Analysis of candidate genes for genotypic diagnosis in the long QT syndrome

Patients with the long QT syndrome (LQTS) suffer from cardiac arrhythmias that can lead to abrupt loss of consciousness and sudden death, already in young individuals. Thus, an early diagnosis of LQTS is essential for patients and their family members. So far, six genes (KCNQ1, HERG, SCN5A, ANK2, KCNE1, KCNE2) have been demonstrated to be involved in the development of LQTS. Since this syndrome is genetically heterogeneous and large-sized families are often not available for linkage analysis, alternative tools are required for a genetic diagnosis. To investigate genes with numerous exons, like KCNQ1, HERG, SCN5A and ANK2, segregation analysis of a Polish Romano-Ward family with eight members was performed as a reliable method faster than linkage analysis or direct sequencing. To test these four LQT loci, an appropriate selection of microsatellite markers covering different chromosomal regions was applied. Furthermore, two small genes KCNE1 and KCNE2 (at the LQT5 and LQT6 loci), and the SGK1 gene (encoding a kinase regulating KCNE1 and SCN5A channels) were sequenced. All six LQT loci and the SGK1 gene were excluded by these analyses, thus a different pathogenic mechanism of LQT syndromes can be presumed.     

Optical mapping of ventricular arrhythmias in LQTS mice with SCN5A mutation N1325S

Transgenic expression of SCN5A mutation N1325S creates a mouse model for type-3 long QT syndrome (LQT3), TG-NS/LQT3. Optical mapping is a high temporal and spatial resolution fluorescence mapping system that records 256 action potentials simultaneously in a Langendorff-perfused heart. Here for the first-time, we provide a spatial view of VT in a genetic LQT3 model using optical mapping. Spontaneous VT was detected in TG-NS/LQT3 hearts, but not in littermate control hearts. VT was initiated primarily by activation of a new firing focus as well as functional conduction block of new activation waves. New firing was initiated at many different Loci in the heart, suggesting that "increased automaticity" is a key mechanism for initiation of VT. The sustained VT was maintained by a reentry mechanism. Nifedipine, an L-type calcium channel blocker, decreased the frequency of VT, indicating the involvement of abnormalities of the calcium homeostasis in the genesis of VT in TG-NS/LQT3 mice.     

Mapping genetic modifiers of survival in a mouse model of Dravet syndrome

Epilepsy is a common neurological disorder affecting approximately 1% of the population. Mutations in voltage‐gated sodium channels are responsible for several monogenic epilepsy syndromes. More than 800 mutations in the voltage‐gated sodium channel SCN1A have been reported in patients with generalized epilepsy with febrile seizures plus and Dravet syndrome. Heterozygous loss‐of‐function mutations in SCN1A result in Dravet syndrome, a severe infant‐onset epileptic encephalopathy characterized by intractable seizures, developmental delays and increased mortality. A common feature of monogenic epilepsies is variable expressivity among individuals with the same mutation, suggesting that genetic modifiers may influence clinical severity. Mice with heterozygous deletion of Scn1a (Scn1a^+/?) model a number of Dravet syndrome features, including spontaneous seizures and premature lethality. Phenotype severity in Scn1a^+/? mice is strongly dependent on strain background. On the 129S6/SvEvTac strain Scn1a^+/? mice exhibit no overt phenotype, whereas on the (C57BL/6J × 129S6/SvEvTac)F1 strain Scn1a^+/? mice exhibit spontaneous seizures and early lethality. To systematically identify loci that influence premature lethality in Scn1a^+/? mice, we performed genome scans on reciprocal backcrosses. Quantitative trait locus mapping revealed modifier loci on mouse chromosomes 5, 7, 8 and 11. RNA‐seq analysis of strain‐dependent gene expression, regulation and coding sequence variation provided a list of potential functional candidate genes at each locus. Identification of modifier genes that influence survival in Scn1a^+/? mice will improve our understanding of the pathophysiology of Dravet syndrome and may suggest novel therapeutic strategies for improved treatment of human patients.     

Altered Function of the SCN1A Voltage-gated Sodium Channel Leads to ��-Aminobutyric Acid-ergic (GABAergic) Interneuron Abnormalities

Voltage-gated sodium channels are required for the initiation and propagation of action potentials. Mutations in the neuronal voltage-gated sodium channel SCN1A are associated with a growing number of disorders including generalized epilepsy with febrile seizures plus (GEFS+),⁷ severe myoclonic epilepsy of infancy, and familial hemiplegic migraine. To gain insight into the effect of SCN1A mutations on neuronal excitability, we introduced the human GEFS+ mutation SCN1A-R1648H into the orthologous mouse gene. Scn1a^RH/RH mice homozygous for the R1648H mutation exhibit spontaneous generalized seizures and premature death between P16 and P26, whereas Scn1a^RH/+ heterozygous mice exhibit infrequent spontaneous generalized seizures, reduced threshold and accelerated propagation of febrile seizures, and decreased threshold to flurothyl-induced seizures. Inhibitory cortical interneurons from P5-P15 Scn1a^RH/+ and Scn1a^RH/RH mice demonstrated slower recovery from inactivation, greater use-dependent inactivation, and reduced action potential firing compared with wild-type cells. Excitatory cortical pyramidal neurons were mostly unaffected. These results suggest that this SCN1A mutation predominantly impairs sodium channel activity in interneurons, leading to decreased inhibition. Decreased inhibition may be a common mechanism underlying clinically distinct SCN1A-derived disorders.     

Altered sinoatrial node function and intra-atrial conduction in murine gain-of-function Scn5a+/ΔKPQ hearts suggest an overlap syndrome

Mutations in SCN5A, the gene encoding the pore-forming subunit of cardiac Na(+) channels, cause a spectrum of arrhythmic syndromes. Of these, sinoatrial node (SAN) dysfunction occurs in patients with both loss- and gain-of-function SCN5A mutations. We explored for corresponding alterations in SAN function and intracardiac conduction and clarified possible mechanisms underlying these in an established mouse long QT syndrome type 3 model carrying a mutation equivalent to human SCN5A-ΔKPQ. Electrophysiological characterizations of SAN function in living animals and in vitro sinoatrial preparations were compared with cellular SAN and two-dimensional tissue models exploring the consequences of Scn5a+/ΔKPQ mutations. Scn5a+/ΔKPQ mice showed prolonged electrocardiographic QT and corrected QT intervals confirming long QT phenotypes. They showed frequent episodes of sinus bradycardia, sinus pause/arrest, and significantly longer sinus node recovery times, suggesting compromised pacemaker activity compared with wild-type mice. Electrocardiographic waveforms suggested depressed intra-atrial, atrioventricular node, and intraventricular conduction in Scn5a+/ΔKPQ mice. Isolated Scn5a+/ΔKPQ sinoatrial preparations similarly showed lower mean intrinsic heart rates and overall slower conduction through the SAN to the surrounding atrium than did wild-type preparations. Computer simulations of both single SAN cells as well as two-dimensional SAN-atrial models could reproduce the experimental observations of impaired pacemaker and sinoatrial conduction in terms of changes produced by both augmented tail and reduced total Na(+) currents, respectively. In conclusion, the gain-of-function long QT syndrome type 3 murine Scn5a+/ΔKPQ cardiac system, in overlap with corresponding features reported in loss-of-function Na(+) channel mutations, shows compromised SAN pacemaker and conduction function explicable in modeling studies through a combination of augmented tail and reduced peak Na(+) currents.     

Variable Nav1.5 Protein Expression from the Wild-Type Allele Correlates with the Penetrance of Cardiac Conduction Disease in the Scn5a +/? Mouse Model

Background

Loss-of-function mutations in SCN5A, the gene encoding Na_v1.5 Na⁺ channel, are associated with inherited cardiac conduction defects and Brugada syndrome, which both exhibit variable phenotypic penetrance of conduction defects. We investigated the mechanisms of this heterogeneity in a mouse model with heterozygous targeted disruption of Scn5a (Scn5a ^+/− mice) and compared our results to those obtained in patients with loss-of-function mutations in SCN5A.

Methodology/Principal Findings

Based on ECG, 10-week-old Scn5a ^+/− mice were divided into 2 subgroups, one displaying severe ventricular conduction defects (QRS interval>18 ms) and one a mild phenotype (QRS≤18 ms; QRS in wild-type littermates: 10–18 ms). Phenotypic difference persisted with aging. At 10 weeks, the Na⁺ channel blocker ajmaline prolonged QRS interval similarly in both groups of Scn5a ^+/− mice. In contrast, in old mice (>53 weeks), ajmaline effect was larger in the severely affected subgroup. These data matched the clinical observations on patients with SCN5A loss-of-function mutations with either severe or mild conduction defects. Ventricular tachycardia developed in 5/10 old severely affected Scn5a ^+/− mice but not in mildly affected ones. Correspondingly, symptomatic SCN5A–mutated Brugada patients had more severe conduction defects than asymptomatic patients. Old severely affected Scn5a ^+/− mice but not mildly affected ones showed extensive cardiac fibrosis. Mildly affected Scn5a ^+/− mice had similar Na_v1.5 mRNA but higher Na_v1.5 protein expression, and moderately larger I_Na current than severely affected Scn5a ^+/− mice. As a consequence, action potential upstroke velocity was more decreased in severely affected Scn5a ^+/− mice than in mildly affected ones.

Conclusions

Scn5a ^+/− mice show similar phenotypic heterogeneity as SCN5A-mutated patients. In Scn5a ^+/− mice, phenotype severity correlates with wild-type Na_v1.5 protein expression.     

Calcium-dependent arrhythmias in transgenic mice with heart failure

Transgenic mice overexpressing the inflammatory cytokine tumor necrosis factor (TNF)-alpha (TNF-alpha mice) in the heart develop a progressive heart failure syndrome characterized by biventricular dilatation, decreased ejection fraction, atrial and ventricular arrhythmias on ambulatory telemetry monitoring, and decreased survival compared with nontransgenic littermates. Programmed stimulation in vitro with single extra beats elicits reentrant ventricular arrhythmias in TNF-alpha (n = 12 of 13 hearts) but not in control hearts. We performed optical mapping of voltage and Ca(2+) in isolated perfused ventricles of TNF-alpha mice to study the mechanisms that lead to the initiation and maintenance of the arrhythmias. When compared with controls, hearts from TNF-alpha mice have prolonged of action potential durations (action potential duration at 90% repolarization: 23 +/- 2 ms, n = 7, vs. 18 +/- 1 ms, n = 5; P < 0.05), no increased dispersion of refractoriness between apex and base, elevated diastolic and depressed systolic [Ca(2+)], and prolonged Ca(2+) transients (72 +/- 6 ms, n = 10, vs. 54 +/- 5 ms, n = 8; P < 0.01). Premature beats have diminished action potential amplitudes and conduct in a slow, heterogeneous manner. Lowering extracellular [Ca(2+)] normalizes conduction and prevents inducible arrhythmias. Thus both action potential prolongation and abnormal Ca(2+) handling may contribute to the initiation of reentrant arrhythmias in this heart failure model by mechanisms distinct from enhanced dispersion of refractoriness or triggered activity.     

Cardiac-specific overexpression of SCN5A gene leads to shorter P wave duration and PR interval in transgenic mice

The Cardiac sodium channel gene SCN5A plays a critical role in cardiac electrophysiology and its mutations, either gain- or loss-of-functions, are associated with lethal arrhythmias. In this study, we investigated the effect of overexpression of SCN5A on the cardiac phenotype in a transgenic mouse model (TG-WT L10). Compared to NTG mice, heart rate, QRS duration, and QT intervals remained unchanged in TG-WT mice. Moreover, no spontaneous ventricular arrhythmias were detected in TG-WT hearts. Despite these results, a mild, irregular cardiac phenotype was observed in TG-WT mice. The P wave and PR interval were significantly shorter in TG-WT compared with NTG mice (P, 8.8+/-0.8 ms vs. 12.6+/-0.9 ms; PR, 12.5+/-2 ms vs. 33.5+/-0.7 ms). Furthermore, spontaneous premature atrial contractions were often detected in TG-WT mice. These results suggest that the expression level of the SCN5A gene is a determinant for the length of the P wave duration and PR interval on electrocardiograms (ECG).     

心脏钠通道疾病

自从心脏钠离子通道 基因 (SC N 5A )突变 被首次鉴定 以来,人们对 SC N 5A 突变进行 了一系列研 究.SC N 5A突 变是在 两种明 显不同但 都与突 发性死 亡相关 联的疾病 ———长 Q T 波综合 症 (LQ T3)的 一种形 式和 B rugada 综合症 中被 鉴定的.后来 ,Lev-Lenegre 综 合症 进行 性的 心脏传 导缺 陷)也增 加到 LQ T3中.基因型 和表 型相 互关 系的 (研 究以及体外 表达研究提 供证据认为 SC N 5A 蛋白的结构 和功能相互 关系远比最 初预期的复 杂.心脏钠通 道的生物 物理特征与不同 的表型相关, 基因型和表型 相互关系的研究 使我们注意到 即使是单个 氨基酸的置换 都可能显而 易见的影响心脏 的兴奋性 .由 隐藏有 SC N 5A 突变的病 人提供的证 据以及临床呈 现 “重叠”现象的证 据显示已经 需要对上述提及 的疾病的传统 分类进行修改 .现在认为 钠通道综合症”作 为唯一的临 床称谓表示这 类疾病可 “能 的表型范围更合 适 .     

Inducible polymorphic ventricular tachyarrhythmias in a transgenic mouse model with a long Q-T phenotype

We created a mouse model with a prolonged Q-T interval and spontaneous arrhythmias by overexpressing the NH(2) terminus and first transmembrane segment (Kv1.1N206Tag) of a delayed rectifier potassium channel (LQT(+/-) mouse). Analyses were performed using whole cell recordings of cardiac myocytes, surface electrocardiography, and programmed electrical stimulation. Action potential duration (APD) was prolonged to the same extent and was more highly variable in myocytes derived from LQT(+/-) and LQT(+/+) mice than in myocytes derived from wild-type (WT) FVB mice. Under ketamine anesthesia, the Q-T interval of both LQT(+/+) and LQT(+/-) mice was comparably prolonged versus that of WT mice. Stimulation of the right ventricle using an intracardiac catheter induced polymorphic ventricular tachyarrhythmias in 50% of the LQT(+/-) mice and 36% of the LQT(+/+) mice, whereas polymorphic ventricular tachyarrhythmias were not inducible in WT mice. The analyses of LQT(+/-) and LQT(+/+) mice indicate that prolongation of the Q-T interval in LQT mice is associated with prolonged APD, increased dispersion of APD among cardiocytes, and inducibility of polymorphic ventricular tachycardia, providing the substrate for spontaneous arrhythmias in these animals.     

Arrhythmogenic substrate and its modification by nicorandil in a murine model of long QT type 3 syndrome

The gain-of-function Scn5a+/ΔKPQ mutation in the cardiac Na⁺ channel causes human long QT type 3 syndrome (LQT3) associated with ventricular arrhythmogenesis. The K_ATP channel-opener nicorandil (20 μM) significantly reduced arrhythmic incidence in Langendorff-perfused Scn5a+/Δ hearts during programmed electrical stimulation; wild-types (WTs) showed a total absence of arrhythmogenicity. These observations precisely correlated with alterations in recently established criteria for re-entrant excitation reflected in: (1) shortened left-ventricular epicardial but not endocardial monophasic action potential durations at 90% repolarization (APD₉₀) that (2) restored transmural repolarization gradients, ΔAPD₉₀. Scn5a+/Δ hearts showed longer epicardial but not endocardial APD₉₀s, giving shorter ΔAPD₉₀s than WT hearts. Nicorandil reduced epicardial APD₉₀ in both Scn5a+/Δ and WT hearts thereby increasing ΔAPD₉₀. (3) Reduced epicardial critical intervals for re-excitation; Scn5a+/Δ hearts showed greater differences between APD₉₀ and ventricular effective refractory period than WT hearts that were reduced by nicorandil. (4) Reduced APD₉₀ alternans. Scn5a+/Δ hearts showed greater epicardial and endocardial alternans than WTs, which increased with pacing rate. Nicorandil reduced these in Scn5a+/Δ hearts to levels indistinguishable from untreated WTs. (5) Flattened restitution curves. Scn5a+/Δ hearts showed larger epicardial and endocardial critical diastolic intervals than WT hearts. Nicorandil decreased these in Scn5a+/Δ and WT hearts. The presence or absence of arrhythmogenesis in Scn5a+/Δ and WT hearts thus agreed with previously established criteria for re-entrant excitation, and alterations in these precisely correlated with the corresponding antiarrhythmic effects of nicorandil. Together these findings implicate spatial and temporal re-entrant mechanisms in arrhythmogenesis in LQT3 and their reversal by nicorandil.     

Treatment of cardiac arrhythmias in a mouse model of Rett syndrome with Na+-channel-blocking antiepileptic drugs

One quarter of deaths associated with Rett syndrome (RTT), an X-linked neurodevelopmental disorder, are sudden and unexpected. RTT is associated with prolonged QTc interval (LQT), and LQT-associated cardiac arrhythmias are a potential cause of unexpected death. The standard of care for LQT in RTT is treatment with β-adrenergic antagonists; however, recent work indicates that acute treatment of mice with RTT with a β-antagonist, propranolol, does not prevent lethal arrhythmias. In contrast, acute treatment with the Na⁺ channel blocker phenytoin prevented arrhythmias. Chronic dosing of propranolol may be required for efficacy; therefore, we tested the efficacy of chronic treatment with either propranolol or phenytoin on RTT mice. Phenytoin completely abolished arrhythmias, whereas propranolol showed no benefit. Surprisingly, phenytoin also normalized weight and activity, but worsened breathing patterns. To explore the role of Na⁺ channel blockers on QT in people with RTT, we performed a retrospective analysis of QT status before and after Na⁺ channel blocker antiepileptic therapies. Individuals with RTT and LQT significantly improved their QT interval status after being started on Na⁺ channel blocker antiepileptic therapies. Thus, Na⁺ channel blockers should be considered for the clinical management of LQT in individuals with RTT.KEY WORDS: Long QT, Rett syndrome, Propranolol, Phenytoin, Arrhythmia, MECP2     

Re-entrant cardiac arrhythmias in computational models of long QT myocardium

The long QT syndrome (LQTS) is an inherited disorder in which repolarization of cardiac ventricular cells is prolonged. Patients with the LQTS are at an increased risk of ventricular cardiac arrhythmias. Two phenotypes of the inherited LQTS are caused by defects in K(+)channels (LQT1 and LQT2) and one by defects in Na(+)channels (LQT3). Patients with LQT1 are more likely to have self-terminating arrhythmias than those with LQT3. The aim of this computational study was to propose an explanation for this finding by comparing the vulnerability of normal and LQT tissue to re-entry, and estimating the likelihood of self-termination by motion of re-entrant waves to an inexcitable boundary in simulated LQT1, LQT2 and LQT3 tissue. We modified a model of mammalian cardiac cells to simulate LQT1 by reducing maximal I(K(s))conductance, LQT2 by reducing maximal I(K(r))conductance, and LQT3 by preventing complete inactivation of I(Na)channels. Each simulated phenotype was incorporated into a computational model of action potential propagation in one- and two-dimensional homogeneous tissue. Simulated LQT tissue was no more vulnerable to re-entry than simulated normal tissue, but the motion of re-entrant waves in simulated LQT1 tissue was between 2 and 5 times greater than the motion of re-entrant waves in simulated LQT2 and LQT3 tissue. These findings suggest that LQT arrhythmias do not result from increased vulnerability to re-entry, and that re-entry once initiated is more likely to self-terminate by moving to an inexcitable tissue boundary in LQT1 than in LQT2 and LQT3. This finding is consistent with clinical observations.     

The molecular basis of long QT syndrome and prospects for therapy

Long QT syndrome (LQT) is a cardiac disorder that causes sudden death from ventricular tachyarrhythmias, specifically torsade de pointes. Two types of LQT have been reported, autosomal-dominant LQT (Romano–Ward syndrome) and autosomal-recessive LQT (Jervell and Lange-Nielsen syndrome); Jervell and Lange-Nielsen syndrome is also associated with deafness. Four LQT genes have been identified for autosomal-dominant LQT: K⁺ channel genes KVLQT1 on chromosome 11p15.5, HERG on 7q35–36 and minK on 21q22, and the cardiac Na⁺ channel gene SCN5A on chromosome 3p21–24. Two genes, KVLQT1 and minK, have been identified for Jervell and Lange-Nielsen syndrome. Genetic testing and gene-specific therapies are available for some LQT patients.     

Genetic linkage analyses of Romano-Ward syndrome (RWS) in 13 Japanese families

Romano-Ward syndrome (RWS) is an autosomal dominant disorder characterized by prolongation of the electrocardiographic QT interval, with clinical manifestations that include recurrent syncope and sudden death from ventricular arrhythmias. Presymptomatic diagnosis is difficult because of the variability in these signs among carriers, but it is important for clinical management to prevent sudden cardiac death. To find an LQT (long QT) locus in Japanese patients and to identify DNA markers useful for presymptomatic diagnosis, linkage analyses were undertaken in 13 Japanese families with RWS patients by means of two DNA markers located on 11p15.5. One of these marker loci, HRAS, was previously reported to be tightly linked to the LQT locus in another ethnic group. Our analyses of homogeneity suggest evidence for genetic heterogeneity of RWS within the Japanese population.     

Biophysical characterization of a new SCN5A mutation S1333Y in a SIDS infant linked to long QT syndrome

Various entities and genetic etiologies, including inherited long QT syndrome type 3 (LQT3), contribute to sudden infant death syndrome (SIDS). The goal of our research was to biophysically characterize a new SCN5A mutation (S1333Y) in a SIDS infant. S1333Y channels showed the gain of Na⁺ channel function characteristic of LQT3, including a persistent inward Na⁺ current and an enhanced window current that was generated by a −8 mV shift in activation and a +7 mV shift in inactivation. The correlation between the biophysical data and arrhythmia susceptibility suggested that the SIDS was secondary to the LQT3-associated S1333Y mutation.     

Tandem promoters and developmentally regulated 5'- and 3'-mRNA untranslated regions of the mouse Scn5a cardiac sodium channel

The SCN5A gene encodes a voltage-sensitive sodium channel expressed in cardiac and skeletal muscle. Coding region mutations cause cardiac sudden death syndromes and conduction system failure. Polymorphisms in the 5'-sequence adjacent to the SCN5A gene have been linked to cardiac arrhythmias. We identified three alternative 5'-splice variants (1A, 1B, and 1C) of the untranslated exon 1 and two 3'-variants in the murine Scn5a mRNA. Two of the exon 1 isoforms (1B and 1C) were novel when compared with the published human and rat SCN5A sequences. Quantitative real time PCR results showed that the abundance of the isoforms varied during cardiac development. The 1A, 1B, and 1C mRNA splice variants increased 7.8 +/- 1.7-fold (E1A), 6.0 +/- 1.0-fold (E1B), and 20.6 +/- 3.7-fold (E1C) from fetal to adult heart, respectively. Promoter deletion and luciferase reporter gene analysis using cardiac and skeletal muscle cell lines demonstrated a pattern of distinct cardiac-specific enhancer elements associated with exons 1A and 1C. In the case of exon 1C, the enhancer element appeared to be within the exon. A 5'-repressor preceded each cardiac enhancer element. We concluded that the murine Na(+) channel has both 5'- and 3'-untranslated region mRNA variants that are developmentally regulated and that the promoter region contains two distinct cardiac-specific enhancer regions. The presence of homologous human splicing suggests that that these regions may be fruitful new areas of study in understanding cardiac sodium channel regulation and the genetic susceptibility to sudden death.     

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 α-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.     

Altered Cardiac Electrophysiology and SUDEP in a Model of Dravet Syndrome

Objective

Dravet syndrome is a severe form of intractable pediatric epilepsy with a high incidence of SUDEP: Sudden Unexpected Death in epilepsy. Cardiac arrhythmias are a proposed cause for some cases of SUDEP, yet the susceptibility and potential mechanism of arrhythmogenesis in Dravet syndrome remain unknown. The majority of Dravet syndrome patients have de novo mutations in SCN1A, resulting in haploinsufficiency. We propose that, in addition to neuronal hyperexcitability, SCN1A haploinsufficiency alters cardiac electrical function and produces arrhythmias, providing a potential mechanism for SUDEP.

Methods

Postnatal day 15-21 heterozygous SCN1A-R1407X knock-in mice, expressing a human Dravet syndrome mutation, were used to investigate a possible cardiac phenotype. A combination of single cell electrophysiology and in vivo electrocardiogram (ECG) recordings were performed.

Results

We observed a 2-fold increase in both transient and persistent Na⁺ current density in isolated Dravet syndrome ventricular myocytes that resulted from increased activity of a tetrodotoxin-resistant Na⁺ current, likely Na_v1.5. Dravet syndrome myocytes exhibited increased excitability, action potential duration prolongation, and triggered activity. Continuous radiotelemetric ECG recordings showed QT prolongation, ventricular ectopic foci, idioventricular rhythms, beat-to-beat variability, ventricular fibrillation, and focal bradycardia. Spontaneous deaths were recorded in 2 DS mice, and a third became moribund and required euthanasia.

Interpretation

These data from single cell and whole animal experiments suggest that altered cardiac electrical function in Dravet syndrome may contribute to the susceptibility for arrhythmogenesis and SUDEP. These mechanistic insights may lead to critical risk assessment and intervention in human patients.     

Mutational screening of SCN5A linked disorders in Polish patients and their family members

Mutations in SCN5A lead to a broad spectrum of phenotypes, including the Long QT syndrome, Brugada syndrome, Idiopathic ventricular fibrillation (IVF), Sudden infant death syndrome (SIDS) (probably regarded as a form of LQT3), Sudden unexplained nocturnal death syndrome (SUNDS) and isolated progressive cardiac conduction defect (PCCD) (Lev-Lenegre disease). Brugada Syndrome (BS) is a form of idiopathic ventricular fibrillation characterized by the right bundle-branch block pattern and ST elevation (STE) in the right precordial leads of the ECG. Mutations of the cardiac sodium channel SCN5A cause the disorder, and an implantable cardioverter-defibrillator is often recommended for affected individuals. In this study sequences of the coding region of the SCN5A gene were analysed in patients with the LQT3, Brugada Syndrome and other arrythmogenic disorders. Different mSSCP patterns are described with no disease-related SSCP conformers in any sample. Direct sequencing of the SCN5A gene confirmed the absence of mutations. This suggests that the analysed region of the SCN5A gene is not commonly involved in the pathogenesis of the Brugada Syndrome and associated disorders.