Reduced cardiac repolarizing K+ currents, mainly IKr (hERG) and IKs, prolong the duration of the QTinterval, which is a hallmark of the congenital long QT syndrome (LQTS). In this study, a novel heterozygous mutation in the hERG gene was discovered in a 37-year-old woman with history of Torsades de pointes and prolonged QTc. Here, we characterized the 7 amino-acid duplication (343 363dup) located in the PAS domain at hERG N-terminus.

Since the duplication is in the PAS domain, which is involved in deactivation of hERG, we first studied this process using the patch-clamp method in CHO cells expressing wild-type (WT) and mutant (Dupl) hERG channels. The main deactivation time constant was reduced by 1.4–1.8-fold for hERG Dupl compared to WT in tail currents recorded at -150 and -90 mV, respectively. Deactivation- corrected tail current density at 120 mV was decreased by 62±10 % for Dupl and by 74±9 % for the heterozygous condition. Steady-state inactivation showed no significant difference, but activation curve displayed a hyperpolarizing shift of 5 mV. Cell surface biotinylation experiments showed a 50 % decrease of the mature form of Dupl in the membrane fraction, whereas the immature form was not affected. Incubation of the cells at lower temperature (28°C,36h) resulted in partial rescue of current for the heterozygous and Dupl conditions. Low-temperature rescue was also observed in biotinylation experiments where mature/immature hERG ratios of heterozygous and Dupl conditions were restored to WT-values.

This hERG 343-363dup mutation causes a reduction of the peak current density and hastening of deactivation process. Both alterations lead to a loss-of-function of the channel. Furthermore, amplitude of current measured in the heterozygous model suggests dominant negative effect of the mutation. The shift towards hyperpolarizing voltages in the activation curve may reflect a gain-of-function; however, we speculate that its contribution is negligible. Biotinylation experiments showed that only the mature form of hERG Dupl is affected by the mutation, unveiling a likely defective maturation/trafficking process that can be rescued at lower incubation temperature. Our findings illustrate that N-terminally located mutations can also lead to severe ventricular arrhythmias.