GABAA receptor β3 subunit mutation D120N causes Lennox-Gastaut syndrome in knock-in mice

Cannabinoid-like compounds found in non-cannabis plants exhibit antiseizure activity in genetic mouse models of drug-resistant epilepsy

OBJECTIVE

The cannabinoid cannabidiol has established antiseizure effects in drug-resistant epilepsies such as Dravet syndrome and Lennox-Gastaut syndrome. Amorfrutin 2, honokiol, and magnolol are structurally similar to cannabinoids (cannabis-like drugs) but derive from non-cannabis plants. We aimed to study the antiseizure potential of these compounds in various mouse seizure models. In addition, we aimed to characterize their molecular pharmacology at cannabinoid CB1 and CB2 receptors and at T-type calcium channels, which are known targets of the cannabinoids.

METHODS

Brain and plasma pharmacokinetic profiles were determined. Antiseizure activity was assessed against hyperthermia-induced seizures in a Scn1a+/- mouse model of Dravet syndrome. We then elaborated on the most promising compounds in the maximal electroshock (MES) test in mice and the Gabrb3+/D120N mouse model of Lennox-Gastaut syndrome. Fluorescence-based assays were used to examine modulatory activity at cannabinoid CB1 and CB2 receptors and T-type calcium channel subtypes CaV3.1, CaV3.2, and CaV3.3 overexpressed in mammalian cells. Automated patch-clamp electrophysiology was then used to confirm inhibitory activity on CaV3.1, CaV3.2, and CaV3.3 channels.

RESULTS

Magnolol and honokiol had high brain-to-plasma ratios (3.55 and 7.56, respectively), unlike amorfrutin 2 (0.06). Amorfrutin 2 and magnolol but not honokiol significantly increased the body temperature threshold at which Scn1a+/- mice had a generalized tonic-clonic seizure. Both amorfrutin 2 and magnolol significantly decreased the proportion of mice exhibiting hindlimb extension in the MES test. Furthermore, magnolol reduced the number and duration of atypical absence seizures in Gabrb3+/D120N mice. The three compounds inhibited all T-type calcium channel subtypes but were without specific activity at cannabinoid receptors.

SIGNIFICANCE

We show for the first time that amorfrutin 2 and magnolol display novel antiseizure activity in mouse drug-resistant epilepsy models. Our results justify future drug discovery campaigns around these structural scaffolds that aim to develop novel antiseizure drugs for intractable epilepsies.

Unraveling the genetic basis of epilepsy: Recent advances and implications for diagnosis and treatment

Epilepsy, affecting approximately 1% of the global population, manifests as recurring seizures and is heavily influenced by genetic factors. Recent advancements in genetic technologies have revolutionized our understanding of epilepsy's genetic landscape. Key studies, such as the discovery of mutations in ion channels (e.g., SCN1A and SCN2A), neurotransmitter receptors (e.g., GABRA1), and synaptic proteins (e.g., SYNGAP1, KCNQ2), have illuminated critical pathways underlying epilepsy susceptibility and pathogenesis. Genome-wide association studies (GWAS) have identified specific genetic variations linked to epilepsy risk, such as variants near SCN1A and PCDH7, enhancing diagnostic accuracy and enabling personalized treatment strategies. Moreover, epigenetic mechanisms, including DNA methylation (e.g., MBD5), histone modifications (e.g., HDACs), and non-coding RNAs (e.g., miR-134), play pivotal roles in altering gene expression and synaptic plasticity, contributing to epileptogenesis. These discoveries offer promising avenues for therapeutic interventions aimed at improving outcomes for epilepsy patients. Genetic testing has become essential in clinical practice, facilitating precise diagnosis and tailored management approaches based on individual genetic profiles. Furthermore, insights into epigenetic regulation suggest novel therapeutic targets for developing more effective epilepsy treatments. In summary, this review highlights significant progress in understanding the genetic and epigenetic foundations of epilepsy. By integrating findings from key studies and specifying genes involved in epigenetic modifications, we underscore the potential for advanced therapeutic strategies in this complex neurological disorder, emphasizing the importance of personalized medicine approaches in epilepsy management.

Clinical trials for Lennox-Gastaut syndrome: Challenges and priorities

OBJECTIVE

Lennox-Gastaut syndrome (LGS) is a severe, childhood-onset epilepsy that is typically refractory to treatment. We surveyed the current landscape of LGS treatment, aiming to identify challenges to the development of efficacious therapies, and to articulate corresponding priorities toward clinical trials that improve outcomes.

METHODS

The LGS Special Interest Group of the Pediatric Epilepsy Research Consortium integrated evidence from the literature and expert opinion, into a narrative review.

RESULTS

We provide an overview of approved and emerging medical, dietary, surgical and neuromodulation approaches for LGS. We note that quality of care could be improved by standardizing LGS treatment based on expert consensus and empirical data. Whereas LGS natural history is incompletely understood, prospective studies and use of large retrospective datasets to understand LGS across the lifespan would enable clinical trials that address these dynamics. Recent discoveries related to LGS pathophysiology should enable development of disease-modifying therapies, which are currently lacking. Finally, clinical trials have focused chiefly on seizures involving "drops," but should incorporate additional patient-centered outcomes, using emerging measures adapted to people with LGS.

INTERPRETATION

Clinicians and researchers should enact these priorities, with the goal of patient-centered clinical trials that are tailored to LGS pathophysiology and natural history.

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

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

Targeting thalamocortical circuits for closed-loop stimulation in Lennox-Gastaut syndrome

This paper outlines the therapeutic rationale and neurosurgical targeting technique for bilateral, closed-loop, thalamocortical stimulation in Lennox-Gastaut syndrome, a severe form of childhood-onset epilepsy. Thalamic stimulation can be an effective treatment for Lennox-Gastaut syndrome, but complete seizure control is rarely achieved. Outcomes may be improved by stimulating areas beyond the thalamus, including cortex, but the optimal targets are unknown. We aimed to identify a cortical target by synthesizing prior neuroimaging studies, and to use this knowledge to advance a dual thalamic (centromedian) and cortical (frontal) approach for closed-loop stimulation. Multi-modal brain network maps from three group-level studies of Lennox-Gastaut syndrome were averaged to define the area of peak overlap: simultaneous EEG-functional MRI of generalized paroxysmal fast activity, [18F]fluorodeoxyglucose PET of cortical hypometabolism and diffusion MRI structural connectivity associated with clinical efficacy in a previous trial of thalamic deep brain stimulation. The resulting 'hotspot' was used as a seed in a normative functional MRI connectivity analysis to identify connected networks. Intracranial electrophysiology was reviewed in the first two trial patients undergoing bilateral implantations guided by this hotspot. Simultaneous recordings from cortex and thalamus were analysed for presence and synchrony of epileptiform activity. The peak overlap was in bilateral premotor cortex/caudal middle frontal gyrus. Functional connectivity of this hotspot revealed a distributed network of frontoparietal cortex resembling the diffuse abnormalities seen on EEG-functional MRI and PET. Intracranial electrophysiology showed characteristic epileptiform activity of Lennox-Gastaut syndrome in both the cortical hotspot and thalamus; most detected events occurred first in the cortex before appearing in the thalamus. Premotor frontal cortex shows peak involvement in Lennox-Gastaut syndrome and functional connectivity of this region resembles the wider epileptic brain network. Thus, it may be an optimal target for a range of neuromodulation therapies, including thalamocortical stimulation and emerging non-invasive treatments like focused ultrasound or transcranial magnetic stimulation. Compared to thalamus-only approaches, the addition of this cortical target may allow more rapid detections of seizures, more diverse stimulation paradigms and broader modulation of the epileptic network. A prospective, multi-centre trial of closed-loop thalamocortical stimulation for Lennox-Gastaut syndrome is currently underway.

Pharmacokinetics, tissue distribution, and plasma protein binding ratio of bicuculline following intragastric and intravenous administration in rats using ultra-high-performance liquid chromatography-tandem mass spectrometry

Bicuculline is a natural isoquinoline alkaloid that works as a gamma-aminobutyric acid receptor antagonist. It is widely found in Papaveraceae plants used in traditional Chinese medicines. Bicuculline not only has been shown to have favorable analgesic, memory-improving, and anxiolytic effects but may also cause adverse effects such as convulsions and epilepsy. A simple, rapid, and sensitive method was developed and validated for the determination of bicuculline in the plasma and tissue samples in rats by ultra-high-performance liquid chromatography-tandem mass spectrometry (MS/MS). The chromatographic separation was performed on a Thermo Scientific C18 column. The MS/MS system was operated in the positive multiple reaction monitoring mode, and the precursor-product ion transitions were optimized as m/z 368.0 → 307.1 for bicuculline and as 354.1 → 188.1 for protopine (internal standard). The linearity, accuracy, precision, recovery, and matrix effect were within acceptable limits. The experimental data showed that bicuculline was rapidly absorbed and eliminated in rats, with a moderate plasma protein binding ratio and low bioavailability. The main tissues of distribution were the kidney, liver, and brain; bicuculline could exert its pharmacological effects across the blood-brain barrier. This study has positive implications for the clinical use of herbal medicines containing bicuculline and for further development.

GABAA Receptor β3 Subunit Mutation N328D Heterozygous Knock-in Mice Have Lennox-Gastaut Syndrome

Lennox-Gastaut Syndrome (LGS) is a developmental and epileptic encephalopathy (DEE) characterized by multiple seizure types, electroencephalogram (EEG) patterns, and cognitive decline. Its etiology has a prominent genetic component, including variants in GABRB3 that encodes the GABAA receptor (GABAAR) β3 subunit. LGS has an unknown pathophysiology, and few animal models are available for studying LGS. The objective of this study was to evaluate Gabrb3+/N328D knock-in mice as a model for LGS. We generated a heterozygous knock-in mouse expressing Gabrb3 (c.A982G, p.N238D), a de novo mutation identified in a patient with LGS. We investigated Gabrb3+/N328D mice for features of LGS. In 2-4-month-old male and female C57BL/J6 wild-type and Gabrb3+/N328D mice, we investigated seizure severity using video-monitored EEG, cognitive impairment using a suite of behavioral tests, and profiled GABAAR subunit expression by Western blot. Gabrb3+/N328D mice showed spontaneous seizures and signs of cognitive impairment, including deficits in spatial learning, memory, and locomotion. Moreover, Gabrb3+/N328D mice showed reduced β3 subunit expression in the cerebellum, hippocampus, and thalamus. This phenotype of epilepsy and neurological impairment resembles the LGS patient phenotype. We conclude that Gabrb3+/N328D mice provide a good model for investigating the pathophysiology and therapeutic intervention of LGS and DEEs.

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.

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

Mutations in GABAA 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.

Pharmacological activation of ATF6 remodels the proteostasis network to rescue pathogenic GABAA receptors

BACKGROUND

Genetic variants in the subunits of the gamma-aminobutyric acid type A (GABAA) receptors are implicated in the onset of multiple pathologic conditions including genetic epilepsy. Previous work showed that pathogenic GABAA subunits promote misfolding and inefficient assembly of the GABAA receptors, limiting receptor expression and activity at the plasma membrane. However, GABAA receptors containing variant subunits can retain activity, indicating that enhancing the folding, assembly, and trafficking of these variant receptors offers a potential opportunity to mitigate pathology associated with genetic epilepsy.

RESULTS

Here, we demonstrate that pharmacologically enhancing endoplasmic reticulum (ER) proteostasis using small molecule activators of the ATF6 (Activating Transcription Factor 6) signaling arm of the unfolded protein response (UPR) increases the assembly, trafficking, and surface expression of variant GABAA receptors. These improvements are attributed to ATF6-dependent remodeling of the ER proteostasis environment, which increases protein levels of pro-folding ER proteostasis factors including the ER chaperone BiP (Immunoglobulin Binding Protein) and trafficking receptors, such as LMAN1 (Lectin Mannose-Binding 1) and enhances their interactions with GABAA receptors. Importantly, we further show that pharmacologic ATF6 activators increase the activity of GABAA receptors at the cell surface, revealing the potential for this strategy to restore receptor activity to levels that could mitigate disease pathogenesis.

CONCLUSIONS

These results indicate that pharmacologic ATF6 activators offer an opportunity to restore GABAA receptor activity in diseases including genetic epilepsy and point to the potential for similar pharmacologic enhancement of ER proteostasis to improve trafficking of other disease-associated variant ion channels implicated in etiologically-diverse diseases.

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.

Current Pharmacologic Strategies for Treatment of Intractable Epilepsy in Children

Epileptic encephalopathy (EE) is a devastating pediatric disease that features medically resistant seizures, which can contribute to global developmental delays. Despite technological advancements in genetics, the neurobiological mechanisms of EEs are not fully understood, leaving few therapeutic options for affected patients. In this review, we introduce the most common EEs in pediatrics (i.e., Ohtahara syndrome, Dravet syndrome, and Lennox-Gastaut syndrome) and their molecular mechanisms that cause excitation/inhibition imbalances. We then discuss some of the essential molecules that are frequently dysregulated in EEs. Specifically, we explore voltage-gated ion channels, synaptic transmission-related proteins, and ligand-gated ion channels in association with the pathophysiology of Ohtahara syndrome, Dravet syndrome, and Lennox-Gastaut syndrome. Finally, we review currently available antiepileptic drugs used to treat seizures in patients with EEs. Since these patients often fail to achieve seizure relief even with the combination therapy, further extensive research efforts to explore the involved molecular mechanisms will be required to develop new drugs for patients with intractable epilepsy.

Modelling epilepsy in the mouse: challenges and solutions

In most mouse models of disease, the outward manifestation of a disorder can be measured easily, can be assessed with a trivial test such as hind limb clasping, or can even be observed simply by comparing the gross morphological characteristics of mutant and wild-type littermates. But what if we are trying to model a disorder with a phenotype that appears only sporadically and briefly, like epileptic seizures? The purpose of this Review is to highlight the challenges of modelling epilepsy, in which the most obvious manifestation of the disorder, seizures, occurs only intermittently, possibly very rarely and often at times when the mice are not under direct observation. Over time, researchers have developed a number of ways in which to overcome these challenges, each with their own advantages and disadvantages. In this Review, we describe the genetics of epilepsy and the ways in which genetically altered mouse models have been used. We also discuss the use of induced models in which seizures are brought about by artificial stimulation to the brain of wild-type animals, and conclude with the ways these different approaches could be used to develop a wider range of anti-seizure medications that could benefit larger patient populations.

Summary: This Review discusses the challenges of modelling epilepsy in mice, a condition in which the outward manifestation of the disorder appears only sporadically, and reviews possible solutions encompassing both genetic and induced models.

Overlaps, gaps, and complexities of mouse models of Developmental and Epileptic Encephalopathy

Mouse models have made innumerable contributions to understanding the genetic basis of neurological disease and pathogenic mechanisms and to therapy development. Here we consider the current state of mouse genetic models of Developmental and Epileptic Encephalopathy (DEE), representing a set of rare but devastating and largely intractable childhood epilepsies. By examining the range of mouse lines available in this rapidly moving field and by detailing both expected and unusual features in representative examples, we highlight lessons learned in an effort to maximize the full potential of this powerful resource for preclinical studies.