A GABR epileptic encephalopathy potpourri

Møller RS, Wuttke TV, Helbig I, Marini C, Johannesen KM, Brilstra EH, Vaher U, Borggraefe I, Talvik I, Talvik T, Kluger G, Francois LL, Lesca G, de Bellescize J, Blichfeldt S, Chatron N, Holert N, Jacobs J, Swinkels M, Betzler C, Syrbe S, Nikanorova M, Myers CT, Larsen LH, Vejzovic S, Pendziwiat M, von Spiczak S, Hopkins S, Dubbs H, Mang Y, Mukhin K, Holthausen H, van Gassen KL, Dahl HA, Tommerup N, Mefford HC, Rubboli G, Guerrini R, Lemke JR, Lerche H, Muhle H,

Maljevic S. Mutations in GABRB3: From febrile seizures to epileptic encephalopathies. Neurology. 2017 Jan 4. pii: 10.1212/WNL.0000000000003565. doi:10.1212/WNL.0000000000003565. [Epub ahead of print]

Abstract

OBJECTIVE:

To examine the role of mutations in GABRB3 encoding the β3 subunit of the GABAA receptor in individual patients with epilepsy with regard to causality, the spectrum of genetic variants, their pathophysiology, and associated phenotypes.

METHODS:

We performed massive parallel sequencing of GABRB3 in 416 patients with a range of epileptic encephalopathies and childhood-onset epilepsies and recruited additional patients with epilepsy with GABRB3 mutations from other research and diagnostic programs.

RESULTS:

We identified 22 patients with heterozygous mutations in GABRB3, including 3 probands from multiplex families. The phenotypic spectrum of the mutation carriers ranged from simple febrile seizures, genetic epilepsies with febrile seizures plus, and epilepsy with myoclonic-atonic seizures to West syndrome and other types of severe, early-onset epileptic encephalopathies. Electrophysiologic analysis of 7 mutations in Xenopus laevis oocytes, using coexpression of wild-type or mutant β3, together with α5 and γ2s subunits and an automated 2-microelectrode voltage-clamp system, revealed reduced GABA-induced current amplitudes or GABA sensitivity for 5 of 7 mutations.

CONCLUSIONS:

Our results indicate that GABRB3 mutations are associated with a broad phenotypic spectrum of epilepsies and that reduced receptor function causing GABAergic disinhibition represents the relevant disease mechanism.

Courtesy of:  https://www.mdlinx.com/neurology/medical-news-article/2017/01/09/mutations-gabrb3/6999171/?category=latest&page_id=7

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Shen D, Hernandez CC, Shen W, Hu N, Poduri A, Shiedley B, Rotenberg A, Datta AN, Leiz S, Patzer S, Boor R, Ramsey K, Goldberg E, Helbig I, Ortiz-Gonzalez XR, Lemke JR, Marsh ED, Macdonald RL. De novo GABRG2 mutations associated with epileptic encephalopathies. Brain. 2017 Jan;140(Pt 1):49-67.

Abstract

Epileptic encephalopathies are a devastating group of severe childhood onset epilepsies with medication-resistant seizures and poor developmental outcomes. Many epileptic encephalopathies have a genetic aetiology and are often associated with de novo mutations in genes mediating synaptic transmission, including GABAA receptor subunit genes. Recently, we performed next generation sequencing on patients with a spectrum of epileptic encephalopathy phenotypes, and we identified five novel (A106T, I107T, P282S, R323W and F343L) and one known (R323Q) de novo GABRG2 pathogenic variants (mutations) in eight patients. To gain insight into the molecular basis for how these mutations contribute to epileptic encephalopathies, we compared the effects of the mutations on the properties of recombinant α1β2γ2L GABAA receptors transiently expressed in HEK293T cells. Using a combination of patch clamp recording, immunoblotting, confocal imaging and structural modelling, we characterized the effects of these GABRG2 mutations on GABAA receptor biogenesis and channel function. Compared with wild-type α1β2γ2L receptors, GABAA receptors containing a mutant γ2 subunit had reduced cell surface expression with altered subunit stoichiometry or decreased GABA-evoked whole-cell current amplitudes, but with different levels of reduction. While a causal role of these mutations cannot be established directly from these results, the functional analysis together with the genetic information suggests that these GABRG2 variants may be major contributors to the epileptic encephalopathy phenotypes. Our study further expands the GABRG2 phenotypic spectrum and supports growing evidence that defects in GABAergic neurotransmission participate in the pathogenesis of genetic epilepsies including epileptic encephalopathies.