A novel SCN5A mutation manifests as a malignant form of long QT syndrome with perinatal onset of tachycardia/bradycardia

Biophysical mechanisms of myocardium sodium channelopathies

Genetic variants of gene SCN5A encoding the alpha-subunit of cardiac voltage-gated sodium channel Nav1.5 are associated with various diseases, including long QT syndrome (LQT3), Brugada syndrome (BrS1), and progressive cardiac conduction disease (PCCD). In the last decades, the great progress in understanding molecular and biophysical mechanisms of these diseases has been achieved. The LQT3 syndrome is associated with gain-of-function of sodium channels Nav1.5 due to impaired inactivation, enhanced activation, accelerated recovery from inactivation or the late current appearance. In contrast, BrS1 and PCCD are associated with the Nav1.5 loss-of-function, which in electrophysiological experiments can be manifested as reduced current density, enhanced fast or slow inactivation, impaired activation, or decelerated recovery from inactivation. Genetic variants associated with congenital arrhythmias can also disturb interactions of the Nav1.5 channel with different proteins or drugs and cause unexpected reactions to drug administration. Furthermore, mutations can affect post-translational modifications of the channels and their sensitivity to pH and temperature. Here we briefly review the current knowledge on biophysical mechanisms of LQT3, BrS1 and PCCD. We focus on limitations of studies that use heterologous expression systems and induced pluripotent stem cells (iPSC) derived cardiac myocytes and summarize our understanding of genotype-phenotype relations of SCN5A mutations.

2023 HRS expert consensus statement on the management of arrhythmias during pregnancy

This international multidisciplinary expert consensus statement is intended to provide comprehensive guidance that can be referenced at the point of care to cardiac electrophysiologists, cardiologists, and other health care professionals, on the management of cardiac arrhythmias in pregnant patients and in fetuses. This document covers general concepts related to arrhythmias, including both brady- and tachyarrhythmias, in both the patient and the fetus during pregnancy. Recommendations are provided for optimal approaches to diagnosis and evaluation of arrhythmias; selection of invasive and noninvasive options for treatment of arrhythmias; and disease- and patient-specific considerations when risk stratifying, diagnosing, and treating arrhythmias in pregnant patients and fetuses. Gaps in knowledge and new directions for future research are also identified.

Modeling incomplete penetrance in long QT syndrome type 3 through ion channel heterogeneity: an in silico population study

Many cardiac diseases are characterized by an increased late sodium current, including heart failure, hypertrophic cardiomyopathy, and inherited long QT syndrome type 3 (LQT3). The late sodium current in LQT3 is caused by a gain-of-function mutation in the voltage-gated sodium channel Nav1.5. Despite a well-defined genetic cause of LQT3, treatment remains inconsistent because of incomplete penetrance of the mutation and variability of antiarrhythmic efficacy. Here, we investigate the relationship between LQT3-associated mutation incomplete penetrance and variability in ion channel expression, simulating a population of 1,000 individuals with the O'Hara-Rudy model of the human ventricular myocyte. We first simulate healthy electrical activity (i.e., in the absence of a mutation) and then incorporate heterozygous expression for three LQT3-associated mutations (Y1795C, I1768V, and ΔKPQ), to directly compare the effects of each mutation on individuals across a diverse population. For all mutations, we find that susceptibility, defined by either the presence of an early afterdepolarization (EAD) or prolonged action potential duration (APD), primarily depends on the balance between the conductance of IKr and INa, for which individuals with a higher IKr-to-INa ratio are less susceptible. Furthermore, we find distinct differences across the population, observing individuals susceptible to zero, one, two, or all three mutations. Individuals tend to be less susceptible with an appropriate balance of repolarizing currents, typically via increased IKs or IK1. Interestingly, the more critical repolarizing current is mutation specific. We conclude that the balance between key currents plays a significant role in mutant-specific presentation of the disease phenotype in LQT3.NEW & NOTEWORTHY An in silico population approach investigates the relationship between variability in ion channel expression and gain-of-function mutations in the voltage-gated sodium channel associated with the congenital disorder long QT syndrome type 3 (LQT3). We find that ion channel variability can contribute to incomplete penetrance of the mutation, with mutant-specific differences in ion channel conductances leading to susceptibility to proarrhythmic action potential duration prolongation or early afterdepolarizations.

Fetal Bradycardia Caused by Monogenic Disorders-A Review of the Literature

Introduction: The standard obstetric definition of fetal bradycardia is a sustained fetal heart rate < 110 bpm over at least 10 min. Fetal bradycardia can be the first and only prenatal presentation of a heart disease. We present an overview on different genetic disorders that should be taken into consideration in case of diagnosed fetal bradycardia. Methods: A literature review was conducted using a PubMed- and OMIM-based search for monogenetic disorders causing fetal bradycardia in September 2022. Results: The review on the literature identified nine monogenic diseases that could lead to fetal bradycardia. Four of these disorders can be associated with extracardiac findings. Discussion: Genetic testing should be considered in cases with fetal bradycardia, especially in cases of additional extracardiac findings. Broad sequencing techniques and improved prenatal phenotyping could help to establish a diagnosis in an increasing number of cases.

Effectiveness and safety of mexiletine in patients at risk for (recurrent) ventricular arrhythmias: a systematic review

While mexiletine has been used for over 40 years for prevention of (recurrent) ventricular arrhythmias and for myotonia, patient access has recently been critically endangered. Here we aim to demonstrate the effectiveness and safety of mexiletine in the treatment of patients with (recurrent) ventricular arrhythmias, emphasizing the absolute necessity of its accessibility. Studies were included in this systematic review (PROSPERO, CRD42020213434) if the efficacy or safety of mexiletine in any dose was evaluated in patients at risk for (recurrent) ventricular arrhythmias with or without comparison with alternative treatments (e.g. placebo). A systematic search was performed in Ovid MEDLINE, Embase, and in the clinical trial registry databases ClinicalTrials.gov and ICTRP. Risk of bias were assessed and tailored to the different study designs. Large heterogeneity in study designs and outcome measures prompted a narrative synthesis approach. In total, 221 studies were included reporting on 8970 patients treated with mexiletine. Age ranged from 0 to 88 years. A decrease in ventricular arrhythmias of >50% was observed in 72% of the studies for pre-mature ventricular complexes, 64% for ventricular tachycardia, and 33% for ventricular fibrillation. Electrocardiographic effects of mexiletine were small; only in a subset of patients with primary arrhythmia syndromes, a relative (desired) QTc decrease was reproducibly observed. As for adverse events, gastrointestinal complaints were most frequently observed (33% of the patients). In this systematic review, we present all the currently available knowledge of mexiletine in patients at risk for (recurrent) ventricular arrhythmias and show that mexiletine is both effective and safe.

Fenestropathy of Voltage-Gated Sodium Channels

Voltage-gated sodium channels (Na_v) are responsible for the initiation and propagation of action potentials in excitable cells. From pain to heartbeat, these integral membrane proteins are the ignition stations for every sensation and action in human bodies. They are large (>200 kDa, 24 transmembrane helices) multi-domain proteins that couple changes in membrane voltage to the gating cycle of the sodium-selective pore. Na_v mutations lead to a multitude of diseases - including chronic pain, cardiac arrhythmia, muscle illnesses, and seizure disorders - and a wide variety of currently used therapeutics block Na_v. Despite this, the mechanisms of action of Na_v blocking drugs are only modestly understood at this time and many questions remain to be answered regarding their state- and voltage-dependence, as well as the role of the hydrophobic membrane access pathways, or fenestrations, in drug ingress or egress. Na_v fenestrations, which are pathways that connect the plasma membrane to the central cavity in the pore domain, were discovered through functional studies more than 40 years ago and once thought to be simple pathways. A variety of recent genetic, structural, and pharmacological data, however, shows that these fenestrations are actually key functional regions of Na_v that modulate drug binding, lipid binding, and influence gating behaviors. We discovered that some of the disease mutations that cause arrhythmias alter amino acid residues that line the fenestrations of Nav1.5. This indicates that fenestrations may play a critical role in channel’s gating, and that individual genetic variation may also influence drug access through the fenestrations for resting/inactivated state block. In this review, we will discuss the channelopathies associated with these fenestrations, which we collectively name “Fenestropathy,” and how changes in the fenestrations associated with the opening of the intracellular gate could modulate the state-dependent ingress and egress of drugs binding in the central cavity of voltage gated sodium channels.

Mexiletine infusion challenge test for neonatal long QT syndrome with 2:1 atrioventricular block

For applying a genotype-based treatment in neonatal long QT syndrome (LQTS), early detection of the genotype becomes an important issue. We report a case of a neonate with LQTS type 3 that presented with 2:1 atrioventricular block and underwent a mexiletine infusion challenge test, and achieved shortening of the QTc and 1:1 atrioventricular conduction. The mexiletine infusion challenge test was helpful to make an early detection of the genotype of the LQTS and predicted the drug efficacy in a neonatal patient.

Distinct modulation of inactivation by a residue in the pore domain of voltage-gated Na+ channels: mechanistic insights from recent crystal structures

Inactivation of voltage-gated Na⁺ channels (VGSC) is essential for the regulation of cellular excitability. The molecular rearrangement underlying inactivation is thought to involve the intracellular linker between domains III and IV serving as inactivation lid, the receptor for the lid (domain III S4-S5 linker) and the pore-lining S6 segements. To better understand the role of the domain IV S6 segment in inactivation we performed a cysteine scanning mutagenesis of this region in rNav 1.4 channels and screened the constructs for perturbations in the voltage-dependence of steady state inactivation. This screen was performed in the background of wild-type channels and in channels carrying the mutation K1237E, which profoundly alters both permeation and gating-properties. Of all tested constructs the mutation I1581C was unique in that the mutation-induced gating changes were strongly influenced by the mutational background. This suggests that I1581 is involved in specific short-range interactions during inactivation. In recently published crystal structures VGSCs the respective amino acids homologous to I1581 appear to control a bend of the S6 segment which is critical to the gating process. Furthermore, I1581 may be involved in the transmission of the movement of the DIII voltage-sensor to the domain IV S6 segment.

Tachycardia-bradycardia syndrome: Electrophysiological mechanisms and future therapeutic approaches (Review)

Sick sinus syndrome (SSS) encompasses a group of disorders whereby the heart is unable to perform its pacemaker function, due to genetic and acquired causes. Tachycardia‑bradycardia syndrome (TBS) is a complication of SSS characterized by alternating tachycardia and bradycardia. Techniques such as genetic screening and molecular diagnostics together with the use of pre-clinical models have elucidated the electrophysiological mechanisms of this condition. Dysfunction of ion channels responsible for initiation or conduction of cardiac action potentials may underlie both bradycardia and tachycardia; bradycardia can also increase the risk of tachycardia, and vice versa. The mainstay treatment option for SSS is pacemaker implantation, an effective approach, but has disadvantages such as infection, limited battery life, dislodgement of leads and catheters to be permanently implanted in situ. Alternatives to electronic pacemakers are gene‑based bio‑artificial sinoatrial node and cell‑based bio‑artificial pacemakers, which are promising techniques whose long-term safety and efficacy need to be established. The aim of this article is to review the different ion channels involved in TBS, examine the three‑way relationship between ion channel dysfunction, tachycardia and bradycardia in TBS and to consider its current and future therapies.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

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

Early somatic mosaicism is a rare cause of long-QT syndrome

Somatic mosaicism, the occurrence and propagation of genetic variation in cell lineages after fertilization, is increasingly recognized to play a causal role in a variety of human diseases. We investigated the case of life-threatening arrhythmia in a 10-day-old infant with long QT syndrome (LQTS). Rapid genome sequencing suggested a variant in the sodium channel Na_V1.5 encoded by SCN5A, NM_000335:c.5284G > T predicting p.(V1762L), but read depth was insufficient to be diagnostic. Exome sequencing of the trio confirmed read ratios inconsistent with Mendelian inheritance only in the proband. Genotyping of single circulating leukocytes demonstrated the mutation in the genomes of 8% of patient cells, and RNA sequencing of cardiac tissue from the infant confirmed the expression of the mutant allele at mosaic ratios. Heterologous expression of the mutant channel revealed significantly delayed sodium current with a dominant negative effect. To investigate the mechanism by which mosaicism might cause arrhythmia, we built a finite element simulation model incorporating Purkinje fiber activation. This model confirmed the pathogenic consequences of cardiac cellular mosaicism and, under the presenting conditions of this case, recapitulated 2:1 AV block and arrhythmia. To investigate the extent to which mosaicism might explain undiagnosed arrhythmia, we studied 7,500 affected probands undergoing commercial gene-panel testing. Four individuals with pathogenic variants arising from early somatic mutation events were found. Here we establish cardiac mosaicism as a causal mechanism for LQTS and present methods by which the general phenomenon, likely to be relevant for all genetic diseases, can be detected through single-cell analysis and next-generation sequencing.

Physiological and Pathophysiological Insights of Nav1.4 and Nav1.5 Comparison

Mutations in Nav1.4 and Nav1.5 α-subunits have been associated with muscular and cardiac channelopathies, respectively. Despite intense research on the structure and function of these channels, a lot of information is still missing to delineate the various physiological and pathophysiological processes underlying their activity at the molecular level. Nav1.4 and Nav1.5 sequences are similar, suggesting structural and functional homologies between the two orthologous channels. This also suggests that any characteristics described for one channel subunit may shed light on the properties of the counterpart channel subunit. In this review article, after a brief clinical description of the muscular and cardiac channelopathies related to Nav1.4 and Nav1.5 mutations, respectively, we compare the knowledge accumulated in different aspects of the expression and function of Nav1.4 and Nav1.5 α-subunits: the regulation of the two encoding genes (SCN4A and SCN5A), the associated/regulatory proteins and at last, the functional effect of the same missense mutations detected in Nav1.4 and Nav1.5. First, it appears that more is known on Nav1.5 expression and accessory proteins. Because of the high homologies of Nav1.5 binding sites and equivalent Nav1.4 sites, Nav1.5-related results may guide future investigations on Nav1.4. Second, the analysis of the same missense mutations in Nav1.4 and Nav1.5 revealed intriguing similarities regarding their effects on membrane excitability and alteration in channel biophysics. We believe that such comparison may bring new cues to the physiopathology of cardiac and muscular diseases.

The genetic basis for inherited forms of sinoatrial dysfunction and atrioventricular node dysfunction

The sinoatrial node (SAN) and the atrioventricular node (AVN) are the anatomical and functional regions of the heart which play critical roles in the generation and conduction of the electrical impulse. Their functions are ensured by peculiar structural cytological properties and specific collections of ion channels. Impairment of SAN and AVN activity is generally acquired,but in some cases familial inheritance has been established and therefore a genetic cause is involved. In recent years, combined efforts of clinical practice and experimental basic science studies have identified and characterized several causative gene mutations associated with the nodal syndromes. Channelopathies, i.e., diseases associated with defective ion channels, remain the major cause of genetically determined nodal arrhythmias; however, it is becoming increasingly evident that mutations in other classes of regulatory and structural proteins also have profound pathophysiological roles. In this review, we will present some aspects of the genetic identification of the molecular mechanism underlying both SAN and AVN dysfunctions with a particular focus on mutations of the Na, pacemaker (HCN), and Ca channels. Genetic defects in regulatory proteins and calcium-handling proteins will be also considered. In conclusion, the identification of the genetic defects associated with familial nodal dysfunction is an essential step for implementing an appropriate therapeutic treatment.

In silico assessment of drug safety in human heart applied to late sodium current blockers

Drug-induced action potential (AP) prolongation leading to Torsade de Pointes is a major concern for the development of anti-arrhythmic drugs. Nevertheless the development of improved anti-arrhythmic agents, some of which may block different channels, remains an important opportunity. Partial block of the late sodium current (I(NaL)) has emerged as a novel anti-arrhythmic mechanism. It can be effective in the settings of free radical challenge or hypoxia. In addition, this approach can attenuate pro-arrhythmic effects of blocking the rapid delayed rectifying K(+) current (I(Kr)). The main goal of our computational work was to develop an in-silico tool for preclinical anti-arrhythmic drug safety assessment, by illustrating the impact of I(Kr)/I(NaL) ratio of steady-state block of drug candidates on "torsadogenic" biomarkers. The O'Hara et al. AP model for human ventricular myocytes was used. Biomarkers for arrhythmic risk, i.e., AP duration, triangulation, reverse rate-dependence, transmural dispersion of repolarization and electrocardiogram QT intervals, were calculated using single myocyte and one-dimensional strand simulations. Predetermined amounts of block of I(NaL) and I(Kr) were evaluated. "Safety plots" were developed to illustrate the value of the specific biomarker for selected combinations of IC(50)s for I(Kr) and I(NaL) of potential drugs. The reference biomarkers at baseline changed depending on the "drug" specificity for these two ion channel targets. Ranolazine and GS967 (a novel potent inhibitor of I(NaL)) yielded a biomarker data set that is considered safe by standard regulatory criteria. This novel in-silico approach is useful for evaluating pro-arrhythmic potential of drugs and drug candidates in the human ventricle.

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.

Modeling type 3 long QT syndrome with cardiomyocytes derived from patient-specific induced pluripotent stem cells

BACKGROUND

Type 3 long QT syndrome (LQT3) is the third most common form of LQT syndrome and is characterized by QT-interval prolongation resulting from a gain-of-function mutation in SCN5A. We aimed to establish a patient-specific human induced pluripotent stem cell (hiPSC) model of LQT3, which could be used for future drug testing and development of novel treatments for this inherited disorder.

METHODS AND RESULTS

Dermal fibroblasts obtained from a patient with LQT3 harboring a SCN5A mutation (c.5287G>A; p.V1763M) were reprogrammed to hiPSCs via repeated transfection of mRNA encoding OCT-4, SOX-2, KLF-4, C-MYC and LIN-28. hiPSC-derived cardiomyocytes (hiPSC-CMs) were obtained via cardiac differentiation. hiPSC-CMs derived from the patient's healthy sister were used as a control. Compared to the control, patient hiPSC-CMs exhibited dominant mutant SCN5A allele gene expression, significantly prolonged action potential duration or APD (paced CMs of control vs. patient: 226.50 ± 17.89 ms vs. 536.59 ± 37.1 ms; mean ± SEM, p < 0.005), an increased tetrodotoxin (TTX)-sensitive late or persistent Na(+) current (control vs. patient: 0.65 ± 0.11 vs. 3.16 ± 0.27 pA/pF; n = 9, p < 0.01), a positive shift of steady state inactivation and a faster recovery from inactivation. Mexiletine, a NaV1.5 blocker, reversed the elevated late Na(+) current and prolonged APD in LQT3 hiPSC-CMs.

CONCLUSIONS

We demonstrate that hiPSC-CMs derived from a LQT3 patient recapitulate the biophysical abnormalities that define LQT3. The clinical significance of such an in vitro model is in the development of novel therapeutic strategies and a more personalized approach in testing drugs on patients with LQT3.

Impact of genetics on the clinical management of channelopathies

There are few areas in cardiology in which the impact of genetics and genetic testing on clinical management has been as great as in cardiac channelopathies, arrhythmic disorders of genetic origin related to the ionic control of the cardiac action potential. Among the growing number of diseases identified as channelopathies, 3 are sufficiently prevalent to represent significant clinical and societal problems and to warrant adequate understanding by practicing cardiologists: long QT syndrome, catecholaminergic polymorphic ventricular tachycardia, and Brugada syndrome. This review will focus selectively on the impact of genetic discoveries on clinical management of these 3 diseases. For each disorder, we will discuss to what extent genetic knowledge and clinical genetic test results modify the way cardiologists should approach and manage affected patients. We will also address the optimal use of genetic testing, including its potential limitations and the potential medico-legal implications when such testing is not performed. We will highlight how important it is to understand the ways that genotype can affect clinical manifestations, risk stratification, and responses to the therapy. We will also illustrate the close bridge between molecular biology and clinical medicine, and will emphasize that consideration of the genetic basis for these heritable arrhythmia syndromes and the proper use and interpretation of clinical genetic testing should remain the standard of care.

Developmentally regulated SCN5A splice variant potentiates dysfunction of a novel mutation associated with severe fetal arrhythmia

BACKGROUND

Congenital long-QT syndrome (LQTS) may present during fetal development and can be life-threatening. The molecular mechanism for the unusual early onset of LQTS during fetal development is unknown.

OBJECTIVE

We sought to elucidate the molecular basis for severe fetal LQTS presenting at 19 weeks' gestation, the earliest known presentation of this disease.

METHODS

Fetal magnetocardiography was used to demonstrated torsades de pointes and a prolonged rate-corrected QT interval. In vitro electrophysiological studies were performed to determine functional consequences of a novel SCN5A mutation found in the fetus.

RESULTS

The fetus presented with episodes of ventricular ectopy progressing to incessant ventricular tachycardia and hydrops fetalis. Genetic analysis disclosed a novel, de novo heterozygous mutation (L409P) and a homozygous common variant (R558 in SCN5A). In vitro electrophysiological studies demonstrated that the mutation in combination with R558 caused significant depolarized shifts in the voltage dependence of inactivation and activation, faster recovery from inactivation, and a 7-fold higher level of persistent current. When the mutation was engineered in a fetal-expressed SCN5A splice isoform, channel dysfunction was markedly potentiated. Also, R558 alone in the fetal splice isoform evoked a large persistent current, and hence both alleles were dysfunctional.

CONCLUSION

We report the earliest confirmed diagnosis of symptomatic LQTS and present evidence that mutant cardiac sodium channel dysfunction is potentiated by a developmentally regulated alternative splicing event in SCN5A. Our findings provide a plausible mechanism for the unusual severity and early onset of cardiac arrhythmia in fetal LQTS.

Multiple arrhythmic syndromes in a newborn, owing to a novel mutation in SCN5A

BACKGROUND

Mutations in the SCN5A gene have been linked to Brugada syndrome (BrS), conduction disease, Long QT syndrome (LQT3), atrial fibrillation (AF), and to pre- and neonatal ventricular arrhythmias.

OBJECTIVE

The objective of this study is to characterize a novel mutation in Na(v)1.5 found in a newborn with fetal chaotic atrial tachycardia, post-partum intraventricular conduction delay, and QT interval prolongation.

METHODS

Genomic DNA was isolated and all exons and intron borders of 15 ion-channel genes were sequenced, revealing a novel missense mutation (Q270K) in SCN5A. Na(v)1.5 wild type (WT) and Q270K were expressed in CHO-K1 with and without the Na(v)β1 subunit. Results. Patch-clamp analysis showed ∼40% reduction in peak sodium channel current (I(Na)) density for Q270K compared with WT. Fast and slow decay of I(Na) were significantly slower in Q270K. Steady-state activation and inactivation of Q270K channels were shifted to positive potentials, and window current was increased. The tetrodotoxin-sensitive late I(Na) was increased almost 3-fold compared with WT channels. Ranolazine reduced late I(Na) in WT and Q270K channels, while exerting minimal effects on peak I(Na).

CONCLUSION

The Q270K mutation in SCN5A reduces peak I(Na) while augmenting late I(Na), and may thus underlie the development of atrial tachycardia, intraventricular conduction delay, and QT interval prolongation in an infant.

Y1767C, a novel SCN5A mutation, induces a persistent Na+ current and potentiates ranolazine inhibition of Nav1.5 channels

Long QT syndrome type 3 (LQT3) has been traced to mutations of the cardiac Na(+) channel (Na(v)1.5) that produce persistent Na(+) currents leading to delayed ventricular repolarization and torsades de pointes. We performed mutational analyses of patients suffering from LQTS and characterized the biophysical properties of the mutations that we uncovered. One LQT3 patient carried a mutation in the SCN5A gene in which the cysteine was substituted for a highly conserved tyrosine (Y1767C) located near the cytoplasmic entrance of the Na(v)1.5 channel pore. The wild-type and mutant channels were transiently expressed in tsA201 cells, and Na(+) currents were recorded using the patch-clamp technique. The Y1767C channel produced a persistent Na(+) current, more rapid inactivation, faster recovery from inactivation, and an increased window current. The persistent Na(+) current of the Y1767C channel was blocked by ranolazine but not by many class I antiarrhythmic drugs. The incomplete inactivation, along with the persistent activation of Na(+) channels caused by an overlap of voltage-dependent activation and inactivation, known as window currents, appeared to contribute to the LQTS phenotype in this patient. The blocking effect of ranolazine on the persistent Na(+) current suggested that ranolazine may be an effective therapeutic treatment for patients with this mutation. Our data also revealed the unique role for the Y1767 residue in inactivating and forming the intracellular pore of the Na(v)1.5 channel.

Atrioventricular block-induced Torsades de Pointes with clinical and molecular backgrounds similar to congenital long QT syndrome

BACKGROUND

Atrioventricular block (AVB) sometimes complicates QT prolongation and torsades de pointes (TdP).

METHODS AND RESULTS

The clinical and genetic background of 14 AVB patients (57±21 years, 13 females) who developed QT prolongation and TdP was analyzed. Electrophysiological characteristics of mutations were analyzed using heterologous expression in Chinese hamster ovary cells, together with computer simulation models. Every patient received a pacemaker or implantable cardioverter defibrillator; 3 patients had recurrence of TdP during follow-up because of pacing failure. Among the ECG parameters, QTc interval was prolonged to 561±76ms in the presence of AVB, but shortened to 495±42ms in the absence of AVB. Genetic screening for KCNQ1, KCNH2, SCN5A, KCNE1, and KCNE2 revealed four heterozygous missense mutations of KCNQ1 or KCNH2 in 4 patients (28.6%). Functional analyses showed that all mutations had loss of functions and various gating dysfunctions of I(Ks) or I(Kr). Finally, action potential simulation based on the Luo-Rudy model demonstrated that most mutant channels induced bradycardia-related early afterdepolarizations.

CONCLUSIONS

Incidental AVB, as a trigger of TdP, can manifest as clinical phenotypes of long QT syndrome (LQTS), and that some patients with AVB-induced TdP share a genetic background with those with congenital LQTS.

Autonomic modulation and antiarrhythmic therapy in a model of long QT syndrome type 3

Aims

Clinical observations in patients with long QT syndrome carrying sodium channel mutations (LQT3) suggest that bradycardia caused by parasympathetic stimulation may provoke torsades de pointes (TdP). β-Adrenoceptor blockers appear less effective in LQT3 than in other forms of the disease.

Methods and results

We studied effects of autonomic modulation on arrhythmias in vivo and in vitro and quantified sympathetic innervation by autoradiography in heterozygous mice with a knock-in deletion (ΔKPQ) in the Scn5a gene coding for the cardiac sodium channel and increased late sodium current (LQT3 mice). Cholinergic stimulation by carbachol provoked bigemini and TdP in freely roaming LQT3 mice. No arrhythmias were provoked by physical stress, mental stress, isoproterenol, or atropine. In isolated, beating hearts, carbachol did not prolong action potentials per se, but caused bradycardia and rate-dependent action potential prolongation. The muscarinic inhibitor AFDX116 prevented effects of carbachol on heart rate and arrhythmias. β-Adrenoceptor stimulation suppressed arrhythmias, shortened rate-corrected action potential duration, increased rate, and minimized difference in late sodium current between genotypes. β-Adrenoceptor density was reduced in LQT3 hearts. Acute β-adrenoceptor blockade by esmolol, propranolol or chronic propranolol in vivo did not suppress arrhythmias. Chronic flecainide pre-treatment prevented arrhythmias (all P < 0.05).

Conclusion

Cholinergic stimulation provokes arrhythmias in this model of LQT3 by triggering bradycardia. β-Adrenoceptor density is reduced, and β-adrenoceptor blockade does not prevent arrhythmias. Sodium channel blockade and β-adrenoceptor stimulation suppress arrhythmias by shortening repolarization and minimizing difference in late sodium current.

Sodium channel mutations and arrhythmias

Since the identification of the first SCN5A mutation associated with long QT syndrome in 1995, several mutations in this gene for the alpha subunit of the cardiac sodium channel have been identified in a heterogeneous subset of cardiac rhythm syndromes, including Brugada syndrome, progressive cardiac conduction defect, sick sinus node syndrome, atrial fibrillation and dilated cardiomyopathy. Robust clinical evidence has been accompanied by bench studies performed in different models spanning from in vitro expression systems to transgenic mice. Together, these studies have helped establish genotype-phenotype correlations and have shaped our understanding of the role of the cardiac sodium channel in health and in disease. Remarkably, these advances in understanding have impacted on clinical management by allowing us to start developing gene-specific risk stratification schemes and mutation-specific management strategies. In this Review, we summarize the current understanding of the molecular mechanism of SCN5A-associated inherited arrhythmias, focusing on the most recent development of mutation-specific management in SCN5A-associated long QT syndrome type 3. We also briefly discuss arrhythmia-causing mutations in the genes encoding the beta subunit of the cardiac sodium channel and in those encoding proteins in the associated macromolecular complex.

A novel SCN5A mutation associated with the linker between III and IV domains of Nav1.5 in a neonate with fatal long QT syndrome

A male newborn weighing 2334 g was delivered at 37 weeks of gestation by caesarean section because of prenatal ultrasound findings of fetal hydrops with atrioventricualr block, ventriucular tachycardia (VT), and impaired ventricular function. In spite of the intravenous administration of lidocaine, VT continued. He developed poor perfusion and systemic hypotension. After the intravenous administration of amiodarone, VT was terminated. The electrocardiogram revealed an extremely prolonged corrected QT interval (860 ms) with 2:1 atrioventricular block. Unfortunately, he died 18 h after birth in spite of the administration of lidocaine, beta-blocker and magnesium. Mutational analysis identified a novel heterozygous de novo mutation (F1486del) in SCN5A. This mutation is associated with the IFM motif in the linker between III and IV domains of Na(v)1.5, which serves as an inactivation particle binding within the pore of sodium channels. This report demonstrates an interesting relationship between the clinical phenotype and the location of the mutation in long QT syndrome.

Phenotypic overlap of cardiac sodium channelopathies: individual-specific or mutation-specific?

Mutations in the cardiac sodium channel gene SCN5A are responsible for a spectrum of hereditary arrhythmias, including type-3 long QT syndrome (LQT3), Brugada syndrome (BrS), conduction disturbance and sinus node dysfunction. These syndromes were originally regarded as independent entities with distinct clinical manifestations and biophysical properties, but recent evidence shows considerable clinical overlap, implying a new disease entity known as an overlap syndrome of cardiac sodium channelopathy. Class IC sodium-channel blockers often induced the BrS phenotype in some patients with LQT3, confirming the clinical overlap of LQT3 and BrS. It also raises a concern about the safety of the class IC drug and questions about the determinants of overlap. Here, an overview is given of current knowledge on the clinical features, prevalence, and molecular and biophysical mechanisms underlying overlap syndrome to gain more insight into this complex issue and generate better therapeutic strategies for patient management.

Prenatal diagnosis and management of fetal Long QT syndrome

This report describes a fetus presenting with second-degree atrioventricular block, sinus bradycardia, and transient ventricular tachycardia with ventriculoatrial dissociation. Long QT syndrome (LQTS) was suspected due to the association of heart rhythm disturbances and very short transmitral early deceleration time. This impaired relaxation of the left ventricle was explained by the extreme prolongation of the refractory period caused by the prolonged relaxation time. The infant was treated successfully with beta-blockers and implantation of a pacemaker. The prognosis is poor when LQTS presents utero or during the first week of life. To date, only a few case reports of a fetus with LQTS have been published.

Malignant perinatal variant of long-QT syndrome caused by a profoundly dysfunctional cardiac sodium channel

BACKGROUND

Inherited cardiac arrhythmia susceptibility contributes to sudden death during infancy and may contribute to perinatal and neonatal mortality, but the molecular basis of this risk and the relationship to genetic disorders presenting later in life is unclear. We studied the functional and pharmacological properties of a novel de novo cardiac sodium channel gene (SCN5A) mutation associated with an extremely severe perinatal presentation of long-QT syndrome in unrelated probands of different ethnicity.

METHODS AND RESULTS

Two subjects exhibiting severe fetal and perinatal ventricular arrhythmias were screened for SCN5A mutations, and the functional properties of a novel missense mutation (G1631D) were determined by whole-cell patch clamp recording. In vitro electrophysiological studies revealed a profound defect in sodium channel function characterized by approximately 10-fold slowing of inactivation, increased persistent current, slowing of recovery from inactivation, and depolarized voltage dependence of activation and inactivation. Single-channel recordings demonstrated increased frequency of late openings, prolonged mean open time, and increased latency to first opening for the mutant. Subjects carrying this mutation responded clinically to the combination of mexiletine with propranolol and survived. Pharmacologically, the mutant exhibited 2-fold greater tonic and use-dependent mexiletine block than wild-type channels. The mutant also exhibited enhanced tonic (2.4-fold) and use-dependent block ( approximately 5-fold) by propranolol, and we observed additive effects of the 2 drugs on the mutant.

CONCLUSIONS

Our study demonstrates the molecular basis for a malignant perinatal presentation of long-QT syndrome, illustrates novel functional and pharmacological properties of SCN5A-G1631D, which caused the disorder, and reveals therapeutic benefits of propranolol block of mutant sodium channels in this setting.

Genetic Na+ channelopathies and sinus node dysfunction

Voltage-gated Na+ channels are transmembrane proteins that produce the fast inward Na+ current responsible for the depolarization phase of the cardiac action potential. They play fundamental roles in the initiation, propagation, and maintenance of normal cardiac rhythm. Inherited mutations in SCN5A, the gene encoding the pore-forming alpha-subunit of the cardiac-type Na+ channel, result in a spectrum of disease entities termed Na+ channelopathies. These include multiple arrhythmic syndromes, such as the long QT syndrome type 3 (LQT3), Brugada syndrome (BrS), an inherited cardiac conduction defect (CCD), sudden infant death syndrome (SIDS) and sick sinus syndrome (SSS). To date, mutational analyses have revealed more than 200 distinct mutations in SCN5A, of which at least 20 mutations are associated with sinus node dysfunction including SSS. This review summarizes recent findings bearing upon: (i) the functional role of distinct voltage-gated Na+ currents in sino-atrial node pacemaker function; (ii) genetic Na+ channelopathy and its relationship to sinus node dysfunction.

SCN5A channelopathies--an update on mutations and mechanisms

Voltage-gated Na+ channels mediate the rapid upstroke of the action potential in excitable tissues. Na(v)1.5, encoded by the SCN5A gene, is the predominant isoform in the heart. Mutations in SCN5A are associated with distinct cardiac excitation disorders often resulting in life-threatening arrhythmias. This review outlines the currently known SCN5A mutations linked to three distinct cardiac rhythm disorders: long QT syndrome subtype 3 (LQT3), Brugada syndrome (BS), and cardiac conduction disease (CCD). Electrophysiological properties of the mutant channels are summarized and discussed in terms of Na+ channel structure-function relationships and regarding molecular mechanisms underlying the respective cardiac dysfunction. Possible reasons for less convincing genotype-phenotype correlations are suggested.

In utero onset of long QT syndrome with atrioventricular block and spontaneous or lidocaine-induced ventricular tachycardia: compound effects of hERG pore region mutation and SCN5A N-terminus variant

BACKGROUND

Mexiletine may protect patients with long QT syndrome (LQTS) type 3 from arrhythmias. However, we found an unusual in utero presentation of intermittent atrioventricular block and ventricular tachycardia (spontaneous or lidocaine-induced) in a fetus and his sibling with LQTS.

OBJECTIVE

The purpose of this study was to investigate the underlying channelopathy and functional alteration.

METHODS

Mutations were searched in KCNQ1, HERG, KCNE1, KCNE2, and SCN5A genes. In expressed mutants, whole-cell voltage clamp defined the electrophysiologic properties.

RESULTS

Novel missense mutations involving hERG (F627L) at the pore region and SCN5A (R43Q) at the N-terminus were found in the proband and in family members with prolonged QT interval. In oocytes injected with mRNA encoding hERG/ F627L, almost zero K(+) currents were elicited. In coinjected oocytes, the currents were decreased to half. In tsA201 cells transfected with SCN5A/R43Q, although the baseline kinetics of the Na current were similar to wild type, lidocaine caused a unique hyperpolarizing shift of the activation and increased the availability of Na currents at resting voltages. Window currents were enhanced due to a right shift of steady-state inactivation. These electrophysiologic alterations after lidocaine may lead to the development of ventricular tachycardia.

CONCLUSION

We identified a novel hERG/F627L mutation that results in LQTS with fetal onset of atrioventricular block and ventricular tachycardia. A coexisting SCN5A/R43Q variant, although it per se does not prolong repolarization, contributes to the development of ventricular tachyarrhythmias after lidocaine. Patients with such latent lidocaine-induced phenotype who are given lidocaine or mexiletine may be at risk.

Cardiac sodium channel overlap syndromes: different faces of SCN5A mutations

Cardiac sodium channel dysfunction caused by mutations in the SCN5A gene is associated with a number of relatively uncommon arrhythmia syndromes, including long-QT syndrome type 3 (LQT3), Brugada syndrome, conduction disease, sinus node dysfunction, and atrial standstill, which potentially lead to fatal arrhythmias in relatively young individuals. Although these various arrhythmia syndromes were originally considered separate entities, recent evidence indicates more overlap in clinical presentation and biophysical defects of associated mutant channels than previously appreciated. Various SCN5A mutations are now known to present with mixed phenotypes, a presentation that has become known as "overlap syndrome of cardiac sodium channelopathy." In many cases, multiple biophysical defects of single SCN5A mutations are suspected to underlie the overlapping clinical manifestations. Here, we provide an overview of current knowledge on SCN5A mutations associated with sodium channel overlap syndromes and discuss a possible role for modifiers in determining disease expressivity in the individual patient.

Mutation of sodium channel SCN3A in a patient with cryptogenic pediatric partial epilepsy

Mutations in the sodium channel genes SCN1A and SCN2A have been identified in monogenic childhood epilepsies, but SCN3A has not previously been investigated as a candidate gene for epilepsy. We screened a consecutive cohort of 18 children with cryptogenic partial epilepsy that was classified as pharmacoresistant because of nonresponse to carbamazepine or oxcarbazepine, antiepileptic drugs that bind sodium channels. The novel coding variant SCN3A-K354Q was identified in one patient and was not present in 295 neurological normal controls. Twelve novel SNPs were also detected. K354Q substitutes glutamine for an evolutionarily conserved lysine residue in the pore domain of SCN3A. Functional analysis of this mutation in the backbone of the closely related gene SCN5A demonstrated an increase in persistent current that is similar in magnitude to epileptogenic mutations of SCN1A and SCN2A. This observation of a potentially pathogenic mutation of SCN3A (Nav1.3) indicates that this gene should be further evaluated for its contribution to childhood epilepsy.

Inherited arrhythmias: a National Heart, Lung, and Blood Institute and Office of Rare Diseases workshop consensus report about the diagnosis, phenotyping, molecular mechanisms, and therapeutic approaches for primary cardiomyopathies of gene mutations affecting ion channel function

The National Heart, Lung, and Blood Institute and Office of Rare Diseases at the National Institutes of Health organized a workshop (September 14 to 15, 2006, in Bethesda, Md) to advise on new research directions needed for improved identification and treatment of rare inherited arrhythmias. These included the following: (1) Na+ channelopathies; (2) arrhythmias due to K+ channel mutations; and (3) arrhythmias due to other inherited arrhythmogenic mechanisms. Another major goal was to provide recommendations to support, enable, or facilitate research to improve future diagnosis and management of inherited arrhythmias. Classifications of electric heart diseases have proved to be exceedingly complex and in many respects contradictory. A new contemporary and rigorous classification of arrhythmogenic cardiomyopathies is proposed. This consensus report provides an important framework and overview to this increasingly heterogeneous group of primary cardiac membrane channel diseases. Of particular note, the present classification scheme recognizes the rapid evolution of molecular biology and novel therapeutic approaches in cardiology, as well as the introduction of many recently described diseases, and is unique in that it incorporates ion channelopathies as a primary cardiomyopathy in consensus with a recent American Heart Association Scientific Statement.

Association of congenital, diffuse electrical disease in children with normal heart: sick sinus syndrome, intraventricular conduction block, and monomorphic ventricular tachycardia

INTRODUCTION

Rhythm disturbances in children with structurally normal hearts are usually associated with abnormalities in cardiac ion channels. The phenotypic expression of these abnormalities ("channelopathies") includes: long and short QT syndromes, Brugada syndrome, congenital sick sinus syndrome, catecholaminergic polymorphic ventricular tachycardia, Lènegre-Lev disease, and/or different degrees of cardiac conduction disease.

METHODS

The study group consisted of three male patients with sick sinus syndrome, intraventricular conduction disease, and monomorphic sustained ventricular tachycardia. Clinical data and results of electrocardiography, Holter monitoring, electrophysiology, and echocardiography are described.

RESULTS

In all patients, the ECG during sinus rhythm showed right bundle branch block and long QT intervals. First-degree AV block was documented in two subjects, and J point elevation in one. A pacemaker was implanted in all cases due to symptomatic bradycardia (sick sinus syndrome). Atrial tachyarryhthmias were observed in two patients. The common characteristic ventricular arrhythmia was a monomorphic sustained ventricular tachycardia, inducible with ventricular stimulation and sensitive to lidocaine. In one patient, radiofrequency catheter ablation was successfully performed. No structural abnormalities were found in echocardiography in the study group.

CONCLUSION

Common clinical and ECG features suggest a common pathophysiology in this group of patients with congenital severe electrical disease.

Changes in action potentials and intracellular ionic homeostasis in a ventricular cell model related to a persistent sodium current in SCN5A mutations underlying LQT3

In LQT3 patients, SCN5A mutations induce ultraslow inactivation of a small fraction of the hNav1.5 current, i.e. persistent Na+ current (IpNa). We explored the time course of effects of such a change on the intracellular ionic homeostasis in a model of guinea-pig cardiac ventricular cell [Pasek, M., Simurda, J., Orchard, C.H., Christé, G., 2007b. A model of the guinea-pig ventricular cardiomyocyte incorporating a transverse-axial tubular system. Prog. Biophys. Mol. Biol., this issue]. Sudden addition of IpNa prevented action potential (AP) repolarization when its conductance (gpNa) exceeded 0.12% of the maximal conductance of fast INa (gNa). With gpNa at 0.1% gNa, the AP duration at 90% repolarization (APD90) was initially lengthened to 2.6-fold that in control. Under regular stimulation at 1 Hz it shortened progressively to 1.37-fold control APD90, and intracellular [Na+]i increased by 6% with a time constant of 106 s. Further increasing gpNa to 0.2% gNa caused an immediate increase in APD90 to 5.7-fold that in control, which decreased to 2.2-fold that in control in 30s stimulation at 1 Hz. At this time diastolic [Na+]i and [Ca2+]i were, respectively, 34% and 52% higher than in control and spontaneous erratic SR Ca release occurred. In the presence of IpNa causing 46% lengthening of APD90, the model cell displayed arrhythmogenic behaviour when external [K+] was lowered to 5 mM from an initial value at 5.4 mM. By contrast, when K+ currents IKr and IKs were lowered in the model cell to produce the same lengthening of APD90, no proarrhythmic behaviour was observed, even when external [K+] was lowered to 2.5 mM.