CHRNA1 gene, encoding the muscle nicotinic acetylcholine receptor alpha subunit, harbors an inframe exon P3A. Inclusion of exon P3A disables assembly of the acetylcholine receptor subunits. A single nucleotide mutation in exon P3A identified in congenital myasthenic syndrome causes exclusive inclusion of exon P3A. The mutation gains a de novo binding affinity for a splicing enhancing RNA-binding protein, hnRNP LL, and displaces binding of a splicing suppressing RNA-binding protein, hnRNP L. The hnRNP L binds to another splicing repressor PTB through the proline-rich region and promotes PTB binding to the polypyrimidine tract upstream of exon P3A, whereas hnRNP LL lacking the proline-rich region cannot bind to PTB. Interaction of hnRNP L with PTB inhibits association of U2AF 65 and U1 snRNP with the upstream and downstream of P3A, respectively, which causes a defect in exon P3A definition. HnRNP L and hnRNP LL thus antagonistically modulate PTB-mediated splicing suppression of exon P3A.
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