Unwinding the roles of DEAD-box RNA helicases DDX39A and DDX39B in alternative RNA splicing
Dead-box RNA helicases are crucial in mRNA processing, specifically in RNA splicing. Our previous work has shown that DDX39B is responsible for regulating the splicing of IL7R exon 6 and several FOXP3 introns, which rely on DDX39B's helicase and ATPase activities, respectively [this is not accurate since exon 6 also must require the ATPase activity, which is required for helicase activity]. In this study, we aimed to investigate whether DDX39A, a highly homologous paralog of DDX39B, plays a similar role in regulating alternative RNA splicing. We find that DDX39A and DDX39B have significant redundancy in their gene targets, however, DDX39A is incapable of complementing defective splicing of IL7R exon 6 when DDX39B is knockdown. Conversely, overexpressing DDX39A can rescue FOXP3 intron 11 splicing under DDX39B-depleted conditions. In this work we also confirm that introns containing C-rich/U-poor polypyrimidine tract are very sensitive to DDX39B levels. We also observed that cassette exons with C-rich/U-poor py tracts in upstream introns were also sensitive to DDX39B levels and were skipped more upon depletion of DDX39B, but not DDX39A. Among the introns retained more upon DDX39A and DDX39B depletion were DDX39A and DDX39B intron 6, which depend on DDX39A and DDX39B levels, respectively. Therefore, we identified an autoregulatory mechanism through which DDX39A and DDX39B control their respective expression. This study presents evidence that while DDX39A and DDX39B differentially impact certain RNA splicing events, they have many shared targets. Overall design: HeLa cells were treated with two independent siRNAs targeting DDX39A expression (siD06 and siD09) and two independent siRNAs targeting DDX39B expression (siD11 and siD13). Control siRNA NSC at two different concentrations 5nM and 25 nM as a non-targeting control. The changes in DDX39A expression were compared with NSC-25 samples and changes in DDX39B expression were compared to NSC-5nM samples.