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

Human C2orf69 is an evolutionarily conserved gene whose function is unknown. Here, we report eight unrelated families from which 20 children presented with a fatal syndrome consisting of severe autoinflammation and progredient leukoencephalopathy with recurrent seizures; 12 of these subjects, whose DNA was available, segregated homozygous loss-of-function C2orf69 variants. C2ORF69 bears homology to esterase enzymes, and orthologs can be found in most eukaryotic genomes, including that of unicellular phytoplankton. We found that endogenous C2ORF69 (1) is loosely bound to mitochondria, (2) affects mitochondrial membrane potential and oxidative respiration in cultured neurons, and (3) controls the levels of the glycogen branching enzyme 1 (GBE1) consistent with a glycogen-storage-associated mitochondriopathy. We show that CRISPR-Cas9-mediated inactivation of zebrafish C2orf69 results in lethality by 8 months of age due to spontaneous epileptic seizures, which is preceded by persistent brain inflammation. Collectively, our results delineate an autoinflammatory Mendelian disorder of C2orf69 deficiency that disrupts the development/homeostasis of the immune and central nervous systems.

Loss of C2orf69 defines a fatal autoinflammatory syndrome in humans and zebrafish that evokes a glycogen storage-associated mitochondriopathy

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

Structural mapping of GABRB3 variants reveals genotype-phenotype correlation

BackgroundDeciphering the function of the many genes previously classified as uncharacterized "open reading frame" (orf) completes our understanding of cell function and its pathophysiology. MethodsWhole-exome sequencing, yeast 2-hybrid and transcriptome analyses together with molecular characterization are used here to uncover the function of the C2orf69 gene. ResultsWe identify loss-of-function mutations in the uncharacterized C2orf69 gene in eight individuals with brain abnormalities involving hypomyelination and microcephaly, liver dysfunction and recurrent autoinflammation. C2orf69 contains an N-terminal signal peptide that is required and sufficient for mitochondrial localization. Consistent with mitochondrial dysfunction, patients show signs of respiratory chain defect and a CRISPR-Cas9 knockout cell model of C2orf69 shows comparable respiratory chain defects. Patient-derived cells reveal alterations in immunological signaling pathways. Deposits of PAS-positive material in tissues from affected individuals together with decreased glycogen branching enzyme 1 (GBE1) activity indicate an additional impact of C2orf69 on glycogen metabolism. ConclusionOur study identifies C2orf69 as an important regulator of human mitochondrial function and suggests an additional influence on other metabolic pathways. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=178 SRC="FIGDIR/small/21252805v1_ufig1.gif" ALT="Figure 1"> View larger version (31K): org.highwire.dtl.DTLVardef@ee7042org.highwire.dtl.DTLVardef@c1e5borg.highwire.dtl.DTLVardef@19d1cb4org.highwire.dtl.DTLVardef@1c780b6_HPS_FORMAT_FIGEXP M_FIG C_FIG

https://www.medrxiv.org/content/medrxiv/early/2021/03/12/2021.03.08.21252805.full.pdf

M. Haeusler

Papers

Structural mapping of GABRB3 variants reveals genotype-phenotype correlation

A human multisystem disorder with autoinflammation, leukoencephalopathy and hepatopathy is caused by mutations in C2orf69