Biophysical mechanisms of myocardium sodium channelopathies

Genetic variants of gene SCN5A encoding the alpha-subunit of cardiac voltage-gated sodium channel Nav1.5 are associated with various diseases, including long QT syndrome (LQT3), Brugada syndrome (BrS1), and progressive cardiac conduction disease (PCCD). In the last decades, the great progress in understanding molecular and biophysical mechanisms of these diseases has been achieved. The LQT3 syndrome is associated with gain-of-function of sodium channels Nav1.5 due to impaired inactivation, enhanced activation, accelerated recovery from inactivation or the late current appearance. In contrast, BrS1 and PCCD are associated with the Nav1.5 loss-of-function, which in electrophysiological experiments can be manifested as reduced current density, enhanced fast or slow inactivation, impaired activation, or decelerated recovery from inactivation. Genetic variants associated with congenital arrhythmias can also disturb interactions of the Nav1.5 channel with different proteins or drugs and cause unexpected reactions to drug administration. Furthermore, mutations can affect post-translational modifications of the channels and their sensitivity to pH and temperature. Here we briefly review the current knowledge on biophysical mechanisms of LQT3, BrS1 and PCCD. We focus on limitations of studies that use heterologous expression systems and induced pluripotent stem cells (iPSC) derived cardiac myocytes and summarize our understanding of genotype-phenotype relations of SCN5A mutations.

Translation reinitiation in c.453delC frameshift mutation of KCNH2 producing functional hERG K+ channels with mild dominant negative effect in the heterozygote patient-derived iPSC cardiomyocytes

The c.453delC (p.Thr152Profs*14) frameshift mutation in KCNH2 is associated with an elevated risk of Long QT syndrome (LQTS) and fatal arrhythmia. Nevertheless, the loss-of-function mechanism underlying this mutation remains unexplored and necessitates an understanding of electrophysiology. To gain insight into the mechanism of the LQT phenotype, we conducted whole-cell patch-clamp and immunoblot assays, utilizing both a heterologous expression system and patient-derived induced pluripotent stem cell-cardiomyocytes (iPSC-CMs) with 453delC-KCNH2. We also explored the site of translational reinitiation by employing LC/MS mass spectrometry. Contrary to the previous assumption of early termination of translation, the findings of this study indicate that the 453delC-KCNH2 leads to an N-terminally truncated hERG channel, a potential from a non-canonical start codon, with diminished expression and reduced current (IhERG). The co-expression with wildtype KCNH2 produced heteromeric hERG channel with mild dominant-negative effect. Additionally, the heterozygote patient-derived iPSC-CMs exhibited prolonged action potential duration and reduced IhERG, which was ameliorated with the use of a hERG activator, PD-118057. The results of our study offer novel insights into the mechanisms involved in congenital LQTS associated with the 453delC mutation of KCNH2. The mutant results in the formation of less functional N-terminal-truncated channels with reduced amount of membrane expression. A hERG activator is capable of correcting abnormalities in both the heterologous expression system and patient-derived iPSC-CMs.

Calmodulin Mutations in Human Disease

Calcium ions (Ca²⁺) are the basis of a unique and potent array of cellular responses. Calmodulin (CaM) is a small but vital protein that is able to rapidly transmit information about changes in Ca²⁺ concentrations to its regulatory targets. CaM plays a critical role in cellular Ca²⁺ signaling, and interacts with a myriad of target proteins. Ca²⁺-dependent modulation by CaM is a major component of a diverse array of processes, ranging from gene expression in neurons to the shaping of the cardiac action potential in heart cells. Furthermore, the protein sequence of CaM is highly evolutionarily conserved, and identical CaM proteins are encoded by three independent genes (CALM1-3) in humans. Mutations within any of these three genes may lead to severe cardiac deficits including severe long QT syndrome (LQTS) and/or catecholaminergic polymorphic ventricular tachycardia (CPVT). Research into disease-associated CaM variants has identified several proteins modulated by CaM that are likely to underlie the pathogenesis of these calmodulinopathies, including the cardiac L-type Ca²⁺ channel (LTCC) Ca_V1.2, and the sarcoplasmic reticulum Ca²⁺ release channel, ryanodine receptor 2 (RyR2). Here, we review the research that has been done to identify calmodulinopathic CaM mutations and evaluate the mechanisms underlying their role in disease.

Prediction of anesthetic torsadogenicity using a human ventricular cell model

This simulation study was designed to predict the torsadogenicity of sevoflurane and propofol in healthy control, as well as type 1 and type 2 long QT syndrome (LQT1 and LQT2, respectively), using the O'Hara-Rudy dynamic model. LQT1 and LQT2 models were simulated by decreasing the conductances of slowly and rapidly activating delayed rectifier K+ currents (IKs and IKr, respectively) by 50%, respectively. Action potential duration at 50% repolarization level (APD50) and diastolic intracellular Ca2+ concentration were measured in epicardial cell during administration of sevoflurane (1 ~ 5%) and propofol (1 ~ 10 μM). Torsadogenicity can be predicted from the relationship between APD50 and diastolic intracellular Ca2+ concentration, which is classified by the decision boundary. Whereas the relationships in control and LQT1 models were distributed on nontorsadogenic side in the presence of sevoflurane at all tested concentrations, those in LQT2 models were shifted to torsadogenic side by concentrations of ≥ 2%. In all three models, propofol shifted the relationships in a direction away from the decision boundary on nontorsadogenic side. Our findings suggest that sevoflurane, but not propofol, exerts torsadogenicity in patients with reduced IKr, such as LQT2 patients. Caution should be paid to the occurrence of arrhythmia during sevoflurane anesthesia in patients with reduced IKr.

Generation of patient-specific induced pluripotent stem cell lines with Type 2 Long QT Syndrome and the KCNH2 c.379C > T pathogenic variant

Type 2 Long QT Syndrome (LQT2) is a rare genetic heart rhythm disorder causing life-threatening arrhythmias. We derived induced pluripotent stem cell (iPSC) lines from two patients with LQT2, aged 18 and 6, both carrying a heterozygous missense mutation on the 3rd and 11th exons of KCNH2. The iPSC lines exhibited normal genomes, expressed pluripotent markers, and differentiated into trilineage embryonic layers. These patient-specific iPSC lines provide a valuable model to study the molecular and functional impact of the hERG channel gene mutation in LQT2 and to develop personalized therapeutic approaches for this syndrome.

Theoretical prediction of early afterdepolarization-evoked triggered activity formation initiating ventricular reentrant arrhythmias

BACKGROUND AND OBJECTIVE

Excessive prolongation of QT interval on ECGs in patients with congenital/acquired long QT syndrome and heart failure is a sign suggesting the development of early afterdepolarization (EAD), an abnormal repolarization in the action potential of ventricular cardiomyocytes. The development of EAD has been believed to be a trigger for fatal tachyarrhythmia, which can be a risk for sudden cardiac death. The role of EAD in triggering ventricular tachycardia (VT) remains unclear. The aim of this study was to elucidate the mechanism of EAD-induced triggered activity formation that leads to the VT such as Torsades de Pointes.

METHODS

We investigated the relationship between EAD and tachyarrhythmia initiation by constructing homogeneous myocardial sheet models consisting of the mid-myocardial cell version of a human ventricular myocyte model and performing simulations of excitation propagation.

RESULTS

A solitary island-like (clustering) occurrence of EADs in the homogeneous myocardial sheet could induce a focal excitation wave. However, reentrant excitation, an entity of tachyarrhythmia, was not able to be triggered regardless of the EAD cluster size when the focal excitation wave formed a repolarization potential difference boundary consisting of only a convex surface. The discontinuous distribution of multiple EAD clusters in the ventricular tissue formed a specific repolarization heterogeneity due to the repolarization potential difference, the shape of which depended on EAD cluster size and placed intervals. We found that the triggered activity was formed in such a manner that the repolarization potential difference boundary included a concave surface.

CONCLUSIONS

The formation of triggered activity that led to tachyarrhythmia required not only the occurrence of EAD onset-mediated focal excitation wave but also a repolarization heterogeneity-based specific repolarization potential difference boundary shape formed within the tissue.

Differential Effects of Remdesivir and Lumacaftor on Homomeric and Heteromeric hERG Channels

The human ether-a-go-go-related gene (hERG) encodes for the pore-forming subunit of the channel that conducts the rapidly activating delayed K+ current (IKr) in the heart. The hERG channel is important for cardiac repolarization, and reduction of its expression in the plasma membrane due to mutations causes long QT syndrome type 2 (LQT2). As such, promoting hERG membrane expression is a strategy to rescue mutant channel function. In the present study, we applied patch clamp, western blots, immunocytochemistry, and quantitative reverse transcription polymerase chain reaction techniques to investigate the rescue effects of two drugs, remdesivir and lumacaftor, on trafficking-defective mutant hERG channels. As our group has recently reported that the antiviral drug remdesivir increases wild-type (WT) hERG current and surface expression, we studied the effects of remdesivir on trafficking-defective LQT2-causing hERG mutants G601S and R582C expressed in HEK293 cells. We also investigated the effects of lumacaftor, a drug used to treat cystic fibrosis, that promotes CFTR protein trafficking and has been shown to rescue membrane expression of some hERG mutations. Our results show that neither remdesivir nor lumacaftor rescued the current or cell-surface expression of homomeric mutants G601S and R582C. However, remdesivir decreased while lumacaftor increased the current and cell-surface expression of heteromeric channels formed by WT hERG and mutant G601S or R582C hERG. We concluded that drugs can differentially affect homomeric WT and heteromeric WT+G601S (or WT+R582C) hERG channels. These findings extend our understanding of drug-channel interaction and may have clinical implications for patients with hERG mutations. SIGNIFICANCE STATEMENT: Various naturally occurring mutations in a cardiac potassium channel called hERG can impair channel function by decreasing cell-surface channel expression, resulting in cardiac electrical disturbances and even sudden cardiac death. Promotion of cell-surface expression of mutant hERG channels represents a strategy to rescue channel function. This work demonstrates that drugs such as remdesivir and lumacaftor can differently affect homomeric and heteromeric mutant hERG channels, which have biological and clinical implications.

Elucidation of ALG10B as a Novel Long-QT Syndrome-Susceptibility Gene

BACKGROUND

Long-QT syndrome (LQTS) is characterized by QT prolongation and increased risk for syncope, seizures, and sudden cardiac death. The majority of LQTS stems from pathogenic mutations in KCNQ1, KCNH2, or SCN5A. However, ≈10% of patients with LQTS remain genetically elusive. We utilized genome sequencing to identify a novel LQTS genetic substrate in a multigenerational genotype-negative LQTS pedigree.

METHODS

Genome sequencing was performed on 5 affected family members. Only rare nonsynonymous variants present in all affected family members were considered. The candidate variant was characterized functionally in patient-derived induced pluripotent stem cell and gene-edited, variant corrected, isogenic control induced pluripotent stem cell-derived cardiomyocytes.

RESULTS

A missense variant (p.G6S) was identified in ALG10B-encoded α-1,2-glucosyltransferase B protein. ALG10B (alpha-1,2-glucosyltransferase B protein) is a known interacting protein of KCNH2-encoded Kv11.1 (HERG [human Ether-à-go-go-related gene]). Compared with isogenic control, ALG10B-p.G6S induced pluripotent stem cell-derived cardiomyocytes showed (1) decreased protein expression of ALG10B (p.G6S, 0.7±0.18, n=8 versus control, 1.25±0.16, n=9; P<0.05), (2) significant retention of HERG in the endoplasmic reticulum (P<0.0005), and (3) a significantly prolonged action potential duration confirmed by both patch clamp (p.G6S, 531.1±38.3 ms, n=15 versus control, 324.1±21.8 ms, n=13; P<0.001) and multielectrode assay (P<0.0001). Lumacaftor-a compound known to rescue HERG trafficking-shortened the pathologically prolonged action potential duration of ALG10B-p.G6S induced pluripotent stem cell-derived cardiomyocytes by 10.6% (n=31 electrodes; P<0.001).

CONCLUSIONS

Here, we demonstrate that ALG10B-p.G6S downregulates ALG10B, resulting in defective HERG trafficking and action potential duration prolongation. Therefore, ALG10B is a novel LQTS-susceptibility gene underlying the LQTS phenotype observed in a multigenerational pedigree. ALG10B mutation analysis may be warranted, especially in genotype-negative patients with an LQT2-like phenotype.

Caveolin-3 and Caveolae regulate ventricular repolarization

RATIONALE

Flask-shaped invaginations of the cardiomyocyte sarcolemma called caveolae require the structural protein caveolin-3 (Cav-3) and host a variety of ion channels, transporters, and signaling molecules. Reduced Cav-3 expression has been reported in models of heart failure, and variants in CAV3 have been associated with the inherited long-QT arrhythmia syndrome. Yet, it remains unclear whether alterations in Cav-3 levels alone are sufficient to drive aberrant repolarization and increased arrhythmia risk.

OBJECTIVE

To determine the impact of cardiac-specific Cav-3 ablation on the electrophysiological properties of the adult mouse heart.

METHODS AND RESULTS

Cardiac-specific, inducible Cav3 homozygous knockout (Cav-3KO) mice demonstrated a marked reduction in Cav-3 expression by Western blot and loss of caveolae by electron microscopy. However, there was no change in macroscopic cardiac structure or contractile function. The QTc interval was increased in Cav-3KO mice, and there was an increased propensity for ventricular arrhythmias. Ventricular myocytes isolated from Cav-3KO mice exhibited a prolonged action potential duration (APD) that was due to reductions in outward potassium currents (Ito, Iss) and changes in inward currents including slowed inactivation of ICa,L and increased INa,L. Mathematical modeling demonstrated that the changes in the studied ionic currents were adequate to explain the prolongation of the mouse ventricular action potential. Results from human iPSC-derived cardiomyocytes showed that shRNA knockdown of Cav-3 similarly prolonged APD.

CONCLUSION

We demonstrate that Cav-3 and caveolae regulate cardiac repolarization and arrhythmia risk via the integrated modulation of multiple ionic currents.

Reclassification of a likely pathogenic Dutch founder variant in KCNH2; implications of reduced penetrance

BACKGROUND

Variants in KCNH2, encoding the human ether a-go-go (hERG) channel that is responsible for the rapid component of the cardiac delayed rectifier K+ current (IKr), are causal to long QT syndrome type 2 (LQTS2). We identified eight index patients with a new variant of unknown significance (VUS), KCNH2:c.2717C > T:p.(Ser906Leu). We aimed to elucidate the biophysiological effect of this variant, to enable reclassification and consequent clinical decision-making.

METHODS

A genotype-phenotype overview of the patients and relatives was created. The biophysiological effects were assessed independently by manual-, and automated calibrated patch clamp. HEK293a cells expressing (i) wild-type (WT) KCNH2, (ii) KCNH2-p.S906L alone (homozygous, Hm) or (iii) KCNH2-p.S906L in combination with WT (1:1) (heterozygous, Hz) were used for manual patching. Automated patch clamp measured the variants function against known benign and pathogenic variants, using Flp-In T-rex HEK293 KCNH2-variant cell lines.

RESULTS

Incomplete penetrance of LQTS2 in KCNH2:p.(Ser906Leu) carriers was observed. In addition, some patients were heterozygous for other VUSs in CACNA1C, PKP2, RYR2 or AKAP9. The phenotype of carriers of KCNH2:p.(Ser906Leu) ranged from asymptomatic to life-threatening arrhythmic events. Manual patch clamp showed a reduced current density by 69.8 and 60.4% in KCNH2-p.S906L-Hm and KCNH2-p.S906L-Hz, respectively. The time constant of activation was significantly increased with 80.1% in KCNH2-p.S906L-Hm compared with KCNH2-WT. Assessment of KCNH2-p.S906L-Hz by calibrated automatic patch clamp assay showed a reduction in current density by 35.6%.

CONCLUSION

The reduced current density in the KCNH2-p.S906L-Hz indicates a moderate loss-of-function. Combined with the reduced penetrance and variable phenotype, we conclude that KCNH2:p.(Ser906Leu) is a low penetrant likely pathogenic variant for LQTS2.

The role of native cysteine residues in the oligomerization of KCNQ1 channels

KCNQ1, the major component of the slow-delayed rectifier potassium channel, is responsible for repolarization of cardiac action potential. Mutations in this channel can lead to a variety of diseases, most notably long QT syndrome. It is currently unknown how many of these mutations change channel function and structure on a molecular level. Since tetramerization is key to proper function and structure of the channel, it is likely that mutations modify the stability of KCNQ1 oligomers. Presently, the C-terminal domain of KCNQ1 has been noted as the driving force for oligomer formation. However, truncated versions of this protein lacking the C-terminal domain still tetramerize. Therefore, we explored the role of native cysteine residues in a truncated construct of human KCNQ1, amino acids 100-370, by blocking potential interactions of cysteines with a nitroxide based spin label. Mobility of the spin labels was investigated with continuous wave electron paramagnetic resonance (CW-EPR) spectroscopy. The oligomerization state was examined by gel electrophoresis. The data provide information on tetramerization of human KCNQ1 without the C-terminal domain. Specifically, how blocking the side chains of native cysteines residues reduces oligomerization. A better understanding of tetramer formation could provide improved understanding of the molecular etiology of long QT syndrome and other diseases related to KCNQ1.

Enzastaurin cardiotoxicity: QT interval prolongation, negative inotropic responses and negative chronotropic action

Several clinical trials observed that enzastaurin prolonged QT interval in cancer patients. However, the mechanism of enzastaurin-induced QT interval prolongation is unclear. Therefore, this study aimed to assess the effect and mechanism of enzastaurin on QT interval and cardiac function. The Langendorff and Ion-Optix MyoCam systems were used to assess the effects of enzastaurin on QT interval, cardiac systolic function and intracellular Ca²⁺ transient in guinea pig hearts and ventricular myocytes. The effects of enzastaurin on the rapid delayed rectifier (I_Kr), the slow delayed rectifier K⁺ current (I_Ks), transient outward potassium current (I_to), action potentials, Ryanodine Receptor 2 (RyR2) and the sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPase 2a (SERCA2a) expression and activity in HEK 293 cell system and primary cardiomyocytes were investigated using whole-cell recording technique and western blotting. We found that enzastaurin significantly prolonged QT interval in guinea pig hearts and increased the action potential duration (APD) in guinea pig cardiomyocytes in a dose-dependent manner. Enzastaurin potently inhibited I_Kr by binding to the human Ether-à-go-go-Related gene (hERG) channel in both open and closed states, and hERG mutant channels, including S636A, S631A, and F656V attenuated the inhibitory effect of enzastaurin. Enzastaurin also moderately decreased I_Ks. Additionally, enzastaurin also induced negative chronotropic action. Moreover, enzastaurin impaired cardiac systolic function and reduced intracellular Ca²⁺ transient via inhibition of RyR2 phosphorylation. Taken together, we found that enzastaurin prolongs QT, reduces heart rate and impairs cardiac systolic function. Therefore, we recommend that electrocardiogram (ECG) and cardiac function should be continuously monitored when enzastaurin is administered to cancer patients.

Generation of two iPSC lines from long QT syndrome patients carrying SNTA1 variants

Long QT syndrome (LQTS) is an inherited cardiovascular disorder characterized by electrical conduction abnormalities leading to arrhythmia, fainting, seizures, and an increased risk of sudden death. There are over 15 genes involved in causing LQTS, including SNTA1. Here we generated two human-induced pluripotent stem cell (iPSC) lines from two LQT patients carrying a missense mutation in SNTA1 (c.1088A > C). Both lines showed normal morphological properties, expressed pluripotency markers, showed a normal karyotype profile, and had the ability to differentiate into the three germ layers, making them a valuable tool to model LQTS to investigate the pathological mechanisms related to this SNTA1 variant.

SARS-CoV-2 spike protein-mediated cardiomyocyte fusion may contribute to increased arrhythmic risk in COVID-19

BACKGROUND

SARS-CoV-2-mediated COVID-19 may cause sudden cardiac death (SCD). Factors contributing to this increased risk of potentially fatal arrhythmias include thrombosis, exaggerated immune response, and treatment with QT-prolonging drugs. However, the intrinsic arrhythmic potential of direct SARS-CoV-2 infection of the heart remains unknown.

OBJECTIVE

To assess the cellular and electrophysiological effects of direct SARS-CoV-2 infection of the heart using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs).

METHODS

hiPSC-CMs were transfected with recombinant SARS-CoV-2 spike protein (CoV-2 S) or CoV-2 S fused to a modified Emerald fluorescence protein (CoV-2 S-mEm). Cell morphology was visualized using immunofluorescence microscopy. Action potential duration (APD) and cellular arrhythmias were measured by whole cell patch-clamp. Calcium handling was assessed using the Fluo-4 Ca2+ indicator.

RESULTS

Transfection of hiPSC-CMs with CoV-2 S-mEm produced multinucleated giant cells (syncytia) displaying increased cellular capacitance (75±7 pF, n = 10 vs. 26±3 pF, n = 10; P<0.0001) consistent with increased cell size. The APD90 was prolonged significantly from 419±26 ms (n = 10) in untransfected hiPSC-CMs to 590±67 ms (n = 10; P<0.05) in CoV-2 S-mEm-transfected hiPSC-CMs. CoV-2 S-induced syncytia displayed delayed afterdepolarizations, erratic beating frequency, and calcium handling abnormalities including calcium sparks, large "tsunami"-like waves, and increased calcium transient amplitude. After furin protease inhibitor treatment or mutating the CoV-2 S furin cleavage site, cell-cell fusion was no longer evident and Ca2+ handling returned to normal.

CONCLUSION

The SARS-CoV-2 spike protein can directly perturb both the cardiomyocyte's repolarization reserve and intracellular calcium handling that may confer the intrinsic, mechanistic substrate for the increased risk of SCD observed during this COVID-19 pandemic.

Understanding Calmodulin Variants Affecting Calcium-Dependent Inactivation of L-Type Calcium Channels through Whole-Cell Simulation of the Cardiac Ventricular Myocyte

Mutations in the calcium-sensing protein calmodulin (CaM) have been linked to two cardiac arrhythmia diseases, Long QT Syndrome 14 (LQT14) and Catecholaminergic Polymorphic Ventricular Tachycardia Type 4 (CPVT4), with varying degrees of severity. Functional characterization of the CaM mutants most strongly associated with LQT14 show a clear disruption of the calcium-dependent inactivation (CDI) of the L-Type calcium channel (LCC). CPVT4 mutants on the other hand are associated with changes in their affinity to the ryanodine receptor. In clinical studies, some variants have been associated with both CPVT4 and LQT15. This study uses simulations in a model for excitation-contraction coupling in the rat ventricular myocytes to understand how LQT14 variant might give the functional phenotype similar to CPVT4. Changing the CaM-dependent transition rate by a factor of 0.75 corresponding to the D96V variant and by a factor of 0.90 corresponding to the F142L or N98S variants, in a physiologically based stochastic model of the LCC prolonger, the action potential duration changed by a small amount in a cardiac myocyte but did not disrupt CICR at 1, 2, and 4 Hz. Under beta-adrenergic simulation abnormal excitation-contraction coupling was observed above 2 Hz pacing for the mutant CaM. The same conditions applied under beta-adrenergic stimulation led to the rapid onset of arrhythmia in the mutant CaM simulations. Simulations with the LQT14 mutations under the conditions of rapid pacing with beta-adrenergic stimulation drives the cardiac myocyte toward an arrhythmic state known as Ca2+ overload. These simulations provide a mechanistic link to a disease state for LQT14-associated mutations in CaM to yield a CPVT4 phenotype. The results show that small changes to the CaM-regulated inactivation of LCC promote arrhythmia and underscore the significance of CDI in proper heart function.

Impaired CaV1.2 inactivation reduces the efficacy of calcium channel blockers in the treatment of LQT8

Mutations in the CaV1.2 L-type calcium channel can cause a profound form of long-QT syndrome known as long-QT type 8 (LQT8), which results in cardiac arrhythmias that are often fatal in early childhood. A growing number of such pathogenic mutations in CaV1.2 have been identified, increasing the need for targeted therapies. As many of these mutations reduce channel inactivation; resulting in excess Ca2+ entry during the action potential, calcium channel blockers (CCBs) would seem to represent a promising treatment option. Yet CCBs have been unsuccessful in the treatment of LQT8. Here, we demonstrate that this lack of efficacy likely stems from the impact of the mutations on CaV1.2 channel inactivation. As CCBs are known to preferentially bind to the inactivated state of the channel, mutation-dependent deficits in inactivation result in a decrease in use-dependent block of the mutant channel. Further, application of the CCB verapamil to induced pluripotent stem cell (iPSC) derived cardiomyocytes from an LQT8 patient demonstrates that this loss of use-dependent block translates to a lack of efficacy in correcting the LQT phenotype. As a growing number of channelopathic mutations demonstrate effects on channel inactivation, reliance on state-dependent blockers may leave a growing population of patients without a viable treatment option. This biophysical understanding of the interplay between inactivation deficits and state-dependent block may provide a new avenue to guide the development of improved therapies.

The N-linker region of hERG1a upregulates hERG1b potassium channels

A major physiological role of hERG1 (human Ether-á-go-go-Related Gene 1) potassium channels is to repolarize cardiac action potentials. Two isoforms, hERG1a and hERG1b, associate to form the potassium current IKr in cardiomyocytes. Inherited mutations in hERG1a or hERG1b cause prolonged cardiac repolarization, long QT syndrome, and sudden death arrhythmia. hERG1a subunits assemble with and enhance the number of hERG1b subunits at the plasma membrane, but the mechanism for the increase in hERG1b by hERG1a is not well understood. Here, we report that the hERG1a N-terminal region expressed in trans with hERG1b markedly increased hERG1b currents and increased biotin-labeled hERG1b protein at the membrane surface. hERG1b channels with a deletion of the N-terminal 1b domain did not have a measurable increase in current or biotinylated protein when coexpressed with hERG1a N-terminal regions, indicating that the 1b domain was required for the increase in hERG1b. Using a biochemical pull-down interaction assay and a FRET hybridization experiment, we detected a direct interaction between the hERG1a N-terminal region and the hERG1b N-terminal region. Using engineered deletions and alanine mutagenesis, we identified a short span of amino acids at positions 216 to 220 within the hERG1a "N-linker" region that were necessary for the upregulation of hERG1b. We propose that direct structural interactions between the hERG1a N-linker region and the hERG1b 1b domain increase hERG1b at the plasma membrane. Mechanisms regulating hERG1a and hERG1b are likely critical for cardiac function, may be disrupted by long QT syndrome mutants, and serve as potential targets for therapeutics.

A massively parallel assay accurately discriminates between functionally normal and abnormal variants in a hotspot domain of KCNH2

Many genes, including KCNH2, contain "hotspot" domains associated with a high density of variants associated with disease. This has led to the suggestion that variant location can be used as evidence supporting classification of clinical variants. However, it is not known what proportion of all potential variants in hotspot domains cause loss of function. Here, we have used a massively parallel trafficking assay to characterize all single-nucleotide variants in exon 2 of KCNH2, a known hotspot for variants that cause long QT syndrome type 2 and an increased risk of sudden cardiac death. Forty-two percent of KCNH2 exon 2 variants caused at least 50% reduction in protein trafficking, and 65% of these trafficking-defective variants exerted a dominant-negative effect when co-expressed with a WT KCNH2 allele as assessed using a calibrated patch-clamp electrophysiology assay. The massively parallel trafficking assay was more accurate (AUC of 0.94) than bioinformatic prediction tools (REVEL and CardioBoost, AUC of 0.81) in discriminating between functionally normal and abnormal variants. Interestingly, over half of variants in exon 2 were found to be functionally normal, suggesting a nuanced interpretation of variants in this "hotspot" domain is necessary. Our massively parallel trafficking assay can provide this information prospectively.

A short focus, azithromycin in the treatment of respiratory viral infection COVID-19: efficacy or inefficacy?

Azithromycin is a macrolide antibiotic. Recent evidence has demonstrated in vitro activity against a wide variety of respiratory tract viruses, including SARS-CoV-2 responsible for the current global pandemic COVID-19. A mechanism of action acting on different phases of the viral cycle is assumed. In addition to its in vitro antiviral properties, some evidence also suggests immunomodulatory and antifibrotic activity. These properties of azithromycin could be useful in the treatment of viral respiratory tract infections such as COVID-19. However, clinical data on the antiviral efficacy of azithromycin in the treatment of respiratory tract infections are inconsistent, both when used as monotherapy and in polypharmacological combination. In addition, cases of azithromycin-induced QT long and malignant arrhythmias are reported. In this short review, we attempt to determine the role of azithromycin in the treatment of viral respiratory tract infections such as COVID-19, therapeutic efficacy, or inefficacy?

Investigation of PAS and CNBH domain interactions in hERG channels and effects of long-QT syndrome-causing mutations with surface plasmon resonance

Human ether-á-go-go-related gene (hERG) channels are key regulators of cardiac repolarization, neuronal excitability, and tumorigenesis. hERG channels contain N-terminal Per-Arnt-Sim (PAS) and C-terminal cyclic nucleotide-binding homology (CNBH) domains with many long-QT syndrome (LQTS)-causing mutations located at the interface between these domains. Despite the importance of PAS/CNBH domain interactions, little is known about their affinity. Here, we used the surface plasmon resonance (SPR) technique to investigate interactions between isolated PAS and CNBH domains and the effects of LQTS-causing mutations R20G, N33T, and E58D, located at the PAS/CNBH domain interface, on these interactions. We determined that the affinity of the PAS/CNBH domain interactions was ∼1.4 μM. R20G and E58D mutations had little effect on the domain interaction affinity, while N33T abolished the domain interactions. Interestingly, mutations in the intrinsic ligand, a conserved stretch of amino acids occupying the beta-roll cavity in the CNBH domain, had little effect on the affinity of PAS/CNBH domain interactions. Additionally, we determined that the isolated PAS domains formed oligomers with an interaction affinity of ∼1.6 μM. Coexpression of the isolated PAS domains with the full-length hERG channels or addition of the purified PAS protein inhibited hERG currents. These PAS/PAS interactions can have important implications for hERG function in normal and pathological conditions associated with increased surface density of channels or interaction with other PAS-domain-containing proteins. Taken together, our study provides the first account of the binding affinities for wild-type and mutant hERG PAS and CNBH domains and highlights the potential functional significance of PAS/PAS domain interactions.

Obesity Potentiates the Risk of Drug-Induced Long QT Syndrome - Preliminary Evidence from WNIN/Ob Spontaneously Obese Rat

Drug-induced long QT syndrome (DI-LQTS) is fatal and known to have a higher incidence in women rather than in men. Multiple risk factors potentiate the incidence of DI-LQTS, but the actual contribution of obesity remains largely unexplored. Correspondingly, the present study is aimed to evaluate the susceptibility of DI-LQTS in WNIN/Ob rat in comparison with its lean counterpart using 3-lead electrocardiography. Four- and eight-month-old female WNIN/Ob and their lean controls were used for the experimentation. Non-invasive blood pressure measurement and total body electric conductivity (TOBEC) analysis were carried out. After the baseline evaluations, animals were anesthetized with Ketamine (50 mg/kg). Haloperidol (12.5 mg/kg single dose) was administered intraperitoneally and ECG was taken at 0, 10, 20, 30, 60 min, and 24 h time points. Myocardial lystes were used to assess the BNP, protein carbonylation, and hydroxyproline content. Adiposity, as assessed by TOBEC, is higher in obese rats with elevated mean arterial blood pressure. Baseline-corrected QT interval (QTc) is significantly higher in the obese rat with a wider QRS complex. The incidence of PVC and VT are more intense in the obese rat. Haloperidol-induced QT prolongation in obese rats was rapidly induced than in lean, which was observed to remain till 24 h in obese groups while normalized in lean controls. Higher levels of BNP, protein carbonylation, hydroxyproline content, and relative heart weights indicated the presence of cardiac hypertrophy. The study provides preliminary evidence that obesity can be a potential risk factor for DI-LQTS with faster onset and longer subsistence.

Molecular determinants of pro-arrhythmia proclivity of d- and l-sotalol via a multi-scale modeling pipeline

Drug isomers may differ in their proarrhythmia risk. An interesting example is the drug sotalol, an antiarrhythmic drug comprising d- and l- enantiomers that both block the hERG cardiac potassium channel and confer differing degrees of proarrhythmic risk. We developed a multi-scale in silico pipeline focusing on hERG channel – drug interactions and used it to probe and predict the mechanisms of pro-arrhythmia risks of the two enantiomers of sotalol. Molecular dynamics (MD) simulations predicted comparable hERG channel binding affinities for d- and l-sotalol, which were validated with electrophysiology experiments. MD derived thermodynamic and kinetic parameters were used to build multi-scale functional computational models of cardiac electrophysiology at the cell and tissue scales. Functional models were used to predict inactivated state binding affinities to recapitulate electrocardiogram (ECG) QT interval prolongation observed in clinical data. Our study demonstrates how modeling and simulation can be applied to predict drug effects from the atom to the rhythm for dl-sotalol and also increased proarrhythmia proclivity of d- vs. l-sotalol when accounting for stereospecific beta-adrenergic receptor blocking.

Mechanisms underlying age-associated manifestation of cardiac sodium channel gain-of-function

Cardiac action potentials are initiated by sodium ion (Na+) influx through voltage-gated Na+ channels. Na+ channel gain-of-function (GOF) can arise in inherited conditions due to mutations in the gene encoding the cardiac Na+ channel, such as Long QT syndrome type 3 (LQT3). LQT3 can be a "concealed" disease, as patients with LQT3-associated mutations can remain asymptomatic until later in life; however, arrhythmias can also arise early in life in LQT3 patients, demonstrating a complex age-associated manifestation. We and others recently demonstrated that cardiac Na+ channels preferentially localize at the intercalated disc (ID) in adult cardiac tissue, which facilitates ephaptic coupling and formation of intercellular Na+ nanodomains that regulate pro-arrhythmic early afterdepolarization (EAD) formation in tissue with Na+ channel GOF. Several properties related to ephaptic coupling vary with age, such as cell size and Na+ channel and gap junction (GJ) expression and distribution: neonatal cells have immature IDs, with Na+ channels and GJs primarily diffusively distributed, while adult myocytes have mature IDs with preferentially localized Na+ channels and GJs. Here, we perform an in silico study varying critical age-dependent parameters to investigate mechanisms underlying age-associated manifestation of Na+ channel GOF in a model of guinea pig cardiac tissue. Simulations predict that total Na+ current conductance is a critical factor in action potential duration (APD) prolongation. We find a complex cell size/ Na+ channel expression relationship: increases in cell size (without concurrent increases in Na+ channel expression) suppress EAD formation, while increases in Na+ channel expression (without concurrent increases in cell size) promotes EAD formation. Finally, simulations with neonatal and early age-associated parameters predict normal APD with minimal dependence on intercellular cleft width; however, variability in cellular properties can lead to EADs presenting in early developmental stages. In contrast, for adult-associated parameters, EAD formation is highly dependent on cleft width, consistent with a mechanism underlying the age-associated manifestation of the Na+ channel GOF.

Long QT Syndrome KCNH2 Variant Induces hERG1a/1b Subunit Imbalance in Patient-Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes

[Figure: see text].

A Novel Mutation in the KCNH2 Gene Associatedwith Long QT Syndrome: A Case Report

OBJECTIVE

Long QT syndrome is a cardiovascular disease with a prolonged QT interval.

CASE REPORT

We report a 22-year-old woman presenting with frequent syncopal episodes two months after childbirth. Electrocardiography showed a sinus rhythm, QT interval prolongation, and Torsade de Pointes. Her mother had experienced an episode of syncope, but her father had not. Genetic analyses revealed that a new mutation in the KCNH2 gene, the c.2108dupA mutation (p.H703Qfs*20, exon8, M_000238), was found in the patient and in her mother and sister.

CONCLUSION

The c.2108dupA mutation (p.H703Qfs*20, exon8, M_000238) is the first reported case of a KCNH2 mutation at this site.

Voltage-Gated Calcium Channels in Nonexcitable Tissues

The identification of a gain-of-function mutation in CACNA1C as the cause of Timothy syndrome, a rare disorder characterized by cardiac arrhythmias and syndactyly, highlighted roles for the L-type voltage-gated Ca2+ channel CaV1.2 in nonexcitable cells. Previous studies in cells and animal models had suggested that several voltage-gated Ca2+ channels (VGCCs) regulated critical signaling events in various cell types that are not expected to support action potentials, but definitive data were lacking. VGCCs occupy a special position among ion channels, uniquely able to translate membrane excitability into the cytoplasmic Ca2+ changes that underlie the cellular responses to electrical activity. Yet how these channels function in cells not firing action potentials and what the consequences of their actions are in nonexcitable cells remain critical questions. The development of new animal and cellular models and the emergence of large data sets and unbiased genome screens have added to our understanding of the unanticipated roles for VGCCs in nonexcitable cells. Here, we review current knowledge of VGCC regulation and function in nonexcitable tissues and cells, with the goal of providing a platform for continued investigation.

Insights into Cardiac IKs (KCNQ1/KCNE1) Channels Regulation

The delayed rectifier potassium IKs channel is an important regulator of the duration of the ventricular action potential. Hundreds of mutations in the genes (KCNQ1 and KCNE1) encoding the IKs channel cause long QT syndrome (LQTS). LQTS is a heart disorder that can lead to severe cardiac arrhythmias and sudden cardiac death. A better understanding of the IKs channel (here called the KCNQ1/KCNE1 channel) properties and activities is of great importance to find the causes of LQTS and thus potentially treat LQTS. The KCNQ1/KCNE1 channel belongs to the superfamily of voltage-gated potassium channels. The KCNQ1/KCNE1 channel consists of both the pore-forming subunit KCNQ1 and the modulatory subunit KCNE1. KCNE1 regulates the function of the KCNQ1 channel in several ways. This review aims to describe the current structural and functional knowledge about the cardiac KCNQ1/KCNE1 channel. In addition, we focus on the modulation of the KCNQ1/KCNE1 channel and its potential as a target therapeutic of LQTS.

In utero exposure to nicotine abolishes the postnatal response of the cardiac sodium current to isoproterenol in newborn rabbit atrium

BACKGROUND

In utero exposure to tobacco smoke is associated with sudden infant death syndrome (SIDS) and cardiac arrhythmias in newborns. The arrhythmogenic mechanisms seem linked to alterations of the cardiac sodium current (I_Na). We previously reported that in utero exposure to nicotine delays the postnatal development of the heart sinoatrial node in rabbits and altered expression of the sodium channels Na_V1.5 and Na_V1.1 in the atrium surrounding it. These channels react differently to sympathetic stimulation.

OBJECTIVE

The purpose of this study was to test whether nicotine altered the response of I_Na to stimulation by the β-adrenoreceptor agonist isoproterenol in atrial myocytes. Our hypothesis is that changes in the sympathetic response of sinoatrial node peripheral cells may create a substrate for arrhythmia.

METHODS

Using the patch-clamp technique we measured the effect of nicotine on the response of I_Na to adrenergic stimulation in isolated cardiomyocytes.

RESULTS

Isoproterenol increased I_Na by 50% in newborn sham rabbits but had no effect in newborn rabbits exposed to nicotine in utero. Our data also show that nicotine increases the late sodium current, an effect that may promote QT prolongation.

CONCLUSION

We provide the first evidence linking fetal exposure to nicotine to long-term alterations of I_Na response to isoproterenol. These changes may impair I_Na adaptation to sympathetic tone and prevent awakening from sleep apnea, thus leading to arrhythmias that could potentially be involved in SIDS. Our data also raise concerns about the use of nicotine replacement therapies for pregnant women.

Functional Impact of BeKm-1, a High-Affinity hERG Blocker, on Cardiomyocytes Derived from Human-Induced Pluripotent Stem Cells

I_Kr current, a major component of cardiac repolarization, is mediated by human Ether-à-go-go-Related Gene (hERG, K_v11.1) potassium channels. The blockage of these channels by pharmacological compounds is associated to drug-induced long QT syndrome (LQTS), which is a life-threatening disorder characterized by ventricular arrhythmias and defects in cardiac repolarization that can be illustrated using cardiomyocytes derived from human-induced pluripotent stem cells (hiPS-CMs). This study was meant to assess the modification in hiPS-CMs excitability and contractile properties by BeKm-1, a natural scorpion venom peptide that selectively interacts with the extracellular face of hERG, by opposition to reference compounds that act onto the intracellular face. Using an automated patch-clamp system, we compared the affinity of BeKm-1 for hERG channels with some reference compounds. We fully assessed its effects on the electrophysiological, calcium handling, and beating properties of hiPS-CMs. By delaying cardiomyocyte repolarization, the peptide induces early afterdepolarizations and reduces spontaneous action potentials, calcium transients, and contraction frequencies, therefore recapitulating several of the critical phenotype features associated with arrhythmic risk in drug-induced LQTS. BeKm-1 exemplifies an interesting reference compound in the integrated hiPS-CMs cell model for all drugs that may block the hERG channel from the outer face. Being a peptide that is easily modifiable, it will serve as an ideal molecular platform for the design of new hERG modulators displaying additional functionalities.

Late INa Blocker GS967 Supresses Polymorphic Ventricular Tachycardia in a Transgenic Rabbit Model of Long QT Type 2

BACKGROUND

Long QT syndrome has been associated with sudden cardiac death likely caused by early afterdepolarizations (EADs) and polymorphic ventricular tachycardias (PVTs). Suppressing the late sodium current (INaL) may counterbalance the reduced repolarization reserve in long QT syndrome and prevent EADs and PVTs.

METHODS

We tested the effects of the selective INaL blocker GS967 on PVT induction in a transgenic rabbit model of long QT syndrome type 2 using intact heart optical mapping, cellular electrophysiology and confocal Ca2+ imaging, and computer modeling.

RESULTS

GS967 reduced ventricular fibrillation induction under a rapid pacing protocol (n=7/14 hearts in control versus 1/14 hearts at 100 nmol/L) without altering action potential duration or restitution and dispersion. GS967 suppressed PVT incidences by reducing Ca2+-mediated EADs and focal activity during isoproterenol perfusion (at 30 nmol/L, n=7/12 and 100 nmol/L n=8/12 hearts without EADs and PVTs). Confocal Ca2+ imaging of long QT syndrome type 2 myocytes revealed that GS967 shortened Ca2+ transient duration via accelerating Na+/Ca2+ exchanger (INCX)-mediated Ca2+ efflux from cytosol, thereby reducing EADs. Computer modeling revealed that INaL potentiates EADs in the long QT syndrome type 2 setting through (1) providing additional depolarizing currents during action potential plateau phase, (2) increasing intracellular Na+ (Nai) that decreases the depolarizing INCX thereby suppressing the action potential plateau and delaying the activation of slowly activating delayed rectifier K+ channels (IKs), suggesting important roles of INaL in regulating Nai.

CONCLUSIONS

Selective INaL blockade by GS967 prevents EADs and abolishes PVT in long QT syndrome type 2 rabbits by counterbalancing the reduced repolarization reserve and normalizing Nai. Graphic Abstract: A graphic abstract is available for this article.

Sex-specific IKAS activation in rabbit ventricles with drug-induced QT prolongation

BACKGROUND

Female sex is a known risk factor for drug-induced long QT syndrome (diLQTS). We recently demonstrated a sex difference in apamin-sensitive small-conductance Ca²⁺-activated K⁺ current (I_KAS) activation during β-adrenergic stimulation.

OBJECTIVE

The purpose of this study was to test the hypothesis that there is a sex difference in I_KAS in the rabbit models of diLQTS.

METHODS

We evaluated the sex difference in ventricular repolarization in 15 male and 22 female Langendorff-perfused rabbit hearts with optical mapping techniques during atrial pacing. HMR1556 (slowly activating delayed rectifier K⁺ current [I_Ks] blocker), E4031 (rapidly activating delayed rectifier K⁺ current [I_Kr] blocker) and sea anemone toxin (ATX-II, late Na⁺ current [I_NaL] activator) were used to simulate types 1-3 long QT syndrome, respectively. Apamin, an I_KAS blocker, was then added to determine the magnitude of further QT prolongation.

RESULTS

HMR1556, E4031, and ATX-II led to the prolongation of action potential duration at 80% repolarization (APD₈₀) in both male and female ventricles at pacing cycle lengths of 300-400 ms. Apamin further prolonged APD₈₀ (pacing cycle length 350 ms) from 187.8±4.3 to 206.9±7.1 (P=.014) in HMR1556-treated, from 209.9±7.8 to 224.9±7.8 (P=.003) in E4031-treated, and from 174.3±3.3 to 188.1±3.0 (P=.0002) in ATX-II-treated female hearts. Apamin did not further prolong the APD₈₀ in male hearts. The Ca_i transient duration (Ca_iTD) was significantly longer in diLQTS than baseline but without sex differences. Apamin did not change Ca_iTD.

CONCLUSION

We conclude that I_KAS is abundantly increased in female but not in male ventricles with diLQTS. Increased I_KAS helps preserve the repolarization reserve in female ventricles treated with I_Ks and I_Kr blockers or I_NaL activators.

Silencing of CCR4-NOT complex subunits affects heart structure and function

The identification of genetic variants that predispose individuals to cardiovascular disease and a better understanding of their targets would be highly advantageous. Genome-wide association studies have identified variants that associate with QT-interval length (a measure of myocardial repolarization). Three of the strongest associating variants (single-nucleotide polymorphisms) are located in the putative promotor region of CNOT1, a gene encoding the central CNOT1 subunit of CCR4-NOT: a multifunctional, conserved complex regulating gene expression and mRNA stability and turnover. We isolated the minimum fragment of the CNOT1 promoter containing all three variants from individuals homozygous for the QT risk alleles and demonstrated that the haplotype associating with longer QT interval caused reduced reporter expression in a cardiac cell line, suggesting that reduced CNOT1 expression might contribute to abnormal QT intervals. Systematic siRNA-mediated knockdown of CCR4-NOT components in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) revealed that silencing CNOT1 and other CCR4-NOT genes reduced their proliferative capacity. Silencing CNOT7 also shortened action potential duration. Furthermore, the cardiac-specific knockdown of Drosophila orthologs of CCR4-NOT genes in vivo (CNOT1/Not1 and CNOT7/8/Pop2) was either lethal or resulted in dilated cardiomyopathy, reduced contractility or a propensity for arrhythmia. Silencing CNOT2/Not2, CNOT4/Not4 and CNOT6/6L/twin also affected cardiac chamber size and contractility. Developmental studies suggested that CNOT1/Not1 and CNOT7/8/Pop2 are required during cardiac remodeling from larval to adult stages. To summarize, we have demonstrated how disease-associated genes identified by GWAS can be investigated by combining human cardiomyocyte cell-based and whole-organism in vivo heart models. Our results also suggest a potential link of CNOT1 and CNOT7/8 to QT alterations and further establish a crucial role of the CCR4-NOT complex in heart development and function.This article has an associated First Person interview with the first author of the paper.

The Citrus Flavonoid Hesperetin Has an Inadequate Anti-Arrhythmic Profile in the ΔKPQ NaV1.5 Mutant of the Long QT Type 3 Syndrome

Type 3 long QT syndromes (LQT3) are associated with arrhythmogenic gain-of-function mutations in the cardiac voltage-gated Na⁺ channel (hNa_V1.5). The citrus flavanone hesperetin (HSP) was previously suggested as a template molecule to develop new anti-arrhythmic drugs, as it blocks slowly-inactivating currents carried by the LQT3-associated hNa_V1.5 channel mutant R1623Q. Here we investigated whether HSP also has potentially beneficial effects on another LQT3 hNa_V1.5 channel variant, the ΔKPQ, which is associated to lethal ventricular arrhythmias. We used whole-cell patch-clamp to record Na⁺ currents (I_Na) in HEK293T cells transiently expressing hNa_V1.5 wild type or ΔKPQ mutant channels. HSP blocked peak I_Na and the late I_Na carried by ΔKPQ mutant channels with an effective concentration of ≈300 μM. This inhibition was largely voltage-independent and tonic. HSP decreased the rate of inactivation of ΔKPQ channels and, consequently, was relatively weak in reducing the intracellular Na⁺ load in this mutation. We conclude that, although HSP has potential value for the treatment of the R1623Q LQT3 variant, this compound is inadequate to treat the LQT3 associated to the ΔKPQ genetic variant. Our results underscore the precision medicine rationale of better understanding the basic pathophysiological and pharmacological mechanisms to provide phenotype- genotype-directed individualization of treatment.

Manifestations of left ventricular dysfunction and arrhythmia in patients with chronic hypoparathyroidism and pseudohypoparathyroidism: a preliminary study

BACKGROUND

Cardiac damage triggered by severe hypocalcemia is well known. However, the role of chronic hypoparathyroidism (HP) and pseudohypoparathyroidism (PHP) in cardiac health is still unclear. We investigated the effect of chronic HP and PHP on cardiac structure and conductive function in patients compiling with treatment.

METHODS

The study included 18 patients with HP and eight with PHP aged 45.4 ± 15.4 and 22.1 ± 6.4 years, respectively with a previously regular follow-up. In addition, 26 age- and sex-matched healthy controls were included. General characteristics and biochemical indices were recorded. Cardiac function and structure were assessed by estimation of myocardial enzymes, B-type natriuretic peptide (BNP), and echocardiography. The 12-lead electrocardiogram and 24-h Holter electrocardiography were performed to evaluate the conductive function.

RESULTS

Levels of serum calcium in HP and PHP were 2.05 ± 0.16 mmol/L and 2.25 ± 0.19 mmol/L, respectively. The levels of myocardial enzyme and BNP were within the normal range. Adjusting for age at evaluation and body mass index, all M-mode measurements, left ventricular mass (LVM), LVM index (LVMI) and relative wall thickness (RWT) were comparable between patients and controls. Prolongation of corrected QT (QTc) intervals occurred in 52.6% (10/19) of patients, and 6.7% (1/15) of patients manifested more than 100 episodes of supraventricular and ventricular extrasystoles, as well as supraventricular tachycardia. None of the above arrhythmias was related to a severe clinical event.

CONCLUSIONS

From this pilot study, patients diagnosed with HP and PHP and well-controlled serum calcium levels manifested normal cardiac morphology and ventricular function, except for prolonged QTc intervals, and a small percentage of mild arrhythmias needing further investigation.

Generation of two human induced pluripotent stem cell (hiPSC) lines from a long QT syndrome South African founder population

We generated PSMi001-A and PSMi008-A hiPSC lines from two individuals belonging to a South African (SA) founder population in which the malignant KCNQ1-A341V mutation cosegregates with the Long QT Syndrome (LQTS) phenotype. PSMi001-A was derived from an asymptomatic KCNQ1-A341V mutation carrier, whereas PSMi008-A was derived from a healthy non-mutation carrier, heterozygous for the minor variant rs16847548 on the NOS1AP gene, associated with QT prolongation in the general population, and with a greater risk for cardiac arrest in the affected members of the SA founder population. The hiPSCs, generated using the Yamanaka's retroviruses, display pluripotent stem cell features and trilineage differentiation potential.

Long QT Syndrome: Genetics and Future Perspective

Long QT syndrome (LQTS) is an inherited primary arrhythmia syndrome that may present with malignant arrhythmia and, rarely, risk of sudden death. The clinical symptoms include palpitations, syncope, and anoxic seizures secondary to ventricular arrhythmia, classically torsade de pointes. This predisposition to malignant arrhythmia is from a cardiac ion channelopathy that results in delayed repolarization of the cardiomyocyte action potential. The QT interval on the surface electrocardiogram is a summation of the individual cellular ventricular action potential durations, and hence is a surrogate marker of the abnormal cellular membrane repolarization. Severely affected phenotypes administered current standard of care therapies may not be fully protected from the occurrence of cardiac arrhythmias. There are 17 different subtypes of LQTS associated with monogenic mutations of 15 autosomal dominant genes. It is now possible to model the various LQTS phenotypes through the generation of patient-specific induced pluripotent stem cell-derived cardiomyocytes. RNA interference can silence or suppress the expression of mutant genes. Thus, RNA interference can be a potential therapeutic intervention that may be employed in LQTS to knock out mutant mRNAs which code for the defective proteins. CRISPR/Cas9 is a genome editing technology that offers great potential in elucidating gene function and a potential therapeutic strategy for monogenic disease. Further studies are required to determine whether CRISPR/Cas9 can be employed as an efficacious and safe rescue of the LQTS phenotype. Current progress has raised opportunities to generate in vitro human cardiomyocyte models for drug screening and to explore gene therapy through genome editing.

Interleukin-6 inhibition of hERG underlies risk for acquired long QT in cardiac and systemic inflammation

Increased proinflammatory interleukin-6 (IL-6) levels are associated with acquired long QT-syndrome (LQTS) in patients with systemic inflammation, leading to higher risks for life-threatening polymorphic ventricular tachycardia such as Torsades de Pointes. However, the functional and molecular mechanisms of this association are not known. In most cases of acquired LQTS, the target ion channel is the human ether-á-go-go-related gene (hERG) encoding the rapid component of the delayed rectifier K current, I_Kr, which plays a critical role in cardiac repolarization. Here, we tested the hypothesis that IL-6 may cause QT prolongation by suppressing I_Kr. Electrophysiological and biochemical assays were used to assess the impact of IL-6 on the functional expression of I_Kr in HEK293 cells and adult guinea-pig ventricular myocytes (AGPVM). In HEK293 cells, IL-6 alone or in combination with the soluble IL-6 receptor (IL-6R), produced a significant depression of I_Kr peak and tail current densities. Block of IL-6R or Janus kinase (JAK) reversed the inhibitory effects of IL-6 on I_Kr. In AGPVM, IL-6 prolonged action potential duration (APD) which was further prolonged in the presence of IL-6R. Similar to heterologous cells, IL-6 reduced endogenous guinea pig ERG channel mRNA and protein expression. The data are first to demonstrate that IL-6 inhibition of I_Kr and the resulting prolongation of APD is mediated via IL-6R and JAK pathway activation and forms the basis for the observed clinical QT interval prolongation. These novel findings may guide the development of targeted anti-arrhythmic therapeutic interventions in patients with LQTS and inflammatory disorders.

Corrected QT interval on the electrocardiogram after liver transplantation: Surrogate marker of poor clinical outcomes?

BACKGROUND

Prolongation of corrected QT interval (QTc) on the electrocardiogram is associated with cardiac arrhythmia and sudden death. Changes in the QTc (corrected QT) interval before and after liver transplantation (LT) for the treatment of liver cirrhosis (LC) and its association with clinical outcomes have not been fully evaluated.

METHODS

From January 2011 to May 2016, consecutive 516 consecutive recipients were enrolled into LT registry and the median follow-up was 31 months (IQR 12-52). Patients with an available electrocardiogram before LT and 1 month after from LT were analyzed. Patients were divided into 2 groups according to prolonged QTc interval. The patient groups were analyzed separately according whether the electrocardiogram was preoperative or postoperative. The primary outcome was all-cause death during the follow-up period.

RESULTS

A total of 283 patients were enrolled in the study. In the preoperative QTc prolongation group, there was not a significant rate difference in all-cause mortality in multivariate analysis (hazard ratio [HR], 0.94; 95% confidence interval [CI], 0.53-1.66; P = 0.26). However, in the postoperative QTc prolongation group, mortality was significantly increased (HR, 1.78; 95%CI, 1.05-3.03; P = 0.03) in patients who underwent LT.

CONCLUSION

In patients who underwent LT for LC, postoperative QTc prolongation on ECG, rather than preoperative, is associated with mortality. Larger clinical trials are needed to support this finding.

Predicting the Functional Impact of KCNQ1 Variants of Unknown Significance

BACKGROUND

An emerging standard-of-care for long-QT syndrome uses clinical genetic testing to identify genetic variants of the KCNQ1 potassium channel. However, interpreting results from genetic testing is confounded by the presence of variants of unknown significance for which there is inadequate evidence of pathogenicity.

METHODS AND RESULTS

In this study, we curated from the literature a high-quality set of 107 functionally characterized KCNQ1 variants. Based on this data set, we completed a detailed quantitative analysis on the sequence conservation patterns of subdomains of KCNQ1 and the distribution of pathogenic variants therein. We found that conserved subdomains generally are critical for channel function and are enriched with dysfunctional variants. Using this experimentally validated data set, we trained a neural network, designated Q1VarPred, specifically for predicting the functional impact of KCNQ1 variants of unknown significance. The estimated predictive performance of Q1VarPred in terms of Matthew's correlation coefficient and area under the receiver operating characteristic curve were 0.581 and 0.884, respectively, superior to the performance of 8 previous methods tested in parallel. Q1VarPred is publicly available as a web server at http://meilerlab.org/q1varpred.

CONCLUSIONS

Although a plethora of tools are available for making pathogenicity predictions over a genome-wide scale, previous tools fail to perform in a robust manner when applied to KCNQ1. The contrasting and favorable results for Q1VarPred suggest a promising approach, where a machine-learning algorithm is tailored to a specific protein target and trained with a functionally validated data set to calibrate informatics tools.

Complex interactions in a novel SCN5A compound mutation associated with long QT and Brugada syndrome: Implications for Na+ channel blocking pharmacotherapy for de novo conduction disease

Background

The SCN5A mutation, P1332L, is linked to a malignant form of congenital long QT syndrome, type 3 (LQT3), and affected patients are highly responsive to the Na⁺ channel blocking drug, mexiletine. In contrast, A647D is an atypical SCN5A mutation causing Brugada syndrome. An asymptomatic male with both P1332L and A647D presented with varying P wave/QRS aberrancy and mild QTc prolongation which did not shorten measurably with mexiletine.

Objective

We characterized the biophysical properties of P1332L, A647D and wild-type (WT) Na⁺ channels as well as their combinations in order to understand our proband’s phenotype and to guide mexilitine therapy.

Methods

Na⁺ channel biophysics and mexilitine-binding kinetics were assessed using heterologous expression studies in CHO-K1 cells and human ventricular myocyte modeling.

Results

Compared to WT, P1332L channels displayed a hyperpolarizing shift in inactivation, slower inactivation and prominent late Na⁺ currents (I_Na). While A647D had no effect on the biophysical properties of I_Na, it reduced peak and late I_Na density when co-expressed with either WT or P1332L. Additionally, while P1332L channels had greater sensitivity to block by mexiletine compared to WT, this was reduced in the presence of A647D. Modelling studies revealed that mixing P1332L with A647D channels, action potential durations were shortened compared to P1332L, while peak I_Na was reduced compared to either A647D coexpressing with WT or WT alone.

Conclusions

While A647D mitigates the lethal LQT3 phenotype seen with P1332L, it also reduces mexilitine sensitivity and decreases I_Na density. These results explain our proband’s mild repolarization abnormality and prominent conduction defect in the atria and ventricles, but also suggest that expression of P1332L with A647D yields a novel disease phenotype for which mexiletine pharmacotherapy is no longer suitable.

The efficacy of Ranolazine on E1784K is altered by temperature and calcium

E1784K is the most common mixed syndrome SCN5a mutation underpinning both Brugada syndrome type 1 (BrS1) and Long-QT syndrome type 3 (LQT3). The charge reversal mutant enhances the late sodium current (INa) passed by the cardiac voltage-gated sodium channel (NaV1.5), delaying cardiac repolarization. Exercise-induced triggers, like elevated temperature and cytosolic calcium, exacerbate E1784K late INa. In this study, we tested the effects of Ranolazine, the late INa blocker, on voltage-dependent and kinetic properties of E1784K at elevated temperature and cytosolic calcium. We used whole-cell patch clamp to measure INa from wild type and E1784K channels expressed in HEK293 cells. At elevated temperature, Ranolazine attenuated gain-of-function in E1784K by decreasing late INa, hyperpolarizing steady-state fast inactivation, and increasing use-dependent inactivation. Both elevated temperature and cytosolic calcium hampered the capacity of Ranolazine to suppress E1784K late INa. In-silico action potential (AP) simulations were done using a modified O'Hara Rudy (ORd) cardiac model. Simulations showed that Ranolazine failed to shorten AP duration, an effect augmented at febrile temperatures. The drug-channel interaction is clearly affected by external triggers, as reported previously with ischemia. Determining drug efficacy under various physiological states in SCN5a cohorts is crucial for accurate management of arrhythmias.

Lidocaine attenuation testing: An in vivo investigation of putative LQT3-associated variants in the SCN5A-encoded sodium channel

BACKGROUND

Long QT syndrome type 3 (LQT3) accounts for 5%-10% of long QT syndrome and results from gain-of-function mutations in the SCN5A-encoded sodium channel. Approximately 2% of healthy individuals host rare SCN5A variants of uncertain significance (VUS). Distinction of true LQT3-causative mutations from background genetic noise is essential.

OBJECTIVE

The purpose of this study was to assess the use of the lidocaine attenuation test (LAT) in evaluating patients with possible LQT3.

METHODS

We reviewed the LAT results and medical records for 25 patients with a possible LQT3-associated SCN5A variant. The LAT involved a loading dose of 1 mg/kg of intravenous lidocaine followed by continuous infusion at 50 μg/(kg⋅min) for 20 minutes. If the corrected QT interval shortened by ≥30 ms, the LAT was defined as positive.

RESULTS

Sixteen patients (64%) had a positive LAT, 6 of which demonstrated the E1784K variant. A positive LAT correlated in 86% of cases with abnormal in vitro channel function (mean corrected QT interval attenuation 43 ± 3 ms vs 25 ± 5 ms for wild-type variants; P = .03). Four of 5 patients (80%) with a VUS had a positive LAT (T1304M [2 patients], L1786P, and R800L). The T1304M variant demonstrated abnormal in vitro function and a positive LAT, opening the door for a potential variant promotion from VUS to likely pathogenic.

CONCLUSION

The LAT may help distinguish true LQT3-causative mutations from an otherwise noncontributory VUS. Given that lidocaine acts as a late sodium current blocker, a positive LAT may enable the early identification of a pathological accentuation of the late sodium current that could be targeted therapeutically.

Electrocardiographic Biomarkers for Detection of Drug-Induced Late Sodium Current Block

BACKGROUND

Drugs that prolong the heart rate corrected QT interval (QTc) on the electrocardiogram (ECG) by blocking the hERG potassium channel and also block inward currents (late sodium or L-type calcium) are not associated with torsade de pointes (e.g. ranolazine and verapamil). Thus, identifying ECG signs of late sodium current block could aid in the determination of proarrhythmic risk for new drugs. A new cardiac safety paradigm for drug development (the "CiPA" initiative) will involve the preclinical assessment of multiple human cardiac ion channels and ECG biomarkers are needed to determine if there are unexpected ion channel effects in humans.

METHODS AND RESULTS

In this study we assess the ability of eight ECG morphology biomarkers to detect late sodium current block in the presence of QTc prolongation by analyzing a clinical trial where a selective hERG potassium channel blocker (dofetilide) was administered alone and then in combination with two late sodium current blockers (lidocaine and mexiletine). We demonstrate that late sodium current block has the greatest effect on the heart-rate corrected J-Tpeak interval (J-Tpeakc), followed by QTc and then T-wave flatness. Furthermore, J-Tpeakc is the only biomarker that improves detection of the presence of late sodium current block compared to using QTc alone (AUC: 0.83 vs. 0.72 respectively, p<0.001).

CONCLUSIONS

Analysis of the J-Tpeakc interval can differentiate drug-induced multichannel block involving the late sodium current from selective hERG potassium channel block. Future methodologies assessing drug effects on cardiac ion channel currents on the ECG should use J-Tpeakc to detect the presence of late sodium current block.

TRIAL REGISTRATION

NCT02308748 and NCT01873950.

Utility of hERG Assays as Surrogate Markers of Delayed Cardiac Repolarization and QT Safety

HERG (human-ether-a-go-go-related gene) encodes for a cardiac potassium channel that plays a critical role in defining ventricular repolarization. Noncardiovascular drugs associated with a rare but potentially lethal ventricular arrhythmia (Torsades de Pointes) have been linked to delayed cardiac repolarization and block of hERG current. This brief overview will discuss the role of hERG current in cardiac electrophysiology, its involvement in drug-induced delayed repolarization, and approaches used to define drug effects on hERG current. In addition, examples of hERG blocking drugs acting differently (i.e., overt and covert hERG blockade due to multichannel block) together with the utility and limitations of hERG assays as tools to predict the risk of delayed repolarization and proarrhythmia are discussed.

Targeted capture sequencing in a large LQTS family reveals a new pathogenic mutation c.2038delG in KCNH2 initially missed due to allelic dropout

We present a new mutation in KCNH2 (c.2038delG) resulting in a frameshift and premature truncation of the IKr channel protein in a large LQTS family with several sudden death cases. This mutation was initially missed by mutation scanning with DHPLC due to allelic dropout and only retrieved after repeat genetic testing with targeted capture and massive parallel sequencing. There was full penetrance of this mutation, only if an individualized QT correction derived from 24-hour Holter data was used. This case again underscores the importance of repeat genetic testing in robust cases of LQTS that remained genotype negative with mutation scanning techniques.

Modeling developmental neuropsychiatric disorders with iPSC technology: challenges and opportunities

The development of cellular reprogramming methods to generate human induced pluripotent stem cells (iPSC) has led to the establishment of lines from hundreds of patients with a variety of neurologic and psychiatric diseases. One of the fundamental powers of iPSC technology lies in the competency of these cells to be directed to become any cell type in the body, thus allowing researchers to examine disease mechanisms and identify and test novel therapeutics in relevant cell types. The field has now exited the phase of 'proof-of-principle' studies showing the potential of the model systems, and it has now entered an exciting new era where iPSC studies are contributing to the field's understanding of mechanisms of disease. Here, we describe the challenges of iPSC modeling of neuropsychiatric disorders, and highlight studies where some of these challenges have been addressed to provide novel insights into disease mechanisms.

Eag Domains Regulate LQT Mutant hERG Channels in Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes

Human Ether á go-go Related Gene potassium channels form the rapid component of the delayed-rectifier (IKr) current in the heart. The N-terminal 'eag' domain, which is composed of a Per-Arnt-Sim (PAS) domain and a short PAS-cap region, is a critical regulator of hERG channel function. In previous studies, we showed that isolated eag (i-eag) domains rescued the dysfunction of long QT type-2 associated mutant hERG R56Q channels, by substituting for defective eag domains, when the channels were expressed in Xenopus oocytes or HEK 293 cells.Here, our goal was to determine whether the rescue of hERG R56Q channels by i-eag domains could be translated into the environment of cardiac myocytes. We expressed hERG R56Q channels in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) and measured electrical properties of the cells with whole-cell patch-clamp recordings. We found that, like in non-myocyte cells, hERG R56Q had defective, fast closing (deactivation) kinetics when expressed in hiPSC-CMs. We report here that i-eag domains slowed the deactivation kinetics of hERG R56Q channels in hiPSC-CMs. hERG R56Q channels prolonged the AP of hiPSCs, and the AP was shortened by co-expression of i-eag domains and hERG R56Q channels. We measured robust Förster Resonance Energy Transfer (FRET) between i-eag domains tagged with Cyan fluorescent protein (CFP) and hERG R56Q channels tagged with Citrine fluorescent proteins (Citrine), indicating their close proximity at the cell membrane in live iPSC-CMs. Together, functional regulation and FRET spectroscopy measurements indicated that i-eag domains interacted directly with hERG R56Q channels in hiPSC-CMs. These results mean that the regulatory role of i-eag domains is conserved in the cellular environment of human cardiomyocytes, indicating that i-eag domains may be useful as a biological therapeutic.