Recent advances in treatment of epilepsy-related sodium channelopathies

When rare meets common: Treatable genetic diseases are enriched in the general psychiatric population

Mental illnesses are one of the biggest contributors to the global disease burden. Despite the increased recognition, diagnosis and ongoing research of mental health disorders, the etiology and underlying molecular mechanisms of these disorders are yet to be fully elucidated. Moreover, despite many treatment options available, a large subset of the psychiatric patient population is nonresponsive to standard medications and therapies. There has not been a comprehensive study to date examining the burden and impact of treatable genetic disorders (TGDs) that can present with neuropsychiatric features in psychiatric patient populations. In this study, we test the hypothesis that TGDs that present with psychiatric symptoms are more prevalent within psychiatric patient populations compared to the general population by performing targeted next-generation sequencing of 129 genes associated with 108 TGDs in a cohort of 2301 psychiatric patients. In total, 48 putative affected and 180 putative carriers for TGDs were identified, with known or likely pathogenic variants in 79 genes. Despite screening for only 108 genetic disorders, this study showed a two-fold (2.09%) enrichment for genetic disorders within the psychiatric population relative to the estimated 1% cumulative prevalence of all single gene disorders globally. This strongly suggests that the prevalence of these, and most likely all, genetic diseases is greatly underestimated in psychiatric populations. Increasing awareness and ensuring accurate diagnosis of TGDs will open new avenues to targeted treatment for a subset of psychiatric patients.

A Novel SCN1A Mutation Associated With Reflex Seizures Induced by Movements

A 14-year-old male patient was admitted to the hospital due to epileptic seizures, which occurred at the beginning of running exercise after being stopped and fast walking. Seizures were consistently characterized by a dystonic posture of the distal portion of the left arm-flexed and adducted by the chest without loss of consciousness. We suspected that this was movement-induced reflex epilepsy and performed whole exome sequencing. Whole exome sequencing revealed a novel SCN1A missense mutation, c.5549T>G (p.Ile1850Ser). SCN1A mutations have not been reported in patients with reflex epilepsy induced by movement. This report enriches the genotypes and phenotypes of SCN1A-related epilepsy and provides further insight into the etiology of reflex epilepsy induced by movement.

SCN1A Polymorphisms and Haplotypes Are Associated With Valproic Acid Treatment Outcomes in Chinese Children With Epilepsy

BACKGROUND

Sodium channel genes, especially SCN1A, were reported to play an important role in the treatment outcomes of antiseizure medications. The aim of this study was to explore the association of SCN1A polymorphisms with efficacy and adverse drug reactions (ADRs) related to valproic acid (VPA) among Chinese children with epilepsy.

METHODS

A total of 126 children with epilepsy treated with VPA for at least 12 months were enrolled in this study. Three single nucleotide polymorphisms (SNPs) of SCN1A including rs2298771, rs10167228, and rs3812718 were genotyped using Sequenom MassArray system. Bioinformatics tools were used to explore the potential targets and pathways of SCN1A in VPA-related ADRs.

RESULTS

The three SNPs in this study were found to be closely associated with treatment outcomes for VPA. Carriers of SCN1A rs3812718 TT genotype tended to be seizure-free with VPA treatment (P = 0.007). AA genotype of rs10167228 and TT genotype of rs2298771 might be protective factors for weight gain induced by VPA, whereas TA genotype of rs10167228 and CT genotype of rs2298771 increased the risk. TAT haplotype carriers were found to respond better to VPA treatment (P = 0.017), whereas CTC haplotype might be a risk factor for VPA-induced weight gain (P = 0.035). Bioinformatics analysis suggested that SCN1A might play a role in VPA-induced weight gain by regulating gated channel activity and GABAergic synapse pathway.

CONCLUSION

This study revealed that SCN1A rs2298771, rs10167228, and rs3812718 polymorphisms and haplotypes might affect the treatment outcomes of VPA in Chinese children with epilepsy.

Loss or gain of function? Effects of ion channel mutations on neuronal firing depend on the neuron type

Introduction

Clinically relevant mutations to voltage-gated ion channels, called channelopathies, alter ion channel function, properties of ionic currents, and neuronal firing. The effects of ion channel mutations are routinely assessed and characterized as loss of function (LOF) or gain of function (GOF) at the level of ionic currents. However, emerging personalized medicine approaches based on LOF/GOF characterization have limited therapeutic success. Potential reasons are among others that the translation from this binary characterization to neuronal firing is currently not well-understood-especially when considering different neuronal cell types. In this study, we investigate the impact of neuronal cell type on the firing outcome of ion channel mutations.

Methods

To this end, we simulated a diverse collection of single-compartment, conductance-based neuron models that differed in their composition of ionic currents. We systematically analyzed the effects of changes in ion current properties on firing in different neuronal types. Additionally, we simulated the effects of known mutations in KCNA1 gene encoding the K_V1.1 potassium channel subtype associated with episodic ataxia type 1 (EA1).

Results

These simulations revealed that the outcome of a given change in ion channel properties on neuronal excitability depends on neuron type, i.e., the properties and expression levels of the unaffected ionic currents.

Discussion

Consequently, neuron-type specific effects are vital to a full understanding of the effects of channelopathies on neuronal excitability and are an important step toward improving the efficacy and precision of personalized medicine approaches.

Dissecting the 22q13 region to explore the genetic and phenotypic diversity of patients with Phelan-McDermid syndrome

SHANK3-related Phelan-McDermid syndrome (PMS) is caused by a loss of the distal part of chromosome 22, including SHANK3, or by a pathological SHANK3 variant. There is an important genetic and phenotypic diversity among patients who can present with developmental delay, language impairments, autism, epilepsy, and other symptoms. SHANK3, encoding a synaptic scaffolding protein, is deleted in the majority of patients with PMS and is considered a major gene involved in the neurological impairments of the patients. However, differences in deletion size can influence clinical features, and in some rare cases, deletions at the 22q13 locus in individuals with SHANK3-unrelated PMS do not encompass SHANK3. These individuals with SHANK3-unrelated PMS still display a PMS-like phenotype. This suggests the participation of other 22q13 genes in the pathogenesis of PMS. Here, we review the biological function and potential implication in PMS symptoms of 110 genes located in the 22q13 region, focusing on 35 genes with evidence for association with neurodevelopmental disorders, including 13 genes for epilepsy and 11 genes for microcephaly and/or macrocephaly. Our review is restricted to the 22q13 region, but future large-scale studies using whole genome sequencing and deep-phenotyping are warranted to develop predictive models of clinical trajectories and to target specific medical and educational care for each individual with PMS.

Rodent Models of Audiogenic Epilepsy: Genetic Aspects, Advantages, Current Problems and Perspectives

Animal models of epilepsy are of great importance in epileptology. They are used to study the mechanisms of epileptogenesis, and search for new genes and regulatory pathways involved in the development of epilepsy as well as screening new antiepileptic drugs. Today, many methods of modeling epilepsy in animals are used, including electroconvulsive, pharmacological in intact animals, and genetic, with the predisposition for spontaneous or refractory epileptic seizures. Due to the simplicity of manipulation and universality, genetic models of audiogenic epilepsy in rodents stand out among this diversity. We tried to combine data on the genetics of audiogenic epilepsy in rodents, the relevance of various models of audiogenic epilepsy to certain epileptic syndromes in humans, and the advantages of using of rodent strains predisposed to audiogenic epilepsy in current epileptology.

Cancer as a Channelopathy—Appreciation of Complimentary Pathways Provides a Different Perspective for Developing Treatments

Simple Summary

While improvements in technology have improved our ability to treat many forms of cancer when diagnosed at an early stage of the disease, the ability to improve survival and quality of life for patients with late stage disease has been limited, largely due to the ability of cancer cells to evade destruction when treatments block preferred paths for survival. Here, we review the role that ions and ion channels play in normal cell function, the development of disease and their role in the life and death of a cell. It is hoped that viewing cancer from the perspective of altered ion channel expression and ion balance may provide a novel approach for developing more effective treatments for this devastating disease.

Abstract

Life depends upon the ability of cells to evaluate and adapt to a constantly changing environment and to maintain internal stability to allow essential biochemical reactions to occur. Ions and ion channels play a crucial role in this process and are essential for survival. Alterations in the expression of the transmembrane proteins responsible for maintaining ion balance that occur as a result of mutations in the genetic code or in response to iatrogenically induced changes in the extracellular environment is a characteristic feature of oncogenesis and identifies cancer as one of a constellation of diseases known as channelopathies. The classification of cancer as a channelopathy provides a different perspective for viewing the disease. Potentially, it may expand opportunities for developing novel ways to affect or reverse the deleterious changes that underlie establishing and sustaining disease and developing tolerance to therapeutic attempts at treatment. The role of ions and ion channels and their interactions in the cell’s ability to maintain ionic balance, homeostasis, and survival are reviewed and possible approaches that mitigate gain or loss of ion channel function to contribute to new or enhance existing cancer therapies are discussed.

Interactome overlap between risk genes of epilepsy and targets of anti-epileptic drugs

Aanti-epileptic drugs have been used for treating epilepsy for decades, meanwhile, more than one hundred genes have been identified to be associated with risk of epilepsy; however, the interaction mechanism between anti-epileptic drugs and risk genes of epilepsy was still not clearly understood. In this study, we systematically explored the interaction of epilepsy risk genes and anti-epileptic drug targets through a network-based approach. Our results revealed that anti-epileptic drug targets were significantly over-represented in risk genes of epilepsy with 17 overlapping genes and P-value = 2.2 ×10 −16. We identified a significantly localized PPI network with 55 epileptic risk genes and 94 anti-epileptic drug target genes, and network overlap analysis showed significant interactome overlap between risk genes and drug targets with P-value = 0.04. Besides, genes from PPI network were significantly enriched in the co-expression network of epilepsy with 22 enriched genes and P-value = 1.3 ×10 −15; meanwhile, cell type enrichment analysis indicated genes in this network were significantly enriched in 4 brain cell types (Interneuron, Medium Spiny Neuron, CA1 pyramidal Neuron, and Somatosensory pyramidal Neuron). These results provide evidence for significant interactions between epilepsy risk genes and anti-epileptic drug targets from the perspective of network biology.

Impact of Genetic Testing on Therapeutic Decision-Making in Childhood-Onset Epilepsies-a Study in a Tertiary Epilepsy Center

We assessed the frequency of pediatric monogenic epilepsies and precision therapies at a tertiary epilepsy center. We analyzed medical records of children, born in 2006-2011 and followed at the Danish Epilepsy Center from January to December 2015; 357 patients were identified, of whom 27 without epilepsy and 35 with acquired brain damage were excluded. Of the remaining 295 children, 188 were consented for study inclusion and genetic testing. At inclusion, 86/188 had a preexisting genetic diagnosis and did not undergo further genetic testing. The 102 genetically unsolved patients underwent WES, which identified a (likely) pathogenic variant in eight patients and a highly relevant variant of unknown significance (VUS) in seven additional patients. Single nucleotide polymorphism array was performed in the remaining 87 patients and revealed no (likely) pathogenic copy number variants (CNVs). Patients with a genetic diagnosis had a significantly lower median age at seizure onset and more often had febrile seizures, status epilepticus, or neurodevelopmental impairment compared to those who remained genetically unsolved. Most common epilepsies were focal or multifocal epilepsies and developmental and epileptic encephalopathies (DDEs). Fifty-three patients, with a putative genetic diagnosis, were potentially eligible for precision therapy approaches. Indeed, genetic diagnosis enabled treatment adjustment in 32/53 (60%); 30/32 (93%) patients experienced at least a 50% reduction in seizure burden while only 4/32 (12.5%) became seizure-free. In summary, a genetic diagnosis was achieved in approximately 50% of patients with non-acquired epilepsy enabling precision therapy approaches in half of the patients, a strategy that results in > 50% reduction in seizure burden, in the majority of the treated patients.

Neurogenetic and Metabolic Mimics of Common Neonatal Neurological Disorders

Neurogenetic and metabolic diseases often present in the neonatal period, masquerading as other disorders, most commonly as neonatal encephalopathy and seizures. Advancements in our understanding of inborn errors of metabolism are leading to an increasing number of therapeutic options. Many of these treatments can improve long-term neurodevelopment and seizure control. However, the treatments are frequently condition-specific. A high index of suspicion is required for prompt identification and treatment. When suspected, simultaneous metabolic and molecular testing are recommended along with concurrent treatment.

Genetic Landscape of SCN1A Variants in a Turkish Cohort with GEFS+ Spectrum and Dravet Syndrome

Introduction

The α subunit of voltage-gated sodium channels in mammals is encoded by 9 different genes, and variations in the SCN1A, SCN2A, SCN3A, and SCN8A genes highly expressed in the CNS have been associated with epilepsy phenotypes. This study aimed at investigating the frequency of SCN1A gene variations in Dravet syndrome (DS) and GEFS+ spectrum phenotype cases and discussing the molecular results in the context of genotype-phenotype correlation.

Methods

Fifteen patients diagnosed with DS and 54 patients meeting the GEFS+ spectrum criteria were included in this study. All patients were evaluated by next-generation sequencing and multiplex ligation-dependent probe amplification using an SCN1A gene commercial kit.

Results

A total of 17 different variants were detected in 18 index cases (26%), of which 7 were novel variations (p.M1R, p.M147T, p.I767L, p.N1391Ifs*5, p.R1886G, p.E1915G, p.R1933Q). Of the 18 cases with variation in the SCN1A gene, 12 had DS and 6 had GEFS+ phenotype. The variations were de novo in all DS cases and in 1 case with a GEFS+ phenotype; in 5 GEFS+ cases, the variant was inherited from the affected parent.

Discussion

This study contributes to the variation spectrum in cases with DS and GEFS+ phenotype with the novel variants detected. SCN1A genetic analysis can help in determining whether antiseizure medication should be selected or avoided in cases with variations. The elucidation of the molecular etiology makes it possible to provide the family with effective genetic counseling for future pregnancies.

New paradigms for the treatment of pediatric monogenic epilepsies: Progressing toward precision medicine

Despite the availability of 28 antiseizure medications (ASMs), one-third of people with epilepsy fail to achieve sustained freedom from seizures. Clinical outcome is even poorer for children with developmental and epileptic encephalopathies (DEEs), many of which are due to single-gene mutations. Discovery of causative genes, however, has paved the way to understanding the molecular mechanism underlying these epilepsies, and to the rational application, or development, of precision treatments aimed at correcting the specific functional defects or their consequences. This article provides an overview of current progress toward precision medicine (PM) in the management of monogenic pediatric epilepsies, by focusing on four different scenarios, namely (a) rational selection of ASMs targeting specifically the underlying pathogenetic mechanisms; (b) development of targeted therapies based on novel molecules; (c) use of dietary treatments or food constituents aimed at correcting specific metabolic defects; and (d) repurposing of medications originally approved for other indications. This article is part of the Special Issue "Severe Infantile Epilepsies".

Human iPSC Modeling of Genetic Febrile Seizure Reveals Aberrant Molecular and Physiological Features Underlying an Impaired Neuronal Activity

Mutations in SCN1A gene, encoding the voltage-gated sodium channel (VGSC) NaV1.1, are widely recognized as a leading cause of genetic febrile seizures (FS), due to the decrease in the Na+ current density, mainly affecting the inhibitory neuronal transmission. Here, we generated induced pluripotent stem cells (iPSCs)-derived neurons (idNs) from a patient belonging to a genetically well-characterized Italian family, carrying the c.434T > C mutation in SCN1A gene (hereafter SCN1AM145T). A side-by-side comparison of diseased and healthy idNs revealed an overall maturation delay of SCN1AM145T cells. Membranes isolated from both diseased and control idNs were injected into Xenopus oocytes and both GABA and AMPA currents were successfully recorded. Patch-clamp measurements on idNs revealed depolarized action potential for SCN1AM145T, suggesting a reduced excitability. Expression analyses of VGSCs and chloride co-transporters NKCC1 and KCC2 showed a cellular “dysmaturity” of mutated idNs, strengthened by the high expression of SCN3A, a more fetal-like VGSC isoform, and a high NKCC1/KCC2 ratio, in mutated cells. Overall, we provide strong evidence for an intrinsic cellular immaturity, underscoring the role of mutant NaV1.1 in the development of FS. Furthermore, our data are strengthening previous findings obtained using transfected cells and recordings on human slices, demonstrating that diseased idNs represent a powerful tool for personalized therapy and ex vivo drug screening for human epileptic disorders.

Commonalities and distinctions between two neurodevelopmental disorder subtypes associated with SCN2A and SCN8A variants and literature review

This study was aimed to analyze the commonalities and distinctions of voltage‐gated sodium channels, Nav1.2, Nav1.6, in neurodevelopmental disorders. An observational study was performed including two patients with neurodevelopmental disorders. The demographic, electroclinical, genetic, and neuropsychological characteristics were analyzed and compared with each other and then with the subjects carrying the same genetic variants reported in the literature. The clinical features of one of them argued for autism spectrum disorder and developmental delay, the other for intellectual disability, diagnoses confirmed by the neuropsychological assessment. The first patient was a carrier of SCN2A (p.R379H) variant while the second was carrier of SCN8A (p.E936K) variant, both involving the pore loop of the two channels. The results of this study suggest that the neurodevelopmental disorders without overt epilepsy of both patients can be the consequences of loss of function of Nav1.2/Nav1.6 channels. Notably, the SCN2A variant, with an earlier expression timing in brain development, resulted in a more severe phenotype as autism spectrum disorder and developmental delay, while the SCN8A variant, with a later expression timing, resulted in a less severe phenotype as intellectual disability.

Usage of Genetic Panels in an Adult Epilepsy Clinic

BACKGROUND

There is limited data on the utility, yield, and cost efficiency of genetic testing in adults with epilepsy. We aimed to describe the yield and utility of genetic panels in our adult epilepsy clinic.

METHODS

We performed a retrospective, cross-sectional study of all patients followed by an epileptologist at a Canadian tertiary care centre's epilepsy clinic between January 2016 and August 2021 for whom a genetic panel was ordered. A panel was generally ordered when the etiology was unknown or in the presence of a malformation of cortical development. We determined the yield of panel positivity and of confirmed genetic diagnoses. We also estimated the proportion of these diagnoses that were clinically actionable.

RESULTS

In total, 164 panels were ordered in 164 patients. Most had refractory epilepsy (80%), and few had comorbid intellectual disability (10%) or a positive family history of epilepsy (11%). The yield of panel positivity was 11%. Panel results were uncertain 49% of the time and negative 40% of the time. Genetic diagnoses were confirmed in 7 (4.3%) patients. These genetic conditions involved the following genes: SCARB2, DEPDC5, PCDH19, LGI1, SCN1A, MT-TL1, and CHRNA7. Of the seven genetic diagnoses, 5 (71%) were evaluated to be clinically actionable.

CONCLUSION

We report a lower diagnostic yield for genetic panels in adults with epilepsy than what has so far been reported. Although the field of the genetics of epilepsy is a fast-moving one and more data is required, our findings suggest that guidelines for genetic testing in adults are warranted.

De novo GRIN2A variants associated with epilepsy and autism and literature review

N-methyl-D-aspartate receptors (NMDAR) are di- or tri-heterotetrameric ligand-gated ion channels composed of two obligate glycine-binding GluN1 subunits and two glutamate-binding GluN2 or GluN3 subunits, encoded by GRIN1, GRIN2A-D, and GRIN3A-B receptor genes respectively. Each NMDA receptor subtype has different temporal and spatial expression patterns in the brain and varies in the cell types and subcellular localization resulting in different functions. They play a crucial role in mediating the excitatory neurotransmission, but are also involved in neuronal development and synaptic plasticity, essential for learning, memory, and high cognitive functions. Among genes coding NMDAR subunits, GRIN2B is predominantly associated with neurodevelopmental disorders such as intellectual disability, developmental delay, autism, attention-deficit/hyperactivity disorder and, further, schizophrenia, Alzheimer's disease. The GRIN2A seems to be predominantly associated with a more definite phenotype including an epileptic spectrum ranging from Landau-Kleffner syndrome to benign childhood epilepsy with centrotemporal spikes, speech or language impairment, intellectual disability/developmental delay often in comorbidity. On the contrary, the occurrence of autism spectrum disorders, unlike GRIN2B-associated disorders, is questionable. To contribute to elucidate the latter issue and to better define the genotype/phenotype correlation, we report the clinical and neuropsychological profile of two patients featuring autism disorder, intellectual disability, language impairment, and focal epilepsy, associated with previously unreported heterozygous de novo GRIN2A pathogenic variants. We hypothesize that the unusual phenotype may be the result of interactions of tri-heterotetrameric 2GluN1/GluN2A-D/GluN3A-B subunits with mutated GluN2A subunit and/or the dysfunction may be influenced by other unknown modifier genes and/or environmental factors.

Clinical next generation sequencing in developmental and epileptic encephalopathies: Diagnostic relevance of data re-analysis and variants re-interpretation

Developmental and epileptic encephalopathies (DEE) are complex pediatric epilepsies, in which heterogeneous pathogenic factors play an important role. Next-generation-sequencing based tools have shown excellent effectiveness. The constant increase in the number of new genotype-phenotype associations suggests the periodic need for re-interpretation and re-analysis of genetic studies without positive results. In this study, we report the diagnostic utility of targeted gene panel sequencing and whole exome sequencing in 55 Argentine subjects with DEE, focusing on the utility of re-interpretation and re-analysis of undetermined and negative genetic diagnoses. The new information in biomedical literature and databases was used for the re-interpretation. For re-analysis, sequencing data processing was repeated using updated bioinformatics tools. Initially, pathogenic variants were detected in 21 subjects (38%). After an average time of 29 months, 25% of the subjects without a genetic diagnosis were re-categorized as diagnosed. Finally, the overall diagnostic yield increased to 53% (29 subjects). In consequence of the re-interpretation and re-analysis, we identified novel variants in the genes: CHD2, COL4A1, FOXG1, GABRA1, GRIN2B, HNRNPU, KCNQ2, MECP2, PCDH19, SCN1A, SCN2A, SCN8A, SLC6A1, STXBP1 and WWOX. Our results expand the diagnostic yield of this subgroup of infantile and childhood seizures and demonstrate the importance of re-evaluation of genetic tests in subjects without an identified causative etiology.

Neonatal Seizures: An Overview of Genetic Causes and Treatment Options

Seizures are the most frequent neurological clinical symptoms of the central nervous system (CNS) during the neonatal period. Neonatal seizures may be ascribed to an acute event or symptomatic conditions determined by genetic, metabolic or structural causes, outlining the so-called 'Neonatal Epilepsies'. To date, three main groups of neonatal epilepsies are recognised during the neonatal period: benign familial neonatal epilepsy (BFNE), early myoclonic encephalopathy (EME) and 'Ohtahara syndrome' (OS). Recent advances showed the role of several genes in the pathogenesis of these conditions, such as KCNQ2, KCNQ3, ARX, STXBP1, SLC25A22, CDKL5, KCNT1, SCN2A and SCN8A. Herein, we reviewed the current knowledge regarding the pathogenic variants most frequently associated with neonatal seizures, which should be considered when approaching newborns affected by these disorders. In addition, we considered the new possible therapeutic strategies reported in these conditions.

Genetic cause of epilepsy in a Greek cohort of children and young adults with heterogeneous epilepsy syndromes

We describe a cohort of 10 unrelated Greek patients (4 females, 6 males; median age 6.5 years, range 2-18 years) with heterogeneous epilepsy syndromes with a genetic basis. In these patients, causative genetic variants, including two novel ones, were identified in 9 known epilepsy-related genes through whole exome sequencing. A patient with glycine encephalopathy was a compound heterozygote for the p.Arg222Cys and the p.Ser77Leu AMT variant. A patient affected with Lafora disease carried the homozygous p.Arg171His EPM2A variant. A de novo heterozygous variant in the GABRG2 gene (p.Pro282Thr) was found in one patient and a pathogenic variant in the GRIN2B gene (p.Gly820Val) in another patient. Infantile-onset lactic acidosis with seizures was associated with the p.Arg446Ter PDHX gene variant in one patient. In two additional epilepsy patients, the p.Ala1662Val and the novel non-sense p.Phe1330Ter SCN1A gene variants were found. Finally, in 3 patients we observed a novel heterozygous missense variant in SCN2A (p.Ala1874Thr), a heterozygous splice site variant in SLC2A1 (c.517-2A>G), as a cause of Glut1 deficiency syndrome, and a pathogenic variant in STXBP1 (p.Arg292Leu), respectively. In half of our cases (patients with variants in the GRIN2B, SCN1A, SCN2A and SLC2A1 genes), a genetic cause with potential management implications was identified.

[Modulatory effect of 2-arachidonoylglycerol on voltage-gated sodium currents in rat caudate nucleus neurons with kainic acid-induced injury]

OBJECTIVE

To investigate the modulatory effect of 2-arachidonoylglycerol (2-AG) on voltage-gated sodium currents(VGSCs) in rat caudate nucleus (CN) neurons with kainic acid (KA)-induced injury and explore the molecular mechanism underlying the neuroprotective effect of 2-AG.

METHODS

Primary cultures of CN neurons isolated from neonatal SD rats were treated with KA, 2-AG+KA, RIM (a CB1 receptor antagonist) +2-AG+KA, or vehicle only (as control).After 7 days in primary culture, the neurons were treated with corresponding agents for 12 h (RIM and 2-AG were added at the same time; KA was added 30 min later) before recording of current density changes, current-voltage characteristics, activation and inactivation kinetics of VGSCs (I_Na) using whole-cell patch clamp technique.

RESULTS

In cultured CN neurons, KA significantly increased current density of VGSCs (P=0.009) as compared with vehicle treatment.KA also produced a hyperpolarizing shift in the activation curve of I_Na and significantly increased the absolute value of V_1/2 for activation (P=0.008).Addition of 2-AG in the culture medium obviously prevented KA-induced increase of I_Na (P=0.009) and hyperpolarizing shift in the activation curve of I_Na, and significantly reduced the value of V_1/2 for activation(P=0.009)in a CB1 receptor-dependent manner.2-AG alone did not affect the density, activation or deactivation of VGSCs in rat CN neurons.

CONCLUSION

In excitotoxic events, endogenous 2-AG can offer neuroprotection by modulating VGSCs in the CN neurons through a CB1 receptor-dependent pathway.

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.

Monogenic Epilepsies: Channelopathies, Synaptopathies, mTorpathies, and Otheropathies

Epilepsy has been historically defined as the recurrence of two or more seizures, together with typical electroencephalogram (EEG) changes, and significant comorbidities, including cardiac and autonomic changes, injuries, intellectual disability, permanent brain damage, and higher mortality risk. Epilepsy may be the consequence of several causes, including genetic anomalies, structural brain malformations, hypoxic–ischemic encephalopathy, brain tumors, drugs, and all contributing factors to the imbalance between excitatory and inhibitory neurons and modulatory interneurons which in turn provoke abnormal, simultaneous electric discharge(s) involving part, or all the brain. In the pregenetic, pregenomic era, in most cases, the exact cause of such neuronal/interneuronal disequilibrium remained unknown and the term “idiopathic epilepsy” was used to define all the epilepsies without cause. At the same time, some specific epileptic syndromes were indicated by the eponym of the first physician who originally described the condition (e.g., the West syndrome, Dravet syndrome, Ohtahara syndrome, and Lennox–Gastaut syndrome) or by some characteristic clinical features (e.g., nocturnal frontal lobe epilepsy, absence epilepsy, and epilepsy and mental retardation limited to females). In many of these occurrences, the distinct epileptic syndrome was defined mainly by its most relevant clinical feature (e.g., seizure semiology), associated comorbidities, and EEGs patterns. Since the identification of the first epilepsy-associated gene (i.e., CHRNA4 gene: cholinergic receptor neuronal nicotinic α polypeptide 4), one of the genes responsible for autosomal dominant nocturnal frontal lobe epilepsy (currently known as sleep-related hypermotor epilepsy) in 1995, the field of epilepsy and the history of epilepsy gene discoveries have gone through at least three different stages as follows: (1) an early stage of relentless gene discovery in monogenic familial epilepsy syndromes; (2) a relatively quiescent and disappointing period characterized by largely negative genome-wide association candidate gene studies; and (3) a genome-wide era in which large-scale molecular genetic studies have led to the identification of several novel epilepsy genes, especially in sporadic forms of epilepsy. As of 2021, more than 150 epilepsy-associated genes or loci are listed in the Online Mendelian Inheritance in Man database.

SCN1A and Its Related Epileptic Phenotypes

Epilepsy is one of the most common neurological disorders, with a lifetime incidence of 1 in 26. Approximately two-thirds of epilepsy has a substantial genetic component in its etiology. As a result, simultaneous screening for mutations in multiple genes and performing whole exome sequencing (WES) are becoming very frequent in the clinical evaluation of children with epilepsy. In this setting, mutations in voltage-gated sodium channel (SCN) α-subunit genes are the most commonly identified cause of epilepsy, with sodium channel genes (i.e., SCN1A, SCN2A, SCN8A) being the most frequently identified causative genes. SCN1A mutations result in a wide spectrum of epilepsy phenotypes ranging from simple febrile seizures to Dravet syndrome, a severe epileptic encephalopathy. In case of mutation of SCN1A, it is also possible to observe behavioral alterations, such as impulsivity, inattentiveness, and distractibility, which can be framed in an attention deficit hyperactivity disorder (ADHD) like phenotype. Despite more than 1,200 SCN1A mutations being reported, it is not possible to assess a clear phenotype–genotype correlations. Treatment remains a challenge and seizure control is often partial and transitory.

Epilepsy-Related Voltage-Gated Sodium Channelopathies: A Review

Epilepsy is a disease characterized by abnormal brain activity and a predisposition to generate epileptic seizures, leading to neurobiological, cognitive, psychological, social, and economic impacts for the patient. There are several known causes for epilepsy; one of them is the malfunction of ion channels, resulting from mutations. Voltage-gated sodium channels (NaV) play an essential role in the generation and propagation of action potential, and malfunction caused by mutations can induce irregular neuronal activity. That said, several genetic variations in NaV channels have been described and associated with epilepsy. These mutations can affect channel kinetics, modifying channel activation, inactivation, recovery from inactivation, and/or the current window. Among the NaV subtypes related to epilepsy, NaV1.1 is doubtless the most relevant, with more than 1500 mutations described. Truncation and missense mutations are the most observed alterations. In addition, several studies have already related mutated NaV channels with the electrophysiological functioning of the channel, aiming to correlate with the epilepsy phenotype. The present review provides an overview of studies on epilepsy-associated mutated human NaV1.1, NaV1.2, NaV1.3, NaV1.6, and NaV1.7.

Functional and pharmacological evaluation of a novel SCN2A variant linked to early-onset epilepsy

OBJECTIVE

We identified a novel de novo SCN2A variant (M1879T) associated with infantile-onset epilepsy that responded dramatically to sodium channel blocker antiepileptic drugs. We analyzed the functional and pharmacological consequences of this variant to establish pathogenicity, and to correlate genotype with phenotype and clinical drug response.

METHODS

The clinical and genetic features of an infant boy with epilepsy are presented. We investigated the effect of the variant using heterologously expressed recombinant human Na_V 1.2 channels. We performed whole-cell patch clamp recording to determine the functional consequences and response to carbamazepine.

RESULTS

The M1879T variant caused disturbances in channel inactivation including substantially depolarized voltage dependence of inactivation, slower time course of inactivation, and enhanced resurgent current that collectively represent a gain-of-function. Carbamazepine partially normalized the voltage dependence of inactivation and produced use-dependent block of the variant channel at high pulsing frequencies. Carbamazepine also suppresses resurgent current conducted by M1879T channels, but this effect was explained primarily by reducing the peak transient current. Molecular modeling suggests that the M1879T variant disrupts contacts with nearby residues in the C-terminal domain of the channel.

INTERPRETATION

Our study demonstrates the value of conducting functional analyses of SCN2A variants of unknown significance to establish pathogenicity and genotype-phenotype correlations. We also show concordance of in vitro pharmacology using heterologous cells with the drug response observed clinically in a case of SCN2A-associated epilepsy.

Precision medicine and therapies of the future

Precision medicine in the epilepsies has gathered much attention, especially with gene discovery pushing forward new understanding of disease biology. Several targeted treatments are emerging, some with considerable sophistication and individual‐level tailoring. There have been rare achievements in improving short‐term outcomes in a few very select patients with epilepsy. The prospects for further targeted, repurposed, or novel treatments seem promising. Along with much‐needed success, difficulties are also arising. Precision treatments do not always work, and sometimes are inaccessible or do not yet exist. Failures of precision medicine may not find their way to broader scrutiny. Precision medicine is not a new concept: It has been boosted by genetics and is often focused on genetically determined epilepsies, typically considered to be driven in an individual by a single genetic variant. Often the mechanisms generating the full clinical phenotype from such a perceived single cause are incompletely understood. The impact of additional genetic variation and other factors that might influence the clinical presentation represent complexities that are not usually considered. Precision success and precision failure are usually equally incompletely explained. There is a need for more comprehensive evaluation and a more rigorous framework, bringing together information that is both necessary and sufficient to explain clinical presentation and clinical responses to precision treatment in a precision approach that considers the full picture not only of the effects of a single variant, but also of its genomic and other measurable environment, within the context of the whole person. As we may be on the brink of a treatment revolution, progress must be considered and reasoned: One possible framework is proposed for the evaluation of precision treatments.