Odening, Katja E.

Abstract Background Short QT syndrome type 1 (SQT1) is a genetic channelopathy caused by gain-of-function variants in the KCNH2-encoded Kv11.1 potassium channel, responsible for the repolarising IKr current. These variants lead to pathologically shortened cardiac repolarisation and QT intervals on 12-lead ECG, predisposing patients to life-threatening ventricular arrhythmias and sudden cardiac death. Purpose To investigate the therapeutic efficacy of our KCNH2-specific suppression-replacement gene therapy (KCNH2-SupRep) in transgenic SQT1 rabbits generated by cardiac-specific overexpression of KCNH2-N588K. Methods KCNH2-SupRep was developed by combining a custom-designed KCNH2 shRNA with its corresponding shRNA-immune KCNH2 cDNA, packaged into a single AAV9 vector and delivered directly into the aortic root via a Swan Ganz catheter (1E10 vg/kg). Therapeutic efficacy was evaluated in vivo by 12-lead ECG recordings pre- and three weeks post-therapy, ex vivo by optical voltage mapping, and at the cellular level by patch-clamp, calcium imaging, and qPCR in ventricular cardiomyocytes (VCMs) isolated from adult transgenic SQT1 and wild-type (WT) rabbits. Results In vivo, KCNH2-SupRep normalized the heart-rate corrected QT interval (QTc) in transgenic SQT1 rabbits towards WT levels (baseline-SQT1: 153±4ms vs SupRep-SQT1: 167±5ms, p=0.0002; Fig. 1A), without adversely impacting repolarisation heterogeneity. No such QTc changes were observed in sham-treated SQT1 rabbits (sham-SQT1) (baseline-SQT1: 155±4ms vs sham-SQT1: 146±5ms, p=0.1422). Ex vivo, at a 2.5Hz stimulation frequency, KCNH2-SupRep therapeutically prolonged the APD90 in SupRep-SQT1 hearts (176±4ms) near WT levels (179±4ms), compared to untreated (UT)-SQT1 (154±4), and resolved the pathologically increased apicobasal APD90 heterogeneity observed in UT-SQT1 hearts, supporting an anti-arrhythmic effect of KCNH2-SupRep (Fig. 1B). At the cellular level, at a 1Hz stimulation frequency, KCNH2-SupRep prolonged the APD90 in VCMs isolated from SupRep-SQT1 rabbits (363±11ms) closer to WT levels (404±14ms) as compared to UT- (314±12ms) and sham-SQT1 (322±11ms; Fig. 1C). Moreover, KCNH2-SupRep restored the cellular surrogate of the electro-mechanical window (cEMW, Ca2+ transient duration – APD) and normalized IKr in nearly 50% of VCMs (Fig. 1D), in line with a 50-60% suppression of the mutant KCNH2 transcript. This partial therapeutic effect on IKr may explain the incomplete APD90 normalization in individual VCMs. Nonetheless, both in vivo and ex vivo, this partial therapeutic effect was sufficient to completely restore the electrical properties. Conclusion This proof-of-concept study is the first to demonstrate the efficacy of gene therapy in treating SQT1 in a medium-sized animal model. KCNH2-SupRep rescued the disease phenotype in vivo, ex vivo and at cellular levels in transgenic SQT1 rabbits.Therapeutic efficacy of KCNH2-SupRep

Abstract Background and Aims Type 1 short QT syndrome (SQT1) is a genetic channelopathy caused by gain-of-function variants in KCNH2, resulting in shortened cardiac repolarization and QT intervals, which predispose patients to ventricular arrhythmias and sudden cardiac death. This study aimed to investigate the therapeutic efficacy of KCNH2-specific suppression-and-replacement (KCNH2-SupRep) gene therapy in a transgenic rabbit model of SQT1. Methods KCNH2-SupRep was developed by combining a KCNH2-shRNA with its corresponding shRNA-immune KCNH2-cDNA into an AAV9 vector, delivered directly into the aortic root (1 × 1010 vg/kg). Therapeutic efficacy was evaluated in vivo by electrocardiogram, ex vivo by optical mapping, and at cellular levels by patch-clamp, calcium imaging, and qPCR in ventricular cardiomyocytes (VCMs). Results In vivo, KCNH2-SupRep normalized the heart rate-corrected QT interval (QTc) in SQT1 rabbits, without affecting repolarization heterogeneity. Ex vivo, KCNH2-SupRep corrected the action potential duration (APD90) and resolved the increased apicobasal APD90 heterogeneity observed in untreated (UT)-SQT1 hearts, supporting an antiarrhythmic effect, which was further validated by reduced re-entry formation in silico. At cellular levels, KCNH2-SupRep prolonged APD90 in VCMs from SupRep-SQT1 rabbits closer to wildtype levels compared with UT- and sham-SQT1. Additionally, KCNH2-SupRep restored the cellular surrogate of the electro-mechanical window and normalized IKr in nearly 50% of VCMs, in line with a 50%–60% suppression of the mutant KCNH2 transcript. Conclusions This proof-of-concept study is the first to demonstrate the efficacy of gene therapy for SQT1 in a medium-sized animal model. KCNH2-SupRep gene therapy successfully corrected the pathologic phenotype in vivo, ex vivo, and at cellular levels in transgenic SQT1 rabbits.

Abstract Aims Pronounced sex-differences are known in the incidence of atrial fibrillation (AF). In this study, we aimed to investigate the atrial electrophysiological properties that may underlie sex-differences in AF incidence in the younger population, focusing on IK1, a cardiac ion current important for action potential (AP) stability and triggered activity. Methods and results We assessed sex-differences in P-wave morphology in 12-lead ECG in healthy young New Zealand White rabbits. Males presented longer PWD and larger P-wave area compared to females. Patch-clamp experiments were performed in isolated rabbit atrial cardiomyocytes (CMs). Male atrial CMs presented higher delayed after depolarizations (DAD) incidence, amplitude, and area under the curve (AUC) than females, potentially facilitating the presence of atrial triggered activity in males. Male atrial CMs showed a less hyperpolarized resting membrane potential (RMP), a 50% smaller IK1, and a 26% reduction in Kir2.1 protein expression, a pore forming subunit of IK1, than females. Dihydrotestosterone (DHT) effects were investigated acutely and semi-chronically ex vivo. Experiments showed that the sex-difference in IK1 could be mimicked by DHT. In female atrial CMs, acute and semi-chronic (24 h) DHT administration reduced IK1. In the presence of a PKC-inhibitor, DHT-mediated IK1 reduction was not observed in atrial female CMs, suggesting it to be PKC-mediated. Chronic DHT-effects were investigated in vivo in female rabbits after hormone-releasing pellet implantation. After 2 weeks, animals showed a significantly prolonged and larger P-wave, a smaller atrial IK1 and a trend towards an increased DAD amplitude and AUC. Conclusion Sex impacts on atrial electrophysiology, leading to sex-differences in P-wave morphology, triggered activity, RMP, and IK1. These sex-differences can be mimicked by sex hormone-treatment, suggesting that sex hormones—particularly DHT—play a pivotal role in mediating sex-differences in atrial electrophysiology. Such sex-differences might impact on the propensity to develop AF, particularly in the younger population.

Abstract The pharmacological provocation test is a pivotal tool in cardiac electrophysiology for the diagnosis of potential causes of sudden cardiac death, sudden cardiac arrest (SCA), arrhythmias, symptoms, or ECG abnormalities. The 2022 European Society of Cardiology Guidelines for the Treatment of Ventricular Arrhythmias and Prevention of Sudden Cardiac Death offered guidance on provocation testing but did not describe the indications and requirements in depth. This clinical consensus statement, led by the European Heart Rhythm Association and approved by major international stakeholders, aims to advise the general cardiologist and the arrhythmia expert who to test and when, where, and how to do it. The statement focuses on current practice for the diagnosis of subclinical arrhythmia syndromes and the causes of SCA, building upon the recommendations of the Guidelines. We address the sodium channel blocker provocation test for patients suspected of Brugada syndrome as well as the use of epinephrine, isoproterenol, adenosine, ergonovine, and acetylcholine.

Abstract Background and Aims Type 1 long QT syndrome (LQT1) is caused by pathogenic variants in the KCNQ1-encoded Kv7.1 potassium channels, which pathologically prolong ventricular action potential duration (APD). Herein, the pathologic phenotype in transgenic LQT1 rabbits is rescued using a novel KCNQ1 suppression-replacement (SupRep) gene therapy. Methods KCNQ1-SupRep gene therapy was developed by combining into a single construct a KCNQ1 shRNA (suppression) and an shRNA-immune KCNQ1 cDNA (replacement), packaged into adeno-associated virus serotype 9, and delivered in vivo via an intra-aortic root injection (1E10 vg/kg). To ascertain the efficacy of SupRep, 12-lead electrocardiograms were assessed in adult LQT1 and wild-type (WT) rabbits and patch-clamp experiments were performed on isolated ventricular cardiomyocytes. Results KCNQ1-SupRep treatment of LQT1 rabbits resulted in significant shortening of the pathologically prolonged QT index (QTi) towards WT levels. Ventricular cardiomyocytes isolated from treated LQT1 rabbits demonstrated pronounced shortening of APD compared to LQT1 controls, leading to levels similar to WT (LQT1-UT vs. LQT1-SupRep, P < .0001, LQT1-SupRep vs. WT, P = ns). Under β-adrenergic stimulation with isoproterenol, SupRep-treated rabbits demonstrated a WT-like physiological QTi and APD90 behaviour. Conclusions This study provides the first animal-model, proof-of-concept gene therapy for correction of LQT1. In LQT1 rabbits, treatment with KCNQ1-SupRep gene therapy normalized the clinical QTi and cellular APD90 to near WT levels both at baseline and after isoproterenol. If similar QT/APD correction can be achieved with intravenous administration of KCNQ1-SupRep gene therapy in LQT1 rabbits, these encouraging data should compel continued development of this gene therapy for patients with LQT1.

BACKGROUND: Despite major advances in the clinical management of long QT syndrome, some patients are not fully protected by beta-blocker therapy. Mexiletine is a well-known sodium channel blocker, with proven efficacy in patients with sodium channel–mediated long QT syndrome type 3. Our aim was to evaluate the efficacy of mexiletine in long QT syndrome type 2 (LQT2) using cardiomyocytes derived from patient-specific human induced pluripotent stem cells, a transgenic LQT2 rabbit model, and patients with LQT2. METHODS: Heart rate–corrected field potential duration, a surrogate for QTc, was measured in human induced pluripotent stem cells from 2 patients with LQT2 (KCNH2-p.A561V, KCNH2-p.R366X) before and after mexiletine using a multiwell multi-electrode array system. Action potential duration at 90% repolarization (APD 90 ) was evaluated in cardiomyocytes isolated from transgenic LQT2 rabbits (KCNH2-p.G628S) at baseline and after mexiletine application. Mexiletine was given to 96 patients with LQT2. Patients were defined as responders in the presence of a QTc shortening ≥40 ms. Antiarrhythmic efficacy of mexiletine was evaluated by a Poisson regression model. RESULTS: After acute treatment with mexiletine, human induced pluripotent stem cells from both patients with LQT2 showed a significant shortening of heart rate–corrected field potential duration compared with dimethyl sulfoxide control. In cardiomyocytes isolated from LQT2 rabbits, acute mexiletine significantly shortened APD 90 by 113 ms, indicating a strong mexiletine-mediated shortening across different LQT2 model systems. Mexiletine was given to 96 patients with LQT2 either chronically (n=60) or after the acute oral drug test (n=36): 65% of the patients taking mexiletine only chronically and 75% of the patients who performed the acute oral test were responders. There was a significant correlation between basal QTc and ∆QTc during the test ( r = −0.8; P <0.001). The oral drug test correctly predicted long-term effect in 93% of the patients. Mexiletine reduced the mean yearly event rate from 0.10 (95% CI, 0.07–0.14) to 0.04 (95% CI, 0.02–0.08), with an incidence rate ratio of 0.40 (95% CI, 0.16–0.84), reflecting a 60% reduction in the event rate ( P =0.01). CONCLUSIONS: Mexiletine significantly shortens cardiac repolarization in LQT2 human induced pluripotent stem cells, in the LQT2 rabbit model, and in the majority of patients with LQT2. Furthermore, mexiletine showed antiarrhythmic efficacy. Mexiletine should therefore be considered a valid therapeutic option to be added to conventional therapies in higher-risk patients with LQT2.

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&lt;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.

Abstract Background Long-QT syndrome type 1 (LQT1) is a genetic channelopathy characterized by loss-of-function variants in the repolarizing potassium channel KCNQ1/IKs which lead to QT-prolongation and predisposes patients to adverse events such as cardiac arrhythmias. Clincally, during catecholamine infusion or exercise stress test, patients with LQT1 present a paradoxical QT prolongation. Purpose To investigate whether a gene-therapy approach targeting the defective K+-channel, through a KCNQ1 suppression-and-replacement (SupRep) strategy, can restore the healthy phenotype at baseline and under catecholamine infusion in LQT1 transgenic rabbits. Methods Our proprietary dual component suppression-and-replacement (SupRep) gene therapy was created by combining a custom-designed KCNQ1 shRNA and a shRNA-immune (shIMM) KCNQ1 cDNA into AAV9. In vivo KCNQ1-SupRep gene therapy was performed by targeted intra-aortic root injection during balloon occlusion. Two or three weeks post gene therapy, 12-lead ECGs were assessed in adult transgenic LQT1 (KCNQ1-Y315S, loss-of IKs) and wild-type (WT) rabbits to determine the effect of SupRep gene therapy on QT-duration at baseline and under catecholamine infusion (10nM isoproterenol, 5mL/h). Ventricular cardiomyocytes (VCMs) were isolated to investigate the effect of SupRep gene therapy on action potential duration (APD) and calcium transients (Ca2+T) at baseline and under isoproterenol infusion. Results At the cellular level, VCMs isolated from SupRep treated LQT1 rabbits showed a significant shortening of the APD90 as compared to UT-LQT1 (p&lt;0.0001). At the whole animal level, at baseline, the QTi (%) of the UT-LQT1 transgenic rabbits (122±3) was significantly longer compared to the WT rabbits (105±2, p&lt;0.0001). Treatment with AAV9-KCNQ1-SupRep shortened significantly the QTi of the transgenic LQT1 rabbits as compared to the UT-LQT1 rabbits (111±2, p=0.0115). To further investigate the effect of SupRep gene therapy, 12-lead ECGs were obtained in UT-LQT1, sham-treated LQT1, SupRep-treated LQT1, and WT rabbits under ISO infusion. While reaching a 20% increase in heart rate during ISO stimulation, the UT-LQT1 and the sham treated LQT1 rabbits showed a marked QT prolongation, with ΔQTi of 31.4% and 24.5%, respectively, as compared to baseline. In contrast, the SupRep-treated LQT1 rabbit behaved nearly identical to the WT rabbit during ISO infusion, showing a ΔQTi of 16.9% as compared to 16.5% in the WT rabbit. Similarly, in contrast to the UT-LQT1, VCMs isolated from SupRep-treated LQT1 rabbits demonstrated a normalization of the APD and Ca2+T under ISO stimulation, showing a significantly shortened APD90 (p&lt;0.0001) and Ca2+T90 (p=0.0025) which was similar to what was observed in WT VCMs. Conclusion In vivo KCNQ1 suppression-and-replacement gene therapy normalizes the pathologically prolonged APD/QT interval and restores a physiological response to catecholamine infusion in treated LQT1 transgenic rabbits.

Abstract Funding Acknowledgements Type of funding sources: Private company. Main funding source(s): Pfizer Background Type 1 long QT syndrome (LQT1) is a genetic channelopathy characterized by both haploinsufficient and dominant-negative loss-of-function pathogenic variants in the KCNQ1-encoded Kv7.1 K+ channels conducting IKs. Patients with LQT1 may manifest QT prolongation and ventricular arrhythmias that can culminate in sudden cardiac death. Purpose With this project, we aim to investigate whether our mutation independent, KCNQ1-specific suppression-replacement (SupRep) gene therapy can restore the deficient K+-channel and thereby restore the healthy phenotype. Methods Our proprietary dual component SupRep gene therapy was created by combining a custom-designed KCNQ1 shRNA and a shRNA-immune (shIMM) KCNQ1 cDNA into AAV9. In vivo AAV9-mediated KCNQ1SupRep gene therapy was performed by targeted intra-aortic root construct injection (1E12 vg/kg body weight) during balloon occlusion using a Swan Ganz catheter. 2 weeks after in vivo gene therapy, 12-lead ECGs were assessed in adult transgenic LQT1 (KCNQ1-Y315S, loss-of IKs) and wild-type (WT) rabbits to determine the effect of SupRep therapy on the rabbit’s QTc. Patch-clamp and calcium transient measurements were performed in isolated ventricular cardiomyocytes to evaluate the effect of SupRep on the cardiomyocyte’s action potential duration (APD90) and Ca2+ transient duration (Ca2+90) both, in vitro – after plasmid transfection, and in vivo – after AAV9-mediated gene therapy. AAV9-KCNQ1-construct expression was verified with immunohistochemistry targeting the GFP-tag. Results After injection of 3 LQT1 rabbits so far, no significant changes were observed in the LQT1 rabbits’ QTc before and after in vivo AAV9-mediated gene therapy. However, at the cellular level, LQT1 cardiomyocytes transfected in vitro with the SupRep-plasmid demonstrated a significant reduction of APD90 (ms, 1Hz stimulation at 37°) compared to LQT1 control cells (SupRep-in-vitro, 358±21 vs. control 447±22, p&lt;0.05). No differences were observed between sham-plasmid transfected cardiomyocytes and LQT1 control cells after one day of culturing. Similarly, cardiomyocytes isolated from the 3 LQT1 rabbits that underwent in vivo SupRep gene therapy demonstrated pronounced shortening of both APD90 and Ca2+90 compared to LQT1 controls, leading to levels similar to WT controls (APD90, 1Hz, 37°: LQT1: 530±18, WT: 445±18, LQT1-SupRep: 375±25, p&lt;0.0001 LQT1 vs. LQT1-SupRep, p=ns LQT1-SupRep vs. WT) (Ca2+90: LQT1: 487±23, WT: 346±16, LQT1-SupRep: 393±19, p&lt;0.0001 LQT1 vs. LQT1-SupRep, p=ns LQT1-SupRep vs. WT). Conclusion In vivo KCNQ1-suppression-replacement gene therapy normalizes cellular action potential and calcium transient duration in transgenic LQT1 rabbits. Further in vivo experiments will be conducted to evaluate whether this therapeutic correction at the cardiomyocyte level will translate into normalization of the LQT1 rabbits’ QTc both at rest and during stress testing.

Abstract An abundance of literature describes physiological and pathological determinants of cardiac performance, building on the principles of excitation–contraction coupling. However, the mutual influencing of excitation–contraction and mechano-electrical feedback in the beating heart, here designated ‘electromechanical reciprocity’, remains poorly recognized clinically, despite the awareness that external and cardiac-internal mechanical stimuli can trigger electrical responses and arrhythmia. This review focuses on electromechanical reciprocity in the long-QT syndrome (LQTS), historically considered a purely electrical disease, but now appreciated as paradigmatic for the understanding of mechano-electrical contributions to arrhythmogenesis in this and other cardiac conditions. Electromechanical dispersion in LQTS is characterized by heterogeneously prolonged ventricular repolarization, besides altered contraction duration and relaxation. Mechanical alterations may deviate from what would be expected from global and regional repolarization abnormalities. Pathological repolarization prolongation outlasts mechanical systole in patients with LQTS, yielding a negative electromechanical window (EMW), which is most pronounced in symptomatic patients. The electromechanical window is a superior and independent arrhythmia-risk predictor compared with the heart rate-corrected QT. A negative EMW implies that the ventricle is deformed—by volume loading during the rapid filling phase—when repolarization is still ongoing. This creates a ‘sensitized’ electromechanical substrate, in which inadvertent electrical or mechanical stimuli such as local after-depolarizations, after-contractions, or dyssynchrony can trigger abnormal impulses. Increased sympathetic-nerve activity and pause-dependent potentiation further exaggerate electromechanical heterogeneities, promoting arrhythmogenesis. Unraveling electromechanical reciprocity advances the understanding of arrhythmia formation in various conditions. Real-time image integration of cardiac electrophysiology and mechanics offers new opportunities to address challenges in arrhythmia management.