Hyperglycosylation and reduced GABA currents of mutated GABRB3 polypeptide in remitting childhood absence epilepsy

Channelopathies in epilepsy: an overview of clinical presentations, pathogenic mechanisms, and therapeutic insights

Pathogenic variants in genes encoding ion channels are causal for various pediatric and adult neurological conditions. In particular, several epilepsy syndromes have been identified to be caused by specific channelopathies. These encompass a spectrum from self-limited epilepsies to developmental and epileptic encephalopathies spanning genetic and acquired causes. Several of these channelopathies have exquisite responses to specific antiseizure medications (ASMs), while others ASMs may prove ineffective or even worsen seizures. Some channelopathies demonstrate phenotypic pleiotropy and can cause other neurological conditions outside of epilepsy. This review aims to provide a comprehensive exploration of the pathophysiology of seizure generation, ion channels implicated in epilepsy, and several genetic epilepsies due to ion channel dysfunction. We outline the clinical presentation, pathogenesis, and the current state of basic science and clinical research for these channelopathies. In addition, we briefly look at potential precision therapy approaches emerging for these disorders.

Case Report: A developmental and epileptic encephalopathy 45 due to de novo variant of GABRB1

Background

The gamma-aminobutyric acid (GABA) variant causes developmental and epileptic encephalopathy 45 (DEE45), an autosomal dominant disorder that results in oculocortical visual impairment, reduced muscle tone, psychomotor retardation, and epilepsy. Analysis of the clinical features and genetics of DEE45 may be helpful in complementing genotype-phenotype studies.

Case presentation

We collected peripheral blood samples from the affected children and parents and extracted genomic DNA. Whole exome sequencing (WES) was utilized to identify the underlying disease-causing variant. WES showed that the prior carried a heterozygous variant c.686C > T p.(Ala229Val) in exon 7 of the GABRB1 (NM_000812.4), and no variant was detected in either parental sample. The child has DEE45.

Conclusion

The variant c.686C > T of the GABRB1 is a possible cause of DEE45. Gene variant analysis of the relevant family lines using WES provides effective genetic counseling for developing and regressing such patients in the clinic. However, further studies are needed to verify the pathogenic mechanism.

Nacc1 Mutation in Mice Models Rare Neurodevelopmental Disorder with Underlying Synaptic Dysfunction

A missense mutation in the transcription repressor Nucleus accumbens-associated 1 (NACC1) gene at c.892C>T (p.Arg298Trp) on chromosome 19 causes severe neurodevelopmental delay ( Schoch et al., 2017). To model this disorder, we engineered the first mouse model with the homologous mutation (Nacc1+/R284W ) and examined mice from E17.5 to 8 months. Both genders had delayed weight gain, epileptiform discharges and altered power spectral distribution in cortical electroencephalogram, behavioral seizures, and marked hindlimb clasping; females displayed thigmotaxis in an open field. In the cortex, NACC1 long isoform, which harbors the mutation, increased from 3 to 6 months, whereas the short isoform, which is not present in humans and lacks aaR284 in mice, rose steadily from postnatal day (P) 7. Nuclear NACC1 immunoreactivity increased in cortical pyramidal neurons and parvalbumin containing interneurons but not in nuclei of astrocytes or oligodendroglia. Glial fibrillary acidic protein staining in astrocytic processes was diminished. RNA-seq of P14 mutant mice cortex revealed over 1,000 differentially expressed genes (DEGs). Glial transcripts were downregulated and synaptic genes upregulated. Top gene ontology terms from upregulated DEGs relate to postsynapse and ion channel function, while downregulated DEGs enriched for terms relating to metabolic function, mitochondria, and ribosomes. Levels of synaptic proteins were changed, but number and length of synaptic contacts were unaltered at 3 months. Homozygosity worsened some phenotypes including postnatal survival, weight gain delay, and increase in nuclear NACC1. This mouse model simulates a rare form of autism and will be indispensable for assessing pathophysiology and targets for therapeutic intervention.

GABAA receptors in epilepsy: Elucidating phenotypic divergence through functional analysis of genetic variants

Normal brain function requires a tightly regulated balance between excitatory and inhibitory neurotransmissions. γ-Aminobutyric acid type A (GABAA ) receptors represent the major class of inhibitory ion channels in the mammalian brain. Dysregulation of these receptors and/or their associated pathways is strongly implicated in the pathophysiology of epilepsy. To date, hundreds of different GABAA receptor subunit variants have been associated with epilepsy, making them a prominent cause of genetically linked epilepsy. While identifying these genetic variants is crucial for accurate diagnosis and effective genetic counselling, it does not necessarily lead to improved personalised treatment options. This is because the identification of a variant does not reveal how the function of GABAA receptors is affected. Genetic variants in GABAA receptor subunits can cause complex changes to receptor properties resulting in various degrees of gain-of-function, loss-of-function or a combination of both. Understanding how variants affect the function of GABAA receptors therefore represents an important first step in the ongoing development of precision therapies. Furthermore, it is important to ensure that functional data are produced using methodologies that allow genetic variants to be classified using clinical guidelines such as those developed by the American College of Medical Genetics and Genomics. This article will review the current knowledge in the field and provide recommendations for future functional analysis of genetic GABAA receptor variants.

Sub-region analysis of DMD gene in cases with idiopathic generalized epilepsy

Gene sub-region encoded protein domain is the basic unit for protein structure and function. The DMD gene is the largest coding gene in humans, with its phenotype relevant to idiopathic generalized epilepsy. We hypothesized variants clustered in sub-regions of idiopathic generalized epilepsy genes and investigated the relationship between the DMD gene and idiopathic generalized epilepsy. Whole exome sequencing was performed in 106 idiopathic generalized epilepsy individuals. DMD variants were filtered with variant type, allele frequency, in silico prediction, hemizygous or homozygous status in the population, inheritance mode, and domain location. Variants located at the sub-regions were selected by the subRVIS software. The pathogenicity of variants was evaluated by the American College of Medical Genetics and Genomics criteria. Articles on functional studies related to epilepsy for variants clustered protein domains were reviewed. In sub-regions of the DMD gene, two variants were identified in two unrelated cases with juvenile absence epilepsy or juvenile myoclonic epilepsy. The pathogenicity of both variants was uncertain significance. Allele frequency of both variants in probands with idiopathic generalized epilepsy reached statistical significance compared with the population (Fisher's test, p = 2.02 × 10-6, adjusted α = 4.52 × 10-6). The variants clustered in the spectrin domain of dystrophin, which binds to glycoprotein complexes and indirectly affects ion channels contributing to epileptogenesis. Gene sub-region analysis suggests a weak association between the DMD gene and idiopathic generalized epilepsy. Functional analysis of gene sub-region helps infer the pathogenesis of idiopathic generalized epilepsy.

Genetic inhibition of glutamate allosteric potentiation of GABAARs in mice results in hyperexcitability, leading to neurobehavioral abnormalities

The imbalance between neuronal excitation and inhibition (E/I) in neural circuit has been considered to be at the root of numerous brain disorders. We recently reported a novel feedback crosstalk between the excitatory neurotransmitter glutamate and inhibitory γ-aminobutyric acid type A receptor (GABA_AR)-glutamate allosteric potentiation of GABA_AR functions through a direct binding of glutamate to the GABA_AR itself. Here, we investigated the physiological significance and pathological implications of this cross-talk by generating the β3_E182G knock-in (KI) mice. We found that β3_E182G KI, while had little effect on basal GABA_AR-mediated synaptic transmission, significantly reduced glutamate potentiation of GABA_AR-mediated responses. These KI mice displayed lower thresholds for noxious stimuli, higher susceptibility to seizures and enhanced hippocampus-related learning and memory. Additionally, the KI mice exhibited impaired social interactions and decreased anxiety-like behaviors. Importantly, hippocampal overexpression of wild-type β3-containing GABA_ARs was sufficient to rescue the deficits of glutamate potentiation of GABA_AR-mediated responses, hippocampus-related behavioral abnormalities of increased epileptic susceptibility, and impaired social interactions. Our data indicate that the novel crosstalk among excitatory glutamate and inhibitory GABA_AR functions as a homeostatic mechanism in fine-tuning neuronal E/I balance, thereby playing an essential role in ensuring normal brain functioning.

Molecular mechanism of hyperactivation conferred by a truncation of TRPA1

A drastic TRPA1 mutant (R919*) identified in CRAMPT syndrome patients has not been mechanistically characterized. Here, we show that the R919* mutant confers hyperactivity when co-expressed with wild type (WT) TRPA1. Using functional and biochemical assays, we reveal that the R919* mutant co-assembles with WT TRPA1 subunits into heteromeric channels in heterologous cells that are functional at the plasma membrane. The R919* mutant hyperactivates channels by enhancing agonist sensitivity and calcium permeability, which could account for the observed neuronal hypersensitivity-hyperexcitability symptoms. We postulate that R919* TRPA1 subunits contribute to heteromeric channel sensitization by altering pore architecture and lowering energetic barriers to channel activation contributed by the missing regions. Our results expand the physiological impact of nonsense mutations, reveal a genetically tractable mechanism for selective channel sensitization, uncover insights into the process of TRPA1 gating, and provide an impetus for genetic analysis of patients with CRAMPT or other stochastic pain syndromes.

Heterozygous GABAA receptor β3 subunit N110D knock-in mice have epileptic spasms

OBJECTIVE

Infantile spasms is an epileptic encephalopathy of childhood, and its pathophysiology is largely unknown. We generated a heterozygous knock-in mouse with the human infantile spasms-associated de novo mutation GABRB3 (c.A328G, p.N110D) to investigate its molecular mechanisms and to establish the Gabrb3+/N110D knock-in mouse as a model of infantile spasms syndrome.

METHODS

We used electroencephalography (EEG) and video monitoring to characterize seizure types, and a suite of behavioral tests to identify neurological and behavioral impairment in Gabrb3+/N110D knock-in mice. Miniature inhibitory postsynaptic currents (mIPSCs) were recorded from layer V/VI pyramidal neurons in somatosensory cortex, and extracellular multi-unit recordings from the ventral basal nucleus of the thalamus in a horizontal thalamocortical slice were used to assess spontaneous thalamocortical oscillations.

RESULTS

The infantile spasms-associated human de novo mutation GABRB3 (c.A328G, p.N110D) caused epileptic spasms early in development and multiple seizure types in adult Gabrb3+/N110D knock-in mice. Signs of neurological impairment, anxiety, hyperactivity, social impairment, and deficits in spatial learning and memory were also observed. Gabrb3+/N110D mice had reduced cortical mIPSCs and increased duration of spontaneous oscillatory firing in the somatosensory thalamocortical circuit.

SIGNIFICANCE

The Gabrb3+/N110D knock-in mouse has epileptic spasms, seizures, and other neurological impairments that are consistent with infantile spasms syndrome in patients. Multiple seizure types and abnormal behaviors indicative of neurological impairment both early and late in development suggest that Gabrb3+/N110D mice can be used to study the pathophysiology of infantile spasms. Reduced cortical inhibition and increased duration of thalamocortical oscillatory firing suggest perturbations in thalamocortical circuits.

Anti-seizure mechanisms of midazolam and valproate at the β2(L51M) variant of the GABAA receptor

Genetic sequencing is identifying an expanding number of variants of GABAA receptors associated with human epilepsies. We identified a new de novo variant of the β2 subunit (β2L51M) of the inhibitory GABAA receptor associated with seizures. Our analysis determined the pathogenicity of the variant and the effects of anti-seizure medications. Our data demonstrates that the variant reduced cell surface trafficking and peak GABA-gated currents. Synaptic currents mediated by variant-containing receptors decayed faster than wild-type and single receptor currents showed that the variant shortened the duration of receptor activity by decreasing receptor open times. We tested the effects of the anti-seizure medications, midazolam, carbamazepine and valproate and found that all three enhance variant receptor surface expression. Additionally, midazolam restored receptor function by increasing single receptor active periods and synaptic current decay times towards wild-type levels. By contrast, valproate increased synaptic peak currents, event frequency and promoted synaptic bursting. Our study identifies a new disease-causing variant to the GABAA receptor, profiles its pathogenic effects and demonstrates how anti-seizure drugs correct its functional deficits.

Variable Expression of GABAA Receptor Subunit Gamma 2 Mutation in a Nuclear Family Displaying Developmental and Encephalopathic Phenotype

Mutations in GABA_A receptor subunit genes (GABRs) are a major etiology for developmental and epileptic encephalopathies (DEEs). This article reports a case of a genetic abnormality in GABRG2 and updates the pathophysiology and treatment development for mutations in DEEs based on recent advances. Mutations in GABRs, especially in GABRA1, GABRB2, GABRB3, and GABRG2, impair GABAergic signaling and are frequently associated with DEEs such as Dravet syndrome and Lennox-Gastaut syndrome, as GABAergic signaling is critical for early brain development. We here present a novel association of a microdeletion of GABRG2 with a diagnosed DEE phenotype. We characterized the clinical phenotype and underlying mechanisms, including molecular genetics, EEGs, and MRI. We then compiled an update of molecular mechanisms of GABR mutations, especially the mutations in GABRB3 and GABRG2 attributed to DEEs. Genetic therapy is also discussed as a new avenue for treatment of DEEs through employing antisense oligonucleotide techniques. There is an urgent need to define treatment targets and explore new treatment paradigms for the DEEs, as early deployment could alleviate long-term disabilities and improve quality of life for patients. This study highlights biomolecular targets for future therapeutic interventions, including via both pharmacological and genetic approaches.

Genetic variations in GABA metabolism and epilepsy

Epilepsy is a paroxysmal brain disorder that results from an imbalance between neuronal excitation and inhibition. Gamma-aminobutyric acid (GABA) is the most important inhibitory neurotransmitter in the brain and plays an important role in the occurrence and development of epilepsy. Abnormalities in all aspects of GABA metabolism, including GABA synthesis, transport, genes encoding GABA receptors, and GABA inactivation, may lead to epilepsy. GABRA1, GABRA2, GABRA5, GABRB1, GABRB2, GABRB3, GABRG2 and GABBR2 are genes that encode GABA receptors and are commonly associated with epilepsy. Mutations of these genes lead to a variety of epilepsy syndromes with different clinical phenotypes, primarily by down regulating receptor expression and reducing the amplitude of GABA-evoked potentials. GABA is metabolized by GABA transaminase and succinate semi aldehyde dehydrogenase, which are encoded by the ABAT and ALDH5A1 genes, respectively. Mutations of these genes result in symptoms related to deficiency of GABA transaminase and succinate semi aldehyde dehydrogenase, such as epilepsy and cognitive impairment. Most of the variation in genes associated with GABA metabolism are accompanied by developmental disorders. This review focuses on advances in understanding the relationship between genetic variation in GABA metabolism and epilepsy to establish a basis for the accurate diagnosis and treatment of epilepsy.

GABRB3-related epilepsy: novel variants, clinical features and therapeutic implications

OBJECTIVE

This study aimed to comprehensively examine the genetic and phenotypic aspects of GABRB3-related epilepsy and to explore the potential prospects of personalized medicine.

METHODS

Genetic testing was conducted in all epilepsy patients without acquired factors for epilepsy. Through the collaboration of multicenter in China, we analyzed the genotype-phenotype correlation and antiepileptic therapy of 26 patients with GABRB3-related epilepsy.

RESULTS

Thirteen GABRB3 variants were novel, and 25 were de novo. The seizure onset age ranged from 1 to 21 months (median age 3.75 months). Seizure types predominated including focal seizures (92.3%), generalized tonic-clonic seizures (23.1%), and epileptic spasms (15.4%). Clinical features included cluster seizures (80.8%), fever sensitivity (53.8%), and developmental delay (96.2%). Neuroimaging was abnormal in 10 patients, including dysplasia of the cerebral cortex, dysplasia of the frontal and temporal cortex, delayed myelination, and corpus callosum dysplasia. Eleven patients were diagnosed with developmental and epileptic encephalopathy (DEE), four with West syndrome, three with epilepsy of infancy with migrating focal seizures (EIMFS), one with epilepsy with myoclonic-atonic seizures (EMAS), one with Dravet syndrome, and one with febrile seizures plus (FS+). Seizures were controlled in 57.7% of patients by valproate, levetiracetam, or perampanel in the majority.

CONCLUSIONS

The clinical features of GABRB3-related epilepsy included seizure onset in early infancy, cluster seizures and fever sensitivity. Most patients manifest severe epilepsy phenotypes. Valproate, levetiracetam and perampanel seem to have positive effects on seizure control for patients with GABRB3 variants.

Gain-of-function and loss-of-function GABRB3 variants lead to distinct clinical phenotypes in patients with developmental and epileptic encephalopathies

Many patients with developmental and epileptic encephalopathies present with variants in genes coding for GABA_A receptors. These variants are presumed to cause loss-of-function receptors leading to reduced neuronal GABAergic activity. Yet, patients with GABA_A receptor variants have diverse clinical phenotypes and many are refractory to treatment despite the availability of drugs that enhance GABAergic activity. Here we show that 44 pathogenic GABRB3 missense variants segregate into gain-of-function and loss-of-function groups and respective patients display distinct clinical phenotypes. The gain-of-function cohort (n = 27 patients) presented with a younger age of seizure onset, higher risk of severe intellectual disability, focal seizures at onset, hypotonia, and lower likelihood of seizure freedom in response to treatment. Febrile seizures at onset are exclusive to the loss-of-function cohort (n = 47 patients). Overall, patients with GABRB3 variants that increase GABAergic activity have more severe developmental and epileptic encephalopathies. This paradoxical finding challenges our current understanding of the GABAergic system in epilepsy and how patients should be treated.

Structural mapping of GABRB3 variants reveals genotype-phenotype correlations

PURPOSE

Pathogenic variants in GABRB3 have been associated with a spectrum of phenotypes from severe developmental disorders and epileptic encephalopathies to milder epilepsy syndromes and mild intellectual disability (ID). In this study, we analyzed a large cohort of individuals with GABRB3 variants to deepen the phenotypic understanding and investigate genotype-phenotype correlations.

METHODS

Through an international collaboration, we analyzed electro-clinical data of unpublished individuals with variants in GABRB3, and we reviewed previously published cases. All missense variants were mapped onto the 3-dimensional structure of the GABRB3 subunit, and clinical phenotypes associated with the different key structural domains were investigated.

RESULTS

We characterized 71 individuals with GABRB3 variants, including 22 novel subjects, expressing a wide spectrum of phenotypes. Interestingly, phenotypes correlated with structural locations of the variants. Generalized epilepsy, with a median age at onset of 12 months, and mild-to-moderate ID were associated with variants in the extracellular domain. Focal epilepsy with earlier onset (median: age 4 months) and severe ID were associated with variants in both the pore-lining helical transmembrane domain and the extracellular domain.

CONCLUSION

These genotype-phenotype correlations will aid the genetic counseling and treatment of individuals affected by GABRB3-related disorders. Future studies may reveal whether functional differences underlie the phenotypic differences.

Neuropsychiatric features of Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a genetic disorder characterized by hypotonia and poor feeding in infancy which progresses to hyperphagia in early-mid childhood, as well as developmental delays, a spectrum of behavioral and psychiatric concerns, endocrinopathies, orthopedic issues, and less commonly, seizures, sleep apnea, and narcolepsy with or without cataplexy. This study used data in the Global PWS Registry (N = 893) to explore the onset and severity over time of the neuropsychiatric features reported in individuals with PWS and explored its associations with sleep disorders, seizures, and psychiatric symptoms. Results demonstrate that seizures are more common in the deletion subtype and that narcolepsy and cataplexy are more common in individuals who have sleep-related seizures. Finally, this work shows that anxiety and compulsive behaviors are persistent features of PWS that may arise early in childhood, and that anxiety is associated with higher frequency of other comorbid psychiatric diagnoses. In conclusion, this study is one of the largest to date characterizing sleep disorders and neuropsychiatric characteristics of individuals with PWS and reports on the novel association between sleep disorders and seizures. This study is also one of the first to offer details on the nature of the progression of these features in individuals with PWS.

MutTMPredictor: Robust and accurate cascade XGBoost classifier for prediction of mutations in transmembrane proteins

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Prediction of mutations in transmembrane proteins is of significance for diseases diagnosis.

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Building on the evolutionary information, proposed the Gaussian WAPSSM algorithm.

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Based on WAPSSM and sequence and structure-based features, proposed the cascade XGBoost algorithm.

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Webserver is freely at (http://csbio.njust.edu.cn/bioinf/ffmsresmutp/).

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Implement MutTMPredictor to predict mutations in transmembrane proteins.

Transmembrane proteins have critical biological functions and play a role in a multitude of cellular processes including cell signaling, transport of molecules and ions across membranes. Approximately 60% of transmembrane proteins are considered as drug targets. Missense mutations in such proteins can lead to many diverse diseases and disorders, such as neurodegenerative diseases and cystic fibrosis. However, there are limited studies on mutations in transmembrane proteins. In this work, we first design a new feature encoding method, termed weight attenuation position-specific scoring matrix (WAPSSM), which builds upon the protein evolutionary information. Then, we propose a new mutation prediction algorithm (cascade XGBoost) by leveraging the idea learned from consensus predictors and gcForest. Multi-level experiments illustrate the effectiveness of WAPSSM and cascade XGBoost algorithms. Finally, based on WAPSSM and other three types of features, in combination with the cascade XGBoost algorithm, we develop a new transmembrane protein mutation predictor, named MutTMPredictor. We benchmark the performance of MutTMPredictor against several existing predictors on seven datasets. On the 546 mutations dataset, MutTMPredictor achieves the accuracy (ACC) of 0.9661 and the Matthew’s Correlation Coefficient (MCC) of 0.8950. While on the 67,584 dataset, MutTMPredictor achieves an MCC of 0.7523 and area under curve (AUC) of 0.8746, which are 0.1625 and 0.0801 respectively higher than those of the existing best predictor (fathmm). Besides, MutTMPredictor also outperforms two specific predictors on the Pred-MutHTP datasets. The results suggest that MutTMPredictor can be used as an effective method for predicting and prioritizing missense mutations in transmembrane proteins. The MutTMPredictor webserver and datasets are freely accessible at http://csbio.njust.edu.cn/bioinf/muttmpredictor/ for academic use.

Genetics in Epilepsy

The presence of unprovoked, recurrent seizures, particularly when drug resistant and associated with cognitive and behavioral deficits, warrants investigation for an underlying genetic cause. This article provides an overview of the major classes of genes associated with epilepsy phenotypes divided into functional categories along with the recommended work-up and therapeutic considerations. Gene discovery in epilepsy supports counseling and anticipatory guidance but also opens the door for precision medicine guiding therapy with a focus on those with disease-modifying effects.

Gamma-Aminobutyric Acid Type A Receptor Genes and Their Related Epilepsies

Gamma-aminobutyric acid type A (GABA-A) receptor subunit gene mutations, which include GABRA1, GABRB3, GABRD, and GABRG2, are often involved in several genetic epilepsy syndromes and other neuropsychiatric diseases like autism spectrum disorder, schizophrenia, and anxiety. GABA-A are ligand-gated ionic channels, and are involved firstly in the fast inhibitory synaptic transmission of the central nervous system. The GABA receptors include the ionotropic GABA-A and GABA-C receptors and the metabotropic GABA-B receptors. According to the site in which mutations occur, they cause disorders in channel opening, “lock-and-pull” receptor system functioning, and capable of causing a specific epilepsy phenotype. The aim of this article is to summarize the most recent literature findings, considering genetic mutations, clinical features, genotype/phenotype correlation, and therapy about neurodevelopment diseases correlated to GABA receptors dysfunction, in particular epilepsy. According to our findings, we conclude that further mutation analysis could permit genotype–phenotype correlation and give more information about the best efficient treatment, even if—at present—more clinical and genetic studies are necessary.

Intricacies of GABAA Receptor Function: The Critical Role of the β3 Subunit in Norm and Pathology

Neuronal intracellular chloride ([Cl⁻]_i) is a key determinant in γ-aminobutyric acid type A (GABA)ergic signaling. γ-Aminobutyric acid type A receptors (GABA_ARs) mediate both inhibitory and excitatory neurotransmission, as the passive fluxes of Cl⁻ and HCO₃⁻ via pores can be reversed by changes in the transmembrane concentration gradient of Cl⁻. The cation–chloride co-transporters (CCCs) are the primary systems for maintaining [Cl⁻]_i homeostasis. However, despite extensive electrophysiological data obtained in vitro that are supported by a wide range of molecular biological studies on the expression patterns and properties of CCCs, the presence of ontogenetic changes in [Cl⁻]_i—along with the consequent shift in GABA reversal potential—remain a subject of debate. Recent studies showed that the β3 subunit possesses properties of the P-type ATPase that participates in the ATP-consuming movement of Cl⁻ via the receptor. Moreover, row studies have demonstrated that the β3 subunit is a key player in GABA_AR performance and in the appearance of serious neurological disorders. In this review, we discuss the properties and driving forces of CCCs and Cl⁻, HCO₃⁻ATPase in the maintenance of [Cl⁻]_i homeostasis after changes in upcoming GABA_AR function. Moreover, we discuss the contribution of the β3 subunit in the manifestation of epilepsy, autism, and other syndromes.

Epilepsy: A bibliometric analysis (1968-2020) of the Instituto Nacional de Neurología y Neurocirugía "Manuel Velasco Suarez" in Mexico

The Instituto Nacional de Neurología y Neurocirugía "Manuel Velasco Suárez" (INNN) is one of the main institutions in Latin America treating epilepsy; and bibliometric analysis has an increasing role in analyzing the literature, acting as a Google Maps of medical research. We tracked the scientific output in Scopus and the impact of the institution from its foundation to July 2020 in the field of epilepsy. We roughly separated this group by clinical and experimental approach, identifying core journals, type of article, increase with time, and number of citations. A total of 228 papers, from a total of 3,034 produced by the INNN in that period, were found. Additionally, we identified that neurocysticercosis, pharmacology, genetics, and proteins involved in epilepsy were the most investigated topics. Also, there is a sustained growth in the number of papers per year since 1985. The number of authors per paper ranges from one to 15, and neuroscience journals are the preferred target of researchers, with a predilection for "Epilepsy and Behavior".

High Mobility Group Box 1/Toll-like Receptor 4 Signaling Increases GABRB3 Expression in Alcohol Exposure

INTRODUCTION

Prefrontal cortex (PFC) and striatal neurotransmitter homeostasis is affected by alcohol dependence. In this study, the microarray dataset from the Gene Expression Omnibus (GEO) database were downloaded. The prefrontal and striatum data were cross-analyzed to reveal the co-effects of alcohol dependence on the two brain regions of mice.

METHODS

The GSE123114 microarray profile was downloaded from the GEO database, and differentially expressed genes (DEGs) between the two groups were acquired by GEO2R. KEGG analyses were performed to identify the pivotal pathways of these DEGs. Key differential gene expressions and their mechanism associated with alcohol exposure were investigated by an intraperitoneal alcohol model.

RESULTS

A total of 13 overlapping DEGs from the PFC and striatal datasets of the GSE123114 microarray profile were identified, and they were significantly enriched in the morphine addiction pathway. The transcript levels and protein expression of Gabrb3 were consistent with the microarray data both in the PFC and striatum. The transcript levels of HMGB1, TLR4, TNFα and IL-1β were upregulated in the PFC and striatum of mice in the alcohol group. The HMGB1 inhibitor decreased Gabrb3 transcript and protein levels as well as TNFα and IL-1β transcript levels both in the PFC and striatum in the intraperitoneal alcohol model mice.

DISCUSSION

Through the reanalysis of GSE123114 microarray profile, we found that Gabrb3 is a key gene associated with alcohol exposure. In further experiments, our findings suggest that alcohol exposure modulates Gabrb3 expression through the HMGB1/TLR4 pathway. Moreover, inflammation-associated factors, such as IL-1β and TNFα, may be related to the HMGB1/TLR4-mediated regulation of GABRB3 expression in alcohol exposure.

Characterization of the GABRB2-Associated Neurodevelopmental Disorders

OBJECTIVE

We aimed to characterize the phenotypic spectrum and functional consequences associated with variants in the gene GABRB2, coding for the γ-aminobutyric acid type A (GABA_A ) receptor subunit β2.

METHODS

We recruited and systematically evaluated 25 individuals with variants in GABRB2, 17 of whom are newly described and 8 previously reported with additional clinical data. Functional analysis was performed using a Xenopus laevis oocyte model system.

RESULTS

Our cohort of 25 individuals from 22 families with variants in GABRB2 demonstrated a range of epilepsy phenotypes from genetic generalized epilepsy to developmental and epileptic encephalopathy. Fifty-eight percent of individuals had pharmacoresistant epilepsy; response to medications targeting the GABAergic pathway was inconsistent. Developmental disability (present in 84%) ranged from mild intellectual disability to severe global disability; movement disorders (present in 44%) included choreoathetosis, dystonia, and ataxia. Disease-associated variants cluster in the extracellular N-terminus and transmembrane domains 1-3, with more severe phenotypes seen in association with variants in transmembrane domains 1 and 2 and the allosteric binding site between transmembrane domains 2 and 3. Functional analysis of 4 variants in transmembrane domains 1 or 2 (p.Ile246Thr, p.Pro252Leu, p.Ile288Ser, p.Val282Ala) revealed strongly reduced amplitudes of GABA-evoked anionic currents.

INTERPRETATION

GABRB2-related epilepsy ranges broadly in severity from genetic generalized epilepsy to developmental and epileptic encephalopathies. Developmental disability and movement disorder are key features. The phenotypic spectrum is comparable to other GABA_A receptor-encoding genes. Phenotypic severity varies by protein domain. Experimental evidence supports loss of GABAergic inhibition as the mechanism underlying GABRB2-associated neurodevelopmental disorders. ANN NEUROL 2021;89:573-586.

Genetic Landscape of Common Epilepsies: Advancing towards Precision in Treatment

Epilepsy, a neurological disease characterized by recurrent seizures, is highly heterogeneous in nature. Based on the prevalence, epilepsy is classified into two types: common and rare epilepsies. Common epilepsies affecting nearly 95% people with epilepsy, comprise generalized epilepsy which encompass idiopathic generalized epilepsy like childhood absence epilepsy, juvenile myoclonic epilepsy, juvenile absence epilepsy and epilepsy with generalized tonic-clonic seizure on awakening and focal epilepsy like temporal lobe epilepsy and cryptogenic focal epilepsy. In 70% of the epilepsy cases, genetic factors are responsible either as single genetic variant in rare epilepsies or multiple genetic variants acting along with different environmental factors as in common epilepsies. Genetic testing and precision treatment have been developed for a few rare epilepsies and is lacking for common epilepsies due to their complex nature of inheritance. Precision medicine for common epilepsies require a panoramic approach that incorporates polygenic background and other non-genetic factors like microbiome, diet, age at disease onset, optimal time for treatment and other lifestyle factors which influence seizure threshold. This review aims to comprehensively present a state-of-art review of all the genes and their genetic variants that are associated with all common epilepsy subtypes. It also encompasses the basis of these genes in the epileptogenesis. Here, we discussed the current status of the common epilepsy genetics and address the clinical application so far on evidence-based markers in prognosis, diagnosis, and treatment management. In addition, we assessed the diagnostic predictability of a few genetic markers used for disease risk prediction in individuals. A combination of deeper endo-phenotyping including pharmaco-response data, electro-clinical imaging, and other clinical measurements along with genetics may be used to diagnose common epilepsies and this marks a step ahead in precision medicine in common epilepsies management.

Venom-derived modulators of epilepsy-related ion channels

Epilepsy is characterised by spontaneous recurrent seizures that are caused by an imbalance between neuronal excitability and inhibition. Since ion channels play fundamental roles in the generation and propagation of action potentials as well as neurotransmitter release at a subset of excitatory and inhibitory synapses, their dysfunction has been linked to a wide variety of epilepsies. Indeed, these unique proteins are the major biological targets for antiepileptic drugs. Selective targeting of a specific ion channel subtype remains challenging for small molecules, due to the high level of homology among members of the same channel family. As a consequence, there is a growing trend to target ion channels with biologics. Venoms are the best known natural source of ion channel modulators, and venom peptides are increasingly recognised as potential therapeutics due to their high selectivity and potency gained through millions of years of evolutionary selection pressure. Here we describe the major ion channel families involved in the pathogenesis of various types of epilepsy, including voltage-gated Na⁺, K⁺, Ca²⁺ channels, Cys-loop receptors, ionotropic glutamate receptors and P2X receptors, and currently available venom-derived peptides that target these channel proteins. Although only a small number of venom peptides have successfully progressed to the clinic, there is reason to be optimistic about their development as antiepileptic drugs, notwithstanding the challenges associated with development of any class of peptide drug.

HAP1 Modulates Epileptic Seizures by Regulating GABAAR Function in Patients with Temporal Lobe Epilepsy and in the PTZ-Induced Epileptic Model

The number of γ-aminobutyric acid type A receptors (GABA_ARs) expressed on the surface membrane and at synaptic sites is implicated in the enhanced excitation of neuronal circuits and abnormal network oscillations in epilepsy. Huntingtin-associated protein 1 (HAP1), a key mediator of pathological alterations in protein trafficking, directly interacts with GABA_ARs and facilitates their recycling back to synapses after internalization from the surface; thus, HAP1 regulates the strength of inhibitory synaptic transmission. Here, we show that HAP1 modulates epileptic seizures by regulating GABA_AR function in patients with temporal lobe epilepsy (TLE) and in the pentylenetetrazol (PTZ)-induced epileptic model. We demonstrate that GABA_ARβ^2/3 and HAP1 expression are decreased and that the HAP1-GABA_ARβ^2/3 complex is disrupted in the epileptic rat brain. We found that HAP1 upregulation exerts antiepileptic activity in the PTZ-induced seizure and that these changes are associated with increased surface GABA_ARβ^2/3 expression and the amplitude of miniature inhibitory postsynaptic currents (mIPSCs). This study provides evidence that hippocampal HAP1 is linked to GABA_ARs in evoking seizures and suggests that this mechanism is involved in epileptic seizures in the brain, representing a potential therapeutic target for epilepsy.

GABRG2 Deletion Linked to Genetic Epilepsy with Febrile Seizures Plus Affects the Expression of GABAA Receptor Subunits and Other Genes at Different Temperatures

Mutations in γ-aminobutyric acid A receptor (GABA_A) subunits and sodium channel genes, especially GABRG2 and SCN1A, have been reported to be associated with febrile seizures (FS) and genetic epilepsy with febrile seizures plus (GEFS+). GEFS+ is a well-known family of epileptic syndrome with autosomal dominant inheritance in children. Its most common phenotypes are febrile seizures often with accessory afebrile generalized tonic-clonic seizures, febrile seizures plus (FS+), severe epileptic encephalopathy, as well as other types of generalized or localization-related seizures. However, the pathogenesis of febrile seizures remains largely unknown. Here, we generated a GABRG2 gene knockout cell line (HT22^GABRG2KO) by applying the CRISPR/Cas9-mediated genomic deletion in HT-22 mouse hippocampal neuronal cell line to explore the function of GABRG2 in vitro. With mRNA-seq, we found significant changes in the expression profiles of several epilepsy-related genes when GABRG2 was knockout, some of them showing temperature-induced changes as well. Kyoto Encyclopedia Gene and Genomic (KEGG) analysis revealed a significant alteration in the MAPK and PI3K-Akt signaling pathways. We also observed an up-regulation of the matrix metalloproteinases (MMPs) family after GABRG2 knockout. Furthermore, the significant decrease in expression of GABRA1 and CACNA1A (but not others) with an increase in temperature is a novel finding. In summary, mutations in the GABA_A receptor can lead to a decrease in numbers of receptors, which may cause the impairment of GABAergic pathway signaling. This data has been the first time to reveal that GABRG2 mutations would affect the function of other genes, and based on this finding we hope this work would also provide a new direction for the research of GABRG2 in GEFS+. It also may provide a molecular basis for the severity of epilepsy, and guide the clinical medication for the treatment of the epilepsy focused on the function on GABA_A receptors, which, might be a new strategy for genetic diagnosis and targeted treatment of epilepsy.

Synaptic clustering differences due to different GABRB3 mutations cause variable epilepsy syndromes

GABRB3 is highly expressed early in the developing brain, and its encoded β3 subunit is critical for GABAA receptor assembly and trafficking as well as stem cell differentiation in embryonic brain. To date, over 400 mutations or variants have been identified in GABRB3. Mutations in GABRB3 have been increasingly recognized as a major cause for severe paediatric epilepsy syndromes such as Lennox-Gastaut syndrome, Dravet syndrome and infantile spasms with intellectual disability as well as relatively mild epilepsy syndromes such as childhood absence epilepsy. There is no plausible molecular pathology for disease phenotypic heterogeneity. Here we used a very high-throughput flow cytometry assay to evaluate the impact of multiple human mutations in GABRB3 on receptor trafficking. In this study we found that surface expression of mutant β3 subunits is variable. However, it was consistent that surface expression of partnering γ2 subunits was lower when co-expressed with mutant than with wild-type subunits. Because γ2 subunits are critical for synaptic GABAA receptor clustering, this provides an important clue for understanding the pathophysiology of GABRB3 mutations. To validate our findings further, we obtained an in-depth comparison of two novel mutations [GABRB3 (N328D) and GABRB3 (E357K)] associated with epilepsy with different severities of epilepsy phenotype. GABRB3 (N328D) is associated with the relatively severe Lennox-Gastaut syndrome, and GABRB3 (E357K) is associated with the relatively mild juvenile absence epilepsy syndrome. With functional characterizations in both heterologous cells and rodent cortical neurons by patch-clamp recordings, confocal microscopy and immunoblotting, we found that both the GABRB3 (N328D) and GABRB3 (E357K) mutations reduced total subunit expression in neurons but not in HEK293T cells. Both mutant subunits, however, were reduced on the cell surface and in synapses, but the Lennox-Gastaut syndrome mutant β3 (N328D) subunit was more reduced than the juvenile absence epilepsy mutant β3 (E357K) subunit. Interestingly, both mutant β3 subunits impaired postsynaptic clustering of wild-type GABAA receptor γ2 subunits and prevented γ2 subunits from incorporating into GABAA receptors at synapses, although by different cellular mechanisms. Importantly, wild-type γ2 subunits were reduced and less clustered at inhibitory synapses in Gabrb3+/- knockout mice. This suggests that impaired receptor localization to synapses is a common pathophysiological mechanism for GABRB3 mutations, although the extent of impairment may be different among mutant subunits. The study thus identifies the novel mechanism of impaired targeting of receptors containing mutant β3 subunits and provides critical insights into understanding how GABRB3 mutations produce severe epilepsy syndromes and epilepsy phenotypic heterogeneity.

Functional analysis of haplotypes and promoter activity at the 5' region of the human GABRB3 gene and associations with schizophrenia

Background

This study investigated the effects of haplotypes T‐G and C‐A derived from NG_012836.1:g.4160T>C and NG_012836.1:g.4326G>A on protein expression levels in vitro and identified the functional sequence in the regulatory region of the GABRB3 gene linked to possible associations with schizophrenia.

Methods

Recombinant plasmids with haplotypes T‐G and C‐A and 10 recombinant vectors containing deletion fragments from the GABRB3 gene 5′ regulatory region were transfected into HEK‐293, SK‐N‐SH, and SH‐SY5Y cells. The relative fluorescence intensity of the two haplotypes and different sequences was compared using a dual luciferase reporter assay system.

Results

The relative fluorescence intensity of haplotype C‐A was significantly lower than that of T‐G. We shortened the core promoter sequence of the GABRB3 gene 5′ regulation region from −177 bp to −18 bp (ATG+1). We also found an expression suppression region from −1,735 bp to −1,638 bp and an enhanced regulatory region from −1,638 bp to −1,335 bp. Multiple inhibitory functional elements were identified in the region from −680 bp to −177 bp.

Conclusion

We demonstrated that haplotype C‐A might increase the risk of schizophrenia and found multiple regulatory regions that had an effect on GABRB3 receptor expression.

A structural look at GABAA receptor mutations linked to epilepsy syndromes

Understanding the genetic variation in GABA_A receptor subunit genes (GABRs), GABRA1-6, GABRB1-3, GABRG1-3 and GABRD, in individuals affected by epilepsy may improve the diagnosis and treatment of epilepsy syndromes through identification of disease-associated variants. However, the lack of functional analysis and validation of many novel and previously reported familial and de novo mutations have made it challenging to address meaningful gene associations with epilepsy syndromes. GABA_A receptors belong to the Cys-loop receptor family. Even though GABA_A receptor mutant residues are widespread among different GABRs, their frequent occurrence in important structural domains that share common functional features suggests associations between structure and function.

Overview of genetic models of autism spectrum disorders

Autism spectrum disorders (ASDs) are a group of neurodevelopment disorders that are characterized by heterogenous cognitive deficits and genetic factors. As more ASD risk genes are identified, genetic animal models have been developed to parse out the underlying neurobiological mechanisms of ASD. In this review, we discuss a subset of genetic models of ASD, focusing on those that have been widely studied and strongly linked to ASD. We focus our discussion of these models in the context of the theories and potential mechanisms of ASD, including disruptions in cell growth and proliferation, spine dynamics, synaptic transmission, excitation/inhibition balance, intracellular signaling, neuroinflammation, and behavior. In addition to ASD pathophysiology, we examine the limitations and challenges that genetic models pose for the study of ASD biology. We end with a review of innovative techniques and concepts of ASD pathology that can be further applied to and studied using genetic ASD models.

Neural Organization of A3 Mushroom Body Extrinsic Neurons in the Honeybee Brain

In the insect brain, the mushroom body is a higher order brain area that is key to memory formation and sensory processing. Mushroom body (MB) extrinsic neurons leaving the output region of the MB, the lobes and the peduncle, are thought to be especially important in these processes. In the honeybee brain, a distinct class of MB extrinsic neurons, A3 neurons, are implicated in playing a role in learning. Their MB arborisations are either restricted to the lobes and the peduncle, here called A3 lobe connecting neurons, or they provide feedback information from the lobes to the input region of the MB, the calyces, here called A3 feedback neurons. In this study, we analyzed the morphology of individual A3 lobe connecting and feedback neurons using confocal imaging. A3 feedback neurons were previously assumed to innervate each lip compartment homogenously. We demonstrate here that A3 feedback neurons do not innervate whole subcompartments, but rather innervate zones of varying sizes in the MB lip, collar, and basal ring. We describe for the first time the anatomical details of A3 lobe connecting neurons and show that their connection pattern in the lobes resemble those of A3 feedback cells. Previous studies showed that A3 feedback neurons mostly connect zones of the vertical lobe that receive input from Kenyon cells of distinct calycal subcompartments with the corresponding subcompartments of the calyces. We can show that this also applies to the neck of the peduncle and the medial lobe, where both types of A3 neurons arborize only in corresponding zones in the calycal subcompartments. Some A3 lobe connecting neurons however connect multiple vertical lobe areas. Contrarily, in the medial lobe, the A3 neurons only innervate one division. We found evidence for both input and output areas in the vertical lobe. Thus, A3 neurons are more diverse than previously thought. The understanding of their detailed anatomy might enable us to derive circuit models for learning and memory and test physiological data.

Clinical and genetic study of Tunisian families with genetic generalized epilepsy: contribution of CACNA1H and MAST4 genes

Genetic generalized epilepsies (GGE) (childhood absence epilepsy (CAE), juvenile myoclonic epilepsy (JME) and epilepsy with generalized tonic-clonic seizures (GTCS)) are mainly determined by genetic factors. Since few mutations were identified in rare families with autosomal dominant GGE, a polygenic inheritance was suspected in most patients. Recent studies on large American or European cohorts of sporadic cases showed that susceptibility genes were numerous although their variants were rare, making their identification difficult. Here, we reported clinical and genetic characteristics of 30 Tunisian GGE families, including 71 GGE patients. The phenotype was close to that in sporadic cases. Nineteen pedigrees had a homogeneous type of GGE (JME-CAE-CGTS), and 11 combined these epileptic syndromes. Rare non-synonymous variants were selected in probands using a targeted panel of 30 candidate genes and their segregation was determined in families. Molecular studies incriminated different genes, mainly CACNA1H and MAST4. The segregation of at least two variants in different genes in some pedigrees was compatible with the hypothesis of an oligogenic inheritance, which was in accordance with the relatively low frequency of consanguineous probands. Since at least 2 susceptibility genes were likely shared by different populations, genetic factors involved in the majority of Tunisian GGE families remain to be discovered. Their identification should be easier in families with a homogeneous type of GGE, in which an intra-familial genetic homogeneity could be suspected.

Defining the phenotypic spectrum of SLC6A1 mutations

OBJECTIVE

Pathogenic SLC6A1 variants were recently described in patients with myoclonic atonic epilepsy (MAE) and intellectual disability (ID). We set out to define the phenotypic spectrum in a larger cohort of SCL6A1-mutated patients.

METHODS

We collected 24 SLC6A1 probands and 6 affected family members. Four previously published cases were included for further electroclinical description. In total, we reviewed the electroclinical data of 34 subjects.

RESULTS

Cognitive development was impaired in 33/34 (97%) subjects; 28/34 had mild to moderate ID, with language impairment being the most common feature. Epilepsy was diagnosed in 31/34 cases with mean onset at 3.7 years. Cognitive assessment before epilepsy onset was available in 24/31 subjects and was normal in 25% (6/24), and consistent with mild ID in 46% (11/24) or moderate ID in 17% (4/24). Two patients had speech delay only, and 1 had severe ID. After epilepsy onset, cognition deteriorated in 46% (11/24) of cases. The most common seizure types were absence, myoclonic, and atonic seizures. Sixteen cases fulfilled the diagnostic criteria for MAE. Seven further patients had different forms of generalized epilepsy and 2 had focal epilepsy. Twenty of 31 patients became seizure-free, with valproic acid being the most effective drug. There was no clear-cut correlation between seizure control and cognitive outcome. Electroencephalography (EEG) findings were available in 27/31 patients showing irregular bursts of diffuse 2.5-3.5 Hz spikes/polyspikes-and-slow waves in 25/31. Two patients developed an EEG pattern resembling electrical status epilepticus during sleep. Ataxia was observed in 7/34 cases. We describe 7 truncating and 18 missense variants, including 4 recurrent variants (Gly232Val, Ala288Val, Val342Met, and Gly362Arg).

SIGNIFICANCE

Most patients carrying pathogenic SLC6A1 variants have an MAE phenotype with language delay and mild/moderate ID before epilepsy onset. However, ID alone or associated with focal epilepsy can also be observed.

Early onset epileptic encephalopathy with a novel GABRB3 mutation treated effectively with clonazepam: A case report

RATIONALE

Early onset epileptic encephalopathy (EOEE) is one of the most serious early onset epilepsies. The etiopathology of this condition remains unclear, and recent evidence indicated that gamma-aminobutyric acid (GABA) A receptor, subunit beta 3 (GABRB3) gene mutations might be associated with EOEE. Furthermore, the therapeutic regimen for EOEE has yet to be well elucidated. Herein, we reported the clinical and genetic features of a case with GABRB3-related EOEE.

PATIENT CONCERNS

A 6-year-old girl developed epileptic seizures 3 days after birth. She presented with multiple seizure types including myoclonic seizures, spasms, and absence seizures. Serial electroencephalographic examinations showed variable abnormalities, and intellectual evaluation revealed significant development retardation. Conventional antiepileptic drugs were ineffective for the seizure controlling. Genetic screening identified a novel nonsense mutation (C.5G > A, p.W2X) in the GABRB3 gene.

DIAGNOSES

Early onset epileptic encephalopathy.

INTERVENTIONS

We changed the antiepileptic strategy to oral clonazepam (0.5mg twice daily). The patient was followed up once a week and significant declining in the attack frequency was noted 1 week later (2-3 times daily). Subsequently, the dosage was doubled (1mg twice daily), and complete cessation of seizures was achieved 20 days later.

OUTCOMES

Through a 9-month follow up,the girl remained seizure-free.

LESSONS

This study identified a novel nonsensemutation (C.5G>A) in the exon 1 of GABRB3 Gene, which may be associated with EOEE. To our knowledge, this is the first report to use clonazepam in the patient with GABRB3-related EOEE with favorable outcome. Our finding suggested that clonazepam might be a choice for patient with GABRB3-related EOEE. The remarkable efficacy of clonazepam in the control of seizures indicated a potential GABRB3- or GABA-related mechanism involved in the development of EOEE.

GABA A Receptor Coupling Junction and Pore GABRB3 Mutations are Linked to Early-Onset Epileptic Encephalopathy

GABA_A receptors are brain inhibitory chloride ion channels. Here we show functional analyses and structural simulations for three de novo missense mutations in the GABA_A receptor β3 subunit gene (GABRB3) identified in patients with early-onset epileptic encephalopathy (EOEE) and profound developmental delay. We sought to obtain insights into the molecular mechanisms that might link defects in GABA_A receptor biophysics and biogenesis to patients with EOEE. The mutant residues are part of conserved structural domains such as the Cys-loop (L170R) and M2-M3 loop (A305V) that form the GABA binding/channel gating coupling junction and the channel pore (T288N), which are functionally coupled during receptor activation. The mutant coupling junction residues caused rearrangements and formation of new hydrogen bonds in the open state, while the mutant pore residue reshaped the pore cavity. Whereas mutant coupling junction residues uncoupled during activation and caused gain of function, the mutant pore residue favoured low conductance receptors and differential sensitivity to diazepam and loss of function. These data reveal novel molecular mechanisms by which EOEE-linked mutations affect GABA_A receptor function.

Genetic and Molecular Regulation of Extrasynaptic GABA-A Receptors in the Brain: Therapeutic Insights for Epilepsy

GABA-A receptors play a pivotal role in many brain diseases. Epilepsy is caused by acquired conditions and genetic defects in GABA receptor channels regulating neuronal excitability in the brain. The latter is referred to as GABA channelopathies. In the last two decades, major advances have been made in the genetics of epilepsy. The presence of specific GABAergic genetic abnormalities leading to some of the classic epileptic syndromes has been identified. Advances in molecular cloning and recombinant systems have helped characterize mutations in GABA-A receptor subunit genes in clinical neurology. GABA-A receptors are the prime targets for neurosteroids (NSs). However, GABA-A receptors are not static but undergo rapid changes in their number or composition in response to the neuroendocrine milieu. This review describes the recent advances in the genetic and neuroendocrine control of extrasynaptic and synaptic GABA-A receptors in epilepsy and its impact on neurologic conditions. It highlights the current knowledge of GABA genetics in epilepsy, with an emphasis on the neuroendocrine regulation of extrasynaptic GABA-A receptors in network excitability and seizure susceptibility. Recent advances in molecular regulation of extrasynaptic GABA-A receptor-mediated tonic inhibition are providing unique new therapeutic approaches for epilepsy, status epilepticus, and certain brain disorders. The discovery of an extrasynaptic molecular mechanism represents a milestone for developing novel therapies such as NS replacement therapy for catamenial epilepsy.

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.'

[Research advances in hereditary epilepsy and precision drug therapy]

Epilepsy is a common nervous system disease. It has been found that the pathogenesis of epilepsy is associated mutations in various genes, including genes encoding voltage-dependent ion channel, genes encoding ligand-gated ion channel, and solute carrier family genes. Different types of epilepsy caused by different mutations have different responses to drugs, and therefore, diagnosis and medication guidance based on genes are new thoughts for developing therapies. With the application of next-generation sequencing technology, more and more genes will be determined, which helps to further study the pathogenic mechanism of mutant genes and provides a basis for precision drug therapy for epilepsy.

[Research advances in hereditary epilepsy and precision drug therapy]

Epilepsy is a common nervous system disease. It has been found that the pathogenesis of epilepsy is associated mutations in various genes, including genes encoding voltage-dependent ion channel, genes encoding ligand-gated ion channel, and solute carrier family genes. Different types of epilepsy caused by different mutations have different responses to drugs, and therefore, diagnosis and medication guidance based on genes are new thoughts for developing therapies. With the application of next-generation sequencing technology, more and more genes will be determined, which helps to further study the pathogenic mechanism of mutant genes and provides a basis for precision drug therapy for epilepsy.

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.

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.

Ion Channel Genes and Epilepsy: Functional Alteration, Pathogenic Potential, and Mechanism of Epilepsy

Ion channels are crucial in the generation and modulation of excitability in the nervous system and have been implicated in human epilepsy. Forty-one epilepsy-associated ion channel genes and their mutations are systematically reviewed. In this paper, we analyzed the genotypes, functional alterations (funotypes), and phenotypes of these mutations. Eleven genes featured loss-of-function mutations and six had gain-of-function mutations. Nine genes displayed diversified funotypes, among which a distinct funotype-phenotype correlation was found in SCN1A. These data suggest that the funotype is an essential consideration in evaluating the pathogenicity of mutations and a distinct funotype or funotype-phenotype correlation helps to define the pathogenic potential of a gene.

Deleterious variants in TRAK1 disrupt mitochondrial movement and cause fatal encephalopathy

Cellular distribution and dynamics of mitochondria are regulated by several motor proteins and a microtubule network. In neurons, mitochondrial trafficking is crucial because of high energy needs and calcium ion buffering along axons to synapses during neurotransmission. The trafficking kinesin proteins (TRAKs) are well characterized for their role in lysosomal and mitochondrial trafficking in cells, especially neurons. Using whole exome sequencing, we identified homozygous truncating variants in TRAK1 (NM_001042646:c.287-2A > C), in six lethal encephalopathic patients from three unrelated families. The pathogenic variant results in aberrant splicing and significantly reduced gene expression at the RNA and protein levels. In comparison with normal cells, TRAK1-deficient fibroblasts showed irregular mitochondrial distribution, altered mitochondrial motility, reduced mitochondrial membrane potential, and diminished mitochondrial respiration. This study confirms the role of TRAK1 in mitochondrial dynamics and constitutes the first report of this gene in association with a severe neurodevelopmental disorder.

Genetic and epigenetic mechanisms of epilepsy: a review

Epilepsy is a common episodic neurological disorder or condition characterized by recurrent epileptic seizures, and genetics seems to play a key role in its etiology. Early linkage studies have localized multiple loci that may harbor susceptibility genes to epilepsy, and mutational analyses have detected a number of mutations involved in both ion channel and nonion channel genes in patients with idiopathic epilepsy. Genome-wide studies of epilepsy have found copy number variants at 2q24.2-q24.3, 7q11.22, 15q11.2-q13.3, and 16p13.11-p13.2, some of which disrupt multiple genes, such as NRXN1, AUTS2, NLGN1, CNTNAP2, GRIN2A, PRRT2, NIPA2, and BMP5, implicated for neurodevelopmental disorders, including intellectual disability and autism. Unfortunately, only a few common genetic variants have been associated with epilepsy. Recent exome-sequencing studies have found some genetic mutations, most of which are located in nonion channel genes such as the LGI1, PRRT2, EFHC1, PRICKLE, RBFOX1, and DEPDC5 and in probands with rare forms of familial epilepsy, and some of these genes are involved with the neurodevelopment. Since epigenetics plays a role in neuronal function from embryogenesis and early brain development to tissue-specific gene expression, epigenetic regulation may contribute to the genetic mechanism of neurodevelopment through which a gene and the environment interacting with each other affect the development of epilepsy. This review focused on the analytic tools used to identify epilepsy and then provided a summary of recent linkage and association findings, indicating the existence of novel genes on several chromosomes for further understanding of the biology of epilepsy.