The implications of genetic mutations in the sodium channel gene (SCN5A)

Peripartum management of patient with long QT3 after successful implantable cardioverter defibrillator device discharge resulting in device failure: a case report

Background

Long QT3 syndrome type 3 (LQT3) is a gain of function mutation of the SCN5A gene that is inherited in an autosomal dominant fashion. Long QT3 syndrome type 3 results in an increase in arrhythmic events during rest, sleep, and bradycardia by extending the QT interval and inducing Torsades de pointes and sudden cardiac death. Attempting to block the sodium channel with Class I anti-arrhythmics or blocking adrenergic tone with beta-blockers especially in women has shown to be beneficial. There have been few large-scale studies on treating patients with LQT3 due to its lethality and underreported number of cases. Specifically, the safety and efficacy of pharmacologic treatment in pregnant LQT3 patients are unknown.

Case summary

This case demonstrates the safe use of Mexiletine and Propranolol in a 3rd-trimester pregnant LQT3 patient after a presumed ventricular arrhythmia and device-lead electrical short from therapy rendered her implantable cardioverter defibrillator inoperable in a VVI mode (venticular demand pacing). With appropriate medications, the patient was safely monitored through the remainder of her pregnancy and safely delivered at 36 weeks of pregnancy a healthy baby girl. The daughter, heterozygous for LQT3, showed no evidence of intrauterine growth restriction or other side effects from the medications.

Discussion

There are many variants of the SCN5A gene mutations that can lead to different phenotypes and not all mutations are responsive to the same medications. In this case, Mexiletine and Propranolol, both of which have only recently shown to benefit certain variants or LQT3 respectively, were safely started during the 3rd trimester of pregnancy without harming the foetus.

Multiple arrhythmic and cardiomyopathic phenotypes associated with an SCN5A A735E mutation

BACKGROUND

SCN5A mutations are associated with multiple arrhythmic and cardiomyopathic phenotypes including Brugada syndrome (BrS), sinus node dysfunction (SND), atrioventricular block, supraventricular tachyarrhythmias (SVTs), long QT syndrome (LQTS), dilated cardiomyopathy and left ventricular noncompaction. Several single SCN5A mutations have been associated with overlap of some of these phenotypes, but never with overlap of all the phenotypes.

OBJECTIVE

We encountered two pedigrees with multiple arrhythmic phenotypes with or without cardiomyopathic phenotypes, and sought to identify a responsible mutation and reveal its functional abnormalities.

METHODS

Target panel sequencing of 72 genes, including inherited arrhythmia syndromes- and cardiomyopathies-related genes, was employed in two probands. Cascade screening was performed by Saner sequencing. Wild-type or identified mutant SCN5A were expressed in tsA201 cells, and whole-cell sodium currents (I_Na) were recorded using patch-clamp techniques.

RESULTS

We identified an SCN5A A735E mutation in these probands, but did not identify any other mutations. All eight mutation carriers exhibited at least one of the arrhythmic phenotypes. Two patients exhibited multiple arrhythmic phenotypes: one (15-year-old girl) exhibited BrS, SND, and exercise and epinephrine-induced QT prolongation, the other (4-year-old boy) exhibited BrS, SND, and SVTs. Another one (30-year-old male) exhibited all arrhythmic and cardiomyopathic phenotypes, except for LQTS. One male suddenly died at age 22. Functional analysis revealed that the mutant did not produce functional I_Na.

CONCLUSIONS

A non-functional SCN5A A735E mutation could be associated with multiple arrhythmic and cardiomyopathic phenotypes, although there remains a possibility that other unidentified factors may be involved in the phenotypic variability of the mutation carriers.

Machine learning-based QSAR models to predict sodium ion channel (Nav 1.5) blockers

Aim: Conventional experimental approaches used for the evaluation of the proarrhythmic potential of compounds in the drug discovery process are expensive and time consuming but an integral element in the safety profile required for a new drug to be approved. The voltage-gated sodium ion channel 1.5 (Na_v 1.5), a target known for arrhythmic drugs, causes adverse cardiac complications when the channel is blocked. Results: Machine learning classification and regression models were built to predict the possibility of blocking these channels by small molecules. The finalized models tested with balanced accuracies of 0.88, 0.93 and 0.94 at three thresholds (1, 10 and 30 µmol, respectively). The regression model built to predict the pIC₅₀ of compounds had q² of 0.84 (root-mean-square error = 0.46). Conclusion: The machine learning models that have been built can act as effective filters to screen out the potentially toxic compounds in the early stages of drug discovery.

Systematic re-evaluation of SCN5A variants associated with Brugada syndrome

BACKGROUND

A large number of SCN5A variants have been reported to underlie Brugada syndrome (BrS). However, the evidence supporting individual variants is highly heterogeneous.

OBJECTIVE

We systematically re-evaluated all SCN5A variants reported in BrS using the 2015 American college of medical genetics and genomics and the association for molecular pathology (ACMG-AMP) guidelines.

METHODS

A PubMed/Embase search was performed to identify all reported SCN5A variants in BrS. Standardized bioinformatic re-analysis (SIFT, PolyPhen, Mutation Taster, Mutation assessor, FATHMM, GERP, PhyloP, and SiPhy) and re-evaluation of frequency in the gnomAD database were performed. Fourteen ACMG-AMP rules were deemed applicable for SCN5A variant analysis.

RESULTS

Four hundred and eighty unique SCN5A variants were identified, the majority of which 425 (88%) were coding variants. One hundred and fifty-six of 425 (37%) variants were classified as pathogenic/likely pathogenic. Two hundred and fifty-eight (60%) were classified as variants of uncertain significance, while a further 11 (3%) were classified as benign/likely benign. When considering the subset of variants that were considered "null" variants separately, 95% fulfilled criteria for pathogenicity/likely pathogenicity. In contrast, only 17% of missense variants fulfilled criteria for pathogenicity/likely pathogenicity. Importantly, however, only 25% of missense variants had available functional data, which was a major score driver for pathogenic classification.

CONCLUSION

Based on contemporary ACMG-AMP guidelines, only a minority of SCN5A variants implicated in BrS fulfill the criteria for pathogenicity or likely pathogenicity.

Predicting changes to INa from missense mutations in human SCN5A

Mutations in SCN5A can alter the cardiac sodium current I_Na and increase the risk of potentially lethal conditions such as Brugada and long-QT syndromes. The relation between mutations and their clinical phenotypes is complex, and systems to predict clinical severity of unclassified SCN5A variants perform poorly. We investigated if instead we could predict changes to I_Na, leaving the link from I_Na to clinical phenotype for mechanistic simulation studies. An exhaustive list of nonsynonymous missense mutations and resulting changes to I_Na was compiled. We then applied machine-learning methods to this dataset, and found that changes to I_Na could be predicted with higher sensitivity and specificity than most existing predictors of clinical significance. The substituted residues’ location on the protein correlated with channel function and strongly contributed to predictions, while conservedness and physico-chemical properties did not. However, predictions were not sufficiently accurate to form a basis for mechanistic studies. These results show that changes to I_Na, the mechanism through which SCN5A mutations create cardiac risk, are already difficult to predict using purely in-silico methods. This partly explains the limited success of systems to predict clinical significance of SCN5A variants, and underscores the need for functional studies of I_Na in risk assessment.

Is Symptomatic Long QT Syndrome Associated with Depression in Women and Men?

We examined whether long QT syndrome (LQTS) mutation carrier status or symptomatic LQTS are associated with depression, and whether there are sex differences in these potential relationships. The sample comprised 782 participants (252 men). Of the 369 genetically defined LQTS mutation carriers, 169 were symptomatic and 200 were asymptomatic. The control group consisted of 413 unaffected relatives. Depression was assessed using the Beck Depression Inventory-II (BDI-II). No association was found for LQTS mutation carrier status with depression. The multinomial logistic regression showed that LQTS mutation carrier men with arrhythmic events scored higher on depression compared with the control group, even when adjusting for age, β-blockers, antidepressants, and social support (OR = 1.09, 95 % CI [1.02, 1.15], p = .007). The binary logistic regression comparing symptomatic and asymptomatic LQTS mutation carriers showed that symptomatic LQTS was associated with depression in men (OR = 1.10, 95 % CI [1.03, 1.19], p = .009). The results were unchanged when additionally adjusted for education. These findings suggest that symptomatic LQTS is associated with depression in men but not in women. Overall, however, depression is more frequent in women than men. Thus, regular screening for depression in LQTS mutation carriers and their unaffected family members can be important.

An R1632C variant in the SCN5A gene causing Brugada syndrome

Brugada syndrome (BS) is an electrical disease, inherited in an autosomal dominant manner. BS is caused by mutations in up to 13 different genes. SCN5A is the gene most frequently mutated in BS, although this presents an incomplete penetrance. The present case study investigated the SCN5A gene in a family exhibiting BS. Direct sequencing of the SCN5A gene was performed to identify mutations and a familial investigation was performed. A novel variant was identified in the voltage‑sensing domain of the SCN5A protein. This familial investigation revealed one novel asymptomatic carrier in the family. Genetic investigations are useful to classify individuals who require more frequent clinical monitoring and to stratify the risk of developing the disease.

The Brugada syndrome revisited

The Brugada syndrome is a rare but well-defined cause of sudden cardiac death. The key underlying abnormality is a decrease in net depolarising current due to a genetic defect, though recent evidence also implicates structural abnormalities in some patients. Diagnosis requires a Brugada-type ECG as well as typical clinical features: such clinical considerations are currently key in guiding risk stratification and hence management. Whilst pharmacological therapies are under investigation, the only intervention with a robust evidence base remains insertion of an implantable cardioverter defibrillator. Further research will be required to allow more effective risk stratification and hence more rational therapy.

Proteasome inhibitor (MG132) rescues Nav1.5 protein content and the cardiac sodium current in dystrophin-deficient mdx (5cv) mice

The cardiac voltage-gated sodium channel, Na_v1.5, plays a central role in cardiac excitability and impulse propagation and associates with the dystrophin multiprotein complex at the lateral membrane of cardiomyocytes. It was previously shown that Na_v1.5 protein content and the sodium current (l_Na) were both decreased in cardiomyocytes of dystrophin-deficient mdx^5cv mice. In this study, wild-type and mdx^5cv mice were treated for 7 days with the proteasome inhibitor MG132 (10 μg/Kg/24 h) using implanted osmotic mini pumps. MG132 rescued both the total amount of Na_v1.5 protein and l_Na but, unlike in previous studies, de novo expression of dystrophin was not observed in skeletal or cardiac muscle. This study suggests that the reduced expression of Na_v1.5 in dystrophin-deficient cells is dependent on proteasomal degradation.

A study of the SCN5A gene in a cohort of 76 patients with Brugada syndrome

We aim to study the SCN5A gene in a cohort of Brugada syndrome (BS) patients and evaluate the genotype-phenotype correlation. BS is caused by mutations in up to 10 different genes, SCN5A being the most frequently involved. Large genomic rearrangements in SCN5A have been associated with conduction disease, but its prevalence in BS is unknown. Seventy-six non-related patients with BS were studied. Clinical characteristics and family risk profile were recorded. Direct sequencing and multiplex ligation-dependent probe amplification (MLPA) of the SCN5A gene for identification of mutations and larger rearrangements were performed, respectively. Eight patients (10.5%) had point mutations (R27H, E901K, G1743R (detected in three families), V728I, N1443S and E1152X). Patients with mutations had a trend toward a higher proportion of spontaneous type I Brugada electrocardiogram (ECG) (87.5% vs 52.9%, p = 0.06) and had evidence of familial disease (62.5%, vs 23.5%, p = 0.03). The symptoms and risk profile of the carriers were not different from wild-type probands. There were non-significant differences in the prevalence of type I ECG, syncope and history of arrhythmia in carriers of selected polymorphisms. None of the patients had any deletion/duplication in the SCN5A gene. In conclusion, 10.5% of our patients had mutations in the SCN5A gene. Patients with mutations seemed to have more spontaneous type I ECG, but no differences in syncope or arrhythmic events compared with patients without mutations. Larger studies are needed to evaluate the role of polymorphisms in the SCN5A in the expression of the phenotype and prognosis. Large rearrangements were not identified in the SCN5A gene using the MLPA technique.

The genetics of atrial fibrillation: from the bench to the bedside

Atrial fibrillation (AF) has become a growing global epidemic and a financial burden for society. The past 10 years have seen significant advances in our understanding of the genetic aspects of AF: At least 2 chromosomal loci and 17 causal genes have been identified in familial AF, and an additional 7 common variants and single-nucleotide polymorphisms in 11 different genes have been indicated in nonfamilial AF. However, the current management strategies for AF are suboptimal. The integration of genetic information into clinical practice may aid the early identification of AF patients who are at risk as well as the characterization of molecular pathways that culminate in AF, with the eventual result of better treatment. Never before has such an opportunity arisen to advance our understanding of the biology of AF through the translation of genetics findings from the bench to the bedside.

Genetics of the sudden infant death syndrome

The sudden infant death syndrome (SIDS) is currently defined as "the sudden unexpected death of an infant less than 1 year of age with onset of the fatal episode apparently occurring during sleep, that remains unexplained after a thorough investigation". SIDS, whose etiology remains rather vague, is still the major cause of death among infants between 1 month and 1 year of age in industrialized countries with varying incidences in different populations. Herein, after touching on definitory approaches and several current hypotheses concerning SIDS etiology, we focus on the triple risk model of SIDS and discuss two large classes of genetic factors potentially contributing to or predisposing for the generation of a vulnerable infant that, when encountering an environmental trigger, may succumb to SIDS. We conclude by acknowledging that for the integration of the vast and complex genetic evidence concerning SIDS, a lot more research will be required and we briefly discuss the potential use of recently presented animal models for functional studies of SIDS pathology.

Sodium channel mutation in irritable bowel syndrome: evidence for an ion channelopathy

The SCN5A-encoded Na(v)1.5 Na(+) channel is expressed in interstitial cells of Cajal and smooth muscle in the circular layer of the human intestine. Patients with mutations in SCN5A are more likely to report gastrointestinal symptoms, especially abdominal pain. Twin and family studies of irritable bowel syndrome (IBS) suggest a genetic basis for IBS, but no genes have been identified to date. Therefore, our aims were to evaluate SCN5A as a candidate gene involved in the pathogenesis of IBS and to determine physiological consequences of identified mutations. Mutational analysis was performed on genomic DNA obtained from 49 subjects diagnosed with IBS who reported at least moderately severe abdominal pain. One patient hosted a loss-of-function missense mutation, G298S, that was not observed in >3,000 reference alleles derived from 1,500 healthy control subjects. Na(+) currents were recorded from the four common human SCN5A transcripts in transfected HEK-293 cells. Comparing Na(v)1.5 with G298S-SCN5A versus wild type in HEK cells, Na(+) current density was significantly less by 49-77%, and channel activation time was delayed in backgrounds that also contained the common H558R polymorphism. Single-channel measurements showed no change in Na(v)1.5 conductance. Mechanosensitivity was reduced in the H558/Q1077del transcript but not in the other three backgrounds. In conclusion, the G298S-SCN5A missense mutation caused a marked reduction of whole cell Na(+) current and loss of function of Na(v)1.5, suggesting SCN5A as a candidate gene in the pathophysiology of IBS.

Sodium channel β1 subunit mutations associated with Brugada syndrome and cardiac conduction disease in humans

Brugada syndrome is a genetic disease associated with sudden cardiac death that is characterized by ventricular fibrillation and right precordial ST segment elevation on ECG. Loss-of-function mutations in SCN5A, which encodes the predominant cardiac sodium channel alpha subunit NaV1.5, can cause Brugada syndrome and cardiac conduction disease. However, SCN5A mutations are not detected in the majority of patients with these syndromes, suggesting that other genes can cause or modify presentation of these disorders. Here, we investigated SCN1B, which encodes the function-modifying sodium channel beta1 subunit, in 282 probands with Brugada syndrome and in 44 patients with conduction disease, none of whom had SCN5A mutations. We identified 3 mutations segregating with arrhythmia in 3 kindreds. Two of these mutations were located in a newly described alternately processed transcript, beta1B. Both the canonical and alternately processed transcripts were expressed in the human heart and were expressed to a greater degree in Purkinje fibers than in heart muscle, consistent with the clinical presentation of conduction disease. Sodium current was lower when NaV1.5 was coexpressed with mutant beta1 or beta1B subunits than when it was coexpressed with WT subunits. These findings implicate SCN1B as a disease gene for human arrhythmia susceptibility.

Novel mRNA isoforms of the sodium channels Na(v)1.2, Na(v)1.3 and Na(v)1.7 encode predicted two-domain, truncated proteins

The expression of voltage-gated sodium channels is regulated at multiple levels, and in this study we addressed the potential for alternative splicing of the Na(v)1.2, Na(v)1.3, Na(v)1.6 and Na(v)1.7 mRNAs. We isolated novel mRNA isoforms of Na(v)1.2 and Na(v)1.3 from adult mouse and rat dorsal root ganglia (DRG), Na(v)1.3 and Na(v)1.7 from adult mouse brain, and Na(v)1.7 from neonatal rat brain. These alternatively spliced isoforms introduce an additional exon (Na(v)1.2 exon 17A and topologically equivalent Na(v)1.7 exon 16A) or exon pair (Na(v)1.3 exons 17A and 17B) that contain an in-frame stop codon and result in predicted two-domain, truncated proteins. The mouse and rat orthologous exon sequences are highly conserved (94-100% identities), as are the paralogous Na(v)1.2 and Na(v)1.3 exons (93% identity in mouse) to which the Na(v)1.7 exon has only 60% identity. Previously, Na(v)1.3 mRNA has been shown to be upregulated in rat DRG following peripheral nerve injury, unlike the downregulation of all other sodium channel transcripts. Here we show that the expression of Na(v)1.3 mRNA containing exons 17A and 17B is unchanged in mouse following peripheral nerve injury (axotomy), whereas total Na(v)1.3 mRNA expression is upregulated by 33% (P=0.003), suggesting differential regulation of the alternatively spliced transcripts. The alternatively spliced rodent exon sequences are highly conserved in both the human and chicken genomes, with 77-89% and 72-76% identities to mouse, respectively. The widespread conservation of these sequences strongly suggests an additional level of regulation in the expression of these channels, that is also tissue-specific.

Challenges of Diagnosis of Long-QT Syndrome in Children

We describe the clinical and genetic characteristics of the family, in which the diagnosis of LQT1 had been made. The electrocardiogram (ECG) characteristics of this patient indicated the likelihood of LQTS1. Polymorphic ventricular extrasystolies and episodes of polymorphic non-sustained ventricular tachycardia were confirmed by Holter ECG monitoring. On the exertional electrocardiogram polymorphic ventricular tachycardia (torsade de pointes) was recorded. Direct sequencing of both DNA strands revealed the absence of mutations or polymorphisms in the KCNQ1, HERG, and SCN5A genes.

Temperature dependence of erythromelalgia mutation L858F in sodium channel Nav1.7

Background

The disabling chronic pain syndrome erythromelalgia (also termed erythermalgia) is characterized by attacks of burning pain in the extremities induced by warmth. Pharmacological treatment is often ineffective, but the pain can be alleviated by cooling of the limbs. Inherited erythromelalgia has recently been linked to mutations in the gene SCN9A, which encodes the voltage-gated sodium channel Nav1.7. Nav1.7 is preferentially expressed in most nociceptive DRG neurons and in sympathetic ganglion neurons. It has recently been shown that several disease-causing erythromelalgia mutations alter channel-gating behavior in a manner that increases DRG neuron excitability.

Results

Here we tested the effects of temperature on gating properties of wild type Nav1.7 and mutant L858F channels. Whole-cell voltage-clamp measurements on wild type or L858F channels expressed in HEK293 cells revealed that cooling decreases current density, slows deactivation and increases ramp currents for both mutant and wild type channels. However, cooling differentially shifts the midpoint of steady-state activation in a depolarizing direction for L858F but not for wild type channels.

Conclusion

The cooling-dependent shift of the activation midpoint of L858F to more positive potentials brings the threshold of activation of the mutant channels closer to that of wild type Nav1.7 at lower temperatures, and is likely to contribute to the alleviation of painful symptoms upon cooling in affected limbs in patients with this erythromelalgia mutation.

Sodium channel kinetic changes that produce Brugada syndrome or progressive cardiac conduction system disease

Some mutations of the sodium channel gene NaV1.5 are multifunctional, causing combinations of LQTS, Brugada syndrome and progressive cardiac conduction system disease (PCCD). The combination of Brugada syndrome and PCCD is uncommon, although they both result from a reduction in the sodium current. We hypothesize that slow conduction is sufficient to cause S-T segment elevation and undertook a combined experimental and theoretical study to determine whether conduction slowing alone can produce the Brugada phenotype. Deletion of lysine 1479 in one of two positively charged clusters in the III/IV inter-domain linker causes both syndromes. We have examined the functional effects of this mutation using heterologous expression of the wild-type and mutant sodium channel in HEK-293-EBNA cells. We show that ΔK1479 shifts the potential of half-activation, V1/2m, to more positive potentials ( V1/2m = −36.8 ± 0.8 and −24.5 ± 1.3 mV for the wild-type and ΔK1479 mutant respectively, n = 11, 10). The depolarizing shift increases the extent of depolarization required for activation. The potential of half-inactivation, V1/2h, is also shifted to more positive potentials ( V1/2h = −85 ± 1.1 and −79.4 ± 1.2 mV for wild-type and ΔK1479 mutant respectively), increasing the fraction of channels available for activation. These shifts are quantitatively the same as a mutation that produces PCCD only, G514C. We incorporated experimentally derived parameters into a model of the cardiac action potential and its propagation in a one dimensional cable (simulating endo-, mid-myocardial and epicardial regions). The simulations show that action potential and ECG changes consistent with Brugada syndrome may result from conduction slowing alone; marked repolarization heterogeneity is not required. The findings also suggest how Brugada syndrome and PCCD which both result from loss of sodium channel function are sometimes present alone and at other times in combination.

Functional effects of the late sodium current inhibition by AZD7009 and lidocaine in rabbit isolated atrial and ventricular tissue and Purkinje fibre

AZD7009 (tert-Butyl-2-(7-[(2S)-3-(4-cyanophenoxy)-2-hydroxypropyl]-9-oxa-3,7-diazabicyclo[3.3.1]non-3-yl)ethylcarbamate) is an antiarrhythmic agent that increases atrial refractoriness, shows high antiarrhythmic efficacy and has low proarrhythmic potential. This study was primarily undertaken to determine the effects of AZD7009 on the late sodium current and to examine the impact of late sodium current inhibition on action potential duration in various myocardial cells. AZD7009 inhibited the late sodium current in Chinese Hamster Ovary K1 (CHO K1) cells expressing hNa(v)1.5 with an IC(50) of 11+/-2 microM. The late sodium current in isolated rabbit atrial and ventricular myocytes was also concentration dependently inhibited by AZD7009. Action potentials were recorded during exposure to 5 microM E-4031 (1-[2-(6-methyl-2pyridyl)ethyl]-4-(4-methylsulfonyl aminobenzoyl)piperidine), a compound that selectively inhibits the rapid delayed rectifier potassium current (I(Kr)), and to E-4031 in combination with AZD7009 or lidocaine in rabbit atrial and ventricular tissue and Purkinje fibres. In Purkinje fibres, but not in ventricular tissue, AZD7009 and lidocaine attenuated the E-4031-induced action potential duration prolongation. In atrial cells, AZD7009, but not lidocaine, further prolonged the E-4031-induced action potential duration. E-4031 induced early afterdepolarisations (EADs) in Purkinje fibres, EADs that were totally suppressed by AZD7009 or lidocaine. In conclusion, excessive action potential duration prolongation induced by E-4031 was attenuated by AZD7009 and lidocaine in rabbit Purkinje fibre, but not in atrial or ventricular tissue, most likely by inhibiting the late sodium current. Furthermore, the opposite effect by AZD7009 on action potential duration in atrial tissue suggests that AZD7009, in addition to inhibiting I(Kr), also inhibits other repolarising currents in the atria.

Diagnostic Value of Flecainide Testing in Unmasking SCN5A-Related Brugada Syndrome

INTRODUCTION

Provocation tests with sodium channel blockers are often required to unmask ECG abnormalities in Brugada syndrome (BrS). However, their diagnostic value is only partially established, while life-threatening ventricular arrhythmias during these tests were reported. We aimed to establish sensitivity, specificity, and safety of flecainide testing, and to predict a positive test outcome from the baseline ECG.

METHODS AND RESULTS

We performed 160 tests with flecainide in subjects determined to be at risk for BrS. P wave width, PQ duration, QRS width, S wave amplitude and duration in leads II-III, in addition to ST morphology and J point elevation in V1-V3 were measured before and after flecainide administration. Moreover, leads were positioned over the third intercostal space (V1(IC3)-V2(IC3)). Flecainide tests were considered positive if criteria from the First Consensus Report on BrS were fulfilled. In 64 cases, the test was positive, while 95 were negative (1 test was prematurely interrupted). The sensitivity and specificity, calculated in SCN5A-positive probands and their family members, were 77% and 80%, respectively. Baseline ECGs exhibited significant group differences in P, PQ, and QRS duration, J point elevation (leads V1-V2 and V1(IC3)-V2(IC3)), and S duration in II, but an attempt to predict the outcome of flecainide testing from these baseline ECG parameters failed. No malignant arrhythmias were observed.

CONCLUSION

Flecainide testing is a valid and safe tool to identify SCN5A-related BrS patients. Baseline ECGs do not predict test outcomes, but point to conduction slowing as a core mechanism in BrS.

The long QT syndrome family of cardiac ion channelopathies: A HuGE review*

Long QT syndrome (LQTS) refers to a group of "channelopathies"-disorders that affect cardiac ion channels. The "family" concept of syndromes has been applied to the multiple LQTS genotypes, LQT1-8, which exhibit converging mechanisms leading to QT prolongation and slowed ventricular repolarization. The 470+ allelic mutations induce loss-of-function in the passage of mainly K+ ions, and gain-of-function in the passage of Na+ ions through their respective ion channels. Resultant early after depolarizations can lead to a polymorphic form of ventricular tachycardia known as torsade de pointes, resulting in syncope, sudden cardiac death, or near-death (i.e., cardiac arrest aborted either spontaneously or with external defibrillation). LQTS may be either congenital or acquired. The genetic epidemiology of both forms can vary with subpopulation depending on the allele, but as a whole, LQTS appears in every corner of the globe. Many polymorphisms, such as HERG P448R and A915V in Asians, and SCN5A S1102Y in African Americans, show racial-ethnic specificity. At least nine genetic polymorphisms may enhance susceptibility to drug-induced arrhythmia (an "acquired" form of LQTS). Studies have generally demonstrated greater QT prolongation and more severe outcomes among adult females. Gene-gene interactions, e.g., between SCN5A Q1077del mutations and the SCN5A H558B polymorphism, have been shown to seriously reduce ion channel current. While phenotypic ascertainment remains a mainstay in the clinical setting, SSCP and DHPLC-aided DNA sequencing are a standard part of mutational investigation, and direct sequencing on a limited basis is now commercially available for patient diagnosis.

Molecular basis of isolated cardiac conduction disease

Cardiac conduction disorders are among the most common rhythm disturbances causing disability in millions of people worldwide and necessitating pacemaker implantation. Isolated cardiac conduction disease (ICCD) can affect various regions within the heart, and therefore the clinical features also vary from case to case. Typically, it is characterized by progressive alteration of cardiac conduction through the atrioventricular node, His-Purkinje system, with right or left bundle branch block and QRS widening. In some instances, the disorder may progress to complete atrioventricular block, with syncope and even death. While the role of genetic factors in conduction disease has been suggested as early as the 1970s, it was only recently that specific genetic loci have been reported. Multiple mutations in the gene encoding for the cardiac voltage-gated sodium channel (SCN5A), which plays a fundamental role in the initiation, propagation, and maintenance of normal cardiac rhythm, have been linked to conduction disease, allowing for genotype-phenotype correlation. The electrophysiological characterization of heterologously expressed mutant Na+ channels has revealed gating defects that consistently lead to a loss of channel function. However, studies have also revealed significant overlap between aberrant rhythm phenotypes, and single mutations have been identified that evoke multiple distinct rhythm disorders with common gating lesions. These new insights highlight the complexities involved in linking single mutations, ion-channel behavior, and cardiac rhythm but suggest that interplay between multiple factors could underlie the manifestation of the disease phenotype.

Variable Electrocardiographic Effects of Short-Term Quinidine Sulfate Administration in Brugada Syndrome

Quinidine, a class I antiarrhythmic agent with blocking property of transient outward current, is a possible candidate for the suppression of ventricular fibrillation in patients with Brugada syndrome; although there is a concern that its ability to these effects may be proarrhythmic. Therefore, we evaluated the effect of quinidine sulfate on ST-segment elevation in Brugada syndrome. In 8 patients with Brugada syndrome, the magnitude of ST-elevation at the J-point (ST(J)), and the ST-segment configuration in leads V1-V3, were compared before and on day 2 after the initiation of quinidine administration. In 3 patients, quinidine attenuated ST(J) by > or = 0.1 mV. Of these 3 patients, ST-segment elevation was normalized in 2 patients, while the ST-segment configuration was unchanged in another. In another 3 patients, quinidine augmented ST(J) by > or = 0.1 mV without any change of ST-segment configuration, and the augmentation was returned to baseline after the discontinuation of quinidine. Quinidine exhibited no effect on the ST-segment in the remaining 2 patients. The favorable effects of quinidine on the ST-segment tended to be more pronounced in patients with prominent ST-elevation at baseline. In 1 patient, quinidine was effective in eliminating both ST-segment elevation and repetitive tachyarrhythmia episodes. In conclusion, the effects of quinidine on ST-segment elevation were variable. Quinidine may potentially augment the ST-segment elevation in some patients with Brugada syndrome.

Gene-environment interactions: implications for sudden unexpected deaths in infancy

From the perspective of systems biology, genes and proteins interact to produce complex networks, which in turn interact with the environment to influence every aspect of our biological lives. Recent advances in molecular genetics and the identification of gene polymorphisms in victims of sudden infant death syndrome (SIDS) are helping us better to understand that SIDS, like all other human conditions in health and disease, represents the confluence of specific environmental risk factors interacting in complex ways with specific polymorphisms to yield phenotypes susceptible to sudden and unexpected death in infancy. Failure to consider both genetic and environmental risk factors will impede research progress.

Effects of Sodium-Calcium Exchange Inhibitors, KB-R7943 and SEA0400, on Aconitine-Induced Arrhythmias in Guinea Pigs in Vivo, in Vitro, and in Computer Simulation Studies

The sodium-calcium exchange (NCX) plays a pivotal role in regulating contractility and electrical activity in the heart. However, the effects of NCX blockers on ventricular arrhythmias are still controversial. We examined the effects of KB-R7943 (KBR) and SEA0400 (SEA), two NCX blockers, on aconitine-induced arrhythmias in guinea pigs using the ECG recordings and the current-clamp method. Using Luo's and Rudy's computer model (1991 Circ Res 68:1501-1526) for ventricular myocytes, we simulated abnormal membrane activity produced by NCX inhibition. In the whole-animal model, KBR in a dose range of 1 to 30 mg/kg (intravenous) suppressed aconitine-induced arrhythmias dose-dependently, but 10 mg/kg of SEA did not suppress these arrhythmias. There was a difference in isolated ventricular myocytes also. KBR (10 microM) suppressed abnormal electrical activity induced by aconitine, but SEA (100 microM) did not show such effects. KBR (10 microM) significantly changed the shape of the action potential configurations (action potential duration at 50% repolarization), but SEA (1-100 microM) did not change these configurations. In the computer simulation study, the aconitine-induced abnormal electrical activity was mimicked by a negative shift of the kinetics of Na+ channels, and this was followed by additional suppression of NCX activity by 90% (mimicking the effect of NCX inhibitors), which enhanced abnormal membrane activity. Our results indicate that the inhibition of aconitine-induced arrhythmias by KBR, not by SEA, might result from a mechanism other than the inhibition of NCX, and thus the involvement of the NCX system plays an insignificant role in the aconitine-induced arrhythmias.