Variable neurologic phenotype in a GEFS+ family with a novel mutation in SCN1A

An SCN1A gene missense variant in a Chinese Tujia ethnic family with genetic epilepsy with febrile seizures plus

Genetic epilepsy with febrile seizures plus (GEFSP) is a familial epileptic syndrome that is genetically heterogeneous and inherited in an autosomal dominant form in most cases. To date, at least seven genes have been reported to associate with GEFSP. This study aimed to identify the disease-causing variant in a Chinese Tujia ethnic family with GEFSP by using whole exome sequencing, Sanger sequencing, and in silico prediction. A heterozygous missense variant c.5725A>G (p.T1909A) was identified in the sodium voltage-gated channel alpha subunit 1 gene (SCN1A) coding region. The variant co-segregated with the GEFSP phenotype in this family, and it was predicted as disease-causing by multiple in silico programs, which was proposed as the genetic cause of GEFSP, further genetically diagnosed as GEFSP2. These findings expand the genetic and phenotypic spectrum of GEFSP and should contribute to genetic diagnoses, personalized therapies, and prognoses.

Alterations of Neuronal Dynamics as a Mechanism for Cognitive Impairment in Epilepsy

Epilepsy is commonly associated with cognitive and behavioral deficits that dramatically affect the quality of life of patients. In order to identify novel therapeutic strategies aimed at reducing these deficits, it is critical first to understand the mechanisms leading to cognitive impairments in epilepsy. Traditionally, seizures and epileptiform activity in addition to neuronal injury have been considered to be the most significant contributors to cognitive dysfunction. In this review we however highlight the role of a new mechanism: alterations of neuronal dynamics, i.e. the timing at which neurons and networks receive and process neural information. These alterations, caused by the underlying etiologies of epilepsy syndromes, are observed in both animal models and patients in the form of abnormal oscillation patterns in unit firing, local field potentials, and electroencephalogram (EEG). Evidence suggests that such mechanisms significantly contribute to cognitive impairment in epilepsy, independently of seizures and interictal epileptiform activity. Therefore, therapeutic strategies directly targeting neuronal dynamics rather than seizure reduction may significantly benefit the quality of life of patients.

Myoclonus-Ataxia Syndromes: A Diagnostic Approach

Background

A myriad of disorders combine myoclonus and ataxia. Most causes are genetic and an increasing number of genes are being associated with myoclonus-ataxia syndromes (MAS), due to recent advances in genetic techniques. A proper etiologic diagnosis of MAS is clinically relevant, given the consequences for genetic counseling, treatment, and prognosis.

Objectives

To review the causes of MAS and to propose a diagnostic algorithm.

Methods

A comprehensive and structured literature search following PRISMA criteria was conducted to identify those disorders that may combine myoclonus with ataxia.

Results

A total of 135 causes of combined myoclonus and ataxia were identified, of which 30 were charted as the main causes of MAS. These include four acquired entities: opsoclonus-myoclonus-ataxia syndrome, celiac disease, multiple system atrophy, and sporadic prion diseases. The distinction between progressive myoclonus epilepsy and progressive myoclonus ataxia poses one of the main diagnostic dilemmas.

Conclusions

Diagnostic algorithms for pediatric and adult patients, based on clinical manifestations including epilepsy, are proposed to guide the differential diagnosis and corresponding work-up of the most important and frequent causes of MAS. A list of genes associated with MAS to guide genetic testing strategies is provided. Priority should be given to diagnose or exclude acquired or treatable disorders.

Focal Dorsal Hippocampal Nav1.1 Knock Down Alters Place Cell Temporal Coordination and Spatial Behavior

Alterations in the voltage-gated sodium channel Nav.1.1 are implicated in various neurological disorders, including epilepsy, Alzheimer's disease, and autism spectrum disorders. Previous studies suggest that the reduction of Nav1.1 expression leads to a decrease of fast spiking activity in inhibitory neurons. Because interneurons (INs) play a critical role in the temporal organization of neuronal discharge, we hypothesize that Nav1.1 dysfunction will negatively impact neuronal coordination in vivo. Using shRNA interference, we induced a focal Nav1.1 knock-down (KD) in the dorsal region of the right hippocampus of adult rats. Focal, unilateral Nav1.1 KD decreases the performance in a spatial novelty recognition task and the firing rate in INs, but not in pyramidal cells. It reduced theta/gamma coupling of hippocampal oscillations and induced a shift in pyramidal cell theta phase preference. Nav1.1 KD degraded spatial accuracy and temporal coding properties of place cells, such as theta phase precession and compression of ongoing sequences. Aken together, these data demonstrate that a deficit in Nav1.1 alters the temporal coordination of neuronal firing in CA1 and impairs behaviors that rely on the integrity of this network. They highlight the potential contribution of local inhibition in neuronal coordination and its impact on behavior in pathological conditions.

Modifier genes in SCN1A-related epilepsy syndromes

Background

SCN1A is one of the most important epilepsy‐related genes, with pathogenic variants leading to a range of phenotypes with varying disease severity. Different modifying factors have been hypothesized to influence SCN1A‐related phenotypes. We investigate the presence of rare and more common variants in epilepsy‐related genes as potential modifiers of SCN1A‐related disease severity.

Methods

87 patients with SCN1A‐related epilepsy were investigated. Whole‐exome sequencing was performed by the Beijing Genomics Institute (BGI). Functional variants in 422 genes associated with epilepsy and/or neuronal excitability were investigated. Differences in proportions of variants between the epilepsy genes and four control gene sets were calculated, and compared to the proportions of variants in the same genes in the ExAC database.

Results

Statistically significant excesses of variants in epilepsy genes were observed in the complete cohort and in the combined group of mildly and severely affected patients, particularly for variants with minor allele frequencies of <0.05. Patients with extreme phenotypes showed much greater excesses of epilepsy gene variants than patients with intermediate phenotypes.

Conclusion

Our results indicate that relatively common variants in epilepsy genes, which would not necessarily be classified as pathogenic, may play a large role in modulating SCN1A phenotypes. They may modify the phenotypes of both severely and mildly affected patients. Our results may be a first step toward meaningful testing of modifier gene variants in regular diagnostics for individual patients, to provide a better estimation of disease severity for newly diagnosed patients.

Influence of common SCN1A promoter variants on the severity of SCN1A-related phenotypes

Background

Pathogenic variants in SCN1A cause variable epilepsy disorders with different disease severities. We here investigate whether common variation in the promoter region of the unaffected SCN1A allele could reduce normal expression, leading to a decreased residual function of Nav1.1, and therefore to more severe clinical outcomes in patients affected by pathogenic SCN1A variants.

Methods

Five different SCN1A promoter‐haplotypes were functionally assessed in SH‐SY5Y cells using Firefly and Renilla luciferase assays. The SCN1A promoter region was analyzed in a cohort of 143 participants with SCN1A pathogenic variants. Differences in clinical features and outcomes between participants with and without common variants in the SCN1A promoter‐region of their unaffected allele were investigated.

Results

All non‐wildtype haplotypes showed a significant reduction in luciferase expression, compared to the wildtype promoter‐region (65%–80%, p = 0.039–0.0023). No statistically significant differences in clinical outcomes were observed between patients with and without common promoter variants. However, patients with a wildtype promoter‐haplotype on their unaffected SCN1A allele showed a nonsignificant trend for milder phenotypes.

Conclusion

The nonsignificant observed trends in our study warrant replication studies in larger cohorts to explore the potential modifying role of these common SCN1A promoter‐haplotypes.

Outcomes and comorbidities of SCN1A-related seizure disorders

PURPOSE

Differentiating between Dravet syndrome and non-Dravet SCN1A-related phenotypes is important for prognosis regarding epilepsy severity, cognitive development, and comorbidities. When a child is diagnosed with genetic epilepsy with febrile seizures plus (GEFS+) or febrile seizures (FS), accurate prognostic information is essential as well, but detailed information on seizure course, seizure freedom, medication use, and comorbidities is lacking for this milder patient group. In this cross-sectional study, we explore disease characteristics in milder SCN1A-related phenotypes and the nature, occurrence, and relationships of SCN1A-related comorbidities in both patients with Dravet and non-Dravet syndromes.

METHODS

A cohort of 164 Dutch participants with SCN1A-related seizures was evaluated, consisting of 116 patients with Dravet syndrome and 48 patients with either GEFS+, febrile seizures plus (FS+), or FS. Clinical data were collected from medical records, semi-structured telephone interviews, and three questionnaires: the Functional Mobility Scale (FMS), the Pediatric Quality of Life Inventory (PedsQL) Measurement Model, and the Child or Adult Behavior Checklists (CBCL/ABCL).

RESULTS

Walking disabilities and severe behavioral problems affect 71% and 43% of patients with Dravet syndrome respectively and are almost never present in patients with non-Dravet syndromes. These comorbidities are strongly correlated to lower quality-of-life (QoL) scores. Less severe comorbidities occur in patients with non-Dravet syndromes: learning problems and psychological/behavioral problems are reported for 27% and 38% respectively. The average QoL score of the non-Dravet group was comparable with that of the general population. The majority of patients with non-Dravet syndromes becomes seizure-free after 10 years of age (85%).

CONCLUSIONS

Severe behavioral problems and walking disabilities are common in patients with Dravet syndrome and should receive specific attention during clinical management. Although the epilepsy course of patients with non-Dravet syndromes is much more favorable, milder comorbidities frequently occur in this group as well. Our results may be of great value for clinical care and informing newly diagnosed patients and their parents about prognosis.

Early-life febrile seizures worsen adult phenotypes in Scn1a mutants

Mutations in the voltage-gated sodium channel (VGSC) gene SCN1A, encoding the Na_v1.1 channel, are responsible for a number of epilepsy disorders including genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome (DS). Patients with SCN1A mutations often experience prolonged early-life febrile seizures (FSs), raising the possibility that these events may influence epileptogenesis and lead to more severe adult phenotypes. To test this hypothesis, we subjected 21-23-day-old mice expressing the human SCN1A GEFS+ mutation R1648H to prolonged hyperthermia, and then examined seizure and behavioral phenotypes during adulthood. We found that early-life FSs resulted in lower latencies to induced seizures, increased severity of spontaneous seizures, hyperactivity, and impairments in social behavior and recognition memory during adulthood. Biophysical analysis of brain slice preparations revealed an increase in epileptiform activity in CA3 pyramidal neurons along with increased action potential firing, providing a mechanistic basis for the observed worsening of adult phenotypes. These findings demonstrate the long-term negative impact of early-life FSs on disease outcomes. This has important implications for the clinical management of this patient population and highlights the need for therapeutic interventions that could ameliorate disease progression.

Scn1a dysfunction alters behavior but not the effect of stress on seizure response

Mutations in the voltage-gated sodium channel gene SCN1A are responsible for a number of epilepsy disorders, including genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome. In addition, dysfunction in SCN1A is increasingly being linked to neuropsychiatric abnormalities, social deficits and cognitive disabilities. We have previously reported that mice heterozygous for the SCN1A R1648H mutation identified in a GEFS+ family have infrequent spontaneous seizures, increased susceptibility to chemically and hyperthermia-induced generalized seizures and sleep abnormalities. In this study, we characterized the behavior of heterozygous mice expressing the SCN1A R1648H mutation (Scn1a(RH/+)) and the effect of stress on spontaneous and induced seizures. We also examined the effect of the R1648H mutation on the hypothalamic-pituitary-adrenal (HPA) axis response. We confirmed our previous finding that Scn1a(RH/+) mutants are hyperactive, and also identified deficits in social behavior, spatial memory, cued fear conditioning, pre-pulse inhibition and risk assessment. Furthermore, while exposure to a stressor did increase seizure susceptibility, the effect seen in the Scn1a(RH/+) mutants was similar to that seen in wild-type littermates. In addition, Scn1a dysfunction does not appear to alter HPA axis function in adult animals. Our results suggest that the behavioral abnormalities associated with Scn1a dysfunction encompass a wider range of phenotypes than previously reported and factors such as stress exposure may alter disease severity in patients with SCN1A mutations.

Genetic background modulates impaired excitability of inhibitory neurons in a mouse model of Dravet syndrome

Dominant loss-of-function mutations in voltage-gated sodium channel NaV1.1 cause Dravet Syndrome, an intractable childhood-onset epilepsy. NaV1.1(+/-) Dravet Syndrome mice in C57BL/6 genetic background exhibit severe seizures, cognitive and social impairments, and premature death. Here we show that Dravet Syndrome mice in pure 129/SvJ genetic background have many fewer seizures and much less premature death than in pure C57BL/6 background. These mice also have a higher threshold for thermally induced seizures, fewer myoclonic seizures, and no cognitive impairment, similar to patients with Genetic Epilepsy with Febrile Seizures Plus. Consistent with this mild phenotype, mutation of NaV1.1 channels has much less physiological effect on neuronal excitability in 129/SvJ mice. In hippocampal slices, the excitability of CA1 Stratum Oriens interneurons is selectively impaired, while the excitability of CA1 pyramidal cells is unaffected. NaV1.1 haploinsufficiency results in increased rheobase and threshold for action potential firing and impaired ability to sustain high-frequency firing. Moreover, deletion of NaV1.1 markedly reduces the amplification and integration of synaptic events, further contributing to reduced excitability of interneurons. Excitability is less impaired in inhibitory neurons of Dravet Syndrome mice in 129/SvJ genetic background. Because specific deletion of NaV1.1 in forebrain GABAergic interneuons is sufficient to cause the symptoms of Dravet Syndrome in mice, our results support the conclusion that the milder phenotype in 129/SvJ mice is caused by lesser impairment of sodium channel function and electrical excitability in their forebrain interneurons. This mild impairment of excitability of interneurons leads to a milder disease phenotype in 129/SvJ mice, similar to Genetic Epilepsy with Febrile Seizures Plus in humans.

Altered sleep regulation in a mouse model of SCN1A-derived genetic epilepsy with febrile seizures plus (GEFS+)

PURPOSE

Mutations in the voltage-gated sodium channel (VGSC) gene SCN1A are responsible for a number of epilepsy disorders, including genetic epilepsy with febrile seizures plus (GEFS+) and Dravet syndrome. In addition to seizures, patients with SCN1A mutations often experience sleep abnormalities, suggesting that SCN1A may also play a role in the neuronal pathways involved in the regulation of sleep. However, to date, a role for SCN1A in the regulation of sleep architecture has not been directly examined. To fill this gap, we tested the hypothesis that SCN1A contributes to the regulation of sleep architecture, and by extension, that SCN1A dysfunction contributes to the sleep abnormalities observed in patients with SCN1A mutations.

METHODS

Using immunohistochemistry we first examined the expression of mouse Scn1a in regions of the mouse brain that are known to be involved in seizure generation and sleep regulation. Next, we performed detailed analysis of sleep and wake electroencephalography (EEG) patterns during 48 continuous hours of baseline recordings in a knock-in mouse line that expresses the human SCN1A GEFS+ mutation R1648H (RH mutants). We also characterized the sleep-wake pattern following 6 h of sleep deprivation.

KEY FINDINGS

Immunohistochemistry revealed broad expression of Scn1a in the neocortex, hippocampus, hypothalamus, thalamic reticular nuclei, dorsal raphe nuclei, pedunculopontine, and laterodorsal tegmental nuclei. Co-localization between Scn1a immunoreactivity and critical cell types within these regions was also observed. EEG analysis under baseline conditions revealed increased wakefulness and reduced non-rapid eye movement (NREM) and rapid eye movement (REM) sleep amounts during the dark phase in the RH mutants, suggesting a sleep deficit. Nevertheless, the mutants exhibited levels of NREM and REM sleep that were generally similar to wild-type littermates during the recovery period following 6 h of sleep deprivation.

SIGNIFICANCE

These results establish a direct role for SCN1A in the regulation of sleep and suggest that patients with SCN1A mutations may experience chronic alterations in sleep, potentially leading to negative outcomes over time. In addition, the expression of Scn1a in specific cell types/brain regions that are known to play critical roles in seizure generation and sleep now provides a mechanistic basis for the clinical features (seizures and sleep abnormalities) associated with human SCN1A mutations.

De-novo mutations and genetic variation in the SCN1A gene in Malaysian patients with generalized epilepsy with febrile seizures plus (GEFS+)

Generalized epilepsy with febrile seizures plus (GEFS+) comprises a group of clinically and genetically heterogeneous epilepsy syndrome. Here, we provide the first report of clinical presentation and mutational analysis of SCN1A gene in 36 Malaysian GEFS+ patients. Mutational analysis of SCN1A gene revealed twenty seven sequence variants (missense mutation and silent polymorphism also intronic polymorphism), as well as 2 novel de-novo mutations were found in our patients at coding regions, c.5197A>G (N1733D) and c.4748A>G (H1583R). Our findings provide potential genetic insights into the pathogenesis of GEFS+ in Malaysian populations concerning the SCN1A gene mutations.

Preferential inactivation of Scn1a in parvalbumin interneurons increases seizure susceptibility

Voltage-gated sodium channels (VGSCs) are essential for the generation and propagation of action potentials in electrically excitable cells. Dominant mutations in SCN1A, which encodes the Nav1.1 VGSC α-subunit, underlie several forms of epilepsy, including Dravet syndrome (DS) and genetic epilepsy with febrile seizures plus (GEFS+). Electrophysiological analyses of DS and GEFS+ mouse models have led to the hypothesis that SCN1A mutations reduce the excitability of inhibitory cortical and hippocampal interneurons. To more directly examine the relative contribution of inhibitory interneurons and excitatory pyramidal cells to SCN1A-derived epilepsy, we first compared the expression of Nav1.1 in inhibitory parvalbumin (PV) interneurons and excitatory neurons from P22 mice using fluorescent immunohistochemistry. In the hippocampus and neocortex, 69% of Nav1.1 immunoreactive neurons were also positive for PV. In contrast, 13% and 5% of Nav1.1 positive cells in the hippocampus and neocortex, respectively, were found to co-localize with excitatory cells identified by CaMK2α immunoreactivity. Next, we reduced the expression of Scn1a in either a subset of interneurons (mainly PV interneurons) or excitatory cells by crossing mice heterozygous for a floxed Scn1a allele to either the Ppp1r2-Cre or EMX1-Cre transgenic lines, respectively. The inactivation of one Scn1a allele in interneurons of the neocortex and hippocampus was sufficient to reduce thresholds to flurothyl- and hyperthermia-induced seizures, whereas thresholds were unaltered following inactivation in excitatory cells. Reduced interneuron Scn1a expression also resulted in the generation of spontaneous seizures. These findings provide direct evidence for an important role of PV interneurons in the pathogenesis of Scn1a-derived epilepsies.

Rhythm and blues: animal models of epilepsy and depression comorbidity

Clinical evidence shows a strong, bidirectional comorbidity between depression and epilepsy that is associated with decreased quality of life and responsivity to pharmacotherapies. At present, the neurobiological underpinnings of this comorbidity remain hazy. To complicate matters, anticonvulsant drugs can cause mood disturbances, while antidepressant drugs can lower seizure threshold, making it difficult to treat patients suffering from both depression and epilepsy. Animal models have been created to untangle the mechanisms behind the relationship between these disorders and to serve as screening tools for new therapies targeted to treat both simultaneously. These animal models are based on chemical interventions (e.g. pentylenetetrazol, kainic acid, pilocarpine), electrical stimulations (e.g. kindling, electroshock), and genetic/selective breeding paradigms (e.g. genetically epilepsy-prone rats (GEPRs), genetic absence epilepsy rat from Strasbourg (GAERS), WAG/Rij rats, swim lo-active rats (SwLo)). Studies on these animal models point to some potential mechanisms that could explain epilepsy and depression comorbidity, such as various components of the dopaminergic, noradrenergic, serotonergic, and GABAergic systems, as well as key brain regions, like the amygdala and hippocampus. These models have also been used to screen possible therapies. The purpose of the present review is to highlight the importance of animal models in research on comorbid epilepsy and depression and to explore the contributions of these models to our understanding of the mechanisms and potential treatments for these disorders.

Autistic-like behaviour in Scn1a+/- mice and rescue by enhanced GABA-mediated neurotransmission

Haploinsufficiency of the SCN1A gene encoding voltage-gated sodium channel Na_V1.1 causes Dravet Syndrome (DS), a childhood neuropsychiatric disorder including recurrent intractable seizures, cognitive deficit, and autism-spectrum behaviors. The neural mechanisms responsible for cognitive deficit and autism-spectrum behaviors in DS are poorly understood. Here we show that mice with Scn1a haploinsufficiency display hyperactivity, stereotyped behaviors, social interaction deficits, and impaired context-dependent spatial memory. Olfactory sensitivity is retained, but novel food odors and social odors are aversive to Scn1a^+/− mice. GABAergic neurotransmission is specifically impaired by this mutation, and selective deletion of Na_V1.1 channels in forebrain interneurons is sufficient to cause these behavioral and cognitive impairments. Remarkably, treatment with low-dose clonazepam, a positive allosteric modulator of GABA_A receptors, completely rescued the abnormal social behaviors and deficits in fear memory in DS mice, demonstrating that they are caused by impaired GABAergic neurotransmission and not by neuronal damage from recurrent seizures. These results demonstrate a critical role for Na_V1.1 channels in neuropsychiatric functions and provide a potential therapeutic strategy for cognitive deficit and autism-spectrum behaviors in DS.

High incidence of pediatric idiopathic epilepsy is associated with familial and autosomal dominant disease in Eastern Newfoundland

PURPOSE

To describe the incidence and epidemiology of pediatric idiopathic epilepsy (IE) in Newfoundland and Labrador.

METHODS

All children in Newfoundland and Labrador aged 0-15 years with IE were ascertained through the provincial neurology clinic at the Janeway Child Health Centre. Family history, medical history and blood samples were obtained from probands and relatives. Two genes, SCN1A and KCNQ2, were screened for mutations by direct sequencing.

RESULTS

The mean annual incidence of IE for the population of children living in the Avalon region of Newfoundland from 2000 to 2004 was 107 per 100,000. This rate is approximately three-fold greater than rates reported in other developed countries. Of 117 families with IE eligible for study, 86 (74%) provided detailed pedigree data. Multiple different epilepsy phenotypes were identified. Fifty-five families (64%) had a positive family history. Eight of these had family histories compatible with autosomal dominant (AD) inheritance and these families lived in five different geographic isolates. DNA was obtained from 21 families (79 individuals). The two previously identified mutations in Newfoundland families with epilepsy were sequenced and excluded as pathogenic sites in all but one family which had a mutation in SCN1A.

CONCLUSION

The incidence of IE is high in the Avalon Peninsula of Newfoundland and the rate of familial disease is high throughout the province of Newfoundland and Labrador. The distribution of familial and AD IE in different geographic isolates, together with the clinical heterogeneity of disease suggests substantial genetic heterogeneity. It is likely that other novel mutations will be identified in this population.

Protective effect of the ketogenic diet in Scn1a mutant mice

PURPOSE

We evaluated the ability of the ketogenic diet (KD) to improve thresholds to flurothyl-induced seizures in two mouse lines with Scn1a mutations: one that models Dravet syndrome (DS) and another that models genetic (generalized) epilepsy with febrile seizures plus (GEFS+).

METHODS

At postnatal day 21, mouse models of DS and GEFS+ were fasted for 12-14 h and then placed on either a 6:1 (fats to proteins and carbohydrates) KD or a standard diet (SD) for 2 weeks. At the end of the 2-week period, we measured thresholds to seizures induced by the chemiconvulsant flurothyl. Body weight, β-hydroxybutyrate (BHB) levels, and glucose levels were also recorded every 2 days over a 2-week period in separate cohorts of mutant and wild-type mice that were either on the KD or the SD.

KEY FINDINGS

Mice on the KD gained less weight and exhibited significantly higher BHB levels compared to mice on the SD. It is notable that thresholds to flurothyl-induced seizures were restored to more normal levels in both mouse lines after 2 weeks on the KD.

SIGNIFICANCE

These results indicate that the KD may be an effective treatment for refractory patients with SCN1A mutations. The availability of mouse models of DS and GEFS+ also provides an opportunity to better understand the mechanism of action of the KD, which may facilitate the development of improved treatments.

Seizure and epilepsy: studies of seizure disorders in Drosophila

Despite the frequency of seizure disorders in the human population, the genetic and physiological basis for these defects has been difficult to resolve. Although many genetic contributions to seizure susceptibility have been identified, these involve disparate biological processes, many of which are not neural specific. The large number and heterogeneous nature of the genes involved makes it difficult to understand the complex factors underlying the etiology of seizure disorders. Examining the effect known genetic mutations have on seizure susceptibility is one approach that may prove fruitful. This approach may be helpful in both understanding how different physiological processes affect seizure susceptibility and identifying novel therapeutic treatments. We review here factors contributing to seizure susceptibility in Drosophila, a genetically tractable system that provides a model for human seizure disorders. Seizure-like neuronal activities and behaviors in the fruit fly are described, as well as a set of mutations that exhibit features resembling some human epilepsies and render the fly sensitive to seizures. Especially interesting are descriptions of a novel class of mutations that are second-site mutations that act as seizure suppressors. These mutations revert epilepsy phenotypes back to the wild-type range of seizure susceptibility. The genes responsible for seizure suppression are cloned with the goal of identifying targets for lead compounds that may be developed into new antiepileptic drugs.

Drosophila as a model for epilepsy: bss is a gain-of-function mutation in the para sodium channel gene that leads to seizures

We report the identification of bang senseless (bss), a Drosophila melanogaster mutant exhibiting seizure-like behaviors, as an allele of the paralytic (para) voltage-gated Na(+) (Na(V)) channel gene. Mutants are more prone to seizure episodes than normal flies because of a lowered seizure threshold. The bss phenotypes are due to a missense mutation in a segment previously implicated in inactivation, termed the "paddle motif" of the Na(V) fourth homology domain. Heterologous expression of cDNAs containing the bss(1) lesion, followed by electrophysiology, shows that mutant channels display altered voltage dependence of inactivation compared to wild type. The phenotypes of bss are the most severe of the bang-sensitive mutants in Drosophila and can be ameliorated, but not suppressed, by treatment with anti-epileptic drugs. As such, bss-associated seizures resemble those of pharmacologically resistant epilepsies caused by mutation of the human Na(V) SCN1A, such as severe myoclonic epilepsy in infants or intractable childhood epilepsy with generalized tonic-clonic seizures.

Altered function of the SCN1A voltage-gated sodium channel leads to gamma-aminobutyric acid-ergic (GABAergic) interneuron abnormalities

Voltage-gated sodium channels are required for the initiation and propagation of action potentials. Mutations in the neuronal voltage-gated sodium channel SCN1A are associated with a growing number of disorders including generalized epilepsy with febrile seizures plus (GEFS+),(7) severe myoclonic epilepsy of infancy, and familial hemiplegic migraine. To gain insight into the effect of SCN1A mutations on neuronal excitability, we introduced the human GEFS+ mutation SCN1A-R1648H into the orthologous mouse gene. Scn1a(RH/RH) mice homozygous for the R1648H mutation exhibit spontaneous generalized seizures and premature death between P16 and P26, whereas Scn1a(RH/+) heterozygous mice exhibit infrequent spontaneous generalized seizures, reduced threshold and accelerated propagation of febrile seizures, and decreased threshold to flurothyl-induced seizures. Inhibitory cortical interneurons from P5-P15 Scn1a(RH/+) and Scn1a(RH/RH) mice demonstrated slower recovery from inactivation, greater use-dependent inactivation, and reduced action potential firing compared with wild-type cells. Excitatory cortical pyramidal neurons were mostly unaffected. These results suggest that this SCN1A mutation predominantly impairs sodium channel activity in interneurons, leading to decreased inhibition. Decreased inhibition may be a common mechanism underlying clinically distinct SCN1A-derived disorders.