Serine mutation of a conserved threonine in the hERG K(+) channel S6-pore region leads to loss-of-function through trafficking impairment

The human Ether-à-go-go Related Gene (hERG) encodes a potassium channel responsible for the cardiac rapid delayed rectifier K(+) current, I(Kr), which regulates ventricular repolarization. Loss-of-function hERG mutations underpin the LQT2 form of congenital long QT syndrome. This study was undertaken to elucidate the functional consequences of a variant of uncertain significance, T634S, located at a highly conserved position at the top of the S6 helix of the hERG channel. Whole-cell patch-clamp recordings were made at 37 °C of hERG current (I(hERG)) from HEK 293 cells expressing wild-type (WT) hERG, WT+T634S and hERG-T634S alone. When the T634S mutation was expressed alone little or no I(hERG) could be recorded. Co-expressing WT and hERG-T634S suppressed I(hERG) tails by ∼57% compared to WT alone, without significant alteration of voltage dependent activation of I(hERG). A similar suppression of I(hERG) was observed under action potential voltage clamp. Comparable reduction of I(Kr) in a ventricular AP model delayed repolarization and led to action potential prolongation. A LI-COR® based On/In-Cell Western assay showed that cell surface expression of hERG channels in HEK 293 cells was markedly reduced by the T634S mutation, whilst total cellular hERG expression was unaffected, demonstrating impaired trafficking of the hERG-T634S mutant. Incubation with E−4031, but not lumacaftor, rescued defective hERG-T634S channel trafficking and I(hERG) density. In conclusion, these data identify hERG-T634S as a rescuable trafficking defective mutation that reduces I(Kr) sufficiently to delay repolarization and, thereby, potentially produce a LQT2 phenotype.