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

Phase 2 Re-Entry Without Ito: Role of Sodium Channel Kinetics in Brugada Syndrome Arrhythmias

BACKGROUND

In Brugada syndrome (BrS), phase 2 re-excitation/re-entry (P2R) induced by the transient outward potassium current (Ito) is a proposed arrhythmia mechanism; yet, the most common genetic defects are loss-of-function sodium channel mutations.

OBJECTIVES

The authors used computer simulations to investigate how sodium channel dysfunction affects P2R-mediated arrhythmogenesis in the presence and absence of Ito.

METHODS

Computer simulations were carried out in 1-dimensional cables and 2-dimensional tissue using guinea pig and human ventricular action potential models.

RESULTS

In the presence of Ito sufficient to generate robust P2R, reducing sodium current (INa) peak amplitude alone only slightly potentiated P2R. When INa inactivation kinetics were also altered to simulate reported effects of BrS mutations and sodium channel blockers, however, P2R occurred even in the absence of Ito. These effects could be potentiated by delaying L-type calcium channel activation or increasing ATP-sensitive potassium current, consistent with experimental and clinical findings. INa-mediated P2R also accounted for sex-related, day and night-related, and fever-related differences in arrhythmia risk in BrS patients.

CONCLUSIONS

Altered INa kinetics synergize powerfully with reduced INa amplitude to promote P2R-induced arrhythmias in BrS in the absence of Ito, establishing a robust mechanistic link between altered INa kinetics and the P2R-mediated arrhythmia mechanism.

Novel SCN5A frame‑shift mutation underlying in patient with idiopathic ventricular fibrillation manifested with J wave in inferior lead and prolonged S‑wave in precordial lead

Mutations in the SCN5A gene has been recognized as resulting in a series of life-threatening arrhythmias. However, it also causes idiopathic ventricular fibrillation (IVF) with J wave in inferior leads and prolonged S-wave upstroke in precordial leads, which has not been previously reported. The present study aimed to study the mechanisms of a patient with IVF manifested with J wave in inferior leads and prolonged S-wave upstroke in precordial leads. The electrocardiograms (ECG) of the proband were recorded and genetic testing was conducted. Patch-clamp and immunocytochemical studies were performed in heterologously transfected 293 cells. The VF attacks was documented in a 55-year-old male proband with syncope episodes. 12-lead ECG shown the transient J wave in the inferior leads and prolonged S-wave upstroke in precordial V1-V3 leads in the same timeframe. Genetic analysis revealed a novel 1 base deletion (G) at position 839 in exon 2 in SCN5A gene (C280S*fs61), which causes a severe truncation of the sodium channel. The functional study revealed that in 293 cells transfected with mutant channel, no sodium current could be recorded even though the immunocytochemical experiment confirmed the truncated sodium channel existed in cytosol. The kinetics of the wild-type (WT) channel were not altered when co-transfected with C280S*fs61 mutant which suggested a haploinsufficiency effect of sodium channel in the cells. The present study identified a novel C280Sfs*61 mutation that caused the 'loss of function' of the sodium channel by haploinsufficiency mechanism. The reduced sodium channel function in the heart may cause conduction delay that may underlie the manifestation of J wave and prolonged S-wave upstroke associated with IVF.

Rare Compound Heterozygous Missense Mutation of the SCN5A Gene with Childhood-Onset Sick Sinus Syndrome in Two Chinese Sisters

Sick sinus syndrome (SSS) is a group of syndromes characterized by pathological changes in the sinoatrial node and its adjacent tissues. Although several mutations in the SCN5A gene have been associated with early-onset SSS, pediatric patients are still less common. Here, we report a rare compound missense mutation in the SCN5A gene [c.2893C>T (p. R965C) and c.2431C>T (p. R811C) ] in two sisters with childhood-onset SSS in Chinese population. The proband (5 years and 5 months old) was the second child of a clinically normal and nonconsanguineous couple. Her elder sister was 12 years old and had been implanted with a pacemaker because of the diagnosis of SSS at another hospital one year ago. The proband was presented to the hospital with a slowed heart rate and reduced endurance exercise capacity for more than three months. After a comprehensive clinical examination, she was diagnosed with SSS and underwent pacemaker implantation. Exome and Sanger sequencing were used to determine the compound heterozygous missense mutation of [c.2893C>T (p. R965C) and c.2431C>T (p. R811C) ] in the SCN5A in the patient and her elder sister. Each healthy parent carried a different heterozygous missense mutation. The compound heterozygous mutation of c.2893C>T (p. R965C) and c.2431C>T (p. R811C) rather than the single mutation might be the primary cause of familial early-onset SSS in Chinese population. Our current findings expanded the current understanding of the SCN5A gene mutations. We further confirmed the essential role of the SCN5A gene on the diagnosis, family cascade screening, early intervention, and prognostic evaluation of SSS.

Tramadol as a Voltage-Gated Sodium Channel Blocker of Peripheral Sodium Channels Nav1.7 and Nav1.5

Tramadol is an opioid analog used to treat chronic and acute pain. Intradermal injections of tramadol at hundreds of millimoles have been shown to produce a local anesthetic effect. We used the whole-cell patch-clamp technique in this study to investigate whether tramadol blocks the sodium current in HEK293 cells, which stably express the pain threshold sodium channel Na_v1.7 or the cardiac sodium channel Na_v1.5. The half-maximal inhibitory concentration of tramadol was 0.73 mM for Na_v1.7 and 0.43 mM for Na_v1.5 at a holding potential of -100 mV. The blocking effects of tramadol were completely reversible. Tramadol shifted the steady-state inactivation curves of Na_v1.7 and Na_v1.5 toward hyperpolarization. Tramadol also slowed the recovery rate from the inactivation of Na_v1.7 and Na_v1.5 and induced stronger use-dependent inhibition. Because the mean plasma concentration of tramadol upon oral administration is lower than its mean blocking concentration of sodium channels in this study, it is unlikely that tramadol in plasma will have an analgesic effect by blocking Na_v1.7 or show cardiotoxicity by blocking Na_v1.5. However, tramadol could act as a local anesthetic when used at a concentration of several hundred millimoles by intradermal injection and as an antiarrhythmic when injected intravenously at a similar dose, as does lidocaine.

Intersegment Contacts of Potentially Damaging Variants of Cardiac Sodium Channel

Over 1,500 missense variants of sodium channel hNav1.5, which are reported in the ClinVar database, are associated with cardiac diseases. For most of the variants, the clinical significance is uncertain (VUS), not provided (NP), or has conflicting interpretations of pathogenicity (CIP). Reclassifying these variants as pathogenic/likely pathogenic (P/LP) variants is important for diagnosing genotyped patients. In our earlier work, several bioinformatics tools and paralogue annotation method consensually predicted that 74 VUS/NP/CIP variants of 54 wild type residues (set w54) are potentially damaging variants (PDVs). Atomic mechanisms underlying dysfunction of the PDVs are unknown. Here we employed a recent cryo-EM structure of the hNav1.5 channel with likely inactivated pore domain (PD) and activated voltage-sensing domains (VSDs), and ad hoc models of the closed and open PD and resting VSDs to explore intersegment contacts of w54 residues. We found that 44 residues from set w54 contact 84 residues with 118 disease missense variants. These include 104 VUS/NP/CIP variants, most of which are associated with the loss-of-function Brugada syndrome (BrS1) or gain-of-function long QT syndrome (LQT3). Matrix representation of the PDVs and their contact variants facilitated recognition of coupled mutations associated with the same disease. In particular, BrS1-associated coupled mutations, which disturb the P-loops region with the selectivity filter slow inactivation gate, would cause the channel dysfunction. Other likely causes of the channel dysfunction include coupled BrS1-associated variants within VSDs that would destabilize their activated states and coupled LQT3-associated variants, which would stabilize the open PD or activated VSDs. Our study proposes mechanisms of channel dysfunction for scores of BrS1- and LQT3-associated variants, confirms status for 82% of PDVs, and suggests damaging status for their contact variants, which are currently categorized as VUS/NP/CIP variants.

Regulation of Cardiac Conduction and Arrhythmias by Ankyrin/Spectrin-Based Macromolecular Complexes

The cardiac conduction system is an extended network of excitable tissue tasked with generation and propagation of electrical impulses to signal coordinated contraction of the heart. The fidelity of this system depends on the proper spatio-temporal regulation of ion channels in myocytes throughout the conduction system. Importantly, inherited or acquired defects in a wide class of ion channels has been linked to dysfunction at various stages of the conduction system resulting in life-threatening cardiac arrhythmia. There is growing appreciation of the role that adapter and cytoskeletal proteins play in organizing ion channel macromolecular complexes critical for proper function of the cardiac conduction system. In particular, members of the ankyrin and spectrin families have emerged as important nodes for normal expression and regulation of ion channels in myocytes throughout the conduction system. Human variants impacting ankyrin/spectrin function give rise to a broad constellation of cardiac arrhythmias. Furthermore, chronic neurohumoral and biomechanical stress promotes ankyrin/spectrin loss of function that likely contributes to conduction disturbances in the setting of acquired cardiac disease. Collectively, this review seeks to bring attention to the significance of these cytoskeletal players and emphasize the potential therapeutic role they represent in a myriad of cardiac disease states.

Functional analysis of three Nav1.6 mutations causing early infantile epileptic encephalopathy

The voltage-gated sodium channel Na_v1.6 is associated with more than 300 cases of epileptic encephalopathy. Na_v1.6 epilepsy-causing mutations are spread over the entire channel's structure and only 10% of mutations have been characterized at the molecular level, with most of them being gain of function mutations. In this study, we analyzed three previously uncharacterized Na_v1.6 epilepsy-causing mutations: G214D, N215D and V216D, located within a mutation hot-spot at the S3-S4 extracellular loop of Domain1. Voltage clamp experiments showed a 6-16 mV hyperpolarizing shift in the activation mid-point for all three mutants. V216D presented the largest shift along with decreased current amplitude, enhanced inactivation and a lack of persistent current. Recordings at hyperpolarized potentials indicated that all three mutants presented gating pore currents. Furthermore, trafficking experiments performed in cultured hippocampal neurons demonstrated that the mutants trafficked properly to the cell surface, with no significant differences regarding surface expression within the axon initial segment or soma compared to wild-type. These trafficking data suggest that the disease-causing consequences are due to only changes in the biophysical properties of the channel. Interestingly, the patient carrying the V216D mutation, which is the mutant with the greatest electrophysiological changes as compared to wild-type, exhibited the most severe phenotype. These results emphasize that these mutations will mandate unique treatment approaches, for normal sodium channel blockers may not work given that the studied mutations present gating pore currents. This study emphasizes the importance of molecular characterization of disease-causing mutations in order to improve the pharmacological treatment of patients.

A case report of one vasovagal syncope patient with third-degree atrioventricular block caused by SCN5A gene mutation and literature review

BACKGROUND

Vasovagal syncope (VVS) is common in children and significantly affects their quality of life. To our knowledge, this the first case report of SCN5A gene mutation associated with VVS and third-degree atrioventricular block (atrioventricular block, AVB), which could help pediatricians aware that VVS is not always a benign condition and help to identify VVS children at the risk of sudden death.

CASE PRESENTATION

A twelve-year-old male child was admitted to Beijing Children's Hospital of Capital Medical University for chest tightness for 9 days and syncope in July 2018. The child was diagnosed as VVS with third-degree AVB after complete investagations. A heterozygous mutation in the exon coding region of the SCN5A gene, C. 5851G > T (coding region 5551 nucleotide changed from G to T), was detected in the peripheral blood of the child. Electrophysiological examination and modified vagal ganglion radiofrequency ablation were performed in the child. The ECG playback was normal on the second day after operation. Holter showed no abnormality and no chest tightness or syncope occurred after 3 months and 1 year follow-up.

CONCLUSIONS

Our case report firstly reported that SCN5A mutation contributed to the pathogenesis of VVS with third-degree AVB. Vagal ganglion modified ablation have obtained good therapeutic effect. Gene analysis was of great value to the accurate diagnosis and treatment of VVS children.

Polyunsaturated fatty acid analogues differentially affect cardiac NaV, CaV, and KV channels through unique mechanisms

The cardiac ventricular action potential depends on several voltage-gated ion channels, including Na_V, Ca_V, and K_V channels. Mutations in these channels can cause Long QT Syndrome (LQTS) which increases the risk for ventricular fibrillation and sudden cardiac death. Polyunsaturated fatty acids (PUFAs) have emerged as potential therapeutics for LQTS because they are modulators of voltage-gated ion channels. Here we demonstrate that PUFA analogues vary in their selectivity for human voltage-gated ion channels involved in the ventricular action potential. The effects of specific PUFA analogues range from selective for a specific ion channel to broadly modulating cardiac ion channels from all three families (Na_V, Ca_V, and K_V). In addition, a PUFA analogue selective for the cardiac I_Ks channel (Kv7.1/KCNE1) is effective in shortening the cardiac action potential in human-induced pluripotent stem cell-derived cardiomyocytes. Our data suggest that PUFA analogues could potentially be developed as therapeutics for LQTS and cardiac arrhythmia.

Deep Mutational Scan of an SCN5A Voltage Sensor

BACKGROUND

Variants in ion channel genes have classically been studied in low throughput by patch clamping. Deep mutational scanning is a complementary approach that can simultaneously assess function of thousands of variants.

METHODS

We have developed and validated a method to perform a deep mutational scan of variants in SCN5A, which encodes the major voltage-gated sodium channel in the heart. We created a library of nearly all possible variants in a 36 base region of SCN5A in the S4 voltage sensor of domain IV and stably integrated the library into HEK293T cells.

RESULTS

In preliminary experiments, challenge with 3 drugs (veratridine, brevetoxin, and ouabain) could discriminate wild-type channels from gain- and loss-of-function pathogenic variants. High-throughput sequencing of the pre- and postdrug challenge pools was used to count the prevalence of each variant and identify variants with abnormal function. The deep mutational scan scores identified 40 putative gain-of-function and 33 putative loss-of-function variants. For 8 of 9 variants, patch clamping data were consistent with the scores.

CONCLUSIONS

These experiments demonstrate the accuracy of a high-throughput in vitro scan of SCN5A variant function, which can be used to identify deleterious variants in SCN5A and other ion channel genes.

Calcium in Brugada Syndrome: Questions for Future Research

The Brugada syndrome (BrS) is characterized by coved-type ST-segment elevation in the right precordial leads on the electrocardiogram (ECG) and increased risk of sudden cardiac death (SCD). While it is an inheritable disease, determining the true prevalence is a challenge, since patients may report no known family history of the syndrome, present with a normal spontaneous ECG pattern at the time of examination, and test negative for all known BrS-causative genes. In fact, SCD is often the first indication that a person is affected by the syndrome. Men are more likely to be symptomatic than women. Abnormal, low-voltage, fractionated electrograms have been found in the epicardium of the right ventricular outflow tract (RVOT). Ablation of this area abolishes the abnormal electrograms and helps to prevent arrhythmic recurrences. BrS patients are more likely to experience ventricular tachycardia/fibrillation (VT/VF) during fever or during an increase in vagal tone. Isoproterenol helps to reverse the ECG BrS phenotype. In this review, we discuss roles of calcium in various conditions that are relevant to BrS, such as changes in temperature, heart rate, and vagal tone, and the effects of gender and isoproterenol on calcium handling. Studies are warranted to further investigate these mechanisms in models of BrS.

Dysfunctional Nav1.5 channels due to SCN5A mutations

The voltage-gated sodium channel 1.5 (Nav1.5), encoded by the SCN5A gene, is responsible for the rising phase of the action potential of cardiomyocytes. The sodium current mediated by Nav1.5 consists of peak and late components (I_Na-P and I_Na-L). Mutant Nav1.5 causes alterations in the peak and late sodium current and is associated with an increasingly wide range of congenital arrhythmias. More than 400 mutations have been identified in the SCN5A gene. Although the mechanisms of SCN5A mutations leading to a variety of arrhythmias can be classified according to the alteration of I_Na-P and I_Na-L as gain-of-function, loss-of-function and both, few researchers have summarized the mechanisms in this way before. In this review article, we aim to review the mechanisms underlying dysfunctional Nav1.5 due to SCN5A mutations and to provide some new insights into further approaches in the treatment of arrhythmias. Impact statement The field of ion channelopathy caused by dysfunctional Nav1.5 due to SCN5A mutations is rapidly evolving as novel technologies of electrophysiology are introduced and our understanding of the mechanisms of various arrhythmias develops. In this review, we focus on the dysfunctional Nav1.5 related to arrhythmias and the underlying mechanisms. We update SCN5A mutations in a precise way since 2013 and presents novel classifications of SCN5A mutations responsible for the dysfunction of the peak (I_Na-P) and late (I_Na-L) sodium channels based on their phenotypes, including loss-, gain-, and coexistence of gain- and loss-of function mutations in I_Na-P, I_Na-L, respectively. We hope this review will provide a new comprehensive way to better understand the electrophysiological mechanisms underlying arrhythmias from cell to bedside, promoting the management of various arrhythmias in practice.

Multiple genetic variations in sodium channel subunits in a case of sudden infant death syndrome

BACKGROUND

Dysfunction of Na_V 1.5 encoded by SCN5A accounts for approximately half of the channelopathic SIDS cases. We investigated the functional effect of two gene variants identified in the same patient, one in SCN5A and one in SCN1Bb. The aim of the study was to risk stratify the proband's family.

METHODS

The family was referred for cardiovascular genetic evaluation to assess familial risk of cardiac disease. Functional analysis of the identified variants was performed with patch-clamp electrophysiology in HEK293 cells.

RESULTS

A 16-month-old healthy boy died suddenly in the context of nonspecific illness and possible fever. Postmortem genetic testing revealed variants in the SCN5A and SCN1Bb genes. The proband's father carries the same variants but is asymptomatic. Electrophysiological analysis of the Na_V 1.5_1281X truncation revealed complete loss-of-function of the channel. Coexpression of Na_V 1.5 with Na_V β1b significantly increased I_Na density when compared to Na_V 1.5 alone. The Na_V β1b _V268I variant abolished this I_Na density increase. Moreover, it shifted the activation curve toward more depolarized potentials.

CONCLUSIONS

Genetic variation of both sodium channel and its modifiers may contribute to sudden unexplained death in childhood. However, the asymptomatic father suggests that genetic variation of these genes is not sufficient to cause sudden death or clinically detectable SCN5A phenotypes.

Cardiac channelopathies: The role of sodium channel mutations

INTRODUCTION AND OBJECTIVES

The importance of sodium channels for the normal electrical activity of the heart is emphasized by the fact that mutations (inherited or de novo) in genes that encode for these channels or their associated proteins cause arrhythmogenic syndromes such as the Brugada syndrome and the long QT syndrome (LQTS). The aim of this study is to conduct a review of the literature on the mutations in the sodium channel complex responsible for heart disease and the implications of a close relationship between genetics and the clinical aspects of the main cardiac channelopathies, namely at the level of diagnosis, risk stratification, prognosis, screening of family members and treatment.

METHODS

The online Pubmed® database was used to search for articles published in this field in indexed journals. The MeSH database was used to define the following query: "Mutation [Mesh] AND Sodium Channels [Mesh] AND Heart Diseases [Mesh]", and articles published in the last 15 years, written in English or Portuguese and referring to research in human beings were included.

CONCLUSIONS

In the past few years, significant advances have been made to clarify the genetic and molecular basis of these syndromes. A greater understanding of the underlying pathophysiological mechanisms showed the importance of the relationship between genotype and phenotype and led to progress in the clinical approach to these patients. However, it is still necessary to improve diagnostic capacity, optimize risk stratification, and develop new specific treatments according to the genotype-phenotype binomial.

Missense variant pathogenicity predictors generalize well across a range of function-specific prediction challenges

The steady advances in machine learning and accumulation of biomedical data have contributed to the development of numerous computational models that assess the impact of missense variants. Different methods, however, operationalize impact differently. Two common tasks in this context are the prediction of the pathogenicity of variants and the prediction of their effects on a protein's function. These are related but distinct problems, and it is unclear whether methods developed for one are optimized for the other. The Critical Assessment of Genome Interpretation (CAGI) experiment provides a means to address this question empirically. To this end, we participated in various protein-specific challenges in CAGI with two objectives in mind. First, to compare the performance of methods in the MutPred family with the state-of-the-art. Second and more importantly, to investigate the applicability of general-purpose pathogenicity predictors to the classification of specific function-altering variants without additional training or calibration. We find that our pathogenicity predictors performed competitively with other methods, outputting score distributions in agreement with experimental outcomes. Overall, we conclude that binary classifiers learned from disease-causing mutations are capable of modeling important aspects of the underlying biology and the alteration of protein function resulting from mutations.

Brugada syndrome: a fatal disease with complex genetic etiologies - still a long way to go

Brugada syndrome (BrS) is an arrhythmogenic disorder which was first described in 1992. This disease is a channelopathy characterized by ST-segment elevations in the right precordial leads and is susceptible to sudden death. BrS is a fatal disease with gender and age preferences. It occurs mainly in young male subjects with a structurally normal heart and silently progresses to sudden death with no significant symptoms. The prevalence of BrS has been reported in the ranges of 5–20 per 10 000 people. The disease is more prevalent in Asia. Nowadays, numerous variations in 23 genes have been linked to BrS since the first gene SCN5A has been associated with BrS in 1998. Not only can clinical specialists apply these discoveries in risk assessment, diagnosis and personal medicine, but also forensic pathologists can make full use of these variations to conduct death cause identification. However, despite the progress in genetics, these associated genes can only account for approximately 35% of the BrS cases while the etiology of the remaining BrS cases is still unexplained. In this review, we discussed the prevalence, the genes associated with BrS and the application of molecular autopsy in forensic pathology. We also summarized the present obstacles, and provided a new insight into the genetic basis of BrS.

Patient-Specific and Genome-Edited Induced Pluripotent Stem Cell-Derived Cardiomyocytes Elucidate Single-Cell Phenotype of Brugada Syndrome

BACKGROUND

Brugada syndrome (BrS), a disorder associated with characteristic electrocardiogram precordial ST-segment elevation, predisposes afflicted patients to ventricular fibrillation and sudden cardiac death. Despite marked achievements in outlining the organ level pathophysiology of the disorder, the understanding of human cellular phenotype has lagged due to a lack of adequate human cellular models of the disorder.

OBJECTIVES

The objective of this study was to examine single cell mechanism of Brugada syndrome using induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs).

METHODS

This study recruited 2 patients with type 1 BrS carrying 2 different sodium voltage-gated channel alpha subunit 5 variants as well as 2 healthy control subjects. We generated iPSCs from their skin fibroblasts by using integration-free Sendai virus. We used directed differentiation to create purified populations of iPSC-CMs.

RESULTS

BrS iPSC-CMs showed reductions in inward sodium current density and reduced maximal upstroke velocity of action potential compared with healthy control iPSC-CMs. Furthermore, BrS iPSC-CMs demonstrated increased burden of triggered activity, abnormal calcium (Ca²⁺) transients, and beating interval variation. Correction of the causative variant by genome editing was performed, and resultant iPSC-CMs showed resolution of triggered activity and abnormal Ca²⁺ transients. Gene expression profiling of iPSC-CMs showed clustering of BrS compared with control subjects. Furthermore, BrS iPSC-CM gene expression correlated with gene expression from BrS human cardiac tissue gene expression.

CONCLUSIONS

Patient-specific iPSC-CMs were able to recapitulate single-cell phenotype features of BrS, including blunted inward sodium current, increased triggered activity, and abnormal Ca²⁺ handling. This novel human cellular model creates future opportunities to further elucidate the cellular disease mechanism and identify novel therapeutic targets.

Molecular pathological study on LRRC10 in sudden unexplained nocturnal death syndrome in the Chinese Han population

Sudden unexplained nocturnal death syndrome (SUNDS) is a perplexing disorder to both forensic pathologists and clinic physicians. Clinical features of SUNDS survivors suggested that SUNDS is similar to Brugada syndrome (BrS). Leucine-rich repeat containing 10 (LRRC10) gene was a newly identified gene linked to dilated cardiomyopathy, a disease associated with sudden cardiac death. To investigate the prevalence and spectrum of genetic variants of LRRC10 gene in SUNDS and BrS, the coding regions of LRRC10 were genetically screened in 113 sporadic SUNDS victims (from January 2005 to December 2015, 30.7 ± 7.5 years) and ten BrS patients (during January 2010 to December 2014, 38.7 ± 10.3 years) using direct Sanger sequencing. Afterwards, LRRC10 missense variant carriers were screened for a panel of 80 genes known to be associated with inherited cardiac arrhythmia/cardiomyopathy using target-captured next-generation sequencing. In this study, an in silico-predicted malignant LRRC10 mutation p.E129K was detected in one SUNDS victim without pathogenic rare variant in a panel of 80 arrhythmia/cardiomyopathy-related genes. We also provided evidence to show that rare variant p.P69L might contribute to the genetic cause for one SUNDS victim and two BrS family members. This is the first report of genetic screening of LRRC10 in Chinese SUNDS victims and BrS patients. LRRC10 may be a new susceptible gene for SUNDS, and LRRC10 variant was initially and genetically linked to BrS-associated arrhythmia.

Electrophysiological characterization of a large set of novel variants in the SCN5A-gene: identification of novel LQTS3 and BrS mutations

SCN5A encodes for the α-subunit of the cardiac voltage-gated sodium channel Nav1.5. Gain-of-function mutations in SCN5A are related to congenital long QT syndrome (LQTS3) characterized by delayed cardiac repolarization, leading to a prolonged QT interval in the ECG. Loss-of-function mutations in SCN5A are related to Brugada syndrome (BrS), characterized by an ST-segment elevation in the right precordial leads (V1-V3). The aim of this study was the characterization of a large set of novel SCN5A variants found in patients with different cardiac phenotypes, mainly LQTS and BrS. SCN5A variants of 13 families were functionally characterized in Xenopus laevis oocytes using the two-electrode voltage-clamp technique. We found in most of the cases, but not all, that the electrophysiology of the variants correlated with the clinically diagnosed phenotype. A susceptibility to develop LQTS can be suggested in patients carrying the variants S216L, K480N, A572D, F816Y, and G983D. However, taking the phenotype into account, the presence of the variants in genomic data bases, the mutational segregation, combined with our in vitro and in silico experiments, the variants S216L, S262G, K480N, A572D, F816Y, G983D, and T1526P remain as variants of unknown significance. However, the SCN5A variants R568H and A993T can be classified as pathogenic LQTS3 causing mutations, while R222stop and R2012H are novel BrS causing mutations.

Genetics of sudden cardiac death

Sudden cardiac death (SCD) is defined by the World Health Organization (WHO) as death within 1 h of symptom onset (witnessed) or within 24 h of being observed alive and symptom free (unwitnessed). It affects more than 3 million people annually worldwide and affects approximately 1/1000 people each year in the USA. Familial studies of syndromes with Mendelian inheritance, candidate genes analyses, and genome-wide association studies (GWAS) have helped our understanding of the genetics of SCD. We will review the genetics of arrhythmogenic hereditary syndromes with Mendelian inheritance from familial studies with structural heart disease (hypertrophic cardiomyopathy, dilated cardiomyopathy, and arrhythmogenic cardiomyopathy) as well as primary electrical causes (long QT syndrome, Brugada syndrome, catecholaminergic polymorphic ventricular tachycardia, and short QT syndrome). In addition, we will review the genetics of intermediate phenotypes for SCD such as coronary artery disease and electrocardiographic variables (QT interval, QRS duration, and RR interval). Finally, we will review rare and common variants that are associated with SCD in the general population and were identified from candidate gene analyses and GWAS. Our understanding of the genetics of SCD will improve by the use of next-generation sequencing/whole-exome sequencing as well as whole-genome sequencing which have the potential to discover unsuspected common and rare genetic variants that might be associated with SCD.

[Functional analysis of a novel SCN5A mutation G1712C identified in Brugada syndrome]

OBJECTIVE

To elucidate the molecular and electrophysiological mechanisms of Brugada syndrome through functional analysis of a novel SCN5A gene mutation G1712C.

METHODS

A recombinant plasmid pRc<CMV-hH1 containing the mutant human cardiac sodium channel α subunit (hH1) cDNA was constructed using in vitro PCR-based site-directed mutagenesis technique. LipofectamineTM 3000 was used to transfect the plasmid DNA into HEK293 cell line to induce stable expression of Na⁺ channel β1-subunit, and the positive colonies were selected by screening with G418.The standard liposome method was used to transiently transfect HEK293 cells with either the wild-type or mutant Na⁺ channel subunits (hH1 and mhH1, respectively), and the macroscopic Na⁺ currents were recorded using whole-cell patch-clamp technique. Data acquisition and analysis, generation of voltage commands and curve fitting were accomplished with EPC-10, PatchMaster and IGOR Pro 6.0.

RESULTS

An HEK293 cell line that stably expressed Na⁺ channel β1-subunit was successfully established. After transient transfection with the WT subunit, large Na⁺ currents were recorded from the stable β1-cell line. Transient transfection with the G1712C subunit, however, did not elicit a Na⁺ current in the cells.

CONCLUSION

Compared with normal Na⁺ channel, the wild-type channel exhibits a similar sodium current. The characteristic kinetics of sodium channel of WT-hH1 was identical to that in normal cardiac muscle cell, and the missense mutation (G1712C) in the P-loop region of the domain IV may have caused the failure of sodium channel expression.

SCN5A(K817E), a novel Brugada syndrome-associated mutation that alters the activation gating of NaV1.5 channel

BACKGROUND

Brugada syndrome (BrS) is an inherited lethal arrhythmic disorder characterized by syncope and sudden cardiac death from ventricular tachyarrhythmias. Here we identified a novel K817E mutation of SCN5A gene in a man with type 1 BrS electrocardiogram pattern using next-generation sequencing targeted for 73 cardiac disorder-related genes. SCN5A encodes the α-subunit of NaV1.5 voltage-gated Na(+) channel, and some of its mutations are linked to BrS. The proband had no mutation in any of the other arrhythmia-related genes sequenced.

OBJECTIVE

We investigated whether the K817E mutation causes a functional change of NaV1.5 channel responsible for the BrS phenotype.

METHODS

We compared the electrophysiological properties of the whole-cell currents mediated by wild-type and mutant channels heterologously expressed in human embryonic kidney 293 cells by using a voltage-clamp technique.

RESULTS

The K817E mutation reduced the Na(+) current density by 39.0%-91.4% at membrane potentials from -55 to -5 mV. This reduction resulted from a ~24-mV positive shift in the voltage dependence of activation. The mutation also decelerated recovery from both fast and intermediate inactivation, whereas it had little effect on the cell surface expression, single-channel conductance, voltage-dependence of fast inactivation, entry into intermediate inactivation, use-dependent loss of channel availability, or closed-state inactivation.

CONCLUSION

The K817E mutation of SCN5A gene leads to loss of function of NaV1.5 channel and may underlie the BrS phenotype of the proband.

Nav1.5 cardiac sodium channels, regulation and clinical implications

<p>Voltage-gated sodium channels constitute a group of membrane<br />proteins widely distributed thought the body. In the heart, there<br />are at least six different isoforms, being the Nav1.5 the most<br />abundant. The channel is composed of an α subunit that is formed<br />by four domains of six segments each, and four much smaller β<br />subunits that provide stability and integrate other channels into<br />the α subunit. The function of the Nav1.5 channel is modulated<br />by intracellular cytoskeleton proteins, extracellular proteins,<br />calcium concentration, free radicals, and medications, among<br />other things. The study of the channel and its alterations has<br />grown thanks to its association with pathogenic conditions such<br />as Long QT syndrome, Brugada syndrome, atrial fibrillation,<br />arrhythmogenic ventricular dysplasia and complications during<br />ischemic processes.</p>

Utility of the exercise electrocardiogram testing in sudden cardiac death risk stratification

BACKGROUND

Sudden cardiac death (SCD) remains a major public health problem. Current established criteria identifying those at risk of sudden arrhythmic death, and likely to benefit from implantable cardioverter defibrillators (ICDs), are neither sensitive nor specific. Exercise electrocardiogram (ECG) testing was traditionally used for information concerning patients' symptoms, exercise capacity, cardiovascular function, myocardial ischemia detection, and hemodynamic responses during activity in patients with hypertrophic cardiomyopathy.

METHODS

We conducted a systematic review of MEDLINE on the utility of exercise ECG testing in SCD risk stratification.

RESULTS

Exercise testing can unmask suspected primary electrical diseases in certain patients (catecholaminergic polymorphic ventricular tachycardia or concealed long QT syndrome) and can be effectively utilized to risk stratify patients at an increased (such as early repolarization syndrome and Brugada syndrome) or decreased risk of SCD, such as the loss of preexcitation on exercise testing in asymptomatic Wolff-Parkinson-White syndrome.

CONCLUSIONS

Exercise ECG testing helps in SCD risk stratification in patients with and without arrhythmogenic hereditary syndromes.

Electrophysiological characteristics of a SCN5A voltage sensors mutation R1629Q associated with Brugada syndrome

Brugada syndrome (BrS) is an inherited arrhythmogenic syndrome leading to sudden cardiac death, partially associated with autosomal dominant mutations in SCN5A, which encodes the cardiac sodium channel alpha-subunit (Nav1.5). To date some SCN5A mutations related with BrS have been identified in voltage sensor of Nav1.5. Here, we describe a dominant missense mutation (R1629Q) localized in the fourth segment of domain IV region (DIV-S4) in a Chinese Han family. The mutation was identified by direct sequencing of SCN5A from the proband's DNA. Co-expression of Wild-type (WT) or R1629Q Nav1.5 channel and hβ1 subunit were achieved in human embryonic kidney cells by transient transfection. Sodium currents were recorded using whole cell patch-clamp protocols. No significant changes between WT and R1629Q currents were observed in current density or steady-state activation. However, hyperpolarized shift of steady-state inactivation curve was identified in cells expressing R1629Q channel (WT: V1/2 = -81.1 ± 1.3 mV, n = 13; R1629Q: V1/2 = -101.7 ± 1.2 mV, n = 18). Moreover, R1629Q channel showed enhanced intermediate inactivation and prolonged recovery time from inactivation. In summary, this study reveals that R1629Q mutation causes a distinct loss-of-function of the channel due to alter its electrophysiological characteristics, and facilitates our understanding of biophysical mechanisms of BrS.

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.

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.