MOG1: a new susceptibility gene for Brugada syndrome

Risk Stratification and Therapeutic Approach in Brugada Syndrome

Brugada syndrome (BrS) is a clinical entity characterised by an incomplete right bundle branch block associated with an ST segment elevation in the right precordial leads and a risk of ventricular arrhythmia and sudden death in the absence of structural abnormalities. Patients with a personal history of sudden death have an annual arrhythmia risk of recurrence as high as 10 %. Similarly, the presence of syncope is consistently associated with an increased arrhythmic risk. This risk can be estimated at about 1.5 % per year. The risk is lower in asymptomatic patients. Regarding the relatively high rate of complication of Implantable cardioverter defibrillator (ICD) implantation, in most of the cases, asymptomatic patients with a Brugada syndrome revealed during ajmaline challenge do not need to be implanted. The situation is more complex in patients with a spontaneous type 1 aspect since the risk could be estimated to be around 0.8 % per year. For these patients, a careful evaluation of the arrhythmic risk using all the different tools available is mandatory.

Brugada Syndrome: Clinical, Genetic, Molecular, Cellular, and Ionic Aspects

Brugada syndrome (BrS) is an inherited cardiac arrhythmia syndrome first described as a new clinical entity in 1992. Electrocardiographically characterized by distinct coved type ST segment elevation in the right-precordial leads, the syndrome is associated with a high risk for sudden cardiac death in young adults, and less frequently in infants and children. The electrocardiographic manifestations of BrS are often concealed and may be unmasked or aggravated by sodium channel blockers, a febrile state, vagotonic agents, as well as by tricyclic and tetracyclic antidepressants. An implantable cardioverter defibrillator is the most widely accepted approach to therapy. Pharmacologic therapy is designed to produce an inward shift in the balance of currents active during the early phases of the right ventricular action potential (AP) and can be used to abort electrical storms or as an adjunct or alternative to device therapy when use of an implantable cardioverter defibrillator is not possible. Isoproterenol, cilostazol, and milrinone boost calcium channel current and drugs like quinidine, bepridil, and the Chinese herb extract Wenxin Keli inhibit the transient outward current, acting to diminish the AP notch and thus to suppress the substrate and trigger for ventricular tachycardia or fibrillation. Radiofrequency ablation of the right ventricular outflow tract epicardium of patients with BrS has recently been shown to reduce arrhythmia vulnerability and the electrocardiographic manifestation of the disease, presumably by destroying the cells with more prominent AP notch. This review provides an overview of the clinical, genetic, molecular, and cellular aspects of BrS as well as the approach to therapy.

Genetics of channelopathies associated with sudden cardiac death

Recent technological advances in cardiology have resulted in new guidelines for the diagnosis, treatment and prevention of diseases. Despite these improvements, sudden death remains one of the main challenges to clinicians because the majority of diseases associated with sudden cardiac death are characterized by incomplete penetrance and variable expressivity. Hence, patients may be unaware of their illness, and physical activity can be the trigger for syncope as first symptom of the disease. Most common causes of sudden cardiac death are congenital alterations and structural heart diseases, although a significant number remain unexplained after comprehensive autopsy. In these unresolved cases, channelopathies are considered the first potential cause of death. Since all these diseases are of genetic origin, family members could be at risk, despite being asymptomatic. Genetics has also benefited from technological advances, and genetic testing has been incorporated into the sudden death field, identifying the cause in clinically affected patients, asymptomatic family members and post-mortem cases without conclusive diagnosis. This review focuses on recent advances in the genetics of channelopathies associated with sudden cardiac death.

Ion channel macromolecular complexes in cardiomyocytes: roles in sudden cardiac death

The movement of ions across specific channels embedded on the membrane of individual cardiomyocytes is crucial for the generation and propagation of the cardiac electric impulse. Emerging evidence over the past 20 years strongly suggests that the normal electric function of the heart is the result of dynamic interactions of membrane ion channels working in an orchestrated fashion as part of complex molecular networks. Such networks work together with exquisite temporal precision to generate each action potential and contraction. Macromolecular complexes play crucial roles in transcription, translation, oligomerization, trafficking, membrane retention, glycosylation, post-translational modification, turnover, function, and degradation of all cardiac ion channels known to date. In addition, the accurate timing of each cardiac beat and contraction demands, a comparable precision on the assembly and organizations of sodium, calcium, and potassium channel complexes within specific subcellular microdomains, where physical proximity allows for prompt and efficient interaction. This review article, part of the Compendium on Sudden Cardiac Death, discusses the major issues related to the role of ion channel macromolecular assemblies in normal cardiac electric function and the mechanisms of arrhythmias leading to sudden cardiac death. It provides an idea of how these issues are being addressed in the laboratory and in the clinic, which important questions remain unanswered, and what future research will be needed to improve knowledge and advance therapy.

J-wave syndromes: Brugada and early repolarization syndromes

A prominent J wave is encountered in a number of life-threatening cardiac arrhythmia syndromes, including the Brugada syndrome and early repolarization syndromes. Brugada syndrome and early repolarization syndromes differ with respect to the magnitude and lead location of abnormal J waves and are thought to represent a continuous spectrum of phenotypic expression termed J-wave syndromes. Despite two decades of intensive research, risk stratification and the approach to therapy of these 2 inherited cardiac arrhythmia syndromes are still undergoing rapid evolution. Our objective in this review is to provide an integrated synopsis of the clinical characteristics, risk stratifiers, and molecular, ionic, cellular, and genetic mechanisms underlying these 2 fascinating syndromes that have captured the interest and attention of the cardiology community in recent years.

Genetics of Brugada syndrome

The Brugada syndrome is characterized by unique 'coved-type' ST-segment elevation in the right precordial leads of electrocardiogram and ventricular fibrillation, and is responsible for 4 to 12% of sudden cardiac death in the general population. The frequency is higher in Southeast Asia including Japan compared with Western countries. Brugada syndrome is an inherited disease usually transmitted in an autosomal-dominant manner, and incomplete penetrance is frequently seen within affected families. To date, 20 genes have been associated with Brugada syndrome, but pathogenic mutations in the genes are identified in only about 30% of patients. The genetic background includes mutations in genes encoding sodium channel, calcium channels and potassium channels, as well as proteins affecting ion channels. Mutations in SCN5A, encoding the cardiac predominant sodium channel α-subunit, account for 20 to 30% of patients with Brugada syndrome and mutations in other genes only account for about 5% of patients. Furthermore, a recent genome-wide association study has identified new loci associated with the susceptibility of Brugada syndrome.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Protein assemblies of sodium and inward rectifier potassium channels control cardiac excitability and arrhythmogenesis

The understanding of how cardiac ion channels function in the normal and the diseased heart has greatly increased over the last four decades thanks to the advent of patch-clamp technology and, more recently, the emergence of genetics, as well as cellular and molecular cardiology. However, our knowledge of how these membrane-embedded proteins physically interact with each other within macromolecular complexes remains incomplete. This review focuses on how the main cardiac inward sodium channel (NaV1.5) and the strong inward rectifier potassium channel (Kir2.1) function within macromolecular complexes to control cardiac excitability. It has become increasingly clear that these two important ion channel proteins physically interact with multiple other protein partners and with each other from early stages of protein trafficking and targeting through membrane anchoring, recycling, and degradation. Recent findings include compartmentalized regulation of NaV1.5 channel expression and function through a PDZ (postsynaptic density protein, Drosophila disc large tumor suppressor, and zonula occludens-1 protein) domain-binding motif, and interaction of caveolin-3 with Kir2.1 and ankyrin-G as a molecular platform for NaV1.5 signaling. At the cardiomyocyte membrane, NaV1.5 and Kir2.1 interact through at least two distinct PDZ domain-scaffolding proteins (synapse-associated protein-97 and α1-syntrophin), thus modulating reciprocally their cell-surface expression at two different microdomains. Emerging evidence also shows that inheritable mutations in plakophilin-2, ankyrin-G, dystrophin, syntrophin, synapse-associated protein-97, and caveolin-3, among others, modify functional expression and/or localization in the cardiac cell of NaV1.5, Kir2.1 or both to give rise to arrhythmogenic diseases. Unveiling the mechanistic underpinnings of macromolecular interactions should increase our understanding of inherited and acquired arrhythmogenic cardiac diseases and may lead to advances in therapy.

Novel heterozygous mutation c.4282G>T in the SCN5A gene in a family with Brugada syndrome

Brugada syndrome (BrS) is a rare, inherited arrhythmia syndrome. The most well-known gene that is responsible for causing BrS is SCN5A, which encodes the human cardiac Na⁺ channel (Na_v1.5) α subunit. To date, it has been reported that >100 mutations in SCN5A can cause BrS. In the present study, a novel BrS-associated Na_v1.5 mutation, A1428S, was identified in a proband who was successfully resuscitated from an episode of sudden collapse during walking. This mutation was further confirmed by polymerase chain reaction (PCR)-restriction fragment length polymorphism analysis, which showed that the PCR fragment containing the mutation A1428S could be cut by the restriction enzyme Nsi1, yielding two shorter DNA fragments of 329 and 159 bp, which were not present in family members homozygous for the wild-type (WT) allele. Furthermore, the electrophysiological properties were analyzed by patch clamp technique. Current density was decreased in the A1428S mutant compared that in the WT. However, neither the steady-state activation or inactivation, nor the recovery from inactivation exhibited changes between the A1428S mutant and the WT. In conclusion, the results of this study are consistent with the hypothesis that a reduction in Na_v1.5 channel function is involved in the pathogenesis of BrS. The structural-functional study of the Na_v1.5 channel enhances the present understanding the pathophysiological function of the channel.

Diseases caused by mutations in Nav1.5 interacting proteins

Sodium current in the heart flows principally through the pore protein NaV1.5, which is part of a complex of interacting proteins that serve both to target and localize the complex in the membrane, and to modulate function by such post-translational modifications as phosphorylation and nitrosylation. Multiple mutations in seven different NaV1.5 interacting proteins have been associated with dysfunctional sodium current and inherited cardiac diseases, including long QT syndrome, Brugada syndrome, atrial fibrillation, and cardiomyopathy, as well as sudden infant death syndrome (SIDS). Mutations in as yet unidentified interacting proteins may account for cardiac disease for which a genetic basis has not yet been established. Characterizing the mechanisms by which these mutations cause disease may give insight into etiologies and treatments of more common acquired cardiac disease, such as ischemia and heart failure.

Dependence of phase-2 reentry and repolarization dispersion on epicardial and transmural ionic heterogeneity: a simulation study

AIMS

Phase-2 reentry (P2R) is a local arrhythmogenic phenomenon where electrotonic current propagates from a spike-and-dome action potential region to re-excite a loss-of-dome action potential region. While ionic heterogeneity has been shown to underlie P2R within the epicardium and has been hypothesized to occur transmurally, we are unaware of any study that has investigated the effects of combining these heterogeneities as they occur in the heart. Thus, we tested the hypothesis that P2R can result by either epicardial or transmural heterogeneity and that the realistic combination of the two would increase the likelihood of P2R.

METHODS AND RESULTS

We used computational ionic models of cardiac myocyte dynamics to investigate initiation and development of P2R in simulated tissues with different ionic heterogeneities. In one-dimensional transmural cable simulations, P2R occurred when the conductance of the transient outward current in the epicardial region was near the range for which epicardial action potentials switched intermittently between spike-and-dome and loss-of-dome morphologies. Phase-2 reentry was more likely in two-dimensional tissue simulations by both epicardial and transmural heterogeneity and could expand beyond its local initiation site to create a macroscopic reentry.

CONCLUSION

The characteristics and stability of action potential morphology in the epicardium are important determinants of the occurrence of both transmural and epicardial P2R and its associated arrhythmogenesis.

Cardiac sodium channel mutations: why so many phenotypes?

Mutations of the cardiac sodium channel (Nav1.5) can induce gain or loss of channel function. Gain-of-function mutations can cause long QT syndrome type 3 and possibly atrial fibrillation, whereas loss-of-function mutations are associated with a variety of phenotypes, such as Brugada syndrome, cardiac conduction disease, sick sinus syndrome, and possibly dilated cardiomyopathy. The phenotypes produced by Nav1.5 mutations vary according to the direct effect of the mutation on channel biophysics, but also with age, sex, body temperature, and between regions of the heart. This phenotypic variability makes genotype-phenotype correlations difficult. In this Perspectives article, we propose that phenotypic variability not ascribed to mutation-dependent changes in channel function might be the result of additional modifiers of channel behaviour, such as other genetic variation and alterations in transcription, RNA processing, translation, post-translational modifications, and protein degradation. Consideration of these modifiers might help to improve genotype-phenotype correlations and lead to new therapeutic strategies.

Myotonic dystrophy type 1 mimics and exacerbates Brugada phenotype induced by Nav1.5 sodium channel loss-of-function mutation

BACKGROUND

Myotonic dystrophy type 1 (DM1), a muscular dystrophy due to CTG expansion in the DMPK gene, can cause cardiac conduction disorders and sudden death. These cardiac manifestations are similar to those observed in loss-of-function SCN5A mutations, which are also responsible for Brugada syndrome (BrS).

OBJECTIVE

The purpose of this study was to investigate DM1 effects on clinical expression of a loss-of-function SCN5A mutation causing BrS.

METHODS

We performed complete clinical evaluation, including ajmaline test, in 1 family combining DM1 and BrS. We screened the known BrS susceptibility genes. We characterized an SCN5A mutation using whole-cell patch-clamp experiments associated with cell surface biotinylation.

RESULTS

The proband, a 15-year-old female, was a survivor of out-of-hospital cardiac arrest with ventricular fibrillation. She combined a DMPK CTG expansion from the father's side and an SCN5A mutation (S910L) from the mother's side. S910L is a trafficking defective mutant inducing a dominant negative effect when transfected with wild-type Nav1.5. This loss-of-function SCN5A mutation caused a Brugada phenotype during the mother's ajmaline test. Surprisingly, in the father, a DM1 patient without SCN5A mutation, ajmaline also unmasked a Brugada phenotype. Furthermore, association of both genetic abnormalities in the proband exacerbated the response to ajmaline with a massive conduction defect.

CONCLUSION

Our study is the first to describe the deleterious effect of DM1 on clinical expression of a loss-of-function SCN5A mutation and to show a provoked BrS phenotype in a DM1 patient. The modification of the ECG pattern by ajmaline supports the hypothesis of a link between DM1 and Nav1.5 loss of -function.

Brugada syndrome risk loci seem protective against atrial fibrillation

Several studies have shown an overlap between genes involved in the pathophysiological mechanisms of atrial fibrillation (AF) and Brugada Syndrome (BrS). We investigated whether three single-nucleotide polymorphisms (SNPs) (rs11708996; G>C located intronic to SCN5A, rs10428132; T>G located in SCN10A, and rs9388451; T>C located downstream to HEY2) at loci associated with BrS in a recent genome-wide association study (GWAS) also were associated with AF. A total of 657 patients diagnosed with AF and a control group comprising 741 individuals free of AF were included. The three SNPs were genotyped using TaqMan assays. The frequencies of risk alleles in the AF population and the control population were compared in two-by-two models. One variant, rs10428132 at SCN10A, was associated with a statistically significant decreased risk of AF (odds ratio (OR)=0.77, P=0.001). A meta-analysis was performed by enriching the control population with allele frequencies from controls in the recently published BrS GWAS (2230 alleles). In this meta-analysis, both rs10428132 at SCN10A (OR=0.73, P=5.7 × 10(-6)) and rs11708996 at SCN5A (OR=0.80, P=0.02) showed a statistically significant decreased risk of AF. When assessing the additive effect of the three loci, we found that the risk of AF decreased in a dose-responsive manner with increasing numbers of risk alleles (OR=0.50, P=0.001 for individuals carrying ≥4 risk alleles vs ≤1 allele). In conclusion, the prevalence of three risk alleles previously associated with BrS was lower in AF patients than in patients free of AF, suggesting a protective role of these loci in developing AF.

Is sudden unexplained nocturnal death syndrome in Southern China a cardiac sodium channel dysfunction disorder?

Sudden unexplained nocturnal death syndrome (SUNDS) remains an enigma to both forensic pathologists and physicians. Previous epidemiological, clinical, and pilot genetic studies have implicated that SUNDS is most likely a disease allelic to Brugada syndrome (BrS). We have performed postmortem genetic testing to address the spectrum and role of genetic abnormalities in the SCN5A-encoded cardiac sodium channel and its several associated proteins in SUNDS victims from Southern China. Genomic DNA extracted from the blood samples of 123 medico-legal autopsy-negative SUNDS cases and 104 sex-, age- and ethnic-matched controls from Southern China underwent comprehensive amino acid coding region mutational analysis for the BrS associated genes SCN5A, SCN1B, SCN2B, SCN3B, SCN4B, MOG1, and GPD1-L using PCR and direct sequencing. We identified a total of 7 unique (4 novel) putative pathogenic mutations (all in SCN5A; V95I, R121Q [2 cases], R367H, R513H, D870H, V1764D, and S1937F) in 8/123 (6.5%) SUNDS cases. Three SCN5A mutations (V95I, R121Q, and R367H) have been previously implicated in BrS. An additional 8 cases hosted rare variants of uncertain clinical significance (SCN5A: V1098L, V1202M, R1512W; SCN1B: V138I [3 cases], T189M [2 cases]; SCN3B: A195T). There were no non-synonymous mutations found in SCN2B, SCN4B, MOG1, or GPD1-L. This first comprehensive genotyping for SCN5A and related genes in the Chinese Han population with SUNDS discovered 13 mutations, 4 of them novel, in 16 cases, which suggests cardiac sodium channel dysfunction might account for the pathogenesis of 7-13% of SUNDS in Southern China.

Induced pluripotent stem cells as cardiac arrhythmic in vitro models and the impact for drug discovery

INTRODUCTION

The development of new antiarrhythmic agents is challenging and is hampered by high attrition rate of novel drug candidates. One of the reasons for this is limited predictability of existing preclinical models for drug assessment. Cardiomyocytes (CMs) derived from disease-specific induced pluripotent stem cells (iPSC) represent a novel in vitro cellular model of cardiac arrhythmias with an unprecedented potential for generating new mechanistic insight into disease pathophysiology and improving the process of drug development.

AREAS COVERED

This review outlines recent studies demonstrating the suitability and limitations of iPSC-derived CMs (iPS-CMs) for in vitro modeling inherited arrhythmias and drug testing. The authors focus on channelopathies and outline the properties of iPS-CMs, highlighting their utility and limitations for investigating the mechanism of cardiac arrhythmias and drug discovery.

EXPERT OPINION

The iPS-CMs represent a valuable addition to the already existing armamentarium of cardiac arrhythmic models. However, the superiority of iPS-CMs over other arrhythmia models has not yet been rigorously established and the limitations of the model must be overcome before its full potential for antiarrhythmic drug discovery can be realized. Nevertheless, iPS cell-based platforms hold a great potential for increasing our knowledge about cellular arrhythmia mechanisms and improving the drug discovery process.

Identifying potential functional impact of mutations and polymorphisms: linking heart failure, increased risk of arrhythmias and sudden cardiac death

Researchers and clinicians have discovered several important concepts regarding the mechanisms responsible for increased risk of arrhythmias, heart failure, and sudden cardiac death. One major step in defining the molecular basis of normal and abnormal cardiac electrical behavior has been the identification of single mutations that greatly increase the risk for arrhythmias and sudden cardiac death by changing channel-gating characteristics. Indeed, mutations in several genes encoding ion channels, such as SCN5A, which encodes the major cardiac Na⁺ channel, have emerged as the basis for a variety of inherited cardiac arrhythmias such as long QT syndrome, Brugada syndrome, progressive cardiac conduction disorder, sinus node dysfunction, or sudden infant death syndrome. In addition, genes encoding ion channel accessory proteins, like anchoring or chaperone proteins, which modify the expression, the regulation of endocytosis, and the degradation of ion channel a-subunits have also been reported as susceptibility genes for arrhythmic syndromes. The regulation of ion channel protein expression also depends on a fine-tuned balance among different other mechanisms, such as gene transcription, RNA processing, post-transcriptional control of gene expression by miRNA, protein synthesis, assembly and post-translational modification and trafficking. The aim of this review is to inventory, through the description of few representative examples, the role of these different biogenic mechanisms in arrhythmogenesis, HF and SCD in order to help the researcher to identify all the processes that could lead to arrhythmias. Identification of novel targets for drug intervention should result from further understanding of these fundamental mechanisms.

Brugada syndrome: an update

More than 20 years have passed since the description of Brugada syndrome as a clinical entity. The original case series depicted patients who all had coved ST-segment elevation in the right precordial leads, associated with a high risk of sudden death and no apparent structural heart disease. As subsequent registry data were published, it became apparent that the spectrum of risk is wide, with the majority of patients classified as low risk. Two consensus documents have been published that will continue to be updated. Despite intense research efforts, many controversies still exist over its pathophysiology and the risk stratification for sudden death. Management continues to be challenging with a lack of drug therapy and high complication rates from implantable cardioverter defibrillators. In this review, we highlight the current state-of-the-art therapies and their controversies.

Genomics of cardiac electrical function

Proper generation and conduction of the cardiac electrical impulse is essential for the continuous coordinated contraction of the heart. Dysregulation of cardiac electrical function may lead to cardiac arrhythmias, which constitute a huge medical and social burden. Identifying the genetic factors underlying cardiac electrical activity serves the double purpose of allowing the early identification of individuals at risk for arrhythmia and discovering new potential therapeutic targets for prevention. The aim of this review is to provide an overview of the genes and genetic loci linked thus far to cardiac electrical function and arrhythmia. These genes and loci have been primarily uncovered through studies on the familial rhythm disorders and through genome-wide association studies on electrocardiographic parameters in large sets of the general population. An overview of all genes and loci with their respective effect is given.

Current perspectives in genetic cardiovascular disorders: from basic to clinical aspects

We summarize recent advances in the clinical genetics of hypercholesterolemia, hypertrophic cardiomyopathy (HCM), and lethal arrhythmia, all of which are monogenic cardiovascular diseases being essential to understanding the heart and circulatory pathophysiology. Among the issues of hypercholesterolemia which play a pivotal role in development of vascular damages, familial hypercholesterolemia is the common genetic cardiovascular disease; in addition to identifying the gene mutation coding low-density lipoprotein receptor, lipid kinetics in autosomal recessive hypercholesterolemia as well as in proprotein convertase subtilisin/kexin 9 gene mutation were recently demonstrated. As for HCM, some gene mutations were identified to correlate with clinical manifestations. Additionally, a gene polymorphism of the renin-angiotensin system in development of heart failure was identified as a modifier gene. The lethal arrhythmias such as sudden death syndromes, QT prolongation, and Brugada syndrome were found to exhibit gene mutation coding potassium and/or sodium ion channels. Interestingly, functional analysis of these gene mutations helped to identify the role of each gene mutation in developing these cardiovascular disorders. We suggest considering the genetic mechanisms of cardiovascular diseases associated with hyperlipidemia, myocardial hypertrophy, or lethal arrhythmia in terms of not only clinical diagnosis but also understanding pathophysiology of each disease with therapeutic aspects.

Genetic biomarkers in Brugada syndrome

Brugada syndrome is an inherited arrhythmia syndrome predisposing to sudden cardiac death. Six years after its initial description as a clinical entity, the first mutations in SCN5A encoding the cardiac sodium channel Nav1.5 were reported. Over 300 mutations in SCN5A have since been described in addition to mutations in genes encoding Nav1.5 auxiliary units, potassium and calcium channels. This review summarizes the current knowledge on the genetics of Brugada syndrome, focusing on SCN5A, and discusses its use as a biomarker for diagnosis, prognosis and treatment.

Channelopathies from mutations in the cardiac sodium channel protein complex

The cardiac sodium current underlies excitability in heart, and inherited abnormalities of the proteins regulating and conducting this current cause inherited arrhythmia syndromes. This review focuses on inherited mutations in non-pore forming proteins of sodium channel complexes that cause cardiac arrhythmia, and the deduced mechanisms by which they affect function and dysfunction of the cardiac sodium current. Defining the structure and function of these complexes and how they are regulated will contribute to understanding the possible roles for this complex in normal and abnormal physiology and homeostasis. This article is part of a Special Issue entitled "Na(+) Regulation in Cardiac Myocytes".

The genetic component of Brugada syndrome

Brugada syndrome (BrS) is a clinical entity first described in 1992. BrS is characterized by ST-segment elevations in the right precordial leads and susceptibility to ventricular arrhythmias and sudden cardiac death. It affects young subjects, predominantly males, with structurally normal hearts. The prevalence varies with ethnicity ranging from 1:2,000 to 1:100,000 in different parts of the world. Today, hundreds of variants in 17 genes have been associated with BrS of which mutations in SCN5A, coding for the cardiac voltage-gated sodium channel, accounts for the vast majority. Despite this, approximately 70% of BrS cases cannot be explained genetically with the current knowledge. Moreover, the monogenic role of some of the variants previously described as being associated with BrS has been questioned by their occurrence in about 4% (1:23) of the general population as found in NHLBI GO Exome Sequencing Project (ESP) currently including approximately 6500 individuals. If we add the variants described in the five newest identified genes associated with BrS, they appear at an even higher prevalence in the ESP (1:21). The current standard treatment of BrS is an implantable cardioverter-defibrillator (ICD). The risk stratification and indications for ICD treatment are based on the ECG and on the clinical and family history. In this review we discuss the genetic basis of BrS.

Cardiac sodium channelopathy associated with SCN5A mutations: electrophysiological, molecular and genetic aspects

Over the last two decades, an increasing number of SCN5A mutations have been described in patients with long QT syndrome type 3 (LQT3), Brugada syndrome, (progressive) conduction disease, sick sinus syndrome, atrial standstill, atrial fibrillation, dilated cardiomyopathy, and sudden infant death syndrome (SIDS). Combined genetic, electrophysiological and molecular studies have provided insight into the dysfunction and dysregulation of the cardiac sodium channel in the setting of SCN5A mutations identified in patients with these inherited arrhythmia syndromes. However, risk stratification and patient management is hindered by the reduced penetrance and variable disease expressivity in sodium channelopathies. Furthermore, various SCN5A-related arrhythmia syndromes are known to display mixed phenotypes known as cardiac sodium channel overlap syndromes. Determinants of variable disease expressivity, including genetic background and environmental factors, are suspected but still largely unknown. Moreover, it has become increasingly clear that sodium channel function and regulation is more complicated than previously assumed, and the sodium channel may play additional, as of yet unrecognized, roles in cardiac structure and function. Development of cardiac structural abnormalities secondary to SCN5A mutations has been reported, but the clinical relevance and underlying mechanisms are unclear. Increased insight into these issues would enable a major next step in research related to cardiac sodium channel disease, ultimately enabling improved diagnosis, risk stratification and treatment strategies.

Genetic testing in heritable cardiac arrhythmia syndromes: differentiating pathogenic mutations from background genetic noise

PURPOSE OF REVIEW

In this review, we summarize the basic principles governing rare variant interpretation in the heritable cardiac arrhythmia syndromes, focusing on recent advances that have led to disease-specific approaches to the interpretation of positive genetic testing results.

RECENT FINDINGS

Elucidation of the genetic substrates underlying heritable cardiac arrhythmia syndromes has unearthed new arrhythmogenic mechanisms and given rise to a number of clinically meaningful genotype-phenotype correlations. As such, genetic testing for these disorders now carries important diagnostic, prognostic, and therapeutic implications. Recent large-scale systematic studies designed to explore the background genetic 'noise' rate associated with these genetic tests have provided important insights and enhanced how positive genetic testing results are interpreted for these potentially lethal, yet highly treatable, cardiovascular disorders.

SUMMARY

Clinically available genetic tests for heritable cardiac arrhythmia syndromes allow the identification of potentially at-risk family members and contribute to the risk-stratification and selection of therapeutic interventions in affected individuals. The systematic evaluation of the 'signal-to-noise' ratio associated with these genetic tests has proven critical and essential to assessing the probability that a given variant represents a rare pathogenic mutation or an equally rare, yet innocuous, genetic bystander.

Characterization and mechanisms of action of novel NaV1.5 channel mutations associated with Brugada syndrome

BACKGROUND

Brugada syndrome is a heterogeneous heart rhythm disorder characterized by an atypical right bundle block pattern with ST-segment elevation and T-wave inversion in the right precordial leads. Loss-of-function mutations in SCN5A encoding the cardiac sodium channel Na(V)1.5 are associated with Brugada syndrome. We found novel mutations in SCN5A in 2 different families diagnosed with Brugada syndrome and investigated how those affected Na(V)1.5 channel function.

METHODS AND RESULTS

We performed genetic testing of the probands' genomic DNA. After site-directed mutagenesis and transfection, whole-cell currents were recorded for Na(V)1.5 wild type and mutants heterologously expressed in Chinese hamster ovary-K1 cells. Proband 1 had two novel Na(V)1.5 mutations: Na(V)1.5-R811H and Na(V)1.5-R620H. The Na(V)1.5-R811H mutation showed a significant loss of function in peak Na(+) current density and alteration of biophysical kinetic parameters (inactivation and recovery from inactivation), whereas Na(V)1.5-R620H had no significant effect on the current. Proband 2 had a novel Na(V)1.5-S1218I mutation. Na(V)1.5-S1218I had complete loss of function, and 1:1 expression of Na(V)1.5-wild type and Na(V)1.5-S1218I mimicking the heterozygous state revealed a 50% reduction in current compared with wild type, suggesting a functional haploinsufficiency in the patient.

CONCLUSIONS

Na(V)1.5-S1218I and R811H are novel loss-of-function mutations in the SCN5A gene causing Brugada syndrome.

MOG1 rescues defective trafficking of Na(v)1.5 mutations in Brugada syndrome and sick sinus syndrome

BACKGROUND

Loss-of-function mutations in Na(v)1.5 cause sodium channelopathies, including Brugada syndrome, dilated cardiomyopathy, and sick sinus syndrome; however, no effective therapy exists. MOG1 increases plasma membrane (PM) expression of Na(v)1.5 and sodium current (I(Na)) density, thus we hypothesize that MOG1 can serve as a therapeutic target for sodium channelopathies.

METHODS AND RESULTS

Knockdown of MOG1 expression using small interfering RNAs reduced Na(v)1.5 PM expression, decreased I(Na) densities by 2-fold in HEK/Na(v)1.5 cells and nearly abolished I(Na) in mouse cardiomyocytes. MOG1 did not affect Na(v)1.5 PM turnover. MOG1 small interfering RNAs caused retention of Na(v)1.5 in endoplasmic reticulum, disrupted the distribution of Na(v)1.5 into caveolin-3-enriched microdomains, and led to redistribution of Na(v)1.5 to noncaveolin-rich domains. MOG1 fully rescued the reduced PM expression and I(Na) densities by Na(v)1.5 trafficking-defective mutation D1275N associated with sick sinus syndrome/dilated cardiomyopathy/atrial arrhythmias. For Brugada syndrome mutation G1743R, MOG1 restored the impaired PM expression of the mutant protein and restored I(Na) in a heterozygous state (mixture of wild type and mutant Na(v)1.5) to a full level of a homozygous wild-type state.

CONCLUSIONS

Use of MOG1 to enhance Na(v)1.5 trafficking to PM may be a potential personalized therapeutic approach for some patients with Brugada syndrome, dilated cardiomyopathy, and sick sinus syndrome in the future.

Molecular genetics and functional anomalies in a series of 248 Brugada cases with 11 mutations in the TRPM4 channel

Brugada syndrome (BrS) is a condition defined by ST-segment alteration in right precordial leads and a risk of sudden death. Because BrS is often associated with right bundle branch block and the TRPM4 gene is involved in conduction blocks, we screened TRPM4 for anomalies in BrS cases. The DNA of 248 BrS cases with no SCN5A mutations were screened for TRPM4 mutations. Among this cohort, 20 patients had 11 TRPM4 mutations. Two mutations were previously associated with cardiac conduction blocks and 9 were new mutations (5 absent from ∼14′000 control alleles and 4 statistically more prevalent in this BrS cohort than in control alleles). In addition to Brugada, three patients had a bifascicular block and 2 had a complete right bundle branch block. Functional and biochemical studies of 4 selected mutants revealed that these mutations resulted in either a decreased expression (p.Pro779Arg and p.Lys914X) or an increased expression (p.Thr873Ile and p.Leu1075Pro) of TRPM4 channel. TRPM4 mutations account for about 6% of BrS. Consequences of these mutations are diverse on channel electrophysiological and cellular expression. Because of its effect on the resting membrane potential, reduction or increase of TRPM4 channel function may both reduce the availability of sodium channel and thus lead to BrS.

Gene mutations in cardiac arrhythmias: a review of recent evidence in ion channelopathies

Over the past 15 years, molecular genetic studies have linked gene mutations to many inherited arrhythmogenic disorders, in particular, “ion channelopathies”, in which mutations in genes encode functional units of ion channels and/or their transporter-associated proteins in patients without primary cardiac structural abnormalities. These disorders are exemplified by congenital long QT syndrome (LQTS), short QT syndrome, Brugada syndrome (BrS) and catecholaminergic polymorphic ventricular tachycardia (CPVT). Functional and pathophysiological studies have led to better understanding of the clinical spectrum, ion channel structures and cellular electrophysiology involving dynamics of intracellular calcium cycling in many subtypes of these disorders and more importantly, development of potentially more effective pharmacological agents and even curative gene therapy. In this review, we have summarized (1) the significance of unveiling mutations in genes encoding transporter-associated proteins as the cause of congenital LQTS, (2) the technique of catheter ablation applied at the right ventricular outflow tract may be curative for severely symptomatic BrS, (3) mutations with channel function modulated by protein Kinase A-dependent phosphorylation can be the culprit of CPVT mimicry in Andersen-Tawil syndrome (LQT7), (4) ablation of the ion channel anchoring protein may prevent arrhythmogenesis in Timothy syndrome (LQT8), (5) altered intracellular Ca2+ cycling can be the basis of effective targeted pharmacotherapy in CPVT, and (6) the technology of induced pluripotent stem cells is a promising diagnostic and research tool as it has become a new paradigm for pathophysiological study of patient- and disease-specific cells aimed at screening new drugs and eventual clinical application of gene therapy. Lastly, we have discussed (7) genotype-phenotype correlation in relation to risk stratification of patients with congenital LQTS in clinical practice.

Arrhythmogenesis in Brugada syndrome: impact and constrains of current concepts

Brugada syndrome (BrS), an inherited arrhythmogenic disease first described in 1992, is characterized by ST segment elevations on the electrocardiogram in the right precordium and by a high occurrence of arrhythmias including the life-threatening ventricular tachycardia/fibrillation. Knowledge of the underlying mechanisms of formation of arrhythmogenic substrate in BrS is essential, namely for the risk stratification of BrS patients and their therapy which is still restrained almost exclusively to the implantation of cardioverter/defibrillator. In spite of many crucial findings in this field published within recent years, the final consistent view has not been established so far. Hence, BrS described 20 years ago remains an actual topic of both clinical and experimental studies. This review presents an overview of the current knowledge related to the pathogenesis of BrS arrhythmogenic substrate, namely of the genetic basis of BrS, functional consequences of mutations related to BrS, and arrhythmogenic mechanisms in BrS.

Brugada ECG pattern: a physiopathological prospective study based on clinical, electrophysiological, angiographic, and genetic findings

Introduction: Brugada syndrome (BrS) is considered a primary electrical disease. However, morphological abnormalities have been reported and localized arrhythmogenic right ventricular (RV) dysplasia/cardiomyopathy (ARVD/C) may mimic its phenotype, raising the question of an overlap between these two conditions and making difficult the therapeutic management of patients with borderline forms. The main objective of this study was to assess prospectively the prevalence of BrS and ARVD/C on the basis of international criteria, in patients with BrS-ECG and normal echocardiography, looking for a potential overlap between the two pathologies. The secondary objectives were to describe and quantify angiographic structural alterations, hemodynamics, electrophysiology, and genetics in the setting of BrS-ECG. Materials and Methods: Hundred and fourteen consecutive patients matched in age underwent prospectively cardiac catheterization and quantitative biventricular contrast angiography to rule out a structural heart disease. Fifty-one patients with a BrS-ECG (BrS group, 7 F, 44 M, 43 ± 11 y) had a spontaneous or ajmaline-induced BrS coved type ECG. For angiographic comparison, 49 patients with localized ARVD/C but without ST segment elevation in the right precordial leads (14 F, 35 M, 39 ± 13 y) were also studied. They fulfilled international ESC/WHF 2000 criteria and presented angiographic localized forms, mainly confined to hypokinetic anteroapical zone (characterized by trabecular dysarray and hypertrophy), and/or diaphragmatic wall, thus resulting in RV normal volumes and preserved systolic function. These two populations were also compared with 14 control patients (7 F, 7 M, 38 ± 16 y). Among BrS group, we identified three main angiographic phenotypes: BrS group I = patients with normal RV (n = 15, 29%); BrS group II = patients with segmental RV wall motion abnormalities but no structural arguments for ARVD/C (n = 26, 51%); BrS group III = patients with localized abnormalities suggestive of focal ARVD/C (n = 10, 20%). Results: Among BrS group, 34/51 patients (67%) fulfilled BrS HRS/EHRA 2005 criteria. Nineteen (37%) were symptomatic for aborted sudden death, agonal nocturnal respiration or syncope. Ventricular stimulation was positive in 14 patients (28%). Angiography showed RV abnormalities in 36/51 patients (71%) of BrS group (BrS groups II and III). Late potentials were present in 73% (100% sensitivity and NPV for an angiographic ARVD/C, but poor specificity and PPV, both 37%). In BrS group III, 8/10 patients (16% of BrS patients) finally fulfilled international ESC/WHF 2000 ARVD/C criteria and 5/10 (10% of BrS patients) fulfilled BrS diagnostic criteria. An overlap was observed in 4 patients (8% of BrS patients) who fulfilled both ARVD/C and BrS criteria. Among the 45 genotyped patients, only one presented a SCN5A mutation, whereas a TRPM4 mutation was found in another patient. Both belonged to BrS group II. MOG1 gene analysis was negative for all patients, as were PKP2, DSP, DSG2, and DSC2 analyzes performed in BrS group III. Conclusions: Seventy-one percent of patients with a BrS-ECG had abnormal RV wall motion and 16 had structural alterations corresponding to localized (anteroapical and/or diaphragmatic) ARVD/C. Moreover, 8% of BrS-ECG patients fulfilled both BrS and ARVD/C criteria. Our results support the hypothesis of an overlap between BrS and localized forms of ARVD/C. Conversely, genetic screening was poorly contributive for both diseases in the present series.

Cardiac channelopathies: genetic and molecular mechanisms

Channelopathies are diseases caused by dysfunctional ion channels, due to either genetic or acquired pathological factors. Inherited cardiac arrhythmic syndromes are among the most studied human disorders involving ion channels. Since seminal observations made in 1995, thousands of mutations have been found in many of the different genes that code for cardiac ion channel subunits and proteins that regulate the cardiac ion channels. The main phenotypes observed in patients carrying these mutations are congenital long QT syndrome (LQTS), Brugada syndrome (BrS), catecholaminergic polymorphic ventricular tachycardia (CPVT), short QT syndrome (SQTS) and variable types of conduction defects (CD). The goal of this review is to present an update of the main genetic and molecular mechanisms, as well as the associated phenotypes of cardiac channelopathies as of 2012.

Cardiac ion channelopathies and the sudden infant death syndrome

The sudden infant death syndrome (SIDS) causes the sudden death of an apparently healthy infant, which remains unexplained despite a thorough investigation, including the performance of a complete autopsy. The triple risk model for the pathogenesis of SIDS points to the coincidence of a vulnerable infant, a critical developmental period, and an exogenous stressor. Primary electrical diseases of the heart, which may cause lethal arrhythmias as a result of dysfunctioning cardiac ion channels (“cardiac ion channelopathies”) and are not detectable during a standard postmortem examination, may create the vulnerable infant and thus contribute to SIDS. Evidence comes from clinical correlations between the long QT syndrome and SIDS as well as genetic analyses in cohorts of SIDS victims (“molecular autopsy”), which have revealed a large number of mutations in ion channel-related genes linked to inheritable arrhythmogenic syndromes, in particular the long QT syndrome, the short QT syndrome, the Brugada syndrome, and catecholaminergic polymorphic ventricular tachycardia. Combining data from population-based cohort studies, it can be concluded that at least one out of five SIDS victims carries a mutation in a cardiac ion channel-related gene and that the majority of these mutations are of a known malignant phenotype.

Optical and electrical recordings from isolated coronary-perfused ventricular wedge preparations

The electrophysiological heterogeneity that exists across the ventricular wall in the mammalian heart has long been recognized, but remains an area that is incompletely understood. Experimental studies of the mechanisms of arrhythmogenesis in the whole heart often examine the epicardial surface in isolation and thereby disregard transmural electrophysiology. Significant heterogeneity exists in the electrophysiological properties of cardiomyocytes isolated from different layers of the ventricular wall, and given that regional heterogeneities of membrane repolarization properties can influence the electrophysiological substrate for re-entry, the diversity of cell types and characteristics spanning the ventricular wall is important in the study of arrhythmogenesis. For these reasons, coronary-perfused left ventricular wedge preparations have been developed to permit the study of transmural electrophysiology in the intact ventricle. Since the first report by Yan and Antzelevitch in 1996, electrical recordings from the transmural surface of canine wedge preparations have provided a wealth of data regarding the cellular basis for the electrocardiogram, the role of transmural heterogeneity in arrhythmogenesis, and differences in the response of the different ventricular layers to drugs and neurohormones. Use of the wedge preparation has since been expanded to other species and more recently it has also been widely used in optical mapping studies. The isolated perfused wedge preparation has become an important tool in cardiac electrophysiology. In this review, we detail the methodology involved in recording both electrical and optical signals from the coronary-perfused wedge preparation and review the advances in cardiac electrophysiology achieved through study of the wedge.

A novel disease gene for Brugada syndrome: sarcolemmal membrane-associated protein gene mutations impair intracellular trafficking of hNav1.5

BACKGROUND

Mutations in genes including SCN5A encoding the α-subunit of the cardiac sodium channel (hNav1.5) cause Brugada syndrome via altered function of cardiac ion channels, but more than two-thirds of Brugada syndrome remains pathogenetically elusive. T-tubules and sarcoplasmic reticulum are essential in excitation of cardiomyocytes, and sarcolemmal membrane-associated protein (SLMAP) is a protein of unknown function localizing at T-tubules and sarcoplasmic reticulum.

METHODS AND RESULTS

We analyzed 190 unrelated Brugada syndrome patients for mutations in SLMAP. Two missense mutations, Val269Ile and Glu710Ala, were found in heterozygous state in 2 patients but were not found in healthy individuals. Membrane surface expression of hNav1.5 in the transfected cells was affected by the mutations, and silencing of mutant SLMAP by small interfering RNA rescued the surface expression of hNav1.5. Whole-cell patch-clamp recordings of hNav1.5-expressing cells transfected with mutant SLMAP confirmed the reduced hNav1.5 current.

CONCLUSIONS

The mutations in SLMAP may cause Brugada syndrome via modulating the intracellular trafficking of hNav1.5 channel.

Genetics of cardiac electrical disease

Few tragedies compare to the sudden death of a family member. Sadly, this may represent the first sign of a familial vulnerability to such events. One common cause is an inherited cardiac arrhythmia syndrome. Sufferers are prone to premature sudden cardiac death due to altered ion channel function in the heart. Typical causes include Brugada syndrome, long QT syndrome, short QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and the newly recognized early repolarization syndrome. Our knowledge of the genetic underpinnings of each of these disorders has increased markedly in recent years. Genetic screening is now a routine part of clinical care and promises more accurate diagnosis and efficient family screening. This review summarizes the diagnosis and management of each of the listed syndromes in the context of currently available genetic testing.

Spectrum and prevalence of mutations involving BrS1- through BrS12-susceptibility genes in a cohort of unrelated patients referred for Brugada syndrome genetic testing: implications for genetic testing

OBJECTIVES

The aim of this study was to provide the spectrum and prevalence of mutations in the 12 Brugada syndrome (BrS)-susceptibility genes discovered to date in a single large cohort of unrelated BrS patients.

BACKGROUND

BrS is a potentially lethal heritable arrhythmia syndrome diagnosed electrocardiographically by coved-type ST-segment elevation in the right precordial leads (V1 to V3; type 1 Brugada electrocardiographic [ECG] pattern) and the presence of a personal/family history of cardiac events.

METHODS

Using polymerase chain reaction, denaturing high-performance liquid chromatography, and DNA sequencing, comprehensive mutational analysis of BrS1- through BrS12-susceptibility genes was performed in 129 unrelated patients with possible/probable BrS (46 with clinically diagnosed BrS [ECG pattern plus personal/family history of a cardiac event] and 83 with a type 1 BrS ECG pattern only).

RESULTS

Overall, 27 patients (21%) had a putative pathogenic mutation, absent in 1,400 Caucasian reference alleles, including 21 patients with an SCN5A mutation, 2 with a CACNB2B mutation, and 1 each with a KCNJ8 mutation, a KCND3 mutation, an SCN1Bb mutation, and an HCN4 mutation. The overall mutation yield was 23% in the type 1 BrS ECG pattern-only patients versus 17% in the clinically diagnosed BrS patients and was significantly greater among young men<20 years of age with clinically diagnosed BrS and among patients who had a prolonged PQ interval.

CONCLUSIONS

We identified putative pathogenic mutations in ∼20% of our BrS cohort, with BrS genes 2 through 12 accounting for <5%. Importantly, the yield was similar between patients with only a type 1 BrS ECG pattern and those with clinically established BrS. The yield approaches 40% for SCN5A-mediated BrS (BrS1) when the PQ interval exceeds 200 ms. Calcium channel-mediated BrS is extremely unlikely in the absence of a short QT interval.

[Brugada syndrome]

Brugada syndrome is a rare, hereditary and primary electrical disease which is associated with a risk of syncope and sudden cardiac death. Initially, Brugada syndrome was considered to be a very malignant disease; however, in subsequent studies the risk of sudden death especially in asymptomatic patients was much lower than initially expected. In patients with Brugada type 1 electrocardiogram (ECG) findings and rhythmogenic syncope or sudden cardiac arrest, implantable cardioverter-defibrillator (ICD) implantation is indicated. Risk stratification and therapy in asymptomatic patients is controversially discussed and is clinically challenging. Due to the low event rate in asymptomatic patients with Brugada syndrome the identification of predictors of sudden cardiac death is difficult. Thus, risk stratification and therapy in asymptomatic patients has to be performed individually. This manuscript reviews the current data on diagnosis, risk stratification and therapy of Brugada syndrome.

Genetics of ion-channel disorders

PURPOSE OF REVIEW

In this article, we summarize the main features of the most common inherited channelopathies, focusing on the findings that advanced the field in the last few years.

RECENT FINDINGS

The progress in genetics prompted the discovery of several new genes associated with ion-channel disorders, elucidating new molecular pathways and new arrhythmogenic mechanisms. The diffusion and availability of genetic screening gave a new relevance to the application of genetics not only for diagnosis, but also for risk assessment and therapeutic decisions. As a consequence, the present challenge in the field is represented by the need to use genetic data to develop personalized clinical approaches.

SUMMARY

Over a few years, the field of inherited arrhythmogenic diseases has rapidly expanded, thus reshaping clinical management for these conditions. It is now clear that to handle these patients a specialized expertise is needed, able to translate the discoveries derived from basic science studies into the clinical care of the patients.

Dominant-negative effect of SCN5A N-terminal mutations through the interaction of Na(v)1.5 α-subunits

AIMS

Brugada syndrome (BrS) is an autosomal-inherited cardiac arrhythmia characterized by an ST-segment elevation in the right precordial leads of the electrocardiogram and an increased risk of syncope and sudden death. SCN5A, encoding the cardiac sodium channel Na(v)1.5, is the main gene involved in BrS. Despite the fact that several mutations have been reported in the N-terminus of Na(v)1.5, the functional role of this region remains unknown. We aimed to characterize two BrS N-terminal mutations, R104W and R121W, a construct where this region was deleted, ΔNter, and a construct where only this region was present, Nter.

METHODS AND RESULTS

Patch-clamp recordings in HEK293 cells demonstrated that R104W, R121W, and ΔNter abolished the sodium current I(Na). Moreover, R104W and R121W mutations exerted a strong dominant-negative effect on wild-type (WT) channels. Immunocytochemistry of rat neonatal cardiomyocytes revealed that both mutants were mostly retained in the endoplasmic reticulum and that their co-expression with WT channels led to WT channel retention. Furthermore, co-immunoprecipitation experiments showed that Na(v)1.5-subunits were interacting with each other, even when mutated, deciphering the mutation dominant-negative effect. Both mutants were mostly degraded by the ubiquitin-proteasome system, while ΔNter was addressed to the membrane, and Nter expression induced a two-fold increase in I(Na). In addition, the co-expression of N-terminal mutants with the gating-defective but trafficking-competent R878C-Na(v)1.5 mutant gave rise to a small I(Na).

CONCLUSION

This study reports for the first time the critical role of the Na(v)1.5 N-terminal region in channel function and the dominant-negative effect of trafficking-defective channels occurring through α-subunit interaction.

Diagnostic dilemmas: overlapping features of brugada syndrome and arrhythmogenic right ventricular cardiomyopathy

Arrhythmogenic right ventricular cardiomyopathy (ARVC) and Brugada syndrome are distinct clinical entities which diagnostic criteria exclude their coexistence in individual patients. ARVC is a myocardial disorder characterized by fibro-fatty replacement of the myocardium and ventricular arrhythmias. In contrast, the Brugada syndrome has long been considered a functional cardiac disorder: no gross structural abnormalities can be identified in the majority of patients and its electrocardiographic hallmark of coved-type ST-segment elevation in right precordial leads is dynamic. Nonetheless, a remarkable overlap in clinical features has been demonstrated between these conditions. This review focuses on this overlap and discusses its potential causes and consequences.