Mechanistic basis for type 2 long QT syndrome caused by KCNH2 mutations that disrupt conserved arginine residues in the voltage sensor

Deciphering hERG Mutation in Long QT Syndrome Type 2 Using Antisense Oligonucleotide-Mediated Techniques: Lessons from Cystic Fibrosis

Long QT syndrome type 2 (LQT2) is a genetic disorder caused by mutations in the KCNH2 gene, also known as the human ether-a-go-go-related gene (hERG). Over 30% of hERG mutations result in a premature termination codon (PTC) that triggers a process called nonsense-mediated mRNA decay (NMD), where the mRNA transcript is degraded. NMD is a quality control mechanism that removes faulty mRNA to prevent the translation of truncated proteins. Recent advances in antisense oligonucleotide (ASO) technology in the field of cystic fibrosis (CF) have yielded significant progress, including the ASO-mediated comprehensive characterization of key NMD factors and exon-skipping therapy. These advances have contributed to our understanding of the role of PTC-containing mutations in disease phenotypes and have also led to the development of potentially useful therapeutic strategies. Historically, studies of CF have provided valuable insights for the research on LQT2, particularly concerning increasing the expression of hERG. In this article, we outline the current state of knowledge regarding ASO, NMD, and hERG and discuss the introduction of ASO technology in the CF to elucidate the pathogenic mechanisms through targeting NMD. We also discuss the potential clinical therapeutic benefits and limitations of ASO for the management of LQT2. By drawing on lessons learned from CF research, we explore the potential translational values of these advances into LQT2 studies.

A need for exhaustive and standardized characterization of ion channels activity. The case of KV11.1

hERG, the pore-forming subunit of the rapid component of the delayed rectifier K⁺ current, plays a key role in ventricular repolarization. Mutations in the KCNH2 gene encoding hERG are associated with several cardiac rhythmic disorders, mainly the Long QT syndrome (LQTS) characterized by prolonged ventricular repolarization, leading to ventricular tachyarrhythmias, sometimes progressing to ventricular fibrillation and sudden death. Over the past few years, the emergence of next-generation sequencing has revealed an increasing number of genetic variants including KCNH2 variants. However, the potential pathogenicity of the majority of the variants remains unknown, thus classifying them as variants of uncertain significance or VUS. With diseases such as LQTS being associated with sudden death, identifying patients at risk by determining the variant pathogenicity, is crucial. The purpose of this review is to describe, on the basis of an exhaustive examination of the 1322 missense variants, the nature of the functional assays undertaken so far and their limitations. A detailed analysis of 38 hERG missense variants identified in Long QT French patients and studied in electrophysiology also underlies the incomplete characterization of the biophysical properties for each variant. These analyses lead to two conclusions: first, the function of many hERG variants has never been looked at and, second, the functional studies done so far are excessively heterogeneous regarding the stimulation protocols, cellular models, experimental temperatures, homozygous and/or the heterozygous condition under study, a context that may lead to conflicting conclusions. The state of the literature emphasizes how necessary and important it is to perform an exhaustive functional characterization of hERG variants and to standardize this effort for meaningful comparison among variants. The review ends with suggestions to create a unique homogeneous protocol that could be shared and adopted among scientists and that would facilitate cardiologists and geneticists in patient counseling and management.

Modeling long QT syndrome type 2 on-a-chip via in-depth assessment of isogenic gene-edited 3D cardiac tissues

Long QT syndrome (LQTS) is a cardiovascular disease characterized by QT interval prolongation that can lead to sudden cardiac death. Many mutations with heterogeneous mechanisms have been identified in KCNH2, the gene that encodes for hERG (Kv11.1), which lead to onset of LQTS type 2 (LQTS2). In this work, we developed a LQTS2-diseased tissue-on-a-chip model, using 3D coculture of isogenic stem cell-derived cardiomyocytes (CMs) and cardiac fibroblasts (CFs) within an organotypic microfluidic chip technology. Primarily, we created a hiPSC line with R531W mutation in KCNH2 using CRISPR-Cas9 gene-editing technique and characterized the resultant differentiated CMs and CFs. A deficiency in hERG trafficking was identified in KCNH2-edited hiPSC-CMs, revealing a possible mechanism of R531W mutation in LQTS2 pathophysiology. Following creation of a 3D LQTS2 tissue-on-a-chip, the tissues were extensively characterized, through analysis of calcium handling and response to β-agonist. Furthermore, attempted phenotypic rescue via pharmacological intervention of LQTS2 on a chip was investigated.

A calibrated functional patch-clamp assay to enhance clinical variant interpretation in KCNH2-related long QT syndrome

Modern sequencing technologies have revolutionized our detection of gene variants. However, in most genes, including KCNH2, the majority of missense variants are currently classified as variants of uncertain significance (VUSs). The aim of this study was to investigate the utility of an automated patch-clamp assay for aiding clinical variant classification in KCNH2. The assay was designed according to recommendations proposed by the Clinical Genome Sequence Variant Interpretation Working Group. Thirty-one variants (17 pathogenic/likely pathogenic, 14 benign/likely benign) were classified internally as variant controls. They were heterozygously expressed in Flp-In HEK293 cells for assessing the effects of variants on current density and channel gating in order to determine the sensitivity and specificity of the assay. All 17 pathogenic variant controls had reduced current density, and 13 of 14 benign variant controls had normal current density, which enabled determination of normal and abnormal ranges for applying evidence of moderate or supporting strength for VUS reclassification. Inclusion of functional assay evidence enabled us to reclassify 6 out of 44 KCNH2 VUSs as likely pathogenic. The high-throughput patch-clamp assay can provide moderate-strength evidence for clinical interpretation of clinical KCNH2 variants and demonstrates the value of developing automated patch-clamp assays for functional characterization of ion channel gene variants.

Molecular dynamics simulations suggest possible activation and deactivation pathways in the hERG channel

The elusive activation/deactivation mechanism of hERG is investigated, a voltage-gated potassium channel involved in severe inherited and drug-induced cardiac channelopathies, including the Long QT Syndrome. Firstly, the available structural data are integrated by providing a homology model for the closed state of the channel. Secondly, molecular dynamics combined with a network analysis revealed two distinct pathways coupling the voltage sensor domain with the pore domain. Interestingly, some LQTS-related mutations known to impair the activation/deactivation mechanism are distributed along the identified pathways, which thus suggests a microscopic interpretation of their role. Split channels simulations clarify a surprising feature of this channel, which is still able to gate when a cut is introduced between the voltage sensor domain and the neighboring helix S5. In summary, the presented results suggest possible activation/deactivation mechanisms of non-domain-swapped potassium channels that may aid in biomedical applications.

Costa et al. present the electro-mechanical coupling between the voltage sensor and pore domain of the hERG channel using a combination of molecular dynamics simulations and theoretical network analyses.

R534C mutation in hERG causes a trafficking defect in iPSC-derived cardiomyocytes from patients with type 2 long QT syndrome

Patient-specific cardiomyocytes obtained from induced pluripotent stem cells (CM-iPSC) offer unprecedented mechanistic insights in the study of inherited cardiac diseases. The objective of this work was to study a type 2 long QT syndrome (LQTS2)-associated mutation (c.1600C > T in KCNH2, p.R534C in hERG) in CM-iPSC. Peripheral blood mononuclear cells were isolated from two patients with the R534C mutation and iPSCs were generated. In addition, the same mutation was inserted in a control iPSC line by genome editing using CRISPR/Cas9. Cells expressed pluripotency markers and showed spontaneous differentiation into the three embryonic germ layers. Electrophysiology demonstrated that action potential duration (APD) of LQTS2 CM-iPSC was significantly longer than that of the control line, as well as the triangulation of the action potentials (AP), implying a longer duration of phase 3. Treatment with the I_Kr inhibitor E4031 only caused APD prolongation in the control line. Patch clamp showed a reduction of I_Kr on LQTS2 CM-iPSC compared to control, but channel activation was not significantly affected. Immunofluorescence for hERG demonstrated perinuclear staining in LQTS2 CM-iPSC. In conclusion, CM-iPSC recapitulated the LQTS2 phenotype and our findings suggest that the R534C mutation in KCNH2 leads to a channel trafficking defect to the plasma membrane.

Prevailing Effects of Ibutilide on Fast Delayed Rectifier K+ Channel

Effects of ibutilide, a class III antiarrhythmic drug, on delayed rectifier potassium currents (I_K) in freshly isolated guinea pig ventricular myocytes were studied. Experiments were performed using the whole-cell configuration of patch-clamp technique under blockade of L-type calcium currents (Cav1). Ibutilide at concentrations ranging between 10 nM and 100 µM inhibited I_Kr in dose-dependent manner with a half maximal effective concentration of 2.03 ± 0.74 µM (n = 5-10). The amplitude of tail currents activated by prepulse to + 20 mV was decreased from 253 ± 52 to 130 ± 25 pA (n = 8, p < 0.01) in the presence of 1 µM ibutilide. The envelope test revealed time-dependent changes in ratio of I_K-tail/ΔI_K during 0.2-2 s pulse durations in the absence of drug. With ibutilide, regardless of pulse duration, a relatively constant ratio was estimated, indicative of predominant involvement of I_Kr component. The slow I_Ks persisted to greater extent even at 100 μM ibutilide revealing a distinguishable selectivity toward the I_Kr component.

Genetic susceptibility in pharmacodynamic and pharmacokinetic pathways underlying drug-induced arrhythmia and sudden unexplained deaths

Drug-induced arrhythmia is an adverse drug reaction that can be potentially fatal since it is mostly related to drug-induced QT prolongation, a known risk factor for Torsade de Pointes and sudden cardiac death (SCD). Several risk factors have been described in association to these drug-induced events, such as preexistent cardiac disease and genetic variation. Our objective was to study the genetic susceptibility in pharmacodynamic and pharmacokinetic pathways underlying suspected drug-induced arrhythmias and sudden unexplained deaths in 32 patients. The genetic component in the pharmacodynamic pathway was studied by analysing 96 genes associated with higher risk of SCD through massive parallel sequencing. Pharmacokinetic-mediated genetic susceptibility was investigated by studying the genes encoding cytochrome P450 enzymes using medium-throughput genotyping. Pharmacodynamic analysis showed three probably pathogenic variants and 45 variants of uncertain significance in 28 patients, several of them previously described in relation to mild or late onset cardiomyopathies. These results suggest that genetic variants in cardiomyopathy genes, in addition to those related with channelopathies, could be relevant to drug-induced cardiotoxicity and contribute to the arrhythmogenic phenotype. Pharmacokinetic analysis showed three patients that could have an altered metabolism of the drugs they received involving CYP2C19 and/or CYP2D6, probably contributing to the arrhythmogenic phenotype. The study of genetic variants in both pharmacodynamic and pharmacokinetic pathways may be a useful strategy to understand the multifactorial mechanism of drug-induced events in both clinical practice and forensic field. However, it is necessary to comprehensively study and evaluate the contribution of the genetic susceptibility to drug-induced cardiotoxicity.

Tale of tail current

The largest biomass of channel proteins is located in unicellular organisms and bacteria that have no organs. However, orchestrated bidirectional ionic currents across the cell membrane via the channels are important for the functioning of organs of organisms, and equally concern both fauna or flora. Several ion channels are activated in the course of action potentials. One of the hallmarks of voltage-dependent channels is a 'tail current' - deactivation as observed after prior and sufficient activation predominantly at more depolarized potentials e.g. for Kv while upon hyperpolarization for HCN α subunits. Tail current also reflects the timing of channel closure that is initiated upon termination of stimuli. Finally, deactivation of currents during repolarization could be a selective estimate for given channel as in case of HERG, if dedicated long and more depolarized 'tail pulse' is used. Since from a holding potential of e.g. -70 mV are often a family of outward K⁺ currents comprising I_A and I_K are simultaneously activated in native cells.

Limulus and heart rhythm

Great interest in the comparative physiology of hearts and their functions in Animalia has emerged with classic papers on Limulus polyphemus and mollusks. The recurrent cardiac activity-heart rate-is the most important physiological parameter and when present the kardia (Greek) is vital to the development of entire organs of the organisms in the animal kingdom. Extensive studies devoted to the regulation of cardiac rhythm in invertebrates have revealed that the basics of heart physiology are comparable to mammals. The hearts of invertebrates also beat spontaneously and are supplied with regulatory nerves: either excitatory or inhibitory or both. The distinct nerves and the source of excitation/inhibition at the level of single neurons are described for many invertebrate genera. The vertebrates and a majority of invertebrates have myogenic hearts, whereas the horseshoe crab L. polyphemus and a few other animals have a neurogenic cardiac rhythm. Nevertheless, the myogenic nature of heartbeat is precursor, because the contraction of native and stem-cell-derived cardiomyocytes does occur in the absence of any neural elements. Even in L. polyphemus, the heart rhythm is myogenic at embryonic stages.

Functional Invalidation of Putative Sudden Infant Death Syndrome-Associated Variants in the KCNH2-Encoded Kv11.1 Channel

BACKGROUND

Heterologous functional validation studies of putative long-QT syndrome subtype 2-associated variants clarify their pathological potential and identify disease mechanism(s) for most variants studied. The purpose of this study is to clarify the pathological potential for rare nonsynonymous KCNH2 variants seemingly associated with sudden infant death syndrome.

METHODS

Genetic testing of 292 sudden infant death syndrome cases identified 9 KCNH2 variants: E90K, R181Q, A190T, G294V, R791W, P967L, R1005W, R1047L, and Q1068R. Previous studies show R181Q-, P967L-, and R1047L-Kv11.1 channels function similar to wild-type Kv11.1 channels, whereas Q1068R-Kv11.1 channels accelerate inactivation gating. We studied the biochemical and biophysical properties for E90K-, G294V-, R791W-, and R1005W-Kv11.1 channels expressed in human embryonic kidney 293 cells; examined the electronic health records of patients who were genotype positive for the sudden infant death syndrome-linked KCNH2 variants; and simulated their functional impact using computational models of the human ventricular action potential.

RESULTS

Western blot and voltage-clamping analyses of cells expressing E90K-, G294V-, R791W-, and R1005W-Kv11.1 channels demonstrated these variants express and generate peak Kv11.1 current levels similar to cells expressing wild-type-Kv11.1 channels, but R791W- and R1005W-Kv11.1 channels accelerated deactivation and activation gating, respectively. Electronic health records of patients with the sudden infant death syndrome-linked KCNH2 variants showed that the patients had median heart rate-corrected QT intervals <480 ms and none had been diagnosed with long-QT syndrome or experienced cardiac arrest. Simulating the impact of dysfunctional gating variants predicted that they have little impact on ventricular action potential duration.

CONCLUSIONS

We conclude that these rare Kv11.1 missense variants are not long-QT syndrome subtype 2-causative variants and therefore do not represent the pathogenic substrate for sudden infant death syndrome in the variant-positive infants.

Kcnh2 and Kcnj8 interactively regulate skin wound healing and regeneration

Previous studies indicate that ion channels are mediators of bioelectricity promoting wound closure/regeneration in nonmammalian, lower vertebrate systems. The role of ion channels however in regeneration of wounds in mammalian systems that do not regenerate as adults is not yet defined. Using a mammalian model system that allows us to determine differentially expressed genes when skin regenerates and when skin does not regenerate after wound induction, we identified two potassium channels, kcnh2 and kcnj8, to be (1) differentially expressed between the two states and (2) highly expressed after wound induction at the nonregenerative state. We also found that kcnh2 small molecule inhibitor enhanced wound healing while kcnj8 small molecule inhibitor did not. In contrast, kcnj8 activator accelerated wound healing and even augmented the effect of kcnh2 inhibition. These results provide evidence for the first time that potassium channels may mediate skin wound healing and regeneration interactively.

Large-scale mutational analysis of Kv11.1 reveals molecular insights into type 2 long QT syndrome

It has been suggested that deficient protein trafficking to the cell membrane is the dominant mechanism associated with type 2 Long QT syndrome (LQT2) caused by Kv11.1 potassium channel missense mutations, and that for many mutations the trafficking defect can be corrected pharmacologically. However, this inference was based on expression of a small number of Kv11.1 mutations. We performed a comprehensive analysis of 167 LQT2-linked missense mutations in four Kv11.1 structural domains and found that deficient protein trafficking is the dominant mechanism for all domains except for the distal carboxy-terminus. Also, most pore mutations--in contrast to intracellular domain mutations--were found to have severe dominant-negative effects when co-expressed with wild-type subunits. Finally, pharmacological correction of the trafficking defect in homomeric mutant channels was possible for mutations within all structural domains. However, pharmacological correction is dramatically improved for pore mutants when co-expressed with wild-type subunits to form heteromeric channels.