The phenotype of SCN8A developmental and epileptic encephalopathy

Severe Acute Motor Exacerbations (SAME) across Metabolic, Developmental and Genetic Disorders

Acute presentation of severe motor disorders is a diagnostic and management challenge. We define severe acute motor exacerbations (SAME) as acute/subacute motor symptoms that persist for hours-to-days with a severity that compromise vital signs (temperature, breath, and heart rate) and bulbar function (swallowing/dysphagia). Phenomenology includes dystonia, choreoathetosis, combined movement disorders, weakness, and hemiplegic attacks. SAME can develop in diverse diseases and can be preceded by triggers or catabolic states. Recent descriptions of SAME in complex neurodevelopmental and epileptic encephalopathies have broadened appreciation of this presentation beyond inborn errors of metabolism. A high degree of clinical suspicion is required to identify appropriately targeted investigations and management. We conducted a comprehensive literature analysis of etiologies. Reported triggers are described and classified as per pathophysiological mechanism. A video of six cases displaying multiple SAME with diverse outcomes is provided. We identified 50 different conditions that manifest SAME, some associated with developmental regression. Etiologies include disorders of metabolism: energy substrate, amino acids, complex molecules, vitamins/cofactors, minerals, and neurotransmitters/synaptic vesicle cycling. Non-metabolic neurodegenerative and genetic disorders that present with movement disorders and epilepsy can additionally manifest SAME. A limited number of triggers are grouped here, together with an approach to investigations and general management strategies. Several neurogenetic and neurometabolic disorders manifest SAME. Identifying triggers can help in certain cases narrow the differential diagnosis and guide the expeditious application of targeted therapies to minimize adverse developmental and neurological consequences. This process may inform pathogenesis and eventually improve our understanding of the mechanisms that lead to the development of SAME. © 2024 International Parkinson and Movement Disorder Society.

Global modified Delphi consensus on diagnosis, phenotypes, and treatment of SCN8A-related epilepsy and/or neurodevelopmental disorders

OBJECTIVE

We aimed to develop consensus for diagnosis/management of SCN8A-related disorders. Utilizing a modified Delphi process, a global cohort of experienced clinicians and caregivers provided input on diagnosis, phenotypes, treatment, and management of SCN8A-related disorders.

METHODS

A Core Panel (13 clinicians, one researcher, six caregivers), divided into three subgroups (diagnosis/phenotypes, treatment, comorbidities/prognosis), performed a literature review and developed questions for the modified Delphi process. Twenty-eight expert clinicians, one researcher, and 13 caregivers from 16 countries participated in the subsequent three survey rounds. We defined consensus as follows: strong consensus, ≥80% fully agree; moderate consensus, ≥80% fully/partially agree, <10% disagree; and modest consensus, 67%-79% fully/partially agree, <10% disagree.

RESULTS

Early diagnosis is important for long-term clinical outcomes in SCN8A-related disorders. There are five phenotypes: three with early seizure onset (severe developmental and epileptic encephalopathy [DEE], mild/moderate DEE, self-limited (familial) infantile epilepsy [SeL(F)IE]) and two with later/no seizure onset (neurodevelopmental delay with generalized epilepsy [NDDwGE], NDD without epilepsy [NDDwoE]). Caregivers represented six patients with severe DEE, five mild/moderate DEE, one NDDwGE, and one NDDwoE. Phenotypes vary by age at seizures/developmental delay onset, seizure type, electroencephalographic/magnetic resonance imaging findings, and first-line treatment. Gain of function (GOF) versus loss of function (LOF) is valuable for informing treatment. Sodium channel blockers are optimal first-line treatment for GOF, severe DEE, mild/moderate DEE, and SeL(F)IE; levetiracetam is relatively contraindicated in GOF patients. First-line treatment for NDDwGE is valproate, ethosuximide, or lamotrigine; sodium channel blockers are relatively contraindicated in LOF patients.

SIGNIFICANCE

This is the first-ever global consensus for the diagnosis and treatment of SCN8A-related disorders. This consensus will reduce knowledge gaps in disease recognition and inform preferred treatment across this heterogeneous disorder. Consensus of this type allows more clinicians to provide evidence-based care and empowers SCN8A families to advocate for their children.

Global modified-Delphi consensus on comorbidities and prognosis of SCN8A-related epilepsy and/or neurodevelopmental disorders

OBJECTIVES

We aimed to develop consensus on comorbidities (frequency, severity, and prognosis) and overall outcomes in epilepsy, development, and cognition for the five phenotypes of SCN8A-related disorders.

METHODS

A core panel consisting of 13 clinicians, 1 researcher, and 6 caregivers was formed and split into three workgroups. One group focused on comorbidities and prognosis. All groups performed a literature review and developed questions for use in a modified-Delphi process. Twenty-eight clinicians, one researcher, and 13 caregivers from 16 countries participated in three rounds of the modified-Delphi process. Consensus was defined as follows: strong consensus ≥80% fully agree; moderate consensus ≥80% fully or partially agree, <10% disagree; and modest consensus 67%-79% fully or partially agree, <10% disagree.

RESULTS

Consensus was reached on the presence of 14 comorbidities in patients with Severe Developmental and Epileptic Encephalopathy (Severe DEE) spanning non-seizure neurological disorders and other organ systems; impacts were mostly severe and unlikely to improve or resolve. Across Mild/Moderate Developmental and Epileptic Encephalopathy (Mild/Moderate DEE), Neurodevelopmental Delay with Generalized Epilepsy (NDDwGE), and NDD without Epilepsy (NDDwoE) phenotypes, cognitive and sleep-related comorbidities as well as fine and gross motor delays may be present but are less severe and more likely to improve compared to Severe DEE. There was no consensus on comorbidities in the SeL(F)IE phenotype but strong conesensus that seizures would largely resolve. Seizure freedom is rare in patients with Severe DEE but may occur in some with Mild/Moderate DEE and NDDwGE.

SIGNIFICANCE

Significant comorbidities are present in most phenotypes of SCN8A-related disorders but are most severe and pervasive in the Severe DEE phenotype. We hope that this work will improve recognition, early intervention, and long-term management for patients with these comorbidities and provide the basis for future evidence-based studies on optimal treatments of SCN8A-related disorders. Identifying the prognosis of patients with SCN8A-related disorders will also improve care and quality-of-life for patients and their caregivers.

Genotypic and phenotypic characteristics of sodium channel-associated epilepsy in Chinese population

Variants in voltage-gated sodium channel (VGSC) genes are implicated in seizures, epilepsy, and neurodevelopmental disorders, constituting a significant aspect of hereditary epilepsy in the Chinese population. Through retrospective analysis utilizing next-generation sequencing (NGS), we examined the genotypes and phenotypes of VGSC-related epilepsy cases from a cohort of 691 epilepsy subjects. Our findings revealed that 5.1% of subjects harbored VGSC variants, specifically 22 with SCN1A, 9 with SCN2A, 1 with SCN8A, and 3 with SCN1B variants; no SCN3A variants were detected. Among these, 14 variants were previously reported, while 21 were newly identified. SCN1A variant carriers predominantly presented with Dravet Syndrome (DS) and Genetic Epilepsy with Febrile Seizures Plus (GEFS + ), featuring a heightened sensitivity to fever-induced seizures. Statistically significant disparities emerged between the SCN1A-DS and SCN1A-GEFS+ groups concerning seizure onset and genetic diagnosis age, incidence of status epilepticus, mental retardation, anti-seizure medication (ASM) responsiveness, and familial history. Notably, subjects with SCN1A variants affecting the protein's pore region experienced more frequent cluster seizures. All SCN2A variants were of de novo origin, and 88.9% of individuals with SCN2A variations exhibited cluster seizures. This research reveals a significant association between variations in VGSC-related genes and the clinical phenotype diversity of epilepsy subjects in China, emphasizing the pivotal role of NGS screening in establishing accurate disease diagnoses and guiding the selection of ASM.

Carvedilol increases seizure resistance in a mouse model of SCN8A-derived epilepsy

Patients with mutations that alter the function of the sodium channel SCN8A present with a range of clinical features, including mild to severe seizures, developmental delay, intellectual disability, autism, feeding dysfunction, motor impairment, and hypotonia. In an effort to identify compounds that could be potentially beneficial in SCN8A-associated epilepsy, Atkin et al. conducted an in vitro screen which resulted in the identification of 90 compounds that effectively reduced sodium influx into the cells expressing the human SCN8A R1872Q mutation. The top compounds that emerged from this screen included amitriptyline, carvedilol, and nilvadipine. In the current study, we evaluated the ability of these three compounds to increase resistance to 6 Hz or pentylenetetrazole (PTZ)-induced seizures in wild-type CF1 mice and in a mouse line expressing the human SCN8A R1620L mutation. We also evaluated the effects of fenfluramine administration, which was recently associated with a 60%-90% decrease in seizure frequency in three patients with SCN8A-associated epilepsy. While amitriptyline, carvedilol, and fenfluramine provided robust protection against induced seizures in CF1 mice, only carvedilol was able to significantly increase resistance to 6 Hz- and PTZ-induced seizures in RL/+ mutants. These results provide support for further evaluation of carvedilol as a potential treatment for patients with SCN8A mutations.

Channelopathies in epilepsy: an overview of clinical presentations, pathogenic mechanisms, and therapeutic insights

Pathogenic variants in genes encoding ion channels are causal for various pediatric and adult neurological conditions. In particular, several epilepsy syndromes have been identified to be caused by specific channelopathies. These encompass a spectrum from self-limited epilepsies to developmental and epileptic encephalopathies spanning genetic and acquired causes. Several of these channelopathies have exquisite responses to specific antiseizure medications (ASMs), while others ASMs may prove ineffective or even worsen seizures. Some channelopathies demonstrate phenotypic pleiotropy and can cause other neurological conditions outside of epilepsy. This review aims to provide a comprehensive exploration of the pathophysiology of seizure generation, ion channels implicated in epilepsy, and several genetic epilepsies due to ion channel dysfunction. We outline the clinical presentation, pathogenesis, and the current state of basic science and clinical research for these channelopathies. In addition, we briefly look at potential precision therapy approaches emerging for these disorders.

The clinical and genetic landscape of developmental and epileptic encephalopathies in Egyptian children

Developmental and epileptic encephalopathies (DEEs) are a heterogeneous group of epilepsies characterized by early-onset, refractory seizures associated with developmental regression or impairment, with a heterogeneous genetic landscape including genes implicated in various pathways and mechanisms. We retrospectively studied the clinical and genetic data of patients with genetic DEE who presented at two tertiary centers in Egypt over a 10-year period. Exome sequencing was used for genetic testing. We report 74 patients from 63 unrelated Egyptian families, with a high rate of consanguinity (58%). The most common seizure type was generalized tonic-clonic (58%) and multiple seizure types were common (55%). The most common epilepsy syndrome was early infantile DEE (50%). All patients showed variable degrees of developmental impairment. Microcephaly, hypotonia, ophthalmological involvement and neuroimaging abnormalities were common. Eighteen novel variants were identified and the phenotypes of five DEE genes were expanded with novel phenotype-genotype associations. Obtaining a genetic diagnosis had implications on epilepsy management in 17 patients with variants in 12 genes. In this study, we expand the phenotype and genotype spectrum of DEE in a large single ethnic cohort of patients. Reaching a genetic diagnosis guided precision management of epilepsy in a significant proportion of patients.

Beyond the Norm: A Report of a Rare Case of Sodium Channel 8 Alpha (SCN8A) Gene-Related Epilepsy Unveiled in a Nine-Year-Old Child

Sodium channel 8 alpha (SCN8A) mutations encompass a spectrum of epilepsy phenotypes with diverse clinical manifestations, posing diagnostic challenges. We present a case of a nine-year-old male with SCN8A gene-associated developmental and epileptic encephalopathies (DEEs), characterized by generalized tonic-clonic seizures (GTCS) since infancy. Despite treatment with multiple antiepileptic drugs (AEDs), including phenytoin, valproate, levetiracetam, carbamazepine, and clobazam, seizure control remained elusive, prompting genetic testing. Whole exome sequencing confirmed a heterozygous mutation (p.Phe210Ser) in SCN8A exon 6, indicative of DEE-13. Functional studies revealed a gain-of-function mechanism in SCN8A variants, resulting in heightened ion channel activity and altered voltage dependence of activation. Despite treatment adjustments, the patient's seizures persisted until topiramate was introduced, offering partial relief. SCN8A, encoding Nav1.6 sodium channels, modulates neuronal excitability, with mutations leading to increased persistent currents and hyperexcitability. Early seizure onset and developmental delays are hallmarks of SCN8A-related DEE. This case highlights the significance of genetic testing in refractory epilepsy management, guiding personalized treatment strategies. Sodium channel blockers like phenytoin and carbamazepine are often first-line therapies, while topiramate presents as a potential adjunctive option in SCN8A-related DEE. Overall, this case underscores the diagnostic and therapeutic complexities of managing SCN8A-related epileptic encephalopathy, emphasizing the importance of long-term monitoring and personalized treatment approaches for optimizing outcomes in refractory epilepsy.

Emergence of lingual dystonia and strabismus in early-onset SCN8A self-limiting familial infantile epilepsy

Pathogenic variants in SCN8A are associated with a broad phenotypic spectrum, including Self-Limiting Familial Infantile Epilepsy (SeLFIE), characterized by infancy-onset age-related seizures with normal development and cognition. Movement disorders, particularly paroxysmal kinesigenic dyskinesia typically arising after puberty, may represent another core symptom. We present the case of a 1-year-old girl with a familial disposition to self-limiting focal seizures from the maternal side and early-onset orofacial movement disorders associated with SCN8A-SeLFIE. Brain MRI was normal. Genetic testing revealed a maternally inherited SCN8A variant [c.4447G > A; p.(Glu1483Lys)]. After the introduction of valproic acid, she promptly achieved seizure control as well as complete remission of strabismus and a significant decrease in episodes of tongue deviation. Family history, genetic findings, and epilepsy phenotype are consistent with SCN8A-SeLFIE. Movement disorders are an important part of the SCN8A phenotypic spectrum, and this case highlights the novel early-onset orofacial movement disorders associated with this condition. The episodes of tongue deviation and protrusion suggest focal oromandibular (lingual) dystonia. Additionally, while infantile strabismus or esophoria is a common finding in healthy individuals, our case raises the possibility of an ictal origin of the strabismus. This study underscores the importance of recognizing and addressing movement disorders in SCN8A-SeLFIE patients, particularly the rare early-onset orofacial manifestations. It adds to the growing body of knowledge regarding the diverse clinical presentations of SCN8A-associated disorders and suggests potential avenues for clinical management and further research.

Use of ketogenic dietary therapy for drug-resistant epilepsy in early infancy

OBJECTIVE

There is growing evidence that ketogenic dietary therapy (KDT) can be safely and efficiently used in young children, but little evidence exists on its use in newborns. Developmental and epileptic encephalopathies starting in the neonatal period or early infancy usually present a poor prognosis. The aim of this study was to evaluate effectiveness, safety, and survival of infants younger than 3 months of age with drug-resistant epilepsy in whom KDT was used.

METHODS

A retrospective study was conducted to evaluate neonates and infants younger than 3 months who started KDT for drug-resistant developmental and epileptic encephalopathies at three referral centers. Data were collected on demographic features, time of epilepsy onset, epilepsy syndrome, seizure type, seizure frequency at diet onset, etiology, details regarding diet initiation, type of ketogenic formula, breastfeeding, route of administration, blood ketones, growth, length of NICU stay, and survival.

RESULTS

Nineteen infants younger than 12 weeks of life who received KDT with a minimum follow-up of 1 month were included; 13 had early-infantile developmental and epileptic encephalopathy, four epilepsy of infancy with migrating focal seizures, and two focal epilepsy. A >50% response was observed in 73.7% at 1 month on the diet; 37% achieved a > 75% seizure reduction, and 10.5% became seizure free. At 3 months, a >50% decrease in seizure frequency was observed in 72.2%; 15.8% had a >75% reduction; 21% became seizure free. Overall survival was 76% at 1 year on diet. Incidence of acute and late adverse effects was low and most adverse effects were asymptomatic and manageable.

SIGNIFICANCE

Our experience suggests that KDT is safe and effective in newborns and very young infants; however, further studies on the management of the diet in this vulnerable age group are necessary.

Clinical severity is correlated with age at seizure onset and biophysical properties of recurrent gain of function variants associated with SCN8A-related epilepsy

OBJECTIVE

Genetic variants in the SCN8A gene underlie a wide spectrum of neurodevelopmental phenotypes including several distinct seizure types and a host of comorbidities. One of the major challenges facing clinicians and researchers alike is to identify genotype-phenotype (G-P) correlations that may improve prognosis, guide treatment decisions, and lead to precision medicine approaches.

METHODS

We investigated G-P correlations among 270 participants harboring gain-of-function (GOF) variants enrolled in the International SCN8A Registry, a patient-driven online database. We performed correlation analyses stratifying the cohort by clinical phenotypes to identify diagnostic features that differ among patients with varying levels of clinical severity, and that differ among patients with distinct GOF variants.

RESULTS

Our analyses confirm positive correlations between age at seizure onset and developmental skills acquisition (developmental quotient), rate of seizure freedom, and percentage of cohort with developmental delays, and identify negative correlations with number of current and weaned antiseizure medications. This set of features is more detrimentally affected in individuals with a priori expectations of more severe clinical phenotypes. Our analyses also reveal a significant correlation between a severity index combining clinical features of individuals with a particular highly recurrent variant and an independent electrophysiological score assigned to each variant based on in vitro testing.

SIGNIFICANCE

This is one of the first studies to identify statistically significant G-P correlations for individual SCN8A variants with GOF properties. The results suggest that individual GOF variants (1) are predictive of clinical severity for individuals carrying those variants and (2) may underlie distinct clinical phenotypes of SCN8A disease, thus helping to explain the wide SCN8A-related epilepsy disease spectrum. These results also suggest that certain features present at initial diagnosis are predictive of clinical severity, and with more informed treatment plans, may serve to improve prognosis for patients with SCN8A GOF variants.

Genetic Background of Epilepsy and Antiepileptic Treatments

Advanced identification of the gene mutations causing epilepsy syndromes is expected to translate into faster diagnosis and more effective treatment of these conditions. Over the last 5 years, approximately 40 clinical trials on the treatment of genetic epilepsies have been conducted. As a result, some medications that are not regular antiseizure drugs (e.g., soticlestat, fenfluramine, or ganaxolone) have been introduced to the treatment of drug-resistant seizures in Dravet, Lennox-Gastaut, maternally inherited chromosome 15q11.2-q13.1 duplication (Dup 15q) syndromes, and protocadherin 19 (PCDH 19)-clusterig epilepsy. And although the effects of soticlestat, fenfluramine, and ganaxolone are described as promising, they do not significantly affect the course of the mentioned epilepsy syndromes. Importantly, each of these syndromes is related to mutations in several genes. On the other hand, several mutations can occur within one gene, and different gene variants may be manifested in different disease phenotypes. This complex pattern of inheritance contributes to rather poor genotype-phenotype correlations. Hence, the detection of a specific mutation is not synonymous with a precise diagnosis of a specific syndrome. Bearing in mind that seizures develop as a consequence of the predominance of excitatory over inhibitory processes, it seems reasonable that mutations in genes encoding sodium and potassium channels, as well as glutamatergic and gamma-aminobutyric (GABA) receptors, play a role in the pathogenesis of epilepsy. In some cases, different pathogenic variants of the same gene can result in opposite functional effects, determining the effectiveness of therapy with certain medications. For instance, seizures related to gain-of-function (GoF) mutations in genes encoding sodium channels can be successfully treated with sodium channel blockers. On the contrary, the same drugs may aggravate seizures related to loss-of-function (LoF) variants of the same genes. Hence, knowledge of gene mutation-treatment response relationships facilitates more favorable selection of drugs for anticonvulsant therapy.

Molecular and Cellular Context Influences SCN8A Variant Function

Pathogenic variants in SCN8A , which encodes the voltage-gated sodium (Na V ) channel Na V 1.6, are associated with neurodevelopmental disorders including epileptic encephalopathy. Previous approaches to determine SCN8A variant function may be confounded by the use of a neonatal-expressed alternatively spliced isoform of Na V 1.6 (Na V 1.6N), and engineered mutations to render the channel tetrodotoxin (TTX) resistant. In this study, we investigated the impact of SCN8A alternative splicing on variant function by comparing the functional attributes of 15 variants expressed in two developmentally regulated splice isoforms (Na V 1.6N, Na V 1.6A). We employed automated patch clamp recording to enhance throughput, and developed a novel neuronal cell line (ND7/LoNav) with low levels of endogenous Na V current to obviate the need for TTX-resistance mutations. Expression of Na V 1.6N or Na V 1.6A in ND7/LoNav cells generated Na V currents that differed significantly in voltage-dependence of activation and inactivation. TTX-resistant versions of both isoforms exhibited significant functional differences compared to the corresponding wild-type (WT) channels. We demonstrated that many of the 15 disease-associated variants studied exhibited isoform-dependent functional effects, and that many of the studied SCN8A variants exhibited functional properties that were not easily classified as either gain- or loss-of-function. Our work illustrates the value of considering molecular and cellular context when investigating SCN8A variants.

Greater female than male resilience to mortality and morbidity in the Scn8a mouse model of pediatric epilepsy

AIMS

Females and males of all ages are affected by epilepsy; however, unlike many clinical studies, most preclinical research has focused on males. Genetic variants in the voltage-gated sodium channel gene, SCN8A, are associated with a broad spectrum of neurological and epileptic syndromes. Here we investigate sex differences in the natural history of the Scn8a-N1768D knockin mouse model of pediatric epilepsy.

METHODS

We utilize 24/7 video to monitor juveniles and adults of both sexes to investigate variability in seizure activity (e.g. onset and frequency), mortality and morbidity, response to cannabinoids, and mode of death. We also monitor sleep architecture using a noninvasive piezoelectric method in order to identify factors that influence seizure severity and outcome.

RESULTS

Both sexes had nearly 100% penetrance in seizure onset and early mortality. However, adult heterozygous (D/+) females were more resilient as exhibited by the ability to tolerate more seizures over a longer lifespan. Homozygous (D/D) juveniles did not exhibit a sex difference in overall survival. Female estrus cycle was disrupted before seizure onset, while sleep was disrupted in both sexes in association with seizure onset. Females typically died while in convulsive status epilepticus; however, a high proportion of males died while not experiencing behavioral seizures. Only juvenile and adult males benefited from cannabinoid administration.

CONCLUSIONS

These results support the hypothesis that factors associated with sexual differentiation play a role in the neurobiology of epilepsy and point to the importance of including both sexes in the design of studies to identify new epilepsy therapies.

Gastrointestinal and eating problems in SCN1A-related seizure disorders

OBJECTIVE

Our study aimed to describe the prevalence and characteristics of gastrointestinal and eating problems in Dravet syndrome (DS) and other SCN1A-related seizure disorders and to determine the association between the occurrence of gastrointestinal and eating problems and core features of DS.

METHODS

Gastrointestinal and eating problems were assessed with a questionnaire in a Dutch cohort of participants with an SCN1A-related seizure disorder. Associations between the number of gastrointestinal and eating problems and core features of DS, seizure severity, level of intellectual disability, impaired mobility, behavioral problems, and use of anti-seizure medication, were explored by multivariate ordinal regression analyses. Symptoms were divided into the categories dysphagia-related, behavioral, and gastrointestinal, and were assessed separately.

RESULTS

One hundred sixty-nine participants with an SCN1A-related seizure disorder, of whom 118 (69.8%) with DS and 51 (30.2%) with Generalized Epilepsy with Febrile Seizures Plus / Febrile Seizures (GEFS+/FS), the non-DS phenotype, were evaluated. Gastrointestinal and eating problems were highly prevalent in DS participants, 50.8% had more than three symptoms compared to 3.9% of non-DS participants. Of participants with DS, 17.8% were fully or partly fed by a gastric tube. Within the three different symptom categories, the most prevalent dysphagia-related symptom was drooling (60.7%), distraction during mealtimes (61.4%) the most prevalent behavioral symptom, and constipation and loss of appetite (both 50.4%) the most prevalent gastrointestinal symptoms. DS participants who use a wheelchair (odds ratio (OR) 4.9 95%CI (1.9-12.8) compared to walking without aid), who use ≥3 anti-seizure medications (ASM) (OR 5.9 95%CI (1.9-18.2) compared to <3 ASM) and who have behavioral problems (OR 3.0 95%CI (1.1-8.1) compared to no behavioral problems) had more gastrointestinal and eating problems.

CONCLUSION

Gastrointestinal and eating problems are frequently reported symptoms in DS. Distinguishing between symptom categories will lead to tailored management of patients at risk, will improve early detection, and enable a timely referral to a dietitian, behavioral expert, and/or speech therapist, ultimately aiming to improve the quality of life of both patients and caregivers.

Voltage-gated sodium channels in genetic epilepsy: up and down of excitability

The past two decades have witnessed a wide range of studies investigating genetic variants of voltage-gated sodium (NaV ) channels, which are involved in a broad spectrum of diseases, including several types of epilepsy. We have reviewed here phenotypes and pathological mechanisms of genetic epilepsies caused by variants in NaV α and β subunits, as well as of some relevant interacting proteins (FGF12/FHF1, PRRT2, and Ankyrin-G). Notably, variants of all these genes can induce either gain- or loss-of-function of NaV leading to either neuronal hyperexcitability or hypoexcitability. We present the results of functional studies obtained with different experimental models, highlighting that they should be interpreted considering the features of the experimental system used. These systems are models, but they have allowed us to better understand pathophysiological issues, ameliorate diagnostics, orientate genetic counseling, and select/develop therapies within a precision medicine framework. These studies have also allowed us to gain insights into the physiological roles of different NaV channels and of the cells that express them. Overall, our review shows the progress that has been made, but also the need for further studies on aspects that have not yet been clarified. Finally, we conclude by highlighting some significant themes of general interest that can be gleaned from the results of the work of the last two decades.

Epileptic Channelopathies and Neuromuscular Disorders in Newborns: A Narrative Review

Neonates can have ion channel abnormalities known as channelopathies, which can impact any organ system. These abnormalities cause seizures, which can result in developmental delays and lead to early death. For a child's long-term neurodevelopment, early identification as a channelopathy is essential to avoid any brain damage. Therefore, this review aims to focus on early diagnostic criteria. Since it might be difficult for doctors to interpret the presenting symptoms of channelopathies, a thorough diagnostic examination that follows a methodical step-by-step procedure is essential. Skeletal muscle fiber and neuron excitability depend on voltage-gated sodium channels. It is now known that mutations in voltage-gated sodium channel genes can cause a growing variety of fatal or debilitating pediatric neurological diseases. Episodic paralysis, myotonia, newborn hypotonia, respiratory impairment, laryngospasm/stridor, congenital myasthenia, and myopathy are examples of muscle phenotypes. There may be a connection between sodium channel malfunction and abrupt infant death, according to recent findings. Numerous epilepsy syndromes and complex encephalopathies are among the manifestations of different channelopathies that are becoming more widely recognized.

Experimental pharmacology: Targeting metabolic pathways

Since the discovery of the treatment for Wilson disease a growing number of treatable inherited dystonias have been identified and their search and treatment have progressively been implemented in the clinics of patients with dystonia. While waiting for gene therapy to be more widely and adequately translated into the clinical setting, the efforts to divert the natural course of dystonia reside in unveiling its pathogenesis. Specific metabolic treatments can rewrite the natural history of the disease by preventing neurotoxic metabolite accumulation or interfering with the cell accumulation of damaging metabolites, restoring energetic cell fuel, supplementing defective metabolites, and supplementing the defective enzyme. A metabolic derangement of cell homeostasis is part of the progression of many non-metabolic genetic lesions and could be the target for possible metabolic approaches. In this chapter, we provided an update on treatment strategies for treatable inherited dystonias and an overview of genetic dystonias with new experimental therapeutic approaches available or close to clinical translation.

Distinguishing Loss-of-Function and Gain-of-Function SCN8A Variants Using a Random Forest Classification Model Trained on Clinical Features

Background and Objectives

Pathogenic variants at the voltage-gated sodium channel gene, SCN8A, are associated with a wide spectrum of clinical disease outcomes. A critical challenge for neurologists is to determine whether patients carry gain-of-function (GOF) or loss-of-function (LOF) variants to guide treatment decisions, yet in vitro studies to infer channel function are often not feasible in the clinic. In this study, we develop a predictive modeling approach to classify variants based on clinical features present at initial diagnosis.

Methods

We performed an exhaustive search for individuals deemed to carry SCN8A GOF and LOF variants by means of in vitro studies in heterologous cell systems, or because the variant was classified as truncating, and recorded clinical features. This resulted in a total of 69 LOF variants: 34 missense and 35 truncating variants, including 9 nonsense, 13 frameshift, 6 splice site, 6 indels, and 1 large deletion. We then assembled a truth set of variants with known functional effects, excluding individuals carrying variants at other loci associated with epilepsy. We then trained a predictive model based on random forest using this truth set of 45 LOF variants and 45 GOF variants randomly selected from a set of variants tested by in vitro methods.

Results

Phenotypic categories assigned to individuals correlated strongly with GOF or LOF variants. All patients with GOF variants experienced early-onset seizures (mean age at onset = 4.5 ± 3.1 months) while only 64.4% patients with LOF variants had seizures, most of which were late-onset absence seizures (mean age at onset = 40.0 ± 38.1 months). With high accuracy (95.4%), our model including 5 key clinical features classified individuals with GOF and LOF variants into 2 distinct cohorts differing in age at seizure onset, development of seizures, seizure type, intellectual disability, and developmental and epileptic encephalopathy.

Discussion

The results support the hypothesis that patients with SCN8A GOF and LOF variants represent distinct clinical phenotypes. The clinical model developed in this study has great utility because it provides a rapid and highly accurate platform for predicting the functional class of patient variants during SCN8A diagnosis, which can aid in initial treatment decisions and improve prognosis.

In vitro effects of eslicarbazepine (S-licarbazepine) as a potential precision therapy on SCN8A variants causing neuropsychiatric disorders

BACKGROUND AND PURPOSE

Variants in SCN8A, the Na_V 1.6 channel's coding gene, are characterized by a variety of symptoms, including intractable epileptic seizures, psychomotor delay, progressive cognitive decline, autistic features, ataxia or dystonia. Standard anticonvulsant treatment has a limited impact on the course of disease.

EXPERIMENTAL APPROACH

We investigated the therapeutic potential of eslicarbazepine (S-licarbazepine; S-lic), an enhancer of slow inactivation of voltage gated sodium channels, on two variants with biophysical and neuronal gain-of-function (G1475R and M1760I) and one variant with biophysical gain-of-function but neuronal loss-of-function (A1622D) in neuroblastoma cells and in murine primary hippocampal neuron cultures. These three variants cover the broad spectrum of Na_V 1.6-associated disease and are linked to representative phenotypes of mild to moderate epilepsy (G1475R), developmental and epileptic encephalopathy (M1760I) and intellectual disability without epilepsy (A1622D).

KEY RESULTS

Similar to known effects on Na_V 1.6 wildtype channels, S-lic predominantly enhances slow inactivation on all tested variants, irrespective of their particular biophysical mechanisms. Beyond that, S-lic exhibits variant-specific effects including a partial reversal of pathologically slowed fast inactivation dynamics (A1622D and M1760I) and a trend to reduce enhanced persistent Na⁺ current by A1622D variant channels. Furthermore, our data in primary transfected neurons reveal that not only variant-associated hyperexcitability (M1760I and G1475R) but also hypoexcitability (A1622D) can be modulated by S-lic.

CONCLUSIONS AND IMPLICATIONS

S-lic has not only substance-specific effects but also variant-specific effects. Personalized treatment regimens optimized to achieve such variant-specific pharmacological modulation may help to reduce adverse side effects and improve the overall therapeutic outcome of SCN8A-related disease.

Structure of human NaV1.6 channel reveals Na+ selectivity and pore blockade by 4,9-anhydro-tetrodotoxin

The sodium channel Na_V1.6 is widely expressed in neurons of the central and peripheral nervous systems, which plays a critical role in regulating neuronal excitability. Dysfunction of Na_V1.6 has been linked to epileptic encephalopathy, intellectual disability and movement disorders. Here we present cryo-EM structures of human Na_V1.6/β1/β2 alone and complexed with a guanidinium neurotoxin 4,9-anhydro-tetrodotoxin (4,9-ah-TTX), revealing molecular mechanism of Na_V1.6 inhibition by the blocker. The apo-form structure reveals two potential Na⁺ binding sites within the selectivity filter, suggesting a possible mechanism for Na⁺ selectivity and conductance. In the 4,9-ah-TTX bound structure, 4,9-ah-TTX binds to a pocket similar to the tetrodotoxin (TTX) binding site, which occupies the Na⁺ binding sites and completely blocks the channel. Molecular dynamics simulation results show that subtle conformational differences in the selectivity filter affect the affinity of TTX analogues. Taken together, our results provide important insights into Na_V1.6 structure, ion conductance, and inhibition.

SCN8A epileptic encephalopathy mutations display a gain-of-function phenotype and divergent sensitivity to antiepileptic drugs

De novo missense mutations in SCN8A gene encoding voltage-gated sodium channel NaV1.6 are linked to a severe form of early infantile epileptic encephalopathy named early infantile epileptic encephalopathy type13 (EIEE13). The majority of the patients with EIEE13 does not respond favorably to the antiepileptic drugs (AEDs) in clinic and has a significantly increased risk of death. Although more than 60 EIEE13-associated mutations have been discovered, only few mutations have been functionally analyzed. In this study we investigated the functional influences of mutations N1466T and N1466K, two EIEE13-associated mutations located in the inactivation gate, on sodium channel properties. Sodium currents were recorded from CHO cells expressing the mutant and wide-type (WT) channels using the whole-cell patch-clamp technique. We found that, in comparison with WT channels, both the mutant channels exhibited increased window currents, persistent currents (INaP) and ramp currents, suggesting that N1466T and N1466K were gain-of-function (GoF) mutations. Sodium channel inhibition is one common mechanism of currently available AEDs, in which topiramate (TPM) was effective in controlling seizures of patients carrying either of the two mutations. We found that TPM (100 µM) preferentially inhibited INaP and ramp currents but did not affect transient currents (INaT) mediated by N1466T or N1466K. Among the other 6 sodium channel-inhibiting AEDs tested, phenytoin and carbamazepine displayed greater efficacy than TPM in suppressing both INaP and ramp currents. Functional characterization of mutants N1466T and N1466K is beneficial for understanding the pathogenesis of EIEE13. The divergent effects of sodium channel-inhibiting AEDs on INaP and ramp currents provide insight into the development of therapeutic strategies for the N1466T and N1466K-associated EIEE13.

Genotype-phenotype correlations in SCN8A-related disorders reveal prognostic and therapeutic implications

We report detailed functional analyses and genotype-phenotype correlations in 392 individuals carrying disease-causing variants in SCN8A, encoding the voltage-gated Na+ channel Nav1.6, with the aim of describing clinical phenotypes related to functional effects. Six different clinical subgroups were identified: Group 1, benign familial infantile epilepsy (n = 15, normal cognition, treatable seizures); Group 2, intermediate epilepsy (n = 33, mild intellectual disability, partially pharmaco-responsive); Group 3, developmental and epileptic encephalopathy (n = 177, severe intellectual disability, majority pharmaco-resistant); Group 4, generalized epilepsy (n = 20, mild to moderate intellectual disability, frequently with absence seizures); Group 5, unclassifiable epilepsy (n = 127); and Group 6, neurodevelopmental disorder without epilepsy (n = 20, mild to moderate intellectual disability). Those in Groups 1-3 presented with focal or multifocal seizures (median age of onset: 4 months) and focal epileptiform discharges, whereas the onset of seizures in patients with generalized epilepsy was later (median: 42 months) with generalized epileptiform discharges. We performed functional studies expressing missense variants in ND7/23 neuroblastoma cells and primary neuronal cultures using recombinant tetrodotoxin-insensitive human Nav1.6 channels and whole-cell patch-clamping. Two variants causing developmental and epileptic encephalopathy showed a strong gain-of-function (hyperpolarizing shift of steady-state activation, strongly increased neuronal firing rate) and one variant causing benign familial infantile epilepsy or intermediate epilepsy showed a mild gain-of-function (defective fast inactivation, less increased firing). In contrast, all three variants causing generalized epilepsy induced a loss-of-function (reduced current amplitudes, depolarizing shift of steady-state activation, reduced neuronal firing). Functional effects were known for 170 individuals. All 136 individuals carrying a functionally tested gain-of-function variant had either focal (n = 97, Groups 1-3) or unclassifiable (n = 39) epilepsy, whereas 34 individuals with a loss-of-function variant had either generalized (n = 14), no (n = 11) or unclassifiable (n = 6) epilepsy; only three had developmental and epileptic encephalopathy. Computational modelling in the gain-of-function group revealed a significant correlation between the severity of the electrophysiological and clinical phenotypes. Gain-of-function variant carriers responded significantly better to sodium channel blockers than to other anti-seizure medications, and the same applied for all individuals in Groups 1-3. In conclusion, our data reveal clear genotype-phenotype correlations between age at seizure onset, type of epilepsy and gain- or loss-of-function effects of SCN8A variants. Generalized epilepsy with absence seizures is the main epilepsy phenotype of loss-of-function variant carriers and the extent of the electrophysiological dysfunction of the gain-of-function variants is a main determinant of the severity of the clinical phenotype in focal epilepsies. Our pharmacological data indicate that sodium channel blockers present a treatment option in SCN8A-related focal epilepsy with onset in the first year of life.

KETASER01 protocol: What went right and what went wrong

Objective

To discuss the results of the KETASER01 trial and the reasons for its failure, particularly in view of future studies.

Methods

KETASER01 is a multicenter, randomized, controlled, open‐label, sequentially designed, non‐profit Italian study that aimed to assess the efficacy of ketamine compared with conventional anesthetics in the treatment of refractory convulsive status epilepticus (RCSE) in children.

Results

During the 5‐year recruitment phase, a total of 76 RCSEs treated with third‐line therapy were observed in five of the 10 participating Centers; only 10 individuals (five for each study arm; five females, mean age 6.5 ± 6.3 years) were enrolled in the KETASER01 study. Two of the five patients (40%) in the experimental arm were successfully treated with ketamine and two of the five (40%) children in the control arm, where successfully treated with thiopental. In the remaining six (60%) enrolled patients, RCSE was not controlled by the randomized anesthetic(s).

Significance

The KETASER01 study was prematurely halted due to low eligibility of patients and no successful recruitment. No conclusions can be drawn regarding the objectives of the study. Here, we discuss the KETASER01 results and critically analyze the reasons for its failure in view of future trials.

Voltage-gated sodium channel scn8a is required for innervation and regeneration of amputated adult zebrafish fins

Teleost fishes and urodele amphibians can regenerate amputated appendages, whereas this ability is restricted to digit tips in adult mammals. One key component of appendage regeneration is reinnervation of the wound area. However, how innervation is regulated in injured appendages of adult vertebrates has seen limited research attention. From a forward genetics screen for temperature-sensitive defects in zebrafish fin regeneration, we identified a mutation that disrupted regeneration while also inducing paralysis at the restrictive temperature. Genetic mapping and complementation tests identify a mutation in the major neuronal voltage-gated sodium channel (VGSC) gene scn8ab. Conditional disruption of scn8ab impairs early regenerative events, including blastema formation, but does not affect morphogenesis of established regenerates. Whereas scn8ab mutations reduced neural activity as expected, they also disrupted axon regrowth and patterning in fin regenerates, resulting in hypoinnervation. Our findings indicate that the activity of VGSCs plays a proregenerative role by promoting innervation of appendage stumps.

CDKL5 deficiency disorder: clinical features, diagnosis, and management

CDKL5 deficiency disorder (CDD) was first identified as a cause of human disease in 2004. Although initially considered a variant of Rett syndrome, CDD is now recognised as an independent disorder and classified as a developmental epileptic encephalopathy. It is characterised by early-onset (generally within the first 2 months of life) seizures that are usually refractory to polypharmacy. Development is severely impaired in patients with CDD, with only a quarter of girls and a smaller proportion of boys achieving independent walking; however, there is clinical variability, which is probably genetically determined. Gastrointestinal, sleep, and musculoskeletal problems are common in CDD, as in other developmental epileptic encephalopathies, but the prevalence of cerebral visual impairment appears higher in CDD. Clinicians diagnosing infants with CDD need to be familiar with the complexities of this disorder to provide appropriate counselling to the patients' families. Despite some benefit from ketogenic diets and vagal nerve stimulation, there has been little evidence that conventional antiseizure medications or their combinations are helpful in CDD, but further treatment trials are finally underway.

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".

Carbamazepine efficacy in a severe electro‐clinical presentation of SLC13A5 ‐epilepsy

Recessive mutations in the SLC13A5 gene encoding the sodium‐dependent citrate transporter are a recently identified cause of developmental and epileptic encephalopathy. Here, we describe a child harboring a novel homozygous loss‐of‐function mutation in the SLC13A5 gene (c.1496C>T–p.Ser499Phe) and exhibiting an unusual extremely severe neonatal presentation with drug‐resistant seizures and burst‐suppression EEG pattern. Early carbamazepine use resulted in dramatic improvement both clinically and on EEG features. Follow‐up from the neonatal period to the age of 4 years is documented. This case expands the electro‐clinical phenotype associated with SLC13A5‐related disease and confirms the efficacy and safety of carbamazepine in nonstructural early‐onset epilepsies.

ILAE classification and definition of epilepsy syndromes with onset in neonates and infants: Position statement by the ILAE Task Force on Nosology and Definitions

The International League Against Epilepsy (ILAE) Task Force on Nosology and Definitions proposes a classification and definition of epilepsy syndromes in the neonate and infant with seizure onset up to 2 years of age. The incidence of epilepsy is high in this age group and epilepsy is frequently associated with significant comorbidities and mortality. The licensing of syndrome specific antiseizure medications following randomized controlled trials and the development of precision, gene-related therapies are two of the drivers defining the electroclinical phenotypes of syndromes with onset in infancy. The principal aim of this proposal, consistent with the 2017 ILAE Classification of the Epilepsies, is to support epilepsy diagnosis and emphasize the importance of classifying epilepsy in an individual both by syndrome and etiology. For each syndrome, we report epidemiology, clinical course, seizure types, electroencephalography (EEG), neuroimaging, genetics, and differential diagnosis. Syndromes are separated into self-limited syndromes, where there is likely to be spontaneous remission and developmental and epileptic encephalopathies, diseases where there is developmental impairment related to both the underlying etiology independent of epileptiform activity and the epileptic encephalopathy. The emerging class of etiology-specific epilepsy syndromes, where there is a specific etiology for the epilepsy that is associated with a clearly defined, relatively uniform, and distinct clinical phenotype in most affected individuals as well as consistent EEG, neuroimaging, and/or genetic correlates, is presented. The number of etiology-defined syndromes will continue to increase, and these newly described syndromes will in time be incorporated into this classification. The tables summarize mandatory features, cautionary alerts, and exclusionary features for the common syndromes. Guidance is given on the criteria for syndrome diagnosis in resource-limited regions where laboratory confirmation, including EEG, MRI, and genetic testing, might not be available.

Nomenclature of Genetic Movement Disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force - An Update

In 2016, the Movement Disorder Society Task Force for the Nomenclature of Genetic Movement Disorders presented a new system for naming genetically determined movement disorders and provided a criterion-based list of confirmed monogenic movement disorders. Since then, a substantial number of novel disease-causing genes have been described, which warrant classification using this system. In addition, with this update, we further refined the system and propose dissolving the imaging-based categories of Primary Familial Brain Calcification and Neurodegeneration with Brain Iron Accumulation and reclassifying these genetic conditions according to their predominant phenotype. We also introduce the novel category of Mixed Movement Disorders (MxMD), which includes conditions linked to multiple equally prominent movement disorder phenotypes. In this article, we present updated lists of newly confirmed monogenic causes of movement disorders. We found a total of 89 different newly identified genes that warrant a prefix based on our criteria; 6 genes for parkinsonism, 21 for dystonia, 38 for dominant and recessive ataxia, 5 for chorea, 7 for myoclonus, 13 for spastic paraplegia, 3 for paroxysmal movement disorders, and 6 for mixed movement disorder phenotypes; 10 genes were linked to combined phenotypes and have been assigned two new prefixes. The updated lists represent a resource for clinicians and researchers alike and they have also been published on the website of the Task Force for the Nomenclature of Genetic Movement Disorders on the homepage of the International Parkinson and Movement Disorder Society (https://www.movementdisorders.org/MDS/About/Committees--Other-Groups/MDS-Task-Forces/Task-Force-on-Nomenclature-in-Movement-Disorders.htm). © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

New and emerging pharmacologic treatments for developmental and epileptic encephalopathies

PURPOSE OF REVIEW

Summarize evidence on Developmental and Epileptic Encephalopathies (DEEs) treatments focusing on new and emerging pharmacologic therapies (see Video, http://links.lww.com/CONR/A61, Supplementary Digital Content 1, which provides an overview of the review).

RECENT FINDINGS

Advances in the fields of molecular genetics and neurobiology have led to the recognition of underlying pathophysiologic mechanisms involved in an increasing number of DEEs that could be targeted with precision therapies or repurposed drugs, some of which are currently being evaluated in clinical trials. Prompt, optimal therapy is critical, and promising therapies approved or in clinical trials for tuberous sclerosis complex, Dravet and Lennox-Gastaut Syndromes including mammalian target of rapamycin inhibitors, selective membrane channel and antisense oligonucleotide modulation, and repurposed drugs such as fenfluramine, stiripentol and cannabidiol, among others, may improve seizure burden and neurological outcomes. There is an urgent need for collaborative efforts to evaluate the efficacy and safety of emerging DEEs therapies.

SUMMARY

Development of new therapies promise to address unmet needs for patients with DEEs, including improvement of neurocognitive function and quality of life.

Do All Roads Lead to Rome? Genes Causing Dravet Syndrome and Dravet Syndrome-Like Phenotypes

Background

Dravet syndrome (DS) is a severe epileptic encephalopathy mainly caused by haploinsufficiency of the gene SCN1A, which encodes the voltage-gated sodium channel Na_V1. 1 in the brain. While SCN1A mutations are known to be the primary cause of DS, other genes that may cause DS are poorly understood. Several genes with pathogenic mutations result in DS or DS-like phenotypes, which may require different drug treatment approaches. Therefore, it is urgent for clinicians, especially epilepsy specialists to fully understand these genes involved in DS in addition to SCN1A. Particularly for healthcare providers, a deep understanding of these pathogenic genes is useful in properly selecting and adjusting drugs in a more effective and timely manner.

Objective

The purpose of this study was to identify genes other than SCN1A that may also cause DS or DS-like phenotypes.

Methods

A comprehensive search of relevant Dravet syndrome and severe myoclonic epilepsy in infancy was performed in PubMed, until December 1, 2021. Two independent authors performed the screening for potentially eligible studies. Disagreements were decided by a third, more professional researcher or by all three. The results reported by each study were narratively summarized.

Results

A PubMed search yielded 5,064 items, and other sources search 12 records. A total of 29 studies published between 2009 and 2021 met the inclusion criteria. Regarding the included articles, seven studies on PCDH19, three on SCN2A, two on SCN8A, five on SCN1B, two on GABRA1, three on GABRB3, three on GABRG2, and three on STXBP1 were included. Only one study was recorded for CHD2, CPLX1, HCN1 and KCNA2, respectively. It is worth noting that a few articles reported on more than one epilepsy gene.

Conclusion

DS is not only identified in variants of SCN1A, but other genes such as PCDH19, SCN2A, SCN8A, SCN1B, GABRA1, GABRB3, GABRG2, KCNA2, CHD2, CPLX1, HCN1A, STXBP1 can also be involved in DS or DS-like phenotypes. As genetic testing becomes more widely available, more genes associated with DS and DS-like phenotypes may be identified and gene-based diagnosis of subtypes of phenotypes in this spectrum may improve the management of these diseases in the future.

A Review of Targeted Therapies for Monogenic Epilepsy Syndromes

Genetic sequencing technologies have led to an increase in the identification and characterization of monogenic epilepsy syndromes. This increase has, in turn, generated strong interest in developing “precision therapies” based on the unique molecular genetics of a given monogenic epilepsy syndrome. These therapies include diets, vitamins, cell-signaling regulators, ion channel modulators, repurposed medications, molecular chaperones, and gene therapies. In this review, we evaluate these therapies from the perspective of their clinical validity and discuss the future of these therapies for individual syndromes.

Cannabidiol Increases Seizure Resistance and Improves Behavior in an Scn8a Mouse Model

Voltage-gated sodium channel genes are an important family of human epilepsy genes. De novo missense mutations in SCN8A (encoding Na_v1.6) are associated with a spectrum of clinical presentation, including multiple seizure types, movement disorders, intellectual disability, and behavioral abnormalities such as autism. Patients with SCN8A mutations are often treated with multiple antiepileptic drugs, the most common being sodium channel blockers. Cannabidiol (CBD) has been included as a component of treatment regimens for some SCN8A patients; however, to date, there are no clinical trials that have evaluated the therapeutic potential of CBD in patients with SCN8A mutations. In the current manuscript, we demonstrated a dose-dependent increase in seizure resistance following CBD treatment in mice expressing the human SCN8A mutation R1620L (RL/+). We also found that CBD treatment improved social behavior and reduced hyperactivity in the RL/+ mutants. Our findings suggest that CBD may be beneficial in patients with SCN8A-associated disease.

Analysis of Factors That May Affect the Effectiveness of Ketogenic Diet Treatment in Pediatric and Adolescent Patients

Purpose. The aim was to find predictors for ketogenic diet (KD) treatment effectiveness. In addition, recognized factors influencing the efficacy of KD were analyzed based on the ILAE (International League Against Epilepsy) proposed Classification and Definition of the Epilepsy Syndromes. Methods. A sample of 42 patients treated with KD were analyzed. The effectiveness of KD was assessed according to the type of diet, the type of seizures, and the known (KE) or undetermined genetic etiology (UNKE). The group of KE consisted of patients with CACNA1S, CHD2, DEPDC5, KIF1A, PIGN, SCN1A, SCN8A, SLC2A1, SYNGAP1 pathogenic variants. The usefulness of the new Classification and Definition of Epilepsy Syndromes proposed by the ILAE was evaluated. Results. KD therapy was effective in 69.05% of cases. No significant correlation was observed with the type of diet used. KE was related to greater effectiveness after KD treatment. KD treatment was most effective in the reduction of non-focal seizures. Considering the ILAE proposed classification, it was found that KD efficacy was higher in patients with simultaneous focal and tonic-clonic seizures compared to patients with only tonic-clonic or focal seizures. Conclusion. The occurrence of focal seizures does not determine the potential ineffectiveness of treatment with a ketogenic diet. A significant efficacy of ketogenic diet treatment was observed in the group of patients with focal and generalized seizures, as well as epileptic and developmental encephalopathies. The etiology of epileptic seizures plays a more significant role. The new classification will make it easier to select patients who can benefit from this form of treatment.

Sodium channel expression and transcript variation in the developing brain of human, Rhesus monkey, and mouse

Genetic variation in voltage-gated sodium (Na_V) channels is a significant contributor to neurodevelopmental disorders. Na_V channel alpha subunits are encoded by the SCNxA family and four are predominately expressed in the brain: SCN1A, SCN2A, SCN3A, and SCN8A. Gene expression is developmentally regulated, and they are known to express functionally distinct transcript variants. Precision therapies targeting these genes and their transcript variants are currently in preclinical development, yet the developmental expression of these transcripts in the human brain is yet to be fully understood. Additionally, the functional consequences of some mutations differ depending on the studied channel isoform, suggesting differential transcript variant expression can affect disease prognoses. We characterise the expression of the four SCNxAs and their transcript variants in human, Rhesus monkey and mouse brain using publicly available RNA-sequencing data and analysis tools, demonstrating that this approach can be used to answer important biological questions of gene and transcript developmental regulation. We find that gene expression and transcript variant regulation are conserved across species at similar developmental stages and determine the developmental milestones for transcript variant expression. Our study provides a guide to researchers testing therapies and clinicians advising prognoses based on the expression of channel isoforms.

Sudden unexpected death in epilepsy: Respiratory mechanisms

Epilepsy is one of the most common chronic neurologic diseases, with a prevalence of 1% in the US population. Many people with epilepsy live normal lives, but are at risk of sudden unexpected death in epilepsy (SUDEP). This mysterious comorbidity of epilepsy causes premature death in 17%-50% of those with epilepsy. Most SUDEP occurs after a generalized seizure, and patients are typically found in bed in the prone position. Until recently, it was thought that SUDEP was due to cardiovascular failure, but patients who died while being monitored in hospital epilepsy units revealed that most SUDEP is due to postictal central apnea. Some cases may occur when seizures invade the amygdala and activate projections to the brainstem. Evidence suggests that the pathophysiology is linked to defects in the serotonin system and central CO₂ chemoreception, and that there is considerable overlap with mechanisms thought to be involved in sudden infant death syndrome (SIDS). Future work is needed to identify biomarkers for patients at highest risk, improve ascertainment, develop methods to alert caregivers when SUDEP is imminent, and find effective approaches to prevent these fatal events.

Phenotypic and genetic spectrum in Chinese children with SCN8A-related disorders

BACKGROUND

Pathogenic variants in SCN8A have been demonstrated with a wide spectrum of epilepsy phenotypes, ranging from benign infantile epilepsy (BIE), to early onset developmental and epileptic encephalopathy (DEE) with moderate to severe developmental delay. In order to provide further insight on the spectrum of SCN8A-related epilepsy, we aimed to explore the clinical and genetic phenotype in Chinese children.

METHODS

A cohort of fifty Chinese patients with SCN8A-related disorders was included in the retrospective study. Genetic and clinical features and treatment effect of patients were further assessed based on phenotype parameters. The pathogenicity of variants was classified using the next-generation sequencing variation study.

RESULTS

We found 50 patients who presented with severe developmental and epileptic encephalopathy (DEE, 70%), benign infantile epilepsy (BIE, 12%), developmental encephalopathy with epilepsy (16%), and severe developmental delay without epilepsy (2%). The seizure onset age ranged from 1 day to 1 year and 11 months. After anti-seizure treatment, 28% of patients obtained seizure control. Sodium channel blockers showed good prognosis in 26% of patients with severe DEE. Oxcarbazepine (OXC) monotherapy was obviously effective in patients with BIE and developmental encephalopathy with epilepsy, most importantly, 87.5% received the anti-seizure therapy with sodium channel blockers in combination. All the variants were de novo missense with exception of one splice site variant. We reported three new variants, Asn1887Ser, Ile1605Thr, and Met1869Thr, which were associated with SCN8A-BIE.

CONCLUSION

The phenotypic spectrum of SCN8A-related disorders in Chinese children ranged from severe developmental delay without epilepsy to severe DEE. Three new variants were associated with SCN8A-BIE. Sodium channel blockers were effective in treating seizures for some SCN8A-related disorders however may not be relevant to the mutant location.

Pathogenic in-Frame Variants in SCN8A: Expanding the Genetic Landscape of SCN8A-Associated Disease

Numerous SCN8A mutations have been identified, of which, the majority are de novo missense variants. Most mutations result in epileptic encephalopathy; however, some are associated with less severe phenotypes. Mouse models generated by knock-in of human missense SCN8A mutations exhibit seizures and a range of behavioral abnormalities. To date, there are only a few Scn8a mouse models with in-frame deletions or insertions, and notably, none of these mouse lines exhibit increased seizure susceptibility. In the current study, we report the generation and characterization of two Scn8a mouse models (ΔIRL/+ and ΔVIR/+) carrying overlapping in-frame deletions within the voltage sensor of domain 4 (DIVS4). Both mouse lines show increased seizure susceptibility and infrequent spontaneous seizures. We also describe two unrelated patients with the same in-frame SCN8A deletion in the DIV S5-S6 pore region, highlighting the clinical relevance of this class of mutations.

A SCN8A variant associated with severe early onset epilepsy and developmental delay: Loss- or gain-of-function?

SCN8A, encoding the voltage-gated sodium channel subunit Na_V1.6, has been associated with a wide spectrum of neuropsychiatric disorders. Missense variants in SCN8A which increase the channel activity can cause a severe developmental and epileptic encephalopathy (DEE). One DEE variant (p.(Arg223Gly)) was described to cause a predominant loss-of-function (LOF) mechanism when expressed in neuroblastoma cells, which is not consistent with the genotype-phenotype correlations in this gene. To resolve this discrepancy and understand the pathophysiological mechanism of this variant, we performed comprehensive electrophysiological studies in both neuroblastoma cells and primary hippocampal neuronal cultures. Although we also found that p.(Arg223Gly) significantly decreased Na⁺ current density and enhanced fast inactivation compared to the wild type (WT) channel in transfected neuroblastoma cells (both LOF mechanisms), it also caused a strong hyperpolarizing shift of steady-state activation and accelerated the recovery from fast inactivation (both gain-of-function (GOF) mechanisms). In cultured neurons transfected with mutant vs. WT Na_V1.6 channels, we found more depolarized resting membrane potentials and a decreased rheobase leading to enhanced action potential firing. We conclude that SCN8A p.(Arg223Gly) leads to a net GOF resulting in neuronal hyperexcitability and a higher firing rate, fitting with the central role of GOF mechanisms in DEE.

PURA-Related Developmental and Epileptic Encephalopathy: Phenotypic and Genotypic Spectrum

BACKGROUND AND OBJECTIVES

Purine-rich element-binding protein A (PURA) gene encodes Pur-α, a conserved protein essential for normal postnatal brain development. Recently, a PURA syndrome characterized by intellectual disability, hypotonia, epilepsy, and dysmorphic features was suggested. The aim of this study was to define and expand the phenotypic spectrum of PURA syndrome by collecting data, including EEG, from a large cohort of affected patients.

METHODS

Data on unpublished and published cases were collected through the PURA Syndrome Foundation and the literature. Data on clinical, genetic, neuroimaging, and neurophysiologic features were obtained.

RESULTS

A cohort of 142 patients was included. Characteristics of the PURA syndrome included neonatal hypotonia, feeding difficulties, and respiratory distress. Sixty percent of the patients developed epilepsy with myoclonic, generalized tonic-clonic, focal seizures, and/or epileptic spasms. EEG showed generalized, multifocal, or focal epileptic abnormalities. Lennox-Gastaut was the most common epilepsy syndrome. Drug refractoriness was common: 33.3% achieved seizure freedom. We found 97 pathogenic variants in PURA without any clear genotype-phenotype associations.

DISCUSSION

The PURA syndrome presents with a developmental and epileptic encephalopathy with characteristics recognizable from neonatal age, which should prompt genetic screening. Sixty percent have drug-resistant epilepsy with focal or generalized seizures. We collected more than 90 pathogenic variants without observing overt genotype-phenotype associations.

Approach to Neurological Channelopathies and Neurometabolic Disorders in Newborns

Ion channel disorders (channelopathies) can affect any organ system in newborns before 2 months of life, including the skeletal muscle and central nervous system. Channelopathies in newborns can manifest as seizure disorders, which is a critical issue as early onset seizures can mimic the presentation of neurometabolic disorders. Seizures in channelopathies can either be focal or generalized, and range in severity from benign to epileptic encephalopathies that may lead to developmental regression and eventually premature death. The presenting symptoms of channelopathies are challenging for clinicians to decipher, such that an extensive diagnostic survey through a precise step-by-step process is vital. Early diagnosis of a newborn's disease, either as a channelopathy or neurometabolic disorder, is important for the long-term neurodevelopment of the child.

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.

The epilepsy-autism spectrum disorder phenotype in the era of molecular genetics and precision therapy

Autism spectrum disorder (ASD) is frequently associated with infants with epileptic encephalopathy, and early interventions targeting social and cognitive deficits can have positive effects on developmental outcome. However, early diagnosis of ASD among infants with epilepsy is complicated by variability in clinical phenotypes. Commonality in both biological and molecular mechanisms have been suggested between ASD and epilepsy, such as occurs with tuberous sclerosis complex. This review summarizes the current understanding of causal mechanisms between epilepsy and ASD, with a particularly genetic focus. Hypothetical explanations to support the conjugation of the two conditions include abnormalities in synaptic growth, imbalance in neuronal excitation/inhibition, and abnormal synaptic plasticity. Investigation of the probable genetic basis has implemented many genes, although the main risk supports existing hypotheses in that these cluster to abnormalities in ion channels, synaptic function and structure, and transcription regulators, with the mammalian target of rapamycin (mTOR) pathway and "mTORpathies" having been a notable research focus. Experimental models not only have a crucial role in determining gene functions but are also useful instruments for tracing disease trajectory. Precision medicine from gene therapy remains a theoretical possibility, but more contemporary developments continue in molecular tests to aid earlier diagnoses and better therapeutic targeting.

Inherited Developmental and Epileptic Encephalopathies

Epileptic encephalopathies often have a genetic etiology. The epileptic activity itself exerts a direct detrimental effect on neurodevelopment, which may add to the cognitive impairment induced by the underlying mutation (“developmental and epileptic encephalopathy”). The focus of this review is on inherited syndromes. The phenotypes of genetic disorders affecting ion channels, metabolic signalling, membrane trafficking and exocytosis, cell adhesion, cell growth and proliferation are discussed. Red flags suggesting family of genes or even specific genes are highlighted. The knowledge of the phenotypical spectrum can indeed prompt the clinician to suspect specific etiologies, expediting the diagnosis.

Somatostatin-Positive Interneurons Contribute to Seizures in SCN8A Epileptic Encephalopathy

SCN8A epileptic encephalopathy is a devastating epilepsy syndrome caused by mutant SCN8A, which encodes the voltage-gated sodium channel Na_V1.6. To date, it is unclear if and how inhibitory interneurons, which express Na_V1.6, influence disease pathology. Using both sexes of a transgenic mouse model of SCN8A epileptic encephalopathy, we found that selective expression of the R1872W SCN8A mutation in somatostatin (SST) interneurons was sufficient to convey susceptibility to audiogenic seizures. Patch-clamp electrophysiology experiments revealed that SST interneurons from mutant mice were hyperexcitable but hypersensitive to action potential failure via depolarization block under normal and seizure-like conditions. Remarkably, GqDREADD-mediated activation of WT SST interneurons resulted in prolonged electrographic seizures and was accompanied by SST hyperexcitability and depolarization block. Aberrantly large persistent sodium currents, a hallmark of SCN8A mutations, were observed and were found to contribute directly to aberrant SST physiology in computational modeling and pharmacological experiments. These novel findings demonstrate a critical and previously unidentified contribution of SST interneurons to seizure generation not only in SCN8A epileptic encephalopathy, but epilepsy in general.SIGNIFICANCE STATEMENT SCN8A epileptic encephalopathy is a devastating neurological disorder that results from de novo mutations in the sodium channel isoform Na_v1.6. Inhibitory neurons express Na_V1.6, yet their contribution to seizure generation in SCN8A epileptic encephalopathy has not been determined. We show that mice expressing a human-derived SCN8A variant (R1872W) selectively in somatostatin (SST) interneurons have audiogenic seizures. Physiological recordings from SST interneurons show that SCN8A mutations lead to an elevated persistent sodium current which drives initial hyperexcitability, followed by premature action potential failure because of depolarization block. Furthermore, chemogenetic activation of WT SST interneurons leads to audiogenic seizure activity. These findings provide new insight into the importance of SST inhibitory interneurons in seizure initiation, not only in SCN8A epileptic encephalopathy, but for epilepsy broadly.

Autistic-like behavior, spontaneous seizures, and increased neuronal excitability in a Scn8a mouse model

Patients with SCN8A epileptic encephalopathy exhibit a range of clinical features, including multiple seizure types, movement disorders, and behavioral abnormalities, such as developmental delay, mild-to-severe intellectual disability, and autism. Recently, the de novo heterozygous SCN8A R1620L mutation was identified in an individual with autism, intellectual disability, and behavioral seizures without accompanying electrographic seizure activity. To date, the effects of SCN8A mutations that are primarily associated with behavioral abnormalities have not been studied in a mouse model. To better understand the phenotypic and functional consequences of the R1620L mutation, we used CRISPR/Cas9 technology to generate mice expressing the corresponding SCN8A amino acid substitution. Homozygous mutants exhibit tremors and a maximum lifespan of 22 days, while heterozygous mutants (RL/+) exhibit autistic-like behaviors, such as hyperactivity and learning and social deficits, increased seizure susceptibility, and spontaneous seizures. Current clamp analyses revealed a reduced threshold for firing action potentials in heterozygous CA3 pyramidal neurons and reduced firing frequency, suggesting that the R1620L mutation has both gain- and loss-of-function effects. In vivo calcium imaging using miniscopes in freely moving RL/+ mutants showed hyperexcitability of cortical excitatory neurons that is likely to increase seizure susceptibility. Finally, we found that oxcarbazepine and Huperzine A, a sodium channel blocker and reversible acetylcholinesterase inhibitor, respectively, were capable of conferring robust protection against induced seizures in RL/+ mutants. This mouse line will provide the opportunity to better understand the range of clinical phenotypes associated with SCN8A mutations and to develop new therapeutic approaches.

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.