The LQT1 Phenotype of the KCNQ1 H258R Mutant is Unmasked by Faster Stimulation Rates

Abstract

The long QT syndrome is a cardiac disorder caused by a delayed ventricular repolarization. LQT1 is linked to mutations in the KCNQ1 gene that codes for the six transmembrane spanning α-subunit of the channel complex that underlies IKsin vivo. The LQT1 mutation H258R, located in the S4-S5 linker, resulted in subunits that failed to generate current in a homotetrameric condition. However, association with hKCNE1 ‘rescued’ the mutant subunit and generated IKs-like currents. Compared to WT hKCNQ1/hKCNE1, H258R/hKCNE1 displayed accelerated activation kinetics, slowed channel closure and a hyperpolarizing shift of the voltage-dependence of activation, thus predicting an increased K+ current. However, current density analysis combined with subcellular localization indicated that the H258R subunit exerted a dominant negative effect on channel trafficking. The co-expression hKCNQ1/H258R/hKCNE1, mimicking the heterozygous state of a patient, displayed similar properties. During repetitive stimulation the mutant yielded more current compared to WT at 1 Hz but this effect was counteracted by the trafficking defect at faster frequencies. Thus at faster stimulation rates there would be less repolarizing K+ current compared to WT, explaining the disease causing effect of the mutation. In terms of H258R being ‘rescued’ by hKCNE1, it seems less likely that this occurs through a pure chaperone-type mechanism and based on the altered gating kinetics we suggest that hKCNE1 rescues H258R by restoring the gating machinery. It has been proposed that hKCNE1 modulates hKCNQ1 kinetics by stabilizing the interaction between the S4-S5 linker and bottom part of S6. Therefore, we speculate that the H258R mutation disrupts the contact with S6 resulting in distorted subunit folding. The association with hKCNE1 then stabilizes the electromechanic coupling and in this way compensates for the destabilization caused by the H258R mutant.