The rate-dependent biophysical properties of the LQT1 H258R mutant are counteracted by a dominant negative effect on channel trafficking.

The long QT syndrome (LQTS) is a cardiac disorder caused by a prolonged ventricular repolarization. The co-assembly of the pore-forming human KCNQ1 alpha-subunits with the modulating hKCNE1 beta-subunits generates I(Ks)in vivo, explaining why mutations in the hKCNQ1 gene underlie the LQT1 form of congenital LQT. Here we describe the functional defects of the LQT1 mutation H258R located in the S4-S5 linker, a segment important for channel gating. Mutant subunits with this arginine substitution generated no or barely detectable currents in a homotetrameric condition, but did generate I(Ks)-like currents in association with hKCNE1. Compared to the WT hKCNQ1/hKCNE1 complex, the H258R/hKCNE1 complex 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 to the plasma membrane. 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. These rate-dependent effects may be relevant given the larger contribution of I(Ks) to the "repolarization reserve" at higher action potential rates. The combination of complex kinetics that counteract the trafficking problem represents a particular mechanism underlying LQT1.