Complex biophysical changes and reduced neuronal firing in an SCN8A variant associated with developmental delay and epilepsy

Mutations in the SCN8A gene, encoding the voltage-gated sodium channel NaV1.6, are associated with a range of neurodevelopmental syndromes. The p.(Gly1625Arg) (G1625R) mutation was identified in a patient diagnosed with developmental epileptic encephalopathy (DEE). While most of the characterized DEE-associated SCN8A mutations were shown to cause a gain-of-channel function, we show that the G1625R variant, positioned within the S4 segment of domain IV, results in complex effects. Voltage-clamp analyses of NaV1.6G1625R demonstrated a mixture of gain- and loss-of-function properties, including reduced current amplitudes, increased time constant of fast voltage-dependent inactivation, a depolarizing shift in the voltage dependence of activation and inactivation, and increased channel availability with high-frequency repeated depolarization. Current-clamp analyses in transfected cultured neurons revealed that these biophysical properties caused a marked reduction in the number of action potentials when firing was driven by the transfected mutant NaV1.6. Accordingly, computational modeling of mature cortical neurons demonstrated a mild decrease in neuronal firing when mimicking the patients' heterozygous SCN8A expression. Structural modeling of NaV1.6G1625R suggested the formation of a cation-π interaction between R1625 and F1588 within domain IV. Double-mutant cycle analysis revealed that this interaction affects the voltage dependence of inactivation in NaV1.6G1625R. Together, our studies demonstrate that the G1625R variant leads to a complex combination of gain and loss of function biophysical changes that result in an overall mild reduction in neuronal firing, related to the perturbed interaction network within the voltage sensor domain, necessitating personalized multi-tiered analysis for SCN8A mutations for optimal treatment selection.

Expanding the genotype-phenotype spectrum in SCN8A-related disorders

BACKGROUND

SCN8A-related disorders are a group of variable conditions caused by pathogenic variations in SCN8A. Online Mendelian Inheritance in Man (OMIM) terms them as developmental and epileptic encephalopathy 13, benign familial infantile seizures 5 or cognitive impairment with or without cerebellar ataxia.

METHODS

In this study, we describe clinical and genetic results on eight individuals from six families with SCN8A pathogenic variants identified via exome sequencing.

RESULTS

Clinical findings ranged from normal development with well-controlled epilepsy to significant developmental delay with treatment-resistant epilepsy. Three novel and three reported variants were observed in SCN8A. Electrophysiological analysis in transfected cells revealed a loss-of-function variant in Patient 4.

CONCLUSIONS

This work expands the clinical and genotypic spectrum of SCN8A-related disorders and provides electrophysiological results on a novel loss-of-function SCN8A variant.

Clinical and electrophysiological features of SCN8A variants causing episodic or chronic ataxia

BACKGROUND

Variants in SCN8A are associated with a spectrum of epilepsies and neurodevelopmental disorders. Ataxia as a predominant symptom of SCN8A variation has not been well studied. We set out to investigate disease mechanisms and genotype-phenotype correlations of SCN8A-related ataxia.

METHODS

We collected genetic and electro-clinical data of ten individuals from nine unrelated families carrying novel SCN8A variants associated with chronic progressive or episodic ataxia. Electrophysiological characterizations of these variants were performed in ND7/23 cells and cultured neurons.

FINDINGS

Variants associated with chronic progressive ataxia either decreased Na+ current densities and shifted activation curves towards more depolarized potentials (p.Asn995Asp, p.Lys1498Glu and p.Trp1266Cys) or resulted in a premature stop codon (p.Trp937Ter). Three variants (p.Arg847Gln and biallelic p.Arg191Trp/p.Asp1525Tyr) were associated with episodic ataxia causing loss-of-function by decreasing Na+ current densities or a hyperpolarizing shift of the inactivation curve. Two additional episodic ataxia-associated variants caused mixed gain- and loss-of function effects in ND7/23 cells and were further examined in primary murine hippocampal neuronal cultures. Neuronal firing in excitatory neurons was increased by p.Arg1629His, but decreased by p.Glu1201Lys. Neuronal firing in inhibitory neurons was decreased for both variants. No functional effect was observed for p.Arg1913Trp. In four individuals, treatment with sodium channel blockers exacerbated symptoms.

INTERPRETATION

We identified episodic or chronic ataxia as predominant phenotypes caused by variants in SCN8A. Genotype-phenotype correlations revealed a more pronounced loss-of-function effect for variants causing chronic ataxia. Sodium channel blockers should be avoided under these conditions.

FUNDING

BMBF, DFG, the Italian Ministry of Health, University of Tuebingen.

Expanding the genotype-phenotype spectrum in SCN8A-related disorders

Background

SCN8A-related disorders are a group of variable conditions caused by pathogenic variations in SCN8A. Online Mendelian Inheritance in Man (OMIM) terms them as developmental and epileptic encephalopathy 13, benign familial infantile seizures 5 or cognitive impairment with or without cerebellar ataxia.

Methods

In this study, we describe clinical and genetic results on eight individuals from six families with SCN8A pathogenic variants identified via exome sequencing.

Results

Clinical findings ranged from normal development with well-controlled epilepsy to significant developmental delay with treatment-resistant epilepsy. Three novel and three reported variants were observed in SCN8A. Electrophysiological analysis in transfected cells revealed a loss-of-function variant in Patient 4.

Conclusions

This work expands the clinical and genotypic spectrum of SCN8A-related disorders and provides electrophysiological results on a novel loss-of-function SCN8A variant.

A novel SCN8A variant of unknown significance in pediatric epilepsy: a case report

Variants in SCN8A are associated with several diseases, including developmental and epileptic encephalopathy, intermediate epilepsy or mild-to-moderate developmental and epileptic encephalopathy, self-limited familial infantile epilepsy, neurodevelopmental delays with generalized epilepsy, neurodevelopmental disorder without epilepsy, hypotonia, and movement disorders. Herein, we report an 8-year-old Moroccan boy with intermediate epilepsy of unknown origin, intellectual disability, autism spectrum disorder, and hyperactivity. The patient presented a normal 46, XY karyotype and a normal comparative genomic hybridization profile. Whole-exome sequencing was performed, and heterozygous variants were identified in KCNK4 and SCN8A. The SCN8A variant [c.4499C > T (p.Pro1500Leu)] was also detected in the healthy mother and was classified as a variant of uncertain clinical significance. This variant occurs in a highly conserved domain, which may affect the function of the encoded protein. More studies are needed to confirm the pathogenicity of this novel variant to establish the effective care, management, and genetic counselling of affected individuals.

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.

Genetic analysis of suicide: a sample study in Tuscany (Central Italy)

Many studies have examined the genetic contribution to suicide. However, data on suicide in the Italian population are scarce. We therefore aimed to address this gap by investigating a cohort of 111 Italians for whom a verdict of suicide had been declared in court in Florence, Italy between 2007 and 2017. This cohort included 86 men and 25 women. DNA samples were obtained from tissues or blood, and 22 genes from multiple neurobiological pathways previously shown to be associated with the pathogenesis of suicide were analysed. Next-generation sequencing was used to compare these gene sequences with those from a large, normal population. In this study, we identified 19 gene variants that were present at significantly lower frequencies in our Italian cohort than in the general population. In addition, four missense mutations were identified in four different genes: Monoamine Oxidase A (MAOA), 5-Hydroxytryptamine Receptor 2 A (HTR2A), Sodium Voltage-Gated Channel Alpha Subunit 8 (SCN8A), and Nitric Oxide Synthase 3 (NOS3). Our study identified several potential genetic links with suicide in a cohort of Italians and supports a relationship between specific genetic variants and suicidal behaviour in this population.

Gene variant effects across sodium channelopathies predict function and guide precision therapy

Pathogenic variants in the voltage-gated sodium channel gene family lead to early onset epilepsies, neurodevelopmental disorders, skeletal muscle channelopathies, peripheral neuropathies and cardiac arrhythmias. Disease-associated variants have diverse functional effects ranging from complete loss-of-function to marked gain-of-function. Therapeutic strategy is likely to depend on functional effect. Experimental studies offer important insights into channel function but are resource intensive and only performed in a minority of cases. Given the evolutionarily conserved nature of the sodium channel genes, we investigated whether similarities in biophysical properties between different voltage-gated sodium channels can predict function and inform precision treatment across sodium channelopathies. We performed a systematic literature search identifying functionally assessed variants in any of the nine voltage-gated sodium channel genes until 28 April 2021. We included missense variants that had been electrophysiologically characterized in mammalian cells in whole-cell patch-clamp recordings. We performed an alignment of linear protein sequences of all sodium channel genes and correlated variants by their overall functional effect on biophysical properties. Of 951 identified records, 437 sodium channel-variants met our inclusion criteria and were reviewed for functional properties. Of these, 141 variants were epilepsy-associated (SCN1/2/3/8A), 79 had a neuromuscular phenotype (SCN4/9/10/11A), 149 were associated with a cardiac phenotype (SCN5/10A) and 68 (16%) were considered benign. We detected 38 missense variant pairs with an identical disease-associated variant in a different sodium channel gene. Thirty-five out of 38 of those pairs resulted in similar functional consequences, indicating up to 92% biophysical agreement between corresponding sodium channel variants (odds ratio = 11.3; 95% confidence interval = 2.8 to 66.9; P < 0.001). Pathogenic missense variants were clustered in specific functional domains, whereas population variants were significantly more frequent across non-conserved domains (odds ratio = 18.6; 95% confidence interval = 10.9-34.4; P < 0.001). Pore-loop regions were frequently associated with loss-of-function variants, whereas inactivation sites were associated with gain-of-function (odds ratio = 42.1, 95% confidence interval = 14.5-122.4; P < 0.001), whilst variants occurring in voltage-sensing regions comprised a range of gain- and loss-of-function effects. Our findings suggest that biophysical characterisation of variants in one SCN-gene can predict channel function across different SCN-genes where experimental data are not available. The collected data represent the first gain- versus loss-of-function topological map of SCN proteins indicating shared patterns of biophysical effects aiding variant analysis and guiding precision therapy. We integrated our findings into a free online webtool to facilitate functional sodium channel gene variant interpretation (http://SCN-viewer.broadinstitute.org).

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.

Case report: A novel de novo variant of SCN8A in a child with benign convulsions with mild gastroenteritis

Benign convulsions with mild gastroenteritis (CwG) is characterized by afebrile convulsions accompanied by mild gastroenteritis, and it can be considered after central nervous system infection, hypoglycemia, electrolyte disturbance, and moderate and severe dehydration are excluded. Previous studies have suggested that genetics may be involved in CWG. Herein, we reported a novel de novo variant of SCN8A in a child with CwG. This is the first report that SCN8A may be associated with CwG. Our report may provides evidence for the genetic etiology of CwG and expands the phenotypic and genetic spectrum of SCN8A-related disorders, which previously included severe developmental and epileptic encephalopathy (DEE) phenotype, benign epilepsy phenotype, spectrum of intermediate epilepsies, and patients with cognitive and/or behavioral disturbances without epilepsy. Phenotype of CwG has a good prognosis, and it does not require long-term antiepileptic therapy. Overtreatment should be avoided clinically. However, the conclusion needs to be further defined by long-term follow-up and similar clinical reports. In spite of this, our clinical observation provides possible evidence for future studies on the relationship between SCN8A and CwG.

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.

Modulating effects of FGF12 variants on Na(V)1.2 and Na(V)1.6 being associated with developmental and epileptic encephalopathy and Autism spectrum disorder: A case series

OBJECTIVE

Fibroblast Growth Factor 12 (FGF12) may represent an important modulator of neuronal network activity and has been associated with developmental and epileptic encephalopathy (DEE). We sought to identify the underlying pathomechanism of FGF12-related disorders.

METHODS

Patients with pathogenic variants in FGF12 were identified through published case reports, GeneMatcher and whole exome sequencing of own case collections. The functional consequences of two missense and two copy number variants (CNVs) were studied by co-expression of wildtype and mutant FGF12 in neuronal-like cells (ND7/23) with the sodium channels Na_V1.2 or Na_V1.6, including their beta-1 and beta-2 sodium channel subunits (SCN1B and SCN2B).

RESULTS

Four variants in FGF12 were identified for functional analysis: one novel FGF12 variant in a patient with autism spectrum disorder and three variants from previously published patients affected by DEE. We demonstrate the differential regulating effects of wildtype and mutant FGF12 on Na_V1.2 and Na_V1.6 channels. Here, FGF12 variants lead to a complex kinetic influence on Na_V1.2 and Na_V1.6, including loss- as well as gain-of function changes in fast and slow inactivation.

INTERPRETATION

We could demonstrate the detailed regulating effect of FGF12 on Na_V1.2 and Na_V1.6 and confirmed the complex effect of FGF12 on neuronal network activity. Our findings expand the phenotypic spectrum related to FGF12 variants and elucidate the underlying pathomechanism. Specific variants in FGF12-associated disorders may be amenable to precision treatment with sodium channel blockers.

FUNDING

DFG, BMBF, Hartwell Foundation, National Institute for Neurological Disorders and Stroke, IDDRC, ENGIN, NIH, ITMAT, ILAE, RES and GRIN.

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.

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.

Social Deficits and Cerebellar Degeneration in Purkinje Cell Scn8a Knockout Mice

Mutations in the SCN8A gene encoding the voltage-gated sodium channel α-subunit Nav1. 6 have been reported in individuals with epilepsy, intellectual disability and features of autism spectrum disorder. SCN8A is widely expressed in the central nervous system, including the cerebellum. Cerebellar dysfunction has been implicated in autism spectrum disorder. We investigated conditional Scn8a knockout mice under C57BL/6J strain background that specifically lack Scn8a expression in cerebellar Purkinje cells (Scn8a ^flox/flox , L7Cre ⁺ mice). Cerebellar morphology was analyzed by immunohistochemistry and MR imaging. Mice were subjected to a battery of behavioral tests including the accelerating rotarod, open field, elevated plus maze, light-dark transition box, three chambers, male-female interaction, social olfaction, and water T-maze tests. Patch clamp recordings were used to evaluate evoked action potentials in Purkinje cells. Behavioral phenotyping demonstrated that Scn8a ^flox/flox , L7Cre ⁺ mice have impaired social interaction, motor learning and reversal learning as well as increased repetitive behavior and anxiety-like behaviors. By 5 months of age, Scn8a ^flox/flox , L7Cre ⁺ mice began to exhibit cerebellar Purkinje cell loss and reduced molecular thickness. At 9 months of age, Scn8a ^flox/flox , L7Cre ⁺ mice exhibited decreased cerebellar size and a reduced number of cerebellar Purkinje cells more profoundly, with evidence of additional neurodegeneration in the molecular layer and deep cerebellar nuclei. Purkinje cells in Scn8a ^flox/flox , L7Cre ⁺ mice exhibited reduced repetitive firing. Taken together, our experiments indicated that loss of Scn8a expression in cerebellar Purkinje cells leads to cerebellar degeneration and several ASD-related behaviors. Our study demonstrated the specific contribution of loss of Scn8a in cerebellar Purkinje cells to behavioral deficits characteristic of ASD. However, it should be noted that our observed effects reported here are specific to the C57BL/6 genome type.

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.

De novo SCN8A and inherited rare CACNA1H variants associated with severe developmental and epileptic encephalopathy

Developmental and epileptic encephalopathies (DEEs) are a group of severe epilepsies that are characterized by seizures and developmental delay. DEEs are primarily attributed to genetic causes and an increasing number of cases have been correlated with variants in ion channel genes. In this study, we report a child with an early severe DEE. Whole exome sequencing showed a de novo heterozygous variant (c.4873–4881 duplication) in the SCN8A gene and an inherited heterozygous variant (c.952G > A) in the CACNA1H gene encoding for Na_v1.6 voltage-gated sodium and Ca_v3.2 voltage-gated calcium channels, respectively. In vitro functional analysis of human Na_v1.6 and Ca_v3.2 channel variants revealed mild but significant alterations of their gating properties that were in general consistent with a gain- and loss-of-channel function, respectively. Although additional studies will be required to confirm the actual pathogenic involvement of SCN8A and CACNA1H, these findings add to the notion that rare ion channel variants may contribute to the etiology of DEEs.

Antisense Oligonucleotide Therapy for Neurodevelopmental Disorders

Antisense oligonucleotides (ASOs) are short oligonucleotides that can modify gene expression and mRNA splicing in the nervous system. The FDA has approved ASOs for treatment of ten genetic disorders, with many applications currently in the pipeline. We describe the molecular mechanisms of ASO treatment for four neurodevelopmental and neuromuscular disorders. The ASO nusinersen is a general treatment for mutations of SMN1 in spinal muscular atrophy that corrects the splicing defect in the SMN2 gene. Milasen is a patient-specific ASO that rescues splicing of CNL7 in Batten's disease. STK-001 is an ASO that increases expression of the sodium channel gene SCN1A by exclusion of a poison exon. An ASO that reduces the abundance of the SCN8A mRNA is therapeutic in mouse models of developmental and epileptic encephalopathy. These examples demonstrate the variety of mechanisms and range of applications of ASOs for treatment of neurodevelopmental disorders.

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

INTRODUCTION

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

CASE REPORT

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

CONCLUSIONS

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

SCN8A Encephalopathy: Case Report and Literature Review

Epileptic encephalopathy is a condition resulting from extreme forms of intractable childhood epilepsy. The disease can cause severe delays in cognitive, sensory, and motor function development, in addition to being fatal in some cases. Missense mutations of SCN8A, which encodes Nav1.6, one of the main voltage-gated sodium channel subunits in neurons and muscles, have been linked to early infantile SCN8A encephalopathy. Herein, we report the case of a 5-month-old girl with SCN8A encephalopathy with a novel missense mutation. Apart from intractable seizures and autistic phenotypes, the results of blood and biochemical tests, electroencephalogram (EEG) results, and brain magnetic resonance imaging (MRI) results were all normal. As the phenotypes caused by these mutations cannot be identified by any clinical, neuroimaging, or electrophysiological features, genetic sequencing should be considered to identify the underlying genetic causes. Although phenytoin is recommended as a last-resort treatment for SCN8A encephalopathy, the administration of the oxcarbazepine, instead of phenytoin, mitigated this patient's intractable seizures.

Alterations of functional connectivity density in a Chinese family with a mild phenotype associated with a novel inherited variant of SCN8A

OBJECTIVE

Only a few heritable SCN8A variants have been described in patients with a mild phenotype of epilepsy. Here, we describe a Chinese family with a novel inherited SCN8A variant and investigate changes in spontaneous cerebral activity during the resting-state in magnetic resonance imaging (MRI)-negative patients with epilepsy and their unaffected siblings.

METHODS

A gene panel targeting 535 epilepsy genes was performed on the proband and his parents. The identified variant was confirmed in other affected members by Sanger sequencing. Resting-state functional MRI (fMRI) data were gathered from the family (4 affected individuals and 3 unaffected siblings) and 72 healthy controls (HCs). Functional connectivity density (FCD) was used to assess whether distant or local functional network changes occurred in patients with epilepsy.

RESULTS

A heterozygous missense variant (c.4568C>A; p.A1523D) in SCN8A was identified in the Chinese family, with a total of 7 members who presented with a mild phenotype (childhood seizures and normal cognition). All patients remained seizure-free, and one patient remained seizure-free without medication. Increased FCD values in the thalamocortical network and basal ganglia network were observed in both patients with epilepsy and their unaffected siblings compared with the HCs. Direct comparison between SCN8A variant patients and unaffected siblings showed that more serious and distributed abnormal changes occurred in the mesial frontal regions of patients with epilepsy.

CONCLUSIONS

We identified a novel SCN8A variant with a mild familial epilepsy phenotype. A similar pattern of FCD alterations in patients and their unaffected siblings might represent an endophenotype of benign epilepsy associated with the SCN8A inherited variant, and more extensive alterations in mesial frontal regions may help us to further understand the pathogenesis of SCN8A-related mild epilepsy.

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

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

Neuropsychological profiles of two patients with differing SCN8A-pathogenic variants

The impact of gene-related early infancy onset epilepsies in cognitive development can be potentially devastating. Here we report two cases of SCN8A-related epilepsy that highlight the neuropsychological heterogeneity seen with differing de-novo pathogenic variants. Case 1 is a 6-year-old right-handed girl who presented with SCN8A-developmental and epileptic encephalopathy (SCN8A-DEE) and a missense pathogenic variant (c.802A > C), not previously documented in the literature. Her history includes speech and motor delay, with focal motor seizures starting at 4-months. Early EEG showed bilateral centroparietal epileptiform discharges. She shows motor and language delays and prominent motor tics. Testing documented Intellectual Disability (ID) (Mild) with widespread neuropsychological deficits (i.e., academics, attention/executive functions, memory, visual-spatial skills, fine motor, language). Case 2 is an 8-year-old right-handed girl who presented with SCN8A-related epilepsy with c.5630A > G pathogenic variant with seizure onset at 5-months. Her initial EEG showed right occipital spikes. She shows low average intellect and average academics, but evaluation documented attention deficits, fine motor delays, and behavioral issues in addition to tics; she was diagnosed with Attention-Deficit/Hyperactivity Disorder, Oppositional Defiant Disorder, Obsessive Compulsive Disorder, and Tourette's. These cases expand limited knowledge regarding neuropsychological functioning of children with SCN8A-related epilepsy with unique de-novo pathogenic variants. While SCN8A-DEE is clearly associated with ID, other pathogenic variants may show better preserved intellect, despite other neuropsychological and behavioral concerns.

SCN8A encephalopathy: Mechanisms and models

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

Differential excitatory vs inhibitory SCN expression at single cell level regulates brain sodium channel function in neurodevelopmental disorders

The four voltage-gated sodium channels SCN1/2/3/8A have been associated with heterogeneous types of developmental disorders, each presenting with disease specific temporal and cell type specific gene expression. Using single-cell RNA sequencing transcriptomic data from humans and mice, we observe that SCN1A is predominantly expressed in inhibitory neurons. In contrast, SCN2/3/8A are profoundly expressed in excitatory neurons with SCN2/3A starting prenatally, followed by SCN1/8A neonatally. In contrast to previous observations from low resolution RNA screens, we observe that all four genes are expressed in both excitatory and inhibitory neurons, however, exhibit differential expression strength. These findings provide molecular evidence, at single-cell resolution, to support the hypothesis that the excitatory/inhibitory (E/I) neuronal expression ratios of sodium channels are important regulatory mechanisms underlying brain homeostasis and neurological diseases. Modulating the E/I expression balance within cell types of sodium channels could serve as a potential strategy to develop targeted treatment for Na_V-associated neuronal developmental disorders.

Distinct functional alterations in SCN8A epilepsy mutant channels

KEY POINTS

Mutations in the SCN8A gene cause early infantile epileptic encephalopathy. We characterize a new epilepsy-related SCN8A mutation, R850Q, in the human SCN8A channel and present gain-of-function properties of the mutant channel. Systematic comparison of R850Q with three other SCN8A epilepsy mutations, T761I, R1617Q and R1872Q, identifies one common dysfunction in resurgent current, although these mutations alter distinct properties of the channel. Computational simulations in two different neuron models predict an increased excitability of neurons carrying these mutations, which explains the over-excitation that underlies seizure activities in patients. These data provide further insight into the mechanism of SCN8A-related epilepsy and reveal subtle but potentially important distinction of functional characterization performed in the human vs. rodent channels.

ABSTRACT

SCN8A is a novel causal gene for early infantile epileptic encephalopathy. It is well accepted that gain-of-function mutations in SCN8A underlie the disorder, although the remarkable heterogeneity of its clinical presentation and poor treatment response demand a better understanding of the disease mechanisms. Here, we characterize a new epilepsy-related SCN8A mutation, R850Q, in human Nav1.6. We show that it is a gain-of-function mutation, with a hyperpolarizing shift in voltage dependence of activation, a two-fold increase of persistent current and a slowed decay of resurgent current. We systematically compare its biophysics with three other SCN8A epilepsy mutations, T767I, R1617Q and R1872Q, in the human Nav1.6 channel. Although all of these mutations are gain-of-function, the mutations affect different aspects of channel properties. One commonality that we discovered is an alteration of resurgent current kinetics, although the mechanisms by which resurgent currents are augmented remain unclear for all of the mutations. Computational simulations predict an increased excitability of neurons carrying these mutations with differential enhancement by open channel blockade.

Recent advances in treatment of epilepsy-related sodium channelopathies

Voltage-gated sodium channels (VGSCs) play a crucial role in generation of action potentials. Pathogenic variants in the five human brain expressed VGSC genes, SCN1A, SCN2A, SCN3A, SCN8A and SCN1B have been associated with a spectrum of epilepsy phenotypes and neurodevelopmental disorders. In the last decade, next generation sequencing techniques have revolutionized the way we diagnose these channelopathies, which is paving the way towards precision medicine. Knowing the functional effect (Loss-of-function versus Gain-of-function) of a variant is not only important for understanding the underlying pathophysiology, but it is particularly crucial to orient therapeutic decisions. Here we provide a review of the literature dealing with treatment options in epilepsy-related sodium channelopathies, including the current and emerging medications.

A multi-disciplinary clinic for SCN8A-related epilepsy

OBJECTIVE

We endeavored to evaluate a cohort of patients diagnosed with SCN8A-related epilepsy in a multi-disciplinary clinic and to create a bio-repository.

METHODS

We recruited patients with epilepsy due to SCN8A variants at Children's National Medical Center, through family organizations, or SCN8A.net. Study procedures included medical record review, review of EEG and MRI data, clinical evaluation, the Vineland Adaptive Behavior Scales, Third Edition (VABS-3), DNA extraction, and preparation of peripheral blood mononuclear cells.

RESULTS

Seventeen patients (9 months - 19 years) completed the study. Age at seizure onset was 1 day to 4 years old (median age 4 months). Epilepsy phenotype ranged from mild epilepsy to severe developmental and epileptic encephalopathy. Medications targeting the voltage-gated sodium channel were most often effective, while levetiracetam resulted in worsening seizures and/or developmental regression in 7/16 (p < 0.05). VABS-3 scores were below age expectations for most children; older children had lower scores. Neurological examination revealed hypotonia (13), spastic quadriparesis (1), ataxia (9), dyskinesia (2)/ dystonia (7), and four non-ambulatory.

CONCLUSIONS

This is the first report of a large series of patients with epilepsy due to SCN8A variants evaluated in a single multi-disciplinary clinic. By utilizing a more comprehensive and consistent evaluation, we clarify specific seizure and epilepsy types, describe a distinct epilepsy phenotype in a patient with a nonsense variant, delineate patterns of developmental delay, language, and swallow function (specifically anomic aphasia and flaccid dysarthria), identify and characterize movement disorders, report common findings on physical exam, and demonstrate clinical worsening with levetiracetam.

A de novo SCN8A heterozygous mutation in a child with epileptic encephalopathy: a case report

Background

Epilepsy is a complex disorder caused by various factors, including genetic aberrance. Recent studies have identified an essential role of the sodium channel Nav1.6, encoded by the gene SCN8A, in epileptic encephalopathy.

Case presentation

Using parent-offspring trio targeted-exome sequencing, we identified a de novo heterozygous missense mutation c.3953A > G (p.N1318S) in SCN8A in a 3-year-and-9-month Chinese female patient with early infantile epileptic encephalopathy and a normal magnetic resonance imaging of the brain.

Conclusions

This de novo mutation was only detected in the patient but not in her parents. Bioinformatic analysis indicates the pathogenicity of this mutation. Administration of the sodium channel blocker well controlled seizures in the patient. Therefore, we recommend trio targeted-exome sequencing as a routine method for pathogenic variant screening in patients with intractable epilepsy and a normal MRI.

Prax330 reduces persistent and resurgent sodium channel currents and neuronal hyperexcitability of subiculum neurons in a mouse model of SCN8A epileptic encephalopathy

SCN8A epileptic encephalopathy is a severe genetic epilepsy syndrome caused by de novo gain-of-function mutations of SCN8A encoding the voltage-gated sodium (Na) channel (VGSC) Na_V1.6. Therapeutic management is difficult in many patients, leading to uncontrolled seizures and risk of sudden unexpected death in epilepsy (SUDEP). There is a need to develop novel anticonvulsants that can specifically target aberrant VGSC activity associated with SCN8A gain-of-function mutations. In this study, we investigate the effects of Prax330, a novel VGSC inhibitor, on the biophysical properties of wild-type (WT) Na_V1.6 and the patient mutation p.Asn1768Asp (N1768D) in ND7/23 cells. The effects of Prax330 on persistent (I_NaP) and resurgent (I_NaR) Na currents and neuronal excitability in subiculum neurons from a knock-in mouse model of the Scn8a-N1768D mutation (Scn8a^D/+) were also examined. In ND7/23 cells, Prax330 reduced I_NaP currents recorded from cells expressing Scn8a-N1768D and hyperpolarized steady-state inactivation curves. Recordings from brain slices demonstrated elevated I_NaP and I_NaR in subiculum neurons from Scn8a^D/+ mutant mice and abnormally large action potential (AP) burst-firing events in a subset of neurons. Prax330 (1 μM) reduced both I_NaP and I_NaR and suppressed AP bursts, with a smaller effect on AP waveforms that had similar morphology to WT neurons. Prax330 (1 μM) also reduced synaptically-evoked APs in Scn8a^D/+ subiculum neurons but not in WT neurons. Our results highlight the efficacy of targeting I_NaP and I_NaR and inactivation parameters in controlling subiculum excitability and suggest Prax330 as a promising novel therapy for SCN8A epileptic encephalopathy.

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

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

De novo SCN1A, SCN8A, and CLCN2 mutations in childhood absence epilepsy

This study aimed to identify monogenic mutations from Chinese patients with childhood absence epilepsy (CAE) and summarize their characteristics. A total of 100 patients with CAE were recruited in Peking University First Hospital from 2005 to 2016 and underwent telephone and outpatient follow-up review. We used targeted disease-specific gene capture sequencing (involving 300 genes) to identify pathogenic variations for these patients. We identified three de novo epilepsy-related gene mutations, including missense mutations of SCN1A (c. 5399 T > A; p. Val1800Asp), SCN8A (c. 2371 G > T; p. Val791Phe), and CLCN2 (c. 481 G > A; p. Gly161Ser), from three patients, separately. All recruited patients presented typical CAE features and good prognosis. To date, CAE has been considered a complex disease caused by multiple susceptibility genes. In this study, we observed that 3% of typical CAE patients had a de novo mutation of a known monogenic epilepsy-related gene. Our study suggests that a significant proportion of typical CAE cases may be monogenic forms of epilepsy. For genetic generalized epilepsies, such as CAE, further studies are needed to clarify the contributions of de novo or inherited rare monogenic coding, noncoding and copy number variants.

Neonatal tremor episodes and hyperekplexia-like presentation at onset in a child with SCN8A developmental and epileptic encephalopathy

SCN8A encephalopathy is a newly defined epileptic encephalopathy caused by de novo mutations of the SCN8A gene. We report herein a four-year-old boy presenting with severe non-epileptic abnormal movements, of possibly antenatal onset, progressively associated with pharmacoresistant epilepsy and regression, associated with a de novo heterozygous missense mutation of SCN8A. This case shows that paroxysmal non-epileptic episodes of severe tremor and hyperekplexia-like startles and a striking vegetative component can be the first early symptoms of severe SCN8A developmental and epileptic encephalopathy. Clinicians should be aware of these symptoms in order to avoid misdiagnosis and ensure early appropriate therapeutic management. [Published with video sequences on www.epilepticdisorders.com].

12q13.12q13.13 microdeletion encompassing ACVRL1 and SCN8A genes: Clinical report of a new contiguous gene syndrome

Hereditary hemorrhagic telangiectasia is usually linked to the presence of a pathogenic mutation ACVRL1 or ENG. Thus, apparently there is no benefit to perform an array CGH in case of HHT. However, ENG has been involved in a contiguous gene syndrome due to a de novo 9q33.3q34.11 microdeletion. We describe here a new contiguous gene syndrome involving ACVRL1 gene. A 50-year-old female patient had a typical clinical presentation of hereditary hemorrhagic telangiectasia (HHT) with epistaxis, cutaneous-mucous telangiectases, arteriovenous malformation. She also presented a cognitive disability. Cognitive assessment showed a heterogeneous cognitive disorder predominating in the executive sphere without intellectual deficiency. She had no peculiar morphological feature. Neurological examination disclosed the presence of contralateral mirror movements during voluntary movement of each hand. A heterozygous deletion of the whole ACVRL1 gene (exons 1 to 10) was found to be responsible for the HHT features. To investigate further the dysexecutive syndrome and the mirror movements, we performed oligonucleotide array comparative genomic hybridization (array CGH) study (180K, Agilent, Santa-Clara, CA, USA). This study revealed a de novo 1.58 Mb deletion on chromosome 12q13.12q13.13 encompassing the ACVRL1 and SCN8A genes. To our knowledge, this deletion has not been previously reported and defines a new contiguous gene syndrome. The loss of one ACVRL1 allele is likely to be responsible for the HHT phenotype, while the deletion of the SCN8A gene is likely to be the cause of the mild cognitive disorder. SCN8A haploinsufficiency might also be involved in the occurrence of mirror movements. This report highlights the benefit of searching for large rearrangements in cases including unusual symptoms in association with HHT. On the other hand, an early diagnosis of 12q13.12q13.13 microdeletion based on the presence of a dysexecutive syndrome and/or mirror movement may allow to prevent HHT complications.

Whole-Exome Sequencing Implicates SCN2A in Episodic Ataxia, but Multiple Ion Channel Variants May Contribute to Phenotypic Complexity

Although the clinical use of targeted gene sequencing-based diagnostics is valuable, whole-exome sequencing has also emerged as a successful diagnostic tool in molecular genetics laboratories worldwide. Molecular genetic tests for episodic ataxia type 2 (EA2) usually target only the specific calcium channel gene (CACNA1A) that is known to cause EA2. In cases where no mutations are identified in the CACNA1A gene, it is important to identify the causal gene so that more effective treatment can be prioritized for patients. Here we present a case of a proband with a complex episodic ataxias (EA)/seizure phenotype with an EA-affected father; and an unaffected mother, all negative for CACNA1A gene mutations. The trio was studied by whole-exome sequencing to identify candidate genes responsible for causing the complex EA/seizure phenotype. Three rare or novel variants in Sodium channel α2-subunit; SCN2A (c.3973G>T: p.Val1325Phe), Potassium channel, Kv3.2; KCNC2 (c.1006T>C: p.Ser336Pro) and Sodium channel Nav1.6; SCN8A (c.3421C>A: p.Pro1141Thr) genes were found in the proband. While the SCN2A variant is likely to be causal for episodic ataxia, each variant may potentially contribute to the phenotypes observed in this family. This study highlights that a major challenge of using whole-exome/genome sequencing is the identification of the unique causative mutation that is associated with complex disease.

A comprehensive approach to identifying repurposed drugs to treat SCN8A epilepsy

OBJECTIVE

Many previous studies of drug repurposing have relied on literature review followed by evaluation of a limited number of candidate compounds. Here, we demonstrate the feasibility of a more comprehensive approach using high-throughput screening to identify inhibitors of a gain-of-function mutation in the SCN8A gene associated with severe pediatric epilepsy.

METHODS

We developed cellular models expressing wild-type or an R1872Q mutation in the Na_v 1.6 sodium channel encoded by SCN8A. Voltage clamp experiments in HEK-293 cells expressing the SCN8A R1872Q mutation demonstrated a leftward shift in sodium channel activation as well as delayed inactivation; both changes are consistent with a gain-of-function mutation. We next developed a fluorescence-based, sodium flux assay and used it to assess an extensive library of approved drugs, including a panel of antiepileptic drugs, for inhibitory activity in the mutated cell line. Lead candidates were evaluated in follow-on studies to generate concentration-response curves for inhibiting sodium influx. Select compounds of clinical interest were evaluated by electrophysiology to further characterize drug effects on wild-type and mutant sodium channel functions.

RESULTS

The screen identified 90 drugs that significantly inhibited sodium influx in the R1872Q cell line. Four drugs of potential clinical interest-amitriptyline, carvedilol, nilvadipine, and carbamazepine-were further investigated and demonstrated concentration-dependent inhibition of sodium channel currents.

SIGNIFICANCE

A comprehensive drug repurposing screen identified potential new candidates for the treatment of epilepsy caused by the R1872Q mutation in the SCN8A gene.

Neurotransmitters and Sodium Channelopathies; Possible Link?

Investigators from the University of British Columbia, Great Ormond Street Hospital for Children, and the National Hospital reported their findings on neurotransmitter deficiencies in two patients with mutations in voltage-gated sodium genes (SCN2A and SCN8A) discovered by whole exome sequencing.

Severe bone loss and multiple fractures in SCN8A-related epileptic encephalopathy

Mutations in the SCN8A gene encoding the neuronal voltage-gated sodium channel Nav1.6 are known to be associated with epileptic encephalopathy type 13. We identified a novel de novo SCN8A mutation (p.Phe360Ala, c.1078_1079delTTinsGC, Exon 9) in a 6-year-old girl with epileptic encephalopathy accompanied by severe juvenile osteoporosis and multiple skeletal fractures, similar to three previous case reports. Skeletal assessment using dual energy X-ray absorptiometry (DXA), high-resolution peripheral quantitative computed tomography (HR-pQCT) and serum analyses revealed a combined trabecular and cortical bone loss syndrome with elevated bone resorption. Likewise, when we analyzed the skeletal phenotype of 2week-old Scn8a-deficient mice we observed reduced trabecular and cortical bone mass, as well as increased osteoclast indices by histomorphometric quantification. Based on this cumulative evidence the patient was treated with neridronate (2mg/kg body weight administered every 3months), which fully prevented additional skeletal fractures for the next 25months. Taken together, our data provide evidence for a negative impact of SCN8A mutations on bone mass, which can be positively influenced by anti-resorptive treatment.

SCN8A mutations in Chinese patients with early onset epileptic encephalopathy and benign infantile seizures

Background

SCN8A mutations have recently been associated with epilepsy and neurodevelopmental disorders. This study aimed to broaden the phenotypic-spectrum of disease related with SCN8A mutations.

Methods

To identify the pathogenic gene of a Chinese family, in which six members suffered from epilepsy, whole-exome sequencing was performed. In addition, target next-generation sequencing (NGS) was performed on 178 sporadic patients, who had epilepsy of unknown etiology within 6 months after birth. A detailed clinical history was obtained.

Results

A heterozygous missense mutation of SCN8A was identified in the Chinese family. Six de novo mutations of SCN8A were detected in 6 sporadic patients with epilepsy. In the family, six members developed seizures within a few years after birth. Five of them had milder clinical performance, that they had normal cognition and developmental milestones, and seizure-free was achieved by mono-therapy. The other one affected member presented with refractory epilepsy and developmental regression. She died from sudden unexpected death in epilepsy (SUDEP) at 17-year-old. Clinical features of six sporadic patients with SCN8A mutations were diverse, ranging from severe epileptic encephalopathy to benign epilepsy with normal cognition. Seizures started at the mean age of 3.9 months (from 2 months to 6 months). Seizure-free was achieved in four of them by mono- or multi-antiepileptic drugs. Five of them demonstrated mild or severe psychomotor retardation, whereas the other one was normal in development and intelligence.

Conclusions

Our findings extend the spectrum of SCN8A mutations and the clinical features of patients with SCN8A mutations. The majority of SCN8A mutations were de novo, inherited mutations from the heterozygous parents can also occur. The phenotypic spectrum of SCN8A mutation varied largely. Most affected patients manifested as refractory epilepsy and severe intellectual disability, only a small number of patients presented with milder clinical patterns. Additionally, our study confirmed that the same mutation can lead to different phenotypes.

Electronic supplementary material

The online version of this article (10.1186/s12881-017-0460-1) contains supplementary material, which is available to authorized users.

Aberrant Sodium Channel Currents and Hyperexcitability of Medial Entorhinal Cortex Neurons in a Mouse Model of SCN8A Encephalopathy

SCN8A encephalopathy, or early infantile epileptic encephalopathy 13 (EIEE13), is caused predominantly by de novo gain-of-function mutations in the voltage-gated Na channel Na_v1.6. Affected individuals suffer from refractory seizures, developmental delay, cognitive disability, and elevated risk of sudden unexpected death in epilepsy (SUDEP). A knock-in mouse model carrying the patient mutation p.Asn1768Asp (N1768D) reproduces many features of the disorder, including spontaneous seizures and SUDEP. We used the mouse model to examine the effects of the mutation on layer II stellate neurons of the medial entorhinal cortex (mEC), which transmit excitatory input to the hippocampus. Heterozygous (Scn8a^D/+), homozygous (Scn8a^D/D)), and WT (Scn8a^+/+) littermates were compared at 3 weeks of age, the time of seizure onset for homozygous mice. Heterozygotes remain seizure free for another month. mEC layer II neurons of heterozygous and homozygous mice were hyperexcitable and generated long-lasting depolarizing potentials with bursts of action potentials after synaptic stimulation. Recording of Na currents revealed proexcitatory increases in persistent and resurgent currents and rightward shifts in inactivation parameters, leading to significant increases in the magnitude of window currents. The proexcitatory changes were more pronounced in homozygous mice than in heterozygotes, consistent with the earlier age of seizure onset in homozygotes. These studies demonstrate that the N1768D mutation increases the excitability of mEC layer II neurons by increasing persistent and resurgent Na currents and disrupting channel inactivation. The aberrant activities of mEC layer II neurons would provide excessive excitatory input to the hippocampus and contribute to hyperexcitability of hippocampal neurons in this model of SCN8A encephalopathy.SIGNIFICANCE STATEMENTSCN8A encephalopathy is a devastating neurological disorder that results from de novo mutations in the Na channel Na_v1.6. In addition to seizures, patients suffer from cognitive and developmental delays and are at high risk for sudden unexpected death in epilepsy (SUDEP). A mouse knock-in model expressing the patient mutation N1768D reproduces several pathological phenotypes, including spontaneous seizures and sudden death. We demonstrate that medial entorhinal cortex (mEC) neurons from the mouse model exhibit proexcitatory alterations in Na channel activity, some of which were not seen in hippocampal or cortical neurons, and resulting in neuronal hyperexcitability. Because mEC neurons regulate the activity of the hippocampus, which plays an important role in seizure onset, we propose that these profound changes in mEC neuron excitability associated with the gain-of-function mutation of Na_v1.6 may increase excitatory drive into the hippocampus, culminating in seizure activity and SUDEP.

Comparison and optimization of in silico algorithms for predicting the pathogenicity of sodium channel variants in epilepsy

OBJECTIVE

Variants in neuronal voltage-gated sodium channel α-subunits genes SCN1A, SCN2A, and SCN8A are common in early onset epileptic encephalopathies and other autosomal dominant childhood epilepsy syndromes. However, in clinical practice, missense variants are often classified as variants of uncertain significance when missense variants are identified but heritability cannot be determined. Genetic testing reports often include results of computational tests to estimate pathogenicity and the frequency of that variant in population-based databases. The objective of this work was to enhance clinicians' understanding of results by (1) determining how effectively computational algorithms predict epileptogenicity of sodium channel (SCN) missense variants; (2) optimizing their predictive capabilities; and (3) determining if epilepsy-associated SCN variants are present in population-based databases. This will help clinicians better understand the results of indeterminate SCN test results in people with epilepsy.

METHODS

Pathogenic, likely pathogenic, and benign variants in SCNs were identified using databases of sodium channel variants. Benign variants were also identified from population-based databases. Eight algorithms commonly used to predict pathogenicity were compared. In addition, logistic regression was used to determine if a combination of algorithms could better predict pathogenicity.

RESULTS

Based on American College of Medical Genetic Criteria, 440 variants were classified as pathogenic or likely pathogenic and 84 were classified as benign or likely benign. Twenty-eight variants previously associated with epilepsy were present in population-based gene databases. The output provided by most computational algorithms had a high sensitivity but low specificity with an accuracy of 0.52-0.77. Accuracy could be improved by adjusting the threshold for pathogenicity. Using this adjustment, the Mendelian Clinically Applicable Pathogenicity (M-CAP) algorithm had an accuracy of 0.90 and a combination of algorithms increased the accuracy to 0.92.

SIGNIFICANCE

Potentially pathogenic variants are present in population-based sources. Most computational algorithms overestimate pathogenicity; however, a weighted combination of several algorithms increased classification accuracy to >0.90.

Phenytoin as a last-resort treatment in SCN8A encephalopathy

SCN8A encodes Nav1.6, one of the main voltage‐gated sodium channel subunits in the brain, and SCN8A mutations lead to epileptic encephalopathy. Particular mutations render the mutant channel more susceptible to inhibition by phenytoin. Yet, the potentially severe side effects of phenytoin maintenance therapy, especially cognitive impairment, are undesirable in these already cognitively impaired patients. We describe a 5‐year‐old patient with SCN8A encephalopathy in whom phenytoin proved successful as emergency treatment to prevent clustering of seizures and status epilepticus, thus hospital stays. The ketogenic diet, levetiracetam, zonisamide, topiramate, and phenytoin maintenance therapy resulted in adverse reactions not previously documented in SCN8A encephalopathy.

De novo and inherited SCN8A epilepsy mutations detected by gene panel analysis

OBJECTIVES

To determine the incidence of pathogenic SCN8A variants in a cohort of epilepsy patients referred for clinical genetic testing. We also investigated the contribution of SCN8A to autism spectrum disorder, intellectual disability, and neuromuscular disorders in individuals referred for clinical genetic testing at the same testing laboratory.

METHODS

Sequence data from 275 epilepsy panels screened by Emory Genetics Laboratory were reviewed for variants in SCN8A. Two additional cases with variants in SCN8A were ascertained from other testing laboratories. Parental samples were tested for variant segregation and clinical histories were examined. SCN8A variants detected from gene panel analyses for autism spectrum disorder, intellectual disability, and neuromuscular disorders were also examined.

RESULTS

Five variants in SCN8A were identified in five individuals with epilepsy. Three variants were de novo, one was inherited from an affected parent, and one was inherited from an unaffected parent. Four of the individuals have epilepsy and developmental delay/intellectual disability. The remaining individual has a milder epilepsy presentation without cognitive impairment. We also identified an amino acid substitution at an evolutionarily conserved SCN8A residue in a patient who was screened on the autism spectrum disorder panel. Additionally, we examined the distribution of pathogenic SCN8A variants across the Na_v1.6 channel and identified four distinct clusters of variants. These clusters are primarily located in regions of the channel that are important for the kinetics of channel inactivation.

CONCLUSIONS

Variants in SCN8A may be responsible for a spectrum of epilepsies as well as other neurodevelopmental disorders without seizures. The predominant pathogenic mechanism appears to involve disruption of channel inactivation, leading to gain-of-function effects.

SCN8A Epileptic Encephalopathy: Detection of Fetal Seizures Guides Multidisciplinary Approach to Diagnosis and Treatment

BACKGROUND

SCN8A mutations are rare and cause a phenotypically heterogeneous early onset epilepsy known as early infantile epileptic encephalopathy type 13 (EIEE13, OMIM #614558). There are currently no clear genotype-phenotype correlations to help guide patient counseling and management.

PATIENT DESCRIPTION

We describe a patient with EIEE13 (de novo heterozygous pathogenic mutation in SCN8A - p.Ile240Val (ATT>GTT)) who presented prenatally with maternally reported intermittent, rhythmic movements that, when observed on ultrasound, were concerning for fetal seizures. Ultrasound also revealed abnormal developmental states. With maternal administration of levetiracetam, the rhythmic fetal movements stopped. After birth, the patient developed treatment-refractory multi-focal epilepsy confirmed by electroencephalogram. Neuroimaging revealed restricted diffusion in the superior cerebellar peduncles, a finding not reported previously in EIEE13.

CONCLUSION

This is the first report of EIEE13 associated with clinical prenatal-onset seizures. Ultrasonography can be useful for identifying fetal seizures, which may be treatable in utero. Ideally, the clinical approach to fetal seizures should involve a multidisciplinary team spanning the pre- and postnatal course to expedite early diagnosis and optimize management, as illustrated by this patient.

Autosomal dominant SCN8A mutation with an unusually mild phenotype

BACKGROUND

Mutations in SCN8A, coding for the voltage-gated sodium channel Nav 1.6, have been described in relation to infantile onset epilepsy with developmental delay and cognitive impairment, in particular early onset epileptic encephalopathy (EIEE) type 13.

CASE REPORT

Here we report an infant and his father with early onset focal epileptic seizures but without cognitive or neurological impairment in whom next generation sequence analysis identified a heterozygous mutation (c.5630A > G, p. (Asn1877Ser)) in the SCN8A gene. This mutation, confirmed by Sanger sequence analysis, affects a highly conserved amino acid and in silico tools predicts that it may be pathogenic. The reported infant has a normal developmental profile at 16-month follow-up. His father also had normal development and has no cognitive impairment at 42 years. This is the second known SCN8A mutation associated with a phenotype of benign familial infantile epilepsy. Good seizure control was achieved in our patients with sodium channel blockers.

CONCLUSION

Based on our proband and a recently described group of families with benign familial infantile epilepsy and SCN8A variant we suggest expanding testing to patients with infantile epilepsy and no cognitive impairment. In addition, the same SCN8A variant (c.5630A > G, p. (Asn1877Ser)) is also found in patients with epilepsy and developmental delay highlighting the phenotypic variability and the possible role of other protective genetic factors.

The SCN8A encephalopathy mutation p.Ile1327Val displays elevated sensitivity to the anticonvulsant phenytoin

OBJECTIVE

SCN8A encephalopathy (early infantile epileptic encephalopathy; EIEE13) is caused by gain-of-function mutations resulting in hyperactivity of the voltage-gated sodium channel Nav 1.6. The channel is concentrated at the axon initial segment (AIS) and is involved in establishing neuronal excitability. Clinical features of SCN8A encephalopathy include seizure onset between 0 and 18 months of age, intellectual disability, and developmental delay. Seizures are often refractory to treatment with standard antiepileptic drugs, and sudden unexpected death in epilepsy (SUDEP) has been reported in approximately 10% of patients. In a recent study, high doses of phenytoin were effective in four patients with SCN8A encephalopathy. In view of this observation, we have investigated the relationship between the functional effect of the SCN8A mutation p.Ile1327Val and its response to phenytoin.

METHODS

The mutation was introduced into the Scn8a cDNA by site-directed mutagenesis. Channel activity was characterized in transfected ND7/23 cells. The effects of phenytoin (100 μm) on mutant and wild-type (WT) channels were compared.

RESULTS

Channel activation parameters were shifted in a hyperpolarizing direction in the mutant channel, whereas inactivation parameters were shifted in a depolarizing direction, increasing Na channel window current. Macroscopic current decay was slowed in I1327V channels, indicating an impairment in the transition from open state to inactivated state. Channel deactivation was also delayed, allowing more channels to remain in the open state. Phenytoin (100 μm) resulted in hyperpolarized activation and inactivation curves as well as greater tonic block and use-dependent block of I1327V mutant channels relative to WT.

SIGNIFICANCE

SCN8A - I1327V is a gain-of-function mutation with altered features that are predicted to increase neuronal excitability and seizure susceptibility. Phenytoin is an effective inhibitor of the mutant channel and may be of use in treating patients with gain-of-function mutations of SCN8A.

SCN8A encephalopathy: Research progress and prospects

On April 21, 2015, the first SCN8A Encephalopathy Research Group convened in Washington, DC, to assess current research into clinical and pathogenic features of the disorder and prepare an agenda for future research collaborations. The group comprised clinical and basic scientists and representatives of patient advocacy groups. SCN8A encephalopathy is a rare disorder caused by de novo missense mutations of the sodium channel gene SCN8A, which encodes the neuronal sodium channel Nav 1.6. Since the initial description in 2012, approximately 140 affected individuals have been reported in publications or by SCN8A family groups. As a result, an understanding of the severe impact of SCN8A mutations is beginning to emerge. Defining a genetic epilepsy syndrome goes beyond identification of molecular etiology. Topics discussed at this meeting included (1) comparison between mutations of SCN8A and the SCN1A mutations in Dravet syndrome, (2) biophysical properties of the Nav 1.6 channel, (3) electrophysiologic effects of patient mutations on channel properties, (4) cell and animal models of SCN8A encephalopathy, (5) drug screening strategies, (6) the phenotypic spectrum of SCN8A encephalopathy, and (7) efforts to develop a bioregistry. A panel discussion of gaps in bioregistry, biobanking, and clinical outcomes data was followed by a planning session for improved integration of clinical and basic science research. Although SCN8A encephalopathy was identified only recently, there has been rapid progress in functional analysis and phenotypic classification. The focus is now shifting from identification of the underlying molecular cause to the development of strategies for drug screening and prioritized patient care.