Digenic heterozygosity in KCNQ1 and KCNH2 genes causes severe long QT phenotype

Long QT syndrome (LQTS) is an inherited arrhythmic disorder associated with QT interval prolongation on ECG, syncope, torsade de pointes and sudden cardiac death in young patients. Heritable LQTS is classified in 12 types depending on mutations in a specific gene. In most cases, the mutations involve the genes encoding a-subunits of repolarizing K+ channels (KCNQ1 and KCNH2). Prolongation of the QT interval and action potential duration is associated with the decrease of outward K+ current in cardiomyocytes. The screening for LQTS causing mutations in a family from South Italy revealed two mutations in two genes: c.G1748A (p.R583H) in KCNQ1 and c.G323A (p.C108Y) in KCNH2. Interestingly, all subjects carrying just one of these two mutations did not show the LQT phenotype, whereas the only two subjects carrying both were clinically affected (QTc > 530ms). To analyze the genotype-phenotype association and to shed light on the pathogenetic defects responsible for these LQTS cases we characterized the biophysical features of the two mutant channels. We generated the KCNQ1 and KCNH2 mutants by site-directed mutagenesis. Each mutant was transiently expressed in CHO cells and functionally characterized by whole-cell patch clamp recordings. The biophysical studies showed that in the homozygous condition, mutation c.G323A led to a nonfunctional KCNH2 channel, whereas, in the heterozygous condition, mutant KCNH2 had significantly reduced current density at positive potentials and a negative shift in the voltage dependence of activation compared to the wild type. Furthermore, mutant KCNQ1-p.R583H had significantly reduced tail current density compared to the wild-type channel, but no significant changes in activating current density and voltage-dependence of activation. In conclusion, our results demonstrate that mutation KCNH2-p.C108Y induced a dominant negative effect in the heterozygous condition. Moreover, both KCNH2-p.C108Y and KCNQ1-p.R583H had some biophysical defects that could be combined and lead to the LQTS phenotype, as observed in the pedigree of the family reported herein.