Exercise Triggers ARVC Phenotype in Mice Expressing a Disease-Causing Mutated Version of Human Plakophilin-2

In Vivo Approaches to Understand Arrhythmogenic Cardiomyopathy: Perspectives on Animal Models

Arrhythmogenic cardiomyopathy (AC) is a hereditary cardiac disorder characterized by the gradual replacement of cardiomyocytes with fibrous and adipose tissue, leading to ventricular wall thinning, chamber dilation, arrhythmias, and sudden cardiac death. Despite advances in treatment, disease management remains challenging. Animal models, particularly mice and zebrafish, have become invaluable tools for understanding AC's pathophysiology and testing potential therapies. Mice models, although useful for scientific research, cannot fully replicate the complexity of the human AC. However, they have provided valuable insights into gene involvement, signalling pathways, and disease progression. Zebrafish offer a promising alternative to mammalian models, despite the phylogenetic distance, due to their economic and genetic advantages. By combining animal models with in vitro studies, researchers can comprehensively understand AC, paving the way for more effective treatments and interventions for patients and improving their quality of life and prognosis.

Animal Models and Molecular Pathogenesis of Arrhythmogenic Cardiomyopathy Associated with Pathogenic Variants in Intercalated Disc Genes

Arrhythmogenic cardiomyopathy (ACM) is a rare genetic cardiac disease characterized by the progressive substitution of myocardium with fibro-fatty tissue. Clinically, ACM shows wide variability among patients; symptoms can include syncope and ventricular tachycardia but also sudden death, with the latter often being its sole manifestation. Approximately half of ACM patients have been found with variations in one or more genes encoding cardiac intercalated discs proteins; the most involved genes are plakophilin 2 (PKP2), desmoglein 2 (DSG2), and desmoplakin (DSP). Cardiac intercalated discs provide mechanical and electro-metabolic coupling among cardiomyocytes. Mechanical communication is guaranteed by the interaction of proteins of desmosomes and adheren junctions in the so-called area composita, whereas electro-metabolic coupling between adjacent cardiac cells depends on gap junctions. Although ACM has been first described almost thirty years ago, the pathogenic mechanism(s) leading to its development are still only partially known. Several studies with different animal models point to the involvement of the Wnt/β-catenin signaling in combination with the Hippo pathway. Here, we present an overview about the existing murine models of ACM harboring variants in intercalated disc components with a particular focus on the underlying pathogenic mechanisms. Prospectively, mechanistic insights into the disease pathogenesis will lead to the development of effective targeted therapies for ACM.

The pig as an optimal animal model for cardiovascular research

Cardiovascular disease is a worldwide health problem and a leading cause of morbidity and mortality. Preclinical cardiovascular research using animals is needed to explore potential targets and therapeutic options. Compared with rodents, pigs have many advantages, with their anatomy, physiology, metabolism and immune system being more similar to humans. Here we present an overview of the available pig models for cardiovascular diseases, discuss their advantages over other models and propose the concept of standardized models to improve translation to the clinical setting and control research costs.

The Kir2.1E299V mutation increases atrial fibrillation vulnerability while protecting the ventricles against arrhythmias in a mouse model of short QT syndrome type 3

AIMS

Short QT syndrome type 3 (SQTS3) is a rare arrhythmogenic disease caused by gain-of-function mutations in KCNJ2, the gene coding the inward rectifier potassium channel Kir2.1. We used a multidisciplinary approach and investigated arrhythmogenic mechanisms in an in-vivo model of de-novo mutation Kir2.1E299V identified in a patient presenting an extremely abbreviated QT interval and paroxysmal atrial fibrillation.

METHODS AND RESULTS

We used intravenous adeno-associated virus-mediated gene transfer to generate mouse models, and confirmed cardiac-specific expression of Kir2.1WT or Kir2.1E299V. On ECG, the Kir2.1E299V mouse recapitulated the QT interval shortening and the atrial-specific arrhythmia of the patient. The PR interval was also significantly shorter in Kir2.1E299V mice. Patch-clamping showed extremely abbreviated action potentials in both atrial and ventricular Kir2.1E299V cardiomyocytes due to a lack of inward-going rectification and increased IK1 at voltages positive to -80 mV. Relative to Kir2.1WT, atrial Kir2.1E299V cardiomyocytes had a significantly reduced slope conductance at voltages negative to -80 mV. After confirming a higher proportion of heterotetrameric Kir2.x channels containing Kir2.2 subunits in the atria, in-silico 3D simulations predicted an atrial-specific impairment of polyamine block and reduced pore diameter in the Kir2.1E299V-Kir2.2WT channel. In ventricular cardiomyocytes, the mutation increased excitability by shifting INa activation and inactivation in the hyperpolarizing direction, which protected the ventricle against arrhythmia. Moreover, Purkinje myocytes from Kir2.1E299V mice manifested substantially higher INa density than Kir2.1WT, explaining the abbreviation in the PR interval.

CONCLUSION

The first in-vivo mouse model of cardiac-specific SQTS3 recapitulates the electrophysiological phenotype of a patient with the Kir2.1E299V mutation. Kir2.1E299V eliminates rectification in both cardiac chambers but protects against ventricular arrhythmias by increasing excitability in both Purkinje-fiber network and ventricles. Consequently, the predominant arrhythmias are supraventricular likely due to the lack of inward rectification and atrial-specific reduced pore diameter of the Kir2.1E299V-Kir2.2WT heterotetramer.

Endurance Training Provokes Arrhythmogenic Right Ventricular Cardiomyopathy Phenotype in Heterozygous Desmoglein-2 Mutants: Alleviation by Preload Reduction

Desmoglein-2 mutations are detected in 5-10% of patients with arrhythmogenic right ventricular cardiomyopathy (ARVC). Endurance training accelerates the development of the ARVC phenotype, leading to earlier arrhythmic events. Homozygous Dsg2 mutant mice develop a severe ARVC-like phenotype. The phenotype of heterozygous mutant (Dsg2mt/wt) or haploinsufficient (Dsg20/wt) mice is still not well understood. To assess the effects of age and endurance swim training, we studied cardiac morphology and function in sedentary one-year-old Dsg2mt/wt and Dsg20/wt mice and in young Dsg2mt/wt mice exposed to endurance swim training. Cardiac structure was only occasionally affected in aged Dsg20/wt and Dsg2mt/wt mice manifesting as small fibrotic foci and displacement of Connexin 43. Endurance swim training increased the right ventricular (RV) diameter and decreased RV function in Dsg2mt/wt mice but not in wild types. Dsg2mt/wt hearts showed increased ventricular activation times and pacing-induced ventricular arrhythmia without obvious fibrosis or inflammation. Preload-reducing therapy during training prevented RV enlargement and alleviated the electrophysiological phenotype. Taken together, endurance swim training induced features of ARVC in young adult Dsg2mt/wt mice. Prolonged ventricular activation times in the hearts of trained Dsg2mt/wt mice are therefore a potential mechanism for increased arrhythmia risk. Preload-reducing therapy prevented training-induced ARVC phenotype pointing to beneficial treatment options in human patients.

Extracellular Kir2.1C122Y Mutant Upsets Kir2.1-PIP2 Bonds and Is Arrhythmogenic in Andersen-Tawil Syndrome

BACKGROUND

Andersen-Tawil syndrome type 1 is a rare heritable disease caused by mutations in the gene coding the strong inwardly rectifying K+ channel Kir2.1. The extracellular Cys (cysteine)122-to-Cys154 disulfide bond in the channel structure is crucial for proper folding but has not been associated with correct channel function at the membrane. We evaluated whether a human mutation at the Cys122-to-Cys154 disulfide bridge leads to Kir2.1 channel dysfunction and arrhythmias by reorganizing the overall Kir2.1 channel structure and destabilizing its open state.

METHODS

We identified a Kir2.1 loss-of-function mutation (c.366 A>T; p.Cys122Tyr) in an ATS1 family. To investigate its pathophysiological implications, we generated an AAV9-mediated cardiac-specific mouse model expressing the Kir2.1C122Y variant. We employed a multidisciplinary approach, integrating patch clamping and intracardiac stimulation, molecular biology techniques, molecular dynamics, and bioluminescence resonance energy transfer experiments.

RESULTS

Kir2.1C122Y mice recapitulated the ECG features of ATS1 independently of sex, including corrected QT prolongation, conduction defects, and increased arrhythmia susceptibility. Isolated Kir2.1C122Y cardiomyocytes showed significantly reduced inwardly rectifier K+ (IK1) and inward Na+ (INa) current densities independently of normal trafficking. Molecular dynamics predicted that the C122Y mutation provoked a conformational change over the 2000-ns simulation, characterized by a greater loss of hydrogen bonds between Kir2.1 and phosphatidylinositol 4,5-bisphosphate than wild type (WT). Therefore, the phosphatidylinositol 4,5-bisphosphate-binding pocket was destabilized, resulting in a lower conductance state compared with WT. Accordingly, on inside-out patch clamping, the C122Y mutation significantly blunted Kir2.1 sensitivity to increasing phosphatidylinositol 4,5-bisphosphate concentrations. In addition, the Kir2.1C122Y mutation resulted in channelosome degradation, demonstrating temporal instability of both Kir2.1 and NaV1.5 proteins.

CONCLUSIONS

The extracellular Cys122-to-Cys154 disulfide bond in the tridimensional Kir2.1 channel structure is essential for the channel function. We demonstrate that breaking disulfide bonds in the extracellular domain disrupts phosphatidylinositol 4,5-bisphosphate-dependent regulation, leading to channel dysfunction and defects in Kir2.1 energetic stability. The mutation also alters functional expression of the NaV1.5 channel and ultimately leads to conduction disturbances and life-threatening arrhythmia characteristic of Andersen-Tawil syndrome type 1.

AAV-Mediated Delivery of Plakophilin-2a Arrests Progression of Arrhythmogenic Right Ventricular Cardiomyopathy in Murine Hearts: Preclinical Evidence Supporting Gene Therapy in Humans

BACKGROUND

Pathogenic variants in PKP2 (plakophilin-2) cause arrhythmogenic right ventricular cardiomyopathy, a disease characterized by life-threatening arrhythmias and progressive cardiomyopathy leading to heart failure. No effective medical therapy is available to prevent or arrest the disease. We tested the hypothesis that adeno-associated virus vector-mediated delivery of the human PKP2 gene to an adult mammalian heart deficient in PKP2 can arrest disease progression and significantly prolong survival.

METHODS

Experiments were performed using a PKP2-cKO (cardiac-specific, tamoxifen-activated PKP2 knockout murine model). The potential therapeutic, adeno-associated virus vector of serotype rh.74 (AAVrh.74)-PKP2a (PKP2 variant A; RP-A601) is a recombinant AAVrh.74 gene therapy viral vector encoding the human PKP2 variant A. AAVrh.74-PKP2a was delivered to adult mice by a single tail vein injection either before or after tamoxifen-activated PKP2-cKO. PKP2 expression was confirmed by molecular and histopathologic analyses. Cardiac function and disease progression were monitored by survival analyses, echocardiography, and electrocardiography.

RESULTS

Consistent with prior findings, loss of PKP2 expression caused 100% mortality within 50 days after tamoxifen injection. In contrast, AAVrh.74-PKP2a-mediated PKP2a expression resulted in 100% survival for >5 months (at study termination). Echocardiographic analysis revealed that AAVrh.74-PKP2a prevented right ventricle dilation, arrested left ventricle functional decline, and mitigated arrhythmia burden. Molecular and histological analyses showed AAVrh.74-PKP2a-mediated transgene mRNA and protein expression and appropriate PKP2 localization at the cardiomyocyte intercalated disc. Importantly, the therapeutic benefit was shown in mice receiving AAVrh.74-PKP2a after disease onset.

CONCLUSIONS

These preclinical data demonstrate the potential for AAVrh.74-PKP2a (RP-A601) as a therapeutic for PKP2-related arrhythmogenic right ventricular cardiomyopathy in both early and more advanced stages of the disease.

Sudden Cardiac Death in Young Athletes: JACC State-of-the-Art Review

Athletes epitomize the healthiest segment of society. Despite this premise, sudden cardiac death may occur in apparently healthy athletes, attracting significant attention not only in the medical community but also in laypersons and media. The incidence of sudden cardiac death is variably reported, and epidemiological burden differs among cohorts. Athletes appear to be at risk of developing fatal arrhythmias when harboring a quiescent cardiac disorder. Primary cardiomyopathies, ion channelopathies, and coronary artery anomalies are prevalent causes in young individuals. Cardiac assessment of athletes can be challenging because these individuals exhibit a plethora of electrical, structural, and functional physiological changes that overlap with cardiac pathology. A diagnosis of cardiac disease in a young athlete is not necessarily an indication to terminate competition and sports participation. International guidelines, traditionally focused on disqualification of individuals with cardiac disease, have recently adopted a more liberal attitude, based on a careful assessment of the risk and on a shared-decision making approach.

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.

Arrhythmogenic Cardiomyopathy: from Preclinical Models to Genotype-phenotype Correlation and Pathophysiology

Arrhythmogenic cardiomyopathy (ACM) is a hereditary myocardial disease characterized by the replacement of the ventricular myocardium with fibrous fatty deposits. ACM is usually inherited in an autosomal dominant pattern with variable penetrance and expressivity, which is mainly related to ventricular tachyarrhythmia and sudden cardiac death (SCD). Importantly, significant progress has been made in determining the genetic background of ACM due to the development of new techniques for genetic analysis. The exact molecular pathomechanism of ACM, however, is not completely clear and the genotype-phenotype correlations have not been fully elucidated, which are useful to predict the prognosis and treatment of ACM patients. Different gene-targeted and transgenic animal models, human-induced pluripotent stem cell-derived cardiomyocyte (hiPSC-CM) models, and heterologous expression systems have been developed. Here, this review aims to summarize preclinical ACM models and platforms promoting our understanding of the pathogenesis of ACM and assess their value in elucidating the ACM genotype-phenotype relationship.

MYH10 activation rescues contractile defects in arrhythmogenic cardiomyopathy (ACM)

The most prevalent genetic form of inherited arrhythmogenic cardiomyopathy (ACM) is caused by mutations in desmosomal plakophilin-2 (PKP2). By studying pathogenic deletion mutations in the desmosomal protein PKP2, here we identify a general mechanism by which PKP2 delocalization restricts actomyosin network organization and cardiac sarcomeric contraction in this untreatable disease. Computational modeling of PKP2 variants reveals that the carboxy-terminal (CT) domain is required for N-terminal domain stabilization, which determines PKP2 cortical localization and function. In mutant PKP2 cells the expression of the interacting protein MYH10 rescues actomyosin disorganization. Conversely, dominant-negative MYH10 mutant expression mimics the pathogenic CT-deletion PKP2 mutant causing actin network abnormalities and right ventricle systolic dysfunction. A chemical activator of non-muscle myosins, 4-hydroxyacetophenone (4-HAP), also restores normal contractility. Our findings demonstrate that activation of MYH10 corrects the deleterious effect of PKP2 mutant over systolic cardiac contraction, with potential implications for ACM therapy.

Understanding Arrhythmogenic Cardiomyopathy: Advances through the Use of Human Pluripotent Stem Cell Models

Cardiomyopathies (CMPs) represent a significant healthcare burden and are a major cause of heart failure leading to premature death. Several CMPs are now recognized to have a strong genetic basis, including arrhythmogenic cardiomyopathy (ACM), which predisposes patients to arrhythmic episodes. Variants in one of the five genes (PKP2, JUP, DSC2, DSG2, and DSP) encoding proteins of the desmosome are known to cause a subset of ACM, which we classify as desmosome-related ACM (dACM). Phenotypically, this disease may lead to sudden cardiac death in young athletes and, during late stages, is often accompanied by myocardial fibrofatty infiltrates. While the pathogenicity of the desmosome genes has been well established through animal studies and limited supplies of primary human cells, these systems have drawbacks that limit their utility and relevance to understanding human disease. Human induced pluripotent stem cells (hiPSCs) have emerged as a powerful tool for modeling ACM in vitro that can overcome these challenges, as they represent a reproducible and scalable source of cardiomyocytes (CMs) that recapitulate patient phenotypes. In this review, we provide an overview of dACM, summarize findings in other model systems linking desmosome proteins with this disease, and provide an up-to-date summary of the work that has been conducted in hiPSC-cardiomyocyte (hiPSC-CM) models of dACM. In the context of the hiPSC-CM model system, we highlight novel findings that have contributed to our understanding of disease and enumerate the limitations, prospects, and directions for research to consider towards future progress.

Spatiotemporal cell junction assembly in human iPSC-CM models of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited cardiac disorder that causes life-threatening arrhythmias and myocardial dysfunction. Pathogenic variants in Plakophilin-2 (PKP2), a desmosome component within specialized cardiac cell junctions, cause the majority of ACM cases. However, the molecular mechanisms by which PKP2 variants induce disease phenotypes remain unclear. Here we built bioengineered platforms using genetically modified human induced pluripotent stem cell-derived cardiomyocytes to model the early spatiotemporal process of cardiomyocyte junction assembly in vitro. Heterozygosity for truncating variant PKP2R413X reduced Wnt/β-catenin signaling, impaired myofibrillogenesis, delayed mechanical coupling, and reduced calcium wave velocity in engineered tissues. These abnormalities were ameliorated by SB216763, which activated Wnt/β-catenin signaling, improved cytoskeletal organization, restored cell junction integrity in cell pairs, and improved calcium wave velocity in engineered tissues. Together, these findings highlight the therapeutic potential of modulating Wnt/β-catenin signaling in a human model of ACM.

Extracellular cysteine disulfide bond break at Cys122 disrupts PIP2-dependent Kir2.1 channel function and leads to arrhythmias in Andersen-Tawil Syndrome

Background

Andersen-Tawil Syndrome Type 1 (ATS1) is a rare heritable disease caused by mutations in the strong inwardly rectifying K+ channel Kir2.1. The extracellular Cys122-to-Cys154 disulfide bond in the Kir2.1 channel structure is crucial for proper folding, but has not been associated with correct channel function at the membrane. We tested whether a human mutation at the Cys122-to-Cys154 disulfide bridge leads to Kir2.1 channel dysfunction and arrhythmias by reorganizing the overall Kir2.1 channel structure and destabilizing the open state of the channel.

Methods and Results

We identified a Kir2.1 loss-of-function mutation in Cys122 (c.366 A>T; p.Cys122Tyr) in a family with ATS1. To study the consequences of this mutation on Kir2.1 function we generated a cardiac specific mouse model expressing the Kir2.1C122Y mutation. Kir2.1C122Y animals recapitulated the abnormal ECG features of ATS1, like QT prolongation, conduction defects, and increased arrhythmia susceptibility. Kir2.1C122Y mouse cardiomyocytes showed significantly reduced inward rectifier K+ (IK1) and inward Na+ (INa) current densities independently of normal trafficking ability and localization at the sarcolemma and the sarcoplasmic reticulum. Kir2.1C122Y formed heterotetramers with wildtype (WT) subunits. However, molecular dynamic modeling predicted that the Cys122-to-Cys154 disulfide-bond break induced by the C122Y mutation provoked a conformational change over the 2000 ns simulation, characterized by larger loss of the hydrogen bonds between Kir2.1 and phosphatidylinositol-4,5-bisphosphate (PIP2) than WT. Therefore, consistent with the inability of Kir2.1C122Y channels to bind directly to PIP2 in bioluminescence resonance energy transfer experiments, the PIP2 binding pocket was destabilized, resulting in a lower conductance state compared with WT. Accordingly, on inside-out patch-clamping the C122Y mutation significantly blunted Kir2.1 sensitivity to increasing PIP2 concentrations.

Conclusion

The extracellular Cys122-to-Cys154 disulfide bond in the tridimensional Kir2.1 channel structure is essential to channel function. We demonstrated that ATS1 mutations that break disulfide bonds in the extracellular domain disrupt PIP2-dependent regulation, leading to channel dysfunction and life-threatening arrhythmias.

Impact of stress on cardiac phenotypes in mice harboring an ankyrin-B disease variant

Encoded by ANK2, ankyrin-B (AnkB) is a multifunctional adapter protein critical for the expression and targeting of key cardiac ion channels, transporters, cytoskeletal-associated proteins, and signaling molecules. Mice deficient for AnkB expression are neonatal lethal, and mice heterozygous for AnkB expression display cardiac structural and electrical phenotypes. Human ANK2 loss-of-function variants are associated with diverse cardiac manifestations; however, human clinical 'AnkB syndrome' displays incomplete penetrance. To date, animal models for human arrhythmias have generally been knock-out or transgenic overexpression models and thus the direct impact of ANK2 variants on cardiac structure and function in vivo is not clearly defined. Here, we directly tested the relationship of a single human ANK2 disease-associated variant with cardiac phenotypes utilizing a novel in vivo animal model. At baseline, young AnkBp.E1458G+/+ mice lacked significant structural or electrical abnormalities. However, aged AnkBp.E1458G+/+ mice displayed both electrical and structural phenotypes at baseline including bradycardia and aberrant heart rate variability, structural remodeling, and fibrosis. Young and old AnkBp.E1458G+/+ mice displayed ventricular arrhythmias following acute (adrenergic) stress. In addition, young AnkBp.E1458G+/+ mice displayed structural remodeling following chronic (transverse aortic constriction) stress. Finally, AnkBp.E1458G+/+ myocytes harbored alterations in expression and/or localization of key AnkB-associated partners, consistent with the underlying disease mechanism. In summary, our findings illustrate the critical role of AnkB in in vivo cardiac function as well as the impact of single AnkB loss-of-function variants in vivo. However, our findings illustrate the contribution and in fact necessity of secondary factors (aging, adrenergic challenge, pressure-overload) to phenotype penetrance and severity.

Desmosomal protein degradation as an underlying cause of arrhythmogenic cardiomyopathy

Arrhythmogenic cardiomyopathy (ACM) is an inherited progressive cardiac disease. Many patients with ACM harbor mutations in desmosomal genes, predominantly in plakophilin-2 (PKP2). Although the genetic basis of ACM is well characterized, the underlying disease-driving mechanisms remain unresolved. Explanted hearts from patients with ACM had less PKP2 compared with healthy hearts, which correlated with reduced expression of desmosomal and adherens junction (AJ) proteins. These proteins were also disorganized in areas of fibrotic remodeling. In vitro data from human-induced pluripotent stem cell-derived cardiomyocytes and microtissues carrying the heterozygous PKP2 c.2013delC pathogenic mutation also displayed impaired contractility. Knockin mice carrying the equivalent heterozygous Pkp2 c.1755delA mutation recapitulated changes in desmosomal and AJ proteins and displayed cardiac dysfunction and fibrosis with age. Global proteomics analysis of 4-month-old heterozygous Pkp2 c.1755delA hearts indicated involvement of the ubiquitin-proteasome system (UPS) in ACM pathogenesis. Inhibition of the UPS in mutant mice increased area composita proteins and improved calcium dynamics in isolated cardiomyocytes. Additional proteomics analyses identified lysine ubiquitination sites on the desmosomal proteins, which were more ubiquitinated in mutant mice. In summary, we show that a plakophilin-2 mutation can lead to decreased desmosomal and AJ protein expression through a UPS-dependent mechanism, which preceded cardiac remodeling. These findings suggest that targeting protein degradation and improving desmosomal protein stability may be a potential therapeutic strategy for the treatment of ACM.

ELITE: rationale and design of a longitudinal elite athlete, extreme cardiovascular phenotyping, prospective cohort study

Introduction

The cardiovascular benefits of physical exercise are well-known. However, vigorous exercise has also been associated with adverse cardiac effects. To improve our understanding of cardiovascular adaptation to exercise versus maladaptation and pathology, the limits of adaptation should be firmly established using state-of-the-art diagnostic modalities. We therefore initiated the Evaluation of Lifetime participation in Intensive Top-level sports and Exercise (ELITE) cohort to investigate the longitudinal (beneficial and pathological) cardiovascular effects of intensive elite sports and exercise.

Methods and analysis

ELITE is a prospective, multicentre, longitudinal cohort study. Elite athletes, from the age of sixteen, are recruited in The Netherlands. The primary objective is to determine the association between elite sports and exercise-induced cardiac remodelling, cardiac pathology, and health benefits over time. Secondary objectives include determining and identifying genetic profiles of elite athletes, and how these are associated with cardiac indices. ELITE will collect data from consultations, electrocardiography, echocardiography and cardiac magnetic resonance imaging, and training- and injury data. ELITE will also collect blood for biobanking and cardiogenetics. Follow-up will take place at intervals of two to five years, and after the elite athletes' professional careers have ended. In addition, a subcohort of ELITE has been established to investigate cardiac sequelae following infections associated with myocardial involvement, including SARS-CoV-2. ELITE is a prospective observational study; therefore, analyses will be primarily explorative.

Ethics and dissemination

This study has been approved by the Medical Ethics Review Board of the Amsterdam University Medical Centers (NL71682.018.19). The results of the study will be disseminated by publication in peer-reviewed journals (Netherlands Trial Register number: NL9328).

Beta-3 adrenergic receptor overexpression reverses aortic stenosis-induced heart failure and restores balanced mitochondrial dynamics

Aortic stenosis (AS) is associated with left ventricular (LV) hypertrophy and heart failure (HF). There is a lack of therapies able to prevent/revert AS-induced HF. Beta3 adrenergic receptor (β3AR) signaling is beneficial in several forms of HF. Here, we studied the potential beneficial effect of β3AR overexpression on AS-induced HF. Selective β3AR stimulation had a positive inotropic effect. Transgenic mice constitutively overexpressing human β3AR in the heart (c-hβ3tg) were protected from the development of HF in response to induced AS, and against cardiomyocyte mitochondrial dysfunction (fragmented mitochondria with remodeled cristae and metabolic reprogramming featuring altered substrate use). Similar beneficial effects were observed in wild-type mice inoculated with adeno-associated virus (AAV9) inducing cardiac-specific overexpression of human β3AR before AS induction. Moreover, AAV9-hβ3AR injection into wild-type mice at late disease stages, when cardiac hypertrophy and metabolic reprogramming are already advanced, reversed the HF phenotype and restored balanced mitochondrial dynamics, demonstrating the potential of gene-therapy-mediated β3AR overexpression in AS. Mice with cardiac specific ablation of Yme1l (cYKO), characterized by fragmented mitochondria, showed an increased mortality upon AS challenge. AAV9-hβ3AR injection in these mice before AS induction reverted the fragmented mitochondria phenotype and rescued them from death. In conclusion, our results step out that β3AR overexpression might have translational potential as a therapeutic strategy in AS-induced HF.

Adipogenic Signaling Promotes Arrhythmia Substrates before Structural Abnormalities in TMEM43 ARVC

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is a genetic disorder of desmosomal and structural proteins that is characterized by fibro-fatty infiltrate in the ventricles and fatal arrhythmia that can occur early before significant structural abnormalities. Most ARVC mutations interfere with β-catenin-dependent transcription that enhances adipogenesis; however, the mechanistic pathway to arrhythmogenesis is not clear. We hypothesized that adipogenic conditions play an important role in the formation of arrhythmia substrates in ARVC. Cardiac myocyte monolayers co-cultured for 2-4 days with mesenchymal stem cells (MSC) were derived from human-induced pluripotent stem cells with the ARVC5 TMEM43 p.Ser358Leu mutation. The TMEM43 mutation in myocyte co-cultures alone had no significant effect on impulse conduction velocity (CV) or APD. In contrast, when co-cultures were exposed to pro-adipogenic factors for 2-4 days, CV and APD were significantly reduced compared to controls by 49% and 31%, respectively without evidence of adipogenesis. Additionally, these arrhythmia substrates coincided with a significant reduction in IGF-1 expression in MSCs and were mitigated by IGF-1 treatment. These findings suggest that the onset of enhanced adipogenic signaling may be a mechanism of early arrhythmogenesis, which could lead to personalized treatment for arrhythmias associated with TMEM43 and other ARVC mutations.

Elucidating arrhythmogenic right ventricular cardiomyopathy with stem cells

Human stems cells have sparked many novel strategies for treating heart disease and for elucidating their underlying mechanisms. For example, arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited heart muscle disorder that is associated with fatal arrhythmias often occurring in healthy young adults. Fibro-fatty infiltrate, a clinical hallmark, progresses with the disease and can develop across both ventricles. Pathogenic variants in genes have been identified, with most being responsible for encoding cardiac desmosome proteins that reside at myocyte boundaries that are critical for cell-to-cell coupling. Despite some understanding of the molecular signaling mechanisms associated with ARVC mutations, their relationship with arrhythmogenesis is complex and not well understood for a monogenetic disorder. This review article focuses on arrhythmia mechanisms in ARVC based on clinical and animal studies and their relationship with disease causing variants. We also discuss the ways in which stem cells can be leveraged to improve our understanding of the role cardiac myocytes, nonmyocytes, metabolic signals, and inflammatory mediators play in an early onset disease such as ARVC.

Humanized Dsp ACM Mouse Model Displays Stress-Induced Cardiac Electrical and Structural Phenotypes

Arrhythmogenic cardiomyopathy (ACM) is an inherited disorder characterized by fibro-fatty infiltration with an increased propensity for ventricular arrhythmias and sudden death. Genetic variants in desmosomal genes are associated with ACM. Incomplete penetrance is a common feature in ACM families, complicating the understanding of how external stressors contribute towards disease development. To analyze the dual role of genetics and external stressors on ACM progression, we developed one of the first mouse models of ACM that recapitulates a human variant by introducing the murine equivalent of the human R451G variant into endogenous desmoplakin (DspR451G/+). Mice homozygous for this variant displayed embryonic lethality. While DspR451G/+ mice were viable with reduced expression of DSP, no presentable arrhythmogenic or structural phenotypes were identified at baseline. However, increased afterload resulted in reduced cardiac performance, increased chamber dilation, and accelerated progression to heart failure. In addition, following catecholaminergic challenge, DspR451G/+ mice displayed frequent and prolonged arrhythmic events. Finally, aberrant localization of connexin-43 was noted in the DspR451G/+ mice at baseline, becoming more apparent following cardiac stress via pressure overload. In summary, cardiovascular stress is a key trigger for unmasking both electrical and structural phenotypes in one of the first humanized ACM mouse models.

Interplay between the Chd4/NuRD Complex and the Transcription Factor Znf219 Control Cardiac Cell Identity

The sarcomere regulates striated muscle contraction. This structure is composed of several myofibril proteins, isoforms of which are encoded by genes specific to either the heart or skeletal muscle. The chromatin remodeler complex Chd4/NuRD regulates the transcriptional expression of these specific sarcomeric programs by repressing genes of the skeletal muscle sarcomere in the heart. Aberrant expression of skeletal muscle genes induced by the loss of Chd4 in the heart leads to sudden death due to defects in cardiomyocyte contraction that progress to arrhythmia and fibrosis. Identifying the transcription factors (TFs) that recruit Chd4/NuRD to repress skeletal muscle genes in the myocardium will provide important information for understanding numerous cardiac pathologies and, ultimately, pinpointing new therapeutic targets for arrhythmias and cardiomyopathies. Here, we sought to find Chd4 interactors and their function in cardiac homeostasis. We therefore describe a physical interaction between Chd4 and the TF Znf219 in cardiac tissue. Znf219 represses the skeletal-muscle sarcomeric program in cardiomyocytes in vitro and in vivo, similarly to Chd4. Aberrant expression of skeletal-muscle sarcomere proteins in mouse hearts with knocked down Znf219 translates into arrhythmias, accompanied by an increase in PR interval. These data strongly suggest that the physical and genetic interaction of Znf219 and Chd4 in the mammalian heart regulates cardiomyocyte identity and myocardial contraction.

Arrhythmogenic Cardiomyopathy: Exercise Pitfalls, Role of Connexin-43, and Moving beyond Antiarrhythmics

Arrhythmogenic Cardiomyopathy (ACM), a Mendelian disorder that can affect both left and right ventricles, is most often associated with pathogenic desmosomal variants that can lead to fibrofatty replacement of the myocardium, a pathological hallmark of this disease. Current therapies are aimed to prevent the worsening of disease phenotypes and sudden cardiac death (SCD). Despite the use of implantable cardioverter defibrillators (ICDs) there is no present therapy that would mitigate the loss in electrical signal and propagation by these fibrofatty barriers. Recent studies have shown the influence of forced vs. voluntary exercise in a variety of healthy and diseased mice; more specifically, that exercised mice show increased Connexin-43 (Cx43) expression levels. Fascinatingly, increased Cx43 expression ameliorated the abnormal electrical signal conduction in the myocardium of diseased mice. These findings point to a major translational pitfall in current therapeutics for ACM patients, who are advised to completely cease exercising and already demonstrate reduced Cx43 levels at the myocyte intercalated disc. Considering cardiac dysfunction in ACM arises from the loss of cardiomyocytes and electrical signal conduction abnormalities, an increase in Cx43 expression-promoted by low to moderate intensity exercise and/or gene therapy-could very well improve cardiac function in ACM patients.

Accuracy of the "International Criteria" for ECG screening in athletes in comparison with previous published criteria: rationale and design of a diagnostic meta-analysis

INTRODUCTION

The impact of the International Criteria for ECG interpretation in athletes has further improved the diagnostic accuracy of the 12-lead ECG use for preparticipation screening (PPS); moreover, these criteria have been evaluated in different populations of athletes and settings proving good results.

EVIDENCE ACQUISITION

We aimed to perform a comprehensive review of the use of the International Criteria for ECG interpretation in athletes, stemming from a systematic review to diagnostic meta-analysis, limiting our inclusion only to observational studies to determine the diagnostic accuracy of ECG for detecting cardiac anomalies related to sudden cardiac death in athletes.

EVIDENCE SYNTHESIS

This meta-analysis is expected to include several important studies related to PPS on different populations of athletes comparing different ECG criteria and detail important data on the diagnostic accuracy of ECG in PPS. Furthermore, we intend to highlight the advantage of using ECG in PPS.

CONCLUSIONS

The present diagnostic meta-analysis results will aid sports medicine physicians and cardiologist in adhering to the most accurate criteria for ECG evaluation in athletes and it may help to solve controversies aroused regarding the excess cost of ECG in PPS related to the amount of false positive cases.

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.

Population Prevalence of Premature Truncating Variants in Plakophilin-2 and Association With Arrhythmogenic Right Ventricular Cardiomyopathy: A UK Biobank Analysis

BACKGROUND

Truncating variants in the desmosomal gene PKP2 (PKP2tv) cause arrhythmogenic right ventricular cardiomyopathy (ARVC) yet display varied penetrance and expressivity.

METHODS

We identified individuals with PKP2tv from the UK Biobank (UKB) and determined the prevalence of an ARVC phenotype and other cardiovascular traits based on clinical and procedural data. The PKP2tv minor allelic frequency in the UKB was compared with a second cohort of probands with a clinical diagnosis of ARVC (ARVC cohort), with a figure of 1:5000 assumed for disease prevalence. In silico predictors of variant pathogenicity (combined annotation-dependent depletion and Splice AI [Illumina, Inc.]) were assessed.

RESULTS

PKP2tv were identified in 193/200 643 (0.10%) UKB participants, with 47 unique PKP2tv. Features consistent with ARVC were present in 3 (1.6%), leaving 190 with PKP2tv without manifest disease (UKB cohort; minor allelic frequency 4.73×10-4). The ARVC cohort included 487 ARVC probands with 144 distinct PKP2tv, with 25 PKP2tv common to both cohorts. The odds ratio for ARVC for the 25 common PKP2tv was 0.047 (95% CI, 0.001-0.268; P=2.43×10-6), and only favored ARVC (odds ratio >1) for a single variant, p.Arg79*. In silico variant analysis did not differentiate PKP2tv between the 2 cohorts. Atrial fibrillation was over-represented in the UKB cohort in those with PKP2tv (7.9% versus 4.3%; odds ratio, 2.11; P=0.005).

CONCLUSIONS

PKP2tv are prevalent in the population and associated with ARVC in only a small minority, necessitating a more detailed understanding of how PKP2tv cause ARVC in combination with associated genetic and environmental risk factors.

Progressive Reduction in Right Ventricular Contractile Function Due to Altered Actin Expression in an Aging Mouse Model of Arrhythmogenic Cardiomyopathy

BACKGROUND

Arrhythmogenic cardiomyopathy (ACM) is an inherited genetic disorder of desmosomal dysfunction, and plakophilin-2 (PKP2) has been reported to be the most common disease-causing gene when mutation-positive. In the early "concealed" phase, the ACM heart is at high risk of sudden cardiac death before cardiac remodeling occurs due to mistargeted ion channels and altered Ca²⁺ handling. However, the results of pathogenic PKP2 variants on myocyte contraction in ACM pathogenesis remain unknown.

METHODS

We studied the outcomes of a human truncating variant of PKP2 on myocyte contraction using a novel knock-in mouse model with insertion of thymidine in exon 5 of Pkp2, which mimics a familial case of ACM (PKP2-L404fsX5). We used serial echocardiography, electrocardiography, blood pressure measurements, histology, cardiomyocyte contraction, intracellular calcium measurements, and gene and protein expression studies.

RESULTS

Serial echocardiography of Pkp2 heterozygous (Pkp2-Het) mice revealed progressive failure of the right ventricle (RV) in animals older than three months of age. By contrast, left ventricular (LV) function remained normal. Electrocardiograms of six-month-old anesthetized Pkp2-Het mice showed normal baseline heart rates and QRS complexes. Cardiac responses to β-adrenergic agonist isoproterenol (2 mg.kg^-1) plus caffeine (120 mg.kg^-1) were also normal. However, adrenergic stimulation enhanced the susceptibility of Pkp2-Het hearts to tachyarrhythmia and sudden cardiac death. Histologic staining showed no significant fibrosis or adipocyte infiltration in the RVs and LVs of six- and twelve-month-old Pkp2-Het hearts. Contractility assessment of isolated myocytes demonstrated progressively reduced Pkp2-Het RV cardiomyocyte function consistent with RV failure measured by echocardiography. However, aging Pkp2-Het and control RV myocytes loaded with intracellular Ca²⁺ indicator Fura-2 showed comparable Ca²⁺ transients. Western blotting of Pkp2-RV homogenates revealed a 40% decrease in actin, while actin immunoprecipitation followed by a 2, 4-dinitrophenylhydrazine staining showed doubled oxidation level. This correlated with a 39% increase in troponin-I phosphorylation. In contrast, Pkp2-Het LV myocytes had normal contraction, actin expression and oxidation, and troponin-I phosphorylation. Finally, Western blotting of cardiac biopsies revealed actin expression was 40% decreased in RVs of end-stage ACM patients.

CONCLUSIONS

During the early "concealed" phase of ACM, reduced actin expression drives loss of RV myocyte contraction, contributing to progressive RV dysfunction.

Paclitaxel mitigates structural alterations and cardiac conduction system defects in a mouse model of Hutchinson-Gilford progeria syndrome

AIMS

Hutchinson-Gilford progeria syndrome (HGPS) is an ultrarare laminopathy caused by expression of progerin, a lamin A variant, also present at low levels in non-HGPS individuals. HGPS patients age and die prematurely, predominantly from cardiovascular complications. Progerin-induced cardiac repolarization defects have been described previously, although the underlying mechanisms are unknown.

METHODS AND RESULTS

We conducted studies in heart tissue from progerin-expressing LmnaG609G/G609G (G609G) mice, including microscopy, intracellular calcium dynamics, patch-clamping, in vivo magnetic resonance imaging, and electrocardiography. G609G mouse cardiomyocytes showed tubulin-cytoskeleton disorganization, t-tubular system disruption, sarcomere shortening, altered excitation-contraction coupling, and reductions in ventricular thickening and cardiac index. G609G mice exhibited severe bradycardia, and significant alterations of atrio-ventricular conduction and repolarization. Most importantly, 50% of G609G mice had altered heart rate variability, and sinoatrial block, both significant signs of premature cardiac aging. G609G cardiomyocytes had electrophysiological alterations, which resulted in an elevated action potential plateau and early afterdepolarization bursting, reflecting slower sodium current inactivation and long Ca+2 transient duration, which may also help explain the mild QT prolongation in some HGPS patients. Chronic treatment with low-dose paclitaxel ameliorated structural and functional alterations in G609G hearts.

CONCLUSIONS

Our results demonstrate that tubulin-cytoskeleton disorganization in progerin-expressing cardiomyocytes causes structural, cardiac conduction, and excitation-contraction coupling defects, all of which can be partially corrected by chronic treatment with low dose paclitaxel.

Modeling reduced contractility and impaired desmosome assembly due to plakophilin-2 deficiency using isogenic iPS cell-derived cardiomyocytes

Loss-of-function mutations in PKP2, which encodes plakophilin-2, cause arrhythmogenic cardiomyopathy (AC). Restoration of deficient molecules can serve as upstream therapy, thereby requiring a human model that recapitulates disease pathology and provides distinct readouts in phenotypic analysis for proof of concept for gene replacement therapy. Here, we generated isogenic induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) with precisely adjusted expression of plakophilin-2 from a patient with AC carrying a heterozygous frameshift PKP2 mutation. After monolayer differentiation, plakophilin-2 deficiency led to reduced contractility, disrupted intercalated disc structures, and impaired desmosome assembly in iPSC-CMs. Allele-specific fluorescent labeling of endogenous DSG2 encoding desmoglein-2 in the generated isogenic lines enabled real-time desmosome-imaging under an adjusted dose of plakophilin-2. Adeno-associated virus-mediated gene replacement of PKP2 recovered contractility and restored desmosome assembly, which was sequentially captured by desmosome-imaging in plakophilin-2-deficient iPSC-CMs. Our isogenic set of iPSC-CMs recapitulates AC pathology and provides a rapid and convenient cellular platform for therapeutic development.

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Generation of isogenic iPSC-CMs with a precise dose of plakophilin-2

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Modeling reduced contractility and impaired desmosome assembly using iPSC-CMs

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Generation of isogenic iPSC-CMs for desmosome-imaging

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Proof of concept of PKP2 replacement using isogenic plakophilin-2-deficient iPSC-CMs

The clinical utility of pediatric cardiomyopathy genetic testing: From diagnosis to a precision medicine-based approach to care

Background

Pediatric-onset cardiomyopathies are rare yet cause significant morbidity and mortality in affected children. Genetic testing has a major role in the clinical evaluation of pediatric-onset cardiomyopathies, and identification of a variant in an associated gene can be used to confirm the clinical diagnosis and exclude syndromic causes that may warrant different treatment strategies. Further, risk-predictive testing of first-degree relatives can assess who is at-risk of disease and requires continued clinical follow-up.

Aim of Review

In this review, we seek to describe the current role of genetic testing in the clinical diagnosis and management of patients and families with the five major cardiomyopathies. Further, we highlight the ongoing development of precision-based approaches to diagnosis, prognosis, and treatment.

Key Scientific Concepts of Review

Emerging application of genotype-phenotype correlations opens the door for genetics to guide a precision medicine-based approach to prognosis and potentially for therapies. Despite advances in our understanding of the genetic etiology of cardiomyopathy and increased accessibility of clinical genetic testing, not all pediatric cardiomyopathy patients have a clear genetic explanation for their disease. Expanded genomic studies are needed to understand the cause of disease in these patients, improve variant classification and genotype-driven prognostic predictions, and ultimately develop truly disease preventing treatment.

Facts and Gaps in Exercise Influence on Arrhythmogenic Cardiomyopathy: New Insights From a Meta-Analysis Approach

Arrhythmogenic cardiomyopathy (ACM) is a genetic cardiac condition characterized by fibrofatty myocardial replacement, either at the right ventricle, at the left ventricle, or with biventricular involvement. Ventricular arrhythmias and heart failure represent its main clinical features. Exercise benefits on mental and physical health are worldwide recognized. However, patients with ACM appear to be an exception. A thorough review of the literature was performed in PubMed searching for original papers with the terms “ARVC AND sports/exercise” and “sudden cardiac death AND sports/exercise.” Additional papers were then identified through other sources and incorporated to the list. All of them had to be based on animal models or clinical series. Information was structured in a regular format, although some data were not available in some papers. A total of 34 papers were selected and processed regarding sports-related sudden cardiac death, pre-clinical models of ACM and sport, and clinical series of ACM patients engaged in sports activities. Eligible papers were identified to obtain pooled data in order to build representative figures showing the global incidence of the most important causes of sudden cardiac death in sports and the global estimates of life-threatening arrhythmic events in ACM patients engaged in sports. Tables and figures illustrate their major characteristics. The scarce points of controversy were discussed in the text. Fundamental concepts were summarized in three main issues: sports may accelerate ACM phenotype with either structural and/or arrhythmic features, restriction may soften the progression, and these rules also apply to phenotype-negative mutation carriers. Additionally, remaining gaps in the current knowledge were also highlighted, namely, the applicability of those fundamental concepts to non-classical ACM phenotypes since left dominant ACM or non-plakophillin-2 genotypes were absent or very poorly represented in the available studies. Hopefully, future research endeavors will provide solid evidence about the safest exercise dose for each patient from a personalized medicine perspective, taking into account a big batch of genetic, epigenetic, and epidemiological variables, for instance, in order to assist clinicians to provide a final tailored recommendation.

p38γ and p38δ regulate postnatal cardiac metabolism through glycogen synthase 1

During the first weeks of postnatal heart development, cardiomyocytes undergo a major adaptive metabolic shift from glycolytic energy production to fatty acid oxidation. This metabolic change is contemporaneous to the up-regulation and activation of the p38γ and p38δ stress-activated protein kinases in the heart. We demonstrate that p38γ/δ contribute to the early postnatal cardiac metabolic switch through inhibitory phosphorylation of glycogen synthase 1 (GYS1) and glycogen metabolism inactivation. Premature induction of p38γ/δ activation in cardiomyocytes of newborn mice results in an early GYS1 phosphorylation and inhibition of cardiac glycogen production, triggering an early metabolic shift that induces a deficit in cardiomyocyte fuel supply, leading to whole-body metabolic deregulation and maladaptive cardiac pathogenesis. Notably, the adverse effects of forced premature cardiac p38γ/δ activation in neonate mice are prevented by maternal diet supplementation of fatty acids during pregnancy and lactation. These results suggest that diet interventions have a potential for treating human cardiac genetic diseases that affect heart metabolism.

Beneficial effects of cardiomyopathy-associated genetic variants on physical performance: a hypothesis-generating scoping review

Background Genetic variants associated with cardiomyopathies (CMPs) are prevalent in the general population. In young athletes, CMPs account for roughly a quarter of sudden cardiac death, with further unexplained clustering in specific sports. Consequently, most CMPs form a contra-indication for competitive sports. We hypothesized that genetic variants might (paradoxically) improve physical performance early in life while impairing cardiac function later in life. Methods Systematic PubMed search to investigate whether genetic variants in genes associated with CMPs could be related to beneficial performance phenotypes. Summary In a limited number of studies (n=6), 2860 individuals/ subjects with genetic variants were able to outperform those without said variants, as measured by running speed (~38 m/min in heterozygous (HET) mice, n=6 vs ~32 m/min in wild type (WT) mice, n=7, P=0.004) and distance (966±169 km HET mice vs 561±144 km WT mice, P=0.0035, n=10), elite athlete status in endurance athletes (n=1672, P=1.43*10-8), maximal oxygen uptake in elite athletes (absolute difference not provided, n=32, P=0.005), maximal oxygen uptake in unrelated individuals (n=473, P=0.0025), personal records in highly trained marathon runners (2:26:28±0:06:23 min HET, n=32 vs. 2:28:53±0:05:50 min without polymorphism, n=108, P=0.020), and peripheral muscle force contraction in patients following a cardiac rehabilitation programme (absolute values not provided, n=260). Key message Beneficial effects in genetic variants associated with CMPs could hypothetically play a role in the selection of young athletes, consequently explaining the prevalence of such genetic variants in athletes and the general population.

Sustained Elevated Blood Pressure Accelerates Atherosclerosis Development in a Preclinical Model of Disease

The continuous relationship between blood pressure (BP) and cardiovascular events makes the distinction between elevated BP and hypertension based on arbitrary cut-off values for BP. Even mild BP elevations manifesting as high-normal BP have been associated with cardiovascular risk. We hypothesize that persistent elevated BP increases atherosclerotic plaque development. To evaluate this causal link, we developed a new mouse model of elevated BP based on adeno-associated virus (AAV) gene transfer. We constructed AAV vectors to support transfer of the hRenin and hAngiotensinogen genes. A single injection of AAV-Ren/Ang (1011 total viral particles) induced sustained systolic BP increase (130 ± 20 mmHg, vs. 110 ± 15 mmHg in controls; p = 0.05). In ApoE-/- mice, AAV-induced mild BP elevation caused larger atherosclerotic lesions evaluated by histology (10-fold increase vs. normotensive controls). In this preclinical model, atheroma plaques development was attenuated by BP control with a calcium channel blocker, indicating that a small increase in BP within a physiological range has a substantial impact on plaque development in a preclinical model of atherosclerosis. These data support that non-optimal BP represents a risk for atherosclerosis development. Earlier intervention in elevated BP may prevent or delay morbidity and mortality associated with atherosclerosis.

Dynamic loading of human engineered heart tissue enhances contractile function and drives a desmosome-linked disease phenotype

The role that mechanical forces play in shaping the structure and function of the heart is critical to understanding heart formation and the etiology of disease but is challenging to study in patients. Engineered heart tissues (EHTs) incorporating human induced pluripotent stem cell (hiPSC)-derived cardiomyocytes have the potential to provide insight into these adaptive and maladaptive changes. However, most EHT systems cannot model both preload (stretch during chamber filling) and afterload (pressure the heart must work against to eject blood). Here, we have developed a new dynamic EHT (dyn-EHT) model that enables us to tune preload and have unconstrained contractile shortening of >10%. To do this, three-dimensional (3D) EHTs were integrated with an elastic polydimethylsiloxane strip providing mechanical preload and afterload in addition to enabling contractile force measurements based on strip bending. Our results demonstrated that dynamic loading improves the function of wild-type EHTs on the basis of the magnitude of the applied force, leading to improved alignment, conduction velocity, and contractility. For disease modeling, we used hiPSC-derived cardiomyocytes from a patient with arrhythmogenic cardiomyopathy due to mutations in the desmoplakin gene. We demonstrated that manifestation of this desmosome-linked disease state required dyn-EHT conditioning and that it could not be induced using 2D or standard 3D EHT approaches. Thus, a dynamic loading strategy is necessary to provoke the disease phenotype of diastolic lengthening, reduction of desmosome counts, and reduced contractility, which are related to primary end points of clinical disease, such as chamber thinning and reduced cardiac output.

Desmoplakin cardiomyopathy and arrhythmogenic right ventricular cardiomyopathy: two distinct forms of cardiomyopathy?

The confirmation of a hypothesis that desmoplakin-related (DSP) cardiomyopathy could represent a distinct clinical entity from the classical, RV-dominant, form of arrhythmogenic cardiomyopathy (ACM), most frequently caused by PKP2 mutations, would without any shadow of doubt signify a turning point in the history of this disease. The concept of gene-specific diseases underneath the umbrella diagnosis of ACM would bring fundamental changes not only in the clinical, diagnostic and therapeutic approach, but also in terms of risk stratification, pushing the scientific community towards a more patient-centred view of the disease, similarly to what has already been done in other inherited arrhythmogenic disease (e.g., Long QT Syndrome; LQTS). We provide a state-of-the-art review, starting with a brief historical framework to give the necessary context and better focus the question. Then, we proceed with a novel, genotype-tophenotype-based comparison of the most important aspects of DSP-related cardiomyopathy with the classical, RV-dominant ACM: this allows us to ascertain not only that the differences between the forms exist, but are also clinically relevant and actionable, leading to the underrecognition of the atypical, DSP-related, LV-dominant forms when applying the current diagnostic criteria. These findings will usher an exciting era, in which the scientific community will try to answer a range of questions, starting from the reasons why different desmosomal mutations cause such different phenotypes.

Cardiomyopathies: An Overview

Background: Cardiomyopathies are a heterogeneous group of pathologies characterized by structural and functional alterations of the heart. Aims: The purpose of this narrative review is to focus on the most important cardiomyopathies and their epidemiology, diagnosis, and management. Methods: Clinical trials were identified by Pubmed until 30 March 2021. The search keywords were “cardiomyopathies, sudden cardiac arrest, dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy, arrhythmogenic cardiomyopathy (ARCV), takotsubo syndrome”. Results: Hypertrophic cardiomyopathy (HCM) is the most common primary cardiomyopathy, with a prevalence of 1:500 persons. Dilated cardiomyopathy (DCM) has a prevalence of 1:2500 and is the leading indication for heart transplantation. Restrictive cardiomyopathy (RCM) is the least common of the major cardiomyopathies, representing 2% to 5% of cases. Arrhythmogenic cardiomyopathy (ARCV) is a pathology characterized by the substitution of the myocardium by fibrofatty tissue. Takotsubo cardiomyopathy is defined as an abrupt onset of left ventricular dysfunction in response to severe emotional or physiologic stress. Conclusion: In particular, it has been reported that HCM is the most important cause of sudden death on the athletic field in the United States. It is needless to say how important it is to know which changes in the heart due to physical activity are normal, and when they are pathological.

Beneficial effect of voluntary physical exercise in Plakophilin2 transgenic mice

Arrhythmogenic right ventricular cardiomyopathy is a hereditary, rare disease with an increased risk for sudden cardiac death. The disease-causing mutations are located within the desmosomal complex and the highest incidence is found in plakophilin2. However, there are other factors playing a role for the disease progression unrelated to the genotype such as inflammation or exercise. Competitive sports have been identified as risk factor, but the type and extend of physical activity as cofactor for arrhythmogenesis remains under debate. We thus studied the effect of light voluntary exercise on cardiac health in a mouse model. Mice with a heterozygous PKP2 loss-of-function mutation were given the option to exercise in a running wheel which was monitored 24 h/d. We analyzed structural and functional development in vivo by echocardiography which revealed that neither the genotype nor the exercise caused any significant structural changes. Ejection fraction and fractional shortening were not influenced by the genotype itself, but exercise did cause a drop in both parameters after 8 weeks, which returned to normal after 16 weeks of training. The electrophysiological analysis revealed that the arrhythmogenic potential was slightly higher in heterozygous animals (50% vs 18% in wt littermates) and that an additional stressor (isoprenaline) did not lead to an increase of arrhythmogenic events pre run or after 8 weeks of running but the vulnerability was increased after 16 weeks. Exercise-induced alterations in Ca handling and contractility of isolated myocytes were mostly abolished in heterozygous animals. No fibrofatty replacements or rearrangement of gap junctions could be observed. Taken together we could show that light voluntary exercise can cause a transient aggravation of the mutation-induced phenotype which is abolished after long term exercise indicating a beneficial effect of long term light exercise.

Arrhythmogenic right ventricular cardiomyopathy and sports activity: from molecular pathways in diseased hearts to new insights into the athletic heart mimicry

Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inherited disease associated with a high risk of sudden cardiac death. Among other factors, physical exercise has been clearly identified as a strong determinant of phenotypic expression of the disease, arrhythmia risk, and disease progression. Because of this, current guidelines advise that individuals with ARVC should not participate in competitive or frequent high-intensity endurance exercise. Exercise-induced electrical and morphological para-physiological remodelling (the so-called 'athlete's heart') may mimic several of the classic features of ARVC. Therefore, the current International Task Force Criteria for disease diagnosis may not perform as well in athletes. Clear adjudication between the two conditions is often a real challenge, with false positives, that may lead to unnecessary treatments, and false negatives, which may leave patients unprotected, both of which are equally inacceptable. This review aims to summarize the molecular interactions caused by physical activity in inducing cardiac structural alterations, and the impact of sports on arrhythmia occurrence and other clinical consequences in patients with ARVC, and help the physicians in setting the two conditions apart.

Role of Exercise as a Modulating Factor in Arrhythmogenic Cardiomyopathy

PURPOSE OF REVIEW

The review addresses the role of exercise in triggering ventricular arrhythmias and promoting disease progression in arrhythmogenic cardiomyopathy (AC) patients and gene-mutation carriers, the differential diagnosis between AC and athlete's heart and current recommendations on exercise activity in AC.

RECENT FINDINGS

AC is an inherited heart muscle disease caused by genetically defective cell-to-cell adhesion structures (mainly desmosomes). The pathophysiological hallmark of the disease is progressive myocyte loss and replacement by fibro-fatty tissue, which creates the substrates for ventricular arrhythmias. Animal and human studies demonstrated that intense exercise, but not moderate physical activity, may increase disease penetrance, worsen the phenotype, and favor life-threatening ventricular arrhythmias. It has been proposed that in some individuals prolonged endurance sports activity may in itself cause AC (so-called exercise-induced AC). The studies agree that intense physical activity should be avoided in patients with AC and healthy gene-mutation carriers. However, low-to-moderate intensity exercise does not appear detrimental and these patients should not be entirely deprived from the many health benefits of physical activity.

Connexins in the Heart: Regulation, Function and Involvement in Cardiac Disease

Connexins are a family of transmembrane proteins that play a key role in cardiac physiology. Gap junctional channels put into contact the cytoplasms of connected cardiomyocytes, allowing the existence of electrical coupling. However, in addition to this fundamental role, connexins are also involved in cardiomyocyte death and survival. Thus, chemical coupling through gap junctions plays a key role in the spreading of injury between connected cells. Moreover, in addition to their involvement in cell-to-cell communication, mounting evidence indicates that connexins have additional gap junction-independent functions. Opening of unopposed hemichannels, located at the lateral surface of cardiomyocytes, may compromise cell homeostasis and may be involved in ischemia/reperfusion injury. In addition, connexins located at non-canonical cell structures, including mitochondria and the nucleus, have been demonstrated to be involved in cardioprotection and in regulation of cell growth and differentiation. In this review, we will provide, first, an overview on connexin biology, including their synthesis and degradation, their regulation and their interactions. Then, we will conduct an in-depth examination of the role of connexins in cardiac pathophysiology, including new findings regarding their involvement in myocardial ischemia/reperfusion injury, cardiac fibrosis, gene transcription or signaling regulation.

RNA decay in processing bodies is indispensable for adipogenesis

The RNA decay pathway plays key regulatory roles in cell identities and differentiation processes. Although adipogenesis is transcriptionally and epigenetically regulated and has been thoroughly investigated, how RNA metabolism that contributes to the stability of phenotype-shaping transcriptomes participates in differentiation remains elusive. In this study, we investigated Ddx6, an essential component of processing bodies (PBs) that executes RNA decay and translational repression in the cytoplasm and participates in the cellular transition of reprogramming. Upon adipogenic induction, Ddx6 dynamically accumulated to form PBs with a binding partner, 4E-T, at the early phase prior to emergence of intracellular lipid droplets. In contrast, preadipocytes with Ddx6 knockout (KO) or 4E-T knockdown (KD) failed to generate PBs, resulting in significant suppression of adipogenesis. Transcription factors related to preadipocytes and negative regulators of adipogenesis that were not expressed under adipogenic stimulation were maintained in Ddx6-KO and 4E-T-KD preadipocytes under adipogenic induction. Elimination of Dlk1, a major negative regulator of adipogenesis, in 3T3L1 Ddx6-KO cells did not restore adipogenic differentiation capacity to any extent. Similar to murine cells, human primary mesenchymal stem cells, which can differentiate into adipocytes upon stimulation with adipogenic cocktails, required DDX6 to maturate into adipocytes. Therefore, RNA decay of the entire parental transcriptome, rather than removal of a strong negative regulator, could be indispensable for adipogenesis.

Tbx5 variants disrupt Nav1.5 function differently in patients diagnosed with Brugada or Long QT Syndrome

AIMS

The transcription factor Tbx5 controls cardiogenesis and drives Scn5a expression in mice. We have identified two variants in TBX5 encoding p.D111Y and p.F206L Tbx5, respectively, in two unrelated patients with structurally normal hearts diagnosed with Long QT (LQTS) and Brugada (BrS) Syndrome. Here we characterized the consequences of each variant to unravel the underlying disease mechanisms.

METHODS AND RESULTS

We combined clinical analysis with in vivo and in vitro electrophysiological and molecular techniques in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs), HL-1 cells, and cardiomyocytes from mice trans-expressing human wildtype (WT) or mutant proteins. Tbx5 increased transcription of SCN5A encoding cardiac Nav1.5 channels, while repressing CAMK2D and SPTBN4 genes encoding Ca-calmodulin kinase IIδ (CaMKIIδ) and βIV-spectrin, respectively. These effects significantly increased Na current (INa) in hiPSC-CMs and in cardiomyocytes from mice trans-expressing Tbx5. Consequently, action potential (AP) amplitudes increased and QRS interval narrowed in the mouse electrocardiogram. p.F206L Tbx5 bound to the SCN5A promoter failed to transactivate it, thus precluding the pro-transcriptional effect of WT Tbx5. Therefore, p.F206L markedly decreased INa in hiPSC-CM, HL-1 cells, and mouse cardiomyocytes. The INa decrease in p.F206L trans-expressing mice translated into QRS widening and increased flecainide sensitivity. p.D111Y Tbx5 increased SCN5A expression but failed to repress CAMK2D and SPTBN4. The increased CaMKIIδ and βIV-spectrin significantly augmented the late component of INa (INaL) which, in turn, significantly prolonged AP duration in both hiPSC-CMs and mouse cardiomyocytes. Ranolazine, a selective INaL inhibitor, eliminated the QT and QTc intervals prolongation seen in p.D111Y trans-expressing mice.

CONCLUSIONS

In addition to peak INa, Tbx5 critically regulates INaL and the duration of repolarization in human cardiomyocytes. Our original results suggest that TBX5 variants associate with and modulate the intensity of the electrical phenotype in LQTS and BrS patients.

Mechanotransduction and Adrenergic Stimulation in Arrhythmogenic Cardiomyopathy: An Overview of in vitro and in vivo Models

Arrhythmogenic Cardiomyopathy (AC) is a rare inherited heart disease, manifesting with progressive myocardium degeneration and dysfunction, and life-threatening arrhythmic events that lead to sudden cardiac death. Despite genetic determinants, most of AC patients admitted to hospital are athletes or very physically active people, implying the existence of other disease-causing factors. It is recognized that AC phenotypes are enhanced and triggered by strenuous physical activity, while excessive mechanical stretch and load, and repetitive adrenergic stimulation are mechanisms influencing disease penetrance. Different approaches have been undertaken to recapitulate and study both mechanotransduction and adrenergic signaling in AC, including the use of in vitro cellular and tissue models, and the development of in vivo models (particularly rodents but more recently also zebrafish). However, it remains challenging to reproduce mechanical load stimuli and physical activity in laboratory experimental settings. Thus, more work to drive the innovation of advanced AC models is needed to recapitulate these subtle physiological influences. Here, we review the state-of-the-art in this field both in clinical and laboratory-based modeling scenarios. Specific attention will be focused on highlighting gaps in the knowledge and how they may be resolved by utilizing novel research methodology.

The Value of Mouse Models of Rare Diseases: A Spanish Experience

Animal models are invaluable for biomedical research, especially in the context of rare diseases, which have a very low prevalence and are often complex. Concretely mouse models provide key information on rare disease mechanisms and therapeutic strategies that cannot be obtained by using only alternative methods, and greatly contribute to accelerate the development of new therapeutic options for rare diseases. Despite this, the use of experimental animals remains controversial. The combination of respectful management, ethical laws and transparency regarding animal experimentation contributes to improve society’s opinion about biomedical research and positively impacts on research quality, which eventually also benefits patients. Here we present examples of current advances in preclinical research in rare diseases using mouse models, together with our perspective on future directions and challenges.

The p.P888L SAP97 polymorphism increases the transient outward current (Ito,f) and abbreviates the action potential duration and the QT interval

Synapse-Associated Protein 97 (SAP97) is an anchoring protein that in cardiomyocytes targets to the membrane and regulates Na⁺ and K⁺ channels. Here we compared the electrophysiological effects of native (WT) and p.P888L SAP97, a common polymorphism. Currents were recorded in cardiomyocytes from mice trans-expressing human WT or p.P888L SAP97 and in Chinese hamster ovary (CHO)-transfected cells. The duration of the action potentials and the QT interval were significantly shorter in p.P888L-SAP97 than in WT-SAP97 mice. Compared to WT, p.P888L SAP97 significantly increased the charge of the Ca-independent transient outward (I_to,f) current in cardiomyocytes and the charge crossing Kv4.3 channels in CHO cells by slowing Kv4.3 inactivation kinetics. Silencing or inhibiting Ca/calmodulin kinase II (CaMKII) abolished the p.P888L-induced Kv4.3 charge increase, which was also precluded in channels (p.S550A Kv4.3) in which the CaMKII-phosphorylation is prevented. Computational protein-protein docking predicted that p.P888L SAP97 is more likely to form a complex with CaMKII than WT. The Na⁺ current and the current generated by Kv1.5 channels increased similarly in WT-SAP97 and p.P888L-SAP97 cardiomyocytes, while the inward rectifier current increased in WT-SAP97 but not in p.P888L-SAP97 cardiomyocytes. The p.P888L SAP97 polymorphism increases the I_to,f, a CaMKII-dependent effect that may increase the risk of arrhythmias.

The arrhythmogenic right ventricular cardiomyopathy in comparison to the athletic heart

Intense exercise-induced right ventricular remodeling is a potential adaptation of cardiac function and structure. The features of the remodeling may overlap with those of a very early form of arrhythmogenic right ventricular cardiomyopathy (ARVC): at this early stage, it could be difficult to discriminate ARVC, from exercise-induced cardiac adaptation that may develop in normal individuals. The purpose of this paper is to discuss which exercise-induced remodeling may be a pathological or a physiological finding. A complete evaluation may be required to identify the pathological features of ARVC that would include potential risk of sudden cardiac death during sport or, to avoid the false diagnosis of ARVC. The most recent expert assessment of arrhythmogenic cardiomyopathy focuses on endurance athletes presenting with clinical features indistinguishable from ARVC.

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.