Artificial transmembrane potassium transporters: designs, functions, mechanisms and applications

Potassium channels represent the most prevalent class of ion channels, exerting regulatory control over numerous vital biological processes, including muscle contraction, neurotransmitter release, cell proliferation, and apoptosis. The seamless integration of astonishing functions into a sophisticated structure, as seen in these protein channels, inspires the chemical community to develop artificial versions, gearing toward simplifying their structure while replicating their key functions. In particular, over the past ten years or so, a number of elegant artificial potassium transporters have emerged, demonstrating high selectivity, high transport efficiency or unprecedented transport mechanisms. In this review, we will provide a detailed exposition of these artificial potassium transporters that are derived from a single molecular backbone or self-assembled from multiple components, with their respective structural designs, channel functions, transport mechanisms and biomedical applications thoroughly reviewed.

Molecular Pathways and Animal Models of Arrhythmias

Arrhythmias account for over 300,000 annual deaths in the United States, and approximately half of all deaths are associated with heart disease. Mechanisms underlying arrhythmia risk are complex; however, work in humans and animal models over the past 25 years has identified a host of molecular pathways linked with both arrhythmia substrates and triggers. This chapter will focus on select arrhythmia pathways solved by linking human clinical and genetic data with animal models.

Congenital Long QT Syndrome: A Review of Genetic and Pathophysiologic Etiologies, Phenotypic Subtypes, and Clinical Management

Congenital Long QT Syndrome (CLQTS) is the most common inherited arrhythmia. The QT interval, which marks the duration of ventricular depolarization and repolarization in the myocardium, can be prolonged due to mutations in genes coding for the ion channel proteins that govern the cardiac action potential. The lengthening of the QT interval can lead to a wide range of clinical symptoms, including seizures, torsades de pointes, and fatal arrhythmias. There is a growing body of evidence that has revealed the genetic mutations responsible for the pathophysiology of CLQTS, and this has led to hypotheses regarding unique triggers and clinical features associated with specific gene mutations. Epidemiologic evidence has revealed a 1-year mortality rate of approximately 20% in untreated CLQTS patients, and a <1% of 1-year mortality rate in treated patients, underscoring the importance of timely diagnosis and effective clinical management. There are many phenotypic syndromes that constitute CLQTS, including but not limited to, Jervell and Lange-Nielsen syndrome, Romano and Ward syndrome, Andersen-Tawil syndrome, and Timothy syndrome. In this review, we aim to (1) summarize the genetic, epidemiologic, and pathophysiological basis of CLQTS and (2) outline the unique features of the phenotypic subtypes and their clinical management.

Using Ribonucleoprotein-based CRISPR/Cas9 to Edit Single Nucleotide on Human Induced Pluripotent Stem Cells to Model Type 3 Long QT Syndrome (SCN5A±)

Human induced pluripotent stem cells (hiPSCs) have been widely used in cardiac disease modelling, drug discovery, and regenerative medicine as they can be differentiated into patient-specific cardiomyocytes. Long QT syndrome type 3 (LQT3) is one of the more malignant congenital long QT syndrome (LQTS) variants with an SCN5A gain-of-function effect on the gated sodium channel. Moreover, the predominant pathogenic variants in LQTS genes are single nucleotide substitutions (missense) and small insertion/deletions (INDEL). CRISPR/Cas9 genome editing has been utilised to create isogenic hiPSCs to control for an identical genetic background and to isolate the pathogenicity of a single nucleotide change. In this study, we described an optimized and rapid protocol to introduce a heterozygous LQT3-specific variant into healthy control hiPSCs using ribonucleoprotein (RNP) and single-stranded oligonucleotide (ssODN). Based on this protocol, we successfully screened hiPSCs carrying a heterozygous LQT3 pathogenic variant (SCN5A±) with high efficiency (6 out of 69) and confirmed no off-target effect, normal karyotype, high alkaline phosphatase activity, unaffected pluripotency, and in vitro embryonic body formation capacity within 2 weeks. In addition, we also provide protocols to robustly differentiate hiPSCs into cardiomyocytes and evaluate the electrophysiological characteristics using Multi-electrode Array. This protocol is also applicable to introduce and/or correct other disease-specific variants into hiPSCs for future pharmacological screening and gene therapeutic development.

Unlocking the Potential of Left Cardiac Sympathetic Denervation: A Scoping Review of a Promising Approach for Long QT Syndrome

Left cardiac sympathetic denervation (LCSD) has emerged as an alternative therapy for individuals diagnosed with long QT syndrome (LQTS), a genetic disorder characterized by abnormal electrical activity in the heart and sudden cardiac death (SCD). This review examines the history and rationale behind LCSD in LQTS treatment, as well as the procedure, its efficacy, and indications along with the adverse effects that may be associated with it. LQTS presents with prolonged QT intervals on an electrocardiogram and can manifest as seizures, fainting, and SCD. Beta-blockers are the primary treatment for LQTS but some patients do not respond well to these medications or experience side effects. Additionally, implantable cardioverter-defibrillators (ICDs) are not always effective in preventing arrhythmias and can lead to complications. LCSD might offer an alternative approach by disrupting sympathetic activity in the heart. In humans, LCSD reduces the release of norepinephrine, normalizes the QT interval, and decreases the likelihood of life-threatening heart rhythms. The procedure does not impair heart rate or cardiac function due to the compensatory effects of the right cardiac sympathetic nerves. The surgical procedure for LCSD involves the removal of the lower half of the stellate ganglion and thoracic ganglia. Complete denervation is essential for optimal outcomes, while incomplete procedures are considered unacceptable. Traditional and minimally invasive approaches, such as video-assisted thoracic surgery (VATS), are available, with VATS offering shorter hospital stays and fewer complications. In conclusion, LCSD provides a viable treatment option for individuals with LQTS who do not respond well to beta-blockers or require additional protection beyond medication or ICDs. Further research and clinical experience are needed to enhance its acceptance and implementation in routine practice.

The Role of TRPM4 in Cardiac Electrophysiology and Arrhythmogenesis

The transient receptor potential melastatin 4 (TRPM4) channel is a non-selective cation channel that activates in response to increased intracellular Ca²⁺ levels but does not allow Ca²⁺ to pass through directly. It plays a crucial role in regulating diverse cellular functions associated with intracellular Ca²⁺ homeostasis/dynamics. TRPM4 is widely expressed in the heart and is involved in various physiological and pathological processes therein. Specifically, it has a significant impact on the electrical activity of cardiomyocytes by depolarizing the membrane, presumably via Na⁺ loading. The TRPM4 channel likely contributes to the development of cardiac arrhythmias associated with specific genetic backgrounds and cardiac remodeling. This short review aims to overview what is known so far about the TRPM4 channel in cardiac electrophysiology and arrhythmogenesis, highlighting its potential as a novel therapeutic target to effectively prevent and treat cardiac arrhythmias.

A novel HECW2 variant in an infant with congenital long QT syndrome

Pathogenic variants of HECW2 have been reported in cases of neurodevelopmental disorder with hypotonia, seizures, and absent language (NDHSAL; OMIM #617268). A novel HECW2 variant (NM_001348768.2:c.4343 T > C,p.Leu1448Ser) was identified in an NDHSAL infant with severe cardiac comorbidities. The patient presented with fetal tachyarrhythmia and hydrops and was postnatally diagnosed with long QT syndrome. This study provides evidence that HECW2 pathogenic variants can cause long QT syndrome along with neurodevelopmental disorders.

Computational Study on Effect of KCNQ1 P535T Mutation in a Cardiac Ventricular Tissue

Heart diseases such as arrhythmia are the main causes of sudden death. Arrhythmias are typically caused by mutations in specific genes, damage in the cardiac tissue, or due to some chemical exposure. Arrhythmias caused due to mutation is called inherited arrhythmia. Induced arrhythmias are caused due to tissue damage or chemical exposure. Mutations in genes that encode ion channels of the cardiac cells usually result in (dysfunction) improper functioning of the channel. Improper functioning of the ion channel may lead to major changes in the action potential (AP) of the cardiac cells. This further leads to distorted electrical activity of the heart. Distorted electrical activity will affect the ECG that results in arrhythmia. KCNQ1 P535T mutation is one such gene mutation that encodes the potassium ion channel (KV7.1) of the cardiac ventricular tissue. Its clinical significance is not known. This study aims to perform a simulation study on P535T mutation in the KCNQ1 gene that encodes the potassium ion channel KV7.1 in the ventricular tissue grid. The effect of P535T mutation on transmural tissue grids for three genotypes (wild type, heterozygous, and homozygous) of cells are studied and the generated pseudo-ECGs are compared. Results show the delayed repolarization in the cells of ventricular tissue grid. Slower propagation of action potential in the transmural tissue grid is observed in the mutated (heterozygous and homozygous) genotypes. Longer QT interval is also observed in the pseudo-ECG of heterozygous and homozygous genotype tissue grids. From the pseudo-ECGs, it is observed that KCNQ1 P535T mutation leads to Long QT Syndrome (LQTS) which may result in life-threatening arrhythmias, such as Torsade de Pointes (TdP), Jervell and Lange-Nielsen syndrome (JLNS), and Romano-Ward syndrome (RWS).

Whole-exome sequencing and electrophysiological study reveal a novel loss-of-function mutation of KCNA10 in epinephrine provoked long QT syndrome with familial history of sudden cardiac death

Congenital long QT syndrome (LQTS) is one type of inherited fatal cardiac arrhythmia that may lead to sudden cardiac death (SCD). Mutations in more than 16 genes have been reported to be associated with LQTS, whereas the genetic causes of about 20% of cases remain unknown. In the present study, we investigated a four-generation pedigree with familial history of syncope and SCD. The proband was a 33-year-old young woman who experienced 3 episodes of syncope when walking at night. The electrocardiogram revealed a markedly epinephrine-provoked prolonged QT interval (QT = 468 ms, QTc = 651 ms) but no obvious arrhythmia in the resting state. Three family members have died of suspected SCD. Whole-exome sequencing and bioinformatic analysis based on pedigree revealed that a novel missense mutation KCNA10 (c.1397G＞A/Arg466Gln) was the potential genetic lesion. Sanger sequencing was performed to confirm the whole-exome sequencing results. This mutation resulted in the K_V1.8 channel amino acid residue 466 changing from arginine to glutamine, and the electrophysiological experiments verified it as a loss-of-function mutation of K_V1.8, which reduced the K⁺ currents of K_V1.8 and might result in the prolonged QT interval. These findings suggested that KCNA10 (c.1397G＞A) mutation was possibly pathogenic in this enrolled LQTS family, and may provide a new potential genetic target for diagnosis and counseling of stress-related LQTS families as well as the postmortem diagnosis of SCD.

Sinus node dysfunction: current understanding and future directions

The sinoatrial node (SAN) is the primary pacemaker of the heart. Normal SAN function is crucial in maintaining proper cardiac rhythm and contraction. Sinus node dysfunction (SND) is due to abnormalities within the SAN, which can affect the heartbeat frequency, regularity, and the propagation of electrical pulses through the cardiac conduction system. As a result, SND often increases the risk of cardiac arrhythmias. SND is most commonly seen as a disease of the elderly given the role of degenerative fibrosis as well as other age-dependent changes in its pathogenesis. Despite the prevalence of SND, current treatment is limited to pacemaker implantation, which is associated with substantial medical costs and complications. Emerging evidence has identified various genetic abnormalities that can cause SND, shedding light on the molecular underpinnings of SND. Identification of these molecular mechanisms and pathways implicated in the pathogenesis of SND is hoped to identify novel therapeutic targets for the development of more effective therapies for this disease. In this review article, we examine the anatomy of the SAN and the pathophysiology and epidemiology of SND. We then discuss in detail the most common genetic mutations correlated with SND and provide our perspectives on future research and therapeutic opportunities in this field.

Generation of two iPSC lines from long QT syndrome patients carrying SNTA1 variants

Long QT syndrome (LQTS) is an inherited cardiovascular disorder characterized by electrical conduction abnormalities leading to arrhythmia, fainting, seizures, and an increased risk of sudden death. There are over 15 genes involved in causing LQTS, including SNTA1. Here we generated two human-induced pluripotent stem cell (iPSC) lines from two LQT patients carrying a missense mutation in SNTA1 (c.1088A > C). Both lines showed normal morphological properties, expressed pluripotency markers, showed a normal karyotype profile, and had the ability to differentiate into the three germ layers, making them a valuable tool to model LQTS to investigate the pathological mechanisms related to this SNTA1 variant.

Exploring mutation specific beta blocker pharmacology of the pathogenic late sodium channel current from patient-specific pluripotent stem cell myocytes derived from long QT syndrome mutation carriers

The congenital long QT syndrome (LQTS), one of the most common cardiac channelopathies, is characterized by delayed ventricular repolarization underlying prolongation of the QT interval of the surface electrocardiogram. LQTS is caused by mutations in genes coding for cardiac ion channels or ion channel-associated proteins. The major therapeutic approach to LQTS management is beta blocker therapy which has been shown to be effective in treatment of LQTS variants caused by mutations in K⁺ channels. However, this approach has been questioned in the treatment of patients identified as LQTS variant 3(LQT3) patients who carry mutations in SCN5A, the gene coding for the principal cardiac Na⁺ channel. LQT3 mutations are gain of function mutations that disrupt spontaneous Na⁺ channel inactivation and promote persistent or late Na⁺ channel current (I_NaL) that delays repolarization and underlies QT prolongation. Clinical investigation of patients with the two most common LQT3 mutations, the ΔKPQ and the E1784K mutations, found beta blocker treatment a useful therapeutic approach for managing arrhythmias in this patient population. However, there is little experimental data that reveals the mechanisms underlying these antiarrhythmic actions. Here, we have investigated the effects of the beta blocker propranolol on I_NaL expressed by ΔKPQ and E1784K channels in induced pluripotent stem cells derived from patients carrying these mutations. Our results indicate that propranolol preferentially inhibits I_NaL expressed by these channels suggesting that the protective effects of propranolol in treating LQT3 patients is due in part to modulation of I_NaL.

shRNAs Targeting a Common KCNQ1 Variant Could Alleviate Long-QT1 Disease Severity by Inhibiting a Mutant Allele

Long-QT syndrome type 1 (LQT1) is caused by mutations in KCNQ1. Patients heterozygous for such a mutation co-assemble both mutant and wild-type KCNQ1-encoded subunits into tetrameric Kv7.1 potassium channels. Here, we investigated whether allele-specific inhibition of mutant KCNQ1 by targeting a common variant can shift the balance towards increased incorporation of the wild-type allele to alleviate the disease in human-induced pluripotent stem-cell-derived cardiomyocytes (hiPSC-CMs). We identified the single nucleotide polymorphisms (SNP) rs1057128 (G/A) in KCNQ1, with a heterozygosity of 27% in the European population. Next, we determined allele-specificity of short-hairpin RNAs (shRNAs) targeting either allele of this SNP in hiPSC-CMs that carry an LQT1 mutation. Our shRNAs downregulated 60% of the A allele and 40% of the G allele without affecting the non-targeted allele. Suppression of the mutant KCNQ1 allele by 60% decreased the occurrence of arrhythmic events in hiPSC-CMs measured by a voltage-sensitive reporter, while suppression of the wild-type allele increased the occurrence of arrhythmic events. Furthermore, computer simulations based on another LQT1 mutation revealed that 60% suppression of the mutant KCNQ1 allele shortens the prolonged action potential in an adult cardiomyocyte model. We conclude that allele-specific inhibition of a mutant KCNQ1 allele by targeting a common variant may alleviate the disease. This novel approach avoids the need to design shRNAs to target every single mutation and opens up the exciting possibility of treating multiple LQT1-causing mutations with only two shRNAs.

Novel CALM3 Variant Causing Calmodulinopathy With Variable Expressivity in a 4-Generation Family

BACKGROUND

CaM (calmodulin), encoded by 3 separate genes (CALM1, CALM2, and CALM3), is a multifunctional Ca²⁺-binding protein involved in many signal transduction events including ion channel regulation. CaM variants may present with early-onset long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia, or sudden cardiac death. Most reported variants occurred de novo. We identified a novel CALM3 variant, p.Asn138Lys (N138K), in a 4-generation family segregating with LQTS. The aim of this study was to elucidate its pathogenicity and to compare it with that of p.D130G-CaM-a variant associated with a severe LQTS phenotype.

METHODS

We performed whole exome sequencing for a large, 4-generation family affected by LQTS. To assess the effect of the detected CALM3 variant, the intrinsic Ca²⁺-binding affinity was measured by stoichiometric Ca²⁺ titrations and equilibrium titrations. L-type Ca²⁺ and slow delayed rectifier potassium currents (I_CaL and I_Ks) were recorded by whole-cell patch-clamp. Cav1.2 and Kv7.1 membrane expression were determined by optical fluorescence assays.

RESULTS

We identified 14 p.N138K-CaM carriers in a family where 2 sudden deaths occurred in children. Several members were only mildly affected compared with CaM-LQTS patients to date described in literature. The intrinsic Ca²⁺-binding affinity of the CaM C-terminal domain was 10-fold lower for p.N138K-CaM compared with wild-type-CaM. I_CaL inactivation was slowed in cells expressing p.N138K-CaM but less than in p.D130G-CaM cells. Unexpectedly, a larger I_Ks current density was observed in cells expressing p.N138K-CaM, but not for p.D130G-CaM, compared with wild-type-CaM.

CONCLUSIONS

The p.N138K CALM3 variant impairs Ca²⁺-binding affinity of CaM and I_CaL inactivation but potentiates I_Ks. The variably expressed phenotype of this variant compared with previously published de novo LQTS-CaM variants is likely explained by a milder impairment of I_CaL inactivation combined with I_Ks augmentation.

Frequent torsades de pointes in a child with novel AKAP9 mutation: A case report and literature review

INTRODUCTION

The aim of the present study is to report the diagnosis and treatment of a rare case of frequent torsades de pointes (Tdp) in a child with a novel AKAP9 mutation. A 13-year-old girl suffered from repeated syncope and frequent Tdp. An electrocardiogram (ECG) showed frequent multisource premature ventricular contractions with the R-ON-T phenomenon. The QTc ranged from 410 to 468 ms. The genetic test indicated a heterozygous mutation, namely, c.11714T > C (p.M3905T), in the AKAP9 gene, which is a controversial gene in long QT syndrome. After treatment with propranolol, recurrent syncope occurred, and the patient received an implantable cardioverter defibrillator (ICD). Due to frequent electrical storms at home, the child was additionally treated with propafenone to prevent arrhythmia. The antitachycardia pacing (ATP) function in the ICD was turned off, and the threshold of ventricular tachycardia (VT) assessment was adjusted from 180 beats/min to 200 beats/min. The patient was followed up for 12 months without malignant arrhythmia and electric shock.

CONCLUSION

Genetic testing may be a useful tool to determine the origin of channelopathy, but the results should be interpreted in combination with the actual situation. Rational parameter settings for the ICD and application of antiarrhythmic drugs can reduce the mortality rates of children.

The Outcome of Long QT Syndrome, a Korean Single Center Study

Congenital long QT syndrome is an inherited cardiac channelopathy, causing fatal arrhythmia. In this study, we conducted a retrospective cohort study on 105 congenital LQTS patients and its outcome in a tertiary center. The 10-year event free survival rate was 73.2%, and the outcome was different according to the genotypes. With treatment, all survived except one. The genetic analysis and risk stratification may be essential for better outcome and further nationwide and large scaled studies are required.

Background and Objectives

Although long QT syndrome (LQTS) is a potentially life-threatening inherited cardiac channelopathy, studies documenting the long-term clinical data of Korean patients with LQTS are scarce.

Methods

This retrospective cohort study included 105 patients with LQTS (48 women; 45.7%) from a single tertiary center. The clinical outcomes were analyzed for the rate of freedom from breakthrough cardiac events (BCEs), additional treatment needed, and death.

Results

LQTS was diagnosed at a median age of 11 (range, 0.003–80) years. Genetic testing was performed on 90 patients (yield, 71.1%). The proportions of genetically confirmed patients with LQTS types 1, 2, 3, and others were 34.4%, 12.2%, 12.2%, and 12.2%, respectively. In the symptomatic group (n=70), aborted cardiac arrest was observed in 30% of the patients. Treatments included medications in 60 patients (85.7%), implantable cardioverter-defibrillators in 27 (38.6%; median age, 17 years; range, 2–79 years), and left cardiac sympathetic denervation surgery in 7 (10%; median age, 13 years; range, 2–34). The 10-year BCE-free survival rate was 73.2%. By genotype, significant differences were observed in BCEs despite medication (p<0.001). The 10-year BCE-free survival rate was the highest in patients with LQTS type 1 (81.8%) and the lowest in those with multiple LQTS-associated mutations (LQTM). All patients with LQTS survived, except for one patient who had LQTM.

Conclusions

Good long-term outcomes can be achieved by using recently developed genetically tailored management strategies for patients with LQTS.

Compendium of causative genes and their encoded proteins for common monogenic disorders

A compendium is presented of inherited monogenic disorders that have a prevalence of >1:20,000 in the human population, along with their causative genes and encoded proteins. "Simple" monogenic diseases are those for which the clinical features are caused by mutations impacting a single gene, usually in a manner that alters the sequence of the encoded protein. Of course, for a given "monogenic disorder", there is sometimes more than one potential disease gene, mutations in any one of which is sufficient to cause phenotypes of that disorder. Disease-causing mutations for monogenic disorders are usually passed on from generation to generation in a Mendelian fashion, and originate from spontaneous (de novo) germline founder mutations. In the past monogenic disorders have often been written off as targets for drug discovery because they sometimes are assumed to be rare disorders, for which the meager projected financial payoff of drug discovery and development has discouraged investment. However, not all monogenic diseases are rare. Here, we report that that currently available data identifies 72 disorders with a prevalence of at least 1 in 20,000 humans. For each, we tabulate the gene(s) for which mutations cause the spectrum of phenotypes associated with that disorder. We also identify the gene and protein that most commonly causes each disease. 34 of these disorders are caused exclusively by mutations in only a single gene and encoded protein.

Develop and Apply Electrocardiography-Based Risk Score to Identify Community-Based Elderly Individuals at High-Risk of Mortality

With an aging world population, risk stratification of community-based, elderly population is required for primary prevention. This study proposes a combined score developed using electrocardiographic (ECG) parameters and determines its long-term prognostic value for predicting risk of cardiovascular mortality. A cohort-study, conducted from December 2008 to April 2019, enrolled 5,380 subjects in Taiwan, who were examined, using three-serial-12-lead ECGs, and their health/demographic information were recorded. To understand the predictive effects of ECG parameters on overall-survival, Cox hazard regression analysis were performed. The mean age at enrollment was 69.04 ± 8.14 years, and 47.4% were males. ECG abnormalities, LVH [hazard ratio (HR) = 1.39, 95% confidence intervals (CI) = (1.16–1.67), P = 0.0003], QTc [HR = 1.31, CI = (1.07–1.61), P = 0.007] and PR interval [HR = 1.40, CI = (1.01–1.95), P = 0.04], were significantly associated with primary outcome all-cause death. Furthermore, LVH [HR = 2.37, CI = (1.48–3.79), P = 0.0003] was significantly associated with cardiovascular death, while PR interval [HR = 2.63, CI = (1.24– 5.57), P = 0.01] with unexplained death. ECG abnormality (EA) score was defined based on the number of abnormal ECG parameters for each patient, which was used to divide all patients into sub-groups. Competing risk survival analysis using EA score were performed by using the Gray's test, which reported that high-risk EA groups showed significantly higher cumulative incidence for all three outcomes. Prognostic models using the EA score as predictor were developed and a 10-fold cross validation design was adopted to conduct calibration and discrimination analysis, to establish the efficacy of the proposed models. Overall, ECG model could successfully predict people, susceptible to all three death outcomes (P < 0.05), with high efficacy. Statistically significant (P < 0.001) improvement of the c-indices further demonstrated the robustness of the prediction model with ECG parameters, as opposed to a traditional model with no EA predictor. The EA score is highly associated with increased risk of mortality in elderly population and may be successfully used in clinical practice.

Long QT syndrome with potassium voltage-gated channel subfamily H member 2 gene mutation mimicking refractory epilepsy: case report

BACKGROUND

Epileptic seizures can be difficult to distinguish from other etiologies that cause cerebral hypoxia, especially cardiac diseases. Long QT syndrome (LQTS), especially LQTS type 2 (LQT2), frequently masquerades as seizures because of the transient cerebral hypoxia caused by ventricular arrhythmia. The high rate of sudden death in LQTS highlights the importance of accurate and early diagnosis; correct diagnosis of LQTS also prevents inappropriate treatment with anti-epileptic drugs (AEDs).

CASE PRESENTATION

We report a case of congenital LQT2 with potassium voltage-gated channel subfamily H member 2 gene (KCNH2) mutation misdiagnosed as refractory epilepsy and treated with various AEDs for 22 years. The possibility of cardiac arrhythmia was suspected after the patient presented to the emergency room and the electrocardiograph (ECG) monitor showed paroxysmal ventricular tachycardia during attacks. Atypical seizure like attacks with prodromal uncomfortable chest sensation and palpitation, triggered by auditory stimulation, and typical ventricular tachycardia monitored by ECG raised suspicion for LQT2, which was confirmed by exome sequencing and epileptic seizure was ruled out by 24-h EEG monitoring. Although the patient rejected implantation of an implantable cardioverter defibrillator, β blocker was given and the syncope only attacked 1-2 per year when there was an incentive during the 5 years follow up.

CONCLUSIONS

Our case illustrates how long LQTS can masquerade convincingly as epilepsy and can be treated wrongly with AEDs, putting the patient at high risk of sudden cardiac death. Careful ECG evaluation is recommend for both patients with first seizure and those with refractory epilepsy.

A Systematic Review on the Role of Βeta-Blockers in Reducing Cardiac Arrhythmias in Long QT Syndrome Subtypes 1-3

Long QT syndrome (LQTS) is one of the most common inherited cardiac channelopathies with a prevalence of 1:2000. The condition can be congenital or acquired with 15 recognized genotypes; the most common subtypes are LQTS 1, 2, and 3 making up to 85%-90% of the cases. LQTS is characterized by delayed ventricular cardiomyocyte repolarization manifesting on the surface electrocardiogram (EKG) by a prolonged corrected QT (QTc) interval. The mainstay of treatment for this condition involves in part or combination medical therapy via β-blockers as first-line (or other anti-arrhythmic), left cardiac sympathectomy, or implantable cardiac defibrillator placement. Given the high rate of adverse cardiac events (ACE) or sudden cardiac death (SCD) in this population of patients with this disease, this review seeks to highlight the genotype-specific treatment consensus in β-blocker therapy of the most common subtypes. A database search of PubMed, PMC, and Medline was conducted to ascertain the most recent data in the last five years on the management of LQTS types 1-3 and the role of β-blockers in reducing ACE in these types. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines were adhered to in the study selection, and selected studies focused on humans, written in the English Language, and within the last five years of LQTS subtypes 1, 2, and 3. Eleven relevant studies were selected after considering inclusion criteria, exclusion criteria, and quality appraisal within the last five years, focusing on β-blocker selection directed based on the subtypes of LQTS. Two meta-analyses, one cohort study, and eight reviews provided significant data that non-selective β-blockers unequivocally are of benefit in these LQTS types. Summary of findings suggested nadolol followed by propranolol yields the best results in LQTS 1, while nadolol would yield the best effect in LQTS 2 and 3.

Gene therapy for inherited arrhythmias

Inherited arrhythmias are disorders caused by one or more genetic mutations that increase the risk of arrhythmia, which result in life-long risk of sudden death. These mutations either primarily perturb electrophysiological homeostasis (e.g. long QT syndrome and catecholaminergic polymorphic ventricular tachycardia), cause structural disease that is closely associated with severe arrhythmias (e.g. hypertrophic cardiomyopathy), or cause a high propensity for arrhythmia in combination with altered myocardial structure and function (e.g. arrhythmogenic cardiomyopathy). Currently available therapies offer incomplete protection from arrhythmia and fail to alter disease progression. Recent studies suggest that gene therapies may provide potent, molecularly targeted options for at least a subset of inherited arrhythmias. Here, we provide an overview of gene therapy strategies, and review recent studies on gene therapies for catecholaminergic polymorphic ventricular tachycardia and hypertrophic cardiomyopathy caused by MYBPC3 mutations.

The zebrafish grime mutant uncovers an evolutionarily conserved role for Tmem161b in the control of cardiac rhythm

The establishment of cardiac function in the developing embryo is essential to ensure blood flow and, therefore, growth and survival of the animal. The molecular mechanisms controlling normal cardiac rhythm remain to be fully elucidated. From a forward genetic screen, we identified a unique mutant, grime, that displayed a specific cardiac arrhythmia phenotype. We show that loss-of-function mutations in tmem161b are responsible for the phenotype, identifying Tmem161b as a regulator of cardiac rhythm in zebrafish. To examine the evolutionary conservation of this function, we generated knockout mice for Tmem161b. Tmem161b knockout mice are neonatal lethal and cardiomyocytes exhibit arrhythmic calcium oscillations. Mechanistically, we find that Tmem161b is expressed at the cell membrane of excitable cells and live imaging shows it is required for action potential repolarization in the developing heart. Electrophysiology on isolated cardiomyocytes demonstrates that Tmem161b is essential to inhibit Ca²⁺ and K⁺ currents in cardiomyocytes. Importantly, Tmem161b haploinsufficiency leads to cardiac rhythm phenotypes, implicating it as a candidate gene in heritable cardiac arrhythmia. Overall, these data describe Tmem161b as a highly conserved regulator of cardiac rhythm that functions to modulate ion channel activity in zebrafish and mice.

Comparative Characteristics of Beta-Blockers in Patients with Congenital Long QT Syndrome

Congenital long QT syndrome is a pathology that requires special attention and knowledge about the safety and effectiveness of various medications. Prolongation of the QT interval due to congenital or acquired causes is an important factor in the development of an unfavorable life forecast with the formation of an elongated QT syndrome. With an unfavorable course, patients suffer from loss of consciousness, episodes of tachycardia. Often, stable polymorphic ventricular tachycardia develops. The risk of sudden cardiac death in this pathology can vary from 0.33% to 5%. In people who have suffered an episode of cardiac arrest, and do not have a permanent prescribed antiarrhythmic therapy, the mortality rate reaches 50% within 15 years. Preventive administration of antiarrhythmic drugs is not always effective. A positive result of treatment depends on the severity of long QT syndrome and its genotype. Beta-blockers are often prescribed to patients of different ages with various cardiac pathologies, including for the prevention of arrhythmia in long QT syndrome. Beta-blockers differ in various pharmacokinetic and pharmacodynamic parameters (lipophilicity/hydrophilicity, selectivity, presence/absence of internal sympathomimetic activity), which, along with the variant of the disease genotype, can affect their effectiveness and safety in the considered pathology. This review article presents the results of major studies on the safety and effectiveness of different groups of beta blockers in various variants of long QT syndrome. The preferred beta-blockers for various genotypes of the syndrome were determined, and a comparative characteristic of beta-blockers for their safety and preventive effectiveness was given.

Congenital Long QT Syndrome: A Clinician's Guide

Congenital long QT syndrome (LQTS) is a familial cardiac ion channelopathy first described over sixty years ago. It is characterised by prolonged ventricular repolarization (long QT on ECG), ventricular arrhythmias and associated syncope or sudden cardiac death. As the most closely studied cardiac channelopathy, over the decades we have gained a deep appreciation of the complex genetic model of LQTS. Variability in genetic expression and incomplete penetrance leads to a heterogenous phenotype that can be challenging to clinically classify. In recent times, progress has been made in diagnostic method, risk stratification and treatment options. This review has been written as a guide for the general cardiologist to understand the basic pathophysiology, diagnosis, and management priorities for the most encountered LQTS subtypes: LQT1, LQT2 and LQT3. This article is protected by copyright. All rights reserved.

Successful Postnatal Cardiopulmonary Resuscitation Due to Defibrillation

An asphyxiated term neonate required postnatal resuscitation. After six minutes of cardio-pulmonary resuscitation (CPR) and two doses of epinephrine, spontaneous circulation returned, but was shortly followed by ventricular fibrillation. CPR and administration of magnesium, calcium gluconate, and sodium bicarbonate did not improve the neonate’s condition. A counter shock of five Joule was delivered and the cardiac rhythm immediately converted to sinus rhythm. The neonate was transferred to the neonatal intensive care unit and received post-resuscitation care. Due to prolonged QTc and subsequently suspected long-QT syndrome propranolol treatment was initiated. The neonate was discharged home on day 14 without neurological sequelae.

Early Identification of Prolonged QT Interval for Prevention of Sudden Infant Death

Introduction: Long QT syndrome is the main arrhythmogenic disease responsible for sudden death in infants, especially in the first days of life. Performing an electrocardiogram in newborns could enable early diagnosis and adoption of therapeutic measures focused on preventing lethal arrhythmogenic events. However, the inclusion of an electrocardiogram in neonatal screening protocols still remains a matter of discussion. To comprehensively analyse the potential clinical value of performing an electrocardiogram and subsequent follow-up in a cohort of newborns. Methods: Electrocardiograms were performed in 685 neonates within the first week of life. One year follow-up was performed if QTc > 450 ms identified. Comprehensive genetic analysis using massive sequencing was performed in all cases with QTc > 470 ms. Results: We identified 54 neonates with QTc > 450 ms/ <470 ms; all normalized QTc values within 6 months. Eight cases had QTc > 480 ms at birth and, if persistent, pharmacological treatment was administrated during follow-up. A rare variant was identified as the potential cause of long QT syndrome in five cases. Three cases showed a family history of sudden arrhythmogenic death. Conclusions: Our prospective study identifies 0.14% of cases with a definite long QT, supporting implementation of electrocardiograms in routine pediatric protocols. It is an effective, simple and non-invasive approach that can help prevent sudden death in neonates and their relatives. Genetic analyses help to unravel the cause of arrhythmogenic disease in diagnosing neonates. Further, clinical assessment and genetic analysis of relatives allowed early identification of family members at risk of arrhythmias helping to adopt preventive personalized measures.

Torsades de Pointes During Myringotomy in a Child with Congenital Long QT Syndrome: A Case Report

Patient: Male, 7-year-old

Final Diagnosis: Torsades de pointes

Symptoms: Ventricular tachycardia

Medication: Propranolol • Magnesium

Clinical Procedure: Bilateral myringotomy tubes

Specialty: Anesthesiology • Cardiology • Pediatrics and Neonatology

Propranolol Attenuates Late Sodium Current in a Long QT Syndrome Type 3-Human Induced Pluripotent Stem Cell Model

Background

Long QT syndrome type 3 (LQT3) is caused by gain-of-function mutations in the SCN5A gene, which encodes the α subunit of the cardiac voltage-gated sodium channel. LQT3 patients present bradycardia and lethal arrhythmias during rest or sleep. Further, the efficacy of β-blockers, the drug used for their treatment, is uncertain. Recently, a large multicenter LQT3 cohort study demonstrated that β-blocker therapy reduced the risk of life-threatening cardiac events in female patients; however, the detailed mechanism of action remains unclear.

Objectives

This study aimed to establish LQT3-human induced pluripotent stem cells (hiPSCs) and to investigate the effect of propranolol in this model.

Method

An hiPSCs cell line was established from peripheral blood mononuclear cells of a boy with LQT3 carrying the SCN5A-N1774D mutation. He had suffered from repetitive torsades de pointes (TdPs) with QT prolongation since birth (QTc 680 ms), which were effectively treated with propranolol, as it suppressed lethal arrhythmias. Furthermore, hiPSCs were differentiated into cardiomyocytes (CMs), on which electrophysiological functional assays were performed using the patch-clamp method.

Results

N1774D-hiPSC-CMs exhibited significantly prolonged action potential durations (APDs) in comparison to those of the control cells (N1774D: 440 ± 37 ms vs. control: 272 ± 22 ms; at 1 Hz pacing; p < 0.01). Furthermore, N1774D-hiPSC-CMs presented gain-of-function features: a hyperpolarized shift of steady-state activation and increased late sodium current compared to those of the control cells. 5 μM propranolol shortened APDs and inhibited late sodium current in N1774D-hiPSC-CMs, but did not significantly affect in the control cells. In addition, even in the presence of intrapipette guanosine diphosphate βs (GDPβs), an inhibitor of G proteins, propranolol reduced late sodium current in N1774D cells. Therefore, these results suggested a unique inhibitory effect of propranolol on late sodium current unrelated to β-adrenergic receptor block in N1774D-hiPSC-CMs.

Conclusion

We successfully recapitulated the clinical phenotype of LQT3 using patient-derived hiPSC-CMs and determined that the mechanism, by which propranolol inhibited the late sodium current, was independent of β-adrenergic receptor signaling pathway.

Dynamic QT response to cold-water face immersion in long-QT syndrome type 3

BACKGROUND

Abnormal dynamics of QT intervals in response to sympathetic nervous system stimulation are used to diagnose long-QT syndrome (LQTS). We hypothesized that parasympathetic stimulation with cold-water face immersion following exercise would influence QT dynamics in patients with LQTS type 3 (LQT3).

METHODS

Study participants (n = 42; mean age = 11.2 years) comprised 20 genotyped LQTS children and 22 healthy children. The LQTS group was divided into LQT3 (n = 12) and non-LQT3 (n = 8) subgroups. Provocative testing for assessing QT dynamics comprised a treadmill exercise followed by cold-water face immersion. The QT intervals were automatically measured at rest and during exercise, recovery, and cold-water face immersion. The QT/heart rate (HR) relationship was visualized by plotting beat-to-beat confluence of the data.

RESULTS

The QT/HR slopes, determined by linear regression analysis, were steeper in the LQTS group than in the control group during exercise and immersion tests: -2.16 ± 0.63 versus -1.21 ± 0.28, P < 0.0001, and -2.02 ± 0.76 vs -0.75 ± 0.24, P < 0.0001, respectively. The LQT3 patients had steeper slopes in the immersion test than did non-LQT3 and control individuals: -2.42 ± 0.52 vs -1.40 ± 0.65, P < 0.0001, and vs -0.75 ± 0.24, P < 0.0001.

CONCLUSIONS

The QT dynamics of LQT3 patients differ from those of other LQTS subtypes during the post-exercise cold-water face immersion test in this study. Abnormal QT dynamics during the parasympathetic provocative test are concordant with the fact that cardiac events occur when HRs are lower or during sleep in LQT3 patients.

An NGS-based genotyping in LQTS; minor genes are no longer minor

Mutations in KCNQ1, KCNH2, and SCN5A are the major cause of long QT syndrome (LQTS). More than 90% of the genotyped patients have been reported to carry mutations in any of these three genes. Thanks to increasing popularity of next generation sequencer (NGS), novel CACNA1C mutations have been identified among LQTS patients without extra-cardiac phenotypes. We aimed to clarify the frequency of genotypes in LQTS patients in the era of NGS. The study comprised 160 congenital LQTS patients (71 males) registered from November 2015 to September 2018. Inclusion criteria was QTc > 460 ms and Schwartz score ≥ 3. We performed genetic analysis using target gene method by NGS and confirmed the mutations by Sanger method. The median age for genetic screening was 13 (0-68) years. Sixteen patients suffered cardiac arrest, 47 syncope, and 97 were asymptomatic. We identified genetic mutations in 111 (69.4%) patients including 6 CACNA1C (5.4% of the genotyped patients) with 4 asymptomatic patients. Five (3.1%) patients carried double mutations; three out of them with RYR2 and KCNQ1 or KCNH2. In conclusion, CACNA1C screening would be recommended even if the patient is asymptomatic to elucidate the genetic background of the LQTS patients.

Pilsicainide Administration Unmasks a Phenotype of Brugada Syndrome in a Patient with Overlap Syndrome due to the E1784K SCN5A Mutation

Mutations in the cardiac sodium channel SCN5A can cause phenotypic overlap syndrome of long QT syndrome and Brugada syndrome. However, Brugada-type ST elevations in patients with overlap syndrome are often concealed, which creates a diagnostic challenge. A 38-year-old man was admitted due to ventricular fibrillation (VF). The 12-lead electrocardiogram showed a prolonged QT interval and saddleback-type ST elevation. Pilsicainide administration induced coved-type ST elevation and VF triggered by a single premature ventricular contraction. A genetic analysis showed an SCN5A c.5350G>A p.E1784K mutation. The present case suggests the importance of a drug administration test being performed in the clinical management of overlap syndrome.

Long QT syndrome with a de novo CALM2 mutation in a 4-year-old boy

BACKGROUND

Human calmodulin (CALM) gene mutation has been reported to be related to inherited arrhythmia syndromes, but the genotype-phenotype relationship remains unclear.

METHODS AND RESULTS

We report here a 4-year-old boy who had cardiac arrest while playing in a kindergarten playground. Cardiopulmonary resuscitation was initiated immediately. Eleven minutes after the cardiac arrest, ambulance crews arrived and an automated external defibrillator was attached. His heart rhythm, which was ventricular fibrillation (VF), was returned to sinus rhythm after only one shock delivery. The boy was brought to hospital by air ambulance. During transfer, electrocardiogram (ECG) showed transient VF. On arrival, chest radiograph showed a cardiothoracic ratio of 55% without pulmonary congestion. A 12-lead ECG showed a normal sinus rhythm, biphasic T wave, and prolongation of the corrected QT interval. On ECG, VF was preceded by torsade de pointes or frequent polymorphic premature ventricular contractions (PVC). Echocardiography showed a normal heart structure with decreased cardiac function. On the second day of hospitalization, ECG showed remarkable QT prolongation, T-wave alternans, and frequent PVC. Thereafter, propranolol was started. The ECG showed rapid improvement of QT prolongation and T-wave abnormality. Genetic test indicated a CALM2 mutation, and he was diagnosed with long QT syndrome-15 (LQT15).

CONCLUSIONS

CALM mutations cause long QT syndrome (LQTS), catecholaminergic polymorphic ventricular tachycardia (CPVT) and idiopathic VF. This patient with a CALM2 p.N98S mutation had both phenotypes of LQTS and CPVT.

Mutation Screening of KCNQ1 and KCNE1 Genes in Iranian Patients With Jervell and Lange-Nielsen Syndrome

Background: Jervell and Lange-Nielsen syndrome (JLNS) is an autosomal recessive genetic disease with deafness and QT prolongation. Mutations in KCNQ1 and KCNE1 genes are a cause of JLNS. Our objective was to perform mutational analysis of the KCNQ1 and KCNE1 genes to determine the frequency of mutations in the Iranian population. Material and methods: Fourteen patients and their families were investigated. Mutational screening of the KCNQ1 and KCNE1 genes was performed by a polymerase chain reaction (PCR) followed by direct Sanger sequencing. Results: We identified two frameshift mutations in the KCNQ1 gene, including a novel mutation, c.1356 1356delG, and a known mutation, c.1534_1534delG. A common single nucleotide polymorphism (SNP), c.112G > A, was also found in KCNE1 in seven probands. Conclusion: A novel mutation in the KCNQ1 gene is described. There may be less frequency of mutations in the KCNQ1 and of KCNE1 genes in Iranian JLNS patients compared with other populations.

The Taiwan Heart Registries: Its Influence on Cardiovascular Patient Care

Taiwanese heart registries for the main cardiovascular diseases have been conducted in the past 10 years, with the goal of examining the quality of cardiovascular patient care, which cannot be guaranteed by the universal Taiwan National Health Insurance. The results show suboptimal adherence to guideline recommendations. Door-to-balloon time and dual antiplatelet therapy use in acute coronary syndrome, standard medications for management of heart failure, low-density lipoprotein cholesterol levels in dyslipidemia, anticoagulant agent use in atrial fibrillation, and the understanding of sudden arrhythmia death syndrome were all found to be inadequate. However, all were improved, either by changing National Health Insurance policy or through continuous education for physicians and patients. Thus, specific cardiovascular disease registries could help examine the status of real-world practice, find inadequacies in guideline implementation and understanding of rare diseases, facilitate lobbying to policy makers and education for physicians and patients, and influence and improve cardiovascular patient care.

Long QT Syndrome: Genetics and Future Perspective

Long QT syndrome (LQTS) is an inherited primary arrhythmia syndrome that may present with malignant arrhythmia and, rarely, risk of sudden death. The clinical symptoms include palpitations, syncope, and anoxic seizures secondary to ventricular arrhythmia, classically torsade de pointes. This predisposition to malignant arrhythmia is from a cardiac ion channelopathy that results in delayed repolarization of the cardiomyocyte action potential. The QT interval on the surface electrocardiogram is a summation of the individual cellular ventricular action potential durations, and hence is a surrogate marker of the abnormal cellular membrane repolarization. Severely affected phenotypes administered current standard of care therapies may not be fully protected from the occurrence of cardiac arrhythmias. There are 17 different subtypes of LQTS associated with monogenic mutations of 15 autosomal dominant genes. It is now possible to model the various LQTS phenotypes through the generation of patient-specific induced pluripotent stem cell-derived cardiomyocytes. RNA interference can silence or suppress the expression of mutant genes. Thus, RNA interference can be a potential therapeutic intervention that may be employed in LQTS to knock out mutant mRNAs which code for the defective proteins. CRISPR/Cas9 is a genome editing technology that offers great potential in elucidating gene function and a potential therapeutic strategy for monogenic disease. Further studies are required to determine whether CRISPR/Cas9 can be employed as an efficacious and safe rescue of the LQTS phenotype. Current progress has raised opportunities to generate in vitro human cardiomyocyte models for drug screening and to explore gene therapy through genome editing.

Impact of disease state on arrhythmic event detection by action potential modelling in cardiac safety pharmacology

INTRODUCTION

The use of in silico cardiac action potential simulations is one of the pillars of the CiPA initiative (Comprehensive in vitro Proarrhythmia Assay) currently under evaluation designed to detect more accurately proarrhythmic liabilities of new drug candidate. In order to take into account the variability of clinical situations, we propose to improve this method by studying the impact of various disease states on arrhythmic events induced by 30 torsadogenic or non-torsadogenic compounds.

METHOD

In silico modelling was done on the human myocytes using the Dutta revised O'Hara-Rudy algorithm. Results were analysed using a new metric based on the compound IC_50s against the seven cardiac ionic currents considered to be the most important by the CiPA initiative (I_Kr, I_Ks, I_Na, I_NaL, I_K1, I_to, I_CaL) and the minimal rate of action potential voltage decrease calculated at the early-afterdepolarization (EAD) take-off membrane voltage (V_min).

RESULTS

The specific threshold at which each torsadogenic compounds induced EAD, was exacerbated by the presence of cardiac risk factors ranked as follows: congestive heart failure > hypertrophic cardiomyopathy > cardiac pause > no risk factor. Non-torsadogenic compounds induced no EAD even in the presence of cardiac risk factors.

DISCUSSION

The present study highlighted the impact of pre-existing cardiovascular disease on arrhythmic event detection suggesting that disease state modelling may need to be incorporated in order to fully realize the goal of the CiPA paradigm in a more accurate predictability of proarrhythmic liabilities of new drug candidate.

Functional analysis of KCNH2 gene mutations of type 2 long QT syndrome in larval zebrafish using microscopy and electrocardiography

Heterologous expression systems play a vital role in the characterization of potassium voltage-gated channel subfamily H member 2 (KCNH2) gene mutations, such as E637K which is associated with long QT syndrome type 2 (LQT2). In vivo assays using zebrafish provide a means for testing genetic variants of cardiac disease; however, limited information on the role of the E637K mutation is available from in vivo systems and their utility has yet to be fully exploited in the context of LQT2. We sought to evaluate the ability of the E637K mutant channel to restore normal repolarization in larval zebrafish with a human KCNH2 orthologue, kcnh2a-knockdown. A morpholino (MO) targeting kcnh2a was injected alone or with wild type (WT) or E637K KCNH2 cRNA into zebrafish embryos at the 1-2 cell stage. Cardiac repolarization phenotypes were screened using light microscopy and the QT interval was measured by single lead electrocardiograph (ECG) analysis at 72-h post-fertilization. In the MO alone group, 17% of zebrafish had a normal phenotype; this rate increased to 60% in the WT KCNH2 cRNA injected zebrafish and to 35% in the E637K injected zebrafish. The ECG of larval zebrafish revealed that QTc was significantly prolonged in the MO alone group compared to the control group. Co-injection of WT KCNH2 cRNA shortened the QTc interval, however, that of the E637K did not. We suggest that this in vivo cardiac assay using microscopy and ECG in larval zebrafish offers a reliable approach for risk discrimination of KCNH2 mutations.

Detection of a new KCNQ1 frameshift mutation associated with Jervell and Lange-Nielsen syndrome in 2 Iranian families

Jervell-Lange Nielsen syndrome (JLNS) with autosomal recessive inheritance is a congenital cardiovascular disorder characterized by prolongation of QT interval on the ECG and deafness. We have performed molecular investigation by haplotype analysis and DNA Sanger sequencing in 2 unrelated Iranian families with a history of syncope. Mutational screening of KCNQ1 gene revealed the novel homozygous frameshift mutation c.733-734delGG (p.G245Rfs*39) in 2 obviously unrelated cases of JLNS which is probably a founder mutation in Iran. The novel mutation detected in this study is the first time reported among Iranian population and will be beneficial in the tribe and region-specific cascade screening of LQTS in Iran.

Clinical Spectrum of SCN5A Mutations: Long QT Syndrome, Brugada Syndrome, and Cardiomyopathy

SCN5A gene encodes the pore-forming ion-conducting α-subunit of the cardiac sodium channel (Na_v1.5), which is responsible for the initiation and propagation of action potentials and thereby determines cardiac excitability and conduction of electrical stimuli through the heart. The importance of Na_v1.5 for normal cardiac electricity is reflected by various disease entities that can be caused by mutations in SCN5A. Gain-of-function mutations in SCN5A lead to more sodium influx into cardiomyocytes through aberrant channel gating and cause long QT syndrome, a primary electrical disease of the heart. Loss-of-function mutations in SCN5A lead to lower expression levels of SCN5A or production of defective Na_v1.5 proteins and cause Brugada syndrome, an electrical disease with minor structural changes in the heart. In addition, both loss- and gain-of-function mutations may cause dilated cardiomyopathy, which is an arrhythmogenic disease with gross structural defects of the left ventricle (and sometimes both ventricles). Other SCN5A-related diseases are multifocal ectopic premature Purkinje-related complexes (gain-of-function mutations), isolated cardiac conduction defect (loss-of-function mutations), sick sinus syndrome (loss-of-function mutations), atrial fibrillation (loss-of-function or gain-of-function mutations), and overlap syndromes (mutations with both loss-of-function and gain-of-function effects). Growing insights into the role of SCN5A in health and disease has enabled clinicians to lay out gene-specific risk stratification schemes and mutation-specific diagnostic and therapeutic strategies in the management of patients with a SCN5A mutation. This review summarizes currently available knowledge about the pathophysiological mechanisms of SCN5A mutations and describes how this knowledge can be used to manage patients suffering from potentially lethal cardiac diseases.

Mode of initiation and clinical significance of malignant rapid ventricular arrhythmias: An observational study

The purpose of this study was to explore the modes of initiation and clinical significance of malignant rapid ventricular arrhythmias (MRVAs).The surface 12-lead electrocardiogram (ECG) or sustained electrocardiomonitor graph was analyzed in 79 patients. All patients had at least 1 MRVA after being admitted to the hospital.According to the length of coupling interval of the initial premature ventricular contraction of MRVA, the modes of initiation of MRVA were divided into the following types: those initiated by premature ventricular contraction with short coupling intervals in patients with normal Q-T interval, and for which short-long-short sequences before MRVA precipitation were not observed; those initiated following short-long-short sequences, which were divided into 2 types according to the length of Q-T interval: a normal Q-T interval and a long Q-T interval. On the basis of the different modes of onset, treatments of MRVA were different.MRVAs have different modes of onset depending on the patients' underlying condition. Prompt recognition of the mode of onset is necessary to facilitate appropriate management. These findings could have important pathophysiologic and clinical implications.

Cardiac channelopathies: The role of sodium channel mutations

INTRODUCTION AND OBJECTIVES

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

METHODS

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

CONCLUSIONS

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