Regulation of FAS Exon Definition and Apoptosis by the Ewing Sarcoma Protein

• The Ewing sarcoma protein (EWS) binds to exonic sequences and promotes exon definition
• EWS regulates alternative splicing of FAS/CD95, favoring the proapoptotic isoform
• EWS levels regulate FAS/CD95-mediated apoptosis in cancer (including Ewing sarcoma) cells

SummaryThe Ewing sarcoma protein EWS is an RNA and DNA binding protein implicated in transcription, pre-mRNA splicing, and DNA damage response. Using CLIP-seq, we identified EWS RNA binding sites in exonic regions near 5′ splice sites. A prominent target was exon 6 of the FAS/CD95 receptor, which is alternatively spliced to generate isoforms with opposing activities in programmed cell death. Depletion and overexpression experiments showed that EWS promotes exon 6 inclusion and consequently the synthesis of the proapoptotic FAS/CD95 isoform, whereas an EWS-FLI1 fusion protein characteristic of Ewing sarcomas shows decreased activity. Biochemical analyses revealed that EWS binding promotes the recruitment of U1snRNP and U2AF65 to the splice sites flanking exon 6 and therefore exon definition. Consistent with a role for EWS in the regulation of programmed cell death, cells depleted of EWS show decreased sensitivity to FAS-induced apoptosis, and elevated EWS expression enhances apoptosis in EWS-haploinsufficient Ewing sarcoma cells.

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