Antiarrhythmic Effects of Dantrolene in Patients with Catecholaminergic Polymorphic Ventricular Tachycardia and Replication of the Responses Using iPSC Models

Human heart disease: lessons from human pluripotent stem cell-derived cardiomyocytes

Technical advances in generating and phenotyping cardiomyocytes from human pluripotent stem cells (hPSC-CMs) are now driving their wider acceptance as in vitro models to understand human heart disease and discover therapeutic targets that may lead to new compounds for clinical use. Current literature clearly shows that hPSC-CMs recapitulate many molecular, cellular, and functional aspects of human heart pathophysiology and their responses to cardioactive drugs. Here, we provide a comprehensive overview of hPSC-CMs models that have been described to date and highlight their most recent and remarkable contributions to research on cardiovascular diseases and disorders with cardiac traits. We conclude discussing immediate challenges, limitations, and emerging solutions.

Induced pluripotent stem cell technology and inherited arrhythmia syndromes

Inherited arrhythmia syndromes, including familial long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and Brugada syndrome, can cause life-threatening arrhythmias and are responsible for a significant proportion of sudden deaths in the young. Identification of genetic mutations and pathophysiological changes that underlie disease development can inform clinical practice and guide novel drug development. However, disease mechanisms in a large number of patients remain elusive and pharmacologic treatment is suboptimal, so many patients rely on implantable cardioverter-defibrillator therapy. Induced pluripotent stem cell models of disease facilitate analysis of disease mechanisms in patient-specific cardiomyocytes, overcoming limitations of animal models and human tissue restrictions. This review outlines how studies using induced pluripotent stem cell-derived cardiomyocytes are contributing to our understanding of the mechanisms that underpin disease pathogenesis and their potential to facilitate new pharmacologic therapies and personalized medicine.

Patient-Specific Drug Screening Using a Human Induced Pluripotent Stem Cell Model of Catecholaminergic Polymorphic Ventricular Tachycardia Type 2

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia type 2 (CPVT2) results from autosomal recessive CASQ2 mutations, causing abnormal Ca²⁺-handling and malignant ventricular arrhythmias. We aimed to establish a patient-specific human induced pluripotent stem cell (hiPSC) model of CPVT2 and to use the generated hiPSC-derived cardiomyocytes to gain insights into patient-specific disease mechanism and pharmacotherapy.

METHODS AND RESULTS

hiPSC cardiomyocytes were derived from a CPVT2 patient (D307H-CASQ2 mutation) and from healthy controls. Laser-confocal Ca²⁺ and voltage imaging showed significant Ca²⁺-transient irregularities, marked arrhythmogenicity manifested by early afterdepolarizations and triggered arrhythmias, and reduced threshold for store overload-induced Ca²⁺-release events in the CPVT2-hiPSC cardiomyocytes when compared with healthy control cells. Pharmacological studies revealed the prevention of adrenergic-induced arrhythmias by β-blockers (propranolol and carvedilol), flecainide, and the neuronal sodium-channel blocker riluzole; a direct antiarrhythmic action of carvedilol (independent of its α/β-adrenergic blocking activity), flecainide, and riluzole; and suppression of abnormal Ca²⁺ cycling by the ryanodine stabilizer JTV-519 and carvedilol. Mechanistic insights were gained on the different antiarrhythmic actions of the aforementioned drugs, with carvedilol and JTV-519 (but not flecainide or riluzole) acting primarily through sarcoplasmic reticulum stabilization. Finally, comparable outcomes were found between flecainide and labetalol antiarrhythmic effects in vitro and the clinical results in the same patient.

CONCLUSIONS

These results demonstrate the ability of hiPSCs cardiomyocytes to recapitulate CPVT2 disease phenotype and drug response in the culture dish, to provide novel insights into disease and drug therapy mechanisms, and potentially to tailor patient-specific drug therapy.

Reduced threshold for store overload-induced Ca2+ release is a common defect of RyR1 mutations associated with malignant hyperthermia and central core disease

Mutations in the skeletal muscle ryanodine receptor (RyR1) cause malignant hyperthermia (MH) and central core disease (CCD), whereas mutations in the cardiac ryanodine receptor (RyR2) lead to catecholaminergic polymorphic ventricular tachycardia (CPVT). Most disease-associated RyR1 and RyR2 mutations are located in the N-terminal, central, and C-terminal regions of the corresponding ryanodine receptor (RyR) isoform. An increasing body of evidence demonstrates that CPVT-associated RyR2 mutations enhance the propensity for spontaneous Ca²⁺ release during store Ca²⁺ overload, a process known as store overload-induced Ca²⁺ release (SOICR). Considering the similar locations of disease-associated RyR1 and RyR2 mutations in the RyR structure, we hypothesize that like CPVT-associated RyR2 mutations, MH/CCD-associated RyR1 mutations also enhance SOICR. To test this hypothesis, we determined the impact on SOICR of 12 MH/CCD-associated RyR1 mutations E2347-del, R2163H, G2434R, R2435L, R2435H, and R2454H located in the central region, and Y4796C, T4826I, L4838V, A4940T, G4943V, and P4973L located in the C-terminal region of the channel. We found that all these RyR1 mutations reduced the threshold for SOICR. Dantrolene, an acute treatment for MH, suppressed SOICR in HEK293 cells expressing the RyR1 mutants R164C, Y523S, R2136H, R2435H, and Y4796C. Interestingly, carvedilol, a commonly used β-blocker that suppresses RyR2-mediated SOICR, also inhibits SOICR in these RyR1 mutant HEK293 cells. Therefore, these results indicate that a reduced SOICR threshold is a common defect of MH/CCD-associated RyR1 mutations, and that carvedilol, like dantrolene, can suppress RyR1-mediated SOICR. Clinical studies of the effectiveness of carvedilol as a long-term treatment for MH/CCD or other RyR1-associated disorders may be warranted.

Calcium Signaling and Cardiac Arrhythmias

There has been a significant progress in our understanding of the molecular mechanisms by which calcium (Ca2+) ions mediate various types of cardiac arrhythmias. A growing list of inherited gene defects can cause potentially lethal cardiac arrhythmia syndromes, including catecholaminergic polymorphic ventricular tachycardia, congenital long QT syndrome, and hypertrophic cardiomyopathy. In addition, acquired deficits of multiple Ca2+-handling proteins can contribute to the pathogenesis of arrhythmias in patients with various types of heart disease. In this review article, we will first review the key role of Ca2+ in normal cardiac function-in particular, excitation-contraction coupling and normal electric rhythms. The functional involvement of Ca2+ in distinct arrhythmia mechanisms will be discussed, followed by various inherited arrhythmia syndromes caused by mutations in Ca2+-handling proteins. Finally, we will discuss how changes in the expression of regulation of Ca2+ channels and transporters can cause acquired arrhythmias, and how these mechanisms might be targeted for therapeutic purposes.

Adeno-associated virus-mediated CASQ2 delivery rescues phenotypic alterations in a patient-specific model of recessive catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic Polymorphic Ventricular Tachycardia type 2 (CPVT2) is a highly lethal recessive arrhythmogenic disease caused by mutations in the calsequestrin-2 (CASQ2) gene. We have previously demonstrated that viral transfer of the wild-type (WT) CASQ2 gene prevents the development of CPVT2 in a genetically induced mouse model of the disease homozygous carrier of the R33Q mutation. In the present study, we investigated the efficacy of the virally mediated gene therapy in cardiomyocytes (CMs) differentiated from induced pluripotent stem cells (iPSCs) obtained from a patient carrying the homozygous CASQ2-G112+5X mutation. To this end, we infected cells with an Adeno-Associated Viral vector serotype 9 (AAV9) encoding the human CASQ2 gene (AAV9-hCASQ2). Administration of the human WT CASQ2 gene was capable and sufficient to restore the physiological expression of calsequestrin-2 protein and to rescue functional defects of the patient-specific iPSC-derived CMs. Indeed, after viral gene transfer, we observed a remarkable decrease in the percentage of delayed afterdepolarizations (DADs) developed by the diseased CMs upon adrenergic stimulation, the calcium transient amplitude was re-established and the density and duration of calcium sparks were normalized. We therefore demonstrate the efficacy of the AAV9-mediated gene replacement therapy for CPVT2 in a human cardiac-specific model system, supporting the view that the gene-therapy tested is curative in models with different human mutations of CPVT.

Antiarrhythmic effects of dantrolene in human diseased cardiomyocytes

BACKGROUND

Cardiac type 2 ryanodine receptors (RyR₂s) play a pivotal role in cellular electrophysiology and contractility. Increased RyR₂-mediated diastolic sarcoplasmic reticulum (SR) Ca²⁺ release is linked to heart failure (HF) and arrhythmias. Dantrolene, a drug used for the treatment of malignant hyperthermia, is known to stabilize RyRs in skeletal muscle.

OBJECTIVE

The purpose of this study was to investigate the effects of dantrolene on arrhythmogenic triggers and contractile function in human atrial fibrillation (AF) and HF cardiomyocytes (CM).

METHODS

Human CM were isolated from either patients with HF (ventricular) or patients with AF (atrial), and Ca²⁺ imaging, patch-clamp, or muscle strip experiments were performed.

RESULTS

After exposure to dantrolene, human atrial AF and left ventricular HF CM showed significant reductions in proarrhythmic SR Ca²⁺ spark frequency and diastolic SR Ca²⁺ leak. Moreover, dantrolene decreased the frequency of Ca²⁺ waves and spontaneous Ca²⁺ transients in HF CM. Patch-clamp experiments revealed that dantrolene significantly suppressed delayed afterdepolarizations in HF and AF CM. Importantly, dantrolene had no effect on action potential duration in AF or in HF CM. In addition, dantrolene had neutral effects on contractile force of human isometrically twitching ventricular HF trabeculae.

CONCLUSION

Our study showed that dantrolene beneficially influenced disrupted SR Ca²⁺ homeostasis in human HF and AF CM. Cellular arrhythmogenic triggers were potently suppressed by dantrolene, whereas action potential duration and contractility were not affected. As a clinically approved drug for the treatment of malignant hyperthermia, dantrolene may be a potential antiarrhythmic drug for patients with rhythm disorders and merits further clinical investigation.

A human pluripotent stem cell model of catecholaminergic polymorphic ventricular tachycardia recapitulates patient-specific drug responses

Although β-blockers can be used to eliminate stress-induced ventricular arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), this treatment is unsuccessful in ∼25% of cases. Induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) generated from these patients have potential for use in investigating the phenomenon, but it remains unknown whether they can recapitulate patient-specific drug responses to β-blockers. This study assessed whether the inadequacy of β-blocker therapy in an individual can be observed in vitro using patient-derived CPVT iPSC-CMs. An individual with CPVT harboring a novel mutation in the type 2 cardiac ryanodine receptor (RyR2) was identified whose persistent ventricular arrhythmias during β-blockade with nadolol were abolished during flecainide treatment. iPSC-CMs generated from this patient and two control individuals expressed comparable levels of excitation-contraction genes, but assessment of the sarcoplasmic reticulum Ca(2+) leak and load relationship revealed intracellular Ca(2+) homeostasis was altered in the CPVT iPSC-CMs. β-adrenergic stimulation potentiated spontaneous Ca(2+) waves and unduly frequent, large and prolonged Ca(2+) sparks in CPVT compared with control iPSC-CMs, validating the disease phenotype. Pursuant to the patient's in vivo responses, nadolol treatment during β-adrenergic stimulation achieved negligible reduction of Ca(2+) wave frequency and failed to rescue Ca(2+) spark defects in CPVT iPSC-CMs. In contrast, flecainide reduced both frequency and amplitude of Ca(2+) waves and restored the frequency, width and duration of Ca(2+) sparks to baseline levels. By recapitulating the improved response of an individual with CPVT to flecainide compared with β-blocker therapy in vitro, these data provide new evidence that iPSC-CMs can capture basic components of patient-specific drug responses.

Patient Specific Induced Pluripotent Stem Cell-Derived Cardiomyocytes for Drug Development and Screening In Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited arrhythmia often leading to sudden cardiac death in children and young adults, is characterized by polymorphic/bidirectional ventricular tachycardia induced by adrenergic stimulation associated with emotionally stress or physical exercise. There are two forms of CPVT: 1. CPVT1 is caused by mutations in the RYR2 gene, encoding for ryanodine receptor type 2. CPVT1 is the most common form of CPVT in the population, and is inherited by a dominant mechanism. 2. CPVT2 is caused by mutations in the CASQ2 gene, encoding for cardiac calsequestrin 2 and is inherited by recessive mechanism. Patient-specific induced Pluripotent Stem Cells (iPSC) have the ability to differentiate into cardiomyocytes carrying the patient's genome including CPVT-linked mutations and expressing the disease phenotype in vitro at the cellular level. The potency for in vitro modeling using iPSC-derived cardiomyocytes (iPSC-CMs) has been exploited to investigate a variety of inherited diseases including cardiac arrhythmias such as CPVT. In this review we attempted to cover the majority of CPVT patient specific iPSC research studies previously published. CPVT patient-specific iPSC model enables the in vitro investigation of the molecular and cellular disease-mechanisms by the means of electrophysiologycal and Ca⁺² imaging methodologies. Furthermore, this in vitro model allows the screening of various antiarrhythmic drugs, specifically for each patient, also known as "personalized medicine".

Ion Channel Diseases: an Update for 2016

OPINION STATEMENT

Ion channelopathies are a frequent cause of sudden cardiac death (SCD) in patients with structurally normal hearts. These are generally Mendelian inherited electrical disorders with variable penetrance and expressivity. The ability to predict the development of life threatening arrhythmias in these patients is challenging. This chapter will present an update on the genetics, the role of genetic testing, and management of the inherited cardiac channelopathies with a focus on the relatively more common syndromes associated with an increased risk of SCD.

Methods for in vitro functional analysis of iPSC derived cardiomyocytes - Special focus on analyzing the mechanical beating behavior

A rapidly increasing number of papers describing novel iPSC models for cardiac diseases are being published. To be able to understand the disease mechanisms in more detail, we should also take the full advantage of the various methods for analyzing these cell models. The traditionally and commonly used electrophysiological analysis methods have been recently accompanied by novel approaches for analyzing the mechanical beatingbehavior of the cardiomyocytes. In this review, we provide first a concise overview on the methodology for cardiomyocyte functional analysis and then concentrate on the video microscopy, which provides a promise for a new faster yet reliable method for cardiomyocyte functional analysis. We also show how analysis conditions may affect the results. Development of the methodology not only serves the basic research on the disease models, but could also provide the much needed efficient early phase screening method for cardiac safety toxicology. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Calcium signaling in human stem cell-derived cardiomyocytes: Evidence from normal subjects and CPVT afflicted patients

Derivation of cardiomyocyte cell lines from human fibroblasts (induced pluripotent stem cells, iPSCs) has made it possible not only to investigate the electrophysiological and Ca(2+) signaling properties of these cells, but also to determine the altered electrophysiological and Ca(2+)-signaling profiles of such cells lines derived from patients expressing mutation-inducing pathologies. This approach has the potential of generating in vitro human models of cardiovascular diseases where cellular pathology can be investigated in detail and possibly specific pharmacotherapy developed. Although this approach has been applied to a number of mutations in channel proteins that cause arrhythmias, there are only few detailed reports addressing Ca(2+) signaling pathologies beyond measurements of Ca(2+) transients in intact non-voltage clamped cells. Unfortunately, full understanding of Ca(2+) signaling pathologies remains elusive, not only because of the plethora of Ca(2+) signaling proteins defects that cause arrhythmias and cardiomyopathies, but also because detailed functional properties of Ca(2+) signaling proteins are difficult to obtain. Catecholaminergic polymorphic ventricular tachycardia (CPVT1) is a malignant inherited arrhythmogenic disorder predominantly caused by mutations in the cardiac ryanodine receptor (RyR2). Thus far over 150 mutations in RyR2 have been identified that appear to cause this arrhythmia, a number of which have been expressed and studied in transgenic mice or cell-line models. The development of human iPSC-technology makes it possible to create human heart cell-lines carrying these mutations, making detailed identification of Ca(2+) signaling defects and its specific pharmacotherapy possible. In this review we shall first briefly summarize the essential characteristics of the mammalian cardiac Ca(2+) signaling, then compare them to Ca(2+) signaling phenotypes of human induced pluripotent stem cell derived cardiomyocytes (hiPSC-CM) and to those of rat neonatal cardiomyocytes, and categorize the possible variance in Ca(2+) signaling defects caused by different CPVT-inducing mutations as expressed in hiPSC-CMs.

Novel Analysis Software for Detecting and Classifying Ca2+ Transient Abnormalities in Stem Cell-Derived Cardiomyocytes

Comprehensive functioning of Ca2+ cycling is crucial for excitation-contraction coupling of cardiomyocytes (CMs). Abnormal Ca2+ cycling is linked to arrhythmogenesis, which is associated with cardiac disorders and heart failure. Accordingly, we have generated spontaneously beating CMs from induced pluripotent stem cells (iPSC) derived from patients with catecholaminergic polymorphic ventricular tachycardia (CPVT), which is an inherited and severe cardiac disease. Ca2+ cycling studies have revealed substantial abnormalities in these CMs. Ca2+ transient analysis performed manually lacks accepted analysis criteria, and has both low throughput and high variability. To overcome these issues, we have developed a software tool, AnomalyExplorer based on interactive visualization, to assist in the classification of Ca2+ transient patterns detected in CMs. Here, we demonstrate the usability and capability of the software, and we also compare the analysis efficiency to manual analysis. We show that AnomalyExplorer is suitable for detecting normal and abnormal Ca2+ transients; furthermore, this method provides more defined and consistent information regarding the Ca2+ abnormality patterns and cell line specific differences when compared to manual analysis. This tool will facilitate and speed up the analysis of CM Ca2+ transients, making it both more accurate and user-independent. AnomalyExplorer can be exploited in Ca2+ cycling analysis to study basic disease pathology and the effects of different drugs.