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. 

/pdf/gain-of-function-and-loss-of-function-gabrb3-variants-lead-1lzez6cs.pdf

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

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

/pdf/molecular-and-clinical-descriptions-of-patients-with-gabaa-18ryrg6c.pdf

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

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

/pdf/gabrg2-variants-associated-with-febrile-seizures-2icnlwbo.pdf

GABRG2 Variants Associated with Febrile Seizures

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.

No Gain, Less Pain: GABRB3 Mutations in Epileptic Encephalopathy

Objective: 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. In the present 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 3D structure of the GABRB3 subunit and clinical phenotypes associated with the different key structural domains were investigated.
Results: We characterize 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 10.5 months, and mild-to-moderate intellectual disability were associated with variants in the extracellular domain. Focal epilepsy with early onset (median: 2.75 months of age) and severe intellectual disability were associated with variants in the pore-lining helical transmembrane domain.
Significance: 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.

/pdf/structural-mapping-of-gabrb3-variants-reveals-genotype-1r437mxovj.pdf

Braakman Hmh

Papers

Structural mapping of GABRB3 variants reveals genotype-phenotype correlation