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

Trace amine associated receptor 1: predicted effects of single nucleotide variants on structure-function in geographically diverse populations

Trace Amine Associated Receptor 1 (TAAR1) is a novel pharmaceutical target under investigation for the treatment of several neuropsychiatric conditions. TAAR1 single nucleotide variants (SNV) have been found in patients with schizophrenia and metabolic disorders. However, the frequency of variants in geographically diverse populations and the functional effects of such variants are unknown. In this study, we aimed to characterise the distribution of TAAR1 SNVs in five different WHO regions using the Database of Genotypes and Phenotypes (dbGaP) and conducted a critical computational analysis using available TAAR1 structural data to identify SNVs affecting ligand binding and/or functional regions. Our analysis shows 19 orthosteric, 9 signalling and 16 micro-switch SNVs hypothesised to critically influence the agonist induced TAAR1 activation. These SNVs may non-proportionally influence populations from discrete regions and differentially influence the activity of TAAR1-targeting therapeutics in genetically and geographically diverse populations. Notably, our dataset presented with orthosteric SNVs D1033.32N (found only in the South-East Asian Region and Western Pacific Region) and T1945.42A (found only in South-East Asian Region), and 2 signalling SNVs (V1253.54A/T2526.36A, found in African Region and commonly, respectively), all of which have previously demonstrated to influence ligand induced functions of TAAR1. Furthermore, bioinformatics analysis using SIFT4G, MutationTaster 2, PROVEAN and MutationAssessor predicted all 16 micro-switch SNVs are damaging and may further influence the agonist activation of TAAR1, thereby possibly impacting upon clinical outcomes. Understanding the genetic basis of TAAR1 function and the impact of common mutations within clinical populations is important for the safe and effective utilisation of novel and existing pharmacotherapies.

Pathogenic variants of human GABRA1 gene associated with epilepsy: A computational approach

Critical for brain development, neurodevelopmental and network disorders, the GABRA1 gene encodes for the α1 subunit, an abundantly and developmentally expressed subunit of heteropentameric gamma-aminobutyric acid A receptors (GABAARs) mediating primary inhibition in the brain. Mutations of the GABAAR subunit genes including GABRA1 gene are associated with epilepsy, a group of syndromes, characterized by unprovoked seizures and diagnosed by integrative approach, that involves genetic testing. Despite the diagnostic use of genetic testing, a large fraction of the GABAAR subunit gene variants including the variants of GABRA1 gene is not known in terms of their molecular consequence, a challenge for precision and personalized medicine. Addressing this, one hundred thirty-seven GABRA1 gene variants of unknown clinical significance have been extracted from the ClinVar database and computationally analyzed for pathogenicity. Eight variants (L49H, P59L, W97R, D99G, G152S, V270G, T294R, P305L) are predicted as pathogenic and mapped to the α1 subunit's extracellular domain (ECD), transmembrane domains (TMDs) and extracellular linker. This is followed by the integration with relevant data for cellular pathology and severity of the epilepsy syndromes retrieved from the literature. Our results suggest that the pathogenic variants in the ECD of GABRA1 (L49H, P59L, W97R, D99G, G152S) will probably manifest decreased surface expression and reduced current with mild epilepsy phenotypes while V270G, T294R in the TMDs and P305L in the linker between the second and the third TMDs will likely cause reduced cell current with severe epilepsy phenotypes. The results presented in this study provides insights for clinical genetics and wet lab experimentation.

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.

No Gain, Less Pain: GABRB3 Mutations in Epileptic Encephalopathy

Gain-of-Function and Loss-of-Function GABRB3 Variants Lead to Distinct Clinical Phenotypes in Patients With Developmental and Epileptic Encephalopathies

Absalom NL, Liao VW, Johannesen KM, Gardella E, Jacobs J, Lesca G, Gokce-Samar Z, Arzimanoglou A, Zeidler S, Striano P, Meyer P, Benkel-Herrenbrueck I, Mero I-L, Rummel J, Chebib M, Møller RS, Ahring PK. Nat Commun. 2022;13(1):1822. doi:10.1038/s41467-022-29280-x

Many patients with developmental and epileptic encephalopathies present with variants in genes coding for GABAA receptors. These variants are presumed to cause loss-of-function receptors leading to reduced neuronal GABAergic activity. Yet, patients with GABAA 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.

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.

Gabra2 is a genetic modifier of Dravet syndrome in mice

Pathogenic variants in epilepsy genes result in a spectrum of clinical severity. One source of phenotypic heterogeneity is modifier genes that affect expressivity of a primary pathogenic variant. Mouse epilepsy models also display varying degrees of clinical severity on different genetic backgrounds. Mice with heterozygous deletion of Scn1a (Scn1a^+/−) model Dravet syndrome, a severe epilepsy most often caused by SCN1A haploinsufficiency. Scn1a^+/− mice recapitulate features of Dravet syndrome, including spontaneous seizures, sudden death, and cognitive/behavioral deficits. Scn1a^+/− mice maintained on the 129S6/SvEvTac (129) strain have normal lifespan and no spontaneous seizures. In contrast, admixture with C57BL/6J (B6) results in epilepsy and premature lethality. We previously mapped Dravet Survival Modifier loci (Dsm1-Dsm5) responsible for strain-dependent differences in survival. Gabra2, encoding the GABA_A α2 subunit, was nominated as a candidate modifier at Dsm1. Direct measurement of GABA_A receptors found lower abundance of α2-containing receptors in hippocampal synapses of B6 mice relative to 129. We also identified a B6-specific single nucleotide deletion within Gabra2 that lowers mRNA and protein by nearly 50%. Repair of this deletion reestablished normal levels of Gabra2 expression. In this study, we used B6 mice with a repaired Gabra2 allele to evaluate Gabra2 as a genetic modifier of severity in Scn1a^+/− mice. Gabra2 repair restored transcript and protein expression, increased abundance of α2-containing GABA_A receptors in hippocampal synapses, and rescued epilepsy phenotypes of Scn1a^+/− mice. These findings validate Gabra2 as a genetic modifier of Dravet syndrome, and support enhancing function of α₂-containing GABA_A receptors as treatment strategy for Dravet syndrome.

Dravet syndrome-associated mutations in GABRA1, GABRB2 and GABRG2 define the genetic landscape of defects of GABAA receptors

Dravet syndrome is a rare, catastrophic epileptic encephalopathy that begins in the first year of life, usually with febrile or afebrile hemiclonic or generalized tonic-clonic seizures followed by status epilepticus. De novo variants in genes that mediate synaptic transmission such as SCN1A and PCDH19 are often associated with Dravet syndrome. Recently, GABAA receptor subunit genes (GABRs) encoding α1 (GABRA1), β3 (GABRB3) and γ2 (GABRG2), but not β2 (GABRB2) or β1 (GABRB1), subunits are frequently associated with Dravet syndrome or Dravet syndrome-like phenotype. We performed next generation sequencing on 870 patients with Dravet syndrome and identified nine variants in three different GABRs. Interestingly, the variants were all in genes encoding the most common GABAA receptor, the α1β2γ2 receptor. Mutations in GABRA1 (c.644T>C, p. L215P; c.640C>T, p. R214C; c.859G>A; V287I; c.641G>A, p. R214H) and GABRG2 (c.269C>G, p. T90R; c.1025C>T, p. P342L) presented as de novo cases, while in GABRB2 two variants were de novo (c.992T>C, p. F331S; c.542A>T, p. Y181F) and one was autosomal dominant and inherited from the maternal side (c.990_992del, p.330_331del). We characterized the effects of these GABR variants on GABAA receptor biogenesis and channel function. We found that defects in receptor gating were the common deficiency of GABRA1 and GABRB2 Dravet syndrome variants, while mainly trafficking defects were found with the GABRG2 (c.269C>G, p. T90R) variant. It seems that variants in α1 and β2 subunits are less tolerated than in γ2 subunits, since variant α1 and β2 subunits express well but were functionally deficient. This suggests that all of these GABR variants are all targeting GABR genes that encode the assembled α1β2γ2 receptor, and regardless of which of the three subunits are mutated, variants in genes coding for α1, β2 and γ2 receptor subunits make them candidate causative genes in the pathogenesis of Dravet syndrome.

Phenotypic spectrum of patients with GABRB2 variants: from mild febrile seizures to severe epileptic encephalopathy

AIM

To characterize the different phenotypes of GABRB2-related epilepsy and to establish a genotype-phenotype correlation.

METHOD

We used next-generation sequencing to identify GABRB2 variants in 15 patients.

RESULTS

Eleven GABRB2 variants were novel and 12 were de novo. The age at the onset of seizures ranged from 1 day to 26 months. Nine patients had multiple seizure types, including focal seizures, generalized tonic-clonic seizures, myoclonic seizures, epileptic spasms, and atonic seizures. Seizures were fever-sensitive in 13 out of the 15 patients. Eleven patients displayed developmental delay, while 11 had abnormal video electroencephalography. Abnormalities in the brain images included dysplasia of the frontal and temporal cortex, dysplasia of the corpus callosum, and delayed myelination in four patients. One patient was diagnosed with febrile seizures, three with febrile seizures plus, three with Dravet syndrome, three with West syndrome, one with Ohtahara syndrome, three with developmental delays and epilepsy, and one with non-specific early-onset epileptic encephalopathy.

INTERPRETATION

The most common phenotypes of patients with GABRB2 variants include early onset of seizure and fever sensitivity. Febrile seizures and febrile seizures plus are new phenotypes of GABRB2 variants. The phenotypic spectrum of GABRB2 variants ranges from mild febrile seizures to severe epileptic encephalopathy.

Rare variants in the GABAA receptor subunit ε identified in patients with a wide spectrum of epileptic phenotypes

BACKGROUND

Epilepsy belongs to a group of chronic and highly heterogeneous brain disorders. Many types of epilepsy and epileptic syndromes are caused by genetic factors. The neural amino acid y-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian central nervous system. It regulates activity of channel pores by binding to transmembrane GABA-receptors (GABRs). The GABRs are heteropentamers assembled from different receptor subunits (α1-6, β1-3, γ1-3, δ, ε, θ, π, and ρ1-3). Several epileptic disorders are caused by mutations in genes encoding single GABRs.

METHODS

We applied trio- and single-whole exome sequencing to search for genetic sequence variants associated with a wide range of epileptic phenotypes accompanied by intellectual disability and/or global developmental delay in the investigated patients.

RESULTS

We identified four hemizygous sequence variants in the GABAA receptor subunit ε gene (GABRE), including one nonsense (NM_004961.3: c.399C>A, p.Tyr133*), two missense variants (NM_004961.3: c.664G>A, p.Glu222Lys; NM_004961.3: c.1045G>A, p.Val349Ile), and one variant affecting the translation initiation codon (NM_004961.3: c.1A>G, p.Met1?) in four unrelated families.

CONCLUSION

Our clinical and molecular genetic findings suggest that GABRE is a likely candidate gene for epilepsy. Nevertheless, functional studies are necessary to better understand pathogenicity of the GABRE-mutations and their associations with epileptic phenotypes.

Differential Coassembly of α1-GABAARs Associated with Epileptic Encephalopathy

GABAA receptors (GABAARs) are profoundly important for controlling neuronal excitability. Spontaneous and familial mutations to these receptors feature prominently in excitability disorders and neurodevelopmental deficits following disruption to GABA-mediated inhibition. Recent genotyping of an individual with severe epilepsy and Williams-Beuren syndrome identified a frameshifting de novo variant in a major GABAAR gene, GABRA1 This truncated the α1 subunit between the third and fourth transmembrane domains and introduced 24 new residues forming the mature protein, α1Lys374Serfs*25 Cell surface expression of mutant murine GABAARs is severely impaired compared with WT, due to retention in the endoplasmic reticulum. Mutant receptors were differentially coexpressed with β3, but not with β2, subunits in mammalian cells. Reduced surface expression was reflected by smaller IPSCs, which may underlie the induction of seizures. The mutant does not have a dominant-negative effect on native neuronal GABAAR expression since GABA current density was unaffected in hippocampal neurons, although mutant receptors exhibited limited GABA sensitivity. To date, the underlying mechanism is unique for epileptogenic variants and involves differential β subunit expression of GABAAR populations, which profoundly affected receptor function and synaptic inhibition.SIGNIFICANCE STATEMENT GABAARs are critical for controlling neural network excitability. They are ubiquitously distributed throughout the brain, and their dysfunction underlies many neurologic disorders, especially epilepsy. Here we report the characterization of an α1-GABAAR variant that results in severe epilepsy. The underlying mechanism is structurally unusual, with the loss of part of the α1 subunit transmembrane domain and part-replacement with nonsense residues. This led to compromised and differential α1 subunit cell surface expression with β subunits resulting in severely reduced synaptic inhibition. Our study reveals that disease-inducing variants can affect GABAAR structure, and consequently subunit assembly and cell surface expression, critically impacting on the efficacy of synaptic inhibition, a property that will orchestrate the extent and duration of neuronal excitability.

Clinical Characterization of 2 Siblings with a Homozygous SPAST Variant

Case series

Patients: —

Final Diagnosis: Hereditary spastic paraplegia type 4

Symptoms: Progressive psychomotor deterioration • mixed seizure patterns • corneal opacity • dysostotic bones • limb spasticity with extensor plantar responses • axial hypotonia

Medication: —

Clinical Procedure: Phenotype-genotype correlation

Specialty: Genetics • Neurology

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.

Neurobiology and Therapeutic Potential of α5-GABA Type A Receptors

α5 subunit containing GABA type A receptors (GABA_ARs) have long been an enigmatic receptor subtype of interest due to their specific brain distribution, unusual surface localization and key role in synaptic plasticity, cognition and memory. These receptors are uniquely positioned to sculpt both the developing and mature hippocampal circuitry due to high overall expression and a distinct peak within the critical synapse formation period during the second postnatal week. Unlike the majority of other GABA_ARs, they exhibit both receptor clustering at extrasynaptic sites via interactions with the radixin scaffold as well as synaptic sites via gephyrin, thus contributing respectively to tonic currents and synaptic GABAergic neurotransmission. α5 GABA_AR signaling can be altered in neurodevelopmental disorders including autism and mental retardation and by inflammation in CNS injury and disease. Due to the unique physiology and pharmacology of α5 GABA_ARs, drugs targeting these receptors are being developed and tested as treatments for neurodevelopmental disorders, depression, schizophrenia, and mild cognitive impairment. This review article focuses on advances in understanding how the α5 subunit contributes to GABA_AR neurobiology. In particular, I discuss both recent insights and remaining knowledge gaps for the functional role of these receptors, pathologies associated with α5 GABA_AR dysfunction, and the effects and potential therapeutic uses of α5 receptor subtype targeted drugs.

Altered inhibitory synapses in de novo GABRA5 and GABRA1 mutations associated with early onset epileptic encephalopathies

We performed next generation sequencing on 1696 patients with epilepsy and intellectual disability using a gene panel with 480 epilepsy-related genes including all GABAA receptor subunit genes (GABRs), and we identified six de novo GABR mutations, two novel GABRA5 mutations (c.880G>T, p.V294F and c.1238C>T, p.S413F), two novel GABRA1 mutations (c.778C>T, p.P260S and c.887T>C, p.L296S/c.944G>T, p.W315L) and two known GABRA1 mutations (c.335G>A, p.R112Q and c.343A>G, p.N115D) in six patients with intractable early onset epileptic encephalopathy. The α5(V294F and S413F) and α1(P260S and L296S/W315L) subunit residue substitutions were all in transmembrane domains, while the α1(R112Q and N115R) subunit residue substitutions were in the N-terminal GABA binding domain. Using multidisciplinary approaches, we compared effects of mutant GABAA receptor α5 and α1 subunits on the properties of recombinant α5β3γ2 and α1β3γ2 GABAA receptors in both neuronal and non-neuronal cells and characterized their effects on receptor clustering, biogenesis and channel function. GABAA receptors containing mutant α5 and α1 subunits all had reduced cell surface and total cell expression with altered endoplasmic reticulum processing, impaired synaptic clustering, reduced GABAA receptor function and decreased GABA binding potency. Our study identified GABRA5 as a causative gene for early onset epileptic encephalopathy and expands the mutant GABRA1 phenotypic spectrum, supporting growing evidence that defects in GABAergic neurotransmission contribute to early onset epileptic encephalopathy phenotypes.

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.

De novo variants in GABRA2 and GABRA5 alter receptor function and contribute to early-onset epilepsy

GABAA receptors are ligand-gated anion channels that are important regulators of neuronal inhibition. Mutations in several genes encoding receptor subunits have been identified in patients with various types of epilepsy, ranging from mild febrile seizures to severe epileptic encephalopathy. Using whole-genome sequencing, we identified a novel de novo missense variant in GABRA5 (c.880G > C, p.V294L) in a patient with severe early-onset epilepsy and developmental delay. Targeted resequencing of 279 additional epilepsy patients identified 19 rare variants from nine GABAA receptor genes, including a novel de novo missense variant in GABRA2 (c.875C > A, p.T292K) and a recurrent missense variant in GABRB3 (c.902C > T, p.P301L). Patients with the GABRA2 and GABRB3 variants also presented with severe epilepsy and developmental delay. We evaluated the effects of the GABRA5, GABRA2 and GABRB3 missense variants on receptor function using whole-cell patch-clamp recordings from human embryonic kidney 293T cells expressing appropriate α, β and γ subunits. The GABRA5 p.V294L variant produced receptors that were 10-times more sensitive to GABA but had reduced maximal GABA-evoked current due to increased receptor desensitization. The GABRA2 p.T292K variant reduced channel expression and produced mutant channels that were tonically open, even in the absence of GABA. Receptors containing the GABRB3 p.P301L variant were less sensitive to GABA and produced less GABA-evoked current. These results provide the first functional evidence that de novo variants in the GABRA5 and GABRA2 genes contribute to early-onset epilepsy and developmental delay, and demonstrate that epilepsy can result from reduced neuronal inhibition via a wide range of alterations in GABAA receptor function.

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.