EWSR1::FLI1 fusion oncoproteins' regulation of ETS1 (RNA-seq)
Ewing sarcomas harbor few mutations beyond the chromosomal translocation that initiates disease and the mechanistic basis for the metastasis of these tumors remains poorly understood. The epigenome of Ewing sarcoma (EWS) cells reflects the regulatory state of genes associated with the DNA binding activity of the fusion oncoproteins EWSR1::FLI1 or EWSR1::ERG. In this study, we examined the repressive activities of the EWSR1::FLI1/ERG fusion oncoproteins. Focusing on one of the repressed EWSR1::FLI1/ERG target genes, ETS1, we detected EWSR1::FLI1 binding and a H3K27me3 repressive mark at this locus. Depletion of EWSR1::FLI1 results in ETS1's binding of promoter regions, and we show ETS1 regulates the expression of multiple proteins that function in extracellular matrix organization including TENSIN3 (TSN3). Interestingly, TSN3 expression in EWS tumors significantly correlates with that of ETS1 (0.85, FDR <0.01). TNS3 is a focal adhesion protein that contributes to tumor cell migration by connecting the cytoplasmic tail of integrins to the actin cytoskeleton. EWS cell lines, in which we activated ETS1 expression (CRISPRa) exhibited increased TNS3 expression and a migratory phenotype. Critically, the activated ETS1 EWS cell lines show TNS3 accumulation at leading cell edges, with F-actin cytoskeletal reorganization, a phenotype associated with cell migration. Overall design: Ewing sarcoma cell lines (TC-32) were analyzed by RNA-seq. To interrogate ETS1 regulation of gene expression in the presence of EWSR1::FLI1, we ectopically expressed ETS1 in TC-32 cells, generating single cell clones that expressed ETS1 at levels comparable to EWSR1::FLI1-depleted cells (TC-32-ETS1-High) and cells expressing lower levels of ETS1 (TC-32-ETS-Low).