Mutant GABA(A) receptor gamma2-subunit in childhood absence epilepsy and febrile seizures

Models for discovery of targeted therapy in genetic epileptic encephalopathies

Epileptic encephalopathies are severe disorders emerging in the first days to years of life that commonly include refractory seizures, various types of movement disorders, and different levels of developmental delay. In recent years, many de novo occurring variants have been identified in individuals with these devastating disorders. To unravel disease mechanisms, the functional impact of detected variants associated with epileptic encephalopathies is investigated in a range of cellular and animal models. This review addresses efforts to advance and use such models to identify specific molecular and cellular targets for the development of novel therapies. We focus on ion channels as the best-studied group of epilepsy genes. Given the clinical and genetic heterogeneity of epileptic encephalopathy disorders, experimental models that can reflect this complexity are critical for the development of disease mechanisms-based targeted therapy. The convergence of technological advances in gene sequencing, stem cell biology, genome editing, and high throughput functional screening together with massive unmet clinical needs provides unprecedented opportunities and imperatives for precision medicine in epileptic encephalopathies.

Inhibitory synapse deficits caused by familial α1 GABAA receptor mutations in epilepsy

Epilepsy is a spectrum of neurological disorders with many causal factors. The GABA type-A receptor (GABA_AR) is a major genetic target for heritable human epilepsies. Here we examine the functional effects of three epilepsy-causing mutations to the α1 subunit (α1^T10'I, α1^D192N and α1^A295D) on inhibitory postsynaptic currents (IPSCs) mediated by the major synaptic GABA_AR isoform, α1β2γ2L. We employed a neuron - HEK293 cell heterosynapse preparation to record IPSCs mediated by mutant-containing GABA_ARs in isolation from other GABA_AR isoforms. IPSCs were recorded in the presence of the anticonvulsant drugs, carbamazepine and midazolam, and at elevated temperatures (22, 37 and 40°C) to gain insight into mechanisms of febrile seizures. The mutant subunits were also transfected into cultured cortical neurons to investigate changes in synapse formation and neuronal morphology using fluorescence microscopy. We found that IPSCs mediated by α1^T10'Iβ2γ2L, α1^D192Nβ2γ2L GABA_ARs decayed faster than those mediated by α1β2γ2L receptors. IPSCs mediated by α1^D192Nβ2γ2L and α1^A295Dβ2γ2L receptors also exhibited a heightened temperature sensitivity. In addition, the α1^T10'Iβ2γ2L GABA_ARs were refractory to modulation by carbamazepine or midazolam. In agreement with previous studies, we found that α1^A295Dβ2γ2L GABA_ARs were retained intracellularly in HEK293 cells and neurons. However, pre-incubation with 100nM suberanilohydroxamic acid (SAHA) induced α1^A295Dβ2γ2L GABA_ARs to mediate IPSCs that were indistinguishable in magnitude and waveform from those mediated by α1β2γ2L receptors. Finally, mutation-specific changes to synaptic bouton size, synapse number and neurite branching were also observed. These results provide new insights into the mechanisms of epileptogenesis of α1 epilepsy mutations and suggest possible leads for improving treatments for patients harbouring these mutations.

Defects at the crossroads of GABAergic signaling in generalized genetic epilepsies

Seizure disorders are very common and affect 3% of the general population. The recurrent unprovoked seizures that are also called epilepsies are highly diverse as to both underlying genetic basis and clinic presentations. Recent genetic advances and sequencing technologies indicate that many epilepsies previously thought to be without known causes, or idiopathic generalized epilepsies (IGEs), are virtually genetic epilepsy as they are caused by genetic variations. IGEs are estimated to account for ∼15-20% of all epilepsies. Initially IGEs were primarily considered channelopathies, because the first genetic defects identified in IGEs involved ion channel genes. However, new findings indicate that mutations in many non ion channel genes are also involved in addition to those in ion channel genes. Interestingly, mutations in many genes associated with epilepsy affect GABAergic signaling, a major biological pathway in epilepsy. Additionally, many antiepileptic drugs work via enhancing GABAergic signaling. Hence, the review will focus on the mutations that impair GABAergic signaling and selectively discuss the newly identified STXBP1, PRRT2, and DNM1 in addition to those long-established epilepsy ion channel genes that also impair GABAergic signaling like SCN1A and GABA_A receptor subunit genes. GABAergic signaling includes the pre- and post- synaptic mechanisms. Some mutations, such as STXBP1, PRRT2, DNM1, and SCN1A, impair GABAergic signaling mainly via pre-synaptic mechanisms while those mutations in GABA_A receptor subunit genes impair GABAergic signaling via post-synaptic mechanisms. Nevertheless, these findings suggest impaired GABAergic signaling is a converging pathway of defects for many ion channel or non ion channel mutations associated with genetic epilepsies.

Early Correlated Network Activity in the Hippocampus: Its Putative Role in Shaping Neuronal Circuits

Synchronized neuronal activity occurring at different developmental stages in various brain structures represents a hallmark of developmental circuits. This activity, which differs in its specific patterns among animal species may play a crucial role in de novo formation and in shaping neuronal networks. In the rodent hippocampus in vitro, the so-called giant depolarizing potentials (GDPs) constitute a primordial form of neuronal synchrony preceding more organized forms of activity such as oscillations in the theta and gamma frequency range. GDPs are generated at the network level by the interaction of the neurotransmitters glutamate and GABA which, immediately after birth, exert both a depolarizing and excitatory action on their targets. GDPs are triggered by GABAergic interneurons, which in virtue of their extensive axonal branching operate as functional hubs to synchronize large ensembles of cells. Intrinsic bursting activity, driven by a persistent sodium conductance and facilitated by the low expression of Kv7.2 and Kv7.3 channel subunits, responsible for I_M, exerts a permissive role in GDP generation. Here, we discuss how GDPs are generated in a probabilistic way when neuronal excitability within a local circuit reaches a certain threshold and how GDP-associated calcium transients act as coincident detectors for enhancing synaptic strength at emerging GABAergic and glutamatergic synapses. We discuss the possible in vivo correlate of this activity. Finally, we debate recent data showing how, in several animal models of neuropsychiatric disorders including autism, a GDPs dysfunction is associated to morphological alterations of neuronal circuits and behavioral deficits reminiscent of those observed in patients.

Epileptic Encephalopathies-Clinical Syndromes and Pathophysiological Concepts

Epileptic encephalopathies account for a large proportion of the intractable early-onset epilepsies and are characterized by frequent seizures and poor developmental outcome. The epileptic encephalopathies can be loosely divided into two related groups of named syndromes. The first comprises epilepsies where continuous EEG changes directly result in cognitive and developmental dysfunction. The second includes patients where cognitive impairment is present at seizure onset and is due to the underlying etiology but the epileptic activity may then worsen the cognitive abilities over time. Recent, large-scale exome studies have begun to establish the genetic architecture of the epileptic encephalopathies, resulting in a re-consideration of the boundaries of these named syndromes. The emergence of this genetic architecture has lead to three main pathophysiological concepts to provide a mechanistic framework for these disorders. In this article, we will review the classic syndromes, the most significant genetic findings, and relate both to the pathophysiological understanding of epileptic encephalopathies.

Functional variants in HCN4 and CACNA1H may contribute to genetic generalized epilepsy

Objective

Genetic generalized epilepsy (GGE) encompasses seizure disorders characterized by spike‐and‐wave discharges (SWD) originating within thalamo‐cortical circuits. Hyperpolarization‐activated (HCN) and T‐type Ca²⁺ channels are key modulators of rhythmic activity in these brain regions. Here, we screened HCN4 and CACNA1H genes for potentially contributory variants and provide their functional analysis.

Methods

Targeted gene sequencing was performed in 20 unrelated familial cases with different subtypes of GGE, and the results confirmed in 230 ethnically matching controls. Selected variants in CACNA1H and HCN4 were functionally assessed in tsA201 cells and Xenopus laevis oocytes, respectively.

Results

We discovered a novel CACNA1H (p.G1158S) variant in two affected members of a single family. One of them also carried an HCN4 (p.P1117L) variant inherited from the unaffected mother. In a separate family, an HCN4 variant (p.E153G) was identified in one of several affected members. Voltage‐clamp analysis of CACNA1H (p.G1158S) revealed a small but significant gain‐of‐function, including increased current density and a depolarizing shift of steady‐state inactivation. HCN4 p.P1117L and p.G153E both caused a hyperpolarizing shift in activation and reduced current amplitudes, resulting in a loss‐of‐function.

Significance

Our results are consistent with a model suggesting cumulative contributions of subtle functional variations in ion channels to seizure susceptibility and GGE.

Overexpressing wild-type γ2 subunits rescued the seizure phenotype in Gabrg2+/Q390X Dravet syndrome mice

OBJECTIVE

The mutant γ-aminobutyric acid type A (GABA_A ) receptor γ2(Q390X) subunit (Q351X in the mature peptide) has been associated with the epileptic encephalopathy, Dravet syndrome, and the epilepsy syndrome genetic epilepsy with febrile seizures plus (GEFS+). The mutation generates a premature stop codon that results in translation of a stable truncated and misfolded γ2 subunit that accumulates in neurons, forms intracellular aggregates, disrupts incorporation of γ2 subunits into GABA_A receptors, and affects trafficking of partnering α and β subunits. Heterozygous Gabrg2^+/Q390X knock-in (KI) mice had reduced cortical inhibition, spike wave discharges on electroencephalography (EEG), a lower seizure threshold to the convulsant drug pentylenetetrazol (PTZ), and spontaneous generalized tonic-clonic seizures. In this proof-of-principal study, we attempted to rescue these deficits in KI mice using a γ2 subunit gene (GABRG2) replacement therapy.

METHODS

We introduced the GABRG2 allele by crossing Gabrg2^+/Q390X KI mice with bacterial artificial chromosome (BAC) transgenic mice overexpressing HA (hemagglutinin)-tagged human γ2^HA subunits, and compared GABA_A receptor subunit expression by Western blot and immunohistochemical staining, seizure threshold by monitoring mouse behavior after PTZ-injection, and thalamocortical inhibition and network oscillation by slice recording.

RESULTS

Compared to KI mice, adult mice carrying both mutant allele and transgene had increased wild-type γ2 and partnering α1 and β2/3 subunits, increased miniature inhibitory postsynaptic current (mIPSC) amplitudes recorded from layer VI cortical neurons, reduced thalamocortical network oscillations, and higher PTZ seizure threshold.

SIGNIFICANCE

Based on these results we suggest that seizures in a genetic epilepsy syndrome caused by epilepsy mutant γ2(Q390X) subunits with dominant negative effects could be rescued potentially by overexpression of wild-type γ2 subunits.

Molecular Pathogenic Basis for GABRG2 Mutations Associated With a Spectrum of Epilepsy Syndromes, From Generalized Absence Epilepsy to Dravet Syndrome

OBJECTIVE

In this review article, we focus on the molecular pathogenic basis for genetic generalized epilepsies associated with mutations in the inhibitory γ-aminobutyric acid (GABAA) receptor γ2 subunit gene, GABRG2 (OMIM 137164), an established epilepsy gene.

OBSERVATIONS

The γ-aminobutyric acid (GABAA) receptor γ2 subunit gene, GABRG2, is abundantly expressed in the mammalian brain, and its encoded γ2 subunit is assembled into αβγ2 receptors, which are the major GABAA receptor isoforms in the brain. The γ2 subunits have a critical role in GABAA receptor trafficking and clustering at synapses. They reside inside the endoplasmic reticulum after synthesis, where they oligomerize with other binding partners, such as α and β subunits, and further assemble into pentameric receptors. Only correctly assembled receptors can traffic beyond the endoplasmic reticulum and reach the cell surface and synapses, where they conduct chloride ion current when activated by GABA. Mutations in GABRG2 have been associated with simple febrile seizures and with genetic epilepsy syndromes, including childhood absence epilepsy, generalized epilepsy with febrile seizures plus, and Dravet syndrome or severe myoclonic epilepsy in infancy. The mutations include missense, nonsense, and frameshift mutations, as well as splice-site and deletion mutations. The mutations have been identified in both coding and noncoding sequences like splice sites. In the coding sequence, these mutations are found in multiple locations, including the extracellular N-terminus, transmembrane domains, and transmembrane 3-transmembrane 4 intracellular loop. All of these mutations reduced channel function but to different extents and by diverse mechanisms, including nonsense-mediated messenger RNA decay, endoplasmic reticulum-associated protein degradation, dominant negative suppression of partnering subunits, mutant subunit aggregation causing cell stress and cell death, and gating defects.

CONCLUSIONS AND RELEVANCE

We conclude that the epilepsy phenotypic heterogeneity associated with GABRG2 mutations may be related to the extent of the reduction of GABAA receptor channel function and the differential dominant negative suppression, as well to toxicity related to the metabolism of mutant subunit proteins resulting from each mutant γ2 subunit, in addition to different genetic backgrounds.

The impact of genetic and experimental studies on classification and therapy of the epilepsies

Different types of epilepsy are associated with gene mutations, in which seizures can be the only symptom (genetic epilepsies) or be one of the elements of complex clinical pictures that are often progressive over time (epileptic or epileptogenic encephalopathies). In epileptogenic encephalopathies, epileptic seizures and other neurological and cognitive signs are symptoms of genetically determined neuropathological or neurochemical disorders. In epileptic encephalopathies, epileptic activity itself is thought to contribute to severe cognitive and behavioral impairments above and beyond what might be expected from the underlying pathology alone. The distinction is conceptually clear and clinically relevant, as the different categories have a different prognosis in terms of both epilepsy and associated neurological and cognitive picture, but the boundaries are sometimes difficult to define in the clinical practice. Here we review the genetic epilepsies from the clinician perspective. A monogenic inheritance has been defined only in a minority of idiopathic epilepsies making improper to rename genetic the category of idiopathic epilepsies, until the presumptive multigenic mechanism will be demonstrated. A search for gene mutations must be done in any patient with candidate genetic types of epilepsy or epileptic/epileptogenic encephalopathy (e.g. familial forms) to complete the diagnostic process, define the prognosis and optimize the therapy. Advanced methods are available to express the gene variant in experimental model systems and test its effect on the properties of the affected protein, on neuronal excitability and on phenotypes in model organisms, and may help in identifying treatments with compatible action mechanisms. The influence of genetic studies on epilepsy taxonomy is now a matter of discussion: their impact on the international classification of the epilepsies will hopefully be defined soon.

Expanding the phenotypic spectrum of GABRG2 variants: a recurrent GABRG2 missense variant associated with a severe phenotype

Pathogenic missense and truncating variants in the GABRG2 gene cause a spectrum of epilepsies, from Dravet syndrome to milder simple febrile seizures. In most cases, pathogenic missense variants in the GABRG2 gene segregate with a febrile seizure phenotype. In this case series, we report a recurrent, de novo missense variant (c0.316 G > A; p.A106T) in the GABRG2 gene that was identified in five unrelated individuals. These patients were described to have a more severe phenotype than previously reported for GABRG2 missense variants. Common features include variable early-onset seizures, significant motor and speech delays, intellectual disability, hypotonia, movement disorder, dysmorphic features and vision/ocular issues. Our report further explores a recurrent pathogenic missense variant within the GABRG2 variant family and broadens the spectrum of associated phenotypes for GABRG2-associated disorders.

Heat induced temperature dysregulation and seizures in Dravet Syndrome/GEFS+ Gabrg2+/Q390X mice

It has been established that febrile seizures and its extended syndromes like generalized epilepsy with febrile seizures (FS) plus (GEFS+) and Dravet syndrome have been associated with mutations especially in SCN1A and GABRG2 genes. In patients, the onset of FS is likely due to the combined effect of temperature and inflammation in genetically vulnerable individuals because fever is often associated with infection. Much effort has been spent to understand the mechanisms underlying fever induction of seizures. In addition to the role of cytokines in FS, previous studies in Scn1a^+/- knockout mice, a model of Dravet syndrome, indicated that temperature elevation alone could result in seizure generation, and the effect of elevated temperature inducing seizures was age-dependent. Here, we report the thermal effect in a different mouse model of Dravet syndrome, the Gabrg2^+/Q390X knockin mouse. We demonstrated age-dependent dysregulated temperature control and that temperature elevation produced myoclonic jerks, generalized tonic clonic seizures (GTCSs) and heightened anxiety-like symptoms in Gabrg2^+/Q390X mice. The study indicated that regardless of other inflammatory factors, brief heat alone increased brain excitability and induced multiple types of seizures in Gabrg2^+/Q390X mice, suggesting that mutations like GABRG2(Q390X) may alter brain thermal regulation and precipitate seizures during temperature elevations.

Investigations in GABAA receptor antibody-associated encephalitis

OBJECTIVE

To report the clinical features, comorbidities, receptor subunit targets, and outcome in patients with anti-GABA_A receptor (GABA_AR) encephalitis.

METHODS

Clinical study of 26 patients, including 17 new (April 2013-January 2016) and 9 previously reported patients. Antibodies to α1, β3, and γ2 subunits of the GABA_AR were determined using reported techniques.

RESULTS

Patients' median age was 40.5 years (interquartile range 48.5 [13.75-62.35] years; the youngest 2.5 months old; 13 female). Symptoms included seizures (88%), alteration of cognition (67%), behavior (46%), consciousness (42%), or abnormal movements (35%). Comorbidities were identified in 11 (42%) patients, including 7 tumors (mostly thymomas), 2 herpesvirus encephalitis (herpes simplex virus 1, human herpesvirus 6; coexisting with NMDAR antibodies), and 2 myasthenia without thymoma. Brain MRI was abnormal in 23 (88%) patients, showing in 20 (77%) multifocal, asynchronous, cortical-subcortical T2/fluid-attenuated inversion recovery abnormalities predominantly involving temporal (95%) and frontal (65%) lobes, but also basal ganglia and other regions. Immunologic or tumor therapy resulted in substantial improvement in 18/21 (86%) assessable patients; the other 3 (14%) died (2 status epilepticus, 1 sepsis). Compared with adults, children were more likely to have generalized seizures (p = 0.007) and movement disorders (p = 0.01) and less likely to have a tumor (p = 0.01). The main epitope targets were in the α1/β3 subunits of the GABA_AR.

CONCLUSIONS

Anti-GABA_AR encephalitis is characterized by frequent seizures and distinctive multifocal cortical-subcortical MRI abnormalities that provide an important clue to the diagnosis. The frequency of symptoms and comorbidities differ between children (more viral-related) and adults (more tumor-related). The disorder is severe but most patients respond to treatment.

Reduced local input to fast-spiking interneurons in the somatosensory cortex in the GABAA γ2 R43Q mouse model of absence epilepsy

Objective

Absence seizures in childhood absence epilepsy are initiated in the thalamocortical (TC) system. We investigated if these seizures result from altered development of the TC system before the appearance of seizures in mice containing a point mutation in γ‐aminobutyric acid A (GABA_A) receptor γ2 subunits linked to childhood absence epilepsy (R43Q). Findings from conditional mutant mice indicate that expression of normal γ2 subunits during preseizure ages protect from later seizures. This indicates that altered development in the presence of the R43Q mutation is a key contributor to the R43Q phenotype. We sought to identify the cellular processes affected by the R43Q mutation during these preseizure ages.

Methods

We examined landmarks of synaptic development at the end of the critical period for somatosensory TC plasticity using electrophysiologic recordings in TC brain slices from wild‐type mice and R43Q mice.

Results

We found that the level of TC connectivity to layer 4 (L4) principal cells and the properties of TC synapses were unaltered in R43Q mice. Furthermore, we show that, although TC feedforward inhibition and the total level of GABAergic inhibition were normal, there was a reduction in the local connectivity to cortical interneurons. This reduction leads to altered inhibition during bursts of cortical activity.

Significance

This altered inhibition demonstrates that alterations in cortical circuitry precede the onset of seizures by more than a week.

Altered Channel Conductance States and Gating of GABAA Receptors by a Pore Mutation Linked to Dravet Syndrome

We identified a de novo missense mutation, P302L, in the γ-aminobutyric acid type A (GABA_A) receptor γ2 subunit gene GABRG2 in a patient with Dravet syndrome using targeted next-generation sequencing. The mutation was in the cytoplasmic portion of the transmembrane segment M2 of the γ2 subunit that faces the pore lumen. GABA_A receptor α1 and β3 subunits were coexpressed with wild-type (wt) γ2L or mutant γ2L(P302L) subunits in HEK 293T cells and cultured mouse cortical neurons. We measured currents using whole-cell and single-channel patch clamp techniques, surface and total expression levels using surface biotinylation and Western blotting, and potential structural perturbations in mutant GABA_A receptors using structural modeling. The γ2(P302L) subunit mutation produced an ∼90% reduction of whole-cell current by increasing macroscopic desensitization and reducing GABA potency, which resulted in a profound reduction of GABA_A receptor-mediated miniature IPSCs (mIPSCs). The conductance of the receptor channel was reduced to 24% of control conductance by shifting the relative contribution of the conductance states from high- to low-conductance levels with only slight changes in receptor surface expression. Structural modeling of the GABA_A receptor in the closed, open, and desensitized states showed that the mutation was positioned to slow activation, enhance desensitization, and shift channels to a low-conductance state by reshaping the hour-glass-like pore cavity during transitions between closed, open, and desensitized states. Our study revealed a novel γ2 subunit missense mutation (P302L) that has a novel pathogenic mechanism to cause defects in the conductance and gating of GABA_A receptors, which results in hyperexcitability and contributes to the pathogenesis of the genetic epilepsy Dravet syndrome.

Epilepsy and ion channels

Many mutations of genes for ion channels result in some epilepsies. Their electrophysiological studies reveal pathophysiological mechanisms underlining epilepsy and also mechanism of action of several antiepileptic drugs. In this review, We briefly summarize pathophysiology of epilepsy and the mechanisms of antiepileptic drugs.

Mutations in GABRB3: From febrile seizures to epileptic encephalopathies

OBJECTIVE

To examine the role of mutations in GABRB3 encoding the β₃ subunit of the GABA_A receptor in individual patients with epilepsy with regard to causality, the spectrum of genetic variants, their pathophysiology, and associated phenotypes.

METHODS

We performed massive parallel sequencing of GABRB3 in 416 patients with a range of epileptic encephalopathies and childhood-onset epilepsies and recruited additional patients with epilepsy with GABRB3 mutations from other research and diagnostic programs.

RESULTS

We identified 22 patients with heterozygous mutations in GABRB3, including 3 probands from multiplex families. The phenotypic spectrum of the mutation carriers ranged from simple febrile seizures, genetic epilepsies with febrile seizures plus, and epilepsy with myoclonic-atonic seizures to West syndrome and other types of severe, early-onset epileptic encephalopathies. Electrophysiologic analysis of 7 mutations in Xenopus laevis oocytes, using coexpression of wild-type or mutant β₃, together with α₅ and γ_2s subunits and an automated 2-microelectrode voltage-clamp system, revealed reduced GABA-induced current amplitudes or GABA sensitivity for 5 of 7 mutations.

CONCLUSIONS

Our results indicate that GABRB3 mutations are associated with a broad phenotypic spectrum of epilepsies and that reduced receptor function causing GABAergic disinhibition represents the relevant disease mechanism.

De novo GABRG2 mutations associated with epileptic encephalopathies

Epileptic encephalopathies are a devastating group of severe childhood onset epilepsies with medication-resistant seizures and poor developmental outcomes. Many epileptic encephalopathies have a genetic aetiology and are often associated with de novo mutations in genes mediating synaptic transmission, including GABAA receptor subunit genes. Recently, we performed next generation sequencing on patients with a spectrum of epileptic encephalopathy phenotypes, and we identified five novel (A106T, I107T, P282S, R323W and F343L) and one known (R323Q) de novo GABRG2 pathogenic variants (mutations) in eight patients. To gain insight into the molecular basis for how these mutations contribute to epileptic encephalopathies, we compared the effects of the mutations on the properties of recombinant α1β2γ2L GABAA receptors transiently expressed in HEK293T cells. Using a combination of patch clamp recording, immunoblotting, confocal imaging and structural modelling, we characterized the effects of these GABRG2 mutations on GABAA receptor biogenesis and channel function. Compared with wild-type α1β2γ2L receptors, GABAA receptors containing a mutant γ2 subunit had reduced cell surface expression with altered subunit stoichiometry or decreased GABA-evoked whole-cell current amplitudes, but with different levels of reduction. While a causal role of these mutations cannot be established directly from these results, the functional analysis together with the genetic information suggests that these GABRG2 variants may be major contributors to the epileptic encephalopathy phenotypes. Our study further expands the GABRG2 phenotypic spectrum and supports growing evidence that defects in GABAergic neurotransmission participate in the pathogenesis of genetic epilepsies including epileptic encephalopathies.

GABA Australis, some reflections on the history of GABA receptor research in Australia

Research on GABA receptors has a long history in Australia dating from 1958 with David Curtis and his colleagues in Canberra. This review traces many of the advances made in Australia guided by highly cited publications and some obscure ones. It covers the discovery of key chemicals with which to investigate GABA receptor function including bicuculline, muscimol, phaclofen, THIP and (+)-CAMP. Also described are findings relevant to the involvement of mutant GABA receptors in inherited epilepsy. The modulation of GABA receptors by a bewildering range of chemicals, especially by flavonoids and terpenoids, is discussed.

PET measurement of "GABA shift" in the rat brain: A preclinical application of bolus plus constant infusion paradigm of [18F]flumazenil

INTRODUCTION

We measured the tiagabine-induced enhancement of the GABA_A receptor's affinity for benzodiazepine ligands ("GABA shift") using [¹⁸F]flumazenil (FMZ) PET with preclinical application of bolus plus constant infusion (B/I). Differences in quantified results of [¹⁸F]FMZ binding were compared to that of [¹⁸F]FMZ PET with single bolus injection (SB).

MATERIALS AND METHODS

Sprague-Dawley rats underwent [¹⁸F]FMZ PET scans with B/I, which consisted of baseline and "GABA shift" sessions in a scan, or scans with SB one week apart. Tiagabine (10mg/kg) was intravenously injected after the baseline session. [¹⁸F]FMZ binding potentials (BP_ND) were calculated using an equilibrium ratio method and a modeling method for B/I and SB, respectively. Regional brain BP_ND changes (%) before and after the tiagabine treatment were also calculated.

RESULTS

In PET studies with B/I (K_bol=20min), [¹⁸F]FMZ distribution in the various cortical and subcortical regions rapidly reached equilibrium. After the tiagabine treatment, [¹⁸F]FMZ BP_ND were substantially increased across the regions of interest (the frontal cortex, hippocampus, thalamus, and striatum), ranging from 3% to 7% BP_ND change (B/I) and 6-14% BP_ND change (SB), respectively. In PET studies with SB, a statistically significant increase of [¹⁸F]FMZ BP_ND was found only in the striatum, due to the greater inter-individual variance compared to those with B/I.

CONCLUSIONS

Data demonstrated that an [¹⁸F]FMZ PET study with B/I (K_bol=20min) is both reliable and sensitive for the assessment of altered GABA_A receptor function induced by tiagabine treatment in the rat brain. These results may help to improve the efficiency of the development of new GABA-targeting drugs in the preclinical stage using [¹⁸F]FMZ PET.

[Childhood absence epilepsy: An update]

Childhood absence epilepsy (CAE) is a common pediatric epilepsy syndrome accounting for 10% of all pediatric epilepsies. The aim of this review is to provide an updated overview of this epilepsy syndrome to pediatricians. Most of the patients can be initially managed in private practice or in general pediatric settings. Absence seizures are the only seizure type observed at the time of diagnosis in these patients. An electroencephalogram recording and a clinical evaluation lead to the diagnosis. The underlying mechanisms are not yet fully understood. CAE is considered a self-limited epilepsy syndrome since most of the patients will become seizure free. Only a few patients (5-10%) have resistant CAE. However, CAE, as well as any pediatric epilepsy syndrome, should not be considered a "benign" epilepsy. Attention deficit is observed in about one third of the patients. There is also an increased risk of academic difficulties related to specific cognitive disorders. Therefore, the early detection of children at risk of developing neuropsychological problems can be helpful for preventing school underachievement and poor psychosocial outcome. Recently, several studies including a large randomized controlled trial indicated that ethosuximide should be considered as a first-line treatment and valproate as a second-line treatment. Cognitive side effects should be an important factor in the selection of the antiepileptic drug and should be specifically assessed during the follow-up. This review concludes by discussing the criteria that might lead to referring the patient to a specialist.

A de novo missense mutation of GABRB2 causes early myoclonic encephalopathy

Background

Early myoclonic encephalopathy (EME), a disease with a devastating prognosis, is characterised by neonatal onset of seizures and massive myoclonus accompanied by a continuous suppression-burst EEG pattern. Three genes are associated with EMEs that have metabolic features. Here, we report a pathogenic mutation of an ion channel as a cause of EME for the first time.

Methods

Sequencing was performed for 214 patients with epileptic seizures using a gene panel with 109 genes that are known or suspected to cause epileptic seizures. Functional assessments were demonstrated by using electrophysiological experiments and immunostaining for mutant γ-aminobutyric acid-A (GABA_A) receptor subunits in HEK293T cells.

Results

We discovered a de novo heterozygous missense mutation (c.859A>C [p.Thr287Pro]) in the GABRB2-encoded β2 subunit of the GABA_A receptor in an infant with EME. No GABRB2 mutations were found in three other EME cases or in 166 patients with infantile spasms. GABA_A receptors bearing the mutant β2 subunit were poorly trafficked to the cell membrane and prevented γ2 subunits from trafficking to the cell surface. The peak amplitudes of currents from GABA_A receptors containing only mutant β2 subunits were smaller than that of those from receptors containing only wild-type β2 subunits. The decrease in peak current amplitude (96.4% reduction) associated with the mutant GABA_A receptor was greater than expected, based on the degree to which cell surface expression was reduced (66% reduction).

Conclusion

This mutation has complex functional effects on GABA_A receptors, including reduction of cell surface expression and attenuation of channel function, which would significantly perturb GABAergic inhibition in the brain.

New SMARCA2 mutation in a patient with Nicolaides-Baraitser syndrome and myoclonic astatic epilepsy

We report a de novo SMARCA2 missense mutation discovered on exome sequencing in a patient with myoclonic astatic epilepsy, leading to reassessment and identification of Nicolaides–Baraitser syndrome. This de novo SMARCA2 missense mutation c.3721C>G, p.Gln1241Glu is the only reported mutation on exon 26 outside the ATPase domain of SMARCA2 to be associated with Nicolaides–Baraitser syndrome and adds to chromatin remodeling as a pathway for epileptogenesis. © 2016 The Authors. American Journal of Medical Genetics Part A published by Wiley Periodicals, Inc.

Deleterious Rare Variants Reveal Risk for Loss of GABAA Receptor Function in Patients with Genetic Epilepsy and in the General Population

Genetic epilepsies (GEs) account for approximately 50% of all seizure disorders, and familial forms include mutations in single GABA_A receptor subunit genes (GABRs). In 144 sporadic GE cases (GECs), exome sequencing of 237 ion channel genes identified 520 GABR variants. Among these variants, 33 rare variants in 11 GABR genes were present in 24 GECs. To assess functional risk of variants in GECs, we selected 8 variants found in GABRA, 3 in GABRB, and 3 in GABRG and compared them to 18 variants found in the general population for GABRA1 (n = 9), GABRB3 (n = 7), and GABRG2 (n = 2). To identify deleterious variants and gain insight into structure-function relationships, we studied the gating properties, surface expression and structural perturbations of the 32 variants. Significant reduction of GABA_A receptor function was strongly associated with variants scored as deleterious and mapped within the N-terminal and transmembrane domains. In addition, 12 out of 17 variants mapped along the β+/α- GABA binding interface, were associated with reduction in channel gating and were predicted to cause structural rearrangements of the receptor by in silico simulations. Missense or nonsense mutations of GABRA1, GABRB3 and GABRG2 primarily impair subunit biogenesis. In contrast, GABR variants affected receptor function by impairing gating, suggesting that different mechanisms are operating in GABR epilepsy susceptibility variants and disease-causing mutations. The functional impact of single GABR variants found in individuals with sporadic GEs warrants the use of molecular diagnosis and will ultimately improve the treatment of genetic epilepsies by using a personalized approach.

Differential molecular and behavioural alterations in mouse models of GABRG2 haploinsufficiency versus dominant negative mutations associated with human epilepsy

Genetic epilepsy is a common disorder with phenotypic variation, but the basis for the variation is unknown. Comparing the molecular pathophysiology of mutations in the same epilepsy gene may provide mechanistic insights into the phenotypic heterogeneity. GABRG2 is an established epilepsy gene, and mutations in it produce epilepsy syndromes with varying severities. The disease phenotype in some cases may be caused by simple loss of subunit function (functional haploinsufficiency), while others may be caused by loss-of-function plus dominant negative suppression and other cellular toxicity. Detailed molecular defects and the corresponding seizures and related comorbidities resulting from haploinsufficiency and dominant negative mutations, however, have not been compared. Here we compared two mouse models of GABRG2 loss-of-function mutations associated with epilepsy with different severities, Gabrg2+/Q390X knockin (KI) and Gabrg2+/- knockout (KO) mice. Heterozygous Gabrg2+/Q390X KI mice are associated with a severe epileptic encephalopathy due to a dominant negative effect of the mutation, while heterozygous Gabrg2+/- KO mice are associated with mild absence epilepsy due to simple haploinsufficiency. Unchanged at the transcriptional level, KI mice with severe epilepsy had neuronal accumulation of mutant γ2 subunits, reduced remaining functional wild-type subunits in dendrites and synapses, while KO mice with mild epilepsy had no intracellular accumulation of the mutant subunits and unaffected biogenesis of the remaining wild-type subunits. Consequently, KI mice with dominant negative mutations had much less wild-type receptor expression, more severe seizures and behavioural comorbidities than KO mice. This work provides insights into the pathophysiology of epilepsy syndrome heterogeneity and designing mechanism-based therapies.

Mutations in GABRG2 receptor gene are not a major factor in the pathogenesis of mesial temporal lobe epilepsy in Indian population

AIM

This study is focused on GABRG2 gene sequence variations in patients with mesial temporal lobe epilepsy (mTLE). The GABAAreceptor is a heteropentameric receptor and alpha-1 beta-2 gamma-2 subunits combination is most abundant and present in almost all regions of the brain. The gamma-2 subunit (GABRG2) gene mutations have been reported in different epilepsy pathologies. In the present study we have looked for GABRG2 gene sequence variations in patients with mTLE.

MATERIALS AND METHODS

Twenty patients (12 females and eight males, age 4.6-38 years) with MTLE were recruited for this investigation. Patients were recommended for epilepsy surgery after all clinical investigations as per the epilepsy protocol. Ethnically matched glioma or meningioma patients were considered as nonepileptic controls. During temporal lobectomy of amygdalohippocampectomy, hippocampal brain tissue samples were resected guided by intraoperative electrocorticography (ECoG) activity. All 11 exons of GABRG2 gene with their flanking intronic regions were amplified by polymerase chain reaction (PCR) and screened by DNA sequencing analysis for sequence variations.

STATISTICAL ANALYSIS USED

Comparison of allele frequencies between patient and control groups was determined using a c(2) test.

RESULTS AND CONCLUSIONS

Total five DNA sequence variations were identified, three in exonic regions (c.643A > G, rs211035), (c.T > A, rs424740), and (c.C > T, rs418210) and two in intronic regions (c.751 + 41A > G, rs211034) and (c.751 + 52G > A, rs 34281163). Allele frequencies of variants identified in this study did not differ between patients and normal controls. Thus, we conclude that GABRG2 gene may not be playing significant role in the development of epilepsy or as a susceptibility gene in patients with MTLE in Indian population.

Rats with Malformations of Cortical Development Exhibit Decreased Length of AIS and Hypersensitivity to Pilocarpine-Induced Status Epilepticus

Malformations of cortical development (MCD) are critical brain development disorders associated with varied abnormalities in both anatomic structures and neural functioning. It is also a very common etiology to the epilepsy, in which the alteration on excitability of cortical neurons is hypothesized as one of important causes to the epileptic seizures. Due to the key role in regulating neuron firing properties, the plasticity of axon initial segment (AIS) was investigated in present study to further determine the relation between MCD and epilepsy. Our results showed a prolonged decrease in the length of AIS occurred in MCD animal models. Besides, the AIS was also found greatly shortened in MCD models during the acute, but not chronic phase of status epileptics compared with intact controls. Our findings of identification of AIS plasticity in MCD animal models and its hypersensitivity to status epilepsy are significant in furthering our understanding of the pathophysiological mechanisms involved in this disorder.

Neocortical gamma oscillations in idiopathic generalized epilepsy

OBJECTIVE

Absence seizures in patients with idiopathic generalized epilepsy (IGE) may in part be explained by a decrease in phasic GABAA (type-A γ-aminobutyric acid) receptor function, but the mechanisms are only partly understood. Here we studied the relation between ictal and interictal spike-wave discharges (SWDs) and electroencephalography (EEG) gamma oscillatory activity (30-60 Hz) in patients with IGE.

METHODS

EEG recordings were obtained of 14 children with IGE (mean age, 8.5 ± 5 years) and 14 age- and sex-matched controls. Time-frequency analysis of each seizure and seizure-free control epochs was performed and cross-coherences of neocortical gamma oscillations were calculated to describe interictal and ictal characteristics of generalized seizures.

RESULTS

SWDs were characterized with an abrupt increase of oscillatory activity of 3-4 and 13-60 Hz, peaking at 3-4 and 30-60 Hz, and with a simultaneous decrease in the 8-12 Hz frequency band. The rise in EEG gamma oscillations was short-lasting and decreased before activity declined at lower frequency ranges. Compared to control patients, patients with epilepsy also showed higher interictal values of mean coherence of gamma activity, but this interictal increase was not significant after post hoc analysis.

SIGNIFICANCE

Our data support the hypothesis that gamma oscillatory activity increase concomitantly with rises in activity of lower EEG frequencies during absence seizures and that the activity starts to cease earlier than lower EEG frequencies. The data did not support a change in gamma activity preceding the 3-4 Hz SWDs. SWDs are hypothetically generated by the synchronous interaction between the thalamus and the cortex, whereas the production of gamma activity is the result of activity in local inhibitory networks. Thus, the modification of SWD by gamma activity may be understood in terms of the cellular and synaptic mechanisms involved.

De novo GABRA1 mutations in Ohtahara and West syndromes

OBJECTIVE

GABRA1 mutations have been identified in patients with familial juvenile myoclonic epilepsy, sporadic childhood absence epilepsy, and idiopathic familial generalized epilepsy. In addition, de novo GABRA1 mutations were recently reported in a patient with infantile spasms and four patients with Dravet syndrome. Those reports suggest that GABRA1 mutations are associated with infantile epilepsy including early onset epileptic encephalopathies. In this study, we searched for GABRA1 mutations in patients with infantile epilepsy to investigate the phenotypic spectrum of GABRA1 mutations.

METHODS

In total, 526 and 145 patients with infantile epilepsy were analyzed by whole-exome sequencing and GABRA1-targeted resequencing, respectively.

RESULTS

We identified five de novo missense GABRA1 mutations in six unrelated patients. A p.R112Q mutation in the long extracellular N-terminus was identified in a patient with infantile epilepsy; p.P260L, p.M263T, and p.M263I in transmembrane spanning domain 1 (TM1) were identified in three unrelated patients with West syndrome and a patient with Ohtahara syndrome, respectively; and p.V287L in TM2 was identified in a patient with unclassified early onset epileptic encephalopathy. Four of these mutations have not been observed previously.

SIGNIFICANCE

Our study suggests that de novo GABRA1 mutations can cause early onset epileptic encephalopathies, including Ohtahara syndrome and West syndrome.

Loss of synaptic Zn2+ transporter function increases risk of febrile seizures

Febrile seizures (FS) are the most common seizure syndrome and are potentially a prelude to more severe epilepsy. Although zinc (Zn(2+)) metabolism has previously been implicated in FS, whether or not variation in proteins essential for Zn(2+) homeostasis contributes to susceptibility is unknown. Synaptic Zn(2+) is co-released with glutamate and modulates neuronal excitability. SLC30A3 encodes the zinc transporter 3 (ZNT3), which is primarily responsible for moving Zn(2+) into synaptic vesicles. Here we sequenced SLC30A3 and discovered a rare variant (c.892C > T; p.R298C) enriched in FS populations but absent in population-matched controls. Functional analysis revealed a significant loss-of-function of the mutated protein resulting from a trafficking deficit. Furthermore, mice null for ZnT3 were more sensitive than wild-type to hyperthermia-induced seizures that model FS. Together our data suggest that reduced synaptic Zn(2+) increases the risk of FS and more broadly support the idea that impaired synaptic Zn(2+) homeostasis can contribute to neuronal hyperexcitability.

Early postnatal switch in GABAA receptor α-subunits in the reticular thalamic nucleus

The GABAergic neurons of the thalamic reticular nucleus (nRt) provide the primary source of inhibition within the thalamus. Using physiology, pharmacology, and immunohistochemistry in mice, we characterized postsynaptic developmental changes in these inhibitory projection neurons. First, at postnatal days 3-5 (P3-5), inhibitory postsynaptic currents (IPSCs) decayed very slowly, followed by a biphasic developmental progression, becoming faster at P6-8 and then slower again at P9-11 before stabilizing in a mature form around P12. Second, the pharmacological profile of GABA(A) receptor (GABA(A)R)-mediated IPSCs differed between neonatal and mature nRt neurons, and this was accompanied by reciprocal changes in α3 (late) and α5 (early) subunit expression in nRt. Zolpidem, selective for α1- and α3-containing GABA(A)Rs, augmented only mature IPSCs, whereas clonazepam enhanced IPSCs at all stages. This effect was blocked by the α5-specific inverse agonist L-655,708, but only in immature neurons. In α3(H126R) mice, in which α3-subunits were mutated to become benzodiazepine insensitive, IPSCs were enhanced compared with those in wild-type animals in early development. Third, tonic GABA(A)R activation in nRt is age dependent and more prominent in immature neurons, which correlates with early expression of α5-containing GABA(A)Rs. Thus neonatal nRt neurons show relatively high expression of α5-subunits, which contributes to both slow synaptic and tonic extrasynaptic inhibition. The postnatal switch in GABA(A)R subunits from α5 to α3 could facilitate spontaneous network activity in nRt that occurs at this developmental time point and which is proposed to play a role in early circuit development.

Novel GABRG2 mutations cause familial febrile seizures

Objective:

To identify the genetic cause in a large family with febrile seizures (FS) and temporal lobe epilepsy (TLE) and subsequently search for additional mutations in a cohort of 107 families with FS, with or without epilepsy.

Methods:

The cohort consisted of 1 large family with FS and TLE, 64 smaller French families recruited through a national French campaign, and 43 Italian families. Molecular analyses consisted of whole-exome sequencing and mutational screening.

Results:

Exome sequencing revealed a p.Glu402fs*3 mutation in the γ2 subunit of the GABA_A receptor gene (GABRG2) in the large family with FS and TLE. Three additional nonsense and frameshift GABRG2 mutations (p.Arg136*, p.Val462fs*33, and p.Pro59fs*12), 1 missense mutation (p.Met199Val), and 1 exonic deletion were subsequently identified in 5 families of the follow-up cohort.

Conclusions:

We report GABRG2 mutations in 5.6% (6/108) of families with FS, with or without associated epilepsy. This study provides evidence that GABRG2 mutations are linked to the FS phenotype, rather than epilepsy, and that loss-of-function of GABA_A receptor γ2 subunit is the probable underlying pathogenic mechanism.

The GABAA Receptor as a Therapeutic Target for Neurodevelopmental Disorders

Intellectual disability, autism spectrum disorder, and epilepsy are prime examples of neurodevelopmental disorders that collectively affect a significant percentage of the world population. Recent technological breakthroughs allowed the elucidation of the genetic causes of many of these disorders. As neurodevelopmental disorders are genetically heterogeneous, the development of rational therapy is extremely challenging. Fortunately, many causative genes are interconnected and cluster in specific cellular pathways. Targeting a common node in such a network would allow us to interfere with a series of related neurodevelopmental disorders at once. Here, we argue that the GABAergic system is disturbed in many neurodevelopmental disorders, including fragile X syndrome, Rett syndrome, and Dravet syndrome, and is a key candidate target for therapeutic intervention. Many drugs that modulate the GABAergic system have already been tested in animal models with encouraging outcomes and are readily available for clinical trials.

Disruption of Fgf13 causes synaptic excitatory-inhibitory imbalance and genetic epilepsy and febrile seizures plus

We identified a family in which a translocation between chromosomes X and 14 was associated with cognitive impairment and a complex genetic disorder termed "Genetic Epilepsy and Febrile Seizures Plus" (GEFS(+)). We demonstrate that the breakpoint on the X chromosome disrupted a gene that encodes an auxiliary protein of voltage-gated Na(+) channels, fibroblast growth factor 13 (Fgf13). Female mice in which one Fgf13 allele was deleted exhibited hyperthermia-induced seizures and epilepsy. Anatomic studies revealed expression of Fgf13 mRNA in both excitatory and inhibitory neurons of hippocampus. Electrophysiological recordings revealed decreased inhibitory and increased excitatory synaptic inputs in hippocampal neurons of Fgf13 mutants. We speculate that reduced expression of Fgf13 impairs excitability of inhibitory interneurons, resulting in enhanced excitability within local circuits of hippocampus and the clinical phenotype of epilepsy. These findings reveal a novel cause of this syndrome and underscore the powerful role of FGF13 in control of neuronal excitability.

GABA(A) receptor modulation and neuropharmacological activities of viscosine isolated from Dodonaea viscosa (Linn)

The objective of the present study was to evaluate the modulation of GABA-evoked currents by the flavonoid viscosine at recombinant GABA(A) receptors, and subsequently to study its anxiolytic, sedative and anticonvulsant activities. Viscosine (1-300μM) positively modulated GABA-evoked currents at human α1β2γ2L and α2β2γ2L GABA(A) receptors expressed in Xenopus oocytes in a flumazenil insensitive manner. In behavioral studies, viscosine at doses of 10-100mg/kg (i.p.) exerted significant anxiolytic effects in the elevated plus maze, light-dark and open field tests (*P<0.05, **P<0.01, ***P<0.001 n=6, One-way ANOVA post-Dunnett's test), and sedative effects at high doses (100mg/kg i.p.) in hole board and thiopental induced sleep time tests. The anxiolytic effect in the elevated plus maze test was not blocked by flumazenil whereas pentylenetetrazole (PTZ) completely attenuated the effect, indicating that the activity was mediated via the non-benzodiazepine sites of GABA(A) receptors. Furthermore, viscosine at doses of 10-100mg/kg (i.p.) exerted anticonvulsant effects in a dose-dependent manner in PTZ, picrotoxin and bicuculline induced seizure paradigms (*P<0.05, **P<0.01,***P<0.001 n=6, One-way ANOVA post-Dunnett's test). In conclusion, the results of the present study suggest that the anxiolytic and anticonvulsant actions of viscosine are likely mediated via its positive allosteric modulatory action of GABA at different GABA(A) receptor subtypes.

Dysgraphia as a Mild Expression of Dystonia in Children with Absence Epilepsy

Background

Absence epilepsy (AE) is etiologically heterogeneous and has at times been associated with idiopathic dystonia.

Objectives

Based on the clinical observation that children with AE often exhibit, interictally, a disorder resembling writer’s cramp but fully definable as dysgraphia, we tested the hypothesis that in this particular population dysgraphia would represent a subtle expression of dystonia.

Methods

We ascertained the prevalence of dysgraphia in 82 children with AE (mean age 9.7) and average intelligence and compared them with 89 age-, gender- and class-matched healthy children (mean age 10.57) using tests for handwriting fluency and quality, based on which we divided patients and controls into four subgroups: AE/dysgraphia, AE without dysgraphia, controls with dysgraphia and healthy controls. We compared the blink reflex recovery cycle in children belonging to all four subgroups.

Results

We identified dysgraphia in 17/82 children with AE and in 7/89 controls (20.7 vs 7.8%; P = 0.016) with the former having a 3.4-times higher risk of dysgraphia regardless of age and gender (odd ratio: 3.49; 95% CI 1.2, 8.8%). The AE/dysgraphia subgroup performed worse than controls with dysgraphia in one test of handwriting fluency (P = 0.037) and in most trials testing handwriting quality (P< 0.02). In children with AE/dysgraphia the blink reflex showed no suppression at short interstimulus intervals, with a difference for each value emerging when comparing the study group with the three remaining subgroups (P<0.001).

Conclusions

In children with AE, dysgraphia is highly prevalent and has a homogeneous, distinctive pathophysiological substrate consistent with idiopathic dystonia.

Ionotropic GABA and Glutamate Receptor Mutations and Human Neurologic Diseases

The advent of whole exome/genome sequencing and the technology-driven reduction in the cost of next-generation sequencing as well as the introduction of diagnostic-targeted sequencing chips have resulted in an unprecedented volume of data directly linking patient genomic variability to disorders of the brain. This information has the potential to transform our understanding of neurologic disorders by improving diagnoses, illuminating the molecular heterogeneity underlying diseases, and identifying new targets for therapeutic treatment. There is a strong history of mutations in GABA receptor genes being involved in neurologic diseases, particularly the epilepsies. In addition, a substantial number of variants and mutations have been found in GABA receptor genes in patients with autism, schizophrenia, and addiction, suggesting potential links between the GABA receptors and these conditions. A new and unexpected outcome from sequencing efforts has been the surprising number of mutations found in glutamate receptor subunits, with the GRIN2A gene encoding the GluN2A N-methyl-d-aspartate receptor subunit being most often affected. These mutations are associated with multiple neurologic conditions, for which seizure disorders comprise the largest group. The GluN2A subunit appears to be a locus for epilepsy, which holds important therapeutic implications. Virtually all α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor mutations, most of which occur within GRIA3, are from patients with intellectual disabilities, suggesting a link to this condition. Similarly, the most common phenotype for kainate receptor variants is intellectual disability. Herein, we summarize the current understanding of disease-associated mutations in ionotropic GABA and glutamate receptor families, and discuss implications regarding the identification of human mutations and treatment of neurologic diseases.

Epilepsy genetics: the ongoing revolution

Epilepsies have long remained refractory to gene identification due to several obstacles, including a highly variable inter- and intrafamilial expressivity of the phenotypes, a high frequency of phenocopies, and a huge genetic heterogeneity. Recent technological breakthroughs, such as array comparative genomic hybridization and next generation sequencing, have been leading, in the past few years, to the identification of an increasing number of genomic regions and genes in which mutations or copy-number variations cause various epileptic disorders, revealing an enormous diversity of pathophysiological mechanisms. The field that has undergone the most striking revolution is that of epileptic encephalopathies, for which most of causing genes have been discovered since the year 2012. Some examples are the continuous spike-and-waves during slow-wave sleep and Landau-Kleffner syndromes for which the recent discovery of the role of GRIN2A mutations has finally confirmed the genetic bases. These new technologies begin to be used for diagnostic applications, and the main challenge now resides in the interpretation of the huge mass of variants detected by these methods. The identification of causative mutations in epilepsies provides definitive confirmation of the clinical diagnosis, allows accurate genetic counselling, and sometimes permits the development of new appropriate and specific antiepileptic therapies. Future challenges include the identification of the genetic or environmental factors that modify the epileptic phenotypes caused by mutations in a given gene and the understanding of the role of somatic mutations in sporadic epilepsies.

Mutations in the GABA Transporter SLC6A1 Cause Epilepsy with Myoclonic-Atonic Seizures

GAT-1, encoded by SLC6A1, is one of the major gamma-aminobutyric acid (GABA) transporters in the brain and is responsible for re-uptake of GABA from the synapse. In this study, targeted resequencing of 644 individuals with epileptic encephalopathies led to the identification of six SLC6A1 mutations in seven individuals, all of whom have epilepsy with myoclonic-atonic seizures (MAE). We describe two truncations and four missense alterations, all of which most likely lead to loss of function of GAT-1 and thus reduced GABA re-uptake from the synapse. These individuals share many of the electrophysiological properties of Gat1-deficient mice, including spontaneous spike-wave discharges. Overall, pathogenic mutations occurred in 6/160 individuals with MAE, accounting for ~4% of unsolved MAE cases.

Cortical alterations in a model for absence epilepsy and febrile seizures: in vivo findings in mice carrying a human GABA(A)R gamma2 subunit mutation

Childhood absence epilepsy (CAE) is one of the most common forms of epilepsy among children. The study of a large Australian family demonstrated that a point mutation in the gene encoding the gamma2 subunit of the GABA(A) receptor (G2R43Q) leads to an autosomal dominantly inherited form of CAE and febrile seizures (FS). In a transgenic mouse model carrying the gamma2 (R43Q) mutation heterozygous animals recapitulate the human phenotype. In-vitro experiments indicated that this point mutation impairs cortical inhibition and thus increases the likelihood of seizures. Here, using whole-cell (WC) and extracellular (EC) recordings as well as voltage-sensitive dye imaging (VSDI), we systematically searched for an in vivo correlate of cortical alterations caused by the G2R43Q mutation, as suggested by the mentioned in vitro results. We measured spontaneous and whisker-evoked activity in the primary somatosensory cortex and ventral posteriomedial nucleus of the thalamus (VPM) before and after intraperitoneal injection of the ictogenic substance pentylenetetrazol (PTZ) in urethane-anesthetized G2R43Q mice and controls in a blinded setting. Compared to wildtype controls in G2R43Q mice after PTZ injection we found 1.) Increased cortical spontaneous activity in layer 2/3 and layer 5/6 pyramidal neurons (increased standard deviation of the mean membrane potential in WC recordings), 2.) Increased variance of stimulus evoked cortical responses in VSDI experiments. 3.) The cortical effects are not due to increased strength or precision of thalamic output. In summary our findings support the hypothesis of a cortical pathology in this mouse model of human genetic absence epilepsy. Further study is needed to characterize underlying molecular mechanisms.

"It's good to know": experiences of gene identification and result disclosure in familial epilepsies

Recognition of the role of genetics in the epilepsies has increased dramatically, impacting on clinical practice across many epilepsy syndromes. There is limited research investigating the impact of gene identification on individuals and families with epilepsy. While research has focused on the impact of delivering genetic information to families at the time of diagnosis in genetic diseases more broadly, little is known about how genetic results in epileptic diseases influences people's lives many years after it has been conveyed. This study used qualitative methods to explore the experience of receiving a genetic result in people with familial epilepsy. Interviews were conducted with individuals with familial epilepsies in whom the underlying genetic mutation had been identified. Recorded interviews underwent thematic analysis. 20 individuals from three families with different epilepsy syndromes and causative genes were interviewed. Multiple generations within families were studied. The mean time from receiving the genetic result prior to interview was 10.9 years (range 5-14 years). Three major themes were identified: 1) living with epilepsy: an individual's experience of the severity of epilepsy in their family influenced their view. 2) Clinical utility of the test: participants expressed varying reactions to receiving a genetic result. While for some it provided helpful information and relief, others were not surprised by the finding given the familial context. Some valued the use of genetic information for reproductive decision-making, particularly in the setting of severely affected family members. While altruistic reasons for participating in genetic research were discussed, participants emphasised the benefit of participation to them and their families. 3) 'Talking about the family genes': individuals reported poor communication between family members about their epilepsy and its genetic implications. The results provide important insights into the family experience of genetic epilepsies and communication within families. This information can be used to inform the development of guidelines for genetic result disclosure and genetic counselling for individuals and families with epilepsies.

Novel astrocyte targets: new avenues for the therapeutic treatment of epilepsy

During the last 20 years, it has been well established that a finely tuned, continuous crosstalk between neurons and astrocytes not only critically modulates physiological brain functions but also underlies many neurological diseases. In particular, this novel way of interpreting brain activity is markedly influencing our current knowledge of epilepsy, prompting a re-evaluation of old findings and guiding novel experimentation. Here, we review recent studies that have unraveled novel and unique contributions of astrocytes to the generation and spread of convulsive and nonconvulsive seizures and epileptiform activity. The emerging scenario advocates an overall framework in which a dynamic and reciprocal interplay among astrocytic and neuronal ensembles is fundamental for a fuller understanding of epilepsy. In turn, this offers novel astrocytic targets for the development of those really novel chemical entities for the control of convulsive and nonconvulsive seizures that have been acknowledged as a key priority in the management of epilepsy.

SWDreader: a wavelet-based algorithm using spectral phase to characterize spike-wave morphological variation in genetic models of absence epilepsy

BACKGROUND

Spike-wave discharges (SWD) found in neuroelectrical recordings are pathognomonic to absence epilepsy. The characteristic spike-wave morphology of the spike-wave complex (SWC) constituents of SWDs can be mathematically described by a subset of possible spectral power and phase values. Morlet wavelet transform (MWT) generates time-frequency representations well-suited to identifying this SWC-associated subset.

NEW METHOD

MWT decompositions of SWDs reveal spectral power concentrated at harmonic frequencies. The phase relationships underlying SWC morphology were identified by calculating the differences between phase values at SWD fundamental frequency from the 2nd, 3rd, and 4th harmonics, then using the three phase differences as coordinates to generate a density distribution in a {360°×360°×360°} phase difference space. Strain-specific density distributions were generated from SWDs of mice carrying the Gria4, Gabrg2, or Scn8a mutations to determine whether SWC morphological variants reliably mapped to the same regions of the distribution, and if distribution values could be used to detect SWD.

COMPARISON WITH EXISTING METHODS

To the best of our knowledge, this algorithm is the first to employ spectral phase to quantify SWC morphology, making it possible to computationally distinguish SWC morphological subtypes and detect SWDs.

RESULTS/CONCLUSIONS

Proof-of-concept testing of the SWDfinder algorithm shows: (1) a major pattern of variation in SWC morphology maps to one axis of the phase difference distribution, (2) variability between the strain-specific distributions reflects differences in the proportions of SWC subtypes generated during SWD, and (3) regularities in the spectral power and phase profiles of SWCs can be used to detect waveforms possessing SWC-like morphology.

Endozepines

Since their introduction in the 1960s, benzodiazepines (BZs) remain one of the most commonly prescribed medications, acting as potent sedatives, hypnotics, anxiolytics, anticonvulsants, and muscle relaxants. The primary neural action of BZs and related compounds is augmentation of inhibitory transmission, which occurs through allosteric modulation of the gamma-aminobutyric acid (GABA)-induced current at the gamma-aminobutyric acid receptor (GABAAR). The discovery of the BZ-binding site on GABAARs encouraged many to speculate that the brain produces its own endogenous ligands to this site (Costa & Guidotti, 1985). The romanticized quest for endozepines, endogenous ligands to the BZ-binding site, has uncovered a variety of ligands that might fulfill this role, including oleamides (Cravatt et al., 1995), nonpeptidic endozepines (Rothstein et al., 1992), and the protein diazepam-binding inhibitor (DBI) (Costa & Guidotti, 1985). Of these ligands, DBI, and affiliated peptide fragments, is the most extensively studied endozepine. The quest for the "brain's Valium" over the decades has been elusive as mainly negative allosteric modulatory effects have been observed (Alfonso, Le Magueresse, Zuccotti, Khodosevich, & Monyer, 2012; Costa & Guidotti, 1985), but recent evidence is accumulating that DBI displays regionally discrete endogenous positive modulation of GABA transmission through activation of the BZ receptor (Christian et al., 2013). Herein, we review the literature on this topic, focusing on identification of the endogenous molecule and its region-specific expression and function.

Dodecyl maltopyranoside enabled purification of active human GABA type A receptors for deep and direct proteomic sequencing

The challenge in high-quality membrane proteomics is all about sample preparation prior to HPLC, and the cell-to-protein step poses a long-standing bottleneck. Traditional protein extraction methods apply ionic or poly-disperse detergents, harsh denaturation, and repeated protein/peptide precipitation/resolubilization afterward, but suffer low yield, low reproducibility, and low sequence coverage. Contrary to attempts to subdue, we resolved this challenge by providing proteins nature-and-activity-promoting conditions throughout preparation. Using 285-kDa hetero-pentameric human GABA type A receptor overexpressed in HEK293 as a model, we describe a n-dodecyl-β-d-maltopyranoside/cholesteryl hemisuccinate (DDM/CHS)-based affinity purification method, that produced active receptors, supported protease activity, and allowed high performance with both in-gel and direct gel-free proteomic analyses-without detergent removal. Unlike conventional belief that detergents must be removed before HPLC MS, the high-purity low-dose nonionic detergent DDM did not interfere with peptides, and obviated removal or desalting. Sonication or dropwise addition of detergent robustly solubilized over 90% of membrane pellets. The purification conditions were comparable to those applied in successful crystallizations of most membrane proteins. These results enabled streamlined proteomics of human synaptic membrane proteins, and more importantly, allowed directly coupling proteomics with crystallography to characterize both static and dynamic structures of membrane proteins in crystallization pipelines.