HERG-F463L potassium channels linked to long QT syndrome reduce I(Kr) current by a trafficking-deficient mechanism

Targeted next generation sequencing revealed a novel deletion-frameshift mutation of KCNH2 gene in a Chinese Han family with long QT syndrome: A case report and review of Chinese cases

INTRODUCTION

Long QT syndrome (LQTS) is electrocardiographically characterized by a prolonged QT interval and manifests predisposition to life-threatening arrhythmia which often leads to sudden cardiac death. Type 2 LQTS (LQT2) is the second most common subtype of LQTS and caused by mutations in KCNH2 gene. Up to date, >900 mutations have been reported to be related to LQT2. However, mutational screening of the KCNH2 gene is still far from completeness. Identification of KCNH2 mutations is particularly important in diagnosis of LQT2 and will gain more insights into the molecular basis for the pathogenesis of LQT2.

PATIENT CONCERNS

A Chinese Han family with LQTS phenotypes was examined.

DIAGNOSIS

A novel deletion-frameshift mutation, c.381_408delCAATTTCGAGGTGGTGATGGAGAAGGAC, in exon 3 of KCNH2 gene was identified in a Chinese family with LQTS. On the basis of this finding and clinical manifestations, the final diagnosis of LQT2 was made.

INTERVENTIONS

Next-generation sequencing (NGS) of DNA samples was performed to detect the mutation in the LQTS-related genes on the proband and her mother, which was confirmed by Sanger sequencing. The proband was then implanted with an implantable cardioverter defibrillator and prescribed metoprolol 47.5 mg per day.

OUTCOMES

This novel heterozygous mutation results in a frameshift mutation after the 128 residue (Asparagine), which replaced the original 1031 amino acids with 27 novel amino acids (p.N128fsX156).

CONCLUSION

This novel mutation presumably resulted in a frameshift mutation, p.N128fsX156. Our data expanded the mutation spectrum of KCNH2 gene and facilitated clinic diagnosis and genetic counseling for this family with LQTS.

Experimentally Validated Pharmacoinformatics Approach to Predict hERG Inhibition Potential of New Chemical Entities

The hERG (human ether-a-go-go-related gene) encoded potassium ion (K⁺) channel plays a major role in cardiac repolarization. Drug-induced blockade of hERG has been a major cause of potentially lethal ventricular tachycardia termed Torsades de Pointes (TdPs). Therefore, we presented a pharmacoinformatics strategy using combined ligand and structure based models for the prediction of hERG inhibition potential (IC₅₀) of new chemical entities (NCEs) during early stages of drug design and development. Integrated GRid-INdependent Descriptor (GRIND) models, and lipophilic efficiency (LipE), ligand efficiency (LE) guided template selection for the structure based pharmacophore models have been used for virtual screening and subsequent hERG activity (pIC₅₀) prediction of identified hits. Finally selected two hits were experimentally evaluated for hERG inhibition potential (pIC₅₀) using whole cell patch clamp assay. Overall, our results demonstrate a difference of less than ±1.6 log unit between experimentally determined and predicted hERG inhibition potential (IC₅₀) of the selected hits. This revealed predictive ability and robustness of our models and could help in correctly rank the potency order (lower μM to higher nM range) against hERG.

F463L increases the potential of dofetilide on human ether-a-go-go-related gene (hERG) channels

Mutations in genes related to long QT syndrome (LQTS) is recognized as an independent risk of drug-induced LQTS. We previously screened a mutation F463L in a Chinese patient with LQT2, syncope, and epilepsy. Here, we planned to illustrate how F463L influences the action of dofetilide on hERG channels. F463L-hERG plasmids were transfected into the stable Human Embryonic Kidney 293 (HEK293) cells expressing WT-hERG to generate heterozygous mutant (WT + F463L-hERG). Whole-cell patch clamp and laser confocal scanning microscopy were used to evaluate electrophysiological consequences and the membrane distribution of hERG protein. In comparison of WT-hERG channels exposed to dofetilide, heterozygous F463L-hERG channels showed a reduction in the density of tail currents when exposed amidarone. F463L-hERG also altered the action of dofetilide on the gating properties of hERG channels. Images of dofetilide-treated cells expressing heterozygous F463L showed a severe retention and reduction of protein expression on the membrane compared to WT. In conclusion, dofetilide displays a powerful inhibitory effect on the currents from cells expressing heterozygous F463L, thus showing an additive suppression of currents by F463L with dofetilide.

Association of the hERG mutation with long-QT syndrome type 2, syncope and epilepsy

Mutations in the human ether‑à‑go‑go‑related gene (hERG) are responsible for long‑QT syndrome (LQTS) type 2 (LQT2). In the present study, a heterozygous missense mutation (A561V) linked to LQT2, syncope and epilepsy was identified in the S5/pore region of the hERG protein. The mutation, A561V, was prepared and subcloned into hERG‑pcDNA3.0. Mutant plasmids were co‑transfected into HEK‑293 cells, which stably express wild‑type (WT) hERG, in order to mimic a heterozygous genotype, and the whole‑cell current was recorded using a patch‑clamp technique. Confocal microscopy was performed to evaluate the membrane distribution of the hERG channel protein using a green fluorescent protein tagged to the N‑terminus of hERG. A561V‑hERG decreased the amplitude of the WT‑hERG currents in a concentration‑dependent manner. In addition, A561V‑hERG resulted in alterations to activation, inactivation and recovery from inactivation in the hERG protein channels. Further evaluation of hERG membrane localization indicated that the A561V‑hERG mutant protein was unable to travel to the plasma membrane, which resulted in a trafficking‑deficient WT‑hERG protein. In conclusion, A561V‑hERG exerts a potent dominant‑negative effect on WT‑hERG channels, resulting in decreased hERG currents and impairment of hERG membrane localization. This may partially elucidate the clinical manifestations of LQTS patients who carry the A561V mutation.

Mouse ERG K(+) channel clones reveal differences in protein trafficking and function

Background

The mouse ether‐a‐go‐go‐related gene 1a (mERG1a, mKCNH2) encodes mERG K⁺ channels in mouse cardiomyocytes. The mERG channels and their human analogue, hERG channels, conduct I_Kr. Mutations in hERG channels reduce I_Kr to cause congenital long‐QT syndrome type 2, mostly by decreasing surface membrane expression of trafficking‐deficient channels. Three cDNA sequences were originally reported for mERG channels that differ by 1 to 4 amino acid residues (mERG‐London, mERG‐Waterston, and mERG‐Nie). We characterized these mERG channels to test the postulation that they would differ in their protein trafficking and biophysical function, based on previous findings in long‐QT syndrome type 2.

Methods and Results

The 3 mERG and hERG channels were expressed in HEK293 cells and neonatal mouse cardiomyocytes and were studied using Western blot and whole‐cell patch clamp. We then compared our findings with the recent sequencing results in the Welcome Trust Sanger Institute Mouse Genomes Project (WTSIMGP).

Conclusions

First, the mERG‐London channel with amino acid substitutions in regions of highly ordered structure is trafficking deficient and undergoes temperature‐dependent and pharmacological correction of its trafficking deficiency. Second, the voltage dependence of channel gating would be different for the 3 mERG channels. Third, compared with the WTSIMGP data set, the mERG‐Nie clone is likely to represent the wild‐type mouse sequence and physiology. Fourth, the WTSIMGP analysis suggests that substrain‐specific sequence differences in mERG are a common finding in mice. These findings with mERG channels support previous findings with hERG channel structure–function analyses in long‐QT syndrome type 2, in which sequence changes in regions of highly ordered structure are likely to result in abnormal protein trafficking.

Allele-specific RNA interference rescues the long-QT syndrome phenotype in human-induced pluripotency stem cell cardiomyocytes

Aims

Long-QT syndromes (LQTS) are mostly autosomal-dominant congenital disorders associated with a 1:1000 mutation frequency, cardiac arrest, and sudden death. We sought to use cardiomyocytes derived from human-induced pluripotency stem cells (hiPSCs) as an in vitro model to develop and evaluate gene-based therapeutics for the treatment of LQTS.

Methods and results

We produced LQTS-type 2 (LQT2) hiPSC cardiomyocytes carrying a KCNH2 c.G1681A mutation in a I_Kr ion-channel pore, which caused impaired glycosylation and channel transport to cell surface. Allele-specific RNA interference (RNAi) directed towards the mutated KCNH2 mRNA caused knockdown, while leaving the wild-type mRNA unaffected. Electrophysiological analysis of patient-derived LQT2 hiPSC cardiomyocytes treated with mutation-specific siRNAs showed normalized action potential durations (APDs) and K⁺ currents with the concurrent rescue of spontaneous and drug-induced arrhythmias (presented as early-afterdepolarizations).

Conclusions

These findings provide in vitro evidence that allele-specific RNAi can rescue diseased phenotype in LQTS cardiomyocytes. This is a potentially novel route for the treatment of many autosomal-dominant-negative disorders, including those of the heart.

A novel mutation in the transmembrane nonpore region of the KCNH2 gene causes severe clinical manifestations of long QT syndrome

BACKGROUND

Long QT syndrome (LQTS) is characterized by prolonged ventricular repolarization and variable clinical course with arrhythmia-related syncope and sudden death. Mutations in the nonpore region of the LQTS-associated KCNH2 gene (also known as hERG) are mostly associated with coassembly or trafficking abnormalities, resulting in haplotype insufficiency and milder clinical phenotypes compared with mutations in the pore domain.

OBJECTIVE

To investigate the effect of a nonpore mutation on the channel current, which was identified from an LQTS family with severe clinical phenotypes.

METHODS

Two members of a Japanese family with LQTS were searched for mutations in KCNQ1, KCNH2, SCN5A, KCNE1, KCNE2, and KCNJ2 genes by using automated DNA sequencing. We characterized the electrophysiological properties and glycosylation pattern of the mutant channels by using patch clamp recording and Western blot analysis.

RESULTS

In the LQTS patient with torsades de pointes and cardiopulmonary arrest, we identified the novel T473P mutation in the transmembrane nonpore region of KCNH2. The proband's father carried the same mutation and showed prolonged corrected QT interval and frequent torsades de pointes in the presence of hypokalemia following the administration of garenoxacin. Patch clamp analysis in heterologous cells showed that hERG T473P channels generated no current and exhibited a dominant negative effect when coexpressed with wild-type protein. Only incompletely glycosylated hERG T473P channels were observed by using Western blot analysis, suggesting impaired trafficking.

CONCLUSIONS

These results demonstrated that a trafficking-deficient mutation in the transmembrane nonpore region of KCNH2 causes a dominant negative effect and a severe clinical course in affected patients.

Drug evaluation in cardiomyocytes derived from human induced pluripotent stem cells carrying a long QT syndrome type 2 mutation

AIMS

Congenital long QT syndromes (LQTSs) are associated with prolonged ventricular repolarization and sudden cardiac death. Limitations to existing clinical therapeutic management strategies prompted us to develop a novel human in vitro drug-evaluation system for LQTS type 2 (LQT2) that will complement the existing in vitro and in vivo models.

METHODS AND RESULTS

Skin fibroblasts from a patient with a KCNH2 G1681A mutation (encodes I(Kr) potassium ion channel) were reprogrammed to human induced pluripotent stem cells (hiPSCs), which were subsequently differentiated to functional cardiomyocytes. Relative to controls (including the patient's mother), multi-electrode array and patch-clamp electrophysiology of LQT2-hiPSC cardiomyocytes showed prolonged field/action potential duration. When LQT2-hiPSC cardiomyocytes were exposed to E4031 (an I(Kr) blocker), arrhythmias developed and these presented as early after depolarizations (EADs) in the action potentials. In contrast to control cardiomyocytes, LQT2-hiPSC cardiomyocytes also developed EADs when challenged with the clinically used stressor, isoprenaline. This effect was reversed by β-blockers, propranolol, and nadolol, the latter being used for the patient's therapy. Treatment of cardiomyocytes with experimental potassium channel enhancers, nicorandil and PD118057, caused action potential shortening and in some cases could abolish EADs. Notably, combined treatment with isoprenaline (enhancers/isoprenaline) caused EADs, but this effect was reversed by nadolol.

CONCLUSIONS

Findings from this paper demonstrate that patient LQT2-hiPSC cardiomyocytes respond appropriately to clinically relevant pharmacology and will be a valuable human in vitro model for testing experimental drug combinations.