Mutations in the Voltage Sensors of Domains I and II of Nav1.5 that are Associated with Arrhythmias and Dilated Cardiomyopathy Generate Gating Pore Currents

Dilated cardiomyopathy: from genes and molecules to potential treatments

Dilated cardiomyopathy is a myocardial condition marked by the enlargement of the heart's ventricular chambers and the gradual decline in systolic function, frequently resulting in congestive heart failure. Dilated cardiomyopathy has obvious familial characteristics, and mutations in related pathogenic genes can account for about 50% of patients with dilated cardiomyopathy. The most common genes related to dilated cardiomyopathy include TTN, LMNA, MYH7, etc. With more and more research on these genes, it will undoubtedly provide more potential targets and therapeutic pathways for the treatment of dilated cardiomyopathy. In addition, myocardial inflammation, myocardial metabolism abnormalities and cardiomyocyte apoptosis all have an important impact on the pathogenesis of dilated cardiomyopathy. Approximately half of sudden deaths among children and adolescents, along with the majority of patients undergoing heart transplantation, stem from cardiomyopathy. Therefore, precise and prompt clinical diagnosis holds paramount importance. Currently, diagnosis primarily hinges on the patient's medical background and imaging tests, with the significance of genetic testing steadily gaining prominence. The primary treatment for dilated cardiomyopathy remains heart transplantation. However, the scarcity of donors and the risk of severe immune rejection underscore the pressing need for novel therapies. Presently, research is actively exploring preclinical treatments like stem cell therapy as potential solutions.

The differential impacts of equivalent gating-charge mutations in voltage-gated sodium channels

Voltage-gated sodium (Nav) channels are pivotal for cellular signaling, and mutations in Nav channels can lead to excitability disorders in cardiac, muscular, and neural tissues. A major cluster of pathological mutations localizes in the voltage-sensing domains (VSDs), resulting in either gain-of-function, loss-of-function effects, or both. However, the mechanism behind this functional diversity of mutations at equivalent positions remains elusive. Through hotspot analysis, we identified three gating charges (R1, R2, and R3) as major mutational hotspots in VSDs. The same amino acid substitutions at equivalent gating-charge positions in VSDI and VSDII of the cardiac sodium channel Nav1.5 show differential gating property impacts in electrophysiology measurements. We conducted molecular dynamics (MD) simulations on wild-type channels and six mutants to elucidate the structural basis of their differential impacts. Our 120-µs MD simulations with applied external electric fields captured VSD state transitions and revealed the differential structural dynamics between equivalent R-to-Q mutants. Notably, we observed transient leaky conformations in some mutants during structural transitions, offering a detailed structural explanation for gating-pore currents. Our salt-bridge network analysis uncovered VSD-specific and state-dependent interactions among gating charges, countercharges, and lipids. This detailed analysis revealed how mutations disrupt critical electrostatic interactions, thereby altering VSD permeability and modulating gating properties. By demonstrating the crucial importance of considering the specific structural context of each mutation, our study advances our understanding of structure-function relationships in Nav channels. Our work establishes a robust framework for future investigations into the molecular basis of ion channel-related disorders.

Searching for a Solution: A Case Report on Multifocal Ectopic Purkinje-Related Premature Contractions Syndrome

Multifocal ectopic Purkinje-related premature contractions (MEPPC) syndrome is a recently recognized rare form of arrhythmia involving the entire His-Purkinje system and often coinciding with dilated cardiomyopathy (DCM). Certain variants in the SCN5A gene may be linked to MEPPC syndrome. We present a case of a 32-year-old Caucasian female who exhibited a high burden of premature ventricular contractions (PVCs) and non-sustained episodes of ventricular tachycardia (NSVT) with an alternating QRS pattern, and who was resistant to traditional medical therapy and radiofrequency catheter ablation (RFCA), necessitating implantation of a cardioverter-defibrillator (ICD). A positive family history (father's death at the age of 40 years) and the rapid deterioration of left ventricular function parameters echocardiographically during recurrent arrhythmic episodes raised concern about a potentially complex disease scenario. Genetic testing revealed a heterozygous variant of the SCN5A gene, c.2440C>T, p.(Arg814Trp), confirming the diagnosis of MEPPC syndrome. Treatment with a combination of class I antiarrhythmic drugs, flecainide and mexiletine, concomitant with beta blockers, led to symptomatic improvement, a reduction of PVCs (from 66 491 (44%) to 858 (1%)), and the restoration of left ventricular function (LV EF from 44% to 53%). A lack of defined diagnostic criteria hampers timely diagnosis, leading to ineffective interventions and delayed initiation of treatment with antiarrhythmic drugs. MEPPC patients remain at significant risk for severe heart failure and sudden cardiac death. Our clinical case report underscores the importance of accurate and timely diagnosis, which allows effective treatment with a combination of antiarrhythmic drugs and mitigates the risk associated with MEPPC syndrome.

Two pores instead of one: Gating pore current and the electrical leak in autism and epilepsy

Imagine the brain as a dynamic city, where countless vehicles traverse major arterial roads and branching side streets. The smooth traffic flow depends on a balance between excitatory neurons, which act as main roads encouraging vehicles to move forward, and inhibitory neurons, represented by branching side streets that regulate and control the traffic flow back onto the main route. Both systems work in tandem to maintain efficient operations, preventing gridlock or chaos. Zooming in further, the voltage-gated ion channels within neurons resemble traffic lights on arterial roads or side streets. Green means go, red means stop, and yellow signals caution. These channels regulate the flow of bioelectric signals, coordinating transitions between green, yellow, and red-analogous to an action potential. In excitatory neurons (major roads), voltage-gated sodium channels act as green lights, allowing sodium ions to flow in during depolarization. In contrast, voltage-gated potassium channels serve as yellow lights, eventually signaling red to terminate the action potential. In inhibitory neurons (side streets), sodium influx produces action potentials that ultimately control and limit traffic on the major roads. This analogy can be extended to describe neuropsychiatric and neurological disorders, such as autism spectrum disorder (ASD) and epilepsy, which arise from mutations in voltage-gated ion channels. These mutations alter the channels' ability to open and close properly, disrupting the timing and duration of red, yellow and green signals and impairing traffic flow. Now, picture yourself on a major arterial road with green and red flickering simultaneously. Such a disastrous scenario could lead to even more dangerous outcomes, with cars moving when they should stop or stopping when they should move. This specific analogy illustrates a key feature of certain mutations in voltage-gated ion channels that result in the gating pore current (Igp), a secondary pore that leaks electrical current. This mini-review focuses on Igp caused by mutations in the gating charge residues of voltage-gated ion channels. We will discuss how Igp contributes to the pathophysiology of ASD and epilepsy and explore therapeutic strategies targeting this mechanism.

Voltage Sensor Conformations Induced by LQTS-associated Mutations in hERG Potassium Channels

Voltage sensors are essential for electromechanical coupling in hERG K + channels, critical to cardiac rhythm. These sensors detect changes in membrane voltage and move in response to the transmembrane electric field. Mutations in voltage-sensing arginines of hERG, associated with Long QT syndrome, alter channel gating, though mechanisms in these mutants remain unclear. Using fluorescence lifetime imaging microscopy (FLIM), transition metal FRET (tmFRET), dual stop-codon mediated noncanonical amino acid incorporation, and molecular dynamics (MD) simulations, we identified distinct intermediate voltage-sensor conformations caused by these mutations. Phasor plot analysis of the FLIM-tmFRET donor revealed multiple FRET states in mutant hERG channels, in contrast to the single high-FRET state observed in unmutated controls. These intermediate FRET states correspond to specific mutation sites and align with distinct intermediate voltage-sensor conformations identified in MD simulations. This study provides novel insights into cardiac channelopathies, highlighting structural underpinnings underlying voltage sensing in cardiac arrhythmias.

Mechanisms underlying dilated cardiomyopathy associated with FKBP12 deficiency

Dilated cardiomyopathy (DCM) is a highly prevalent and genetically heterogeneous condition that results in decreased contractility and impaired cardiac function. The FK506-binding protein FKBP12 has been implicated in regulating the ryanodine receptor in skeletal muscle, but its role in cardiac muscle remains unclear. To define the effect of FKBP12 in cardiac function, we generated conditional mouse models of FKBP12 deficiency. We used Cre recombinase driven by either the α-myosin heavy chain, (αMHC) or muscle creatine kinase (MCK) promoter, which are expressed at embryonic day 9 (E9) and E13, respectively. Both conditional models showed an almost total loss of FKBP12 in adult hearts compared with control animals. However, only the early embryonic deletion of FKBP12 (αMHC-Cre) resulted in an early-onset and progressive DCM, increased cardiac oxidative stress, altered expression of proteins associated with cardiac remodeling and disease, and sarcoplasmic reticulum Ca2+ leak. Our findings indicate that FKBP12 deficiency during early development results in cardiac remodeling and altered expression of DCM-associated proteins that lead to progressive DCM in adult hearts, thus suggesting a major role for FKBP12 in embryonic cardiac muscle.

The many faces of SCN5A pathogenic variants: from channelopathy to cardiomyopathy

The SCN5A gene encodes the alpha subunit of the cardiac sodium channel, which plays a fundamental role in the generation and propagation of the action potential in the heart muscle. During the past years our knowledge concerning the function of the cardiac sodium channel and the diseases caused by mutations of the SCN5A gene has grown. Although initially SCN5A pathogenic variants were mainly associated with channelopathies, increasing recent evidence suggests an association with structural heart disease in the form of heart muscle disease. The pathways leading to a cardiomyopathic phenotype remain unclear and require further elucidation. The aim of the present review is to provide a concise summary regarding the mechanisms through which SCN5A pathogenic variants result in heart disease, focusing in cardiomyopathy, highlighting along the way the complex role of the SCN5A gene at the intersection of cardiac excitability and contraction networks.

Impacts of DCM-linked gating pore currents on the electrophysiological characteristics of hiPSC-CM monolayers

BACKGROUND

Variants of the SCN5A gene, which encodes the NaV1.5 cardiac sodium channel, have been linked to arrhythmic disorders associated with dilated cardiomyopathy (DCM). However, the precise pathological mechanisms remain elusive. The present study aimed to elucidate the pathophysiological consequences of the DCM-linked Nav1.5/R219H variant, which is known to generate a gating pore current, using patient-specific human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) cultured in monolayers.

METHODS

Ventricular- and atrial-like hiPSC-CM monolayers were generated from DCM patients carrying the R219H SCN5A variant as well as from healthy control individuals. CRISPR-corrected hiPSC-CMs served as isogenic controls. Simultaneous optical mapping of action potentials (APs) and calcium transients (CaTs) was employed to measure conduction velocities (CVs) and AP durations (APDs) and served as markers of electrical excitability. Calcium handling was evaluated by assessing CaT uptake (half-time to peak), recapture (tau of decay), and durations (TD50 and TD80). A multi-electrode array (MEA) analysis was conducted on hiPSC-CM monolayers to measure field potential (FP) parameters, including corrected Fridericia FP durations (FPDc).

RESULTS

Our results revealed that CVs were significantly reduced by more than 50 % in both ventricular- and atrial-like hiPSC-CM monolayers carrying the R219H variant compared to the control group. APDs were also prolonged in the R219H group compared to the control and CRISPR-corrected groups. CaT uptake, reuptake, and duration were also markedly delayed in the R219H group compared to the control and CRISPR-corrected groups in both the ventricular- and the atrial-like hiPSC-CM monolayers. Lastly, the MEA data revealed a notably prolonged FPDc in the ventricular- and atrial-like hiPSC-CMs carrying the R219H variant compared to the control and isogenic control groups.

CONCLUSIONS

These findings highlight the impact of the gating pore current on AP propagation and calcium homeostasis within a functional syncytium environment and offer valuable insights into the potential mechanisms underlying DCM pathophysiology.

Genetic background determines the severity of age-dependent cardiac structural abnormalities and arrhythmia susceptibility in Scn5a-1798insD mice

AIMS

Patients with mutations in SCN5A encoding NaV1.5 often display variable severity of electrical and structural alterations, but the underlying mechanisms are not fully elucidated. We here investigate the combined modulatory effect of genetic background and age on disease severity in the Scn5a1798insD/+ mouse model.

METHODS AND RESULTS

In vivo electrocardiogram and echocardiograms, ex vivo electrical and optical mapping, and histological analyses were performed in adult (2-7 months) and aged (8-28 months) wild-type (WT) and Scn5a1798insD/+ (mutant, MUT) mice from the FVB/N and 129P2 inbred strains. Atrio-ventricular (AV) conduction, ventricular conduction, and ventricular repolarization are modulated by strain, genotype, and age. An aging effect was present in MUT mice, with aged MUT mice of both strains showing prolonged QRS interval and right ventricular (RV) conduction slowing. 129P2-MUT mice were severely affected, with adult and aged 129P2-MUT mice displaying AV and ventricular conduction slowing, prolonged repolarization, and spontaneous arrhythmias. In addition, the 129P2 strain appeared particularly susceptible to age-dependent electrical, functional, and structural alterations including RV conduction slowing, reduced left ventricular (LV) ejection fraction, RV dilatation, and myocardial fibrosis as compared to FVB/N mice. Overall, aged 129P2-MUT mice displayed the most severe conduction defects, RV dilatation, and myocardial fibrosis, in addition to the highest frequency of spontaneous arrhythmia and inducible arrhythmias.

CONCLUSION

Genetic background and age both modulate disease severity in Scn5a1798insD/+ mice and hence may explain, at least in part, the variable disease expressivity observed in patients with SCN5A mutations. Age- and genetic background-dependent development of cardiac structural alterations furthermore impacts arrhythmia risk. Our findings therefore emphasize the importance of continued assessment of cardiac structure and function in patients carrying SCN5A mutations.

Pathogenic gating pore current conducted by autism-related mutations in the NaV1.2 brain sodium channel

Autism spectrum disorder (ASD) is a complex neurodevelopmental condition characterized by social and communication deficits and repetitive behaviors. The genetic heterogeneity of ASD presents a challenge to the development of an effective treatment targeting the underlying molecular defects. ASD gating charge mutations in the KCNQ/KV7 potassium channel cause gating pore currents (Igp) and impair action potential (AP) firing of dopaminergic neurons in brain slices. Here, we investigated ASD gating charge mutations of the voltage-gated SCN2A/NaV1.2 brain sodium channel, which ranked high among the ion channel genes with mutations in individuals with ASD. Our results show that ASD mutations in the gating charges R2 in Domain-II (R853Q), and R1 (R1626Q) and R2 (R1629H) in Domain-IV of NaV1.2 caused Igp in the resting state of ~0.1% of the amplitude of central pore current. The R1626Q mutant also caused significant changes in the voltage dependence of fast inactivation, and the R1629H mutant conducted proton-selective Igp. These potentially pathogenic Igp were exacerbated by the absence of the extracellular Mg2+ and Ca2+. In silico simulation of the effects of these mutations in a conductance-based single-compartment cortical neuron model suggests that the inward Igp reduces the time to peak for the first AP in a train, increases AP rates during a train of stimuli, and reduces the interstimulus interval between consecutive APs, consistent with increased neural excitability and altered input/output relationships. Understanding this common pathophysiological mechanism among different voltage-gated ion channels at the circuit level will give insights into the underlying mechanisms of ASD.

Anionic omega currents from single countercharge mutants in the voltage-sensing domain of Ci-VSP

The S4 segment of voltage-sensing domains (VSDs) directly responds to voltage changes by reorienting within the electric field as a permion. A narrow hydrophobic "gasket" or charge transfer center at the core of most VSDs focuses the electric field into a narrow region and catalyzes the sequential and reversible translocation of S4 positive gating charge residues across the electric field while preventing the permeation of physiological ions. Mutating specific S4 gating charges can cause ionic leak currents through the VSDs. These gating pores or omega currents play important pathophysiological roles in many diseases of excitability. Here, we show that mutating D129, a key countercharge residue in the Ciona intestinalis voltage-sensing phosphatase (Ci-VSP), leads to the generation of unique anionic omega currents. Neutralizing D129 causes a dramatic positive shift of activation, facilitates the formation of a continuous water path through the VSD, and creates a positive electrostatic potential landscape inside the VSD that contributes to its unique anionic selectivity. Increasing the population or dwell time of the conducting state by a high external pH or an engineered Cd2+ bridge markedly increases the current magnitude. Our findings uncover a new role of countercharge residues in the impermeable VSD of Ci-VSP and offer insights into mechanisms of the conduction of anionic omega currents linked to countercharge residue mutations.

Voltage gated sodium and calcium channels: Discovery, structure, function, and Pharmacology

Voltage-gated sodium channels initiate action potentials in nerve and muscle, and voltage-gated calcium channels couple depolarization of the plasma membrane to intracellular events such as secretion, contraction, synaptic transmission, and gene expression. In this Review and Perspective article, I summarize early work that led to identification, purification, functional reconstitution, and determination of the amino acid sequence of the protein subunits of sodium and calcium channels and showed that their pore-forming subunits are closely related. Decades of study by antibody mapping, site-directed mutagenesis, and electrophysiological recording led to detailed two-dimensional structure-function maps of the amino acid residues involved in voltage-dependent activation and inactivation, ion permeation and selectivity, and pharmacological modulation. Most recently, high-resolution three-dimensional structure determination by X-ray crystallography and cryogenic electron microscopy has revealed the structural basis for sodium and calcium channel function and pharmacological modulation at the atomic level. These studies now define the chemical basis for electrical signaling and provide templates for future development of new therapeutic agents for a range of neurological and cardiovascular diseases.

Development of automated patch clamp assays to overcome the burden of variants of uncertain significance in inheritable arrhythmia syndromes

Advances in next-generation sequencing have been exceptionally valuable for identifying variants in medically actionable genes. However, for most missense variants there is insufficient evidence to permit definitive classification of variants as benign or pathogenic. To overcome the deluge of Variants of Uncertain Significance, there is an urgent need for high throughput functional assays to assist with the classification of variants. Advances in parallel planar patch clamp technologies has enabled the development of automated high throughput platforms capable of increasing throughput 10- to 100-fold compared to manual patch clamp methods. Automated patch clamp electrophysiology is poised to revolutionize the field of functional genomics for inheritable cardiac ion channelopathies. In this review, we outline i) the evolution of patch clamping, ii) the development of high-throughput automated patch clamp assays to assess cardiac ion channel variants, iii) clinical application of these assays and iv) where the field is heading.

Fifty years of gating currents and channel gating

We celebrate this year the 50th anniversary of the first electrophysiological recordings of the gating currents from voltage-dependent ion channels done in 1973. This retrospective tries to illustrate the context knowledge on channel gating and the impact gating-current recording had then, and how it continued to clarify concepts, elaborate new ideas, and steer the scientific debate in these 50 years. The notion of gating particles and gating currents was first put forward by Hodgkin and Huxley in 1952 as a necessary assumption for interpreting the voltage dependence of the Na and K conductances of the action potential. 20 years later, gating currents were actually recorded, and over the following decades have represented the most direct means of tracing the movement of the gating charges and gaining insights into the mechanisms of channel gating. Most work in the early years was focused on the gating currents from the Na and K channels as found in the squid giant axon. With channel cloning and expression on heterologous systems, other channels as well as voltage-dependent enzymes were investigated. Other approaches were also introduced (cysteine mutagenesis and labeling, site-directed fluorometry, cryo-EM crystallography, and molecular dynamics [MD] modeling) to provide an integrated and coherent view of voltage-dependent gating in biological macromolecules. The layout of this retrospective reflects the past 50 years of investigations on gating currents, first addressing studies done on Na and K channels and then on other voltage-gated channels and non-channel structures. The review closes with a brief overview of how the gating-charge/voltage-sensor movements are translated into pore opening and the pathologies associated with mutations targeting the structures involved with the gating currents.

Multifocal ectopic purkinje-related premature contractions and related cardiomyopathy

In the past 20 years, genetic variants in SCN5A encoding the cardiac voltage-gated sodium channel Nav1.5 have been linked to a range of inherited cardiac arrhythmias: variants resulting in loss-of-function of Nav1.5 have been linked to sick sinus syndrome, atrial stand still, atrial fibrillation (AF) impaired pulse generation, progressive and non-progressive conduction defects, the Brugada Syndrome (BrS), and sudden cardiac death. SCN5A variants causing increased sodium current during the plateau phase of the cardiac action potential is associated with Long QT Syndrome type 3 (LQTS3), Torsade de Pointes ventricular tachycardia and SCD. Recently, gain-of-function variants have been linked to complex electrical phenotypes, such as the Multifocal Ectopic Purkinje-related Premature Contractions (MEPPC) syndrome. MEPPC is a rare condition characterized by a high burden of premature atrial contractions (PACs) and/or premature ventricular contractions (PVCs) often accompanied by dilated cardiomyopathy (DCM). MEPPC is inherited in an autosomal dominant fashion with an almost complete penetrance. The onset is often in childhood. The link between SCN5A variants, MEPPC and DCM is currently not well understood, but amino acid substitutions resulting in gain-of-function of Nav1.5 or introduction of gating pore currents potentially play an important role. DCM patients with a MEPPC phenotype respond relatively poorly to standard heart failure medical therapy and catheter ablation as the PVCs originate from all parts of the fascicular Purkinje fiber network. Class 1c sodium channel inhibitors, notably flecainide, have a remarkable positive effect on the ectopic burden and the associated cardiomyopathy. This highlights the importance of genetic screening of DCM patients to identify patients with SCN5A variants associated with MEPPC. Here we review the MEPPC phenotype, MEPPC-SCN5A associated variants, and pathogenesis as well as treatment options.

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.

Domain- and state-specific shape of the electric field tunes voltage sensing in voltage-gated sodium channels

The ability to sense transmembrane voltage underlies most physiological roles of voltage-gated sodium (Nav) channels. Whereas the key role of their voltage-sensing domains (VSDs) in channel activation is well established, the molecular underpinnings of voltage coupling remain incompletely understood. Voltage-dependent energetics of the activation process can be described in terms of the gating charge that is defined by coupling of charged residues to the external electric field. The shape of the electric field within VSDs is therefore crucial for the activation of voltage-gated ion channels. Here, we employed molecular dynamics simulations of cardiac Nav1.5 and bacterial NavAb, together with our recently developed tool g_elpot, to gain insights into the voltage-sensing mechanisms of Nav channels via high-resolution quantification of VSD electrostatics. In contrast to earlier low-resolution studies, we found that the electric field within VSDs of Nav channels has a complex isoform- and domain-specific shape, which prominently depends on the activation state of a VSD. Different VSDs vary not only in the length of the region where the electric field is focused but also differ in their overall electrostatics, with possible implications in the diverse ion selectivity of their gating pores. Due to state-dependent field reshaping, not only translocated basic but also relatively immobile acidic residues contribute significantly to the gating charge. In the case of NavAb, we found that the transition between structurally resolved activated and resting states results in a gating charge of 8e, which is noticeably lower than experimental estimates. Based on the analysis of VSD electrostatics in the two activation states, we propose that the VSD likely adopts a deeper resting state upon hyperpolarization. In conclusion, our results provide an atomic-level description of the gating charge, demonstrate diversity in VSD electrostatics, and reveal the importance of electric-field reshaping for voltage sensing in Nav channels.

Functional evaluation of the tachycardia patient-derived iPSC cardiomyocytes carrying a novel pathogenic SCN5A variant

Tachycardia is characterized by high beating rates that can lead to life-threatening fibrillations. Mutations in several ion-channel genes were implicated with tachycardia; however, the complex genetic contributors and their modes of action are still unclear. Here, we investigated the influence of an SCN5A gene variant on tachycardia phenotype by deriving patient-specific iPSCs and cardiomyocytes (iPSC-CM). Two tachycardia patients were genetically analyzed and revealed to inherit a heterozygous p.F1465L variant in the SCN5A gene. Gene expression and immunocytochemical analysis in iPSC-CMs generated from patients did not show any significant changes in mRNA levels of SCN5A or gross NaV1.5 cellular mislocalization, compared to healthy-derived iPSC-CMs. Electrophysiological and contraction imaging analysis in patient iPSC-CMs revealed intermittent fibrillation-like states, occasional arrhythmic events, and sustained high-paced contractions that could be selectively reduced by flecainide treatment. The patch-clamp analysis demonstrated a negative shift in the voltage-dependent activation at the patient-derived iPSC-CMs compared to the healthy control line, suggestive of a gain-of-function activity associated with the SCN5A^+/p.F1465L variant. Our patient-derived iPSC-CM model recapitulated the clinically relevant characteristics of tachycardia associated with a novel pathogenic SCN5A^+/p.F1465L variant leading to altered Na⁺ channel kinetics as the likely mechanism underlying high excitability and tachycardia phenotype.

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.

Racial Disparities in Ion Channelopathies and Inherited Cardiovascular Diseases Associated With Sudden Cardiac Death

Cardiovascular disease (CVD) continues to be the most common cause of death worldwide, and cardiac arrhythmias account for approximately one half of these deaths. The morbidity and mortality from CVD have been reduced significantly over the past few decades; however, disparities in racial or ethnic populations still exist. This review is based on available literature to date and focuses on known cardiac channelopathies and other inherited disorders associated with sudden cardiac death in African American/Black subjects and the role of epigenetics in phenotypic manifestations of CVD, and illustrates existing disparities in treatment and outcomes. The review also highlights the knowledge gaps that limit understanding of the manifestation of phenotypic abnormalities across racial or ethnic groups and discusses disparities associated with device underuse in the management of patients at risk for sudden cardiac death. We discuss factors related to reports in the United States, that the overall mortality attributed to CVD and the number of out-of-hospital cardiac arrests are higher among African American/Black subjects when compared with other racial or ethnic groups. African American/Black subjects are disproportionally affected by CVD, including cardiac arrhythmias and sudden cardiac death, thus highlighting a major concern in this population that remains underrepresented in clinical trials with limited genetic testing and device underuse. The proposed solutions include (1) early identification of genetic variants, which is crucial in tailoring a preventive management strategy; (2) inclusion of diverse racial or ethnic groups in clinical trials; (3) compliance with guideline-directed medical treatment and referral to cardiovascular subspecialists; and (4) training and mentoring of underrepresented junior faculty in cardiovascular health disparities research.

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.

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.

Voltage-Gated Ca2+-Channel α1-Subunit de novo Missense Mutations: Gain or Loss of Function - Implications for Potential Therapies

This review summarizes our current knowledge of human disease-relevant genetic variants within the family of voltage gated Ca2+ channels. Ca2+ channelopathies cover a wide spectrum of diseases including epilepsies, autism spectrum disorders, intellectual disabilities, developmental delay, cerebellar ataxias and degeneration, severe cardiac arrhythmias, sudden cardiac death, eye disease and endocrine disorders such as congential hyperinsulinism and hyperaldosteronism. A special focus will be on the rapidly increasing number of de novo missense mutations identified in the pore-forming α1-subunits with next generation sequencing studies of well-defined patient cohorts. In contrast to likely gene disrupting mutations these can not only cause a channel loss-of-function but can also induce typical functional changes permitting enhanced channel activity and Ca2+ signaling. Such gain-of-function mutations could represent therapeutic targets for mutation-specific therapy of Ca2+-channelopathies with existing or novel Ca2+-channel inhibitors. Moreover, many pathogenic mutations affect positive charges in the voltage sensors with the potential to form gating-pore currents through voltage sensors. If confirmed in functional studies, specific blockers of gating-pore currents could also be of therapeutic interest.

Structural Mechanism of ω-Currents in a Mutated Kv7.2 Voltage Sensor Domain from Molecular Dynamics Simulations

Activation of voltage-gated ion channels is regulated by conformational changes of the voltage sensor domains (VSDs), four water- and ion-impermeable modules peripheral to the central, permeable pore domain. Anomalous currents, defined as ω-currents, have been recorded in response to mutations of residues on the VSD S4 helix and associated with ion fluxes through the VSDs. In humans, gene defects in the potassium channel Kv7.2 result in a broad range of epileptic disorders, from benign neonatal seizures to severe epileptic encephalopathies. Experimental evidence suggests that the R207Q mutation in S4, associated with peripheral nerve hyperexcitability, induces ω-currents at depolarized potentials, but the fine structural details are still elusive. In this work, we use atom-detailed molecular dynamics simulations and a refined model structure of the Kv7.2 VSD in the active conformation in a membrane/water environment to study the effect of R207Q and four additional mutations of proven clinical importance. Our results demonstrate that the R207Q mutant shows the most pronounced increase of hydration in the internal VSD cavity, a feature favoring the occurrence of ω-currents. Free energy and kinetics calculations of sodium permeation through the native and mutated VSD indicate as more favorable the formation of a cationic current in the latter. Overall, our simulations establish a mechanistic linkage between genetic variations and their physiological outcome, by providing a computational description that includes both thermodynamic and kinetic features of ion permeation associated with ω-currents.

A Congenital Deadly Association: Dilated Cardiomyopathy and Long QT Syndrome

Long QT syndrome is one of the most feared entities in hospitalized patients due to the potential risk for ventricular tachycardia and sudden death. Association between channelopathies and congenital cardiomyopathy is a new entity that has been studied recently. We report an interesting case of this association that maybe related to a genetic mutation.

Functional analysis of three Nav1.6 mutations causing early infantile epileptic encephalopathy

The voltage-gated sodium channel Na_v1.6 is associated with more than 300 cases of epileptic encephalopathy. Na_v1.6 epilepsy-causing mutations are spread over the entire channel's structure and only 10% of mutations have been characterized at the molecular level, with most of them being gain of function mutations. In this study, we analyzed three previously uncharacterized Na_v1.6 epilepsy-causing mutations: G214D, N215D and V216D, located within a mutation hot-spot at the S3-S4 extracellular loop of Domain1. Voltage clamp experiments showed a 6-16 mV hyperpolarizing shift in the activation mid-point for all three mutants. V216D presented the largest shift along with decreased current amplitude, enhanced inactivation and a lack of persistent current. Recordings at hyperpolarized potentials indicated that all three mutants presented gating pore currents. Furthermore, trafficking experiments performed in cultured hippocampal neurons demonstrated that the mutants trafficked properly to the cell surface, with no significant differences regarding surface expression within the axon initial segment or soma compared to wild-type. These trafficking data suggest that the disease-causing consequences are due to only changes in the biophysical properties of the channel. Interestingly, the patient carrying the V216D mutation, which is the mutant with the greatest electrophysiological changes as compared to wild-type, exhibited the most severe phenotype. These results emphasize that these mutations will mandate unique treatment approaches, for normal sodium channel blockers may not work given that the studied mutations present gating pore currents. This study emphasizes the importance of molecular characterization of disease-causing mutations in order to improve the pharmacological treatment of patients.

Structure of the Cardiac Sodium Channel

Voltage-gated sodium channel Na_v1.5 generates cardiac action potentials and initiates the heartbeat. Here, we report structures of Na_V1.5 at 3.2-3.5 Å resolution. Na_V1.5 is distinguished from other sodium channels by a unique glycosyl moiety and loss of disulfide-bonding capability at the Na_Vβ subunit-interaction sites. The antiarrhythmic drug flecainide specifically targets the central cavity of the pore. The voltage sensors are partially activated, and the fast-inactivation gate is partially closed. Activation of the voltage sensor of Domain III allows binding of the isoleucine-phenylalanine-methionine (IFM) motif to the inactivation-gate receptor. Asp and Ala, in the selectivity motif DEKA, line the walls of the ion-selectivity filter, whereas Glu and Lys are in positions to accept and release Na⁺ ions via a charge-delocalization network. Arrhythmia mutation sites undergo large translocations during gating, providing a potential mechanism for pathogenic effects. Our results provide detailed insights into Na_v1.5 structure, pharmacology, activation, inactivation, ion selectivity, and arrhythmias.

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.

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.

A New Cardiac Channelopathy: From Clinical Phenotypes to Molecular Mechanisms Associated With Nav1.5 Gating Pores

Voltage gated sodium channels (NaV) are broadly expressed in the human body. They are responsible for the initiation of action potentials in excitable cells. They also underlie several physiological processes such as cognitive, sensitive, motor, and cardiac functions. The NaV1.5 channel is the main NaV expressed in the heart. A dysfunction of this channel is usually associated with the development of pure electrical disorders such as long QT syndrome, Brugada syndrome, sinus node dysfunction, atrial fibrillation, and cardiac conduction disorders. However, mutations of Nav1.5 have recently been linked to the development of an atypical clinical entity combining complex arrhythmias and dilated cardiomyopathy. Although several Nav1.5 mutations have been linked to dilated cardiomyopathy phenotypes, their pathogenic mechanisms remain to be elucidated. The gating pore may constitute a common biophysical defect for all NaV1.5 mutations located in the channel's VSDs. The creation of such a gating pore may disrupt the ionic homeostasis of cardiomyocytes, affecting electrical signals, cell morphology, and cardiac myocyte function. The main objective of this article is to review the concept of gating pores and their role in structural heart diseases and to discuss potential pharmacological treatments.

A leaky voltage sensor domain of cardiac sodium channels causes arrhythmias associated with dilated cardiomyopathy

Dilated cardiomyopathy (DCM) is a structural heart disease that causes dilatation of cardiac chambers and impairs cardiac contractility. The SCN5A gene encodes Na_v1.5, the predominant cardiac sodium channel alpha subunit. SCN5A mutations have been identified in patients with arrhythmic disorders associated with DCM. The characterization of Na_v1.5 mutations located in the voltage sensor domain (VSD) and associated with DCM revealed divergent biophysical defects that do not fully explain the pathologies observed in these patients. The purpose of this study was to characterize the pathological consequences of a gating pore in the heart arising from the Na_v1.5/R219H mutation in a patient with complex cardiac arrhythmias and DCM. We report its properties using cardiomyocytes derived from patient-specific human induced pluripotent stem cells. We showed that this mutation generates a proton leak (called gating pore current). We also described disrupted ionic homeostasis, altered cellular morphology, electrical properties, and contractile function, most probably linked to the proton leak. We thus propose a novel link between SCN5A mutation and the complex pathogenesis of cardiac arrhythmias and DCM. Furthermore, we suggest that leaky channels would constitute a common pathological mechanism underlying several neuronal, neuromuscular, and cardiac pathologies.

A gene-centric strategy for identifying disease-causing rare variants in dilated cardiomyopathy

PURPOSE

We evaluated strategies for identifying disease-causing variants in genetic testing for dilated cardiomyopathy (DCM).

METHODS

Cardiomyopathy gene panel testing was performed in 532 DCM patients and 527 healthy control subjects. Rare variants in 41 genes were stratified using variant-level and gene-level characteristics.

RESULTS

A majority of DCM cases and controls carried rare protein-altering cardiomyopathy gene variants. Variant-level characteristics alone had limited discriminative value. Differentiation between groups was substantially improved by addition of gene-level information that incorporated ranking of genes based on literature evidence for disease association. The odds of DCM were increased to nearly 9-fold for truncating variants or high-impact missense variants in the subset of 14 genes that had the strongest biological links to DCM (P <0.0001). For some of these genes, DCM-associated variants appeared to be clustered in key protein functional domains. Multiple rare variants were present in many family probands, however, there was generally only one "driver" pathogenic variant that cosegregated with disease.

CONCLUSION

Rare variants in cardiomyopathy genes can be effectively stratified by combining variant-level and gene-level information. Prioritization of genes based on their a priori likelihood of disease causation is a key factor in identifying clinically actionable variants in cardiac genetic testing.

Structural basis for gating pore current in periodic paralysis

Potassium-sensitive Hypokalemic and Normokalemic Periodic Paralysis (HypoPP, NormoPP) are inherited skeletal muscle diseases characterized by episodes of flaccid muscle weakness^1,2. They are caused by mutations in one gating charge in an S4 transmembrane segment in the voltage sensor (VS) of voltage-gated sodium channel Na_v1.4 or calcium channel Ca_v1.1^1,2. Mutations of the outermost arginine gating charges (R1 and R2) cause HypoPP^1,2 by creating a pathogenic gating pore in the VS through which cations leak in the resting state^3,4. Mutations of the third arginine gating charge (R3) cause NormoPP⁵ owing to cationic leak in activated/inactivated states⁶. Here we present high-resolution structures of these pathogenic gating pores in the model bacterial sodium channel Na_VAb^7,8. Mutation of R2 in Na_VAb gives gating pore current in resting states, whereas mutation of R3 gives gating pore current in activated/inactivated states. Mutations R2G and R3G have no effect on backbone structures of VS, but create aqueous space near the hydrophobic constriction site (HCS) that controls gating charge movement through VS. The R3G mutation extends the extracellular aqueous cleft completely through the activated VS. Although the R2G mutation does not create a continuous aqueous pathway in the activated state, molecular modeling of the resting state reveals a complete water-accessible pathway. Crystal structures of Na_VAb/R2G in complex with guanidinium define a potential drug target site. Molecular dynamics simulations illustrate the mechanism of Na⁺ permeation through the mutant gating pore in concert with conformational fluctuations of gating charge R4. Our results reveal pathogenic mechanisms of periodic paralysis at the atomic level and suggest designs of drugs that may prevent ionic leak and provide symptomatic relief from these episodic diseases.

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.

Excitation-Contraction Coupling Alterations in Myopathies

During the complex series of events leading to muscle contraction, the initial electric signal coming from motor neurons is transformed into an increase in calcium concentration that triggers sliding of myofibrils. This process, referred to as excitation-contraction coupling, is reliant upon the calcium-release complex, which is restricted spatially to a sub-compartment of muscle cells ("the triad") and regulated precisely. Any dysfunction in the calcium-release complex leads to muscle impairment and myopathy. Various causes can lead to alterations in excitation-contraction coupling and to muscle diseases. The latter are reviewed and classified into four categories: (i) mutation in a protein of the calcium-release complex; (ii) alteration in triad structure; (iii) modification of regulation of channels; (iv) modification in calcium stores within the muscle. Current knowledge of the pathophysiologic mechanisms in each category is described and discussed.

Gating Pore Currents in Sodium Channels

Voltage-gated sodium channels belong to the superfamily of voltage-gated cation channels. Their structure is based on domains comprising a voltage sensor domain (S1-S4 segments) and a pore domain (S5-S6 segments). Mutations in positively charged residues of the S4 segments may allow protons or cations to pass directly through the gating pore constriction of the voltage sensor domain; these anomalous currents are referred to as gating pore or omega (ω) currents. In the skeletal muscle disorder hypokalemic periodic paralysis, and in arrhythmic dilated cardiomyopathy, inherited mutations of S4 arginine residues promote omega currents that have been shown to be a contributing factor in the pathogenesis of these sodium channel disorders. Characterization of gating pore currents in these channelopathies and with artificial mutations has been possible by measuring the voltage-dependence and selectivity of these leak currents. The basis of gating pore currents and the structural basis of S4 movement through the gating pore has also been studied extensively with molecular dynamics. These simulations have provided valuable insight into the nature of S4 translocation and the physical basis for the effects of mutations that promote permeation of protons or cations through the gating pore.

A novel KCNA1 mutation in a patient with paroxysmal ataxia, myokymia, painful contractures and metabolic dysfunctions

Episodic ataxia type 1 (EA1) is a human dominant neurological syndrome characterized by continuous myokymia, episodic attacks of ataxic gait and spastic contractions of skeletal muscles that can be triggered by emotional stress and fatigue. This rare disease is caused by missense mutations in the KCNA1 gene coding for the neuronal voltage gated potassium channel Kv1.1, which contributes to nerve cell excitability in the cerebellum, hippocampus, cortex and peripheral nervous system. We identified a novel KCNA1 mutation, E283K, in an Italian proband presenting with paroxysmal ataxia and myokymia aggravated by painful contractures and metabolic dysfunctions. The E283K mutation is located in the S3-S4 extracellular linker belonging to the voltage sensor domain of Kv channels. In order to test whether the E283K mutation affects Kv1.1 biophysical properties we transfected HEK293 cells with WT or mutant cDNAs alone or in a 1:1 combination, and recorded relative potassium currents in the whole-cell configuration of patch-clamp. Mutant E283K channels display voltage-dependent activation shifted by 10mV toward positive potentials and kinetics of activation slowed by ~2 fold compared to WT channels. Potassium currents resulting from heteromeric WT/E283K channels show voltage-dependent gating and kinetics of activation intermediate between WT and mutant homomeric channels. Based on homology modeling studies of the mutant E283K, we propose a molecular explanation for the reduced voltage sensitivity and slow channel opening. Overall, our results suggest that the replacement of a negatively charged residue with a positively charged lysine at position 283 in Kv1.1 causes a drop of potassium current that likely accounts for EA-1 symptoms in the heterozygous carrier.

Ion channelopathies associated genetic variants as the culprit for sudden unexplained death

Forensic identification of sudden unexplained death (SUD) has always been a ticklish issue because it used to be defined as sudden death without a conclusive diagnosis after autopsy. However, benefiting from the developments in genome research, a growing body of evidence points to the importance of ion channelopathies associated genetic variants in the pathogenesis of SUD. Genetic diagnosis of the deceased is also a new trend in epidemiological studies, for it enables the undertaking for preventive approach in individuals with high risks. In this review, we briefly discuss the molecular structure of ion channels and the role of genetic variants in regulating their functions as well as the diverse mechanisms underlying the ion channelopathies at gene level.

Sodium channel biophysics, late sodium current and genetic arrhythmic syndromes

Arrhythmias arise from breakdown of orderly action potential (AP) activation, propagation and recovery driven by interactive opening and closing of successive voltage-gated ion channels, in which one or more Na⁺ current components play critical parts. Early peak, Na⁺ currents (I_Na) reflecting channel activation drive the AP upstroke central to cellular activation and its propagation. Sustained late Na⁺ currents (I_Na-L) include contributions from a component with a delayed inactivation timecourse influencing AP duration (APD) and refractoriness, potentially causing pro-arrhythmic phenotypes. The magnitude of I_Na-L can be analysed through overlaps or otherwise in the overall voltage dependences of the steady-state properties and kinetics of activation and inactivation of the Na⁺ conductance. This was useful in analysing repetitive firing associated with paramyotonia congenita in skeletal muscle. Similarly, genetic cardiac Na⁺ channel abnormalities increasing I_Na-L are implicated in triggering phenomena of automaticity, early and delayed afterdepolarisations and arrhythmic substrate. This review illustrates a wide range of situations that may accentuate I_Na-L. These include (1) overlaps between steady-state activation and inactivation increasing window current, (2) kinetic deficiencies in Na⁺ channel inactivation leading to bursting phenomena associated with repetitive channel openings and (3) non-equilibrium gating processes causing channel re-opening due to more rapid recoveries from inactivation. All these biophysical possibilities were identified in a selection of abnormal human SCN5A genotypes. The latter presented as a broad range of clinical arrhythmic phenotypes, for which effective therapeutic intervention would require specific identification and targeting of the diverse electrophysiological abnormalities underlying their increased I_Na-L.

Genetic basis of dilated cardiomyopathy

Dilated cardiomyopathy is a rare cardiac disease characterized by left ventricular dilatation and systolic dysfunction leading to heart failure and sudden cardiac death. Currently, despite several conditions have been reported as aetiologies of the disease, a large number of cases remain classified as idiopathic. Recent studies determine that nearly 60% of cases are inherited, therefore due to a genetic cause. Progressive technological advances in genetic analysis have identified over 60 genes associated with this entity, being TTN the main gene, so far. All these genes encode a wide variety of myocyte proteins, mainly sarcomeric and desmosomal, but physiopathologic pathways are not yet completely unraveled. We review the recent published data about genetics of familial dilated cardiomyopathy.