Abstract 5701: Identification and Functional Characterization of a Novel Intronic Splicing Mutation in KCNQ1 Associated with Long QT Syndrome
Background: KCNQ1 gene encodes the delayed rectifier K+ channel in cardiac muscle, and the genetic mutations cause long QT syndrome type 1 (LQT1). Exercise-related cardiac events predominate in LQT1. We previously reported mechanistic basis for the pathogenesis of LQT1 associated with a KCNQ1 splicing mutation at the last base of exon 7 (c.1032G>A), which causes severe phenotype of LQT1. Here we report a novel splicing mutation at the third base of intron 7 (IVS7+3A>G), identified in a family without syncopal episodes, despite a pronounced exercise-induced QT prolongation.
Methods and Results: Minigene assay in COS7 cells and RT-PCR analysis of patients’ lymphocytes demonstrated the presence of exon 7-deficient mRNA in IVS7+3A>G, as well as c.1032G>A. Quantitative analysis using real-time RT-PCR demonstrated that IVS7+3A>G carrier expressed modest but significant amount of exon-skipping mRNAs (18.8% of total KCNQ1 mRNA), compared with the higher amount in c.1032G>A carriers (44.8%). Current recordings from Xenopus oocytes simulating the cRNA ratios of the IVS7+3A>G carrier showed remarkable reduction in currents compared with those with the ratios of normal individuals, but less pronounced than c.1032G>A carrier; 64.8±4.5% for IVS7+3A>G, 41.4±9.5% for c.1032G>A, versus normal individuals. Finally, computer simulation incorporating these quantitative results revealed the pronounced QT prolongation under beta-adrenergic stimulation in both IVS7+3A>G carrier model and c.1032G>A carrier model, but the occurrence of the monomorphic ventricular tachycardia derived from triggered activities due to delayed afterdepolarization only in c.1032G>A carrier model. These findings coincided well with the clinical phenotypes of IVS7+3A>G and c.1032G>A carriers.
Conclusion: A novel KCNQ1 splicing mutation IVS7+3A>G generates exon-skipping mRNAs, and manifests a mild phenotype of LQT1. The amount of these mRNAs and its functional consequences may determine the clinical severity of the disease.