Molecular and clinical characterization of a novel SCN5A mutation associated with atrioventricular block and dilated cardiomyopathy

Cellular-level analyses of SCN5A mutations in left ventricular noncompaction cardiomyopathy suggest electrophysiological mechanisms for ventricular tachycardia

Left ventricular noncompaction cardiomyopathy (LVNC) is a cardiovascular disease characterized by arrhythmia and heart failure. In this study, LVNC myocardial samples were collected from patients who underwent heart transplantation and were analyzed using exome sequencing. Approximately half of the LVNC patients carried SCN5A variants, which are associated with clinical symptoms of ventricular tachycardia. To investigate the electrophysiological functions of these SCN5A variants and the underlying mechanism by which they increase arrhythmia susceptibility in LVNC patients, functional evaluations were conducted in CHO-K1 cells and human embryonic stem cell-derived cardiomyocytes (hESC-CMs) using patch-clamp or microelectrode array (MEA) techniques. These findings demonstrated that these SCN5A mutants exhibited gain-of-function properties, leading to increased channel activation and enhanced fast inactivation in CHO-K1 cells. Additionally, these mutants enhanced the excitability and contractility of the cardiomyocyte population in hESC-CMs models. All SCN5A variants induced fibrillation-like arrhythmia and increased the heart rate in cardiomyocytes. However, the administration of Lidocaine, an antiarrhythmic drug that acts on sodium ion channels, was able to rescue or alleviate fibrillation-like arrhythmias and secondary beat phenomenon. Based on these findings, it is speculated that SCN5A variants may contribute to susceptibility to arrhythmia in LVNC patients. Furthermore, the construction of cardiomyocyte models with SCN5A variants and their application in drug screening may facilitate the development of precise therapies for arrhythmia in the future.

Single coronary artery presenting dilated cardiomyopathy and hyperlipidemia with the SCN5A and APOA5 gene mutation: A case report and review of the literature

We present a 55-year-old man with chest tightness and dyspnoea after activity lasting for 2 months who was diagnosed with single coronary artery (SCA) and presented with dilated cardiomyopathy (DCM) with the c.1858C > T mutation in the SCN5A gene. The computed tomography coronary angiogram (CTCA) showed congenital absence of the right coronary artery (RCA), and the right heart was nourished by the left coronary artery branch with no apparent stenosis. Transthoracic echocardiography (TTE) revealed enlargement of the left heart and cardiomyopathy. Cardiac magnetic resonance imaging (CMR) revealed DCM. Genetic testing showed that the c.1858C > T variant of the SCN5A gene could lead to Brugada syndrome and DCM. SCA is a rare congenital anomaly of the coronary anatomy, and this case reported as SCA accompanied by DCM is even rarer. We present a rare case of a 55-year-old man with DCM with the c.1858C > T (p. Arg620Cys)/c.1008G > A (p.(Pro336=) variant of the SCN5A gene, congenital absence of RCA, and c.990_993delAACA (p. Asp332Valfs*5) variant of the APOA5 gene. To our knowledge, this is the first report of DCM combined with the SCN5A gene mutation in SCA after searching the PubMed, CNKI and Wanfang databases.

Study of pathogenic genes in a pedigree with familial dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is a genetically heterogeneous cardiac disorder characterized by left ventricular dilation and contractile dysfunction. The substantial genetic heterogeneity evident in patients with DCM contributes to variable disease severity and complicates overall prognosis, which can be very poor.

AIM

To identify pathogenic genes in DCM through pedigree analysis.

METHODS

Our research team identified a patient with DCM in the clinic. Through investigation, we found that the family of this patient has a typical DCM pedigree. High-throughput sequencing technology, next-generation sequencing, was used to sequence the whole exomes of seven samples in the pedigree.

RESULTS

A novel and potentially pathogenic gene mutation-ANK2p.F3067L-was discovered. The mutation was completely consistent with the clinical information for this DCM pedigree. Sanger sequencing was used to further verify the locus of the mutation in pedigree samples. These results were consistent with those of high-throughput sequencing.

CONCLUSIONS

ANK2p.F3067L is considered a novel and potentially pathogenic gene mutation in DCM.

Association between SCN5A R225Q variant and dilated cardiomyopathy: potential role of intracellular pH and WNT/β-catenin pathway

BACKGROUND

The SCN5A variant is a common cause of familial dilated cardiomyopathy (DCM). We previously reported a SCN5A variant (c.674G>A), located in the high-risk S4 segment of domain I (DI-S4) region in patients with idiopathic DCM and R225Q knockin (p.R225Q) mice carrying the c.674G>A variant exhibited prolonged baseline PR intervals without DCM phenotypes. In this study, we explored the association and mechanism between R225Q variant and DCM phenotype.

METHODS

Prevalence of DI-S4 variant was compared between patients with idiopathic DCM and the control participants. R225Q knockin and wild-type (WT) mice were subjected to doxorubicin (DOX), D-galactose (D-gal) or D-gal combined with DOX.

RESULTS

Clinical data suggested that the prevalence of DI-S4 variant was higher in DCM group than in the control group (4/90 (4.4%) vs 3/1339 (0.2%), p<0.001). Cardiomyocytes from R225Q knockin mice treated with D-gal and DOX exhibited more significant hypertrophic phenotype and weaker contraction/dilation function and an increased level of apoptosis as compared with WT mice. Mechanistically, we found that R225Q variant could increase intracellular pH and further induce the activation of the WNT/β-catenin pathway as well as the overexpression of pro-hypertrophic and pro-apoptotic targets. WNT-C59 inhibitor improved cardiac function in the R225Q knockin mice treated with D-gal and DOX.

CONCLUSION

Our results suggest that R225Q variant is associated with increased susceptibility to DCM. Ageing could enhance this process via activating WNT/β-catenin signaling in response to increased intracellular pH. Antagonising the WNT/β-catenin pathway might be a potential therapeutic strategy for mitigating R225Q variant-related DCM pathogenesis.

Arrhythmias as Presentation of Genetic Cardiomyopathy

There is increasing evidence regarding the prevalence of genetic cardiomyopathies, for which arrhythmias may be the first presentation. Ventricular and atrial arrhythmias presenting in the absence of known myocardial disease are often labelled as idiopathic, or lone. While ventricular arrhythmias are well-recognized as presentation for arrhythmogenic cardiomyopathy in the right ventricle, the scope of arrhythmogenic cardiomyopathy has broadened to include those with dominant left ventricular involvement, usually with a phenotype of dilated cardiomyopathy. In addition, careful evaluation for genetic cardiomyopathy is also warranted for patients presenting with frequent premature ventricular contractions, conduction system disease, and early onset atrial fibrillation, in which most detected genes are in the cardiomyopathy panels. Sudden death can occur early in the course of these genetic cardiomyopathies, for which risk is not adequately tracked by left ventricular ejection fraction. Only a few of the cardiomyopathy genotypes implicated in early sudden death are recognized in current indications for implantable cardioverter defibrillators which otherwise rely upon a left ventricular ejection fraction ≤0.35 in dilated cardiomyopathy. The genetic diagnoses impact other aspects of clinical management such as exercise prescription and pharmacological therapy of arrhythmias, and new therapies are coming into clinical investigation for specific genetic cardiomyopathies. The expansion of available genetic information and implications raises new challenges for genetic counseling, particularly with the family member who has no evidence of a cardiomyopathy phenotype and may face a potentially negative impact of a genetic diagnosis. Discussions of risk for both probands and relatives need to be tailored to their numeric literacy during shared decision-making. For patients presenting with arrhythmias or cardiomyopathy, extension of genetic testing and its implications will enable cascade screening, intervention to change the trajectory for specific genotype-phenotype profiles, and enable further development and evaluation of emerging targeted therapies.

Research progress of Nedd4L in cardiovascular diseases

Post-translational modifications (PTMs) are a covalent processing process of proteins after translation. Proteins are capable of playing their roles only after being modified, so as to maintain the normal physiological function of cells. As a key modification of protein post-translational modification, ubiquitination is an essential element, which forms an enzyme-linked reaction through ubiquitin-activating enzyme, ubiquitin binding enzyme, and ubiquitin ligase, aiming to regulate the expression level and function of cellular proteins. Nedd4 family is the largest group of ubiquitin ligases, including 9 members, such as Nedd4-1, Nedd4L (Nedd4-2), WWP1, WWP2, ITCH, etc. They could bind to substrate proteins through their WW domain and play a dominant role in the ubiquitination process, and then participate in various pathophysiological processes of cardiovascular diseases (such as hypertension, myocardial hypertrophy, heart failure, etc.). At present, the role of Nedd4L in the cardiovascular field is not fully understood. This review aims to summarize the progress and mechanism of Nedd4L in cardiovascular diseases, and provide potential perspective for the clinical treatment or prevention of related cardiovascular diseases by targeting Nedd4L.

Late Sodium Current of the Heart: Where Do We Stand and Where Are We Going?

Late sodium current has long been linked to dysrhythmia and contractile malfunction in the heart. Despite the increasing body of accumulating information on the subject, our understanding of its role in normal or pathologic states is not complete. Even though the role of late sodium current in shaping action potential under physiologic circumstances is debated, it’s unquestioned role in arrhythmogenesis keeps it in the focus of research. Transgenic mouse models and isoform-specific pharmacological tools have proved useful in understanding the mechanism of late sodium current in health and disease. This review will outline the mechanism and function of cardiac late sodium current with special focus on the recent advances of the area.

Arrhythmic Phenotypes Are a Defining Feature of Dilated Cardiomyopathy-Associated SCN5A Variants: A Systematic Review

BACKGROUND

Variants in the SCN5A gene, that encodes the cardiac sodium channel, Nav1.5, are associated with a highly arrhythmogenic form of dilated cardiomyopathy (DCM). Our aim was to review the phenotypes, natural history, functional effects, and treatment outcomes of DCM-associated rare SCN5A variants.

METHODS

A systematic review of reported DCM-associated rare SCN5A variants was undertaken using PubMed and Embase.

RESULTS

Eighteen SCN5A rare variants in 29 families with DCM (173 affected individuals) were identified. Eleven variants had undergone experimental evaluation, with 7 of these resulting in increased sustained current flow during the action potential (eg, increased window current) and at resting membrane potentials (eg, creation of a new gating pore current). These variants were located in transmembrane voltage-sensing domains and had a consistent phenotype characterized by frequent multifocal narrow and broad complex ventricular premature beats (VPB; 72% of affected relatives), ventricular arrhythmias (33%), atrial arrhythmias (32%), sudden cardiac death (13%), and DCM (56%). This VPB-predominant phenotype was not seen with 1 variant that increased late sodium current, or with variants that reduced peak current density or had mixed effects. In the latter groups, affected individuals mainly showed sinus node dysfunction, conduction defects, and atrial arrhythmias, with infrequent VPB and ventricular arrhythmias. DCM did not occur in the absence of arrhythmias for any variant. Twelve studies (23 total patients) reported treatment success in the VPB-predominant cardiomyopathy using sodium channel-blocking drug therapy.

CONCLUSIONS

SCN5A variants can present with a diverse spectrum of primary arrhythmic features. A majority of DCM-associated variants cause a multifocal VPB-predominant cardiomyopathy that is reversible with sodium channel blocking drug therapy. Early recognition of the distinctive phenotype and prompt genetic testing to identify variant carriers are needed. Our findings have implications for interpretation and management of SCN5A variants found in DCM patients with and without arrhythmias.

Heritable arrhythmia syndromes associated with abnormal cardiac sodium channel function: ionic and non-ionic mechanisms

The cardiac sodium channel NaV1.5, encoded by the SCN5A gene, is responsible for the fast upstroke of the action potential. Mutations in SCN5A may cause sodium channel dysfunction by decreasing peak sodium current, which slows conduction and facilitates reentry-based arrhythmias, and by enhancing late sodium current, which prolongs the action potential and sets the stage for early afterdepolarization and arrhythmias. Yet, some NaV1.5-related disorders, in particular structural abnormalities, cannot be directly or solely explained on the basis of defective NaV1.5 expression or biophysics. An emerging concept that may explain the large disease spectrum associated with SCN5A mutations centres around the multifunctionality of the NaV1.5 complex. In this alternative view, alterations in NaV1.5 affect processes that are independent of its canonical ion-conducting role. We here propose a novel classification of NaV1.5 (dys)function, categorized into (i) direct ionic effects of sodium influx through NaV1.5 on membrane potential and consequent action potential generation, (ii) indirect ionic effects of sodium influx on intracellular homeostasis and signalling, and (iii) non-ionic effects of NaV1.5, independent of sodium influx, through interactions with macromolecular complexes within the different microdomains of the cardiomyocyte. These indirect ionic and non-ionic processes may, acting alone or in concert, contribute significantly to arrhythmogenesis. Hence, further exploration of these multifunctional effects of NaV1.5 is essential for the development of novel preventive and therapeutic strategies.

Genetic basis and molecular biology of cardiac arrhythmias in cardiomyopathies

Cardiac arrhythmias are common, often the first, and sometimes the life-threatening manifestations of hereditary cardiomyopathies. Pathogenic variants in several genes known to cause hereditary cardiac arrhythmias have also been identified in the sporadic cases and small families with cardiomyopathies. These findings suggest a shared genetic aetiology of a subset of hereditary cardiomyopathies and cardiac arrhythmias. The concept of a shared genetic aetiology is in accord with the complex and exquisite interplays that exist between the ion currents and cardiac mechanical function. However, neither the causal role of cardiac arrhythmias genes in cardiomyopathies is well established nor the causal role of cardiomyopathy genes in arrhythmias. On the contrary, secondary changes in ion currents, such as post-translational modifications, are common and contributors to the pathogenesis of arrhythmias in cardiomyopathies through altering biophysical and functional properties of the ion channels. Moreover, structural changes, such as cardiac hypertrophy, dilatation, and fibrosis provide a pro-arrhythmic substrate in hereditary cardiomyopathies. Genetic basis and molecular biology of cardiac arrhythmias in hereditary cardiomyopathies are discussed.

Multiple arrhythmic and cardiomyopathic phenotypes associated with an SCN5A A735E mutation

BACKGROUND

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

OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Life Cycle of the Cardiac Voltage-Gated Sodium Channel NaV1.5

Cardiac voltage-gated sodium channel Na_V1.5, encoded by SCN5A, is crucial for the upstroke of action potential and excitation of cardiomyocytes. Na_V1.5 undergoes complex processes before it reaches the target membrane microdomains and performs normal functions. A variety of protein partners are needed to achieve the balance between SCN5A transcription and mRNA decay, endoplasmic reticulum retention and export, Golgi apparatus retention and export, selective anchoring and degradation, activation, and inactivation of sodium currents. Subtle alterations can impair Na_V1.5 in terms of expression or function, eventually leading to Na_V1.5-associated diseases such as lethal arrhythmias and cardiomyopathy.

Arrhythmogenic Cardiomyopathy: Molecular Insights for Improved Therapeutic Design

Arrhythmogenic cardiomyopathy (ACM) is an inherited disorder characterized by structural and electrical cardiac abnormalities, including myocardial fibro-fatty replacement. Its pathological ventricular substrate predisposes subjects to an increased risk of sudden cardiac death (SCD). ACM is a notorious cause of SCD in young athletes, and exercise has been documented to accelerate its progression. Although the genetic culprits are not exclusively limited to the intercalated disc, the majority of ACM-linked variants reside within desmosomal genes and are transmitted via Mendelian inheritance patterns; however, penetrance is highly variable. Its natural history features an initial “concealed phase” that results in patients being vulnerable to malignant arrhythmias prior to the onset of structural changes. Lack of effective therapies that target its pathophysiology renders management of patients challenging due to its progressive nature, and has highlighted a critical need to improve our understanding of its underlying mechanistic basis. In vitro and in vivo studies have begun to unravel the molecular consequences associated with disease causing variants, including altered Wnt/β-catenin signaling. Characterization of ACM mouse models has facilitated the evaluation of new therapeutic approaches. Improved molecular insight into the condition promises to usher in novel forms of therapy that will lead to improved care at the clinical bedside.

Reevaluating the Mutation Classification in Genetic Studies of Bradycardia Using ACMG/AMP Variant Classification Framework

PURPOSE

Next-generation sequencing (NGS) has become more accessible, leading to an increasing number of genetic studies of familial bradycardia being reported. However, most of the variants lack full evaluation. The relationship between genetic factors and bradycardia should be summarized and reevaluated.

METHODS

We summarized genetic studies published in the PubMed database from 2008/1/1 to 2019/9/1 and used the ACMG/AMP classification framework to analyze related sequence variants.

RESULTS

We identified 88 articles, 99 sequence variants, and 34 genes after searching the PubMed database and classified ABCC9, ACTN2, CACNA1C, DES, HCN4, KCNQ1, KCNH2, LMNA, MECP2, LAMP2, NPPA, SCN5A, and TRPM4 as high-priority genes causing familial bradycardia. Most mutated genes have been reported as having multiple clinical manifestations.

CONCLUSIONS

For patients with familial CCD, 13 high-priority genes are recommended for evaluation. For genetic studies, variants should be carefully evaluated using the ACMG/AMP variant classification framework before publication.

Cardiac Sodium Channel Dysfunction and Dilated Cardiomyopathy: A Contemporary Reappraisal of Pathophysiological Concepts

A key emerging theme in translational cardiovascular medicine is the need to identify specific causes of arrhythmias and heart failure, defined by phenotype and/or genotype that will respond to a particular intervention. Unlike other genes implicated in hereditary arrhythmias and cardiomyopathies, pathogenic/likely pathogenic variants in the cardiac sodium channel alpha subunit gene (SCN5A) produce a remarkably diverse set of electrical and structural phenotypes, one of them being dilated cardiomyopathy. There has been debate about whether left ventricular remodeling is a bona fide phenotypic feature of cardiac sodium channel dysfunction, or a consequence of tachyarrhythmias or conduction disturbances. In light of recent findings, a critical digest of the available experimental and medical literature is necessary. This paper provides a critical appraisal of the evidence linking a dysfunctional cardiac sodium channel to ventricular dysfunction, and discusses the potential mechanisms involved in shaping this phenotype along with implications for precision therapy.

Protein structure aids predicting functional perturbation of missense variants in SCN5A and KCNQ1

Rare variants in the cardiac potassium channel K_V7.1 (KCNQ1) and sodium channel Na_V1.5 (SCN5A) are implicated in genetic disorders of heart rhythm, including congenital long QT and Brugada syndromes (LQTS, BrS), but also occur in reference populations. We previously reported two sets of Na_V1.5 (n = 356) and K_V7.1 (n = 144) variants with in vitro characterized channel currents gathered from the literature. Here we investigated the ability to predict commonly reported Na_V1.5 and K_V7.1 variant functional perturbations by leveraging diverse features including variant classifiers PROVEAN, PolyPhen-2, and SIFT; evolutionary rate and BLAST position specific scoring matrices (PSSM); and structure-based features including “functional densities” which is a measure of the density of pathogenic variants near the residue of interest. Structure-based functional densities were the most significant features for predicting Na_V1.5 peak current (adj. R² = 0.27) and K_V7.1 + KCNE1 half-maximal voltage of activation (adj. R² = 0.29). Additionally, use of structure-based functional density values improves loss-of-function classification of SCN5A variants with an ROC-AUC of 0.78 compared with other predictive classifiers (AUC = 0.69; two-sided DeLong test p = .01). These results suggest structural data can inform predictions of the effect of uncharacterized SCN5A and KCNQ1 variants to provide a deeper understanding of their burden on carriers.

A common polymorphism in the SCN5A gene is associated with dilated cardiomyopathy

Aims

SCN5A is a disease-causing gene associated with familial dilated cardiomyopathy (FDC). We examined the possible association between a common polymorphism in the SCN5A gene (c.1673A>G-p.H558R; rs1805124) and the risk of dilated cardiomyopathy (DCM) occurrence.

Methods

We genotyped 185 DCM cases (familial DCM, idiopathic DCM and postischemic DCM) and 251 controls for the p.H558R polymorphism in the SCN5A gene, to test the association of the molecular epidemiology of the individuals with the presence/absence of various types of DCM.

Results

Our results showed that the rs1805124 polymorphism was significantly associated with DCM, and the association was more significant in patients with FDC; furthermore, in these individuals, the less frequent GG genotype was associated with a 7.39-fold increased risk of disease [95% confidence interval (95% CI) = 2.88–18.96; P < 0.0001] compared with the AA genotype. Moreover, logistic regression analysis showed that GG carriers had a higher risk of DCM than AA + AG carriers (odds ratio = 5.45, 95% CI = 2.23–13.35; P < 0.001). No association was observed between the rs1805124 and DCM risk in postischemic DCM patients.

Conclusion

Our study demonstrates an association between familial DCM and the rs1805124 polymorphism in the SCN5A gene, which may unravel additional genetic predisposition to the development of a multifactorial disease as DCM.

SCN5A Variants: Association With Cardiac Disorders

The SCN5A gene encodes the alpha subunit of the main cardiac sodium channel Na_v1.5. This channel predominates inward sodium current (INa) and plays a critical role in regulation of cardiac electrophysiological function. Since 1995, SCN5A variants have been found to be causatively associated with Brugada syndrome, long QT syndrome, cardiac conduction system dysfunction, dilated cardiomyopathy, etc. Previous genetic, electrophysiological, and molecular studies have identified the arrhythmic and cardiac structural characteristics induced by SCN5A variants. However, due to the variation of disease manifestations and genetic background, impact of environmental factors, as well as the presence of mixed phenotypes, the detailed and individualized physiological mechanisms in various SCN5A-related syndromes are not fully elucidated. This review summarizes the current knowledge of SCN5A genetic variations in different SCN5A-related cardiac disorders and the newly developed therapy strategies potentially useful to prevent and treat these disorders in clinical setting.

Editor's Highlight: The Altered DNA Methylome of Chronic Doxorubicin Exposure in Sprague Dawley Rats

Doxorubicin (DOX) is a widely used treatment for human cancers, but increases the risk of life-threatening congestive heart failure (CHF). DOX-induced mitochondrial damage is cumulative and persistent, similar to that observed clinically for risk of CHF. Recent evidence suggests the persistent nature of this injury is caused by altered regulation of genes important to normal cardiac functioning. We hypothesize that chronic DOX therapy is associated with epigenetic modifications of DNA methylation status, particularly in critical regulators of mitochondrial function and capacity. Cardiac tissue from Sprague Dawley rats receiving injections of DOX (2 mg/kg, s.c.) or saline once a week for 6 weeks, followed by 5 weeks of drug-free holiday was used for Reduced Representation Bisulfite Sequencing to map specific sites of DNA methylation. Comparison of these methylomes indicated DOX exposure alters DNA methylation landscapes, and identified 14 genes with highly altered methylation status. Preliminary functional effects of DNA methylation changes were characterized by quantifying mRNA expression of selected targets (Rbm20, Nmnat2, Klhl29, Cacna1c, Scn5a.) Gene expression of Rbm20, Klhl29, and Nmnat2 were significantly altered in DOX treated animals; Klhl29 and Nmnat2 demonstrated significant decreases in protein expression corresponding to gene expression. Through an epigenotype-to-phenotype approach, this study identifies potential markers and molecular regulators of irreversible DOX-induced cardiovascular toxicity associated with clinically limiting CHF. However, none of the most prevalent genes identified directly relate to mitochondrial structure or function. Thus, the investigation fails to demonstrate a direct association between this altered methylome and persistent mitochondrionopathy associated with chronic doxorubicin cardiac toxicity.

Genetic Mechanisms Contribute to the Development of Heart Failure in Patients with Atrioventricular Block and Right Ventricular Apical Pacing

Right ventricular apical (RVA) pacing can lead to progressive left ventricular dysfunction and heart failure (HF), even in patients with normal cardiac structure and function. Our study conducted candidate gene screening and lentivirus transfected neonatal rat cardiomyocytes (NRCMs) to explore the genetic and pathogenic mechanisms of RVA pacing induced cardiomyopathy in third degree atrioventricular block (III AVB) patients. We followed 887 III AVB patients with baseline normal cardiac function and RVA pacing. After a median follow-up of 2.5 years, 10 patients (four males, mean age 47.6 ± 10.0 years) were diagnosed with RVA pacing induced HF with left ventricular ejection fraction (LVEF) reducing dramatically to 37.8 ± 7.1% (P  < 0.05). Candidate genes sequencing found cardiomyopathy associated genetic variations in all ten HF patients and six SCN5A variations in 6 of 20 control patients. Transfected NRCMs of Lamin A/C mutations (R216C and L379F) disrupted Lamin A/C location on nucleus membrane and finally resulted in increased apoptotic rate after serum starvation. In conclusion, cardiomyopathy associated genetic variations play an essential role in occurrence of newly onset HF in the III AVB patients with RVA pacing. RVA pacing, serving as extra stimulator, might accelerate the deterioration of cardiac structure and function.

Genetic Determinants of Hereditary Bradyarrhythmias: A Contemporary Review of a Diverse Group of Disorders

Bradyarrhythmia is a common clinical presentation. Although the majority of cases are acquired, genetic screening of families with bradyarrhythmia has led to the discovery of a growing number of causative hereditary mutations. These mutations can interfere with any of the steps required for the occurrence of each cardiac cycle, including generation of an action potential in the sinoatrial node, successful exit of the action potential from the node, propagation of the action potential throughout the atria until the depolarization waves reach the atrioventricular node, and finally transmission of the action potential to the ventricles through the His-Purkinje system. As expected, channelopathies are the predominant culprit for hereditary bradyarrhythmias, because they play a crucial role in action potential generation and propagation. Interestingly, there are an increasing number of genes that encode for various regulatory or structural cellular components that have been linked to hereditary bradyarrhythmias. Furthermore, population-based genetic screening has revealed that age-related conduction defects may in fact be caused by genetic predispositions rather than the simple process of aging. With recent advances in genetic testing and the creation of animal models, not only have we discovered new culprit genes but it has also has become evident that there are still significant gaps in our knowledge of cardiac pathophysiology. In this review, we discuss the clinical presentations of known hereditary bradyarrhythmias and their associated conditions in addition to detailing our current molecular understanding of the mechanisms by which they are manifested.

Murine Electrophysiological Models of Cardiac Arrhythmogenesis

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

Analysis of SCN5A Gene Variants in East Slovak Patients with Cardiomyopathy

OBJECTIVE

Mutations in ion channels genes are potential cause of cardiomyopathy. The SCN5A gene (sodium channel, voltage gated, type V alpha subunit gene; 3p21) belongs to the family of cardiac sodium channel genes. Mutations in SCN5A gene lead to decreased Na+ current and ion unbalance. The SCN5A gene mutations are found in approximately 2% of patients with dilated cardiomyopathy (DCM), and they may be potential phenotype modifiers in hypertrophic cardiomyopathy (HCM). The role of SCN5A gene mutations in cardiomyopathy is not fully elucidated.

METHODS

Three selected exons (12, 20, and 21) of the SCN5A gene in the cohort of 58 East Slovak patients with dilated and HCM were analyzed by the Sanger sequencing method in order to detect etiopathogenic mutations associated with dilated and HCM.

RESULTS

The mutation screening of three selected exons of SCN5A gene in the cohort of 27 DCM, 12 HCM patients, and 16 controls identified 10 missense genetic variants. Three of them (T1247I, A1260D, and G1262S), all in exon 21 of the SCN5A gene, were potentially damaging and disease-causing variants.

CONCLUSION

Data from this study demonstrate that SCN5A gene variants have important role in the etiopathogenesis of dilated and HCM.

Aberrant sialylation causes dilated cardiomyopathy and stress-induced heart failure

Dilated cardiomyopathy (DCM), the third most common cause of heart failure, is often associated with arrhythmias and sudden cardiac death if not controlled. The majority of DCM is of unknown etiology. Protein sialylation is altered in human DCM, with responsible mechanisms not yet described. Here we sought to investigate the impact of clinically relevant changes in sialylation on cardiac function using a novel model for altered glycoprotein sialylation that leads to DCM and to chronic stress-induced heart failure (HF), deletion of the sialyltransferase, ST3Gal4. We previously reported that 12- to 20-week-old ST3Gal4 (-/-) mice showed aberrant cardiac voltage-gated ion channel sialylation and gating that contribute to a pro-arrhythmogenic phenotype. Here, echocardiography supported by histology revealed modest dilated and thinner-walled left ventricles without increased fibrosis in ST3Gal4 (-/-) mice starting at 1 year of age. Cardiac calcineurin expression in younger (16-20 weeks old) ST3Gal4 (-/-) hearts was significantly reduced compared to WT. Transverse aortic constriction (TAC) was used as a chronic stressor on the younger mice to determine whether the ability to compensate against a pathologic insult is compromised in the ST3Gal4 (-/-) heart, as suggested by previous reports describing the functional implications of reduced cardiac calcineurin levels. TAC'd ST3Gal4 (-/-) mice presented with significantly reduced systolic function and ventricular dilation that deteriorated into congestive HF within 6 weeks post-surgery, while constricted WT hearts remained well-adapted throughout (ejection fraction, ST3Gal4 (-/-) = 34 ± 5.2 %; WT = 53.8 ± 7.4 %; p < 0.05). Thus, a novel, sialo-dependent model for DCM/HF is described in which clinically relevant reduced sialylation results in increased arrhythmogenicity and reduced cardiac calcineurin levels that precede cardiomyopathy and TAC-induced HF, suggesting a causal link among aberrant sialylation, chronic arrhythmia, reduced calcineurin levels, DCM in the absence of a pathologic stimulus, and stress-induced HF.

The role of mutations in the SCN5A gene in cardiomyopathies

The SCN5A gene encodes the alpha-subunit of the Nav1.5 ion channel protein, which is responsible for the sodium inward current (INa). Since 1995 several hundred mutations in this gene have been found to be causative for inherited arrhythmias including Long QT syndrome, Brugada syndrome, cardiac conduction disease, sudden infant death syndrome, etc. As expected these syndromes are primarily electrical heart diseases leading to life-threatening arrhythmias with an "apparently normal heart". In 2003 a new form of dilated cardiomyopathy was identified associated with mutations in the SCN5A gene. Recently mutations have been also found in patients with arrhythmogenic right ventricular cardiomyopathy and atrial standstill. The purpose of this review is to outline and analyze the following four topics: 1) SCN5A genetic variants linked to different cardiomyopathies; 2) clinical manifestations of the known mutations; 3) possible molecular mechanisms of myocardial remodeling; and 4) the potential implications of gene-specific treatment for those disorders. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Integration of Developmental and Environmental Cues in the Heart edited by Marcus Schaub and Hughes Abriel.

Sodium MRI in human heart: a review

This paper offers a critical review of the properties, methods and potential clinical application of sodium ((23)Na) MRI in human heart. Because the tissue sodium concentration (TSC) in heart is about ~40 µmol/g wet weight, and the (23)Na gyromagnetic ratio and sensitivity are respectively about one-quarter and one-11th of that of hydrogen ((1)H), the signal-to-noise ratio of (23)Na MRI in the heart is about one-6000th of that of conventional cardiac (1)H MRI. In addition, as a quadrupolar nucleus, (23)Na exhibits ultra-short and multi-component relaxation behavior (T1 ~ 30 ms; T2 ~ 0.5-4 ms and 12-20 ms), which requires fast, specialized, ultra-short echo-time MRI sequences, especially for quantifying TSC. Cardiac (23)Na MRI studies from 1.5 to 7 T measure a volume-weighted sum of intra- and extra-cellular components present at cytosolic concentrations of 10-15 mM and 135-150 mM in healthy tissue, respectively, at a spatial resolution of about 0.1-1 ml in 10 min or so. Currently, intra- and extra-cellular sodium cannot be unambiguously resolved without the use of potentially toxic shift reagents. Nevertheless, increases in TSC attributable to an influx of intra-cellular sodium and/or increased extra-cellular volume have been demonstrated in human myocardial infarction consistent with prior animal studies, and arguably might also be seen in future studies of ischemia and cardiomyopathies--especially those involving defects in sodium transport. While technical implementation remains a hurdle, a central question for clinical use is whether cardiac (23)Na MRI can deliver useful information unobtainable by other more convenient methods, including (1)H MRI.

Sodium channel haploinsufficiency and structural change in ventricular arrhythmogenesis

Normal cardiac excitation involves orderly conduction of electrical activation and recovery dependent upon surface membrane, voltage-gated, sodium (Na(+) ) channel α-subunits (Nav 1.5). We summarize experimental studies of physiological and clinical consequences of loss-of-function Na(+) channel mutations. Of these conditions, Brugada syndrome (BrS) and progressive cardiac conduction defect (PCCD) are associated with sudden, often fatal, ventricular tachycardia (VT) or fibrillation. Mouse Scn5a(+/-) hearts replicate important clinical phenotypes modelling these human conditions. The arrhythmic phenotype is associated not only with the primary biophysical change but also with additional, anatomical abnormalities, in turn dependent upon age and sex, each themselves exerting arrhythmic effects. Available evidence suggests a unified binary scheme for the development of arrhythmia in both BrS and PCCD. Previous biophysical studies suggested that Nav 1.5 deficiency produces a background electrophysiological defect compromising conduction, thereby producing an arrhythmic substrate unmasked by flecainide or ajmaline challenge. More recent reports further suggest a progressive decline in conduction velocity and increase in its dispersion particularly in ageing male Nav 1.5 haploinsufficient compared to WT hearts. This appears to involve a selective appearance of slow conduction at the expense of rapidly conducting pathways with changes in their frequency distributions. These changes were related to increased cardiac fibrosis. It is thus the combination of the structural and biophysical changes both accentuating arrhythmic substrate that may produce arrhythmic tendency. This binary scheme explains the combined requirement for separate, biophysical and structural changes, particularly occurring in ageing Nav 1.5 haploinsufficient males in producing clinical arrhythmia.

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

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

Cardiac Nav 1.5 is modulated by ubiquitin protein ligase E3 component n-recognin UBR3 and 6

The voltage-gated Na⁺ channel Na_v1.5 is essential for action potential (AP) formation and electrophysiological homoeostasis in the heart. The ubiquitin–proteasome system (UPS) is a major degradative system for intracellular proteins including ion channels. The ubiquitin protein ligase E3 component N-recognin (UBR) family is a part of the UPS; however, their roles in regulating cardiac Na_v1.5 channels remain elusive. Here, we found that all of the UBR members were expressed in cardiomyocytes. Individual knockdown of UBR3 or UBR6, but not of other UBR members, significantly increased Na_v1.5 protein levels in neonatal rat ventricular myocytes, and this effect was verified in HEK293T cells expressing Na_v1.5 channels. The UBR3/6-dependent regulation of Na_v1.5 channels was not transcriptionally mediated, and pharmacological inhibition of protein biosynthesis failed to counteract the increase in Na_v1.5 protein caused by UBR3/6 reduction, suggesting a degradative modulation of UBR3/6 on Na_v1.5. Furthermore, the effects of UBR3/6 knockdown on Na_v1.5 proteins were abolished under the inhibition of proteasome activity, and UBR3/6 knockdown reduced Na_v1.5 ubiquitylation. The double UBR3–UBR6 knockdown resulted in comparable increases in Na_v1.5 proteins to that observed for single knockdown of either UBR3 or UBR6. Electrophysiological recordings showed that UBR3/6 reduction-mediated increase in Na_v1.5 protein enhanced the opening of Na_v1.5 channels and thereby the amplitude of the AP. Thus, our findings indicate that UBR3/6 regulate cardiomyocyte Na_v1.5 channel protein levels via the ubiquitin–proteasome pathway. It is likely that UBR3/6 have the potential to be a therapeutic target for cardiac arrhythmias.

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

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

Cellular hyper-excitability caused by mutations that alter the activation process of voltage-gated sodium channels

Voltage-gated sodium channels (Na_v) are widely expressed as macro-molecular complexes in both excitable and non-excitable tissues. In excitable tissues, the upstroke of the action potential is the result of the passage of a large and rapid influx of sodium ions through these channels. Na_V dysfunction has been associated with an increasingly wide range of neurological, muscular and cardiac disorders. The purpose of this review is to summarize the recently identified sodium channel mutations that are linked to hyper-excitability phenotypes and associated with the alteration of the activation process of voltage gated sodium channels. Indeed, several clinical manifestations that demonstrate an alteration of tissue excitability were recently shown to be strongly associated with the presence of mutations that affect the activation process of the Na_v. These emerging genotype-phenotype correlations have expanded the clinical spectrum of sodium channelopathies to include disorders which feature a hyper-excitability phenotype that may or may not be associated with a cardiomyopathy. The p.I141V mutation in SCN4A and SCN5A, as well as its homologous p.I136V mutation in SCN9A, are interesting examples of mutations that have been linked to inherited hyperexcitability myotonia, exercise-induced polymorphic ventricular arrhythmias and erythromelalgia, respectively. Regardless of which sodium channel isoform is investigated, the substitution of the isoleucine to valine in the locus 141 induces similar modifications in the biophysical properties of the Na_v by shifting the voltage-dependence of steady state activation toward more negative potentials.

Wnt signalling suppresses voltage-dependent Na⁺ channel expression in postnatal rat cardiomyocytes

Wnt signalling plays crucial roles in heart development, but is normally suppressed postnatally. In arrhythmogenic conditions, such as cardiac hypertrophy and heart failure, Wnt signalling is reactivated. To explore the potential role of Wnt signalling in arrhythmogenic electrical remodelling, we examined voltage-dependent ion channels in cardiomyocytes. Treatment of neonatal rat ventricular myocytes with either recombinant Wnt3a protein or CHIR-99021 (CHIR, a glycogen synthase kinase-3β inhibitor) caused a dose-dependent increase in Wnt target gene expression (Axin2 and Lef1), indicating activation of the Wnt/β-catenin pathway. Cardiac Na(+) current (INa) density was reduced by Wnt3a (-20 ± 4 vs. control -59 ± 7 pA pF(-1) , at -30 mV) or CHIR (-22 ± 5 pA pF(-1) ), without changes in steady-state activation, inactivation or repriming kinetics. Wnt3a and CHIR also produced dose-dependent reductions in the mRNA level of Scn5a (the cardiac Na(+) channel α subunit gene), as well as a 56% reduction (by Wnt3a) in the Nav 1.5 protein level. Consistent with INa reduction, action potentials in Wnt3a-treated neonatal rat ventricular myocytes had a lower upstroke amplitude (91 ± 3 vs. control 137 ± 2 mV) and decreased maximum upstroke velocity (70 ± 10 vs. control 163 ± 15 V s(-1)). In contrast, inward rectifier K(+) current and L-type Ca(2+) channels were not affected by Wnt3a treatment. Taken together, our data indicate that the Wnt/β-catenin pathway suppresses INa in postnatal cardiomyocytes and may contribute to ion channel remodelling in heart disease.

Cardiac sodium channels and inherited electrophysiological disorders: an update on the pharmacotherapy

INTRODUCTION

Since the recognition of inherited sodium (Na(+)) channel disease, the cardiac Na(+) channel has been extensively studied. Both loss-of-function and gain-of-function mutations of the cardiac Na(+) channel are associated with cardiac arrhythmia and sudden cardiac death. Pathophysiological mechanisms that may induce arrhythmia are unravelled and include alterations in biophysical properties due to the mutation in SCN5A, drug use and circumstantial factors. Insights into the mechanisms of inherited Na(+) channel disease may result in tailored therapy. However, due to the complexity of cardiac electrical activity and pathophysiological mechanisms, pharmacotherapy in cardiac Na(+) channel disease remains challenging.

AREAS COVERED

This review discusses various mechanisms involved in inherited Na(+) channel disorders, focussing on Brugada syndrome (Brs) and long QT syndrome type 3 (LQTS3). It aims to provide an overview of developments in pharmacotherapy, discussing both treatment and which drugs to avoid to prevent arrhythmia.

EXPERT OPINION

Altered biophysical properties of cardiac Na(+) channels are the basis of arrhythmias in patients with inherited Na(+) channel diseases such as BrS and LQTS3. The effects of such biophysical derangements are strongly modulated by concomitant factors. Tailored drug therapy is required to prevent arrhythmia and is best achieved by educating patients affected by Na(+) channel disorders.

Reduced Penetrance and Variable Expression of SCN5A Mutations and the Importance of Co-inherited Genetic Variants: Case Report and Review of the Literature

Mutations in the SCN5A gene are responsible for multiple phenotypical presentations including Brugada syndrome, long QT syndrome, progressive familial heart block, sick sinus syndrome, dilated cardiomyopathy, lone atrial fibrillation and multiple overlap syndromes. These different phenotypic expressions of a mutation in a single gene can be explained by variable expression and reduced penetrance. One of the possible explanations of these phenomena is the co-inheritance of genetic variants. We describe a family where the individuals exhibit a compound heterozygosity in the SCN5A gene including a mutation (R1632H) and a new variant (M858L). Individuals with both the mutation and new variant present with a more severe phenotype including spontaneous atrial tachyarrhythmia at young age. We give an overview of the different phenotypes of "SCN5A disease" and discuss the importance of co-inherited genetic variants in the expression of SCN5A disease.

Biophysics, pathophysiology, and pharmacology of ion channel gating pores

Voltage sensor domains (VSDs) are a feature of voltage gated ion channels (VGICs) and voltage sensitive proteins. They are composed of four transmembrane (TM) segments (S1–S4). Currents leaking through VSDs are called omega or gating pore currents. Gating pores are caused by mutations of the highly conserved positively charged amino acids in the S4 segment that disrupt interactions between the S4 segment and the gating charge transfer center (GCTC). The GCTC separates the intracellular and extracellular water crevices. The disruption of S4–GCTC interactions allows these crevices to communicate and create a fast activating and non-inactivating alternative cation-selective permeation pathway of low conductance, or a gating pore. Gating pore currents have recently been shown to cause periodic paralysis phenotypes. There is also increasing evidence that gating pores are linked to several other familial diseases. For example, gating pores in Na_v1.5 and K_v7.2 channels may underlie mixed arrhythmias associated with dilated cardiomyopathy (DCM) phenotypes and peripheral nerve hyperexcitability (PNH), respectively. There is little evidence for the existence of gating pore blockers. Moreover, it is known that a number of toxins bind to the VSD of a specific domain of Na⁺ channels. These toxins may thus modulate gating pore currents. This focus on the VSD motif opens up a new area of research centered on developing molecules to treat a number of cell excitability disorders such as epilepsy, cardiac arrhythmias, and pain. The purpose of the present review is to summarize existing knowledge of the pathophysiology, biophysics, and pharmacology of gating pore currents and to serve as a guide for future studies aimed at improving our understanding of gating pores and their pathophysiological roles.

The SCN5A mutation A1180V is associated with electrocardiographic features of LQT3

Mutations of the SCN5A gene are associated with several arrhythmic syndromes including the Brugada syndrome, conduction disease, long QT syndrome type 3 (LQT3), atrial fibrillation, and dilated cardiomyopathy. We report LQT3 associated with an A1180V cardiac sodium channel mutation, previously associated with cardiac conduction block, and dilated cardiomyopathy in three generations of a Chinese family. Clinical, electrocardiographic (ECG), and echocardiographic examination was followed by direct sequencing of SCN5A and HERG to screen genomic DNA from blood samples. The proband presented with multiple syncopes from the age of 7 years and was found to share a mutation with two other members of his family. Continuous ECG monitoring after presentation showed prolonged QTc and biphasic T waves, multiple episodes of ventricular tachycardia and torsades de pointes. The other two mutation carriers showed ECG features of LQT3 without clinical symptoms. Transthoracic echocardiography showed normal cardiac structure in all three mutation carriers. This study shows LQT3 features associated with an A1180V cardiac sodium channel mutation, expanding the spectrum of phenotypes resulting from this mutation in which biophysical study has shown a persistent late Na(+) current.

Nav 1.5 mutations linked to dilated cardiomyopathy phenotypes: Is the gating pore current the missing link?

Nav 1.5 dysfunctions are commonly linked to rhythms disturbances that include type 3 long QT syndrome (LQT3), Brugada syndrome (BrS), sick sinus syndrome (SSS) and conduction defects. Recently, this channel protein has been also linked to structural heart diseases such as dilated cardiomyopathy (DCM).

The Connexin40A96S mutation from a patient with atrial fibrillation causes decreased atrial conduction velocities and sustained episodes of induced atrial fibrillation in mice

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia and a major cause of stroke. In the mammalian heart the gap junction proteins connexin40 (Cx40) and connexin43 (Cx43) are strongly expressed in the atrial myocardium mediating effective propagation of electrical impulses. Different heterozygous mutations in the coding region for Cx40 were identified in patients with AF. We have generated transgenic Cx40A96S mice harboring one of these mutations, the loss-of-function Cx40A96S mutation, as a model for atrial fibrillation. Cx40A96S mice were characterized by immunochemical and electrophysiological analyses. Significantly reduced atrial conduction velocities and strongly prolonged episodes of atrial fibrillation were found after induction in Cx40A96S mice. Analyses of the gating properties of Cx40A96S channels in cultured HeLa cells also revealed significantly lower junctional conductance and enhanced sensitivity voltage gating of Cx40A96S in comparison to Cx40 wild-type gap junctions. This is caused by reduced open probabilities of Cx40A96S gap junction channels, while single channel conductance remained the same. Similar to the corresponding patient, heterozygous Cx40A96S mice revealed normal expression levels and localization of the Cx40 protein. We conclude that heterozygous Cx40A96S mice exhibit prolonged episodes of induced atrial fibrillation and severely reduced atrial conduction velocities similar to the corresponding human patient.

A1180V of cardiac sodium channel gene (SCN5A): is it a risk factor for dilated cardiomyopathy or just a common variant in Han Chinese?

Our previous study of a Chinese family with dilated cardiomyopathy (DCM) suggested that A1180V of the cardiac sodium channel gene (SCN5A) was associated with the disease within this family. According to data deposited in dbSNP, however, A1180V has been found in some small samples of the Asian population. In this study, we followed up the affected pedigree and expanded the investigation of the prevalence of A1180V in 460 unrelated healthy Han Chinese. Besides, we searched and analyzed it in other database as well. During the follow-up period, 1 A1180V carrier's condition deteriorated alot, and another 4 carriers progressed to DCM or atrioventricular block (AVB). We also found that the A1180V was absent among the 460 individuals (0%, 0/460), and the carrier frequency of A1180V among Chinese was about 0.51% obtained from the 1000 genome project. In conclusion, our finding suggests that A1180V is a potential risk factor for DCM, and it is extremely rare among Healthy Han Chinese.

Cardiac sodium channelopathy associated with SCN5A mutations: electrophysiological, molecular and genetic aspects

Over the last two decades, an increasing number of SCN5A mutations have been described in patients with long QT syndrome type 3 (LQT3), Brugada syndrome, (progressive) conduction disease, sick sinus syndrome, atrial standstill, atrial fibrillation, dilated cardiomyopathy, and sudden infant death syndrome (SIDS). Combined genetic, electrophysiological and molecular studies have provided insight into the dysfunction and dysregulation of the cardiac sodium channel in the setting of SCN5A mutations identified in patients with these inherited arrhythmia syndromes. However, risk stratification and patient management is hindered by the reduced penetrance and variable disease expressivity in sodium channelopathies. Furthermore, various SCN5A-related arrhythmia syndromes are known to display mixed phenotypes known as cardiac sodium channel overlap syndromes. Determinants of variable disease expressivity, including genetic background and environmental factors, are suspected but still largely unknown. Moreover, it has become increasingly clear that sodium channel function and regulation is more complicated than previously assumed, and the sodium channel may play additional, as of yet unrecognized, roles in cardiac structure and function. Development of cardiac structural abnormalities secondary to SCN5A mutations has been reported, but the clinical relevance and underlying mechanisms are unclear. Increased insight into these issues would enable a major next step in research related to cardiac sodium channel disease, ultimately enabling improved diagnosis, risk stratification and treatment strategies.

Mouse Models of SCN5A-Related Cardiac Arrhythmias

Mutations of SCN5A gene, which encodes the α-subunit of the voltage-gated Na⁺ channel Na_V1.5, underlie hereditary cardiac arrhythmic syndromes such as the type 3 long QT syndrome, cardiac conduction diseases, the Brugada syndrome, the sick sinus syndrome, a trial standstill, and numerous overlap syndromes. Patch-clamp studies in heterologous expression systems have provided important information to understand the genotype-phenotype relationships of these diseases. However, they could not clarify how SCN5A mutations can be responsible for such a large spectrum of diseases, for the late age of onset or the progressiveness of some of these diseases and for the overlapping syndromes. Genetically modified mice rapidly appeared as promising tools for understanding the pathophysiological mechanisms of cardiac SCN5A-related arrhythmic syndromes and several mouse models have been established. This review presents the results obtained on these models that, for most of them, recapitulate the clinical phenotypes of the patients. This includes two models knocked out for Nav1.5 β1 and β3 auxiliary subunits that are also discussed. Despite their own limitations that we point out, the mouse models still appear as powerful tools to elucidate the pathophysiological mechanisms of SCN5A-related diseases and offer the opportunity to investigate the secondary cellular consequences of SCN5A mutations such as the expression remodeling of other genes. This points out the potential role of these genes in the overall human phenotype. Finally, they constitute useful tools for addressing the role of genetic and environmental modifiers on cardiac electrical activity.

A proton leak current through the cardiac sodium channel is linked to mixed arrhythmia and the dilated cardiomyopathy phenotype

Cardiac Na⁺ channels encoded by the SCN5A gene are essential for initiating heart beats and maintaining a regular heart rhythm. Mutations in these channels have recently been associated with atrial fibrillation, ventricular arrhythmias, conduction disorders, and dilated cardiomyopathy (DCM).

We investigated a young male patient with a mixed phenotype composed of documented conduction disorder, atrial flutter, and ventricular tachycardia associated with DCM. Further family screening revealed DCM in the patient's mother and sister and in three of the mother's sisters. Because of the complex clinical phenotypes, we screened SCN5A and identified a novel mutation, R219H, which is located on a highly conserved region on the fourth helix of the voltage sensor domain of Na_v1.5. Three family members with DCM carried the R219H mutation.

The wild-type (WT) and mutant Na⁺ channels were expressed in a heterologous expression system, and intracellular pH (pHi) was measured using a pH-sensitive electrode. The biophysical characterization of the mutant channel revealed an unexpected selective proton leak with no effect on its biophysical properties. The H⁺ leak through the mutated Na_v1.5 channel was not related to the Na⁺ permeation pathway but occurred through an alternative pore, most probably a proton wire on the voltage sensor domain.

We propose that acidification of cardiac myocytes and/or downstream events may cause the DCM phenotype and other electrical problems in affected family members. The identification of this clinically significant H⁺ leak may lead to the development of more targeted treatments.

Dilated cardiomyopathy and sinoatrial dysfunction in an Estrela mountain dog

A 1 yr old male Estrela mountain dog was evaluated as a part of a screening program for dilated cardiomyopathy. The dog came from a family with a history of dilated cardiomyopathy but was asymptomatic. Occult dilated cardiomyopathy and sino-atrial dysfunction were diagnosed based on echocardiography and electrocardiography. These two disorders may be associated given that related dogs have been diagnosed with the same disorders. The dog has remained asymptomatic for 4 years following initial evaluation.

Striking In vivo phenotype of a disease-associated human SCN5A mutation producing minimal changes in vitro

BACKGROUND

The D1275N SCN5A mutation has been associated with a range of unusual phenotypes, including conduction disease and dilated cardiomyopathy, as well as atrial and ventricular tachyarrhythmias. However, when D1275N is studied in heterologous expression systems, most studies show near-normal sodium channel function. Thus, the relationship of the variant to the clinical phenotypes remains uncertain.

METHODS AND RESULTS

We identified D1275N in a patient with atrial flutter, atrial standstill, conduction disease, and sinus node dysfunction. There was no major difference in biophysical properties between wild-type and D1275N channels expressed in Chinese hamster ovary cells or tsA201 cells in the absence or presence of β1 subunits. To determine D1275N function in vivo, the Scn5a locus was modified to knock out the mouse gene, and the full-length wild-type (H) or D1275N (DN) human SCN5A cDNAs were then inserted at the modified locus by recombinase mediated cassette exchange. Mice carrying the DN allele displayed slow conduction, heart block, atrial fibrillation, ventricular tachycardia, and a dilated cardiomyopathy phenotype, with no significant fibrosis or myocyte disarray on histological examination. The DN allele conferred gene-dose-dependent increases in SCN5A mRNA abundance but reduced sodium channel protein abundance and peak sodium current amplitudes (H/H, 41.0±2.9 pA/pF at -30 mV; DN/H, 19.2±3.1 pA/pF, P<0.001 vs. H/H; DN/DN, 9.3±1.1 pA/pF, P<0.001 versus H/H).

CONCLUSIONS

Although D1275N produces near-normal currents in multiple heterologous expression experiments, our data establish this variant as a pathological mutation that generates conduction slowing, arrhythmias, and a dilated cardiomyopathy phenotype by reducing cardiac sodium current.

PITX2 insufficiency leads to atrial electrical and structural remodeling linked to arrhythmogenesis

BACKGROUND

Pitx2 is a homeobox transcription factor that plays a pivotal role in early left/right determination during embryonic development. Pitx2 loss-of-function mouse mutants display early embryonic lethality with severe cardiac malformations, demonstrating the importance of Pitx2 during cardiogenesis. Recently, independent genome-wide association studies have provided new evidence for a putative role of PITX2 in the adult heart. These studies have independently reported several risk variants close to the PITX2 locus on chromosome 4q25 that are strongly associated with atrial fibrillation in humans.

METHODS AND RESULTS

We show for the first time that PITX2C expression is significantly decreased in human patients with sustained atrial fibrillation, thus providing a molecular link between PITX2 loss of function and atrial fibrillation. In addition, morphological, molecular, and electrophysiological characterization of chamber-specific Pitx2 conditional mouse mutants reveals that atrial but not ventricular chamber-specific deletion of Pitx2 results in differences in the action potential amplitude and resting membrane potential in the adult heart as well as ECG characteristics of atrioventricular block. Lack of Pitx2 in atrial myocardium impairs sodium channel and potassium channel expression, mediated in part by miRNA misexpression.

CONCLUSIONS

This study thus identifies Pitx2 as an upstream transcriptional regulator of atrial electric function, the insufficiency of which results in cellular and molecular changes leading to atrial electric and structural remodeling linked to arrhythmogenesis.