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.     

Sodium channel SCN1A and epilepsy: Mutations and mechanisms

Mutations in a number of genes encoding voltage‐gated sodium channels cause a variety of epilepsy syndromes in humans, including genetic (generalized) epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS, severe myoclonic epilepsy of infancy). Most of these mutations are in the SCN1A gene, and all are dominantly inherited. Most of the mutations that cause DS result in loss of function, whereas all of the known mutations that cause GEFS+ are missense, presumably altering channel activity. Family members with the same GEFS+ mutation often display a wide range of seizure types and severities, and at least part of this variability likely results from variation in other genes. Many different biophysical effects of SCN1A‐GEFS+ mutations have been observed in heterologous expression systems, consistent with both gain and loss of channel activity. However, results from mouse models suggest that the primary effect of both GEFS+ and DS mutations is to decrease the activity of GABAergic inhibitory neurons. Decreased activity of the inhibitory circuitry is thus likely to be a major factor contributing to seizure generation in patients with GEFS+ and DS, and may be a general consequence of SCN1A mutations.     

Painful neuropathies: the emerging role of sodium channelopathies

Pain is a frequent debilitating feature reported in peripheral neuropathies with involvement of small nerve (Aδ and C) fibers. Voltage‐gated sodium channels are responsible for the generation and conduction of action potentials in the peripheral nociceptive neuronal pathway where Na_V1.7, Na_V1.8, and Na_V1.9 sodium channels (encoded by SCN9A, SCN10A, and SCN11A) are preferentially expressed. The human genetic pain conditions inherited erythromelalgia and paroxysmal extreme pain disorder were the first to be linked to gain‐of‐function SCN9A mutations. Recent studies have expanded this spectrum with gain‐of‐function SCN9A mutations in patients with small fiber neuropathy and in a new syndrome of pain, dysautonomia, and small hands and small feet (acromesomelia). In addition, painful neuropathies have been recently linked to SCN10A mutations. Patch‐clamp studies have shown that the effect of SCN9A mutations is dependent upon the cell‐type background. The functional effects of a mutation in dorsal root ganglion (DRG) neurons and sympathetic neuron cells may differ per mutation, reflecting the pattern of expression of autonomic symptoms in patients with painful neuropathies who carry the mutation in question. Peripheral neuropathies may not always be length‐dependent, as demonstrated in patients with initial facial and scalp pain symptoms with SCN9A mutations showing hyperexcitability in both trigeminal ganglion and DRG neurons. There is some evidence suggesting that gain‐of‐function SCN9A mutations can lead to degeneration of peripheral axons. This review will focus on the emerging role of sodium channelopathies in painful peripheral neuropathies, which could serve as a basis for novel therapeutic strategies.     

A homozygous mutation of voltage‐gated sodium channel βI gene SCN1B in a patient with Dravet syndrome

Dravet syndrome is a severe form of epileptic encephalopathy characterized by early onset epileptic seizures followed by ataxia and cognitive decline. Approximately 80% of patients with Dravet syndrome have been associated with heterozygous mutations in SCN1A gene encoding voltage‐gated sodium channel (VGSC) α_I subunit, whereas a homozygous mutation (p.Arg125Cys) of SCN1B gene encoding VGSC β_I subunit was recently described in a patient with Dravet syndrome. To further examine the involvement of homozygous SCN1B mutations in the etiology of Dravet syndrome, we performed mutational analyses on SCN1B in 286 patients with epileptic disorders, including 67 patients with Dravet syndrome who have been negative for SCN1A and SCN2A mutations. In the cohort, we found one additional homozygous mutation (p.Ile106Phe) in a patient with Dravet syndrome. The identified homozygous SCN1B mutations indicate that SCN1B is an etiologic candidate underlying Dravet syndrome.     

Partial epilepsy with antecedent febrile seizures and seizure aggravation by antiepileptic drugs: Associated with loss of function of Nav1.1

Purpose: Generalized epilepsy with febrile seizures plus (GEFS+) and severe myoclonic epilepsy in infancy (SMEI) are associated with sodium channel α‐subunit type‐1 gene (SCN1A) mutations. Febrile seizures and partial seizures occur in both GEFS+ and SMEI; sporadic onset and seizure aggravation by antiepileptic drugs (AEDs) are features of SMEI. We thus searched gene mutations in isolated cases of partial epilepsy with antecedent FS (PEFS+) that showed seizure aggravations by AEDs. Methods: Genomic DNA from four patients was screened for mutations in SCN1A, SCN2A, SCN1B, and GABRG2 using denaturing high‐performance liquid chromatography (dHPLC) and sequencing. Whole‐cell patch clamp analysis was used to characterize biophysical properties of two newly defined mutants of Na_v1.1 in tsA201 cells. Results: Two heterozygous de novo mutations of SCN1A (R946H and F1765L) were detected, which were proven to cause loss of function of Na_v1.1. When the functional defects of mutants reported previously are compared, it is found that all mutants from PEFS+ have features of loss of function, whereas GEFS+ shows mild dysfunction excluding loss of function, coincident with mild clinical manifestations. PEFS+ is similar to SMEI clinically with possible AED‐induced seizure aggravation and biophysiologically with features of loss of function, and different from SMEI by missense mutation without changes in hydrophobicity or polarity of the residues. Conclusions: Isolated milder PEFS+ may associate with SCN1A mutations and loss of function of Na_v1.1, which may be the basis of seizure aggravation by sodium channel–blocking AEDs. This study characterized phenotypes biologically, which may be helpful in understanding the pathophysiologic basis, and further in management of the disease.     

Exome sequencing identifies a de novo SCN2A mutation in a patient with intractable seizures,severe intellectual disability,optic atrophy,muscular hypotonia,and brain abnormalities

Epilepsy is a phenotypically and genetically highly heterogeneous disorder with >200 genes linked to inherited forms of the disease. To identify the underlying genetic cause in a patient with intractable seizures, optic atrophy, severe intellectual disability (ID), brain abnormalities, and muscular hypotonia, we performed exome sequencing in a 5‐year‐old girl and her unaffected parents. In the patient, we detected a novel, de novo missense mutation in the SCN2A (c.5645G>T; p.R1882L) gene encoding the α_II‐subunit of the voltage‐gated sodium channel Na_v1.2. A literature review revealed 33 different SCN2A mutations in 14 families with benign forms of epilepsy and in 21 cases with severe phenotypes. Although almost all benign mutations were inherited, the majority of severe mutations occurred de novo. Of interest, de novo SCN2A mutations have also been reported in five patients without seizures but with ID (n = 3) and/or autism (n = 3). In the present study, we successfully used exome sequencing to detect a de novo mutation in a genetically heterogeneous disorder with epilepsy and ID. Using this approach, we expand the phenotypic spectrum of SCN2A mutations. Our own and literature data indicate that SCN2A‐linked severe phenotypes are more likely to be caused by de novo mutations. A PowerPoint slide summarizing this article is available for download in the Supporting Information section here .     

Missense mutation of the sodium channel gene SCN2A causes Dravet syndrome

Mutations of the gene encoding the α2 subunit of the neuronal sodium channel, SCN2A, have been found in benign familial neonatal-infantile seizures (BFNIS). In Dravet syndrome, only one nonsense mutation of SCN2A was identified, while hundreds of mutations were found in the paralogue gene, SCN1A, which encodes the α1 subunit. This study examines whether SCN2A mutations are associated with Dravet syndrome. We screened for mutations of SCN1A, SCN2A and GABRG2 (the gene encoding γ2 subunit of the GABA_A receptor) in 59 patients with Dravet syndrome and found 29 SCN1A mutations and three missense SCN2A mutations. Among the three, one de novo SCN2A mutation (c.3935G>C: R1312T) identified in a patient was thought to affect an arginine residue in a voltage sensor of the channel and hence, to be pathogenic. This finding suggests that both nonsense mutations and missense SCN2A mutations cause Dravet syndrome.     

Confirming an expanded spectrum of SCN2A mutations: a case series

Mutations in sodium channel genes are highly associated with epilepsy. Mutation of SCN1A, the gene encoding the voltage gated sodium channel (VGSC) alpha subunit type 1 (Nav1.1), causes Dravet syndrome spectrum disorders. Mutations in SCN2A have been identified in patients with benign familial neonatal‐infantile epilepsy (BFNIE), generalised epilepsy with febrile seizures plus (GEFS+), and a small number of reported cases of other infantile‐onset severe intractable epilepsy. Here, we report three patients with infantile‐onset severe intractable epilepsy found to have de novo mutations in SCN2A. While a causal role for these mutations cannot be directly established, these findings contribute to growing evidence that mutation of SCN2A is associated with a range of epilepsy phenotypes including severe infantile‐onset epilepsy.     

Role of the sodium channel SCN9A in genetic epilepsy with febrile seizures plus and Dravet syndrome

Mutations of the SCN1A subunit of the sodium channel is a cause of genetic epilepsy with febrile seizures plus (GEFS⁺) in multiplex families and accounts for 70–80% of Dravet syndrome (DS). DS cases without SCN1A mutation inherited have predicted SCN9A susceptibility variants, which may contribute to complex inheritance for these unexplained cases of DS. Compared with controls, DS cases were significantly enriched for rare SCN9A genetic variants. None of the multiplex febrile seizure or GEFS⁺ families could be explained by highly penetrant SCN9A mutations.     

The incidence of SCN1A‐related Dravet syndrome in Denmark is 1:22,000: A population‐based study from 2004 to 2009

Dravet syndrome is a severe infantile‐onset epileptic encephalopathy associated with mutations in the sodium channel alpha‐1 subunit gene SCN1A. We aimed to describe the incidence of Dravet syndrome in the Danish population. Based on a 6‐year birth cohort from 2004 to 2009, we propose an incidence of 1:22,000, which is higher than what has been established earlier. We identified 17 cases with SCN1A mutation–positive Dravet syndrome. Fifteen patients were found, by conventional Sanger sequencing. Two additional patients with clinical Dravet syndrome, but without a detectable SCN1A mutation by Sanger sequencing, were diagnosed with a SCN1A mutation after using a targeted next‐generation sequencing gene panel.     

Sodium channels and the neurobiology of epilepsy

Voltage‐gated sodium channels (VGSCs) are integral membrane proteins. They are essential for normal neurologic function and are, currently, the most common recognized cause of genetic epilepsy. This review summarizes the neurobiology of VGSCs, their association with different epilepsy syndromes, and the ways in which we can experimentally interrogate their function. The most important sodium channel subunit of relevance to epilepsy is SCN1A, in which over 650 genetic variants have been discovered. SCN1A mutations are associated with a variety of epilepsy syndromes; the more severe syndromes are associated with truncation or complete loss of function of the protein. SCN2A is another important subtype associated with epilepsy syndromes, across a range of severe and less severe epilepsies. This subtype is localized primarily to excitatory neurons, and mutations have a range of functional effects on the channel. SCN8A is the other main adult subtype found in the brain and has recently emerged as an epilepsy gene, with the first human mutation discovered in a severe epilepsy syndrome. Mutations in the accessory β subunits, thought to modulate trafficking and function of the α subunits, have also been associated with epilepsy. Genome sequencing is continuing to become more affordable, and as such, the amount of incoming genetic data is continuing to increase. Current experimental approaches have struggled to keep pace with functional analysis of these mutations, and it has proved difficult to build associations between disease severity and the precise effect on channel function. These mutations have been interrogated with a range of experimental approaches, from in vitro, in vivo, to in silico. In vitro techniques will prove useful to scan mutations on a larger scale, particularly with the advance of high‐throughput automated patch‐clamp techniques. In vivo models enable investigation of mutation in the context of whole brains with connected networks and more closely model the human condition. In silico models can help us incorporate the impact of multiple genetic factors and investigate epistatic interactions and beyond.     

Impaired NaV1.2 function and reduced cell surface expression in benign familial neonatal-infantile seizures

Purpose: Mutations in SCN2A, the gene encoding the brain voltage‐gated sodium channel α‐subunit Na _V 1.2, are associated with inherited epilepsies including benign familial neonatal‐infantile seizures (BFNIS). Functional characterization of three BFNIS mutations was performed to identify defects in channel function that underlie this disease. Methods: We examined three BFNIS mutations (R1319Q, L1330F, and L1563V) using whole‐cell patch‐clamp recording of heterologously expressed human Na _V 1.2. Membrane biotinylation was employed to examine the cell surface protein expression of the four Na _V 1.2 alleles. Results: R1319Q displayed mixed effects on activation and fast inactivation gating, consistent with a net loss of channel function. L1563V exhibited impaired fast inactivation predicting a net gain of channel function. The L1330F mutation significantly decreased overall channel availability during repetitive stimulation. Patch‐clamp analysis also revealed that cells expressing BFNIS mutants exhibited lower levels of sodium current compared to wild type (WT) Na _V 1.2. Biochemical experiments demonstrated that all three BFNIS mutations exhibited a significant reduction in cell surface expression compared to WT. Discussion: Our findings indicate that BFNIS is associated with a range of biophysical defects accompanied by reduced levels of channel protein at the plasma membrane.     

An SCN2A mutation in a family with infantile seizures from Madagascar reveals an increased subthreshold Na+ current

Missense mutations in SCN2A, encoding the brain sodium channel Na_V1.2, have been described in benign familial neonatal‐infantile seizures (BFNIS), a self‐limiting disorder, whereas several SCN2A de novo nonsense mutations have been found in patients with more severe phenotypes including epileptic encephalopathy. We report a family with BFNIS originating from Madagascar. Onset extended from 3 to 9 months of age. Interictal EEGs were normal. In two patients, ictal electroencephalography (EEG) studies showed partial seizure patterns with secondary generalization in one. Seizures remitted before 18 months of age, with or without medication. Intellectual development was normal. A novel missense mutation of SCN2A, c.4766A>G/p.Tyr1589Cys, was found in a highly conserved region of Na_V1.2 (D4/S2‐S3). Functional studies using heterologous expression in tsA201 cells and whole‐cell patch clamping revealed a depolarizing shift of steady‐state inactivation, increased persistent Na⁺ current, a slowing of fast inactivation and an acceleration of its recovery, thus a gain‐of‐function. Using an action potential waveform in a voltage‐clamp experiment we indicated an increased inward Na⁺ current at subthreshold voltages, which can explain a neuronal hyperexcitability. Our results suggest that this mutation induces neuronal hyperexcitability, resulting in infantile epilepsy with favorable outcome.     

A CACNB4 mutation shows that altered Ca(v)2.1 function may be a genetic modifier of severe myoclonic epilepsy in infancy

Mutations of SCN1A, encoding the voltage-gated sodium channel α1 subunit, represent the most frequent genetic cause of severe myoclonic epilepsy in infancy (SMEI). The purpose of this study was to determine if mutations in other seizure susceptibility genes are also present and could modify the disease severity. All coding exons of SCN1B, GABRG2, and CACNB4 genes were screened for mutations in 38 SCN1A-mutation-positive SMEI probands. We identified one proband who was heterozygous for a de novo SCN1A nonsense mutation (R568X) and another missense mutation (R468Q) of the CACNB4 gene. The latter mutation was inherited from his father who had a history of febrile seizures. An electrophysiological analysis of heterologous expression system exhibited that R468Q-CACNB4 showed greater Ba²⁺ current density compared with the wild-type CACNB4. The greater Ca_v2.1 currents caused by the R468Q-CACNB4 mutation may increase the neurotransmitter release in the excitatory neurons under the condition of insufficient inhibitory neurons caused primarily by the SCN1A mutation.     

Mutations of the SCN1A gene in acute encephalopathy

Purpose: Acute encephalopathy is the most serious complication of pediatric viral infections, such as influenza and exanthema subitum. It occurs worldwide, but is most prevalent in East Asia. Recently, there have been sporadic case reports of epilepsy/febrile seizure and acute encephalopathy with a neuronal sodium channel alpha 1 subunit (SCN1A) mutation. To determine whether SCN1A mutations are a predisposing factor of acute encephalopathy, we sought to identify SCN1A mutations in a large case series of acute encephalopathy including various syndromes. Methods: We analyzed the SCN1A gene in 87 patients with acute encephalopathy, consisting of 20 with acute necrotizing encephalopathy (ANE), 61 with acute encephalopathy with biphasic seizures and late reduced diffusion (AESD), and six with nonspecific (unclassified) acute encephalopathy. Key Findings: Three patients had distinct point mutations. Two of them had epileptic seizures prior to acute encephalopathy. Clinical and neuroradiologic findings of acute encephalopathy were diverse among the three patients, although all had a prolonged and generalized seizure at its onset. The first patient with V982L had partial epilepsy and AESD. The second patient with M1977L had febrile seizures and nonspecific acute encephalopathy. The third patient with R1575C had no seizures until the onset of ANE. M1977L was a novel mutation, whereas the remaining two, V982L and R1575C, have previously been reported in cases of Dravet syndrome and acute encephalopathy, respectively. Significance: These findings provide further evidence that SCN1A mutations are a predisposing factor for the onset of various types of acute encephalopathy.     

SCN2A channelopathies: Mechanisms and models

Variants in the SCN2A gene, encoding the voltage‐gated sodium channel Na_V1.2, cause a variety of neuropsychiatric syndromes with different severity ranging from self‐limiting epilepsies with early onset to developmental and epileptic encephalopathy with early or late onset and intellectual disability (ID), as well as ID or autism without seizures. Functional analysis of channel defects demonstrated a genotype‐phenotype correlation and suggested effective treatment options for one group of affected patients carrying gain‐of‐function variants. Here, we sum up the functional mechanisms underlying different phenotypes of patients with SCN2A channelopathies and present currently available models that can help in understanding SCN2A‐related disorders.     

A sodium channel gene SCN9A polymorphism that increases nociceptor excitability

Sodium channel Na_V1.7, encoded by the SCN9A gene, is preferentially expressed in nociceptive primary sensory neurons, where it amplifies small depolarizations. In studies on a family with inherited erythromelalgia associated with Na_V1.7 gain‐of‐function mutation A863P, we identified a nonsynonymous single‐nucleotide polymorphism within SCN9A in the affected proband and several unaffected family members; this polymorphism (c. 3448C&T, Single Nucleotide Polymorphisms database rs6746030, which produces the amino acid substitution R1150W in human Na_V1.7 [hNa_V1.7]) is present in 1.1 to 12.7% of control chromosomes, depending on ethnicity. In this study, we examined the effect of the R1150W substitution on function of the hNa_V1.7 channel, and on the firing of dorsal root ganglion (DRG) neurons in which this channel is normally expressed. We show that this polymorphism depolarizes activation (7.9–11mV in different assays). Current‐clamp analysis shows that the 1150W allele depolarizes (6mV) resting membrane potential and increases (～2‐fold) the firing frequency in response to depolarization in DRG neurons in which it is present. Our results suggest that polymorphisms in the Na_V1.7 channel may influence susceptibility to pain. Ann Neurol 2009;66:862–866     

Remarkable Phenytoin Sensitivity in 4 Children with SCN8A-related Epilepsy: A Molecular Neuropharmacological Approach

Mutations in SCN8A are associated with epilepsy and intellectual disability. SCN8A encodes for sodium channel Nav1.6, which is located in the brain. Gain-of-function missense mutations in SCN8A are thought to lead to increased firing of excitatory neurons containing Nav1.6, and therefore to lead to increased seizure susceptibility. We hypothesized that sodium channel blockers could have a beneficial effect in patients with SCN8A-related epilepsy by blocking the overactive Nav1.6 and thereby counteracting the effect of the mutation. Herein, we describe 4 patients with a missense SCN8A mutation and epilepsy who all show a remarkably good response on high doses of phenytoin and loss of seizure control when phenytoin medication was reduced, while side effects were relatively mild. In 2 patients, repeated withdrawal of phenytoin led to the reoccurrence of seizures. Based on the findings in these patients and the underlying molecular mechanism we consider treatment with (high-dose) phenytoin as a possible treatment option in patients with difficult-to-control seizures due to an SCN8A mutation.

Electronic supplementary material

The online version of this article (doi:10.1007/s13311-015-0372-8) contains supplementary material, which is available to authorized users.Key Words: SCN8A, phenytoin, epileptic encephalopathy, sodium channel blockers     

A novel SCN1A mutation in a cytoplasmic loop in intractable juvenile myoclonic epilepsy without febrile seizures

Generalised (genetic) epilepsy with febrile seizures plus (GEFS+) is a familial epilepsy syndrome with various phenotypes. The majority of individuals with GEFS+ have generalised seizure types, in addition to febrile seizures (FS) or febrile seizures plus (FS+), defined as either continued FS after 6 years of age or afebrile seizures following FS. A 27‐year‐old man with no history of FS/FS+ experienced intractable generalised convulsive seizures. The patient's father had a history of similar seizures during puberty and the patient's siblings had only FS. No individual in the family had both generalised seizures and FS/FS+, although GEFS+ might be considered to be present in the family. Analysis of SCN1A, a sodium channel gene, revealed a novel mutation (c.3250A>T [S1084C]) in the cytoplasmic loop 2 of SCN1A in both the patient and his father. Most previously reported SCN1A mutations in GEFS+ patients are located in the conserved homologous domains of SCN1A, whereas mutations in the cytoplasmic loops are very rare. SCN1A gene analysis is not commonly performed in subjects with generalised seizures without FS. SCN1A mutation may be a clinically‐useful genetic marker in order to distinguish GEFS+ patients from those with classic idiopathic generalised epilepsy, even if they present an atypical clinical picture.