Novel GABRG2 mutations cause familial febrile seizures

GABA(A) Receptor Subunit (γ2, δ, β1-3) Variants in Genetic Epilepsy: A Comprehensive Summary of 206 Clinical Cases

Epilepsy is identified in individuals who experienced 2 or more unprovoked seizures occurring over 24 hours apart, which can have a profound impact on a person's neurobiological, cognitive, psychological, and social well-being. Epilepsy is considerably diverse, with classifications such as genetic epilepsy that result directly from a known or presumed genetic variant with the core symptoms of seizures. The GABAA receptor primarily functions as a heteropentamer, containing 3 of 8 subunit types: α, β, γ, δ, ε, π, θ, and ρ. In the adult brain, the GABAA receptor is the primary inhibitory component in neural networks. The involvement of GABAA receptors in the pathogenesis of epilepsy has been proposed. We extensively reviewed all relevant clinical data of previously published cases of GABAA receptor subunit γ2, δ, β1-3 variants included in PubMed up to February 2024, including the variant types, loci, postulated mechanisms, their relevant regions, first onset ages, and phenotypes. We summarized the postulated mechanisms of epileptic pathogenesis. We also divided the collected 206 cases of epilepsy into 4 epileptic phenotypes: genetic generalized epilepsies, focal epilepsy, developmental and epileptic encephalopathies, and epilepsy with fever sensibility. We showed that there were significant differences in the likelihood of the γ2, β2, and β3 subunit variants causing genetic generalized epilepsies, focal epilepsy, developmental and epileptic encephalopathies, and epilepsy with fever sensibility. Patients with the β3 subunit variant seemed related to an earlier first onset age. Our review supports that GABAA receptor subunit variants are a crucial area of epilepsy research and treatment exploration.

GABRG2 mutations in genetic epilepsy with febrile seizures plus: structure, roles, and molecular genetics

Genetic epilepsy with febrile seizures plus (GEFS+) is a genetic epilepsy syndrome characterized by a marked hereditary tendency inherited as an autosomal dominant trait. Patients with GEFS+ may develop typical febrile seizures (FS), while generalized tonic-clonic seizures (GTCSs) with fever commonly occur between 3 months and 6 years of age, which is generally followed by febrile seizure plus (FS+), with or without absence seizures, focal seizures, or GTCSs. GEFS+ exhibits significant genetic heterogeneity, with polymerase chain reaction, exon sequencing, and single nucleotide polymorphism analyses all showing that the occurrence of GEFS+ is mainly related to mutations in the gamma-aminobutyric acid type A receptor gamma 2 subunit (GABRG2) gene. The most common mutations in GABRG2 are separated in large autosomal dominant families, but their pathogenesis remains unclear. The predominant types of GABRG2 mutations include missense (c.983A → T, c.245G → A, p.Met199Val), nonsense (R136*, Q390*, W429*), frameshift (c.1329delC, p.Val462fs*33, p.Pro59fs*12), point (P83S), and splice site (IVS6+2T → G) mutations. All of these mutations types can reduce the function of ion channels on the cell membrane; however, the degree and mechanism underlying these dysfunctions are different and could be linked to the main mechanism of epilepsy. The γ2 subunit plays a special role in receptor trafficking and is closely related to its structural specificity. This review focused on investigating the relationship between GEFS+ and GABRG2 mutation types in recent years, discussing novel aspects deemed to be great significance for clinically accurate diagnosis, anti-epileptic treatment strategies, and new drug development.

Modulating Endoplasmic Reticulum Chaperones and Mutant Protein Degradation in GABRG2(Q390X) Associated with Genetic Epilepsy with Febrile Seizures Plus and Dravet Syndrome

A significant number of patients with genetic epilepsy do not obtain seizure freedom, despite developments in new antiseizure drugs, suggesting a need for novel therapeutic approaches. Many genetic epilepsies are associated with misfolded mutant proteins, including GABRG2(Q390X)-associated Dravet syndrome, which we have previously shown to result in intracellular accumulation of mutant GABAA receptor γ2(Q390X) subunit protein. Thus, a potentially promising therapeutic approach is modulation of proteostasis, such as increasing endoplasmic reticulum (ER)-associated degradation (ERAD). To that end, we have here identified an ERAD-associated E3 ubiquitin ligase, HRD1, among other ubiquitin ligases, as a strong modulator of wildtype and mutant γ2 subunit expression. Overexpressing HRD1 or knockdown of HRD1 dose-dependently reduced the γ2(Q390X) subunit. Additionally, we show that zonisamide (ZNS)-an antiseizure drug reported to upregulate HRD1-reduces seizures in the Gabrg2+/Q390X mouse. We propose that a possible mechanism for this effect is a partial rescue of surface trafficking of GABAA receptors, which are otherwise sequestered in the ER due to the dominant-negative effect of the γ2(Q390X) subunit. Furthermore, this partial rescue was not due to changes in ER chaperones BiP and calnexin, as total expression of these chaperones was unchanged in γ2(Q390X) models. Our results here suggest that leveraging the endogenous ERAD pathway may present a potential method to degrade neurotoxic mutant proteins like the γ2(Q390X) subunit. We also demonstrate a pharmacological means of regulating proteostasis, as ZNS alters protein trafficking, providing further support for the use of proteostasis regulators for the treatment of genetic epilepsies.

A novel GABRG2 variant in Sunflower syndrome: A case report and video EEG monitoring

OBJECTIVE

Sunflower syndrome is a unique photosensitive epilepsy, characterized by heliotropism and stereotyped seizures associated with handwaving. These handwaving events (HWE) are thought to be an ictal phenomenon, although current data are contrasting. Photosensitive epilepsy occurs in 2%-5% of the epilepsy forms and several pathogenic gene variants have been associated with photosensitive epilepsy. However, the genetic etiology of Sunflower syndrome remains unknown. Antiseizure medications (ASM) efficacious in treating photosensitive epilepsy are valproic acid (VPA) and levetiracetam (LEV) although some forms, such as Sunflower syndrome, can be drug-resistant.

METHODS AND RESULTS

Here, we report an 8-year-old boy with an early onset of episodes of HWE that was initially categorized as behavioral problems for which risperidone was started. However, the medical history was suggestive of Sunflower syndrome, and subsequent video EEG showed focal mostly temporal and frontotemporal (right and left) epileptiform activity and confirmed the epileptic nature of the HWE. Thus, VPA was started and initially led to seizure frequency reduction. Molecular analyses showed a pathogenic variant in GABRG2 (c.1287G>A p.(Trp429Ter)), which has been associated with photosensitive and generalized epilepsy.

SIGNIFICANCE

Overall, clinicians worldwide should be cautious by interpreting HWE and/or other tic-like movements, since an epileptic origin cannot be ruled out. A prompt and correct diagnosis can be made by performing a video EEG early on in the diagnostic process when epileptic seizures are part of the differential diagnosis. Even though the genetic etiology of Sunflower syndrome remains poorly understood, this constellation supports further genetic testing since the detection of a pathogenic variant can help in making correct decisions regarding ASM management.

Whole-exome sequencing in searching for novel variants associated with the development of high altitude pulmonary edema

BACKGROUND

High altitude pulmonary edema (HAPE) is a high-altitude idiopathic disease with serious consequences due to hypoxia at high altitude, and there is individual genetic susceptibility. Whole-exome sequencing (WES) is an effective tool for studying the genetic etiology of HAPE and can identify potentially novel mutations that may cause protein instability and may contribute to the development of HAPE.

MATERIALS AND METHODS

A total of 50 unrelated HAPE patients were examined using WES, and the available bioinformatics tools were used to perform an analysis of exonic regions. Using the Phenolyzer program, disease candidate gene analysis was carried out. SIFT, PolyPhen-2, Mutation Taster, CADD, DANN, and I-Mutant software were used to assess the effects of genetic variations on protein function.

RESULTS

The results showed that rs368502694 (p. R1022Q) located in NOS3, rs1595850639 (p. G61S) located in MYBPC3, and rs1367895529 (p. R333H) located in ITGAV were correlated with a high risk of HAPE, and thus could be regarded as potential genetic variations associated with HAPE.

CONCLUSION

WES was used in this study for the first time to directly screen genetic variations related to HAPE. Notably, our study offers fresh information for the subsequent investigation into the etiology of HAPE.

GABRG2 Variants Associated with Febrile Seizures

Febrile seizures (FS) are the most common form of epilepsy in children between six months and five years of age. FS is a self-limited type of fever-related seizure. However, complicated prolonged FS can lead to complex partial epilepsy. We found that among the GABAA receptor subunit (GABR) genes, most variants associated with FS are harbored in the γ2 subunit (GABRG2). Here, we characterized the effects of eight variants in the GABAA receptor γ2 subunit on receptor biogenesis and channel function. Two-thirds of the GABRG2 variants followed the expected autosomal dominant inheritance in FS and occurred as missense and nonsense variants. The remaining one-third appeared as de novo in the affected probands and occurred only as missense variants. The loss of GABAA receptor function and dominant negative effect on GABAA receptor biogenesis likely caused the FS phenotype. In general, variants in the GABRG2 result in a broad spectrum of phenotypic severity, ranging from asymptomatic, FS, genetic epilepsy with febrile seizures plus (GEFS+), and Dravet syndrome individuals. The data presented here support the link between FS, epilepsy, and GABRG2 variants, shedding light on the relationship between the variant topological occurrence and disease severity.

Molecular and clinical descriptions of patients with GABAA receptor gene variants (GABRA1, GABRB2, GABRB3, GABRG2): A cohort study, review of literature, and genotype-phenotype correlation

OBJECTIVE

γ-Aminobutyric acid (GABA)_A -receptor subunit variants have recently been associated with neurodevelopmental disorders and/or epilepsy. The phenotype linked with each gene is becoming better known. Because of the common molecular structure and physiological role of these phenotypes, it seemed interesting to describe a putative phenotype associated with GABA_A -receptor-related disorders as a whole and seek possible genotype-phenotype correlations.

METHODS

We collected clinical, electrophysiological, therapeutic, and molecular data from patients with GABA_A -receptor subunit variants (GABRA1, GABRB2, GABRB3, and GABRG2) through a national French collaboration using the EPIGENE network and compared these data to the one already described in the literature.

RESULTS

We gathered the reported patients in three epileptic phenotypes: 15 patients with fever-related epilepsy (40%), 11 with early developmental epileptic encephalopathy (30%), 10 with generalized epilepsy spectrum (27%), and 1 patient without seizures (3%). We did not find a specific phenotype for any gene, but we showed that the location of variants on the transmembrane (TM) segment was associated with a more severe phenotype, irrespective of the GABA_A -receptor subunit gene, whereas N-terminal variants seemed to be related to milder phenotypes.

SIGNIFICANCE

GABA_A -receptor subunit variants are associated with highly variable phenotypes despite their molecular and physiological proximity. None of the genes described here was associated with a specific phenotype. On the other hand, it appears that the location of the variant on the protein may be a marker of severity. Variant location may have important weight in the development of targeted therapeutics.

Association of ultra-rare coding variants with genetic generalized epilepsy: A case-control whole exome sequencing study

OBJECTIVE

We aimed to identify genes associated with genetic generalized epilepsy (GGE) by combining large cohorts enriched with individuals with a positive family history. Secondarily, we set out to compare the association of genes independently with familial and sporadic GGE.

METHODS

We performed a case-control whole exome sequencing study in unrelated individuals of European descent diagnosed with GGE (previously recruited and sequenced through multiple international collaborations) and ancestry-matched controls. The association of ultra-rare variants (URVs; in 18 834 protein-coding genes) with epilepsy was examined in 1928 individuals with GGE (vs. 8578 controls), then separately in 945 individuals with familial GGE (vs. 8626 controls), and finally in 1005 individuals with sporadic GGE (vs. 8621 controls). We additionally examined the association of URVs with familial and sporadic GGE in two gene sets important for inhibitory signaling (19 genes encoding γ-aminobutyric acid type A [GABAA ] receptors, 113 genes representing the GABAergic pathway).

RESULTS

GABRG2 was associated with GGE (p = 1.8 × 10-5 ), approaching study-wide significance in familial GGE (p = 3.0 × 10-6 ), whereas no gene approached a significant association with sporadic GGE. Deleterious URVs in the most intolerant subgenic regions in genes encoding GABAA receptors were associated with familial GGE (odds ratio [OR] = 3.9, 95% confidence interval [CI] = 1.9-7.8, false discovery rate [FDR]-adjusted p = .0024), whereas their association with sporadic GGE had marginally lower odds (OR = 3.1, 95% CI = 1.3-6.7, FDR-adjusted p = .022). URVs in GABAergic pathway genes were associated with familial GGE (OR = 1.8, 95% CI = 1.3-2.5, FDR-adjusted p = .0024) but not with sporadic GGE (OR = 1.3, 95% CI = .9-1.9, FDR-adjusted p = .19).

SIGNIFICANCE

URVs in GABRG2 are likely an important risk factor for familial GGE. The association of gene sets of GABAergic signaling with familial GGE is more prominent than with sporadic GGE.

Phenotypic Spectrum and Prognosis of Epilepsy Patients With GABRG2 Variants

OBJECTIVE

This study aimed to obtain a comprehensive understanding of the genetic and phenotypic aspects of GABRG2-related epilepsy and its prognosis and to explore the potential prospects for personalized medicine.

METHODS

Through a multicenter collaboration in China, we analyzed the genotype-phenotype correlation and antiseizure medication (ASM) of patients with GABRG2-related epilepsy. The three-dimensional protein structure of the GABRG2 variant was modeled to predict the effect of GABRG2 missense variants using PyMOL 2.3 software.

RESULTS

In 35 patients with GABRG2 variants, 22 variants were de novo, and 18 variants were novel. The seizure onset age was ranged from 2 days after birth to 34 months (median age: 9 months). The seizure onset age was less than 1 year old in 22 patients (22/35, 62.9%). Seizure types included focal seizures (68.6%), generalized tonic-clonic seizures (60%), myoclonic seizures (14.3%), and absence seizures (11.4%). Other clinical features included fever-sensitive seizures (91.4%), cluster seizures (57.1%), and developmental delay (45.7%). Neuroimaging was abnormal in 2 patients, including dysplasia of the frontotemporal cortex and delayed myelination of white matter. Twelve patients were diagnosed with febrile seizures plus, eleven with epilepsy and developmental delay, two with Dravet syndrome, two with developmental and epileptic encephalopathy, two with focal epilepsy, two with febrile seizures, and four with unclassified epilepsy. The proportions of patients with missense variants in the extracellular region and the transmembrane region exhibiting developmental delay were 40% and 63.2%, respectively. The last follow-up age ranged from 11 months to 17 years. Seizures were controlled in 71.4% of patients, and 92% of their seizures were controlled by valproate and/or levetiracetam.

CONCLUSION

The clinical features of GABRG2-related epilepsy included seizure onset, usually in infancy, and seizures were fever-sensitive. More than half of the patients had cluster seizures. Phenotypes of GABRG2-related epilepsy were ranged from mild febrile seizures to severe epileptic encephalopathies. Most patients with GABRG2 variants who experienced seizures had a good prognosis. Valproate and levetiracetam were effective treatments for most patients.

Fever-Associated Seizures or Epilepsy: An Overview of Old and Recent Literature Acquisitions

In addition to central nervous system infections, seizures and fever may occur together in several neurological disorders. Formerly, based on the clinical features and prognostic evolution, the co-association of seizure and fever included classical febrile seizures (FS) divided into simple, complex, and prolonged FS (also called febrile status epilepticus). Later, this group of disorders has been progressively indicated, with a more inclusive term, as "fever-associated seizures or epilepsy" (FASE) that encompasses: (a) FS divided into simple, complex, and prolonged FS; (b) FS plus; (c) severe myoclonic epilepsy in infancy (Dravet syndrome); (d) genetic epilepsy with FS plus; and (e) febrile infection-related epilepsy syndrome (FIRES). Among the FASE disorders, simple FS, the most common and benign condition, is rarely associated with subsequent epileptic seizures. The correlation of FS with epilepsy and other neurological disorders is highly variable. The pathogenesis of FASE is unclear but immunological and genetic factors play a relevant role and the disorders belonging to the FASE group show to have an underlying common clinical, immunological, and genetic pathway. In this study, we have reviewed and analyzed the clinical data of each of the heterogeneous group of disorders belonging to FASE.

Neocortex- and hippocampus-specific deletion of Gabrg2 causes temperature-dependent seizures in mice

Mutations in the GABRG2 gene encoding the γ-aminobutyric acid (GABA) A receptor gamma 2 subunit are associated with genetic epilepsy with febrile seizures plus, febrile seizures plus, febrile seizures, and other symptoms of epilepsy. However, the mechanisms underlying Gabrg2-mediated febrile seizures are poorly understood. Here, we used the Cre/loxP system to generate conditional knockout (CKO) mice with deficient Gabrg2 in the hippocampus and neocortex. Heterozygous CKO mice (Gabrg2^fl/wtCre⁺) exhibited temperature-dependent myoclonic jerks, generalised tonic-clonic seizures, increased anxiety-like symptoms, and a predisposition to induce seizures. Cortical electroencephalography showed the hyperexcitability in response to temperature elevation in Gabrg2^fl/wtCre⁺ mice, but not in wild-type mice. Gabrg2^fl/wtCre⁺ mice exhibited spontaneous seizures and susceptibility to temperature-induced seizures. Loss of neurons were observed in cortical layers V-VI and hippocampus of Gabrg2^fl/wtCre⁺ mice. Furthermore, the latency of temperature- or pentylenetetrazol-induced seizures were significantly decreased in Gabrg2^fl/wtCre⁺ mice compared with wild-type mice. In summary, Gabrg2^fl/wtCre⁺ mice with Gabrg2 deletion in the neocortex and hippocampus reproduce many features of febrile seizures and therefore provide a novel model to further understand this syndrome at the cellular and molecular level.

PGRMC1 Inhibits Progesterone-Evoked Proliferation and Ca2+ Entry Via STIM2 in MDA-MB-231 Cells

Progesterone receptor membrane component 1 (PGRMC1) has been shown to regulate some cancer hallmarks. Progesterone (P₄) evokes intracellular calcium (Ca²⁺) changes in the triple-negative breast cancer cell lines (MDA-MB-231, MDA-MB-468, and BT-20) and in other breast cancer cell lines like the luminal MCF7 cells. PGRMC1 expression is elevated in MDA-MB-231 and MCF7 cells as compared to non-tumoral MCF10A cell line, and PGRMC1 silencing enhances P₄-evoked Ca²⁺ mobilization. Here, we found a new P₄-dependent Ca²⁺ mobilization pathway in MDA-MB-231 cells and other triple-negative breast cancer cells, as well as in MCF7 cells that involved Stromal interaction molecule 2 (STIM2), Calcium release-activated calcium channel protein 1 (Orai1), and Transient Receptor Potential Channel 1 (TRPC1). Stromal interaction molecule 1 (STIM1) was not involved in this novel Ca²⁺ pathway, as evidenced by using siRNA STIM1. PGRMC1 silencing reduced the negative effect of P₄ on cell proliferation and cell death in MDA-MB-231 cells. In line with the latter observation, Nuclear Factor of Activated T-Cells 1 (NFAT1) nuclear accumulation due to P₄ incubation for 48 h was enhanced in cells transfected with the small hairpin siRNA against PGRMC1 (shPGRMC1). These results provide evidence for a novel P₄-evoked Ca²⁺ entry pathway that is downregulated by PGRMC1.

Endoplasmic reticulum stress increases inflammatory cytokines in an epilepsy mouse model Gabrg2+/Q390X knockin: A link between genetic and acquired epilepsy?

OBJECTIVE

Neuroinflammation is a major theme in epilepsy, which has been characterized in acquired epilepsy but is poorly understood in genetic epilepsy. γ-Aminobutyric acid type A receptor subunit gene mutations are significant causes of epilepsy, and we have studied the pathophysiology directly resulting from defective receptor channels. Here, we determined the proinflammatory factors in a genetic mouse model, the Gabrg2^+/Q390X knockin (KI). We have identified increased cytokines in multiple brain regions of the KI mouse throughout different developmental stages and propose that accumulation of the trafficking-deficient mutant protein may increase neuroinflammation, which would be a novel mechanism for genetic epilepsy.

METHODS

We used enzyme-linked immunosorbent assay, immunoprecipitation, nuclei purification, immunoblot, immunohistochemistry, and confocal microscopy to characterize increased neuroinflammation and its potential causes in a Gabrg2^+/Q390X KI mouse and a Gabrg2^+/- knockout (KO) mouse, each associated with a different epilepsy syndrome with different severities.

RESULTS

We found that proinflammatory cytokines such as tumor necrosis factor alpha, interleukin 1-beta (IL-1β), and IL-6 were increased in the KI mice but not in the KO mice. A major underlying basis for the discrepancy in cytokine expression between the two mouse models is likely chronic mutant protein accumulation and endoplasmic reticulum (ER) stress. The presence of mutant protein dampened cytokine induction upon further cellular stimulation or external stress such as elevated temperature. Pharmacological induction of ER stress upregulated cytokine expression in the wild-type and KO but not in the KI mice. The increased cytokine expression was independent of seizure occurrence, because it was upregulated in both mice and cultured neurons.

SIGNIFICANCE

Together, these data demonstrate a novel pathophysiology for genetic epilepsy, increased neuroinflammation, which is a common mechanism for acquired epilepsy. The findings thus provide the first link of neuroinflammation between genetic epilepsy associated with an ion channel gene mutation and acquired epilepsy.

A regulatory variant of CHRM3 is associated with cannabis-induced hallucinations in European Americans

Cannabis, the most widely used illicit drug, can induce hallucinations. Our understanding of the biology of cannabis-induced hallucinations (Ca-HL) is limited. We used the Semi-Structured Assessment for Drug Dependence and Alcoholism (SSADDA) to identify cannabis-induced hallucinations (Ca-HL) among long-term cannabis users (used cannabis ≥1 year and ≥100 times). A genome-wide association study (GWAS) was conducted by analyzing European Americans (EAs) and African Americans (AAs) in Yale-Penn 1 and 2 cohorts individually, then meta-analyzing the two cohorts within population. In the meta-analysis of Yale-Penn EAs (n = 1917), one genome-wide significant (GWS) signal emerged at the CHRM3 locus, represented by rs115455482 (P = 1.66 × 10^-10), rs74722579 (P = 2.81 × 10^-9), and rs1938228 (P = 1.57 × 10^-8); signals were GWS in Yale-Penn 1 EAs (n = 1092) and nominally significant in Yale-Penn 2 EAs (n = 825). Two SNPs, rs115455482 and rs74722579, were available from the Collaborative Study on the Genetics of Alcoholism data (COGA; 3630 long-term cannabis users). The signals did not replicate, but when meta-analyzing Yale-Penn and COGA EAs, the two SNPs' association signals were increased (meta-P-values 1.32 × 10^-10 and 2.60 × 10^-9, respectively; n = 4291). There were no significant findings in AAs, but in the AA meta-analysis (n = 3624), nominal significance was seen for rs74722579. The rs115455482*T risk allele was associated with lower CHRM3 expression in the thalamus. CHRM3 was co-expressed with three psychosis risk genes (GABAG2, CHRNA4, and HRH3) in the thalamus and other human brain tissues and mouse GABAergic neurons. This work provides strong evidence for the association of CHRM3 with Ca-HL and provides insight into the potential involvement of thalamus for this trait.

Targeted knockout of GABA-A receptor gamma 2 subunit provokes transient light-induced reflex seizures in zebrafish larvae

Epilepsy is a common primary neurological disorder characterized by the chronic tendency of a patient to experience epileptic seizures, which are abnormal body movements or cognitive states that result from excessive, hypersynchronous brain activity. Epilepsy has been found to have numerous etiologies and, although about two-thirds of epilepsies were classically considered idiopathic, the majority of those are now believed to be of genetic origin. Mutations in genes involved in gamma-aminobutyric acid (GABA)-mediated inhibitory neurotransmission have been associated with a broad range of epilepsy syndromes. Mutations in the GABA-A receptor gamma 2 subunit gene (GABRG2), for example, have been associated with absence epilepsy and febrile seizures in humans. Several rodent models of GABRG2 loss of function depict clinical features of the disease; however, alternative genetic models more amenable for the study of ictogenesis and for high-throughput screening purposes are still needed. In this context, we generated a gabrg2 knockout (KO) zebrafish model (which we called R23X) that displayed light/dark-induced reflex seizures. Through high-resolution in vivo calcium imaging of the brain, we showed that this phenotype is associated with widespread increases in neuronal activity that can be effectively alleviated by the anti-epileptic drug valproic acid. Moreover, these seizures only occur at the larval stages but disappear after 1 week of age. Interestingly, our whole-transcriptome analysis showed that gabrg2 KO does not alter the expression of genes in the larval brain. As a result, the gabrg2^−/− zebrafish is a novel in vivo genetic model of early epilepsies that opens new doors to investigate ictogenesis and for further drug-screening assays.

Summary: The authors present a novel in vivo genetic model of idiopathic epilepsy in zebrafish (gabrg2^−/−) to aid the study of ictogenesis and provide a convenient genetic tool for drug screening.

Functional genomics of epilepsy-associated mutations in the GABAA receptor subunits reveal that one mutation impairs function and two are catastrophic

A number of epilepsy-causing mutations have recently been identified in the genes of the α1, β3, and γ2 subunits comprising the γ-aminobutyric acid type A (GABA_A) receptor. These mutations are typically dominant, and in certain cases, such as the α1 and β3 subunits, they may lead to a mix of receptors at the cell surface that contain no mutant subunits, a single mutated subunit, or two mutated subunits. To determine the effects of mutations in a single subunit or in two subunits on receptor activation, we created a concatenated protein assembly that links all five subunits of the α1β3γ2 receptor and expresses them in the correct orientation. We created nine separate receptor variants with a single-mutant subunit and four receptors containing two subunits of the γ2^R323Q, β3^D120N, β3^T157M, β3^Y302C, and β3^S254F epilepsy-causing mutations. We found that the singly mutated γ2^R323Q subunit impairs GABA activation of the receptor by reducing GABA potency. A single β3^D120N, β3^T157M, or β3^Y302C mutation also substantially impaired receptor activation, and two copies of these mutants within a receptor were catastrophic. Of note, an effect of the β3^S254F mutation on GABA potency depended on the location of this mutant subunit within the receptor, possibly because of the membrane environment surrounding the transmembrane region of the receptor. Our results highlight that precise functional genomic analyses of GABA_A receptor mutations using concatenated constructs can identify receptors with an intermediate phenotype that contribute to epileptic phenotypes and that are potential drug targets for precision medicine approaches.

γ2 GABAAR Trafficking and the Consequences of Human Genetic Variation

GABA type A receptors (GABA_ARs) mediate the majority of fast inhibitory neurotransmission in the central nervous system (CNS). Most prevalent as heteropentamers composed of two α, two β, and a γ2 subunit, these ligand-gated ionotropic chloride channels are capable of extensive genetic diversity (α1-6, β1-3, γ1-3, δ, 𝜀, 𝜃, π, ρ1-3). Part of this selective GABA_AR assembly arises from the critical role for γ2 in maintaining synaptic receptor localization and function. Accordingly, mutations in this subunit account for over half of the known epilepsy-associated genetic anomalies identified in GABA_ARs. Fundamental structure-function studies and cellular pathology investigations have revealed dynamic GABA_AR trafficking and synaptic scaffolding as critical regulators of GABAergic inhibition. Here, we introduce in vitro and in vivo findings regarding the specific role of the γ2 subunit in receptor trafficking. We then examine γ2 subunit human genetic variation and assess disease related phenotypes and the potential role of altered GABA_AR trafficking. Finally, we discuss new-age imaging techniques and their potential to provide novel insight into critical regulatory mechanisms of GABA_AR function.

Febrile seizures: an overview

Background

Febrile seizures are the most common neurologic disorder in childhood. Physicians should be familiar with the proper evaluation and management of this common condition.

Objective

To provide an update on the current understanding, evaluation, and management of febrile seizures.

Methods

A PubMed search was completed in Clinical Queries using the key terms ‘febrile convulsions’ and ‘febrile seizures’. The search strategy included meta-analyses, randomized controlled trials, clinical trials, observational studies, and reviews.

Results

Febrile seizures, with a peak incidence between 12 and 18 months of age, likely result from a vulnerability of the developing central nervous system to the effects of fever, in combination with an underlying genetic predisposition and environmental factors. The majority of febrile seizures occur within 24 hours of the onset of the fever. Febrile seizures can be simple or complex. Clinical judgment based on variable presentations must direct the diagnostic studies which are usually not necessary in the majority of cases. A lumbar puncture should be considered in children younger than 12 months of age or with suspected meningitis. Children with complex febrile seizures are at risk of subsequent epilepsy. Approximately 30–40% of children with a febrile seizure will have a recurrence during early childhood. The prognosis is favorable as the condition is usually benign and self-limiting. Intervention to stop the seizure often is unnecessary.

Conclusion

Continuous preventative antiepileptic therapy for the prevention of recurrent febrile seizures is not recommended. The use of intermittent anticonvulsant therapy is not routinely indicated. Antipyretics have no role in the prevention of febrile seizures.

Whole-Exome Sequencing in Searching for New Variants Associated With the Development of Parkinson's Disease

Background: Parkinson’s disease (PD) is a complex disease with its monogenic forms accounting for less than 10% of all cases. Whole-exome sequencing (WES) technology has been used successfully to find mutations in large families. However, because of the late onset of the disease, only small families and unrelated patients are usually available. WES conducted in such cases yields in a large number of candidate variants. There are currently a number of imperfect software tools that allow the pathogenicity of variants to be evaluated.

Objectives: We analyzed 48 unrelated patients with an alleged autosomal dominant familial form of PD using WES and developed a strategy for selecting potential pathogenetically significant variants using almost all available bioinformatics resources for the analysis of exonic areas.

Methods: DNA sequencing of 48 patients with excluded frequent mutations was performed using an Illumina HiSeq 2500 platform. The possible pathogenetic significance of identified variants and their involvement in the pathogenesis of PD was assessed using SNP and Variation Suite (SVS), Combined Annotation Dependent Depletion (CADD) and Rare Exome Variant Ensemble Learner (REVEL) software. Functional evaluation was performed using the Pathway Studio database.

Results: A significant reduction in the search range from 7082 to 25 variants in 23 genes associated with PD or neuronal function was achieved. Eight (FXN, MFN2, MYOC, NPC1, PSEN1, RET, SCN3A and SPG7) were the most significant.

Conclusions: The multistep approach developed made it possible to conduct an effective search for potential pathogenetically significant variants, presumably involved in the pathogenesis of PD. The data obtained need to be further verified experimentally.

Genetics update: Monogenetics, polygene disorders and the quest for modifying genes

The genetic channelopathies are a broad collection of diseases. Many ion channel genes demonstrate wide phenotypic pleiotropy, but nonetheless concerted efforts have been made to characterise genotype-phenotype relationships. In this review we give an overview of the factors that influence genotype-phenotype relationships across this group of diseases as a whole, using specific individual channelopathies as examples. We suggest reasons for the limitations observed in these relationships. We discuss the role of ion channel variation in polygenic disease and highlight research that has contributed to unravelling the complex aetiological nature of these conditions. We focus specifically on the quest for modifying genes in inherited channelopathies, using the voltage-gated sodium channels as an example. Epilepsy related to genetic channelopathy is one area in which precision medicine is showing promise. We will discuss the successes and limitations of precision medicine in these conditions. This article is part of the Special Issue entitled 'Channelopathies.'

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.

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.

Comprehensive analysis of the numbers, lengths and amino acid compositions of transmembrane helices in prokaryotic, eukaryotic and viral integral membrane proteins of high-resolution structure

We report a comprehensive analysis of the numbers, lengths and amino acid compositions of transmembrane helices in 235 high-resolution structures of integral membrane proteins. The properties of 1551 transmembrane helices in the structures were compared with those obtained by analysis of the same amino acid sequences using topology prediction tools. Explanations for the 81 (5.2%) missing or additional transmembrane helices in the prediction results were identified. Main reasons for missing transmembrane helices were mis-identification of N-terminal signal peptides, breaks in α-helix conformation or charged residues in the middle of transmembrane helices and transmembrane helices with unusual amino acid composition. The main reason for additional transmembrane helices was mis-identification of amphipathic helices, extramembrane helices or hairpin re-entrant loops. Transmembrane helix length had an overall median of 24 residues and an average of 24.9 ± 7.0 residues and the most common length was 23 residues. The overall content of residues in transmembrane helices as a percentage of the full proteins had a median of 56.8% and an average of 55.7 ± 16.0%. Amino acid composition was analysed for the full proteins, transmembrane helices and extramembrane regions. Individual proteins or types of proteins with transmembrane helices containing extremes in contents of individual amino acids or combinations of amino acids with similar physicochemical properties were identified and linked to structure and/or function. In addition to overall median and average values, all results were analysed for proteins originating from different types of organism (prokaryotic, eukaryotic, viral) and for subgroups of receptors, channels, transporters and others.

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.

Whole-exome sequencing in neurologic practice: Reducing the diagnostic odyssey

The current issue of Neurology® Genetics emphasizes the unparalleled role of next-generation sequencing (NGS) in defining an expanding spectrum of genetic neurologic disorders. Clinically, NGS encompasses the use of large gene panels, whole-exome sequencing (WES), or whole-genome sequencing (WGS). The impact of NGS technology is twofold. First, researchers have discovered novel genes as the cause of neurologic disorders. This research includes the efforts of Martikainen et al.(1) to define further the phenotype of a previously reported SNCA mutation that is associated with autosomal dominant Parkinson disease. Second and more common is the connection of novel phenotypes with previously described genes. Several articles in the current issue highlight the role of NGS in this effort. For example, Schottman et al.(2) identified REEP1 mutations as the cause of a severe axonal neuropathy with a spinal muscular atrophy with respiratory distress (SMARD) phenotype. This gene was previously associated with a hereditary spastic paraplegia phenotype. Similarly, Shieh et al.(3) expand the phenotype associated with mutations in L1CAM to a neuronal migration phenotype.