Biological Heart Rate Reduction Through Genetic Suppression of Gα(s) Protein in the Sinoatrial Node

Gene Therapy Approaches to Biological Pacemakers

Bradycardia arising from pacemaker dysfunction can be debilitating and life threatening. Electronic pacemakers serve as effective treatment options for pacemaker dysfunction. They however present their own limitations and complications. This has motivated research into discovering more effective and innovative ways to treat pacemaker dysfunction. Gene therapy is being explored for its potential to treat various cardiac conditions including cardiac arrhythmias. Gene transfer vectors with increasing transduction efficiency and biosafety have been developed and trialed for cardiovascular disease treatment. With an improved understanding of the molecular mechanisms driving pacemaker development, several gene therapy targets have been identified to generate the phenotypic changes required to correct pacemaker dysfunction. This review will discuss the gene therapy vectors in use today along with methods for their delivery. Furthermore, it will evaluate several gene therapy strategies attempting to restore biological pacing, having the potential to emerge as viable therapies for pacemaker dysfunction.

Cardiac gene therapy with adeno-associated virus-based vectors

PURPOSE OF REVIEW

Cardiac gene therapy with adeno-associated virus (AAV)-based vectors is emerging as an entirely new platform to treat, or even cure, so far intractable cardiac disorders. This review describes our current knowledge of cardiac AAV gene therapy with a particular focus on the biggest obstacle for the successful translation of cardiac AAV gene therapy into the clinic, namely the efficient delivery of the therapeutic gene to the myocardium.

RECENT FINDINGS

We summarize the significant recent progress that has been made in treating heart failure in preclinically relevant animal models with AAV gene therapy and the recent results of clinical trials with cardiac AAV gene therapy for the treatment of heart failure. We also discuss the benefits and shortcomings of the currently available delivery methods of AAV to the heart. Finally, we describe the current state of identifying novel AAV variants that have enhanced tropism for human cardiomyocytes and that show increased resistance to preexisting neutralizing antibodies.

SUMMARY

Here, we describe the successes and challenges in cardiac AAV gene therapy, a treatment modality that has the potential to transform current treatment approaches for cardiac diseases.

Improving lipoprotein profiles by liver-directed gene transfer of low density lipoprotein receptor gene in hypercholesterolaemia mice

The defect of low density lipoprotein receptor disturbs cholesterol metabolism and causes familial hypercholesterolaemia (FH). In this study, we directly delivered exogenous Ldlr gene into the liver of FH model mice (Ldlr(-/-)) by lentiviral gene transfer system. The results showed that the Ldlr gene controlled by hepatocyte-specific human thyroxine-binding globulin (TBG) promoter successfully and exclusively expressed in livers.We found that, although, the content of high density lipoprotein in serum was not significantly affected by the Ldlr gene expression, the serum low density lipoprotein level was reduced by 46%, associated with a 30% and 28% decrease in triglyceride and total cholesterol, respectively, compared to uninjected Ldlr(-/-) mice. Moreover, the TBG directed expression of Ldlr significantly decreased the lipid accumulation in liver and reduced plaque burden in aorta (32%). Our results indicated that the hepatocyte-specific expression of Ldlr gene strikingly lowered serum lipid levels and resulted in amelioration of hypercholesterolaemia.

Antiarrhythmic gene therapy - will biologics replace catheters, drugs and devices?

The clinical management of heart rhythm disorders still constitutes a major challenge. The development of alternatives to current approaches is of significant interest in order to establish more effective therapies that increase quality of life and reduce symptoms and hospitalizations. Over the past two decades the mechanistic understanding of pathophysiological pathways underlying cardiac arrhythmias has advanced profoundly, opening up novel avenues for mechanism-based therapeutic approaches. In particular, gene therapy offers greater selectivity than small molecule-based or interventional treatment. The gene of interest is packaged into viral or non-viral carriers and delivered to the target area via direct injection or using catheter-based techniques, providing the advantage of site-restricted action in contrast to systemic application of drugs. This work summarizes the current knowledge on mechanistic background, application strategies, and preclinical outcome of antiarrhythmic gene therapy for atrial fibrillation, ventricular tachycardia, and modulation of sinus node function.

Gene therapy to restore electrophysiological function in heart failure

INTRODUCTION

Heart failure (HF) is a major public health epidemic and a leading cause of morbidity and mortality in the industrialized world. Existing treatments for patients with HF are often associated with pro-arrhythmic activity and risk of sudden cardiac death. Therefore, development of novel, effective and safe therapeutic options for HF patients is a critical area of unmet need.

AREAS COVERED

In this article, we review recent advances in the emerging field of cardiac gene therapy for the treatment of tachy- and bradyarrhythmias in HF. We provide an overview of gene-based approaches that modulate myocardial conduction, repolarization, calcium cycling and adrenergic signaling to restore heart rate and rhythm.

EXPERT OPINION

We highlight major advantages of gene therapy for arrhythmias, including the ability to selectively target specific cell populations and to limit the therapeutic effect to the region that requires modification. We illustrate how advances in our fundamental understanding of the molecular origins of arrhythmogenic disorders are allowing investigators to use targeted gene-based approaches to successfully correct abnormal excitability in the atria, ventricles and conduction system. Translation of various gene therapy approaches to humans may revolutionize our ability to combat lethal arrhythmias in HF patients.

Atrial fibrillation complicated by heart failure induces distinct remodeling of calcium cycling proteins

Atrial fibrillation (AF) and heart failure (HF) are two of the most common cardiovascular diseases. They often coexist and account for significant morbidity and mortality. Alterations in cellular Ca²⁺ homeostasis play a critical role in AF initiation and maintenance. This study was designed to specifically elucidate AF-associated remodeling of atrial Ca²⁺ cycling in the presence of mild HF. AF was induced in domestic pigs by atrial burst pacing. The animals underwent electrophysiologic and echocardiographic examinations. Ca²⁺ handling proteins were analyzed in right atrial tissue obtained from pigs with AF (day 7; n = 5) and compared to sinus rhythm (SR) controls (n = 5). During AF, animals exhibited reduction of left ventricular ejection fraction (from 73% to 58%) and prolonged atrial refractory periods. AF and HF were associated with suppression of protein kinase A (PKA)_RII (-62%) and Ca²⁺-calmodulin-dependent kinase II (CaMKII) δ by 37%, without changes in CaMKIIδ autophosphorylation. We further detected downregulation of L-type calcium channel (LTCC) subunit α₂ (-75%), sarcoplasmic reticulum Ca²⁺-ATPase (Serca) 2a (-29%), phosphorylated phospholamban (Ser16, -92%; Thr17, -70%), and phospho-ryanodine receptor 2 (RyR2) (Ser2808, -62%). Na⁺-Ca²⁺ exchanger (NCX) levels were upregulated (+473%), whereas expression of Ser2814-phosphorylated RyR2 and LTCCα_1c subunits was not significantly altered. In conclusion, AF produced distinct arrhythmogenic remodeling of Ca²⁺ handling in the presence of tachycardia-induced mild HF that is different from AF without structural alterations. The changes may provide a starting point for personalized approaches to AF treatment.

The Use of Gene Therapy for Ablation of Atrial Fibrillation

Atrial fibrillation is the most common clinically significant cardiac arrhythmia, increasing the risk of stroke, heart failure and morbidity and mortality. Current therapies, including rate control and rhythm control by antiarrhythmic drugs or ablation therapy, are moderately effective but far from optimal. Gene therapy has the potential to become an attractive alternative to currently available therapies for atrial fibrillation. Various gene transfer vectors have been developed for cardiovascular disease with viral vectors being most widely used due to their high efficiency. Several gene delivery methods have been employed on different therapeutic targets. With increasing understanding of arrhythmia mechanisms, novel therapeutic targets have been discovered. This review will evaluate state-of-art gene therapy strategies and approaches including sinus rhythm restoration and ventricular rate control that could eventually prevent or eliminate atrial fibrillation in patients.

Mechanisms of zolpidem-induced long QT syndrome: acute inhibition of recombinant hERG K(+) channels and action potential prolongation in human cardiomyocytes derived from induced pluripotent stem cells

BACKGROUND AND PURPOSE

Zolpidem, a short-acting hypnotic drug prescribed to treat insomnia, has been clinically associated with acquired long QT syndrome (LQTS) and torsade de pointes (TdP) tachyarrhythmia. LQTS is primarily attributed to reduction of cardiac human ether-a-go-go-related gene (hERG)/I(Kr) currents. We hypothesized that zolpidem prolongs the cardiac action potential through inhibition of hERG K(+) channels.

EXPERIMENTAL APPROACH

Two-electrode voltage clamp and whole-cell patch clamp electrophysiology was used to record hERG currents from Xenopus oocytes and from HEK 293 cells. In addition, hERG protein trafficking was evaluated in HEK 293 cells by Western blot analysis, and action potential duration (APD) was assessed in human-induced pluripotent stem cell (hiPSC)-derived cardiomyocytes.

KEY RESULTS

Zolpidem caused acute hERG channel blockade in oocytes (IC(50) = 61.5 μM) and in HEK 293 cells (IC(50) = 65.5 μM). Mutation of residues Y652 and F656 attenuated hERG inhibition, suggesting drug binding to a receptor site inside the channel pore. Channels were blocked in open and inactivated states in a voltage- and frequency-independent manner. Zolpidem accelerated hERG channel inactivation but did not affect I-V relationships of steady-state activation and inactivation. In contrast to the majority of hERG inhibitors, hERG cell surface trafficking was not impaired by zolpidem. Finally, acute zolpidem exposure resulted in APD prolongation in hiPSC-derived cardiomyocytes.

CONCLUSIONS AND IMPLICATIONS

Zolpidem inhibits cardiac hERG K(+) channels. Despite a relatively low affinity of zolpidem to hERG channels, APD prolongation may lead to acquired LQTS and TdP in cases of reduced repolarization reserve or zolpidem overdose.

The in vivo regulation of heart rate in the murine sinoatrial node by stimulatory and inhibitory heterotrimeric G proteins

Reciprocal physiological modulation of heart rate is controlled by the sympathetic and parasympathetic systems acting on the sinoatrial (SA) node. However, there is little direct in vivo work examining the role of stimulatory and inhibitory G protein signaling in the SA node. Thus, we designed a study to examine the role of the stimulatory (Gαs) and inhibitory G protein (Gαi2) in in vivo heart rate regulation in the SA node in the mouse. We studied mice with conditional deletion of Gαs and Gαi2 in the conduction system using cre-loxP technology. We crossed mice in which cre recombinase expression was driven by a tamoxifen-inducible conduction system-specific construct with “Gαs floxed” and “Gαi2 floxed” mice. We studied the heart rate responses of adult mice compared with littermate controls by using radiotelemetry before and after administration of tamoxifen. The mice with conditional deletion of Gαs and Gαi2 had a loss of diurnal variation and were bradycardic or tachycardic, respectively, in the daytime. In mice with conditional deletion of Gαs, there was a selective loss of low-frequency power, while with deletion of Gαi2, there was a loss of high-frequency power in power spectral analysis of heart rate variability. There was no evidence of pathological arrhythmia. Pharmacological modulation of heart rate by isoprenaline was impaired in the Gαs mice, but a muscarinic agonist was still able to slow the heart rate in Gαi2 mice. We conclude that Gαs- and Gαi2-mediated signaling in the sinoatrial node is important in the reciprocal regulation of heart rate through the autonomic nervous system.

Anti-KCNQ1 K⁺ channel autoantibodies increase IKs current and are associated with QT interval shortening in dilated cardiomyopathy

AIMS

Autoimmune-associated proarrhythmia in dilated cardiomyopathy (DCM) is poorly understood. Given the significance of KCNQ1 potassium channels in heart rhythm disorders, we hypothesized that circulating anti-KCNQ1 autoantibodies directly modulate cardiac electrophysiology in DCM patients. The purpose of this pilot study was to characterize ion channel autoantibodies in DCM targeting the cardiac repolarizing K(+) current, IKs, and the underlying KCNQ1 potassium channel.

METHODS AND RESULTS

One hundred and fifty DCM patients were screened for anti-KCNQ1 autoantibodies using an enzyme-linked immunosorbent assay. Autoantibodies targeting the extracellular pore domain of the KCNQ1 channel were detected in 6% of study patients. Seropositive individuals exhibited significantly shorter corrected QT intervals when compared with seronegative patients (371 ± 39.9 ms vs. 408 ± 47.9 ms; P = 0.036). There was no difference in clinical severity of heart failure between groups. The functional significance of anti-KCNQ1 antibodies was determined in human embryonic kidney 293 cells expressing KCNQ1/KCNE1 using the whole-cell patch clamp technique. IKs recordings demonstrated a 2.7-fold increase in mean current density on exposure to patients' sera containing anti-KCNQ1 antibodies in contrast to seronegative controls (8.74 ± 1.44 pA/pF vs. 3.26 ± 0.36 pA/pF; P = 0.003). IKs enhancement was not associated with increased KCNQ1 protein levels or altered cell surface expression of the channel.

CONCLUSION

Anti-KCNQ1 autoantibodies found in a subgroup of DCM patients are associated with QT interval shortening and increased IKs current.