SCN2A-related epilepsy of infancy with migrating focal seizures: report of a variant with apparent gain- and loss-of-function effects.

SCN2A encodes a voltage-gated sodium channel (NaV1.2) expressed throughout the central nervous system in predominantly excitatory neurons. Pathogenic variants in SCN2A are associated with epilepsy and neurodevelopmental disorders. Genotype-phenotype correlations have been described, with loss-of-function variants typically being associated with neurodevelopmental delay and later-onset seizures, whereas gain-of-function variants more often result in early infantile-onset epilepsy. However, the true electrophysiological effects of most disease-causing SCN2A variants have yet to be characterized. We report an infant who presented with migrating focal seizures in the neonatal period. She was found to have a mosaic c.2635G>A, p.Gly879Arg variant in SCN2A. Voltage-clamp studies of the variant expressed on adult and neonatal NaV1.2 isoforms demonstrated a mixed gain and loss of function, with predominantly a loss-of-function effect with reduced cell surface expression and current density. Additional small electrophysiological alterations included a decrease in the voltage dependence of activation and an increase in the voltage dependence of inactivation. This finding of a predominantly loss-of-function effect was unexpected, as the infant's early epilepsy onset would have suggested a predominantly gain-of-function effect. This case illustrates that our understanding of genotype-phenotype correlations is still limited and highlights the complexity of the underlying electrophysiological effects of SCN2A variants.NEW & NOTEWORTHY Voltage-gated sodium channels play an important role in the central nervous system, mutations in which have been reported to be responsible for epilepsy. We report here an infant presenting with epilepsy of infancy with migrating focal seizures (EIMFS) in the neonatal period with a mosaic c.2635G>A, resulting in a p.Gly879Arg missense mutation on the SCN2A gene encoding NaV1.2 sodium channels. Biophysical characterization of this variant revealed a mixture of gain- and loss-of-function effects.

Novel G1481V and Q1491H SCN5A Mutations Linked to Long QT Syndrome Destabilize the Nav1.5 Inactivation State.

BACKGROUND: Nav1.5, which is encoded by the SCN5A gene, is the predominant voltage-gated Na+ channel in the heart. Several mutations of this gene have been identified and reported to be involved in several cardiac rhythm disorders, including type 3 long QT interval syndrome, that can cause sudden cardiac death. We analyzed the biophysical properties of 2 novel variants of the Nav1.5 channel (Q1491H and G1481V) detected in 5- and 12-week-old infants diagnosed with a prolonged QT interval. METHODS: The Nav1.5 wild-type and the Q1491H and G1481V mutant channels were reproduced in vi tr o. Wild-type or mutant channels were cotransfected in human embryonic kidney (HEK) 293 cells with the beta 1 regulatory subunit. Na+ currents were recorded using the whole-cell configuration of the patch-clamp technique. RESULTS: The Q1491H mutant channel exhibited a lower current density, a persistent Na+ current, an enhanced window current due to a +20-mV shift of steady-state inactivation, a +10-mV shift of steady-state activation, a faster onset of slow inactivation, and a recovery from fast inactivation with fast and slow time constants of recovery. The G1481V mutant channel exhibited an increase in current density and a +7-mV shift of steady-state inactivation. The observed defects are characteristic of gain-of-function mutations typical of type 3 long QT interval syndrome. CONCLUSIONS: The 5- and 12-week-old infants displayed prolonged QT intervals. Our analyses of the Q1491H and G1481V mutations correlated with the clinical diagnosis. The observed biophysical dysfunctions associated with both mutations were most likely responsible for the sudden deaths of the 2 infants.

INTRODUCTION: Le canal Nav1.5, codé par le gène SCN5A, est le canal Na+ dépendant du voltage prédominant dans le cœur. Plusieurs mutations de ce gène sont impliquées dans plusieurs anomalies du rythme cardiaque, dont le syndrome du QT long de type 3, qui peut provoquer la mort subite d'origine cardiaque. Nous avons analysé les propriétés biophysiques de deux nouveaux variants du canal Nav1.5 (Q1491H et G1481V) détectés chez deux bébés âgés respectivement de 5 et 12 semaines qui avaient une prolongation de l'intervalle QT. MÉTHODES: Le canal Nav1.5 de type sauvage et les canaux mutants Q1491H et G1481V ont été reproduits in vi tr o. Les canaux de type sauvage ou mutants ont été co-transfectés dans les cellules des reins embryonnaires humains (REH) 293 avec la sous-unité régulatrice bêta 1. Les courants Na+ ont été enregistrés à partir de la configuration en cellule entière via la technique de patch-clamp. RÉSULTATS: Le canal mutant Q1491H montre une densité de courant plus faible, un courant Na+ persistant, un courant fenêtre augmenté en raison d'un changement dép de +20 mV de l'inactivation à l'état stable, un changement de +10 mV de l'activation à l'état stable, une entrée plus rapide de l'inactivation lente et une récupération de l'inactivation rapide avec des constantes de temps rapides et lentes. Le canal mutant G1481V montre une augmentation de la densité de courant et un changement de +7 mV de l'inactivation à l'état stable. Les anomalies observées sont caractéristiques des mutations avec gain de fonction typiques du syndrome du QT long de type 3. CONCLUSIONS: Les deux bébés âgés respectivement de cinq 5 et 12 semaines montraient une prolongation des intervalles QT. Nos analyses des mutations Q1491H et G1481V montrent une corrélation avec le diagnostic clinique. Les dysfonctions biophysiques observées qui sont associées aux deux mutations étaient fort probablement responsables des morts subites des deux bébés.

Biophysical, Molecular, and Pharmacological Characterization of Voltage-Dependent Sodium Channels From Induced Pluripotent Stem Cell-Derived Cardiomyocytes.

BACKGROUND: The ability to differentiate patient-specific human induced pluripotent stem cells in cardiac myocytes (hiPSC-CM) offers novel perspectives for cardiovascular research. A number of studies, that reported mainly on current-voltage curves used hiPSC-CM to model voltage-gated Na+ channel (Nav) dysfunction. However, the expression patterns and precise biophysical and pharmacological properties of Nav channels from hiPSC-CM remain unknown. Our objective was to study the characteristics of Nav channels from hiPSC-CM and assess the appropriateness of this novel cell model. METHODS: We generated hiPSC-CM using the recently described monolayer-based differentiation protocol. RESULTS: hiPSC-CM expressed cardiac-specific markers, exhibited spontaneous electrical and contractile activities, and expressed distinct Nav channels subtypes. Electrophysiological, pharmacological, and molecular characterizations revealed that, in addition to the main Nav1.5 channel, the neuronal tetrodotoxin (TTX)-sensitive Nav1.7 channel was also significantly expressed in hiPSC-CM. Most of the Na+ currents were resistant to TTX block. Therapeutic concentrations of lidocaine, a class I antiarrhythmic drug, also inhibited Na+ currents in a use-dependent manner. Nav1.5 and Nav1.7 expression and maturation patterns of hiPSC-CM and native human cardiac tissues appeared to be similar. The 4 Navß regulatory subunits were expressed in hiPSC-CM, with ß3 being the preponderant subtype. CONCLUSIONS: The findings indicated that hiPSC-CM robustly express Nav1.5 channels, which exhibited molecular and pharmacological properties similar to those in native cardiac tissues. Interestingly, neuronal Nav1.7 channels were also expressed in hiPSC-CM and are likely to be responsible for the TTX-sensitive Nav current.

Differential modulation of Nav1.7 and Nav1.8 channels by antidepressant drugs.

Antidepressant drugs of the SSRI family are used as a third-line treatment for neuropathic pain. In contrast MAOi antidepressants, that also increase extracellular serotonin bioavailability have little or no effects on this condition. In addition to their action of the serotonin transporter, some SSRI have been shown to inhibit voltage gated sodium channels. Here we investigated the potential inhibition of SSRIs and MAOi antidepressants on Nav1.7 or Nav1.8, which are expressed in sensory neurons and play an important role in pain sensation. We used the whole-cell patch-clamp technique on HEK293 cells expressing either Nav1.7 or Nav1.8, and evaluated the effects of the SSRIs fluoxetine, paroxetine, and citalopram as well as one MAOi antidepressants on the electrophysiological properties of the Na(+) channels. Paroxetine exhibited the greatest affinity for Na(+) channels. In ascending order of affinity for Nav1.7 were paroxetine (IC50=10 µM), followed by fluoxetine (IC50=66 µM), then citalopram (IC50=174 µM). In ascending order of affinity for Nav1.8 were paroxetine (IC50=9 µM), followed by fluoxetine (IC50=49 µM), then citalopram (IC50=100 µM). Paroxetine and fluoxetine accelerated the onset of slow-inactivation and delayed the time-course of recovery from inactivation for both channels. Paroxetine and fluoxetine also had a prominent effect on the frequency-dependent inhibition, with a greater effect on Nav1.7. In contrast to SSRIs, MAOi did not affect Na(+) channels currents. These results suggest that, in certain conditions, the analgesic effect of SSRIs may in part be due to their interactions with Na(+) channels.

Correlation of the electrophysiological profiles and sodium channel transcripts of individual rat dorsal root ganglia neurons.

Voltage gated sodium channels (Nav channels) play an important role in nociceptive transmission. They are intimately tied to the genesis and transmission of neuronal firing. Five different isoforms (Nav1.3, Nav1.6, Nav1.7, Nav1.8, and Nav1.9) have been linked to nociceptive responses. A change in the biophysical properties of these channels or in their expression levels occurs in different pathological pain states. However, the precise involvement of the isoforms in the genesis and transmission of nociceptive responses is unknown. The aim of the present study was to investigate the synergy between the different populations of Nav channels that give individual neurons a unique electrophysical profile. We used the patch-clamp technique in the whole-cell configuration to record Nav currents and action potentials from acutely dissociated small diameter DRG neurons (<30 µm) from adult rats. We also performed single cell qPCR on the same neurons. Our results revealed that there is a strong correlation between Nav currents and mRNA transcripts in individual neurons. A cluster analysis showed that subgroups formed by Nav channel transcripts by mRNA quantification have different biophysical properties. In addition, the firing frequency of the neurons was not affected by the relative populations of Nav channel. The synergy between populations of Nav channel in individual small diameter DRG neurons gives each neuron a unique electrophysiological profile. The Nav channel remodeling that occurs in different pathological pain states may be responsible for the sensitization of the neurons.

Fluoxetine blocks Nav1.5 channels via a mechanism similar to that of class 1 antiarrhythmics.

The voltage-gated Nav1.5 channel is essential for the propagation of action potentials in the heart. Malfunctions of this channel are known to cause hereditary diseases. It is a prime target for class 1 antiarrhythmic drugs and a number of antidepressants. Our study investigated the Nav1.5 blocking properties of fluoxetine, a selective serotonin reuptake inhibitor. Nav1.5 channels were expressed in HEK-293 cells, and Na(+) currents were recorded using the patch-clamp technique. Dose-response curves of racemic fluoxetine (IC50 = 39 µM) and its optical isomers had a similar IC50 [40 and 47 µM for the (+) and (-) isomers, respectively]. Norfluoxetine, a fluoxetine metabolite, had a higher affinity than fluoxetine, with an IC50 of 29 µM. Fluoxetine inhibited currents in a frequency-dependent manner, shifted steady-state inactivation to more hyperpolarized potentials, and slowed the recovery of Nav1.5 from inactivation. Mutating a phenylalanine (F1760) and a tyrosine (Y1767) in the S6 segment of domain (D) IV (DIVS6) significantly reduced the affinity of fluoxetine and its frequency-dependent inhibition. We used a noninactivating Nav1.5 mutant to show that fluoxetine displays open-channel block behavior. The molecular model of fluoxetine in Nav1.5 was in agreement with mutational experiments in which F1760 and Y1767 were found to be the key residues in binding fluoxetine. We concluded that fluoxetine blocks Nav1.5 by binding to the class 1 antiarrhythmic site. The blocking of cardiac Na(+) channels should be taken into consideration when prescribing fluoxetine alone or in association with other drugs that may be cardiotoxic or for patients with conduction disorders.

Pyridoxal-5'-phosphate (MC-1), a vitamin B6 derivative, inhibits expressed P2X receptors.

P2X receptors are cation-permeable ligand-gated ion channels that open in response to the binding of ATP. These receptors are present in many excitable cells, including neurons, striated muscle cells, epithelial cells, and leukocytes. They mediate fast excitatory neurotransmission in the central and peripheral nervous systems and are thought to be involved in neuropathic pain, inflammation, and cell damage following ischemia-reperfusion injuries. P2X receptors are thus a target for the development of new therapeutics to treat chronic pain and inflammation. In this study, we characterized the inhibition caused by pyridoxal-5'-phosphate, a natural metabolite of vitamin B6 (MC-1), of P2X2, P2X4, P2X7, and P2X2/3 receptors stably expressed in HEK293 cells using the patch-clamp technique in the whole-cell configuration. We also tested a new approach using VC6.1, a modified cameleon calcium-sensitive fluorescent protein, to characterize the inhibition of P2X2 and P2X2/3. MC-1 blocked these two P2X receptors, with an IC50 of 7 and 13 µmol/L, respectively. P2X2 exhibited the highest affinity for VC6.1, and the chimeric receptor P2X2/3, the lowest. The patch-clamp and imaging approaches gave similar results and indicated that VC6.1 may be useful for high throughput drug screening. Pyridoxal-5'-phosphate is an efficient P2X blocker and can be classified as a P2X antagonist.

Modulation of peripheral Na(+) channels and neuronal firing by n-butyl-p-aminobenzoate.

n-butyl-p-aminobenzoate (BAB), a local anesthetic, is administered epidurally in cancer patients to treat pain that is poorly controlled by other drugs that have a number of adverse effects. The purpose of the study was to unravel the mechanisms underlying the apparent selective pain suppressant effect of BAB. We used the whole-cell patch-clamp technique to record Na(+) currents and action potentials (APs) in dissociated, nociceptive dorsal root ganglion (DRG) cells from rats, two types of peripheral sensory neuron Na(+) channels (Nav1.7 and Nav1.8), and the motor neuron-specific Na(+) channel (Nav1.6) expressed in HEK293 cells. BAB (1-100µM) inhibited, in a concentration-dependent manner, the depolarization evoked repetitive firing in DRG cells, the three types of Na(+) current expressed in HEK293 cells, and the TTXr Na(+) current of the DRG neurons. BAB induced a use-dependent block that caused a shift of the inactivation curve in the hyperpolarizing direction. BAB enhanced the onset of slow inactivation of Nav1.7 and Nav1.8 currents but not of Nav1.6 currents. At clinically relevant concentrations (1-100µM), BAB is thus a more potent inhibitor of peripheral TTX-sensitive TTXs, Nav1.7 and TTX-resistant NaV1.8 Na(+) channels than of motor neuron axonal Nav1.6 Na(+) channels. BAB had similar effects on the TTXr Na(+) channels of rat DRG neurons and Nav1.8 channels expressed in HEK293 cells. The observed selectivity of BAB in treating cancer pain may be due to an enhanced and selective responsiveness of Na(+) channels in nociceptive neurons to this local anesthetic.

A distinct de novo expression of Nav1.5 sodium channels in human atrial fibroblasts differentiated into myofibroblasts.

Fibroblasts play a major role in heart physiology. They are at the origin of the extracellular matrix renewal and production of various paracrine and autocrine factors. In pathological conditions, fibroblasts proliferate, migrate and differentiate into myofibroblasts leading to cardiac fibrosis. This differentiated status is associated with changes in expression profile leading to neo-expression of proteins such as ionic channels. The present study investigates further electrophysiological changes associated with fibroblast differentiation focusing on the activity of voltage-gated sodium channels in human atrial fibroblasts and myofibroblasts. Using the patch clamp technique we show that human atrial myofibroblasts display a fast inward voltage gated sodium current with a density of 13.28 ± 2.88 pA pF(-1) whereas no current was detectable in non-differentiated fibroblasts. Quantitative RT-PCR reveals a large amount of transcripts encoding the Na(v)1.5 α-subunit with a fourfold increased expression level in myofibroblasts when compared to fibroblasts. Accordingly, half of the current was blocked by 1 µm of tetrodotoxin and immunocytochemistry experiments reveal the presence of Na(v)1.5 proteins. Overall, this current exhibits similar biophysical characteristics to sodium currents found in cardiac myocytes except for the window current that is enlarged for potentials between -100 and -20 mV. Since fibrosis is one of the fundamental mechanisms implicated in atrial fibrillation, it is of great interest to investigate how this current could influence myofibroblast properties. Moreover, since several Na(v)1.5 mutations are related to cardiac pathologies, this study offers a new avenue on the fibroblasts involvement of these mutations.

Coexisting mutations/polymorphisms of the long QT syndrome genes in patients with repaired Tetralogy of Fallot are associated with the risks of life-threatening events.

Coexisting long QT gene mutations/polymorphisms in Tetralogy of Fallot (TOF) patients may aggravate the repolarization abnormality from cardiac repair. We investigated the impact of these genes on the risk of life-threatening events. Genetic variants of the three common long QT genes were identified from patients with repaired TOF. Life-threatening events were defined as sudden cardiac death and hemodynamic unstable ventricular arrhythmia. Biophysical characterization of the alleles of the genetic variants was performed using a whole-cell voltage clamp with expression in Xenopus oocytes. A total of 84 patients (56.0 % male with 1,215 patients-year follow-up) were enrolled. Six rare variants and six non-synonymous single nucleotide polymorphisms (SNPs) were found in 40 (47.6 %) patients. Life-threatening events occurred in five patients; four received implantable cardioverter defibrillator and one died of sudden cardiac death. Life-threatening events occurred more often in those with genetic variants than those without (5/40 vs. 0/44, P = 0.021); particularly, the hERG or SCN5A gene mutations/polymorphisms (2/5 vs. 3/79, P = 0.027 and 5/27 vs. 0/57, P = 0.003, respectively). Among the five patients with life-threatening events, three had compound variants (hERG p.M645R/SCN5A p.R1193Q, hERG p.K897T/SCN5A p.H558R, and KVLQT1 p.G645S/SCN5A p.P1090L), that also increased the risk of events. Their QTc and JTc were all prolonged. Functional study of the novel variant (hERG gene p.M645R) from patients with life-threatening events revealed a dominant negative effect. In conclusion, in repaired TOF patients, coexisting long QT mutations/polymorphisms might have additive effects on the repolarization abnormality from surgery and thereby increase the risks of life-threatening events.

A new C-terminal hERG mutation A915fs+47X associated with symptomatic LQT2 and auditory-trigger syncope.

BACKGROUND: A novel mutation of hERG (A915fs+47X) was discovered in a 32-year-old woman with torsades de pointes, long QTc interval (515 ms), and syncope upon auditory trigger. OBJECTIVE: We explored whether the properties of this mutation could explain the pathology. METHODS: Whole-cell A915fs+47X (del) and wild-type (WT) currents were recorded in transiently transfected COS7 cells or Xenopus oocytes. Western blots and sedimentation analysis of del/WT hERG were used to analyze protein expression, assembly, and trafficking. RESULTS: The tail current density at -40 mV after a 2-s depolarization to +40 mV in COS7 cells expressing del was 36% of that for WT. Inactivation was 1.9-fold to 2.8-fold faster in del versus WT between -60 and +60 mV. In the range -60 to -10 mV, we found that a nondeactivating fraction of current was increased in del at the expense of a rapidly deactivating fraction, with a slowly deactivating fraction being unchanged. In Xenopus oocytes, expression of del alone produced 38% of WT currents, whereas coexpression of 1/2 WT + 1/2 del produced 49.8%. Furthermore, the expression of del protein at the cell surface was reduced by about 50%. This suggests that a partial trafficking defect of del contributes to the reduction in del current densities and to the dominant negative effect when coexpressed with WT. In model simulations, the mutation causes a 10% prolongation of action potential duration. CONCLUSION: Decreased current levels caused by a trafficking defect may explain the long QT syndrome observed in our patient.

In utero onset of long QT syndrome with atrioventricular block and spontaneous or lidocaine-induced ventricular tachycardia: compound effects of hERG pore region mutation and SCN5A N-terminus variant.

BACKGROUND: Mexiletine may protect patients with long QT syndrome (LQTS) type 3 from arrhythmias. However, we found an unusual in utero presentation of intermittent atrioventricular block and ventricular tachycardia (spontaneous or lidocaine-induced) in a fetus and his sibling with LQTS. OBJECTIVE: The purpose of this study was to investigate the underlying channelopathy and functional alteration. METHODS: Mutations were searched in KCNQ1, HERG, KCNE1, KCNE2, and SCN5A genes. In expressed mutants, whole-cell voltage clamp defined the electrophysiologic properties. RESULTS: Novel missense mutations involving hERG (F627L) at the pore region and SCN5A (R43Q) at the N-terminus were found in the proband and in family members with prolonged QT interval. In oocytes injected with mRNA encoding hERG/ F627L, almost zero K(+) currents were elicited. In coinjected oocytes, the currents were decreased to half. In tsA201 cells transfected with SCN5A/R43Q, although the baseline kinetics of the Na current were similar to wild type, lidocaine caused a unique hyperpolarizing shift of the activation and increased the availability of Na currents at resting voltages. Window currents were enhanced due to a right shift of steady-state inactivation. These electrophysiologic alterations after lidocaine may lead to the development of ventricular tachycardia. CONCLUSION: We identified a novel hERG/F627L mutation that results in LQTS with fetal onset of atrioventricular block and ventricular tachycardia. A coexisting SCN5A/R43Q variant, although it per se does not prolong repolarization, contributes to the development of ventricular tachyarrhythmias after lidocaine. Patients with such latent lidocaine-induced phenotype who are given lidocaine or mexiletine may be at risk.