Enhancer-dependent splicing of FGFR1 α-exon is repressed by RNA interference-mediated down-regulation of SRp55

The FGFR1 gene transcript is alternatively processed to produce functionally different receptor forms. Previously, we identified a 69-nucleotide exonic splicing enhancer (ESE) required for alpha-exon inclusion in JEG3 cells. In the present study, we found that this sequence is composed of three independent elements, two smaller ESE sequences flanking an exonic splicing silencer sequence. Ultraviolet cross-linking and immunoprecipitation identified ESE-specific binding of the splicing regulator SRp55. A RNA interference-mediated decrease in SRp55 confirmed the significance of this interaction. There was a 6- to 14-fold decrease in exon inclusion on ablation of SRp55. In SNB19 glioblastoma cells, which normally skip this exon, SRp55 was also demonstrated to play a role in exon inclusion after the removal of intronic splicing silencer sequences. These observations indicate that SRp55 plays a major role in maintaining normal FGFR1 a-exon inclusion, which is subject to dominant intronic splicing silencer-mediated and exonic splicing silencer-mediated inhibition in SNB19 cells.