually expresses the EWS/FLI fusion transcription factor oncoprotein. EWS/FLI regulates myriad genes required for Ewing sarcoma development. EWS/FLI binds GGAA-microsatellite sequences in vivo and in vitro. These sequences provide EWS/FLI-mediated activation to reporter constructs, suggesting that they function as EWS/FLI-response elements. We now demonstrate the critical role of an EWS/FLI-bound GGAA-microsatellite in regulation of the NR0B1 gene as well as for Ewing sarcoma proliferation and anchorage-independent growth. Clinically, genomic GGAA-microsatellites are highly variable and polymorphic. Current data suggest that there is an optimal "sweet-spot" GGAA-microsatellite length (of 18-26 GGAA repeats) that confers maximal EWS/FLI-responsiveness to target genes, but the mechanistic basis for this remains unknown. Our biochemical studies, using recombinant Δ22 (a version of EWS/FLI containing only the FLI portion), demonstrate a stoichiometry of one Δ22-monomer binding to every two consecutive GGAA-repeats on shorter microsatellite sequences. Surprisingly, the affinity for Δ22 binding to GGAA-microsatellites significantly decreased, and ultimately became unmeasureable, when the size of the microsatellite was increased to the sweet-spot length. In contrast, a fully functional EWS/FLI mutant (Mut9, which retains approximately half of the EWS portion of the fusion) showed low affinity for smaller GGAA-microsatellites but instead significantly increased its affinity at sweet-spot microsatellite lengths. Single-gene ChIP and genome-wide ChIP-sequencing (ChIP-seq) and RNA-seq studies extended these findings to the in vivo setting. Together, these data demonstrate the critical requirement of GGAA-microsatellites as EWS/FLI activating response elements in vivo and reveal an unexpected role for the EWS portion of the EWS/FLI fusion in binding to sweet-spot GGAA-microsatellites.
Keywords: EWS/FLI fusion; Ewing sarcoma; microsatellites; transcriptional activation.