SRSF1 and hnRNP H antagonistically regulate splicing of COLQ exon 16 in a congenital myasthenic syndrome

The catalytic subunits of acetylcholinesterase (AChE) are anchored in the basal lamina of the neuromuscular junction using a collagen-like tail subunit (ColQ) encoded by COLQ. Mutations in COLQ cause endplate AChE deficiency. An A-to-G mutation predicting p.E415G in COLQ exon 16 identified in a patient with endplate AChE deficiency causes exclusive skipping of exon 16. RNA affinity purification, mass spectrometry, and siRNA-mediated gene knocking down disclosed that the mutation disrupts binding of a splicing-enhancing RNA-binding protein, SRSF1, and de novo gains binding of a splicing-suppressing RNA-binding protein, hnRNP H. MS2-mediated artificial tethering of each factor demonstrated that SRSF1 and hnRNP H antagonistically modulate splicing by binding exclusively to the target in exon 16. Further analyses with artificial mutants revealed that SRSF1 is able to bind to degenerative binding motifs, whereas hnRNP H strictly requires an uninterrupted stretch of poly(G). The mutation compromised splicing of the downstream intron. Isolation of early spliceosome complex revealed that the mutation impairs binding of U1-70K (snRNP70) to the downstream 5' splice site. Global splicing analysis with RNA-seq revealed that exons carrying the hnRNP H-binding GGGGG motif are predisposed to be skipped compared to those carrying the SRSF1-binding GGAGG motif in both human and mouse brains.