Mice with the R176Q cardiac ryanodine receptor mutation exhibit catecholamine-induced ventricular tachycardia and cardiomyopathy

An inherited life-threatening arrhythmia model established by screening randomly mutagenized mice

Inherited arrhythmia syndromes (IASs) can cause life-threatening arrhythmias and are responsible for a significant proportion of sudden cardiac deaths (SCDs). Despite progress in the development of devices to prevent SCDs, the precise molecular mechanisms that induce detrimental arrhythmias remain to be fully investigated, and more effective therapies are desirable. In the present study, we screened a large-scale randomly mutagenized mouse library by electrocardiography to establish a disease model of IASs and consequently found one pedigree that exhibited spontaneous ventricular arrhythmias (VAs) followed by SCD within 1 y after birth. Genetic analysis successfully revealed a missense mutation (p.I4093V) of the ryanodine receptor 2 gene to be a cause of the arrhythmia. We found an age-related increase in arrhythmia frequency accompanied by cardiomegaly and decreased ventricular contractility in the Ryr2I4093V/+ mice. Ca2+ signaling analysis and a ryanodine binding assay indicated that the mutant ryanodine receptor 2 had a gain-of-function phenotype and enhanced Ca2+ sensitivity. Using this model, we detected the significant suppression of VA following flecainide or dantrolene treatment. Collectively, we established an inherited life-threatening arrhythmia mouse model from an electrocardiogram-based screen of randomly mutagenized mice. The present IAS model may prove feasible for use in investigating the mechanisms of SCD and assessing therapies.

Long-term efficacy and safety of cardiac genome editing for catecholaminergic polymorphic ventricular tachycardia

Introduction

Heterozygous autosomal-dominant single nucleotide variants in RYR2 account for 60% of cases of catecholaminergic polymorphic ventricular tachycardia (CPVT), an inherited arrhythmia disorder associated with high mortality rates. CRISPR/Cas9-mediated genome editing is a promising therapeutic approach that can permanently cure the disease by removing the mutant RYR2 allele. However, the safety and long-term efficacy of this strategy have not been established in a relevant disease model.

Aim

The purpose of this study was to assess whether adeno-associated virus type-9 (AAV9)-mediated somatic genome editing could prevent ventricular arrhythmias by removal of the mutant allele in mice that are heterozygous for Ryr2 variant p.Arg176Gln (R176Q/+).

Methods and Results

Guide RNA and SaCas9 were delivered using AAV9 vectors injected subcutaneously in 10-day-old mice. At 6 weeks after injection, R176Q/+ mice had a 100% reduction in ventricular arrhythmias compared to controls. When aged to 12 months, injected R176Q/+ mice maintained a 100% reduction in arrhythmia induction. Deep RNA sequencing revealed the formation of insertions/deletions at the target site with minimal off-target editing on the wild-type allele. Consequently, CRISPR/SaCas9 editing resulted in a 45% reduction of total Ryr2 mRNA and a 38% reduction in RyR2 protein. Genome editing was well tolerated based on serial echocardiography, revealing unaltered cardiac function and structure up to 12 months after AAV9 injection.

Conclusion

Taken together, AAV9-mediated CRISPR/Cas9 genome editing could efficiently disrupt the mutant Ryr2 allele, preventing lethal arrhythmias while preserving normal cardiac function in the R176Q/+ mouse model of CPVT.

Are we there yet? A critical evaluation of sudden and unexpected death in epilepsy models

Although animal models have helped to elaborate meaningful hypotheses about the pathophysiology of sudden and unexpected death in epilepsy (SUDEP), specific prevention strategies are still lacking, potentially reflecting the limitations of these models and the intrinsic difficulties of investigating SUDEP. The interpretation of preclinical data and their translation to diagnostic and therapeutic developments in patients thus require a high level of confidence in their relevance to model the human situation. Preclinical models of SUDEP are heterogeneous and include rodent and nonrodent species. A critical aspect is whether the animals have isolated seizures exclusively induced by a specific trigger, such as models where seizures are elicited by electrical stimulation, pharmacological intervention, or DBA mouse strains, or whether they suffer from epilepsy with spontaneous seizures, with or without spontaneous SUDEP, either of nongenetic epilepsy etiology or from genetically based developmental and epileptic encephalopathies. All these models have advantages and potential disadvantages, but it is important to be aware of these limitations to interpret data appropriately in a translational perspective. The majority of models with spontaneous seizures are of a genetic basis, whereas SUDEP cases with a genetic basis represent only a small proportion of the total number. In almost all models, cardiorespiratory arrest occurs during the course of the seizure, contrary to that in patients observed at the time of death, potentially raising the issue of whether we are studying models of SUDEP or models of periseizure death. However, some of these limitations are impossible to avoid and can in part be dependent on specific features of SUDEP, which may be difficult to model. Several preclinical tools are available to address certain gaps in SUDEP pathophysiology, which can be used to further validate current preclinical models.

Molecular Pathways and Animal Models of Arrhythmias

Arrhythmias account for over 300,000 annual deaths in the United States, and approximately half of all deaths are associated with heart disease. Mechanisms underlying arrhythmia risk are complex; however, work in humans and animal models over the past 25 years has identified a host of molecular pathways linked with both arrhythmia substrates and triggers. This chapter will focus on select arrhythmia pathways solved by linking human clinical and genetic data with animal models.

Screening for Novel Type 2 Ryanodine Receptor Inhibitors by Endoplasmic Reticulum Ca2+ Monitoring

Type 2 ryanodine receptor (RyR2) is a Ca2+ release channel on the endoplasmic (ER)/sarcoplasmic reticulum that plays a central role in the excitation-contraction coupling in the heart. Hyperactivity of RyR2 has been linked to ventricular arrhythmias in patients with catecholaminergic polymorphic ventricular tachycardia and heart failure, where spontaneous Ca2+ release via hyperactivated RyR2 depolarizes diastolic membrane potential to induce triggered activity. In such cases, drugs that suppress RyR2 activity are expected to prevent the arrhythmias, but there is no clinically available RyR2 inhibitors at present. In this study, we searched for RyR2 inhibitors from a well-characterized compound library using a recently developed ER Ca2+-based assay, where the inhibition of RyR2 activity was detected by the increase in ER Ca2+ signals from R-CEPIA1er, a genetically encoded ER Ca2+ indicator, in RyR2-expressing HEK293 cells. By screening 1535 compounds in the library, we identified three compounds (chloroxylenol, methyl orsellinate, and riluzole) that greatly increased the ER Ca2+ signal. All of the three compounds suppressed spontaneous Ca2+ oscillations in RyR2-expressing HEK293 cells and correspondingly reduced the Ca2+-dependent [3H]ryanodine binding activity. In cardiomyocytes from RyR2-mutant mice, the three compounds effectively suppressed abnormal Ca2+ waves without substantial effects on the action-potential-induced Ca2+ transients. These results confirm that ER Ca2+-based screening is useful for identifying modulators of ER Ca2+ release channels and suggest that RyR2 inhibitors have potential to be developed as a new category of antiarrhythmic drugs. SIGNIFICANCE STATEMENT: We successfully identified three compounds having RyR2 inhibitory action from a well-characterized compound library using an endoplasmic reticulum Ca2+-based assay, and demonstrated that these compounds suppressed arrhythmogenic Ca2+ wave generation without substantially affecting physiological action-potential induced Ca2+ transients in cardiomyocytes. This study will facilitate the development of RyR2-specific inhibitors as a potential new class of drugs for life-threatening arrhythmias induced by hyperactivation of RyR2.

Sex and Gene Influence Arrhythmia Susceptibility in Murine Models of Calmodulinopathy

BACKGROUND

Pathogenic variants in genes encoding CaM (calmodulin) are associated with a life-threatening ventricular arrhythmia syndrome (calmodulinopathy). The in vivo consequences of CaM variants have not been studied extensively and there is incomplete understanding of the genotype-phenotype relationship for recurrent variants. We investigated effects of different factors on calmodulinopathy phenotypes using 2 mouse models with a recurrent pathogenic variant (N98S) in Calm1 or Calm2.

METHODS

Genetically engineered mice with heterozygous N98S pathogenic variants in Calm1 or Calm2 were generated. Differences between the sexes and affected genes were assessed using multiple physiological assays at the cellular and whole animal levels. Statistical significance among groups was evaluated using 1-way ANOVA or the Kruskal-Wallis test when data were not normally distributed.

RESULTS

Calm1N98S/+ (Calm1S/+) or Calm2N98S/+ (Calm2S/+) mice exhibited sinus bradycardia and were more susceptible to arrhythmias after exposure to epinephrine and caffeine. Male Calm1S/+ mice had the most severe arrhythmia phenotype with evidence of early embryonic lethality, greater susceptibility for arrhythmic events, frequent premature beats, corrected QT prolongation, and more heart rate variability after epinephrine and caffeine than females with the same genotype. Calm2 S/+ mice exhibited a less severe phenotype, with female Calm2 S/+ mice having the least severe arrhythmia susceptibility. Flecainide was not effective in preventing arrhythmias in heterozygous CaM-N98S mice. Intracellular Ca2+ transients observed in isolated ventricular cardiomyocytes from male heterozygous CaM-N98S mice had lower peak amplitudes and slower sarcoplasmic reticulum Ca2+ release following in vitro exposure to epinephrine and caffeine, which were not observed in cardiomyocytes from heterozygous female CaM-N98S mice.

CONCLUSIONS

We report heterogeneity in arrhythmia susceptibility and cardiomyocyte Ca2+ dynamics among male and female mice heterozygous for a recurrent pathogenic variant in Calm1 or Calm2, illustrating a complex calmodulinopathy phenotype in vivo. Further investigation of sex and genetic differences may help identify the molecular basis for this heterogeneity.

Arrhythmogenic mechanism of a novel ryanodine receptor mutation underlying sudden cardiac death

AIMS

The ryanodine receptor 2 (RyR2) is essential for cardiac muscle excitation-contraction coupling; dysfunctional RyR2 participates in the development of inherited arrhythmogenic cardiac disease. In this study, a novel RyR2 mutation A690E is identified from a patient with family inheritance of sudden cardiac death, and we aimed to investigate the pathogenic basis of the mutation.

METHODS AND RESULTS

We generated a mouse model that carried the A690E mutation. Mice were characterized by adrenergic-induced ventricular arrhythmias similar to clinical manifestation of the patient. Optical mapping studies revealed that isolated A690E hearts were prone to arrhythmogenesis and displayed frequency-dependence calcium transient alternans. Upon β-adrenoceptor challenge, the concordant alternans was shifted towards discordant alternans that favour triggering ectopic beats and Ca2+ re-entry; similar phenomenon was also found in the A690E cardiomyocytes. In addition, we found that A690E cardiomyocytes manifested abnormal Ca2+ release and electrophysiological disorders, including an increased sensitivity to cytosolic Ca2+, an elevated diastolic RyR2-mediated Ca2+ leak, and an imbalance between Ca2+ leak and reuptake. Structural analyses reveal that the mutation directly impacts RyR2-FK506 binding protein interaction.

CONCLUSION

In this study, we have identified a novel mutation in RyR2 that is associated with sudden cardiac death. By characterizing the function defects of mutant RyR2 in animal, whole heat, and cardiomyocytes, we demonstrated the pathogenic basis of the disease-causing mutation and provided a deeper mechanistic understanding of a life-threatening cardiac arrhythmia.

An improved reporter identifies ruxolitinib as a potent and cardioprotective CaMKII inhibitor

Ca2+/calmodulin-dependent protein kinase II (CaMKII) hyperactivity causes cardiac arrhythmias, a major source of morbidity and mortality worldwide. Despite proven benefits of CaMKII inhibition in numerous preclinical models of heart disease, translation of CaMKII antagonists into humans has been stymied by low potency, toxicity, and an enduring concern for adverse effects on cognition due to an established role of CaMKII in learning and memory. To address these challenges, we asked whether any clinically approved drugs, developed for other purposes, were potent CaMKII inhibitors. For this, we engineered an improved fluorescent reporter, CaMKAR (CaMKII activity reporter), which features superior sensitivity, kinetics, and tractability for high-throughput screening. Using this tool, we carried out a drug repurposing screen (4475 compounds in clinical use) in human cells expressing constitutively active CaMKII. This yielded five previously unrecognized CaMKII inhibitors with clinically relevant potency: ruxolitinib, baricitinib, silmitasertib, crenolanib, and abemaciclib. We found that ruxolitinib, an orally bioavailable and U.S. Food and Drug Administration-approved medication, inhibited CaMKII in cultured cardiomyocytes and in mice. Ruxolitinib abolished arrhythmogenesis in mouse and patient-derived models of CaMKII-driven arrhythmias. A 10-min pretreatment in vivo was sufficient to prevent catecholaminergic polymorphic ventricular tachycardia, a congenital source of pediatric cardiac arrest, and rescue atrial fibrillation, the most common clinical arrhythmia. At cardioprotective doses, ruxolitinib-treated mice did not show any adverse effects in established cognitive assays. Our results support further clinical investigation of ruxolitinib as a potential treatment for cardiac indications.

Genome Editing and Cardiac Arrhythmias

This article reviews progress in the field of cardiac genome editing, in particular, its potential utility in treating cardiac arrhythmias. First, we discuss genome editing methods by which DNA can be disrupted, inserted, deleted, or corrected in cardiomyocytes. Second, we provide an overview of in vivo genome editing in preclinical models of heritable and acquired arrhythmias. Third, we discuss recent advancements in cardiac gene transfer, including delivery methods, gene expression optimization, and potential adverse effects associated with therapeutic somatic genome editing. While genome editing for cardiac arrhythmias is still in its infancy, this approach holds great promise, especially for inherited arrhythmia syndromes with a defined genetic defect.

Guidelines for assessment of cardiac electrophysiology and arrhythmias in small animals

Cardiac arrhythmias are a major cause of morbidity and mortality worldwide. Although recent advances in cell-based models, including human-induced pluripotent stem cell-derived cardiomyocytes (iPSC-CM), are contributing to our understanding of electrophysiology and arrhythmia mechanisms, preclinical animal studies of cardiovascular disease remain a mainstay. Over the past several decades, animal models of cardiovascular disease have advanced our understanding of pathological remodeling, arrhythmia mechanisms, and drug effects and have led to major improvements in pacing and defibrillation therapies. There exist a variety of methodological approaches for the assessment of cardiac electrophysiology and a plethora of parameters may be assessed with each approach. This guidelines article will provide an overview of the strengths and limitations of several common techniques used to assess electrophysiology and arrhythmia mechanisms at the whole animal, whole heart, and tissue level with a focus on small animal models. We also define key electrophysiological parameters that should be assessed, along with their physiological underpinnings, and the best methods with which to assess these parameters.

Characterization of N-terminal RYR2 variants outside CPVT1 hotspot regions using patient iPSCs reveal pathogenesis and therapeutic potential

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a cardiac channelopathy causing ventricular tachycardia following adrenergic stimulation. Pathogenic variants in RYR2-encoded ryanodine receptor 2 (RYR2) cause CPVT1 and cluster into domains I–IV, with the most N-terminal domain involving residues 77–466. Patient-specific induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) were generated for RYR2-F13L, -L14P, -R15P, and -R176Q variants. Isogenic control iPSCs were generated using CRISPR-Cas9/PiggyBac. Fluo-4 Ca²⁺ imaging assessed Ca²⁺ handling with/without isoproterenol (ISO), nadolol (Nad), and flecainide (Flec) treatment. CPVT1 iPSC-CMs displayed increased Ca²⁺ sparking and Ca²⁺ transient amplitude following ISO compared with control. Combined Nad treatment/ISO stimulation reduced Ca²⁺ amplitude and sparking in variant iPSC-CMs. Molecular dynamic simulations visualized the structural role of these variants. We provide the first functional evidence that these most proximal N-terminal localizing variants alter calcium handling similar to CPVT1. These variants are located at the N-terminal domain and the central domain interface and could destabilize the RYR2 channel promoting Ca²⁺ leak-triggered arrhythmias.

•

Extreme N-terminal RyR2 variants alter calcium handling similar to classical CPVT1

•

Abnormal Ca²⁺ kinetics as well as uncontrolled Ca²⁺ release underlies CPVT1

•

In vitro arrhythmia studies with iPSCs show nadolol is an effective treatment

•

In silico 3D modeling of RYR2 revealed pathogenicity of N-terminal variants

Animal Models to Study Cardiac Arrhythmias

Cardiac arrhythmias are a significant cause of morbidity and mortality worldwide, accounting for 10% to 15% of all deaths. Although most arrhythmias are due to acquired heart disease, inherited channelopathies and cardiomyopathies disproportionately affect children and young adults. Arrhythmogenesis is complex, involving anatomic structure, ion channels and regulatory proteins, and the interplay between cells in the conduction system, cardiomyocytes, fibroblasts, and the immune system. Animal models of arrhythmia are powerful tools for studying not only molecular and cellular mechanism of arrhythmogenesis but also more complex mechanisms at the whole heart level, and for testing therapeutic interventions. This review summarizes basic and clinical arrhythmia mechanisms followed by an in-depth review of published animal models of genetic and acquired arrhythmia disorders.

Loss-of-function mutations in the co-chaperone protein BAG5 cause dilated cardiomyopathy requiring heart transplantation

Dilated cardiomyopathy (DCM) is a major cause of heart failure, characterized by ventricular dilatation and systolic dysfunction. Familial DCM is reportedly caused by mutations in more than 50 genes, requiring precise disease stratification based on genetic information. However, the underlying genetic causes of 60 to 80% of familial DCM cases remain unknown. Here, we identified that homozygous truncating mutations in the gene encoding Bcl-2-associated athanogene (BAG) co-chaperone 5 (BAG5) caused inherited DCM in five patients among four unrelated families with complete penetrance. BAG5 acts as a nucleotide exchange factor for heat shock cognate 71 kDa protein (HSC70), promoting adenosine diphosphate release and activating HSC70-mediated protein folding. Bag5 mutant knock-in mice exhibited ventricular dilatation, arrhythmogenicity, and poor prognosis under catecholamine stimulation, recapitulating the human DCM phenotype, and administration of an adeno-associated virus 9 vector carrying the wild-type BAG5 gene could fully ameliorate these DCM phenotypes. Immunocytochemical analysis revealed that BAG5 localized to junctional membrane complexes (JMCs), critical microdomains for calcium handling. Bag5-mutant mouse cardiomyocytes exhibited decreased abundance of functional JMC proteins under catecholamine stimulation, disrupted JMC structure, and calcium handling abnormalities. We also identified heterozygous truncating mutations in three patients with tachycardia-induced cardiomyopathy, a reversible DCM subtype associated with abnormal calcium homeostasis. Our study suggests that loss-of-function mutations in BAG5 can cause DCM, that BAG5 may be a target for genetic testing in cases of DCM, and that gene therapy may potentially be a treatment for this disease.

Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation

Cardiac voltage-gated ion channels (VGICs) play critical roles in mediating cardiac electrophysiological signals, such as action potentials, to maintain normal heart excitability and contraction. Inherited or acquired alterations in the structure, expression, or function of VGICs, as well as VGIC-related side effects of pharmaceutical drug delivery can result in abnormal cellular electrophysiological processes that induce life-threatening cardiac arrhythmias or even sudden cardiac death. Hence, to reduce possible heart-related risks, VGICs must be acknowledged as important targets in drug discovery and safety studies related to cardiac disease. In this review, we first summarize the development and application of electrophysiological techniques that are employed in cardiac VGIC studies alone or in combination with other techniques such as cryoelectron microscopy, optical imaging and optogenetics. Subsequently, we describe the characteristics, structure, mechanisms, and functions of various well-studied VGICs in ventricular myocytes and analyze their roles in and contributions to both physiological cardiac excitability and inherited cardiac diseases. Finally, we address the implications of the structure and function of ventricular VGICs for drug safety evaluation. In summary, multidisciplinary studies on VGICs help researchers discover potential targets of VGICs and novel VGICs in heart, enrich their knowledge of the properties and functions, determine the operation mechanisms of pathological VGICs, and introduce groundbreaking trends in drug therapy strategies, and drug safety evaluation.

Gene therapy for inherited arrhythmias

Inherited arrhythmias are disorders caused by one or more genetic mutations that increase the risk of arrhythmia, which result in life-long risk of sudden death. These mutations either primarily perturb electrophysiological homeostasis (e.g. long QT syndrome and catecholaminergic polymorphic ventricular tachycardia), cause structural disease that is closely associated with severe arrhythmias (e.g. hypertrophic cardiomyopathy), or cause a high propensity for arrhythmia in combination with altered myocardial structure and function (e.g. arrhythmogenic cardiomyopathy). Currently available therapies offer incomplete protection from arrhythmia and fail to alter disease progression. Recent studies suggest that gene therapies may provide potent, molecularly targeted options for at least a subset of inherited arrhythmias. Here, we provide an overview of gene therapy strategies, and review recent studies on gene therapies for catecholaminergic polymorphic ventricular tachycardia and hypertrophic cardiomyopathy caused by MYBPC3 mutations.

Cardiac ryanodine receptor calcium release deficiency syndrome

Cardiac ryanodine receptor (RyR2) gain-of-function mutations cause catecholaminergic polymorphic ventricular tachycardia, a condition characterized by prominent ventricular ectopy in response to catecholamine stress, which can be reproduced on exercise stress testing (EST). However, reports of sudden cardiac death (SCD) have emerged in EST-negative individuals who have loss-of-function (LOF) RyR2 mutations. The clinical relevance of RyR2 LOF mutations including their pathogenic mechanism, diagnosis, and treatment are all unknowns. Here, we performed clinical and genetic evaluations of individuals who suffered from SCD and harbored an LOF RyR2 mutation. We carried out electrophysiological studies using a programed electrical stimulation protocol consisting of a long-burst, long-pause, and short-coupled (LBLPS) ventricular extra-stimulus. Linkage analysis of RyR2 LOF mutations in six families revealed a combined logarithm of the odds ratio for linkage score of 11.479 for a condition associated with SCD with negative EST. A RyR2 LOF mouse model exhibited no catecholamine-provoked ventricular arrhythmias as in humans but did have substantial cardiac electrophysiological remodeling and an increased propensity for early afterdepolarizations. The LBLPS pacing protocol reliably induced ventricular arrhythmias in mice and humans having RyR2 LOF mutations, whose phenotype is otherwise concealed before SCD. Furthermore, treatment with quinidine and flecainide abolished LBLPS-induced ventricular arrhythmias in model mice. Thus, RyR2 LOF mutations underlie a previously unknown disease entity characterized by SCD with normal EST that we have termed RyR2 Ca²⁺ release deficiency syndrome (CRDS). Our study provides insights into the mechanism of CRDS, reports a specific CRDS diagnostic test, and identifies potentially efficacious anti-CRDS therapies.

Efficacy of RyR2 inhibitor EL20 in induced pluripotent stem cell-derived cardiomyocytes from a patient with catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited cardiac arrhythmia syndrome that often leads to sudden cardiac death. The most common form of CPVT is caused by autosomal-dominant variants in the cardiac ryanodine receptor type-2 (RYR2) gene. Mutations in RYR2 promote calcium (Ca2+ ) leak from the sarcoplasmic reticulum (SR), triggering lethal arrhythmias. Recently, it was demonstrated that tetracaine derivative EL20 specifically inhibits mutant RyR2, normalizes Ca2+ handling and suppresses arrhythmias in a CPVT mouse model. The objective of this study was to determine whether EL20 normalizes SR Ca2+ handling and arrhythmic events in induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from a CPVT patient. Blood samples from a child carrying RyR2 variant RyR2 variant Arg-176-Glu (R176Q) and a mutation-negative relative were reprogrammed into iPSCs using a Sendai virus system. iPSC-CMs were derived using the StemdiffTM kit. Confocal Ca2+ imaging was used to quantify RyR2 activity in the absence and presence of EL20. iPSC-CMs harbouring the R176Q variant demonstrated spontaneous SR Ca2+ release events, whereas administration of EL20 diminished these abnormal events at low nanomolar concentrations (IC50  = 82 nM). Importantly, treatment with EL20 did not have any adverse effects on systolic Ca2+ handling in control iPSC-CMs. Our results show for the first time that tetracaine derivative EL20 normalized SR Ca2+ handling and suppresses arrhythmogenic activity in iPSC-CMs derived from a CPVT patient. Hence, this study confirms that this RyR2-inhibitor represents a promising therapeutic candidate for treatment of CPVT.

Precision Medicine in Catecholaminergic Polymorphic Ventricular Tachycardia: JACC Focus Seminar 5/5

In this final of a 5-part Focus Seminar series on precision medicine, we focus on catecholaminergic polymorphic ventricular tachycardia (CPVT). This focus on CPVT allows us to take a "deep dive" and explore the full extent of the precision medicine opportunities for a single cardiovascular condition at a level that was not possible in the preceding articles. As a new paradigm presented in this article, it has become clear that CPVT can occur as either a typical or atypical form. Although there is a degree of overlap between the typical and atypical forms, it is notable that they arise due to different underlying genetic changes, likely exhibiting differing mechanisms of action, and presenting with different phenotypic features. The recognition of these differing forms of CPVT and their different etiologies and mechanisms is an important step toward implementing rapidly emerging precision medicine approaches that will tailor novel therapies to specific gene defects.

Identification of loss-of-function RyR2 mutations associated with idiopathic ventricular fibrillation and sudden death

Mutations in cardiac ryanodine receptor (RyR2) are linked to catecholaminergic polymorphic ventricular tachycardia (CPVT). Most CPVT RyR2 mutations characterized are gain-of-function (GOF), indicating enhanced RyR2 function as a major cause of CPVT. Loss-of-function (LOF) RyR2 mutations have also been identified and are linked to a distinct entity of cardiac arrhythmia termed RyR2 Ca²⁺ release deficiency syndrome (CRDS). Exercise stress testing (EST) is routinely used to diagnose CPVT, but it is ineffective for CRDS. There is currently no effective diagnostic tool for CRDS in humans. An alternative strategy to assess the risk for CRDS is to directly determine the functional impact of the associated RyR2 mutations. To this end, we have functionally screened 18 RyR2 mutations that are associated with idiopathic ventricular fibrillation (IVF) or sudden death. We found two additional RyR2 LOF mutations E4146K and G4935R. The E4146K mutation markedly suppressed caffeine activation of RyR2 and abolished store overload induced Ca²⁺ release (SOICR) in human embryonic kidney 293 (HEK293) cells. E4146K also severely reduced cytosolic Ca²⁺ activation and abolished luminal Ca²⁺ activation of single RyR2 channels. The G4935R mutation completely abolished caffeine activation of and [³H]ryanodine binding to RyR2. Co-expression studies showed that the G4935R mutation exerted dominant negative impact on the RyR2 wildtype (WT) channel. Interestingly, the RyR2-G4935R mutant carrier had a negative EST, and the E4146K carrier had a family history of sudden death during sleep, which are different from phenotypes of typical CPVT. Thus, our data further support the link between RyR2 LOF and a new entity of cardiac arrhythmias distinct from CPVT.

International Evidence Based Reappraisal of Genes Associated With Arrhythmogenic Right Ventricular Cardiomyopathy Using the Clinical Genome Resource Framework

BACKGROUND

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disease characterized by ventricular arrhythmias and progressive ventricular dysfunction. Genetic testing is recommended, and a pathogenic variant in an ARVC-associated gene is a major criterion for diagnosis according to the 2010 Task Force Criteria. As incorrect attribution of a gene to ARVC can contribute to misdiagnosis, we assembled an international multidisciplinary ARVC Clinical Genome Resource Gene Curation Expert Panel to reappraise all reported ARVC genes.

METHODS

Following a comprehensive literature search, six 2-member teams conducted blinded independent curation of reported ARVC genes using the semiquantitative Clinical Genome Resource framework.

RESULTS

Of 26 reported ARVC genes, only 6 (PKP2, DSP, DSG2, DSC2, JUP, and TMEM43) had strong evidence and were classified as definitive for ARVC causation. There was moderate evidence for 2 genes, DES and PLN. The remaining 18 genes had limited or no evidence. RYR2 was refuted as an ARVC gene since clinical data and model systems exhibited a catecholaminergic polymorphic ventricular tachycardia phenotype. In ClinVar, only 5 pathogenic/likely pathogenic variants (1.1%) in limited evidence genes had been reported in ARVC cases in contrast to 450 desmosome gene variants (97.4%).

CONCLUSIONS

Using the Clinical Genome Resource approach to gene-disease curation, only 8 genes (PKP2, DSP, DSG2, DSC2, JUP, TMEM43, PLN, and DES) had definitive or moderate evidence for ARVC, and these genes accounted for nearly all pathogenic/likely pathogenic ARVC variants in ClinVar. Therefore, only pathogenic/likely pathogenic variants in these 8 genes should yield a major criterion for ARVC diagnosis. Pathogenic/likely pathogenic variants identified in other genes in a patient should prompt further phenotyping as variants in many of these genes are associated with other cardiovascular conditions.

MIR448 antagomir reduces arrhythmic risk after myocardial infarction by upregulating the cardiac sodium channel

Cardiac ischemia is associated with arrhythmias; however, effective therapies are currently limited. The cardiac voltage-gated sodium channel α subunit (SCN5A), encoding the Na_v1.5 current, plays a key role in the cardiac electrical conduction and arrhythmic risk. Here, we show that hypoxia reduces Na_v1.5 through effects on a miR, miR-448. miR-448 expression is increased in ischemic cardiomyopathy. miR-448 has a conserved binding site in 3′-UTR of SCN5A. miR-448 binding to this site suppressed SCN5A expression and sodium currents. Hypoxia-induced HIF-1α and NF-κB were major transcriptional regulators for MIR448. Moreover, hypoxia relieved MIR448 transcriptional suppression by RE1 silencing transcription factor. Therefore, miR-448 inhibition reduced arrhythmic risk after myocardial infarction. Here, we show that ischemia drove miR-448 expression, reduced Na_v1.5 current, and increased arrhythmic risk. Arrhythmic risk was improved by preventing Na_v1.5 downregulation, suggesting a new approach to antiarrhythmic therapy.

Ischemic induction of miR-448 negatively regulates the cardiac sodium channel Nav1.5, and inhibiting miR-448 raises Nav1.5 and reduces arrhythmic risk after myocardial infarction in mice.

Multisite phosphorylation of the cardiac ryanodine receptor: a random or coordinated event?

Many proteins are phosphorylated at more than one phosphorylation site to achieve precise tuning of protein function and/or integrate a multitude of signals into the activity of one protein. Increasing the number of phosphorylation sites significantly broadens the complexity of molecular mechanisms involved in processing multiple phosphorylation sites by one or more distinct kinases. The cardiac ryanodine receptor (RYR2) is a well-established multiple phospho-target of kinases activated in response to β-adrenergic stimulation because this Ca²⁺ channel is a critical component of Ca²⁺ handling machinery which is responsible for β-adrenergic enhancement of cardiac contractility. Our review presents a selective overview of the extensive, often conflicting, literature which focuses on identifying reliable lines of evidence to establish if multiple RYR2 phosphorylation is achieved randomly or in a specific sequence, and whether phosphorylation at individual sites is functionally specific and additive or similar and can therefore be substituted.

State of the Art Review on Genetics and Precision Medicine in Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited cardiomyopathy characterised by ventricular arrhythmia and an increased risk of sudden cardiac death (SCD). Numerous genetic determinants and phenotypic manifestations have been discovered in ACM, posing a significant clinical challenge. Further to this, wider evaluation of family members has revealed incomplete penetrance and variable expressivity in ACM, suggesting a complex genotype-phenotype relationship. This review details the genetic basis of ACM with specific genotype-phenotype associations, providing the reader with a nuanced perspective of this condition; whilst also proposing a future roadmap to delivering precision medicine-based management in ACM.

Established and Emerging Mechanisms in the Pathogenesis of Arrhythmogenic Cardiomyopathy: A Multifaceted Disease

Arrhythmogenic cardiomyopathy (ACM) is a heritable myocardial disease that manifests with cardiac arrhythmias, syncope, sudden cardiac death, and heart failure in the advanced stages. The pathological hallmark of ACM is a gradual replacement of the myocardium by fibroadiposis, which typically starts from the epicardium. Molecular genetic studies have identified causal mutations predominantly in genes encoding for desmosomal proteins; however, non-desmosomal causal mutations have also been described, including genes coding for nuclear proteins, cytoskeleton componentsand proteins involved in excitation-contraction coupling. Despite the poor prognosis, currently available treatments can only partially control symptoms and to date there is no effective therapy for ACM. Inhibition of the canonical Wnt/β-catenin pathway and activation of the Hippo and the TGF-β pathways have been implicated in the pathogenesis of ACM. Yet, our understanding of the molecular mechanisms involved in the development of the disease and the cell source of fibroadiposis remains incomplete. Elucidation of the pathogenesis of the disease could facilitate targeted approaches for treatment. In this manuscript we will provide a comprehensive review of the proposed molecular and cellular mechanisms of the pathogenesis of ACM, including the emerging evidence on abnormal calcium homeostasis and inflammatory/autoimmune response. Moreover, we will propose novel hypothesis about the role of epicardial cells and paracrine factors in the development of the phenotype. Finally, we will discuss potential innovative therapeutic approaches based on the growing knowledge in the field.

Effects of occlusal disharmony on susceptibility to atrial fibrillation in mice

Tooth loss or incorrect positioning causes occlusal disharmony. Furthermore, tooth loss and atrial fibrillation (AF) are both risk factors for ischemic stroke and coronary heart disease. Therefore, we hypothesized that occlusal disharmony-induced stress increases susceptibility to AF, and we designed the present study to test this idea in mice. Bite-opening (BO) was done by cementing a suitable appliance onto the mandibular incisor to cause occlusal disharmony by increasing the vertical height of occlusion by 0.7 mm for a period of 2 weeks. AF susceptibility, evaluated in terms of the duration of AF induced by transesophageal burst pacing, was significantly increased concomitantly with atrial remodeling, including fibrosis, myocyte apoptosis and oxidative DNA damage, in BO mice. The BO-induced atrial remodeling was associated with increased calmodulin kinase II-mediated ryanodine receptor 2 phosphorylation on serine 2814, as well as inhibition of Akt phosphorylation. However, co-treatment with propranolol, a non-selective β-blocker, ameliorated these changes in BO mice. These data suggest that improvement of occlusal disharmony by means of orthodontic treatment might be helpful in the treatment or prevention of AF.

Structural mechanism of two gain-of-function cardiac and skeletal RyR mutations at an equivalent site by cryo-EM

Mutations in ryanodine receptors (RyRs), intracellular Ca2+ channels, are associated with deadly disorders. Despite abundant functional studies, the molecular mechanism of RyR malfunction remains elusive. We studied two single-point mutations at an equivalent site in the skeletal (RyR1 R164C) and cardiac (RyR2 R176Q) isoforms using ryanodine binding, Ca2+ imaging, and cryo-electron microscopy (cryo-EM) of the full-length protein. Loss of the positive charge had greater effect on the skeletal isoform, mediated via distortion of a salt bridge network, a molecular latch inducing rotation of a cytoplasmic domain, and partial progression to open-state traits of the large cytoplasmic assembly accompanied by alteration of the Ca2+ binding site, which concur with the major "hyperactive" feature of the mutated channel. Our cryo-EM studies demonstrated the allosteric effect of a mutation situated ~85 Å away from the pore and identified an isoform-specific structural effect.

Genetic Animal Models for Arrhythmogenic Cardiomyopathy

Arrhythmogenic cardiomyopathy has been clinically defined since the 1980s and causes right or biventricular cardiomyopathy associated with ventricular arrhythmia. Although it is a rare cardiac disease, it is responsible for a significant proportion of sudden cardiac deaths, especially in athletes. The majority of patients with arrhythmogenic cardiomyopathy carry one or more genetic variants in desmosomal genes. In the 1990s, several knockout mouse models of genes encoding for desmosomal proteins involved in cell-cell adhesion revealed for the first time embryonic lethality due to cardiac defects. Influenced by these initial discoveries in mice, arrhythmogenic cardiomyopathy received an increasing interest in human cardiovascular genetics, leading to the discovery of mutations initially in desmosomal genes and later on in more than 25 different genes. Of note, even in the clinic, routine genetic diagnostics are important for risk prediction of patients and their relatives with arrhythmogenic cardiomyopathy. Based on improvements in genetic animal engineering, different transgenic, knock-in, or cardiac-specific knockout animal models for desmosomal and nondesmosomal proteins have been generated, leading to important discoveries in this field. Here, we present an overview about the existing animal models of arrhythmogenic cardiomyopathy with a focus on the underlying pathomechanism and its importance for understanding of this disease. Prospectively, novel mechanistic insights gained from the whole animal, organ, tissue, cellular, and molecular levels will lead to the development of efficient personalized therapies for treatment of arrhythmogenic cardiomyopathy.

Identification of an amino-terminus determinant critical for ryanodine receptor/Ca2+ release channel function

AIMS

The cardiac ryanodine receptor (RyR2), which mediates intracellular Ca2+ release to trigger cardiomyocyte contraction, participates in development of acquired and inherited arrhythmogenic cardiac disease. This study was undertaken to characterize the network of inter- and intra-subunit interactions regulating the activity of the RyR2 homotetramer.

METHODS AND RESULTS

We use mutational investigations combined with biochemical assays to identify the peptide sequence bridging the β8 with β9 strand as the primary determinant mediating RyR2 N-terminus self-association. The negatively charged side chains of two aspartate residues (D179 and D180) within the β8-β9 loop are crucial for the N-terminal inter-subunit interaction. We also show that the RyR2 N-terminus domain interacts with the C-terminal channel pore region in a Ca2+-independent manner. The β8-β9 loop is required for efficient RyR2 subunit oligomerization but it is dispensable for N-terminus interaction with C-terminus. Deletion of the β8-β9 sequence produces unstable tetrameric channels with subdued intracellular Ca2+ mobilization implicating a role for this domain in channel opening. The arrhythmia-linked R176Q mutation within the β8-β9 loop decreases N-terminus tetramerization but does not affect RyR2 subunit tetramerization or the N-terminus interaction with C-terminus. RyR2R176Q is a characteristic hypersensitive channel displaying enhanced intracellular Ca2+ mobilization suggesting an additional role for the β8-β9 domain in channel closing.

CONCLUSION

These results suggest that efficient N-terminus inter-subunit communication mediated by the β8-β9 loop may constitute a primary regulatory mechanism for both RyR2 channel activation and suppression.

Calcium as a Key Player in Arrhythmogenic Cardiomyopathy: Adhesion Disorder or Intracellular Alteration?

Arrhythmogenic cardiomyopathy (ACM) is an inherited heart disease characterized by sudden death in young people and featured by fibro-adipose myocardium replacement, malignant arrhythmias, and heart failure. To date, no etiological therapies are available. Mutations in desmosomal genes cause abnormal mechanical coupling, trigger pro-apoptotic signaling pathways, and induce fibro-adipose replacement. Here, we discuss the hypothesis that the ACM causative mechanism involves a defect in the expression and/or activity of the cardiac Ca²⁺ handling machinery, focusing on the available data supporting this hypothesis. The Ca²⁺ toolkit is heavily remodeled in cardiomyocytes derived from a mouse model of ACM defective of the desmosomal protein plakophilin-2. Furthermore, ACM-related mutations were found in genes encoding for proteins involved in excitation‒contraction coupling, e.g., type 2 ryanodine receptor and phospholamban. As a consequence, the sarcoplasmic reticulum becomes more eager to release Ca²⁺, thereby inducing delayed afterdepolarizations and impairing cardiac contractility. These data are supported by preliminary observations from patient induced pluripotent stem-cell-derived cardiomyocytes. Assessing the involvement of Ca²⁺ signaling in the pathogenesis of ACM could be beneficial in the treatment of this life-threatening disease.

Somatic mutations and promotor methylation of the ryanodine receptor 2 is a common event in the pathogenesis of head and neck cancer

Genomic sequencing projects unraveled the mutational landscape of head and neck squamous cell carcinoma (HNSCC) and provided a comprehensive catalog of somatic mutations. However, the limited number of significant cancer-related genes obtained so far only partially explains the biological complexity of HNSCC and hampers the development of novel diagnostic biomarkers and therapeutic targets. We pursued a multiscale omics approach based on whole-exome sequencing, global DNA methylation and gene expression profiling data derived from tumor samples of the HIPO-HNC cohort (n = 87), and confirmed new findings with datasets from The Cancer Genome Atlas (TCGA). Promoter methylation was confirmed by MassARRAY analysis and protein expression was assessed by immunohistochemistry and immunofluorescence staining. We discovered a set of cancer-related genes with frequent somatic mutations and high frequency of promoter methylation. This included the ryanodine receptor 2 (RYR2), which showed variable promoter methylation and expression in both tumor samples and cell lines. Immunohistochemical staining of tissue sections unraveled a gradual loss of RYR2 expression from normal mucosa via dysplastic lesion to invasive cancer and indicated that reduced RYR2 expression in adjacent tissue and precancerous lesions might serve as risk factor for unfavorable prognosis and upcoming malignant conversion. In summary, our data indicate that impaired RYR2 function by either somatic mutation or epigenetic silencing is a common event in HNSCC pathogenesis. Detection of RYR2 expression and/or promoter methylation might enable risk assessment for malignant conversion of dysplastic lesions.

Identification of RyR2-PBmice and the effects of transposon insertional mutagenesis of the RyR2 gene on cardiac function in mice

Ryanodine receptor 2 (RyR2) plays an important role in maintaining the normal heart function, and mutantions can lead to arrhythmia, heart failure and other heart diseases. In this study, we successfully identified a piggyBac translocated RyR2 gene heterozygous mouse model (RyR2-PBmice) by tracking red fluorescent protein (RFP) and genotyping PCR. Cardiac function tests showed that there was no significant difference between the RyR2-PBmice and corresponding wild-type mice (WTmice), regardless of whether they were in the basal state or injected with epinephrine and caffeine. However, the sarcoplasmic reticulum Ca²⁺ content was significantly reduced in the cardiomyocytes of RyR2-PBmice as assessed by measuring caffeine-induced [Ca²⁺]_i transients; the cardiac muscle tissue of RyR2-PBmice displayed significant mitochondrial swelling and focal dissolution of mitochondrial cristae, and the tissue ATP content in the RyR2-PBmice heart was significantly reduced. To further analyze the molecular mechanism behind these changes, we tested the expression levels of related proteins using RT-PCR and Western blot analyses. The mRNA level of RyR2 in RyR2-PBmice cardiac tissue decreased significantly compared with the WTmice, and the protein expression associated with the respiratory chain was also downregulated. These results suggested that the piggyBac transposon inserted into the RyR2 gene substantively affected the structure and function of mitochondria in the mouse cardiomyocytes, leading to disorders of energy metabolism.

Unnatural verticilide enantiomer inhibits type 2 ryanodine receptor-mediated calcium leak and is antiarrhythmic

Ca²⁺ leak via ryanodine receptor type 2 (RyR2) can cause potentially fatal arrhythmias in a variety of heart diseases and has also been implicated in neurodegenerative and seizure disorders, making RyR2 an attractive therapeutic target for drug development. Here we synthesized and investigated the fungal natural product and known insect RyR antagonist (-)-verticilide and several congeners to determine their activity against mammalian RyR2. Although the cyclooligomeric depsipeptide natural product (-)-verticilide had no effect, its nonnatural enantiomer [ent-(+)-verticilide] significantly reduced RyR2-mediated spontaneous Ca²⁺ leak both in cardiomyocytes from wild-type mouse and from a gene-targeted mouse model of Ca²⁺ leak-induced arrhythmias (Casq2^-/-). ent-(+)-verticilide selectively inhibited RyR2-mediated Ca²⁺ leak and exhibited higher potency and a distinct mechanism of action compared with the pan-RyR inhibitors dantrolene and tetracaine and the antiarrhythmic drug flecainide. ent-(+)-verticilide prevented arrhythmogenic membrane depolarizations in cardiomyocytes without significant effects on the cardiac action potential and attenuated ventricular arrhythmia in catecholamine-challenged Casq2^-/- mice. These findings indicate that ent-(+)-verticilide is a potent and selective inhibitor of RyR2-mediated diastolic Ca²⁺ leak, making it a molecular tool to investigate the therapeutic potential of targeting RyR2 hyperactivity in heart and brain pathologies. The enantiomer-specific activity and straightforward chemical synthesis of (unnatural) ent-(+)-verticilide provides a compelling argument to prioritize ent-natural product synthesis. Despite their general absence in nature, the enantiomers of natural products may harbor unprecedented activity, thereby leading to new scaffolds for probe and therapeutic development.

Region-resolved proteomics profiling of monkey heart

Nonhuman primates (NHPs) play an indispensable role in biomedical research because of their similarities in genetics, physiological, and neurological function to humans. Proteomics profiling of monkey heart could reveal significant cardiac biomarkers and help us to gain a better understanding of the pathogenesis of heart disease. However, the proteomic study of monkey heart is relatively lacking. Here, we performed the proteomics profiling of the normal monkey heart by measuring three major anatomical regions (vessels, valves, and chambers) based on iTRAQ‐coupled LC‐MS/MS analysis. Over 3,200 proteins were identified and quantified from three heart tissue samples. Furthermore, multiple bioinformatics analyses such as gene ontology analysis, protein–protein interaction analysis, and gene‐diseases association were used to investigate biological network of those proteins from each area. More than 60 genes in three heart regions are implicated with heart diseases such as hypertrophic cardiomyopathy, heart failure, and myocardial infarction. These genes associated with heart disease are mainly enriched in citrate cycle, amino acid degradation, and glycolysis pathway. At the anatomical level, the revelation of molecular characteristics of the healthy monkey heart would be an important starting point to investigate heart disease. As a unique resource, this study can serve as a reference map for future in‐depth research on cardiac disease‐related NHP model and novel biomarkers of cardiac injury.

The clinical toxicology of caffeine: A review and case study

Caffeine is a widely recognized psychostimulant compound with a long history of consumption by humans. While it has received a significant amount of attention there is still much to be learned with respect to its toxicology in humans, especially in cases of overdose. A review of the history of consumption and the clinical toxicology of caffeine including clinical features, pharmacokinetics, toxicokinetics, a thorough examination of mechanism of action and management/treatment strategies are undertaken. While higher (i.e., several grams) quantities of caffeine are known to cause toxicity and potentially lethality, cases of mainly younger individuals who have experienced severe side effects and death despite consuming doses not otherwise known to cause such harm is troubling and deserves further study. An attempted case reconstruction is performed in an effort to shed light on this issue with a focus on the pharmacokinetics and pharmacodynamics of caffeine.

In Vivo Ryr2 Editing Corrects Catecholaminergic Polymorphic Ventricular Tachycardia

RATIONALE

Autosomal-dominant mutations in ryanodine receptor type 2 ( RYR2) are responsible for ≈60% of all catecholaminergic polymorphic ventricular tachycardia. Dysfunctional RyR2 subunits trigger inappropriate calcium leak from the tetrameric channel resulting in potentially lethal ventricular tachycardia. In vivo CRISPR/Cas9-mediated gene editing is a promising strategy that could be used to eliminate the disease-causing Ryr2 allele and hence rescue catecholaminergic polymorphic ventricular tachycardia.

OBJECTIVE

To determine if somatic in vivo genome editing using the CRISPR/Cas9 system delivered by adeno-associated viral (AAV) vectors could correct catecholaminergic polymorphic ventricular tachycardia arrhythmias in mice heterozygous for RyR2 mutation R176Q (R176Q/+).

METHODS AND RESULTS

Guide RNAs were designed to specifically disrupt the R176Q allele in the R176Q/+ mice using the SaCas9 ( Staphylococcus aureus Cas9) genome editing system. AAV serotype 9 was used to deliver Cas9 and guide RNA to neonatal mice by single subcutaneous injection at postnatal day 10. Strikingly, none of the R176Q/+ mice treated with AAV-CRISPR developed arrhythmias, compared with 71% of R176Q/+ mice receiving control AAV serotype 9. Total Ryr2 mRNA and protein levels were significantly reduced in R176Q/+ mice, but not in wild-type littermates. Targeted deep sequencing confirmed successful and highly specific editing of the disease-causing R176Q allele. No detectable off-target mutagenesis was observed in the wild-type Ryr2 allele or the predicted putative off-target site, confirming high specificity for SaCas9 in vivo. In addition, confocal imaging revealed that gene editing normalized the enhanced Ca2+ spark frequency observed in untreated R176Q/+ mice without affecting systolic Ca2+ transients.

CONCLUSIONS

AAV serotype 9-based delivery of the SaCas9 system can efficiently disrupt a disease-causing allele in cardiomyocytes in vivo. This work highlights the potential of somatic genome editing approaches for the treatment of lethal autosomal-dominant inherited cardiac disorders, such as catecholaminergic polymorphic ventricular tachycardia.

Mitochondrial Ca2+ Influx Contributes to Arrhythmic Risk in Nonischemic Cardiomyopathy

Background

Heart failure (HF) is associated with increased arrhythmia risk and triggered activity. Abnormal Ca²⁺ handling is thought to underlie triggered activity, and mitochondria participate in Ca²⁺ homeostasis.

Methods and Results

A model of nonischemic HF was induced in C57BL/6 mice by hypertension. Computer simulations were performed using a mouse ventricular myocyte model of HF. Isoproterenol‐induced premature ventricular contractions and ventricular fibrillation were more prevalent in nonischemic HF mice than sham controls. Isolated myopathic myocytes showed decreased cytoplasmic Ca²⁺ transients, increased mitochondrial Ca²⁺ transients, and increased action potential duration at 90% repolarization. The alteration of action potential duration at 90% repolarization was consistent with in vivo corrected QT prolongation and could be explained by augmented L‐type Ca²⁺ currents, increased Na⁺‐Ca²⁺ exchange currents, and decreased total K⁺ currents. Of myopathic ventricular myocytes, 66% showed early afterdepolarizations (EADs) compared with 17% of sham myocytes (P<0.05). Intracellular application of 1 μmol/L Ru360, a mitochondrial Ca²⁺ uniporter–specific antagonist, could reduce mitochondrial Ca²⁺ transients, decrease action potential duration at 90% repolarization, and ameliorate EADs. Furthermore, genetic knockdown of mitochondrial Ca²⁺ uniporters inhibited mitochondrial Ca²⁺ uptake, reduced Na⁺‐Ca²⁺ exchange currents, decreased action potential duration at 90% repolarization, suppressed EADs, and reduced ventricular fibrillation in nonischemic HF mice. Computer simulations showed that EADs promoted by HF remodeling could be abolished by blocking either the mitochondrial Ca²⁺ uniporter or the L‐type Ca²⁺ current, consistent with the experimental observations.

Conclusions

Mitochondrial Ca²⁺ handling plays an important role in EADs seen with nonischemic cardiomyopathy and may represent a therapeutic target to reduce arrhythmic risk in this condition.

Pathogenic mechanism of a catecholaminergic polymorphic ventricular tachycardia causing-mutation in cardiac calcium release channel RyR2

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a condition that is characterized by an abnormal heart rhythm in response to physical or emotional stress. The majority CPVT patients carry mutations in the RYR2 gene that encodes the calcium release channel/ryanodine receptor (RyR2) in cardiomyocytes. The pathogenic mechanisms that account for the clinical phenotypes of CPVT are still elusive. We have identified a de novo mutation, A165D, from a CPVT patient. We found that CPVT phenotypes are recapitulated in A165D knock-in mice. The mutant RyR2 channels enhanced sarcoplasmic reticulum Ca²⁺ release, triggered delayed afterdepolarization in cardiomyocytes. Structural analysis revealed that the A165D mutation is located in a loop that is involved in inter-subunit interactions in the RyR2 tetrameric structure, it disrupted conformational stability of the RyR2, which favored a closed-to-open state transition, resulting in a leaky channel. The loop also harbors several other CPVT mutations, which suggests a common pathogenic molecular mechanism of CPVT-causing mutations. Our data illustrated disease-relevant functional defects and provide a deeper mechanistic understanding of a life-threatening cardiac arrhythmia.

EL20, a potent antiarrhythmic compound, selectively inhibits calmodulin-deficient ryanodine receptor type 2

BACKGROUND

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an arrhythmogenic disorder caused by mutations in the cardiac ryanodine receptor RyR2 that increase diastolic calcium cation (Ca²⁺) leak from the sarcoplasmic reticulum (SR). Calmodulin (CaM) dissociation from RyR2 has been associated with diastolic Ca²⁺ leak in heart failure.

OBJECTIVE

Determine whether the tetracaine-derivative compound EL20 inhibits abnormal Ca²⁺ release from RyR2 in a CPVT model and investigate the underlying mechanism of inhibition.

METHODS

Spontaneous Ca²⁺ sparks in cardiomyocytes and inducible ventricular tachycardia were assessed in a CPVT mouse model, which is heterozygous for the R176Q mutation in RyR2 (R176Q/+ mice) in the presence of EL20 or vehicle. Single-channel studies using sheep cardiac SR or purified RyR2 reconstituted into proteoliposomes with and without exogenous CaM were used to assess mechanisms of inhibition.

RESULTS

EL20 potently inhibits abnormal Ca²⁺ release in R176Q/+ myocytes (half-maximal inhibitory concentration = 35.4 nM) and diminishes arrhythmia in R176Q/+ mice. EL20 inhibition of single-channel activity of purified RyR2 occurs in a similar range as seen in R176Q/+ myocytes (half-maximal inhibitory concentration = 8.2 nM). Inhibition of single-channel activity for cardiac SR or purified RyR2 supplemented with 100-nM or 1-μM CaM shows a 200- to 1000-fold reduction in potency.

CONCLUSION

This work provides a potential therapeutic mechanism for the development of antiarrhythmic compounds that inhibit leaky RyR2 resulting from CaM dissociation, which is often associated with failing hearts. Our data also suggest that CaM dissociation may contribute to the pathogenesis of arrhythmias with the CPVT-linked R176Q mutation.

An optogenetic arrhythmia model to study catecholaminergic polymorphic ventricular tachycardia mutations

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a condition of abnormal heart rhythm (arrhythmia), induced by physical activity or stress. Mutations in ryanodine receptor 2 (RyR2), a Ca2+ release channel located in the sarcoplasmic reticulum (SR), or calsequestrin 2 (CASQ2), a SR Ca2+ binding protein, are linked to CPVT. For specific drug development and to study distinct arrhythmias, simple models are required to implement and analyze such mutations. Here, we introduced CPVT inducing mutations into the pharynx of Caenorhabditis elegans, which we previously established as an optogenetically paced heart model. By electrophysiology and video-microscopy, we characterized mutations in csq-1 (CASQ2 homologue) and unc-68 (RyR2 homologue). csq-1 deletion impaired pharynx function and caused missed pumps during 3.7 Hz pacing. Deletion mutants of unc-68, and in particular the point mutant UNC-68(R4743C), analogous to the established human CPVT mutant RyR2(R4497C), were unable to follow 3.7 Hz pacing, with progressive defects during long stimulus trains. The pharynx either locked in pumping at half the pacing frequency or stopped pumping altogether, possibly due to UNC-68 leakiness and/or malfunctional SR Ca2+ homeostasis. Last, we could reverse this 'worm arrhythmia' by the benzothiazepine S107, establishing the nematode pharynx for studying specific CPVT mutations and for drug screening.

The role of βII spectrin in cardiac health and disease

Spectrins are large, flexible proteins comprised of α-β dimers that are connected head-to-head to form the canonical heterotetrameric spectrin structure. Spectrins were initially believed to be exclusively found in human erythrocytic membrane and are highly conserved among different species. βII spectrin, the most common isoform of non-erythrocytic spectrin, is found in all nucleated cells and forms larger macromolecular complexes with ankyrins and actins. Not only is βII spectrin a central cytoskeletal scaffolding protein involved in preserving cell structure but it has also emerged as a critical protein required for distinct physiologic functions such as posttranslational localization of crucial membrane proteins and signal transduction. In the heart, βII spectrin plays a vital role in maintaining normal cardiac membrane excitability and proper cardiac development during embryogenesis. Mutations in βII spectrin genes have been strongly linked with the development of serious cardiac disorders such as congenital arrhythmias, heart failure, and possibly sudden cardiac death. This review focuses on our current knowledge of the role βII spectrin plays in the cardiovascular system in health and disease and the potential future clinical implications.

CPVT-associated cardiac ryanodine receptor mutation G357S with reduced penetrance impairs Ca2+ release termination and diminishes protein expression

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is one of the most lethal inherited cardiac arrhythmias mostly linked to cardiac ryanodine receptor (RyR2) mutations with high disease penetrance. Interestingly, a novel RyR2 mutation G357S discovered in a large family of more than 1400 individuals has reduced penetrance. The molecular basis for the incomplete disease penetrance in this family is unknown. To gain insights into the variable disease expression in this family, we determined the impact of the G357S mutation on RyR2 function and expression. We assessed spontaneous Ca²⁺ release in HEK293 cells expressing RyR2 wildtype and the G357S mutant during store Ca²⁺ overload, also known as store overload induced Ca²⁺ release (SOICR). We found that the G357S mutation reduced the percentage of RyR2-expressing cells that showed SOICR. However, in cells that displayed SOICR, G357S reduced the thresholds for the activation and termination of SOICR. Furthermore, G357S decreased the thermal stability of the N-terminal domain of RyR2, and markedly reduced the protein expression of the full-length RyR2. On the other hand, the G357S mutation did not alter the Ca²⁺ activation of [³H]ryanodine binding or the Ca²⁺ induced release of Ca²⁺ from the intracellular stores in HEK293 cells. These data indicate that the CPVT-associated G357S mutation enhances the arrhythmogenic SOICR and reduces RyR2 protein expression, which may be attributable to the incomplete penetrance of CPVT in this family.

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.

Treatment of catecholaminergic polymorphic ventricular tachycardia in mice using novel RyR2-modifying drugs

RATIONALE

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a potentially lethal arrhythmic disorder caused by mutations in the type-2 ryanodine receptor (RyR2). Mutant RyR2 cause abnormal Ca2+ leak from the sarcoplasmic reticulum (SR), which is associated with the development of arrhythmias.

OBJECTIVE

To determine whether derivatives of tetracaine, a local anesthetic drug with known RyR2 inhibiting action, could prevent CPVT induction by suppression of RyR2-mediated SR Ca2+ leak.

METHODS AND RESULTS

Confocal microscopy was used to assess the effects of tetracaine and 9 derivatives (EL1-EL9) on spontaneous Ca2+ sparks in ventricular myocytes isolated from RyR2-R176Q/+ mice with CPVT. Whereas each derivative suppressed the Ca2+ spark frequency, derivative EL9 was most effective at the screening dose of 500nmol/L. At this high dose, the Ca2+ transient amplitude was not affected in myocytes from WT or R176Q/+ mice. The IC50 of EL9 was determined to be 13nmol/L, which is about 400× time lower than known RyR2 stabilizer K201. EL9 prevented the induction of ventricular tachycardia observed in placebo-treated R176Q/+ mice, without affecting heart rate or cardiac contractility.

CONCLUSIONS

Tetracaine derivatives represent a novel class of RyR2 stabilizing drugs that could be used for the treatment of the potentially fatal disorder catecholaminergic polymorphic ventricular tachycardia.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.

RNAseq analysis of heart tissue from mice treated with atenolol and isoproterenol reveals a reciprocal transcriptional response

Background

The transcriptional response to many widely used drugs and its modulation by genetic variability is poorly understood. Here we present an analysis of RNAseq profiles from heart tissue of 18 inbred mouse strains treated with the β-blocker atenolol (ATE) and the β-agonist isoproterenol (ISO).

Results

Differential expression analyses revealed a large set of genes responding to ISO (n = 1770 at FDR = 0.0001) and a comparatively small one responding to ATE (n = 23 at FDR = 0.0001). At a less stringent definition of differential expression, the transcriptional responses to these two antagonistic drugs are reciprocal for many genes, with an overall anti-correlation of r = −0.3. This trend is also observed at the level of most individual strains even though the power to detect differential expression is significantly reduced. The inversely expressed gene sets are enriched with genes annotated for heart-related functions. Modular analysis revealed gene sets that exhibit coherent transcription profiles across some strains and/or treatments. Correlations between these modules and a broad spectrum of cardiovascular traits are stronger than expected by chance. This provides evidence for the overall importance of transcriptional regulation for these organismal responses and explicits links between co-expressed genes and the traits they are associated with. Gene set enrichment analysis of differentially expressed groups of genes pointed to pathways related to heart development and functionality.

Conclusions

Our study provides new insights into the transcriptional response of the heart to perturbations of the β-adrenergic system, implicating several new genes that had not been associated to this system previously.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-3059-6) contains supplementary material, which is available to authorized users.

Leaky RyR2 channels unleash a brainstem spreading depolarization mechanism of sudden cardiac death

Cardiorespiratory failure is the most common cause of sudden unexplained death in epilepsy (SUDEP). Genetic autopsies have detected "leaky" gain-of-function mutations in the ryanodine receptor-2 (RyR2) gene in both SUDEP and sudden cardiac death cases linked to catecholaminergic polymorphic ventricular tachycardia that feature lethal cardiac arrhythmias without structural abnormality. Here we find that a human leaky RyR2 mutation, R176Q (RQ), alters neurotransmitter release probability in mice and significantly lowers the threshold for spreading depolarization (SD) in dorsal medulla, leading to cardiorespiratory collapse. Rare episodes of sinus bradycardia, spontaneous seizure, and sudden death were detected in RQ/+ mutant mice in vivo; however, when provoked, cortical seizures frequently led to apneas, brainstem SD, cardiorespiratory failure, and death. In vitro studies revealed that the RQ mutation selectively strengthened excitatory, but not inhibitory, synapses and facilitated SD in both the neocortex as well as brainstem dorsal medulla autonomic microcircuits. These data link defects in neuronal intracellular calcium homeostasis to the vulnerability of central autonomic brainstem pathways to hypoxic stress and implicate brainstem SD as a previously unrecognized site and mechanism contributing to premature death in individuals with leaky RYR2 mutations.

Human Induced Pluripotent Stem Cell-Derived Cardiomyocytes Afford New Opportunities in Inherited Cardiovascular Disease Modeling

Fundamental studies of molecular and cellular mechanisms of cardiovascular disease pathogenesis are required to create more effective and safer methods of their therapy. The studies can be carried out only when model systems that fully recapitulate pathological phenotype seen in patients are used. Application of laboratory animals for cardiovascular disease modeling is limited because of physiological differences with humans. Since discovery of induced pluripotency generating induced pluripotent stem cells has become a breakthrough technology in human disease modeling. In this review, we discuss a progress that has been made in modeling inherited arrhythmias and cardiomyopathies, studying molecular mechanisms of the diseases, and searching for and testing drug compounds using patient-specific induced pluripotent stem cell-derived cardiomyocytes.