Cardiac Action Potential Duration Research Articles

In vivo KCNH2-suppression-replacement gene therapy attenuates the pathogenic phenotype in transgenic rabbits with short QT syndrome type 1

Abstract Background Short QT syndrome type 1 (SQT1) is a genetic channelopathy characterized by gain-of-function variants in the KCNH2-encoded potassium channel underlying the IKr current, leading to a pathologically shortened cardiac action potential duration (APD). Patients with SQT1 manifest shortening of the QT duration on 12-lead ECG and are prone to cardiac arrhythmias that can culminate in sudden cardiac death. Purpose To investigate the effect of a KCNH2-specific suppression-replacement (KCNH2-SupRep) gene therapy on the pathogenic phenotype in transgenic SQT1 rabbits. Methods A novel dual-component KCNH2-SupRep gene therapy was developed by combining a custom-designed KCNH2 shRNA (suppression) and an shRNA-immune (shIMM) KCNH2 cDNA (replacement) into AAV9. In vivo AAV9-mediated KCNH2-SupRep / sham treatment was performed by intra-aortic root injection of the KCNH2-SupRep AAV9 (1E10 vg/kg) / NaCl 0.9% during balloon occlusion using a Swan Ganz catheter. Three weeks after administration, 12-lead ECGs were assessed in adult transgenic SQT1 (KCNH2-N588K, gain of IKr due to impaired inactivation) and wild-type (WT) rabbits to determine the effect of KCNH2-SupRep / sham treatment on the rabbit’s QTc duration and QT index. Patch-clamp recordings were performed in isolated ventricular cardiomyocytes (CMs) to evaluate the effect of KCNH2-SupRep / sham treatment on the CM’s action potential duration at 90% repolarization (APD90). Results After in vivo AAV9-mediated KCNH2-SupRep gene therapy injection (1E10 vg/kg) in 7 SQT1 rabbits, QTc duration and QT index significantly increased in all SQT1 rabbits (QTc: untreated(UT)-SQT1: 153.2±3.8ms vs treated-SQT1: 166.5±4.6ms, p=0.0002, N=7 each; QTi: UT-SQT1: 92.5±1.9% vs treated-SQT1: 99.7±2.7%, p=0.0007, N=7 each), reaching a QT index of ~100% typically observed in WT rabbits. No such changes were observed in the sham-SQT1 rabbits. Furthermore, cardiomyocytes isolated from 7 SQT1 rabbits that received in vivo SupRep gene therapy demonstrated APD90 prolongation closer to WT rabbits compared to UT-SQT1 and sham-SQT1 rabbits (APD90: UT-SQT1: 314.2±11.9ms, WT: 404.5±13.8ms, treated-SQT1: 362.9±10.6ms; UT-SQT1 vs WT: p<0.0001; UT-SQT1 vs treated-SQT1: p=0.019; WT vs Treated-SQT1: p=0.031). In line with these observations, first IKr measurements demonstrate smaller IKr currents in SupRep-SQT1 than in sham-SQT1 CMs. Conclusion In vivo KCNH2-SupRep gene therapy normalizes the pathologically shortened QTc duration and QT index in transgenic SQT1 rabbits. Moreover, KCNH2-SupRep treatment prolongs APD90 in SQT1 compared to UT-SQT1 and sham-SQT1 reaching levels closer to WT.

In Silico Human Cardiomyocyte Action Potential Modeling: Exploring Ion Channel Input Combinations.

In silico modeling offers an opportunity to supplement and accelerate cardiac safety testing. With in silico modeling, computational simulation methods are used to predict electrophysiological interactions and pharmacological effects of novel drugs on critical physiological processes. The O'Hara-Rudy's model was developed to predict the response to different ion channel inhibition levels on cardiac action potential duration (APD) which is known to directly correlate with the QT interval. APD data at 30% 60% and 90% inhibition were derived from the model to delineate possible ventricular arrhythmia scenarios and the marginal contribution of each ion channel to the model. Action potential values were calculated for epicardial, myocardial, and endocardial cells, with action potential curve modeling. This study assessed cardiac ion channel inhibition data combinations to consider when undertaking in silico modeling of proarrhythmic effects as stipulated in the Comprehensive in Vitro Proarrhythmia Assay (CiPA). As expected, our data highlight the importance of the delayed rectifier potassium channel (IKr) as the most impactful channel for APD prolongation. The impact of the transient outward potassium channel (Ito) inhibition on APD was minimal while the inward rectifier (IK1) and slow component of the delayed rectifier potassium channel (IKs) also had limited APD effects. In contrast, the contribution of fast sodium channel (INa) and/or L-type calcium channel (ICa) inhibition resulted in substantial APD alterations supporting the pharmacological relevance of in silico modeling using input from a limited number of cardiac ion channels including IKr, INa, and ICa, at least at an early stage of drug development.

E3 ubiquitin ligase rififylin has yin and yang effects on rabbit cardiac transient outward potassium currents (Ito) and corresponding channel proteins

Genome-wide association studies (GWAS) have reported a correlation between a single nucleotide polymorphism of the RING finger E3 ubiquitin protein ligase rififylin (RFFL) and QT interval variability in humans (Newton-Cheh et al., 2009). Previously, we have shown that RFFL down-regulates expression and function of the human-like ether-a-go-go-related gene (hERG) potassium channel and corresponding rapidly activating delayed rectifier potassium current (IKr) in adult rabbit ventricular cardiomyocytes (ARbCMs). Here we report that RFFL also affects the transient outward current (Ito), but in a peculiar way. RFFL overexpression in ARbCMs significantly decreases the contribution of its fast component (Ito,f) from 35% to 21%, and increases the contribution of its slow component (Ito,s) from 65% to 79%. Since Ito,f in rabbits is mainly conducted by Kv4.3, we investigated the effect of RFFL on Kv4.3 expressed in HEK293A cells. We found that RFFL overexpression reduced Kv4.3 expression and corresponding Ito,f in a RING domain-dependent manner in the presence or absence of its accessory subunit KChIP2. On the other hand, RFFL overexpression in Kv1.4-expressing HEK cells leads to an increase in both Kv1.4 expression level and Ito,s, similarly in a RING domain-dependent manner. Our physiologically detailed rabbit ventricular myocyte computational model shows that these yin and yang effects of RFFL overexpression on Ito,f, and Ito,s affect phase 1 of the action potential waveform and slightly decrease its duration in addition to suppressing IKr. Thus, RFFL modifies cardiac repolarization reserve via ubiquitination of multiple proteins that differently affect various potassium channels and cardiac action potential duration (APD).

Proof of concept for monoclonal antibody therapy in a cellular model of acquired long QT syndrome type 3.

Long QT syndrome (LQTS) type 3 although less common than the first two forms, differs in that arrhythmic events are less likely triggered by adrenergic stimuli and are more often lethal. Effective pharmacological treatment is challenged by interindividual differences, mutation dependence, and adverse effects, translating into an increased use of invasive measures (implantable cardioverter-defibrillator, sympathetic denervation) in patients with LQTS type 3. Previous studies have demonstrated the therapeutic potential of polyclonal KCNQ1 antibody for LQTS type 2. Here, we sought to identify a monoclonal KCNQ1 antibody that preserves the electrophysiological properties of the polyclonal form. Using hybridoma technology, murine monoclonal antibodies were generated, and patch clamp studies were performed for functional characterization. We identified a monoclonal KCNQ1 antibody able to normalize cardiac action potential duration and to suppress arrhythmias in a pharmacological model of LQTS type 3 using human-induced pluripotent stem cell-derived cardiomyocytes.NEW & NOTEWORTHY Long QT syndrome is a leading cause of sudden cardiac death in the young. Recent research has highlighted KCNQ1 antibody therapy as a new treatment modality for long QT syndrome type 2. Here, we developed a monoclonal KCNQ1 antibody that similarly restores cardiac repolarization. Moreover, the identified monoclonal KCNQ1 antibody suppresses arrhythmias in a cellular model of long QT syndrome type 3, holding promise as a first-in-class antiarrhythmic immunotherapy.

Functional Characterization of a Spectrum of Novel Romano-Ward Syndrome KCNQ1 Variants.

The KCNQ1 gene encodes the α-subunit of the cardiac voltage-gated potassium (Kv) channel KCNQ1, also denoted as Kv7.1 or KvLQT1. The channel assembles with the ß-subunit KCNE1, also known as minK, to generate the slowly activating cardiac delayed rectifier current IKs, a key regulator of the heart rate dependent adaptation of the cardiac action potential duration (APD). Loss-of-function variants in KCNQ1 cause the congenital Long QT1 (LQT1) syndrome, characterized by delayed cardiac repolarization and a QT interval prolongation in the surface electrocardiogram (ECG). Autosomal dominant loss-of-function variants in KCNQ1 result in the LQT syndrome called Romano-Ward syndrome (RWS), while autosomal recessive variants affecting function, lead to Jervell and Lange-Nielsen syndrome (JLNS), associated with deafness. The aim of this study was the characterization of novel KCNQ1 variants identified in patients with RWS to widen the spectrum of known LQT1 variants, and improve the interpretation of the clinical relevance of variants in the KCNQ1 gene. We functionally characterized nine human KCNQ1 variants using the voltage-clamp technique in Xenopus laevis oocytes, from which we report seven novel variants. The functional data was taken as input to model surface ECGs, to subsequently compare the functional changes with the clinically observed QTc times, allowing a further interpretation of the severity of the different LQTS variants. We found that the electrophysiological properties of the variants correlate with the severity of the clinically diagnosed phenotype in most cases, however, not in all. Electrophysiological studies combined with in silico modelling approaches are valuable components for the interpretation of the pathogenicity of KCNQ1 variants, but assessing the clinical severity demands the consideration of other factors that are included, for example in the Schwartz score.

SGK1 inhibition attenuates the action potential duration in reengineered heart cell models of drug-induced QT prolongation

Drug-induced QT prolongation (DI-QTP) is a clinical entity in which administration of a human ether-à-go-go-related gene/rapid delayed rectifier potassium current blocker such as dofetilide prolongs the cardiac action potential duration (APD) and the QT interval on the electrocardiogram. Inhibition of serum and glucocorticoid regulated kinase-1 (SGK1) reduces the APD at 90% repolarization (APD90) in induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) derived from patients with congenital long QT syndrome. Here, we test the efficacy of 2 novel SGK1 inhibitors-SGK1-I1 and SGK1-I2-in iPSC-CM models of dofetilide-induced APD prolongation. Normal iPSC-CMs were treated with dofetilide to produce a DI-QTP iPSC-CM model. SGK1-I1's and SGK1-I2's therapeutic efficacy for shortening the dofetilide-induced APD90 prolongation was compared to mexiletine. The APD90 values were recorded 4hours after treatment using a voltage-sensing dye. The APD90 was prolonged in normal iPSC-CMs treated with dofetilide (673 ± 8 ms vs 436 ± 4 ms; P < .0001). While 10 mM mexiletine shortened the APD90 of dofetilide-treated iPSC-CMs from 673 ± 4 to 563 ± 8 ms (46% attenuation; P < .0001), 30 nM of SGK1-I1 shortened the APD90 from 673 ± 8 to 502 ± 7 ms (72% attenuation; P < .0001). Additionally, 300 nM SGK1-I2 shortened the APD90 of dofetilide-treated iPSC-CMs from 673 ± 8 to 460 ± 7 ms (90% attenuation; P < .0001). These novel SGK1-Is substantially attenuated the pathological APD prolongation in a human heart cell model of DI-QTP. These preclinical data support the development of this therapeutic strategy to counter and neutralize DI-QTP, thereby increasing the safety profile for patients receiving drugs with torsadogenic potential.

Abstract 11556: Sodium/Glucose Co-Transporter 2 Inhibition and Attenuation of the Action Potential Duration in Patient-Specific Re-Engineered Heart Cells With Congenital Long QT Syndrome

Background: Long QT syndrome (LQTS) stems from cardiac action potential duration (APD) prolonging pathogenic variants in KCNQ1 (LQT1), KCNH2 (LQT2), or SCN5A (LQT3). Sodium/glucose co-transporter 2 inhibitor (SGLT2i), a class of drugs used to treat type II diabetes and all forms of heart failure, reduces cardiac late sodium channel current. Objective: To test the ability of three commercially available SGLT2i to reduce APD in patient-specific induced pluripotent stem cell-derived cardiomyocyte (iPSC-CM) models of LQT1, LQT2, and LQT3. Method: Patient-specific iPSC-CMs and a CRISPR/Cas9 gene-edited variant corrected, isogenic control (IC) iPSC-CMs were generated. The variant-corrected IC was used to determine the curative APD in order to establish the degree of APD attenuation. The SCN5A-P1332L (LQT3) and IC iPSC-CMs were tested with 10 μM of either canagliflozin (CANA), dapagliflozin (DAPA), or empagliflozin (EMPA). An ATXII (5μM)-induced APD prolongation iPSC-CM model, mimicking drug-induced LQT3, was also treated. The APD was measured 30 minutes, 1 hour, and 2 hours after treatment using local extracellular action potential recordings on multielectrode array. The efficacy of these SGLT2i were also tested in KCNQ1-V254M (LQT1) and KCNH2-G604S (LQT2) iPSC-CMs. Results: The APD90 was prolonged in SCN5A-P1332L iPSC-CMs compared to its IC (469 ± 4 ms vs 305 ± 2 ms, p&lt;0.0001). While 10 μM of SGLT2i had no APD shortening effect on the KCNQ1-V254M or KCNH2-G604S iPSC-CMs, all three SGLT2i compounds shortened the APD90 of the SCN5A-P1332L iPSC-CMs after 1 hour of treatment from 463±5 ms to 432±6 ms (EMPA, 18.8% attenuation, p=0.0002), 459±8 ms to 430±5 ms (CANA, 17.5%, p=0.004), and 453±6 ms to 434±4 ms (DAPA, 11.4%, p=0.01). Additionally, IC iPSC-CMs treated with 5 μM of ATXII induced significant APD prolongation from 318±9 ms to 379±12 ms (p=0.0003) after 30 min incubation. However, co-incubation of ATXII and SGLT2i abolished the APD prolonging effect of ATXII. Conclusions: Therapeutically inhibiting SGLT2 effectively shortens the cardiomyocyte APD in human heart cell models of the LQT3, but not LQT1 or LQT2. This pre-clinical data compels further assessment of SGLT2i as a novel therapy for patients with congenital LQT3.

Abstract 11027: SGK1 Inhibition and Attenuation of the Action Potential Duration in Re-Engineered Heart Cell Models of Drug-Induced QT Prolongation

Background: Drug-induced QT prolongation (DI-QTP) is a clinical entity in which administration of a HERG/I Kr blocker such as dofetilide prolongs the cardiac action potential duration (APD) at the cellular level and the QT interval clinically, and increases the risk for a potentially lethal, QT-triggered ventricular arrhythmia. There may be a role for inhibition of I NaL to counter DI-QTP. Recently, we have shown that inhibition of serum and glucocorticoid regulated kinase-1 (SGK1) reduces the APD90 in induced pluripotent stem cell derived cardiomyocytes (iPSC-CM) from patients with LQT3 and ameliorates increases in I NaL . Objective: To test the efficacy of novel SGK1 inhibitors (SGK-I) in re-engineered cardiomyocyte models of dofetilide-induced APD prolongation. Methods: Normal iPSC-CMs were treated with dofetilide [5 nM] to produce a DI-QTP iPSC-CM model. The SGK1-I’s therapeutic efficacy for shortening the dofetilide-induced APD90 prolongation was compared to mexiletine. The APD90 values were recorded 4 hours after treatment using FluoVolt, a voltage-sensitive fluorescent dye. Results: The APD90 was significantly prolonged in normal iPSC-CMs treated with dofetilide (673 ± 8 ms with dofetilide vs 436 ± 4 ms with DMSO, p&lt;0.0001). While 10 μM mexiletine shortened the average APD90 of dofetilide-treated normal iPSC-CMs from 673 ± 4 ms to 563 ± 8 ms (46% attenuation, p&lt;0.0001), 30 nM of SGK-I shortened the APD90 from 673 ± 4 ms to 478 ± 10 ms (82% attenuation, p&lt;0.0001). Conclusions: Therapeutically inhibiting SGK1 effectively shortens the cardiomyocyte APD in a human heart cell model of DI-QTP. The novel SGK1 inhibitor normalized the pathological APD prolongation almost fully (&gt; 80%) in the iPSC-CM model treated with dofetilide. This pre-clinical data supports further development of this therapeutic strategy to counter and neutralize the threat of DI-QTP.

Abstract 15036: Suppression-Replacement Gene Therapy for CALM1 -Mediated Long QT Syndrome

Introduction: Calmodulin (CaM)-mediated long QT syndrome (CaM-LQTS) is a genetic arrhythmia disorder characterized by a high prevalence of life-threatening ventricular arrhythmias occurring very early in life, frequent T-wave alternans, markedly prolonged QT intervals (QTc &gt; 600 ms), and intermittent 2:1 AV block. Three distinct genes ( CALM1 , CALM2 , and CALM3 ) all encode for the identical CaM protein. Conventional LQTS pharmacotherapies fail to adequately protect against potentially lethal cardiac events in patients with CaM-LQTS. Here, we developed a CALM1 “ suppression-replacement (SupRep)” gene therapy with the goal of rescuing the prolonged cardiac action potential duration (APD) and increased Ca 2+ amplitude (ΔF/F0) in patient-derived induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from a patient with CALM1 -mediated LQTS. Methods: Custom designed CALM1 -targeting shRNAs were tested for knockdown efficiency using TSA201 cells and RT-qPCR. A dual-component SupRep CALM1 gene therapy was created by cloning into a single construct a custom-designed CALM1 shRNA that produces knockdown (suppression) and a “shRNA-immune” (shIMM) CALM1 cDNA (replacement). Patient-specific CALM1-F142L iPSC-CMs were generated from a patient with severe LQTS (QTc = 515 ms). FluoVolt voltage-sensing dye was used to measure the APD at 90% repolarization (APD 90 ). Calcium transient amplitude was measured using Fluo-4 indicator. Results: Overall, 6 unique candidate CALM1 -targeting shRNAs were tested for knockdown efficiency in a transient transfection CALM1 over-expression TSA201 cell system. One candidate shRNA had an 80% knockdown efficiency of CALM1. Following transfection with CALM1 -SupRep gene therapy, the APD 90 was significantly decreased in F142L iPSC-CMs compared to untreated cells (F142L: 665 ± 9 ms [n=40] vs F142L + SupRep: 353 ± 9 ms [n=20], p&lt;0.0001). Ca 2+ amplitude was significantly decreased following SupRep transfection when compared to untreated cells (0.99 ± 0.04 vs 0.8 ± 0.06, p=0.02). Conclusions: Here, we provide the first proof-of-principle SupRep gene therapy for CALM1-mediated LQTS. CALM1 -SupRep gene therapy shortened significantly the pathologically prolonged APD 90 in CALM1 -F142L patient-derived iPSC-CMs.

Abstract 10986: SGK1 Inhibition Attenuated the Action Potential Duration In-Patient and Genotype-Specific Re-Engineered Heart Cells With Congenital Long QT Syndrome

Background: Long QT syndrome (LQTS) often stems from pathogenic variants in KCNQ1 (LQT1), KCNH2 (LQT2), or SCN5A (LQT3) and is characterized cellularly by prolongation of the cardiac action potential duration (APD). We have shown that inhibition of serum and glucocorticoid regulated kinase-1 (SGK1) reduces I NaL and is proposed as a novel therapeutic strategy for LQT3. Objective: To test the efficacy of a potent and selective SGK1 inhibitor (SGK1-I) in human cardiomyocyte models of LQT1, LQT2, and LQT3. Methods: Induced pluripotent stem cell derived cardiomyocytes (iPSC-CMs) were generated from patients with either LQT1, LQT2, or LQT3. The mexiletine (MEX)-sensitive SCN5A-P1332L iPSC-CMs were tested initially. A CRISPR/Cas9 P1332L variant-corrected isogenic control (IC) was used as a control. The SGK1-I’s therapeutic efficacy was compared to MEX and to a commercially available SGK1 inhibitor (EMD638683). The novel SGK-I was then tested for APD shortening in SCN5A-R1623Q, KCNH2-G604S and KCNQ1-V254M iPSC-CMs. The APD90 values were recorded 4 hours after treatment using FluoVolt. Results: The APD90 was significantly prolonged in SCN5A-P1332L iPSC-CMs compared to its IC (646 ± 7 ms vs 482 ± 23 ms, p&lt;0.0001). MEX shortened the average APD90 to 560 ± 7 ms (52% attenuation). Interestingly, SGK1-I significantly shortened the APD to 518 ± 5 ms (78% attenuation) but did not further shorten the APD in the IC. SGK1-I also shortened the APD90 of the SCN5A-R1623Q iPSC-CMs from 753 ± 8 ms to 475 ± 19 ms compared to 558 ± 19 ms with MEX. The SGK1-I shortened the APD90 in KCNH2-G604S (666 ± 10ms to 574 ± 18ms for SGK1-I versus 538 ± 15 ms after MEX). EMD had limited APD shortening effects in SCN5A and KCNH2 iPSC-CMs. Interestingly, while neither MEX nor EMD reduced the APD90 in the KCNQ1-V254M iPSC-CMs, the novel SGK1-I reduced the APD90 from 544 ± 10 ms to 475 ± 11ms (p=0.0004). Conclusions: Therapeutically inhibiting SGK1 effectively shortens the cardiomyocyte APD in human heart cell models of the 3 major LQTS genotypes. The novel SGK1-I normalized the pathological APD prolongation almost fully (&gt; 70%) in the patient-derived SCN5A-P1332L iPSC-CM model. These pre-clinical data support further development of SGK1-I as a novel, first-in-class therapy for patients with congenital LQTS.

Abstract 12634: A Novel in-vitro Patient-Specific Re-Engineered Cardiomyocyte Model of Multifocal Ectopic Purkinje-Related Premature Contractions

Introduction: Multifocal ectopic Purkinje-related premature contractions (MEPPC), the most recent addition to the spectrum of SCN5A -mediated cardiac channelopathies, is characterized by frequent premature ventricular complexes (PVCs) originating from the Purkinje system, atrial arrhythmias, and a predilection for PVC-mediated dilated cardiomyopathy. Hypothesis: To investigate the electrophysiology phenotype of MEPPC in a novel, patient-specific, re-engineered cardiomyocyte model. Methods: Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated from a 27-year-old male with a history of frequent, multifocal PVCs and atrial tachycardia, positive for a putative MEPPC-causative SCN5A variant (p.Q419P). A CRISPR-Cas9 Q419P variant-corrected isogenic control (IC) was created to establish the normalized electrophysiology recordings for that MEPPC model. The standard whole-cell patch clamp technique was used to measure Q419P-SCN5A- and IC-derived NaV1.5 sodium currents using voltage clamp configuration. In addition, action potential duration (APD) of the iPSC-CMs were measured using FluoVolt optical voltage-sensing dye. Results: Analysis of the current-voltage relationship revealed that neither peak sodium current densities nor late inward sodium current were impacted by Q419P-SCN5A. However, Q419P-SCN5A significantly right-shifted the V 1/2 in inactivation by 8.3 mV from -81.1±1.7 mV (IC, n=5) to -72.8±0.2 mV (Q419P-SCN5A, n=6, p&lt;0.05 vs. IC) and left-shifted the V 1/2 in activation by -4.1 mV from -39.5±0.3 mV (IC, n=5) to -43.6±0.9 mV (Q419P-SCN5A, n=6, p&lt;0.05 vs. IC) leading to a larger hyperpolarized window current. Optical APD90 measurement revealed no difference between the Q419P-containing (476 ± 23 ms) and the IC (474 ± 46 ms, p=0.867) cardiomyocytes. Conclusions: This is the first in-vitro disease model of MEPPC using patient-specific, re-engineered heart cells. Consistent with the findings in the non-cardiac HEK cell system, this MEPPC-causative variant contributes to increased window current in cardiomyocytes without affecting the cardiac APD. This novel cardiac cell model for MEPPC may facilitate new insights into the pathobiology of MEPPC and enable the testing of novel therapies for it.

Incorrectly corrected? QT interval analysis in rats and mice.

QT interval, a surrogate measure for ventricular action potential duration (APD) in the surface ECG, is widely used to identify cardiac abnormalities and drug safety. In humans, cardiac APD and QT interval are prominently affected by heart rate (HR), leading to widely accepted formulas to correct the QT interval for HR changes (QT corrected - QTc). While QTc is widely used in the clinic, the proper way to correct the QT interval in small mammals such as rats and mice is not clear. Over the years, empiric correction formulas were developed for rats and mice, which are widely used in the literature. Recent experimental findings obtained from pharmacological and direct pacing experiments in unanesthetized rodents show that the rate-adaptation properties are markedly different from those in humans and the use of existing QTc formulae can lead to major errors in data interpretation. In the present review, these experimental findings are summarized and discussed.

Two small-molecule activators share similar effector sites in the KCNQ1 channel pore but have distinct effects on voltage sensor movements.

ML277 and R-L3 are two small-molecule activators of KCNQ1, the pore-forming subunit of the slowly activating potassium channel IKs. KCNQ1 loss-of-function mutations prolong cardiac action potential duration and are associated with long QT syndrome, which predispose patients to lethal ventricular arrhythmia. ML277 and R-L3 enhance KCNQ1 current amplitude and slow deactivation. However, the presence of KCNE1, an auxiliary subunit of IKs channels, renders the channel insensitive to both activators. We found that ML277 effects are dependent on several residues in the KCNQ1 pore domain. Some of these residues are also necessary for R-L3 effects. These residues form a putative hydrophobic pocket located between two adjacent KCNQ1 subunits, where KCNE1 subunits are thought to dwell, thus providing an explanation for how KCNE1 renders the IKs channel insensitive to these activators. Our experiments showed that the effect of R-L3 on voltage sensor movement during channel deactivation was much more prominent than that of ML277. Simulations using a KCNQ1 kinetic model showed that the effects of ML277 and R-L3 could be reproduced through two different effects on channel gating: ML277 enhances KCNQ1 channel function through a pore-dependent and voltage sensor–independent mechanism, while R-L3 affects both channel pore and voltage sensor.

Gene therapy targeting protein trafficking regulator MOG1 in mouse models of Brugada syndrome, arrhythmias, and mild cardiomyopathy.

Brugada syndrome (BrS) is a fatal arrhythmia that causes an estimated 4% of all sudden death in high-incidence areas. SCN5A encodes cardiac sodium channel NaV1.5 and causes 25 to 30% of BrS cases. Here, we report generation of a knock-in (KI) mouse model of BrS (Scn5aG1746R/+). Heterozygous KI mice recapitulated some of the clinical features of BrS, including an ST segment abnormality (a prominent J wave) on electrocardiograms and development of spontaneous ventricular tachyarrhythmias (VTs), seizures, and sudden death. VTs were caused by shortened cardiac action potential duration and late phase 3 early afterdepolarizations associated with reduced sodium current density (INa) and increased Kcnd3 and Cacna1c expression. We developed a gene therapy using adeno-associated virus serotype 9 (AAV9) vector-mediated MOG1 delivery for up-regulation of MOG1, a chaperone that binds to NaV1.5 and traffics it to the cell surface. MOG1 was chosen for gene therapy because the large size of the SCN5A coding sequence (6048 base pairs) exceeds the packaging capacity of AAV vectors. AAV9-MOG1 gene therapy increased cell surface expression of NaV1.5 and ventricular INa, reversed up-regulation of Kcnd3 and Cacna1c expression, normalized cardiac action potential abnormalities, abolished J waves, and blocked VT in Scn5aG1746R/+ mice. Gene therapy also rescued the phenotypes of cardiac arrhythmias and contractile dysfunction in heterozygous humanized KI mice with SCN5A mutation p.D1275N. Using a small chaperone protein may have broad implications for targeting disease-causing genes exceeding the size capacity of AAV vectors.

The antidiabetic drug rosiglitazone blocks Kv1.5 potassium channels in an open state.

An antidiabetic drug, rosiglitazone is a member of the drug class of thiazolidinedione. Although restrictions on use due to the possibility of heart toxicity have been removed, it is still a drug that is concerned about side effects on the heart. We here examined, using Chinese hamster ovary cells, the action of rosiglitazone on Kv1.5 channels, which is a major determinant of the duration of cardiac action potential. Rosiglitazone rapidly and reversibly inhibited Kv1.5 currents in a concentration-dependent manner (IC50 = 18.9 µM) and accelerated the decay of Kv1.5 currents without modifying the activation kinetics. In addition, the deactivation of Kv1.5 current, assayed with tail current, was slowed by the drug. All of the results as well as the use-dependence of the rosiglitazone-mediated blockade indicate that rosiglitazone acts on Kv1.5 channels as an open channel blocker. This study suggests that the cardiac side effects of rosiglitazone might be mediated in part by suppression of Kv1.5 channels, and therefore, raises a concern of using the drug for diabetic therapeutics.

Modulatory role of OKG on delayed rectifier potassium channels in ventricular cardiomyocytes from metabolic syndrome rats

Metabolic syndrome (MetS) is a cluster of multifactorial diseases with multiple risk factors and is closely linked to overweight and obesity. Rats treated with a high sucrose diet (32% sucrose in drinking-water, daily, for 30-week) develop MetS-like features that include an elevated blood pressure, insulin resistance and a prolonged cardiac action potential duration (APD). Delayed rectifier potassium channel currents are key determinants of the APD duration and they are under control of several regulatory mechanisms (e.g.

Regulation of cardiac action potential duration by neurotransmitters and co-transmitters

The autonomic nervous system plays a critical role in regulating cardiac contractility, but it must do so while maintaining precise control of action potential duration (APD) in order to ensure orderly propagation of electrical impulses throughout the heart. This requires the orchestration of multiple ion channel responses, but exactly which ones are most important in sustaining this balancing act vary depending on the species. Many studies have addressed this question using the hearts of small animals, such as the mouse or rat, where the ion channels involved in regulating APD are very different from those found in human cardiac myocytes.

MicroRNA-365 regulates human cardiac action potential duration

Abnormalities of ventricular action potential cause malignant cardiac arrhythmias and sudden cardiac death. Here, we aim to identify microRNAs that regulate the human cardiac action potential and ask whether their manipulation allows for therapeutic modulation of action potential abnormalities. Quantitative analysis of the microRNA targetomes in human cardiac myocytes identifies miR-365 as a primary microRNA to regulate repolarizing ion channels. Action potential recordings in patient-specific induced pluripotent stem cell-derived cardiac myocytes show that elevation of miR-365 significantly prolongs action potential duration in myocytes derived from a Short-QT syndrome patient, whereas specific inhibition of miR-365 normalizes pathologically prolonged action potential in Long-QT syndrome myocytes. Transcriptome analyses in these cells at bulk and single-cell level corroborate the key cardiac repolarizing channels as direct targets of miR-365, together with functionally synergistic regulation of additional action potential-regulating genes by this microRNA. Whole-cell patch-clamp experiments confirm miR-365-dependent regulation of repolarizing ionic current Iks. Finally, refractory period measurements in human myocardial slices substantiate the regulatory effect of miR-365 on action potential in adult human myocardial tissue. Our results delineate miR-365 to regulate human cardiac action potential duration by targeting key factors of cardiac repolarization.

Epicatechin Attenuated Vascular Dysfunction, Markers of Oxidative Stress, Hyperlipidemia and Decreased Cardiac Action Potential Duration in Middle-Aged Obese, Hypertensive Rat Models of Metabolic Syndrome

Epicatechin Attenuated Vascular Dysfunction, Markers of Oxidative Stress, Hyperlipidemia and Decreased Cardiac Action Potential Duration in Middle-Aged Obese, Hypertensive Rat Models of Metabolic Syndrome

B-PO01-007 THE EFFICACY OF MEXILETINE TREATMENT IN PATIENT-SPECIFIC RE-ENGINEERED CARDIOMYOCYTE MODELS OF LONG QT SYNDROME TYPE 1, 2, AND 3

Long QT syndrome (LQTS) stems from pathogenic variants in KCNQ1 (LQT1), KCNH2 (LQT2), or SCN5A (LQT3) and is characterized by prolongation of the cardiac action potential duration (APD) at the cellular level. Mexiletine (MEX) is an effective therapy in some patients with LQT3 and may have therapeutic efficacy in LQT2 also. However, the efficacy of MEX to shorten APD in patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) has not been tested. To test the efficacy of MEX in patient-specific re-engineered cardiomyocyte models of LQT1 (KCNQ1-V254M), LQT2 (KCNH2-G604S), and LQT3 (SCN5A-P1332L). iPSC-CMs were generated from a 29-year-old female (KCNQ1-V254M), a 16-year-old male (KCNH2-G604S), and an 8-year-old female (SCN5A-P1332L) with a history of syncope or sudden cardiac arrest. CRISPR-Cas9 G604S and P1332L variant-corrected isogenic controls were created to establish the curative level of APD shortening for LQT2 and LQT3 models. MEX’s therapeutic efficacy was tested for APD shortening in the 3 LQTS genotype-specific iPSC-CMs by recording the APD90 values 4 hours after treatment with 10 μM MEX using FluoVolt. The APD90 was prolonged significantly in KCNH2-G604S and SCN5A-P1332L iPSC-CMs compared to their isogenic controls (726 ± 8ms vs 425 ± 40ms in KCNH2-G604S, 654 ±8 ms vs 482 ± 23 ms in SCN5A-P1332L, p<0.0001). MEX shortened the APD90 on KCNH2-G604S iPSC-CMs from 726 ± 8ms to 605 ± 17 ms. MEX also shortened the average APD90 on SCN5A-P1332L to 570 ± 9 ms (49% attenuation relative to the isogenic control value). However, the APD90 in the KCNQ1-V254M iPSC-CMs was not significantly reduced. Clinically, MEX has shortened the P1332L-SCN5A positive patient’s QTc from 600 ms to 460 ms and the G604S-KCNH2 positive patient’s QTc from > 600 ms to 440 ms. The patient with KCNQ1-V254M has not been treated with MEX. This is the first ‘treatment-in-the-dish’ studies of patient-specific, re-engineered heart cells for all three major LQTS genotypes using MEX. Consistent with its clinical use in the patient herself, MEX normalized the pathological APD prolongation by nearly 50% in her LQT3 iPSC-CM model. Additionally, MEX effectively shortened the APD in the LQT2, but not in LQT1, cardiac cells.