Cardiac Sodium Channel Nav1.5 Mutations and Cardiac Arrhythmia

As a major cardiac voltage-gated sodium channel isoform in the heart, the Nav1.5 channel is essential for cardiac action potential initiation and subsequent propagation throughout the heart. Mutations of Nav1.5 have been linked to a variety of cardiac diseases such as long QT syndrome (LQTs), Brugada syndrome, cardiac conduction defect, atrial fibrillation, and dilated cardiomyopathy. The mutagenesis approach and heterologous expression systems are most frequently used to study the function of this channel. This review focuses primarily on recent findings of Nav1.5 mutations associated with type 3 long QT syndrome (LQT3) in particular. Understanding the functional changes of the Nav1.5 mutation may offer critical insight into the mechanism of long QT3 syndrome. In addition, this review provides the updated information on the current progress of using various experimental model systems to study primarily the long QT3 syndrome.     

A missense mutation (G604S) in the S5/pore region of <Emphasis Type="Italic">HERG</Emphasis> causes long QT syndrome in a Chinese family with a high incidence of sudden unexpected death

Long QT syndrome (LQTS) is characterized by abnormalities in cardiac repolarization that lead to prolongation of the electrocardiographic (ECG) QT interval. Mutations in the human ether-a-go-go-related gene (HERG, KCNH2) cause the chromosome 7-linked LQT2 form of congenital LQTS, which is characterized by a prolonged QT interval and a bifid T-wave with an increased susceptibility to life-threatening cardiac arrhythmias, especially in children. We describe the genotypic and phenotypic pedigree of a large Chinese family (n = 36) in which 11 members were diagnosed with LQTS on the basis of typical ECG patterns for LQT2. Symptomatic syncopal episodes appeared in seven members of this family at a young age; an additional four members had died suddenly at ages of 18, 19, 24 and 70 years, respectively. Screening for SCN5A and HERG candidate genes identified a heterozygous missense mutation 1810G→A in exon 7 of HERG that leads to the substitution of the amino acid glycine by serine (G604S); this mutation was located in the S5/pore region of the HERG protein and was associated with a malignant phenotype. Ten of the family members carrying the mutation showed a prolongation of the corrected QT interval (QTc), and seven of these had experienced multiple syncopal episodes. The retrospective examination of documented ECG records revealed that one family member who had died suddenly also had a prolonged QT interval. This study is the first to demonstrate a close correlation between clinical phenotype and genotype with a 100% penetrance based on the pedigree of a Chinese family with LQT2. Yanmin Zhang and Nan Zhou contributed equally to this investigation.     

Clinical and Genetic Analysis of Long QT Syndrome in Children from Six Families in Saudi Arabia: Are They Different?

Congenital long QT syndrome (LQTS) is an inherited cardiac arrhythmia disorder characterized by prolongation of the QT interval; patients are predisposed to ventricular tachyarrhythmias and fibrillation leading to recurrent syncope or sudden cardiac death. We performed clinical and genetic studies in six Saudi Arabian families with a history of sudden unexplained death of children. Clinical symptoms, ECG phenotypes, and genetic findings led to the diagnosis of LQT1 in two families (recessive) and LQT2 in four families (three recessive and one dominant). Onset of arrhythmia was more severe in the recessive carriers and occurred during early childhood in all recessive LQT1 patients. Arrhythmia originated at the intrauterine stages of life in the recessive LQT2 patients. LQT1, causing mutation c.387-5 T > A in the KCNQ1 gene, and LQT2, causing mutation c.3208 C > T in the KCNH2 gene, are presumably founder mutations in the Assir province of Saudi Arabia. Further, all LQTS causing mutations detected in this study are novel and have not been reported in other populations.     

Electroclinical features of epileptic encephalopathy caused by SCN8A mutation

Voltage‐gated sodium channel Na_v1.6, encoded by the gene SCN8A, plays a crucial role in controlling neuronal excitability. SCN8A mutations that cause increased channel activity are associated with seizures. We describe a patient with epileptic encephalopathy caused by de novo SCN8A mutation (c.5614C>T, p.Arg1872Trp). Seizures began 10 days after birth at which time brain magnetic resonance imaging (MRI) and electroencephalography (EEG) were normal. Seizure recurrence increased with age, leading to the development of frequent status epilepticus from 1 year of age. Seizure type included generalized tonic seizures and focal motor seizures. EEG first showed focal epileptic activity at the age of 4 months, and thereafter showed multifocal spikes. Serial MRI demonstrated brain atrophy, which appeared to progress with seizure aggravation. Clinical features that may give a clue to the diagnosis include normal EEG despite frequent seizures in early infancy and an increase in epileptic activity that occurs with aging.     

Hepatic coma culminating in severe brain damage in a child with a SCN1A mutation

An 11 months old boy, developed liver failure after febrile status epilepticus while being treated with valproic acid for myoclonic epilepsy and recurrent partial and generalized seizures. The diagnosis of Alpers-Huttenlocher disease was considered. A muscle biopsy showed mitochondrial dysfunction. Mitochondrial DNA depletion was ruled out. Sequencing of the polymerase gamma gene (POLG1) did not detect any mutations.Sequencing of the alpha-1 subunit gene of the voltage-gated neuronal sodium channel (SCN1A) revealed a novel, de novo amino acid change p.Val 1637 Glu.This case expands the spectrum of clinical presentations related to mutations in SCN1A. We warn that children with SCN1A mutations may be at risk for developing liver failure following status epilepticus, due to mitochondrial dysfunction.     

先天性Q-T间期延长综合征3型一家系基因突变分析

目的 对一个表现有先天性Q-T间期延长综合征,同时表现有扩张性心肌病、心脏传导障碍性疾病的家系进行相关基因分析,探讨其发病机制.方法 一个中国人3代多表现型的先天性Q-T间期延长综合征家系,采集8名家系成员和100名健康对照血样,采用PCR扩增、DNA直接测序技术进行SCN5A、KCNQ1、KCNH2、LAMIN A/C基因突变分析,采用多聚酶链反应--单链构象多态性进行基因型和表型研究.结果 PCR-DNA直接测序在先证者钠通道SCN5A基因第26号外显子发现了9个碱基CAGAAGCCC缺失突变,该突变位于SCN5A基因DⅢ和DⅣ连接区域,导致第1507位谷氨酰胺、第1508位赖氨酸、第1509位脯氨酸3个氨基酸缺失.家系中患者检出与先证者相同的突变,而家系内参加本研究的健康人和家系外100名健康对照未出现这种突变.结论 SCN5A基因突变是部分先天性Q-T间期延长综合征的分子发病机制,SCN5A基因delQKP1507-1509突变是国内未曾报道过的新突变,国外的相同突变报道仅表现为Q-T间期延长综合征,无其他表型.进一步开展SCN5A delQKP1507-1509突变的功能研究有助于理解相关疾病的分子发病机制.     

Dravet syndrome: The main issues

Dravet syndrome (DS) is a severe form of infantile onset epilepsy characterized by multiple seizure types, prolonged convulsive seizures and frequent episodes of status epilepticus. Seizures precipitated by fever are a main characteristic. Affected children exhibit normal early development. Cognitive impairment, behavioral disturbances with hyperactivity and sometimes autistic traits occur after seizure onset. Seizures persist into adulthood but become less frequent. In about 85% of patients, a mutation of the SCN1A gene is present. DS fully illustrates the concept of epileptic encephalopathy. However, it is difficult to determine the causative role of the underlying sodium channel dysfunction and that of the consequent seizures in influencing cognitive outcome. An overwhelmingly high number of SCN1A mutations have been associated with DS. Intragenic or whole gene deletions, duplications and amplifications are additional rare molecular mechanisms. Most mutations are de novo, but familial mutations also occur. Somatic mosaic mutations should be considered when estimating the recurrence. MRI imaging is usually normal, and no neuropathologic signature of the condition seems to exist. In heterozygous Scn1a+/? mice, GABAergic interneurons exhibit substantially reduced sodium current density with reduced ability for sustained action potential firing. GABAergic output is reduced and excitability of downstream synaptic targets increased. Stiripentol was effective in combination with valproate and clobazam in two pivotal phase III trials. Phenytoin, carbamazepine, and lamotrigine can worsen seizures and should be avoided. Prospective studies will clarify to what extent earlier diagnosis and efforts at seizure control with the most appropriate drug combinations will reduce clinical deterioration.     

Arrhythmia in children with dilated cardiomyopathy (data of 24-hour ECG monitoring)

The authors demonstrate potentialities of long-term ECG monitoring in the diagnosis of heart rhythm and conduction disturbances in children with dilated cardiomyopathy. Provide the results of comparative studies of routine and 24-hour ECG monitoring in children with dilated cardiomyopathy under every-day life conditions. Show that during 24-hour ECG monitoring, all the children with the above pathology manifest heart rhythm and conduction disturbances of varying intensity and prognostic significance.     

Novel SCN1A mutations in Indonesian patients with severe myoclonic epilepsy in infancy

Background: Severe myoclonic epilepsy in infancy (SMEI) and borderline SMEI (SMEB) are caused by a mutation in SCN1A, which encodes a voltage‐gated sodium channel α1‐subunit protein. Although many mutations in SCN1A have been associated with clinical features of SMEI or SMEB from different ethnic groups, there have been no such reports from the South‐East Asian populations so far. Methods: Patients 1 and 2 were Indonesian children diagnosed as having SMEI and SMEB based on their clinical features. SCN1A was screened for mutations using a combination of polymerase chain reaction and denaturing high‐performance liquid chromatography. Nucleotide substitutions were confirmed on direct sequencing. Results: In patient 1, a G‐to‐A heterozygous transition was detected at nucleotide 4834 (c.4834G>A) in exon 25, leading to substitution of valine with isoleucine at amino acid position 1612 (p.V1612I) in the SCN1A protein. In patient 2 a T‐to‐G heterozygous transversion was identified at nucleotide 5266 (c.5266T>G) in exon 26, leading to substitution of cysteine with glycine at amino acid 1756 (p.C1756G) in the SCN1A protein. Both amino acid substitutions might disrupt these highly conserved regions in species from drosophila to human, leading to dysfunction of the protein. p.V1612I and p.C1756G were determined as disease‐causing mutations due to their absence in the control population. Conclusion: The first cases of SMEI and SMEB are reported in South‐East Asian populations. Two novel SCN1A mutations are also identified in these patients, p.V1612I and p.C1756G, which may lead to neuronal excitability or convulsions.     

Paramyotonia congenita: From clinical diagnosis to in silico protein modeling analysis

Background: Paramyotonia congenita (PMC) is an autosomal dominant disorder characterized by cold‐ or exercise‐induced myotonia. PMC is caused by a mutation in SCN4A which encodes the α‐subunit of the skeletal muscle sodium channel. Methods: The patient was an 11‐year‐old Japanese girl who was diagnosed as having PMC. To confirm the diagnosis, an orbital ice‐pack test and blinking tests were performed. Next, to identify the mutation, genetic analysis of SCN4A was performed. Finally, to evaluate the mutation effect on the protein structure, in silico protein modeling analysis was performed. Results: Cold‐ and exercise‐induced myotonia was reproduced in the patient with non‐invasive bedside tests: ice‐pack and blinking tests. In the genetic analysis, a missense mutation, c.4343G>A in SCN4A, was identified, which may result in an arginine to histidine substitution at 1448 in the protein sequence (p.Arg1448His). According to the protein modeling analysis, the mutation neutralized the positive electrostatic charge at 1448 in the DIV/S4 segment and disrupted the beginning of the helical structure in the DIV/S3‐S4 linker of the SCN4A protein. Conclusions: Diagnostic physical interventions in the patient confirmed the phenotype presentation consistent with PMC, and the in silico protein modeling analysis of p.Arg1448His predicted structural changes which can affect function of the protein. All the data confirmed the diagnosis of PMC in the patient and added to existing literature emphasizing the important role of arginine residue at 1448.     

Congenital Long QT Syndrome and 2:1 Atrioventricular Block with a Mutation of the SCN5A Gene

Infants with congenital long QT syndrome (LQTS) and 2:1 atrioventricular block (AVB) have been recognized as a clinical subset of children with LQTS. However, the genotype of this disorder is not well-known. We report an infant with LQTS and 2:1 AVB with a mutation of the SCN5A gene (LQT3). In some patients with LQTS and 2:1 AVB, the disorder may be due to mutation of the SCN5A gene (LQT3).     

Genetische und molekularbiologische Aspekte von Fieberkrämpfen

At a life-time prevalence of 2–5%, febrile convulsions (FC) are the most frequent seizure disorder at all. Several family and twin studies provided clear evidence that genetic factors play a predominant role in the majority of all FC cases. Results of complex segregation analyses showed that in the majority of all FC families a polygenic mode of inheritance fits best the observed family patterns. By assessing few large multigenerational families evidence has been provided for the existence of four FC susceptibility loci, however, genes, which may confer susceptibility to FC have not yet been identified. In the most recently described syndrome “generalized epilepsy with febrile seizures plus” several mutations have already been identified in the sodium channel α-subunit gene SCN1A and in the sodium channel β-subunit gene SCN1B. Further identification of genes conferring susceptibility to FC could contribute to the development of novel strategies for the diagnosis and prevention of FC.     

Fever-induced life-threatening arrhythmias in children harboring an SCN5A mutation

Cardiac channelopathies caused by SCN5A mutation are well tolerated by most patients. However, the dramatic presentation of a previously healthy 4-month-old girl with life-threatening arrhythmias and the subsequent findings in the child and her family provide evidence that loss-of-function sodium channel mutations can present very early in life. An SCN5A mutation was detected in the infant, her brother, and their father. Both the siblings manifested recurrent serious arrhythmias during febrile episodes, which followed immunization, as well as fever of nonspecific origin. Management consisted of prompt antipyretic measures, hospitalization with vigorous monitoring during immunization and febrile episodes, and prevention of tachycardia-induced conduction disturbance with β-blockers.     

Current Concepts in Long QT Syndrome

Sudden cardiac death occurs in the United States with an incidence of more than 300,000 persons per year. The underlying cause of death is commonly considered to be due to primary or secondary arrhythmias. In young persons in whom no structural heart disease can be identified, the long QT syndromes (LQTS) are commonly considered as likely causes. Multiple genes causing LQTS have been identified thus far, all of which encode cardiac ion channels. These include two potassium channel α subunits (KVLQT1 and HERG), two potassium channel β subunits (minK and MiRP1), and one sodium channel gene (SCN5A). The purpose of this review is to describe the current understanding of the molecular genetics of LQTS and the resultant phenotypes, particularly in young patients.     

SCN1A 基因相关癫痫研究进展

SCN1A基因编码电压门控钠离子通道α1亚基,其致病性变异可通过影响钠通道的功能导致癫痫发作。SCN1A基因致病性变异相关的癫痫患者具有高度的临床异质性,可表现出从良性表型到严重表型的一系列癫痫表型谱。基因变异的位置及类型、嵌合体变异以及修饰基因的作用等都是影响癫痫表型的因素。早期识别SCN1A基因相关癫痫的临床特点,及时进行SCN1A基因检测,有助于实现癫痫的精准诊疗及预后评估。文章就SCN1A基因相关癫痫的发病机制、临床表现、基因型与临床表型相关性及治疗等进行综述,以提高对SCN1A基因相关癫痫的认识。     

Low R-wave amplitude in the right precordial leads in children with symptomatic doxorubicin cardiomyopathy

Summary Electrocardiographic (ECG) findings were studied in four patients with doxorubicin cardiomyopathy. In all patients with congestive heart failure (CHF), the ECGs had a low R-wave and low R/S ratio in lead V₁. Our study suggests that increased injury to myocardial cells in the regions of the anterior septum and anterior left ventricular wall may be important in the pathogenesis of doxorubicin cardiomyopathy. Eight years later, cardiac recovery from CHF occurred with a normal ECG and left ventricular ejection fraction in one patient, indicating that CHF may be reversible in certain cases.     

Dilated cardiomyopathy and thrombo-embolism

The purpose of this study was to investigate the incidence, outcome and prevention of thrombo-embolism in children with dilated cardiomyopathy. From 130 patients with dilated cardiomyopathy, 17 (14%) showed evidence of thrombo-embolism. Seven had initial cardiac thrombus, 7 exhibited initial embolus and in 3 thrombo-embolism was only diagnosed at autopsy. All 17 patients showed seriously impaired systolic function of the left ventricle with fractional shortening (FS) of 10?±?3%, range 5%–17%, as compared to those without thrombo-embolism with FS of 17%?±?6%, range 5%–26% (P?<;?0.0001). Seven patients were treated with oral anticoagulants once thrombo-embolism had been diagnosed; one of them experienced a further embolic event as opposed to three out of four patients not treated with anticoagulants. Conclusion?All children with dilated cardiomyopathy and fractional shortening below 20% should be treated with prophylactic anticoagulative agents     

Diagnosis and long-term course of Dravet syndrome

Dravet syndrome is a severe infantile-onset epilepsy syndrome with a distinctive but complex electroclinical presentation. A healthy, developmentally normal infant presents at around 6 months of age with convulsive status epilepticus, which may be hemiclonic or generalized; seizures may be triggered by fever, illness or vaccination. The infant typically has further episodes of status epilepticus every month or two, often triggered by fever. Other seizure types including focal dyscognitive seizures, absence and myoclonic seizures develop between 1 and 4 years. Atonic drop attacks and episodes of non-convulsive status may occur. Early development is normal but slows in the second year. Developmental regression may occur, particularly with status epilepticus. EEG studies are initially normal, but after 2 years they show generalized spike-wave and polyspike-wave activity with multifocal discharges. Photosensitivity may be seen. Imaging is normal or shows non-specific findings such as atrophy.Dravet syndrome is associated with mutations of the gene encoding the alpha-1 subunit of the sodium channel, SCN1A, in >70% of patients. These include sequencing mutations and copy number variant anomalies; 90% of mutations arise de novo. PCDH19 mutational analysis is a second-tier test for girls with a Dravet-like picture who do not have SCN1A mutations.Outcome is poor, with intellectual disability in most patients and ongoing seizures. Intellectual impairment varies from severe in 50% patients, to moderate and mild intellectual disability each accounting for 25% cases. Rare patients have normal intellect. The long-term course involves ongoing, brief nocturnal convulsions and a characteristic deterioration in gait.     

A loss-of-function CACNA1A mutation causing benign paroxysmal torticollis of infancy

Benign paroxysmal torticollis of infancy (BPTI) is a rare paroxysmal disorder characterized by recurrent episodes of head tilt and accompanying general symptoms which remit spontaneously. The rare association with gain-of-function CACNA1A mutations, similar to hemiplegic migraine, has been reported. We report here two new BPTI patients from the same family carrying a heterozygous mutation in the CACNA1A gene leading to the change p.Glu533Lys. Functional analysis revealed that this mutation induces a loss of channel function due to impaired gating by voltage and much lower current density. Our data suggest that BPTI, a periodic syndrome commonly considered a migraine precursor, constitutes an age-specific manifestation of defective neuronal calcium channel activity.