Cardiac arrhythmia in a mouse model of sodium channel SCN8A epileptic encephalopathy

Cardiac arrhythmia and epilepsy genetic variants in sudden unexpected death in epilepsy

Introduction

Sudden Unexpected Death in Epilepsy (SUDEP) is the leading epilepsy-related cause of death, affecting approximately 1 per 1,000 individuals with epilepsy per year. Genetic variants that affect autonomic function, such as genes associated with cardiac arrhythmias, may predispose people with epilepsy to greater risk of both sudden cardiac death and SUDEP. Advances in next generation sequencing allow for the exploration of gene variants as potential biomarkers.

Methods

Genetic testing for the presence of cardiac arrhythmia and epilepsy gene variants was performed via genetic panels in 39 cases of SUDEP identified via autopsy by the Ontario Forensic Pathology Service. Variants were summarized by in-silico evidence for pathogenicity from 4 algorithms (SIFT, PolyPhen-2, PROVEAN, Mutation Taster) and allele frequencies in the general population (GnomAD). A maximum credible population allele frequency of 0.00004 was calculated based on epilepsy prevalence and SUDEP incidence to assess whether a variant was compatible with a pathogenic interpretation.

Results

Median age at the time of death was 33.3 years (range: 2, 60). Fifty-nine percent (n=23) were male. Gene panels detected 62 unique variants in 45 genes: 19 on the arrhythmia panel and 26 on the epilepsy panel. At least one variant was identified in 28 (72%) of decedents. Missense mutations comprised 57 (92%) of the observed variants. At least three in silico models predicted 12 (46%) cardiac arrhythmia panel missense variants and 20 (65%) epilepsy panel missense variants were pathogenic. Population allele frequencies were <0.00004 for 11 (42%) of the cardiac variants and 10 (32%) of the epilepsy variants. Together, these metrics identified 13 SUDEP variants of interest.

Discussion

Nearly three-quarters of decedents in this SUDEP cohort carried variants in comprehensive epilepsy or cardiac arrhythmia gene panels, with more than a third having variants in both panels. The proportion of decedents with cardiac variants aligns with recent studies of the disproportionate cardiac burden the epilepsy community faces compared to the general population and suggests a possible cardiac contribution to epilepsy mortality. These results identified 13 priority targets for future functional studies of these genes potential role in sudden death and demonstrates the necessity for further exploration of potential genetic contributions to SUDEP.

Cardiac-Specific Deletion of Scn8a Mitigates Dravet Syndrome-Associated Sudden Death in Adults

BACKGROUND

Sudden unexpected death in epilepsy (SUDEP) is a fatal complication experienced by otherwise healthy epilepsy patients. Dravet syndrome (DS) is an inherited epileptic disorder resulting from loss of function of the voltage-gated sodium channel, NaV 1.1, and is associated with particularly high SUDEP risk. Evidence is mounting that NaVs abundant in the brain also occur in the heart, suggesting that the very molecular mechanisms underlying epilepsy could also precipitate cardiac arrhythmias and sudden death. Despite marked reduction of NaV 1.1 functional expression in DS, pathogenic late sodium current (INa,L) is paradoxically increased in DS hearts. However, the mechanisms by which DS directly impacts the heart to promote sudden death remain unclear.

OBJECTIVES

In this study, the authors sought to provide evidence implicating remodeling of Na+ - and Ca2+ -handling machinery, including NaV 1.6 and Na+/Ca2+exchanger (NCX) within transverse (T)-tubules in DS-associated arrhythmias.

METHODS

The authors undertook scanning ion conductance microscopy (SICM)-guided patch clamp, super-resolution microscopy, confocal Ca2+ imaging, and in vivo electrocardiography studies in Scn1a haploinsufficient murine model of DS.

RESULTS

DS promotes INa,L in T-tubular nanodomains, but not in other subcellular regions. Consistent with increased NaV activity in these regions, super-resolution microscopy revealed increased NaV 1.6 density near Ca2+release channels, the ryanodine receptors (RyR2) and NCX in DS relative to WT hearts. The resulting INa,L in these regions promoted aberrant Ca2+ release, leading to ventricular arrhythmias in vivo. Cardiac-specific deletion of NaV 1.6 protects adult DS mice from increased T-tubular late NaV activity and the resulting arrhythmias, as well as sudden death.

CONCLUSIONS

These data demonstrate that NaV 1.6 undergoes remodeling within T-tubules of adult DS hearts serving as a substrate for Ca2+ -mediated cardiac arrhythmias and may be a druggable target for the prevention of SUDEP in adult DS subjects.

Cardiac dysfunctions in children with drug-resistant epilepsy

Objective

There were reports of cardiac dysfunction that led to sudden unexpected death in epilepsy (SUDEP) in patients with epilepsy. Early detection of cardiac dysfunction can lead to early management to prevent sudden cardiac death in these patients. The objective of our study is to assess cardiac functions in children with drug-resistant epilepsy (DRE) compared with the normal population by using a standard echocardiogram (SE), tissue Doppler imaging (TDI) and myocardial strain evaluations (MSE).

Method

Twenty-seven children who have been diagnosed with DRE based on the International League against Epilepsy (ILAE) were included in the study, along with 27 children whose ages match those of the normal control group.

Results

Seventeen children, median age 12 years old, were using more than four anti-seizure medications. Structural brain lesions were the most common cause of epilepsy, 55.6% (15). Generalized tonic-clonic seizures were the most common seizure type, 55.6% (15). Children with DRE had a lower early mitral valve E wave inflow velocity compared with the control group (p < 0.05). They also had lowered early diastolic velocities (e') and myocardial performance index (MPI) when compared with the control group (p < 0.05). There was a statistically significant difference in left ventricular myocardial strain in children with DRE, with an average of -21.1 (IQR -23.5 and -19.4) and control, -25.5 (IQR -27.3 and -24.2).

Significance

Children with DRE have an impairment of left ventricular diastolic function and myocardial strain, which could indicate decreased myocardial deformation and contraction compared with controls. These cardiological assessments can be used to evaluate children with DRE for early diagnosis and management of their cardiac dysfunction.

Are we there yet? A critical evaluation of sudden and unexpected death in epilepsy models

Although animal models have helped to elaborate meaningful hypotheses about the pathophysiology of sudden and unexpected death in epilepsy (SUDEP), specific prevention strategies are still lacking, potentially reflecting the limitations of these models and the intrinsic difficulties of investigating SUDEP. The interpretation of preclinical data and their translation to diagnostic and therapeutic developments in patients thus require a high level of confidence in their relevance to model the human situation. Preclinical models of SUDEP are heterogeneous and include rodent and nonrodent species. A critical aspect is whether the animals have isolated seizures exclusively induced by a specific trigger, such as models where seizures are elicited by electrical stimulation, pharmacological intervention, or DBA mouse strains, or whether they suffer from epilepsy with spontaneous seizures, with or without spontaneous SUDEP, either of nongenetic epilepsy etiology or from genetically based developmental and epileptic encephalopathies. All these models have advantages and potential disadvantages, but it is important to be aware of these limitations to interpret data appropriately in a translational perspective. The majority of models with spontaneous seizures are of a genetic basis, whereas SUDEP cases with a genetic basis represent only a small proportion of the total number. In almost all models, cardiorespiratory arrest occurs during the course of the seizure, contrary to that in patients observed at the time of death, potentially raising the issue of whether we are studying models of SUDEP or models of periseizure death. However, some of these limitations are impossible to avoid and can in part be dependent on specific features of SUDEP, which may be difficult to model. Several preclinical tools are available to address certain gaps in SUDEP pathophysiology, which can be used to further validate current preclinical models.

ILAE Genetic Literacy Series: Postmortem Genetic Testing in Sudden Unexpected Death in Epilepsy

A 24-year-old man with non-lesional bitemporal lobe epilepsy since age 16 years was found dead in bed around midday. He was last seen the previous night when he was witnessed to have a tonic-clonic seizure. Before his death, he was experiencing weekly focal impaired awareness seizures and up to two focal-to-bilateral tonic-clonic seizures each year. He had trialed several antiseizure medications and was on levetiracetam 1500 mg/day, lamotrigine 400 mg/day, and clobazam 10 mg/day at the time of death. Other than epilepsy, his medical history was unremarkable. Of note, he had an older brother with a history of febrile seizures and a paternal first cousin with epilepsy. No cause of death was identified following a comprehensive postmortem investigation. The coroner classified the death as "sudden unexpected death in epilepsy" (SUDEP), and it would qualify as "definite SUDEP" using the current definitions.1 This left the family with many questions unanswered; in particular, they wish to know what caused the death and whether it could happen to other family members. Could postmortem genetic testing identify a cause of death, provide closure to the family, and facilitate cascade genetic testing of first-degree family members who may be at risk of sudden death? While grieving family members struggle with uncertainty about the cause of death, we as clinicians also face similar uncertainties about genetic contributions to SUDEP, especially when the literature is sparse, and the utility of genetic testing is still being worked out. We aim to shed some light on this topic, highlighting areas where data is emerging but also areas where uncertainty remains, keeping our case in mind as we examine this clinically important area.

Gene mutations in comorbidity of epilepsy and arrhythmia

Epilepsy is one of the most common neurological disorders, and sudden unexpected death in epilepsy (SUDEP) is the most severe outcome of refractory epilepsy. Arrhythmia is one of the heterogeneous factors in the pathophysiological mechanism of SUDEP with a high incidence in patients with refractory epilepsy, increasing the risk of premature death. The gene co-expressed in the brain and heart is supposed to be the genetic basis between epilepsy and arrhythmia, among which the gene encoding ion channel contributes to the prevalence of "cardiocerebral channelopathy" theory. Nevertheless, this theory could only explain the molecular mechanism of comorbid arrhythmia in part of patients with epilepsy (PWE). Therefore, we summarized the mutant genes that can induce comorbidity of epilepsy and arrhythmia and the possible corresponding treatments. These variants involved the genes encoding sodium, potassium, calcium and HCN channels, as well as some non-ion channel coding genes such as CHD4, PKP2, FHF1, GNB5, and mitochondrial genes. The relationship between genotype and clinical phenotype was not simple linear. Indeed, genes co-expressed in the brain and heart could independently induce epilepsy and/or arrhythmia. Mutant genes in brain could affect cardiac rhythm through central or peripheral regulation, while in the heart it could also affect cerebral electrical activity by changing the hemodynamics or internal environment. Analysis of mutations in comorbidity of epilepsy and arrhythmia could refine and expand the theory of "cardiocerebral channelopathy" and provide new insights for risk stratification of premature death and corresponding precision therapy in PWE.

Toward automated prediction of sudden unexpected death in epilepsy

Sudden unexpected death in epilepsy (SUDEP) is a devastating yet overlooked complication of epilepsy. The rare and complex nature of SUDEP makes it challenging to study. No prediction or prevention of SUDEP is currently available in a clinical setting. In the past decade, significant advances have been made in our knowledge of the pathophysiologic cascades that lead to SUDEP. In particular, studies of brain, heart, and respiratory functions in both human patients at the epilepsy monitoring unit and animal models during fatal seizures provide critical information to integrate computational tools for SUDEP prediction. The rapid advances in automated seizure detection and prediction algorithms provide a fundamental framework for their adaption in predicting SUDEP. If a SUDEP can be predicted, then there will be a potential for medical intervention to be administered, either by their caregivers or via an implanted device automatically delivering electrical stimulation or medication, and finally save lives from fatal seizures. This article presents recent developments of SUDEP studies focusing on the pathophysiologic basis of SUDEP and computational implications of machine learning techniques that can be adapted and extended for SUDEP prediction. This article also discusses some novel ideas for SUDEP prediction and rescue including principal component analysis and closed-loop intervention.

Activin A directly impairs human cardiomyocyte contractile function indicating a potential role in heart failure development

Activin A has been linked to cardiac dysfunction in aging and disease, with elevated circulating levels found in patients with hypertension, atherosclerosis, and heart failure. Here, we investigated whether Activin A directly impairs cardiomyocyte (CM) contractile function and kinetics utilizing cell, tissue, and animal models. Hydrodynamic gene delivery-mediated overexpression of Activin A in wild-type mice was sufficient to impair cardiac function, and resulted in increased cardiac stress markers (N-terminal pro-atrial natriuretic peptide) and cardiac atrophy. In human-induced pluripotent stem cell-derived (hiPSC) CMs, Activin A caused increased phosphorylation of SMAD2/3 and significantly upregulated SERPINE1 and FSTL3 (markers of SMAD2/3 activation and activin signaling, respectively). Activin A signaling in hiPSC-CMs resulted in impaired contractility, prolonged relaxation kinetics, and spontaneous beating in a dose-dependent manner. To identify the cardiac cellular source of Activin A, inflammatory cytokines were applied to human cardiac fibroblasts. Interleukin -1β induced a strong upregulation of Activin A. Mechanistically, we observed that Activin A-treated hiPSC-CMs exhibited impaired diastolic calcium handling with reduced expression of calcium regulatory genes (SERCA2, RYR2, CACNB2). Importantly, when Activin A was inhibited with an anti-Activin A antibody, maladaptive calcium handling and CM contractile dysfunction were abrogated. Therefore, inflammatory cytokines may play a key role by acting on cardiac fibroblasts, causing local upregulation of Activin A that directly acts on CMs to impair contractility. These findings demonstrate that Activin A acts directly on CMs, which may contribute to the cardiac dysfunction seen in aging populations and in patients with heart failure.

Unmasking of Brugada syndrome by lamotrigine in a patient with pre-existing epilepsy: A case report with review of the literature

Brugada syndrome is an inherited cardiac channelopathy arising from mutations in voltage-gated cardiac sodium channels. Idiopathic epilepsy portrays a coalescent underlying pathophysiological mechanism pertaining to the premature excitation of neuronal voltage-gated ion channels resulting in the disruption of presynaptic neurons and the unregulated release of excitatory neurotransmitters. The coexistence of epilepsy and Brugada syndrome may be explained by mutations in voltage-gated ion channels, which are coexpressed in cardiac and neural tissue. Moreover, the incidence of sudden unexpected death in epilepsy has been associated with malignant cardiac arrhythmias in the presence of mutations in voltage-gated ion channels. Lamotrigine is an antiepileptic drug that inhibits neuronal voltage-gated sodium channels, thus stabilizing neural impulse propagation and controlling seizure activity in the brain. However, lamotrigine has been shown to inhibit cardiac voltage-gated sodium channels resulting in a potential arrhythmogenic effect and the ability to unmask Brugada syndrome in genetically susceptible individuals. We are reporting a case of a 27-year-old male patient with a background of presumed idiopathic epilepsy who was initiated on lamotrigine therapy resulting in the unmasking of Brugada syndrome and the onset of syncopal episodes. This case provides further evidence for the arrhythmogenic capacity of lamotrigine and highlights the relationship between epilepsy and Brugada syndrome. In this report, we aim to review the current literature regarding the associations between epilepsy and Brugada syndrome and the impact of lamotrigine therapy on such patients.

Neonatal Scn1b-null mice have sinoatrial node dysfunction, altered atrial structure, and atrial fibrillation

Loss-of-function (LOF) variants in SCN1B, encoding the voltage-gated sodium channel β1/β1B subunits, are linked to neurological and cardiovascular diseases. Scn1b-null mice have spontaneous seizures and ventricular arrhythmias and die by approximately 21 days after birth. β1/β1B Subunits play critical roles in regulating the excitability of ventricular cardiomyocytes and maintaining ventricular rhythmicity. However, whether they also regulate atrial excitability is unknown. We used neonatal Scn1b-null mice to model the effects of SCN1B LOF on atrial physiology in pediatric patients. Scn1b deletion resulted in altered expression of genes associated with atrial dysfunction. Scn1b-null hearts had a significant accumulation of atrial collagen, increased susceptibility to pacing induced atrial fibrillation (AF), sinoatrial node (SAN) dysfunction, and increased numbers of cholinergic neurons in ganglia that innervate the SAN. Atropine reduced the incidence of AF in null animals. Action potential duration was prolonged in null atrial myocytes, with increased late sodium current density and reduced L-type calcium current density. Scn1b LOF results in altered atrial structure and AF, demonstrating the critical role played by Scn1b in atrial physiology during early postnatal mouse development. Our results suggest that SCN1B LOF variants may significantly impact the developing pediatric heart.

Late sodium current: incomplete inactivation triggers seizures, myotonias, arrhythmias, and pain syndromes

Acquired and inherited dysfunction in voltage-gated sodium channels underlies a wide range of diseases. "In addition to the defects in trafficking and expression, sodium channelopathies are also caused by dysfunction in one or several gating properties, for instance activation or inactivation. Disruption of the channel inactivation leads to the increased late sodium current, which is a common defect in seizure disorders, cardiac arrhythmias skeletal muscle myotonia and pain. An increase in late sodium current leads to repetitive action potential in neurons and skeletal muscles, and prolonged action potential duration in the heart. In this topical review, we compare the effects of late sodium current in brain, heart, skeletal muscle, and peripheral nerves. Abstract figure legend Shows cartoon illustration of general Nav channel transitions between (1) resting, (2) open, and (3) fast inactivated states. Disruption of the inactivation process exacerbates (4) late sodium currents. This article is protected by copyright. All rights reserved.

Peri-Ictal Autonomic Control of Cardiac Function and Seizure-Induced Death

Sudden unexpected death in epilepsy (SUDEP) accounts for the deaths of 8–17% of patients with epilepsy. Although the mechanisms of SUDEP are unknown, one proposed mechanism is abnormal control of the heart by the autonomic nervous system (ANS). Our objective was to determine whether the broad changes in ictal heart rate experienced by mouse models of SUDEP are (1) due to the ANS and (2) contribute to seizure-induced death. Seizures were induced by electrical stimulation of the hippocampus of a mouse carrying the human SCN8A encephalopathy mutation p.Asn1768Asp (N1768D; “D/+ mice”). Using standard autonomic pharmacology, the relative roles of the parasympathetic and sympathetic nervous systems on heart rate changes associated with seizures were determined. All induced seizures had pronounced ictal bradycardia and postictal tachycardia. Seizure susceptibility or severity were unchanged by the pharmacological agents. Administration of Atropine, a muscarinic antagonist, eliminated ictal bradycardia, while carbachol, a muscarinic agonist, had no effect on ictal bradycardia, but reduced postictal tachycardia. Sotalol, an adrenergic β-receptor antagonist, had no effect on ictal bradycardia, but did suppress postictal tachycardia. Isoproterenol, a β-receptor agonist, had no effect on either ictal bradycardia or postictal tachycardia. Administration of the α1-receptor antagonist prazosin increases the incidence of seizure-induced death in D/+ mice. Although postictal heart rate was lower for these fatal seizures in the presence of prazosin, rates were not as low as that recorded for carbachol treated mice, which all survived. Both ictal bradycardia and postictal tachycardia are manifestations of the ANS. Bradycardia is mediated by a maximal activation of the parasympathetic arm of the ANS, and tachycardia is mediated by parasympathetic inactivation and sympathetic activation. While the changes in heart rate during seizures are profound, suppression of postictal heart rate did not increase seizure mortality.

SUDEP risk and autonomic dysfunction in genetic epilepsies

The underlying pathophysiology of sudden unexpected death in epilepsy (SUDEP) remains unclear. This phenomenon is likely multifactorial, and there is considerable evidence that genetic factors play a role. There are certain genetic causes of epilepsy in which the risk of SUDEP appears to be increased relative to epilepsy overall. For individuals with pathogenic variants in genes including SCN1A, SCN1B, SCN8A, SCN2A, GNB5, KCNA1 and DEPDC5, there are varying degrees of evidence to suggest an increased risk for sudden death. Why the risk for sudden death is higher is not completely clear; however, in many cases pathogenic variants in these genes are also associated with autonomic dysfunction, which is hypothesized as a contributing factor to SUDEP. We review the evidence for increased SUDEP risk for patients with epilepsy due to pathogenic variants in these genes, and also discuss what is known about autonomic dysfunction in these contexts.

Ventricular voltage-gated ion channels: Detection, characteristics, mechanisms, and drug safety evaluation

Cardiac voltage-gated ion channels (VGICs) play critical roles in mediating cardiac electrophysiological signals, such as action potentials, to maintain normal heart excitability and contraction. Inherited or acquired alterations in the structure, expression, or function of VGICs, as well as VGIC-related side effects of pharmaceutical drug delivery can result in abnormal cellular electrophysiological processes that induce life-threatening cardiac arrhythmias or even sudden cardiac death. Hence, to reduce possible heart-related risks, VGICs must be acknowledged as important targets in drug discovery and safety studies related to cardiac disease. In this review, we first summarize the development and application of electrophysiological techniques that are employed in cardiac VGIC studies alone or in combination with other techniques such as cryoelectron microscopy, optical imaging and optogenetics. Subsequently, we describe the characteristics, structure, mechanisms, and functions of various well-studied VGICs in ventricular myocytes and analyze their roles in and contributions to both physiological cardiac excitability and inherited cardiac diseases. Finally, we address the implications of the structure and function of ventricular VGICs for drug safety evaluation. In summary, multidisciplinary studies on VGICs help researchers discover potential targets of VGICs and novel VGICs in heart, enrich their knowledge of the properties and functions, determine the operation mechanisms of pathological VGICs, and introduce groundbreaking trends in drug therapy strategies, and drug safety evaluation.

Automatic detection of adult cardiomyocyte for high throughput measurements of calcium and contractility

Simultaneous calcium and contractility measurements on isolated adult cardiomyocytes have been the gold standard for the last decades to study cardiac (patho)physiology. However, the throughput of this system is low which limits the number of compounds that can be tested per animal. We developed instrumentation and software that can automatically find adult cardiomyocytes. Cells are detected based on the cell boundary using a Sobel-filter to find the edge information in the field of view. Separately, we detected motion by calculating the variance of intensity for each pixel in the frame through time. Additionally, it detects the best region for calcium and contractility measurements. A sensitivity of 0.66 ± 0.08 and a precision of 0.82 ± 0.03 was reached using our cell finding algorithm. The percentage of cells that were found and had good contractility measurements was 90 ± 10%. In addition, the average time between 2 cardiomyocyte calcium and contractility measurements decreased from 93.5 ± 80.2 to 15.6 ± 8.0 seconds using our software and microscope. This drastically increases throughput and provides a higher statistical reliability when performing adult cardiomyocyte functional experiments.

SCN8A Epilepsy, Developmental Encephalopathy, and Related Disorders

Understanding the precise genetic -basis of disease is one of the critical developments in medicine in the twenty-first century. Genetic testing has revolutionized the diagnosis and treatment of neurological diseases in children. Whole-genome and whole-exome sequencing have particularly been useful in understanding the genetic basis of childhood epileptic encephalopathies characterized by early-onset seizures with significant developmental impairment and regression. In this review we describe the identification of a new epileptic encephalopathy caused by a de novo mutation in the SCN8A gene, which encodes for Na_V1.6, a vital sodium channel in the central nervous system. SCN8A variants in patients with epilepsy result primarily in gain-of-function in Na_v1.6 and hyperexcitability of neurons in the central nervous system. Following the original discovery in 2012 of a de novo mutation in a child with developmental and epileptic encephalopathy (DEE), more than 400 individuals with SCN8A-related disorders have been identified. Clinical manifestations range from movement disorders or intellectual disability only to severe DEE, which includes epileptic encephalopathy with intractable multivariate seizure types, developmental impairment and regression, intellectual disability, and other neurological manifestations. Gain-of-function of the Na_v1.6 channel predicts effectiveness of sodium channel-blocking agents in the treatment of seizures, which has been corroborated by clinical experience. Nevertheless, treatment options remain limited and adverse effects are common. However, with the availability of a growing database of genetic and clinical data along with transfected cell lines and mouse models, more efficacious, targeted, and selective treatments may soon be feasible.

SCN8A-related developmental and epileptic encephalopathy with ictal asystole requiring cardiac pacemaker implantation

INTRODUCTION

SCN8A-related epilepsy has various phenotypes. In particular, patients with developmental and epileptic encephalopathy (DEE) are resistant to antiepileptic drugs and may present with autonomic symptoms, such as marked bradycardia and apnea during seizures, and thus have an increased risk of sudden death. Herein, we report a case of very severe SCN8A-related epilepsy necessitating cardiac pacemaker implantation because of repetitive ictal asystole.

CASE REPORT

The patient was a 14-month-old girl. Tremor and generalized tonic seizure occurred after birth. During seizures, bradycardia and perioral cyanosis occurred, and then, after developing tachycardia and apnea, marked bradycardia and generalized cyanosis occurred, which sometimes resulted in ictal asystole requiring cardiopulmonary resuscitation. Her seizures were refractory to antiepileptic drugs. As the seizures requiring resuscitation did not decrease, cardiac pacemaker implantation was performed four months after birth. Exome sequencing revealed a heterozygous de novo variant in SCN8A (NM_014191.3:c.4934T>C,p.(Met1645Thr)). Even though phenytoin was effective, seizures with bradycardia remained approximately once a month, and pacemaker activity was observed.

CONCLUSIONS

This is, to our knowledge, the first reported case of SCN8A-related DEE in whom pacemaker implantation was performed. Pacemaker implantation should be considered as a treatment option for critical patients with SCN8A-related DEE as in the present case, because the incidence of sudden unexpected death in epilepsy is reported to be approximately 10% in patients with SCN8A-related DEE.

The brain-heart interaction in epilepsy: implications for diagnosis, therapy, and SUDEP prevention

The influence of the central nervous system and autonomic system on cardiac activity is being intensively studied, as it contributes to the high rate of cardiologic comorbidities observed in people with epilepsy. Indeed, neuroanatomic connections between the brain and the heart provide links that allow cardiac arrhythmias to occur in response to brain activation, have been shown to produce arrhythmia both experimentally and clinically. Moreover, seizures may induce a variety of transient cardiac effects, which include changes in heart rate, heart rate variability, arrhythmias, asystole, and other ECG abnormalities, and can trigger the development of Takotsubo syndrome. People with epilepsy are at a higher risk of death than the general population, and sudden unexpected death in epilepsy (SUDEP) is the most important direct epilepsy-related cause of death. Although the cause of SUDEP is still unknown, cardiac abnormalities during and between seizures could play a significant role in its pathogenesis, as highlighted by studies on animal models of SUDEP and registration of SUDEP events. Recently, genetic mutations in genes co-expressed in the heart and brain, which may result in epilepsy and cardiac comorbidity/increased risk for SUDEP, have been described. Recognition and a better understanding of brain-heart interactions, together with new advances in sequencing techniques, may provide new insights into future novel therapies and help in the prevention of cardiac dysfunction and sudden death in epileptic individuals.

SCN8A and Its Related Epileptic Phenotypes

Sodium channelopathies are among the most common single-gene causes of epilepsy and have been considered model disorders for the study of genetic epilepsies. Epilepsies due to SCN8A pathogenic variants can present with a broad range of phenotypes varying from a severe epileptic encephalopathy with multiple types of drug-resistant seizure to neurodevelopmental delay, mental retardation, and electroencephalogram (EEG) findings of multifocal spike and waves (mostly in the temporal/parietal/occipital areas). In rare cases, benign familial infantile seizures and developmental delay with/without ataxia have been reported. A first-level, specific SCN8A Sanger's sequencing, although available, is rarely performed because the clinical phenotype is not strictly characteristic and several overlaps with other genetic epilepsies may occur. Given its indistinctive phenotype, diagnosis is usually performed through a specific gene panel for epileptic encephalopathies, early epilepsies, or genetic epilepsy in general, or through whole exome sequencing (WES) and more rarely through whole genome sequencing (WGS). Mutations in SCN8A occur as an autosomal dominant trait. The great majority of individuals diagnosed with SCN8A epilepsy do not have an affected parent, because usually SCN8A patients do not reproduce, and mutations are inherited as a “de novo” trait. In rare cases, SCN8A mutations may be inherited in the setting of parental germline mosaicism. SCN8A-related epilepsies have not shown a clear genotype–phenotype correlation, the same variants have been described with different clinical expressivity and this could be due to other genetic factors or to interacting environmental factors. There is no standardized treatment for SCN8A-related epilepsy because of the rarity of the disease and the unavailability of specific, targeted drugs. Treatment is based mainly on antiepileptic drugs which include classic wide-spectrum drugs such as valproic acid, levetiracetam, and lamotrigine. Sodium-channel blockers (phenytoin, carbamazepine, oxcarbazepine, and lamotrigine) have shown appreciable results in terms of seizure reduction, in particular, in patients presenting gain-of-function mutations. Nowadays, new potentially transformative gene therapy treatment approaches are currently being explored, allowing in the next future, a precision-based treatment directed against the gene defect and protein alterations.

Early onset epilepsy and sudden unexpected death in epilepsy with cardiac arrhythmia in mice carrying the early infantile epileptic encephalopathy 47 gain-of-function FHF1(FGF12) missense mutation

OBJECTIVE

Fibroblast growth factor homologous factors (FHFs) are brain and cardiac sodium channel-binding proteins that modulate channel density and inactivation gating. A recurrent de novo gain-of-function missense mutation in the FHF1(FGF12) gene (p.Arg52His) is associated with early infantile epileptic encephalopathy 47 (EIEE47; Online Mendelian Inheritance in Man database 617166). To determine whether the FHF1 missense mutation is sufficient to cause EIEE and to establish an animal model for EIEE47, we sought to engineer this mutation into mice.

METHODS

The Arg52His mutation was introduced into fertilized eggs by CRISPR (clustered regularly interspaced short palindromic repeats) editing to generate Fhf1^{R52H /F+} mice. Spontaneous epileptiform events in Fhf1^{R52H /+} mice were assessed by cortical electroencephalography (EEG) and video monitoring. Basal heart rhythm and seizure-induced arrhythmia were recorded by electrocardiography. Modulation of cardiac sodium channel inactivation by FHF1B_R52H protein was assayed by voltage-clamp recordings of FHF-deficient mouse cardiomyocytes infected with adenoviruses expressing wild-type FHF1B or FHF1B_R52H protein.

RESULTS

All Fhf1^{R52H /+} mice experienced seizure or seizurelike episodes with lethal ending between 12 and 26 days of age. EEG recordings in 19-20-day-old mice confirmed sudden unexpected death in epilepsy (SUDEP) as severe tonic seizures immediately preceding loss of brain activity and death. Within 2-53 s after lethal seizure onset, heart rate abruptly declined from 572 ± 16 bpm to 108 ± 15 bpm, suggesting a parasympathetic surge accompanying seizures that may have contributed to SUDEP. Although ectopic overexpression of FHF1B_R52H in cardiomyocytes induced a 15-mV depolarizing shift in voltage of steady-state sodium channel inactivation and slowed the rate of channel inactivation, heart rhythm was normal in Fhf1^{R52H /+} mice prior to seizure.

SIGNIFICANCE

The Fhf1 missense mutation p.Arg52His induces epileptic encephalopathy with full penetrance in mice. Both Fhf1 (p.Arg52His) and Scn8a (p.Asn1768Asp) missense mutations enhance sodium channel Na_v 1.6 currents and induce SUDEP with bradycardia in mice, suggesting an FHF1/Na_v 1.6 functional axis underlying altered brain sodium channel gating in epileptic encephalopathy.

Sodium channelopathies of skeletal muscle and brain

Voltage-gated sodium channels initiate action potentials in nerve, skeletal muscle, and other electrically excitable cells. Mutations in them cause a wide range of diseases. These channelopathy mutations affect every aspect of sodium channel function, including voltage sensing, voltage-dependent activation, ion conductance, fast and slow inactivation, and both biosynthesis and assembly. Mutations that cause different forms of periodic paralysis in skeletal muscle were discovered first and have provided a template for understanding structure, function, and pathophysiology at the molecular level. More recent work has revealed multiple sodium channelopathies in the brain. Here we review the well-characterized genetics and pathophysiology of the periodic paralyses of skeletal muscle and then use this information as a foundation for advancing our understanding of mutations in the structurally homologous α-subunits of brain sodium channels that cause epilepsy, migraine, autism, and related comorbidities. We include studies based on molecular and structural biology, cell biology and physiology, pharmacology, and mouse genetics. Our review reveals unexpected connections among these different types of sodium channelopathies.

Adrenergic Mechanisms of Audiogenic Seizure-Induced Death in a Mouse Model of SCN8A Encephalopathy

Sudden unexpected death in epilepsy (SUDEP) is the leading cause of death amongst patients whose seizures are not adequately controlled by current therapies. Patients with SCN8A encephalopathy have an elevated risk for SUDEP. While transgenic mouse models have provided insight into the molecular mechanisms of SCN8A encephalopathy etiology, our understanding of seizure-induced death has been hampered by the inability to reliably trigger both seizures and seizure-induced death in these mice. Here, we demonstrate that mice harboring an Scn8a allele with the patient-derived mutation N1768D (D/+) are susceptible to audiogenic seizures and seizure-induced death. In adult D/+ mice, audiogenic seizures are non-fatal and have nearly identical behavioral, electrographical, and cardiorespiratory characteristics as spontaneous seizures. In contrast, at postnatal days 20–21, D/+ mice exhibit the same seizure behavior, but have a significantly higher incidence of seizure-induced death following an audiogenic seizure. Seizure-induced death was prevented by either stimulating breathing via mechanical ventilation or by acute activation of adrenergic receptors. Conversely, in adult D/+ mice inhibition of adrenergic receptors converted normally non-fatal audiogenic seizures into fatal seizures. Taken together, our studies show that in our novel audiogenic seizure-induced death model adrenergic receptor activation is necessary and sufficient for recovery of breathing and prevention of seizure-induced death.

Children with refractory epilepsy demonstrate alterations in myocardial strain

OBJECTIVE

To test whether children with epilepsy have impairments in myocardial mechanics compared to controls without epilepsy.

METHODS

Children with refractory epilepsy with epilepsy duration of at least 3 years underwent echocardiography including conventional measurements and speckle tracking to assess longitudinal and circumferential strain. Parent-completed surveys, capturing critical aspects of the children's seizure history and cardiac risk factors, complemented retrospective chart reviews, which also included antiepileptic drug history. Normal echocardiograms from controls, matched for age and gender, were obtained from our institutional database and evaluated for strain.

RESULTS

Forty-one patients (median age = 10 years, interquartile range [IQR] = 5-15; 58.5% male) were enrolled. Epilepsy etiology included genetic (n = 26), structural (n = 6), genetic and structural (n = 5), infection (n = 3), and unknown (n = 1). No cardiac structural abnormalities were identified. Both longitudinal and circumferential strain were impaired (P < .03) in patients compared to controls (median [IQR] = 22.7% [21.2-24.2] vs 23.6% [22.2-26.1] and 22.0% [20.3-25.4] vs 24.5% [22.3-27.0], respectively), indicating decreased myocardial deformation/contraction. Shortening fraction was higher in patients (37.6% [35.7-39.7] vs 34.9% [32.5-38.7], P = .009); mitral valve E wave inflow velocity (84.8 cm/s [78.4-92.8] vs 97.2 cm/s [85.9-105.8], P = .005) and tissue Doppler lateral E' wave (13.9 cm/s [12.3-16.1] vs 17.3 cm/s [15.4-18.5], P < .001) were decreased compared to controls. Findings were similar in the pairs with epilepsy patients distinguished by the ability to independently ambulate. There was no difference between patients and controls in ejection fraction. Among the epilepsy patients, there were no associations between cardiac measurements and epilepsy characteristics, including seizure type and frequency and cardiotoxic antiseizure medication exposure after correction for multiple comparisons.

SIGNIFICANCE

Children with refractory epilepsy had impaired systolic ventricular strain compared to controls, not correlated with epilepsy history. Further studies are needed to determine the significance of these changes.

Ion Channels Involvement in Neurodevelopmental Disorders

Inherited and sporadic mutations in genes encoding for brain ion channels, affecting membrane expression or biophysical properties, have been associated with neurodevelopmental disorders characterized by epilepsy, cognitive and behavioral deficits with significant phenotypic and genetic heterogeneity. Over the years, the screening of a growing number of patients and the functional characterization of newly identified mutations in ion channels genes allowed to recognize new phenotypes and to widen the clinical spectrum of known diseases. Furthermore, advancements in understanding disease pathogenesis at atomic level or using patient-derived iPSCs and animal models have been pivotal to orient therapeutic intervention and to put the basis for the development of novel pharmacological options for drug-resistant disorders. In this review we will discuss major improvements and critical issues concerning neurodevelopmental disorders caused by dysfunctions in brain sodium, potassium, calcium, chloride and ligand-gated ion channels.

SCN8A encephalopathy: Mechanisms and models

De novo mutations of the neuronal sodium channel SCN8A have been identified in approximately 2% of individuals with epileptic encephalopathy. These missense mutations alter the biophysical properties of sodium channel Nav1.6 in ways that lead to neuronal hyperexcitability. We generated two mouse models carrying patient mutations N1768D and R1872W to examine the effects on neuronal function in vivo. The conditional R1872W mutation is activated by expression of CRE recombinase, permitting characterization of the effects of the mutation on different classes of neurons and at different points in postnatal development. Preclinical drug testing in these mouse models provides support for several new therapies for this devastating disorder. In contrast with the gain-of-function mutations in epilepsy, mutations of SCN8A that result in partial or complete loss of function are associated with intellectual disability and other disorders.

The voltage-gated sodium channel inhibitor, 4,9-anhydrotetrodotoxin, blocks human Nav1.1 in addition to Nav1.6

Voltage-gated sodium channels (VGSCs) are responsible for the initiation and propagation of action potentials in neurons. The human genome includes ten human VGSC α-subunit genes, SCN(X)A, encoding Nav1.1-1.9 plus Nax. To understand the unique role that each VGSC plays in normal and pathophysiological function in neural networks, compounds with high affinity and selectivity for specific VGSC subtypes are required. Toward that goal, a structural analog of the VGSC pore blocker tetrodotoxin, 4,9-anhydrotetrodotoxin (4,9-ah-TTX), has been reported to be more selective in blocking Na+ current mediated by Nav1.6 than other TTX-sensitive VGSCs, including Nav1.2, Nav1.3, Nav1.4, and Nav1.7. While SCN1A, encoding Nav1.1, has been implicated in several neurological diseases, the effects of 4,9-ah-TTX on Nav1.1-mediated Na+ current have not been tested. Here, we compared the binding of 4,9-ah-TTX for human and mouse brain preparations, and the effects of 4,9-ah-TTX on human Nav1.1-, Nav1.3- and Nav1.6-mediated Na+ currents using the whole-cell patch clamp technique in heterologous cells. We show that, while 4,9-ah-TTX administration results in significant blockade of Nav1.6-mediated Na+ current in the nanomolar range, it also has significant effects on Nav1.1-mediated Na+ current. Thus, 4,9-ah-TTX is not a useful tool in identifying Nav1.6-specific effects in human brain networks.

Epilepsy Benchmarks Area IV: Limit or Prevent Adverse Consequence of Seizures and Their Treatment Across the Life Span

Epilepsy represents a complex spectrum disorder, with patients sharing seizures as a common symptom and manifesting a broad array of additional clinical phenotypes. To understand this disorder and treat individuals who live with epilepsy, it is important not only to identify pathogenic mechanisms underlying epilepsy but also to understand their relationships with other health-related factors. Benchmarks Area IV focuses on the impact of seizures and their treatment on quality of life, development, cognitive function, and other aspects and comorbidities that often affect individuals with epilepsy. Included in this review is a discussion on sudden unexpected death in epilepsy and other causes of mortality, a major area of research focus with still many unanswered questions. We also draw attention to special populations, such as individuals with nonepileptic seizures and pregnant women and their offspring. In this study, we review the progress made in these areas since the 2016 review of the Benchmarks Area IV and discuss challenges and opportunities for future study.

Epilepsy and brain channelopathies from infancy to adulthood

Genetic brain channelopathies result from inherited or de novo mutations of genes encoding ion channel subunits within the central nervous system. Most neurological channelopathies arise in childhood with paroxysmal or episodic symptoms, likely because of a transient impairment of homeostatic mechanisms regulating membrane excitability, and the prototypical expression of this impairment is epilepsy. Migraine, episodic ataxia and alternating hemiplegia can also occur, as well as chronic phenotypes, such as spinocerebellar ataxias, intellectual disability and autism spectrum disorder. Voltage-gated and ligand-gated channels may be involved. In most cases, a single gene may be associated with a phenotypical spectrum that shows variable expressivity. Different clinical features may arise at different ages and the adult phenotype may be remarkably modified from the syndrome onset in childhood or adolescence. Recognizing the prominent phenotypical traits of brain channelopathies is essential to perform appropriate diagnostic investigations and to provide the better care not only in the paediatric setting but also for adult patients and their caregivers. Herein, we provide an overview of genetic brain channelopathies associated with epilepsy, highlight the different molecular mechanisms and describe the different clinical characteristics which may prompt the clinician to suspect specific syndromes and to possibly establish tailored treatments.

Prominent role of forebrain excitatory neurons in SCN8A encephalopathy

De novo mutations of the sodium channel gene SCN8A result in an epileptic encephalopathy with refractory seizures, developmental delay, and elevated risk of sudden death. p.Arg1872Trp is a recurrent de novo SCN8A mutation reported in 14 unrelated individuals with epileptic encephalopathy that included seizure onset in the prenatal or infantile period and severe verbal and ambulatory comorbidities. The major biophysical effect of the mutation was previously shown to be impaired channel inactivation accompanied by increased current density. We have generated a conditional mouse mutation in which expression of this severe gain-of-function mutation is dependent upon Cre recombinase. Global activation of p.Arg1872Trp by EIIa-Cre resulted in convulsive seizures and lethality at 2 weeks of age. Neural activation of the p.Arg1872Trp mutation by Nestin-Cre also resulted in early onset seizures and death. Restriction of p.Arg1872Trp expression to excitatory neurons using Emx1-Cre recapitulated seizures and juvenile lethality between 1 and 2 months of age. In contrast, activation of p.Arg1872Trp in inhibitory neurons by Gad2-Cre or Dlx5/6-Cre did not induce seizures or overt neurological dysfunction. The sodium channel modulator GS967/Prax330 prolonged survival of mice with global expression of R1872W and also modulated the activity of the mutant channel in transfected cells. Activation of the p.Arg1872Trp mutation in adult mice was sufficient to generate seizures and death, indicating that successful therapy will require lifelong treatment. These findings provide insight into the pathogenic mechanism of this gain-of-function mutation of SCN8A and identify excitatory neurons as critical targets for therapeutic intervention.

Generalized tonic seizures with autonomic signs are the hallmark of SCN8A developmental and epileptic encephalopathy

Developmental and epileptic encephalopathy (DEE) due to SCN8A gene variants is characterized by drug-resistant early onset epilepsy associated with severe intellectual disability. Different seizure types have been reported, and a sequence of autonomic manifestations such as brady-/tachycardia, irregular breathing, and cyanosis. Nevertheless, an exhaustive video-polygraphic documentation is still lacking. In this study, we reviewed the ictal electroencephalograms (EEGs) of five patients with SCN8A-DEE followed-up at the Neuroscience Department at Bambino Gesù Children's Hospital in Rome. We identified generalized tonic seizure as the major seizure type at epilepsy onset. Seizure severity could vary from subtle to marked clinical manifestations, depending from the extent and groups of muscles involved and association with autonomic modifications. We found autonomic signs in 80% of seizures in our cases, and we were able to identify a stereotyped sequence of ictal events for most of seizures. Autonomic signs occurred in rapid sequence: flushing of the face, sometimes associated with sialorrhea, bradycardia, and hypopnea appeared within the first 1-2 s. Tachycardia, polypnea, perioral cyanosis, and pallor occurred later in the course of the seizure. Generalized tonic seizures are rarely described in other genetic epileptic conditions of early infancy because of ion channel mutations, such as in DEE due to KCNQ2 or SCN2A gene mutations, where seizures are most frequently reported as focal to bilateral tonic. Therefore, generalized symmetric tonic seizures with autonomic signs can be considered a clinical hallmark for diagnosis of SCN8A-related DEE and relevant for therapeutic implications.

ZIPK mediates endothelial cell contraction through myosin light chain phosphorylation and is required for ischemic-reperfusion injury

The paracellular gap formed by endothelial cell (EC) contraction is fundamental for endothelium permeability, but the mechanism underlying EC contraction has yet to be determined. Here, we identified the zipper-interacting protein kinase (ZIPK) as the kinase for EC contraction and myosin light chain (MLC) phosphorylation. Inhibition of ZIPK activity by pharmacological inhibitors and small interfering RNAs led to a significant decrease in the mono- and diphosphorylation of MLCs along with a contractile response to thrombin, suggesting an essential role of ZIPK in EC paracellular permeability. To assess the role of ZIPK in vivo, we established mouse lines with conditional deletion of Zipk gene. The endothelium-specific deletion of Zipk led to embryonic lethality, whereas the UBC-Cre^ERT2-mediated deletion of Zipk by tamoxifen induction at adulthood caused no apparent phenotype. The induced deletion of Zipk significantly inhibited ischemia-reperfusion-induced blood-brain barrier dysfunction and neuronal injuries from middle cerebral artery occlusion and reperfusion, as evidenced by reduced infarct and edema volume, attenuated Evans blue dye leakage, and improved neuronal behavior. We thus concluded that ZIPK and its phosphorylation of MLC were required for EC contraction and ischemic neuronal injuries. ZIPK may be a prospective therapeutic target for stroke.-Zhang, Y., Zhang, C., Zhang, H., Zeng, W., Li, S., Chen, C., Song, X., Sun, J., Sun, Z., Cui, C., Cao, X., Zheng, L., Wang, P., Zhao, W., Zhang, Z., Xu, Y., Zhu, M., Chen, H. ZIPK mediates endothelial cell contraction through myosin light chain phosphorylation and is required for ischemic-reperfusion injury.

Exome sequencing identifies molecular diagnosis in children with drug-resistant epilepsy

OBJECTIVE

Early onset drug-resistant epilepsy is a neurologic disorder in which 2 antiepileptic drugs fail to maintain the seizure-free status of the patient. Heterogeneous clinical presentations make the diagnosis challenging. We aim to identify the underlying genetic causes of a pediatric cohort with drug-resistant epilepsy and evaluate whether the findings can provide information on patient management.

METHODS

We include patients with drug-resistant epilepsy onset before 18 years of age. Singleton clinical chromosomal microarray (CMA) followed by whole exome sequencing (WES) was performed using genomic DNA. In the first-tier analysis of the exome data, we aimed to identify disease-causing mutations in 546 genes known to cause, or to be associated with, epilepsy. For negative cases, we proceeded to exome-wide analysis. Rare coding variants were interrogated for pathogenicity based on the American College of Medical Genetics and Genomics (ACMG) guidelines.

RESULTS

We recruited 50 patients. We identified 6 pathogenic or likely pathogenic mutations, giving a diagnostic yield of 12%. Mutations were found in 6 different genes: SCN8A, SCN1A, MECP2, CDKL5, DEPDC5, and CHD2. The CDKL5 variant was found to be mosaic. One variant of unknown significance (VUS) in KCNT1 was found in a patient with compatible clinical features. Of note, a reported pathogenic SCN5A mutation known to contribute to Brugada syndrome, was also found in the patient with an SCN1A mutation.

SIGNIFICANCE

Our study suggests that singleton WES is an effective diagnostic tool for drug-resistant epilepsy. Genetic diagnosis can help to consolidate the clinical diagnosis, to facilitate phenotypic expansion, and to influence treatment and management options for seizure control in our patients. In our study, a significant portion of the genetic findings are known to be associated with an increased risk of sudden unexpected death in epilepsy (SUDEP). These findings could assist with more appropriate management in patients with epilepsy.

Channelopathy as a SUDEP Biomarker in Dravet Syndrome Patient-Derived Cardiac Myocytes

Dravet syndrome (DS) is a severe developmental and epileptic encephalopathy with a high incidence of sudden unexpected death in epilepsy (SUDEP). Most DS patients carry de novo variants in SCN1A, resulting in Nav1.1 haploinsufficiency. Because SCN1A is expressed in heart and in brain, we proposed that cardiac arrhythmia contributes to SUDEP in DS. We generated DS patient and control induced pluripotent stem cell-derived cardiac myocytes (iPSC-CMs). We observed increased sodium current (INa) and spontaneous contraction rates in DS patient iPSC-CMs versus controls. For the subject with the largest increase in INa, cardiac abnormalities were revealed upon clinical evaluation. Generation of a CRISPR gene-edited heterozygous SCN1A deletion in control iPSCs increased INa density in iPSC-CMs similar to that seen in patient cells. Thus, the high risk of SUDEP in DS may result from a predisposition to cardiac arrhythmias in addition to seizures, reflecting expression of SCN1A in heart and brain.

Cardiac Arrhythmias as Manifestations of Nanopathies: An Emerging View

A nanodomain is a collection of proteins localized within a specialized, nanoscale structural environment, which can serve as the functional unit of macroscopic physiologic processes. We are beginning to recognize the key roles of cardiomyocyte nanodomains in essential processes of cardiac physiology such as electrical impulse propagation and excitation–contraction coupling (ECC). There is growing appreciation of nanodomain dysfunction, i.e., nanopathy, as a mechanistic driver of life-threatening arrhythmias in a variety of pathologies. Here, we offer an overview of current research on the role of nanodomains in cardiac physiology with particular emphasis on: (1) sodium channel-rich nanodomains within the intercalated disk that participate in cell-to-cell electrical coupling and (2) dyadic nanodomains located along transverse tubules that participate in ECC. The beat to beat function of cardiomyocytes involves three phases: the action potential, the calcium transient, and mechanical contraction/relaxation. In all these phases, cell-wide function results from the aggregation of the stochastic function of individual proteins. While it has long been known that proteins that exist in close proximity influence each other’s function, it is increasingly appreciated that there exist nanoscale structures that act as functional units of cardiac biophysical phenomena. Termed nanodomains, these structures are collections of proteins, localized within specialized nanoscale structural environments. The nano-environments enable the generation of localized electrical and/or chemical gradients, thereby conferring unique functional properties to these units. Thus, the function of a nanodomain is determined by its protein constituents as well as their local structural environment, adding an additional layer of complexity to cardiac biology and biophysics. However, with the emergence of experimental techniques that allow direct investigation of structure and function at the nanoscale, our understanding of cardiac physiology and pathophysiology at these scales is rapidly advancing. Here, we will discuss the structure and functions of multiple cardiomyocyte nanodomains, and novel strategies that target them for the treatment of cardiac arrhythmias.

Mapt deletion fails to rescue premature lethality in two models of sodium channel epilepsy

Deletion of Mapt, encoding the microtubule‐binding protein Tau, prevents disease in multiple genetic models of hyperexcitability. To investigate whether the effect of Tau depletion is generalizable across multiple sodium channel gene‐linked models of epilepsy, we examined the Scn1b^−/− mouse model of Dravet syndrome, and the Scn8a^N1768D/+ model of Early Infantile Epileptic Encephalopathy. Both models display severe seizures and early mortality. We found no prolongation of survival between Scn1b^−/−,Mapt^+/+, Scn1b^−/−,Mapt^+/−, or Scn1b^−/−,Mapt^−/− mice or between Scn8a^N1768D/+,Mapt^+/+, Scn8a^N1768D/+,Mapt^+/−, or Scn8a^N1768D/+,Mapt^−/− mice. Thus, the effect of Mapt deletion on mortality in epileptic encephalopathy models is gene specific and provides further mechanistic insight.

Second-hit mosaic mutation in mTORC1 repressor DEPDC5 causes focal cortical dysplasia-associated epilepsy

DEP domain-containing 5 protein (DEPDC5) is a repressor of the recently recognized amino acid-sensing branch of the mTORC1 pathway. So far, its function in the brain remains largely unknown. Germline loss-of-function mutations in DEPDC5 have emerged as a major cause of familial refractory focal epilepsies, with case reports of sudden unexpected death in epilepsy (SUDEP). Remarkably, a fraction of patients also develop focal cortical dysplasia (FCD), a neurodevelopmental cortical malformation. We therefore hypothesized that a somatic second-hit mutation arising during brain development may support the focal nature of the dysplasia. Here, using postoperative human tissue, we provide the proof of concept that a biallelic 2-hit - brain somatic and germline - mutational mechanism in DEPDC5 causes focal epilepsy with FCD. We discovered a mutation gradient with a higher rate of mosaicism in the seizure-onset zone than in the surrounding epileptogenic zone. Furthermore, we demonstrate the causality of a Depdc5 brain mosaic inactivation using CRISPR-Cas9 editing and in utero electroporation in a mouse model recapitulating focal epilepsy with FCD and SUDEP-like events. We further unveil a key role of Depdc5 in shaping dendrite and spine morphology of excitatory neurons. This study reveals promising therapeutic avenues for treating drug-resistant focal epilepsies with mTORC1-targeting molecules.

The novel sodium channel modulator GS-458967 (GS967) is an effective treatment in a mouse model of SCN8A encephalopathy

OBJECTIVE

De novo mutations of SCN8A, encoding the voltage-gated sodium channel Na_V 1.6, have been associated with a severe infant onset epileptic encephalopathy. Individuals with SCN8A encephalopathy have a mean age of seizure onset of 4-5 months, with multiple seizure types that are often refractory to treatment with available drugs. Anecdotal reports suggest that high-dose phenytoin is effective for some patients, but there are associated adverse effects and potential for toxicity. Functional characterization of several SCN8A encephalopathy variants has shown that elevated persistent sodium current is one of several common biophysical defects. Therefore, specifically targeting elevated persistent current may be a useful therapeutic strategy in some cases.

METHODS

The novel sodium channel modulator GS967 has greater preference for persistent as opposed to peak current and nearly 10-fold greater potency than phenytoin. We evaluated the therapeutic effect of GS967 in the Scn8a^N1768D/+ mouse model carrying an SCN8A patient mutation that results in elevated persistent sodium current. We also performed patch clamp recordings to assess the effect of GS967 on peak and persistent sodium current and excitability in hippocampal neurons from Scn8a^N1768D/+ mice.

RESULTS

GS967 potently blocked persistent sodium current without affecting peak current, normalized action potential morphology, and attenuated excitability in neurons from heterozygous Scn8a^N1768D/+ mice. Acute treatment with GS967 provided dose-dependent protection against maximal electroshock-induced seizures in Scn8a^N1768D/+ and wild-type mice. Chronic treatment of Scn8a^N1768D/+ mice with GS967 resulted in lower seizure burden and complete protection from seizure-associated lethality observed in untreated Scn8a^N1768D/+ mice. Protection was achieved at a chronic dose that did not cause overt behavioral toxicity or sedation.

SIGNIFICANCE

Persistent sodium current modulators like GS967 may be an effective precision targeting strategy for SCN8A encephalopathy and other functionally similar channelopathies when elevated persistent sodium current is the primary dysfunction.

Differences in ion channel phenotype and function between humans and animal models

Ion channels play crucial roles in regulating a broad range of physiological processes. They form a very large family of transmembrane proteins. Their diversity results from not only a large number of different genes encoding for ion channel subunits but also the ability of subunits to assemble into homo- or heteromultimers, the existence of splice variants, and the expression of different regulatory subunits. These characteristics and the existence of very selective modulators make ion channels very attractive targets for therapy in a wide variety of pathologies. Some ion channels are already being targeted in the clinic while many more are being evaluated as novel drug targets in both clinical and preclinical studies. Advancing ion channel modulators from the bench to the clinic requires their assessment for safety and efficacy in animal models. While extrapolating results from one species to another is tempting, doing such without careful evaluation of the ion channels in different species presents a risk as the translation is not always straightforward. Here, we discuss differences between species in terms of ion channels expressed in selected tissues, differing roles of ion channels in some cell types, variable response to pharmacological agents, and human channelopathies that cannot fully be replicated in animal models.

The effect of atomoxetine, a selective norepinephrine reuptake inhibitor, on respiratory arrest and cardiorespiratory function in the DBA/1 mouse model of SUDEP

Sudden unexpected death in epilepsy (SUDEP) is a significant public health burden. The mechanisms of SUDEP are elusive, although cardiorespiratory dysfunction is a likely contributor. Clinical and animal studies indicate that seizure-induced respiratory arrest (S-IRA) is the primary event leading to death in many SUDEP cases. Our prior studies demonstrated that intraperitoneal (IP) injection of atomoxetine, a norepinephrine reuptake inhibitor (NRI) widely used to treat attention deficit hyperactivity disorder, suppresses S-IRA in DBA/1 mice. In the current study, we injected atomoxetine intracerebroventricularly (ICV) and measured its effect on S-IRA in DBA/1 mice to determine its central effects. Additionally, to test our hypothesis that atomoxetine reduces S-IRA via altering cardiorespiratory function, we examined the effect of atomoxetine on respiratory and cardiac function using non-invasive plethysmography and ECG in anesthetized DBA/1 mice, and on blood pressure and heart rate using a tail-cuff system in conscious DBA/1 mice. ICV administration of atomoxetine at 200-250nmol significantly reduced S-IRA evoked by acoustic stimulation in DBA/1 mice, consistent with a central atomoxetine effect on S-IRA. Peripheral atomoxetine administration at a dosage that reduces S-IRA (15mg/kg, IP) slightly increased basal ventilation and the ventilatory response to 7% CO₂, but exerted no effect on heart rate in anesthetized DBA/1 mice. IP injection of atomoxetine produced no effect on the heart rate and blood pressures in conscious mice. These data suggest that atomoxetine suppresses S-IRA through direct effects on the CNS and potentially through enhanced lung ventilation in DBA/1 mice.

Genetic Basis of Sudden Unexpected Death in Epilepsy

People with epilepsy are at heightened risk of sudden death compared to the general population. The leading cause of epilepsy-related premature mortality is sudden unexpected death in epilepsy (SUDEP). Postmortem investigation of people with SUDEP, including histological and toxicological analysis, does not reveal a cause of death, and the mechanisms of SUDEP remain largely unresolved. In this review we present the possible mechanisms underlying SUDEP, including respiratory dysfunction, cardiac arrhythmia and postictal generalized electroencephlogram suppression. Emerging studies in humans and animal models suggest there may be an underlying genetic basis to SUDEP in some cases. We will highlight a mounting body of evidence for the involvement of genetic risk factors in SUDEP, with a particular focus on the role of cardiac arrhythmia genes in SUDEP.

Neuronal sodium channels: emerging components of the nano-machinery of cardiac calcium cycling

Excitation-contraction coupling is the bridge between cardiac electrical activation and mechanical contraction. It is driven by the influx of Ca²⁺ across the sarcolemma triggering Ca²⁺ release from the sarcoplasmic reticulum (SR) - a process termed Ca²⁺ -induced Ca²⁺ release (CICR) - followed by re-sequestration of Ca²⁺ into the SR. The Na⁺ /Ca²⁺ exchanger inextricably couples the cycling of Ca²⁺ and Na⁺ in cardiac myocytes. Thus, influx of Na⁺ via voltage-gated Na⁺ channels (Na_V ) has emerged as an important regulator of CICR both in health and in disease. Recent insights into the subcellular distribution of cardiac and neuronal Na_V isoforms and their ultrastructural milieu have important implications for the roles of these channels in mediating Ca²⁺ -driven arrhythmias. This review will discuss functional insights into the role of neuronal Na_V isoforms vis-à-vis cardiac Na_V s in triggering such arrhythmias and their potential as therapeutic targets in the context of the aforementioned structural observations.

Neuronal hyperexcitability in a mouse model of SCN8A epileptic encephalopathy

Patients with early infantile epileptic encephalopathy (EIEE) experience severe seizures and cognitive impairment and are at increased risk for sudden unexpected death in epilepsy (SUDEP). EIEE13 [Online Mendelian Inheritance in Man (OMIM) # 614558] is caused by de novo missense mutations in the voltage-gated sodium channel gene SCN8A Here, we investigated the neuronal phenotype of a mouse model expressing the gain-of-function SCN8A patient mutation, p.Asn1768Asp (Na_v1.6-N1768D). Our results revealed regional and neuronal subtype specificity in the effects of the N1768D mutation. Acutely dissociated hippocampal neurons from Scn8a^N1768D/+ mice showed increases in persistent sodium current (I_Na) density in CA1 pyramidal but not bipolar neurons. In CA3, I_Na,P was increased in both bipolar and pyramidal neurons. Measurement of action potential (AP) firing in Scn8a^N1768D/+ pyramidal neurons in brain slices revealed early afterdepolarization (EAD)-like AP waveforms in CA1 but not in CA3 hippocampal or layer II/III neocortical neurons. The maximum spike frequency evoked by depolarizing current injections in Scn8a^N1768D/+ CA1, but not CA3 or neocortical, pyramidal cells was significantly reduced compared with WT. Spontaneous firing was observed in subsets of neurons in CA1 and CA3, but not in the neocortex. The EAD-like waveforms of Scn8a^N1768D/+ CA1 hippocampal neurons were blocked by tetrodotoxin, riluzole, and SN-6, implicating elevated persistent I_Na and reverse mode Na/Ca exchange in the mechanism of hyperexcitability. Our results demonstrate that Scn8a plays a vital role in neuronal excitability and provide insight into the mechanism and future treatment of epileptogenesis in EIEE13.

Pharmacogenomics in epilepsy

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Genetic variation can influence response to antiepileptic drug (AED) treatment through various effector processes.

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Metabolism of many AEDs is mediated by the cytochrome P450 (CYP) family; some of the CYPs have allelic variants that may affect serum AED concentrations.

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‘Precision medicine’ focuses on the identification of an underlying genetic aetiology allowing personalised therapeutic choices.

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Certain human leukocyte antigen, HLA, alleles are associated with an increased risk of idiosyncratic adverse drug reactions.

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New results are emerging from large-scale multinational efforts, likely imminently to add knowledge of value from a pharmacogenetic perspective.

There is high variability in the response to antiepileptic treatment across people with epilepsy. Genetic factors significantly contribute to such variability. Recent advances in the genetics and neurobiology of the epilepsies are establishing the basis for a new era in the treatment of epilepsy, focused on each individual and their specific epilepsy. Variation in response to antiepileptic drug treatment may arise from genetic variation in a range of gene categories, including genes affecting drug pharmacokinetics, and drug pharmacodynamics, but also genes held to actually cause the epilepsy itself.

From a purely pharmacogenetic perspective, there are few robust genetic findings with established evidence in epilepsy. Many findings are still controversial with anecdotal or less secure evidence and need further validation, e.g. variation in genes for transporter systems and antiepileptic drug targets. The increasing use of genetic sequencing and the results of large-scale collaborative projects may soon expand the established evidence.

Precision medicine treatments represent a growing area of interest, focussing on reversing or circumventing the pathophysiological effects of specific gene mutations. This could lead to a dramatic improvement of the effectiveness and safety of epilepsy treatments, by targeting the biological mechanisms responsible for epilepsy in each specific individual.

Whilst much has been written about epilepsy pharmacogenetics, there does now seem to be building momentum that promises to deliver results of use in clinic.