Encephalopathy-causing mutations in Gβ1 (GNB1) alter regulation of neuronal GIRK channels

GNB1 and obesity: Evidence for a correlation between haploinsufficiency and syndromic obesity

Most patients with GNB1 encephalopathy have developmental delay and/or intellectual disability, brain anomalies and seizures. Recently, two cases with GNB1 encephalopathy caused by haploinsufficiency have been reported that also show a Prader-Willi-like phenotype of childhood hypotonia and severe obesity. Here we present three new cases from our expert centre for genetic obesity in which GNB1 truncating and splice variants, probably leading to haploinsufficiency, were identified. They all have obesity, hyperphagia and intellectual deficit. The clinical cases and their weight courses are presented, together with a review of all 68 published cases with GNB1 encephalopathy. Information on weight was not mentioned in most of these articles, so we contacted authors for additional clinical information on weight status and hyperphagia. Of the 42 patients whose weight status we could determine, obesity was present in 8 patients (19%). Obesity is significantly over-represented in the group with truncating and splicing variants. In this group, we see an obesity prevalence of 75%. Since GNB1 has been linked to several key genes in the hypothalamic leptin-melanocortin pathway, which regulates satiety and energy expenditure, our data support the potential association between GNB1 haploinsufficiency and genetic obesity. We also suggest GNB1 is a candidate gene for the known obesity phenotype of the 1p36 microdeletion syndrome given this chromosomal region includes the GNB1 gene. Knowledge of an additional obesity phenotype is important for prognosis, early interventions against obesity and awareness when prescribing weight-inducing medication.

KCTD Proteins Have Redundant Functions in Controlling Cellular Growth

We explored the functional redundancy of three structurally related KCTD (Potassium Channel Tetramerization Domain) proteins, KCTD2, KCTD5, and KCTD17, by progressively knocking them out in HEK 293 cells using CRISPR/Cas9 genome editing. After validating the knockout, we assessed the effects of progressive knockout on cell growth and gene expression. We noted that the progressive effects of knockout of KCTD isoforms on cell growth were most pervasive when all three isoforms were deleted, suggesting some functions were conserved between them. This was also reflected in progressive changes in gene expression. Our previous work indicated that Gβ1 was involved in the transcriptional control of gene expression, so we compared the gene expression patterns between GNB1 and KCTD KO. Knockout of GNB1 led to numerous changes in the expression levels of other G protein subunit genes, while knockout of KCTD isoforms had the opposite effect, presumably because of their role in regulating levels of Gβ1. Our work demonstrates a unique relationship between KCTD proteins and Gβ1 and a global role for this subfamily of KCTD proteins in maintaining the ability of cells to survive and proliferate.

GNB1 Encephalopathy: Clinical Case Report and Literature Review

GNB1 encephalopathy is a rare genetic disease caused by pathogenic variants in the G Protein Subunit Beta 1 (GNB1) gene, with only around 68 cases documented worldwide. Although most cases had been caused by de novo germline mutations, in this case, the pathogenic variant was inherited from patient's mother, indicating an autosomal dominant inheritance pattern. The patient presented at 25 years of age with mild developmental delay and cognitive impairment, prominent generalized dystonia, and horizontal nystagmus which are all characterizing symptoms of GNB1 encephalopathy. Electroencephalography (EEG) showed no epileptiform patterns, and magnetic resonance imaging (MRI) revealed hypointensities in globus pallidus and dentate nucleus areas. The main theory for GNB1 encephalopathy pathogenesis is neuronal hyperexcitability caused by impaired ion channel regulation. Due to low specificity of symptoms, diagnosis relies on genetic testing. As there are no standardized GNB1 encephalopathy treatment guidelines, evaluation of different treatment options is based on anecdotal cases. Reviewing different treatment options, deep brain stimulation and intrathecal baclofen pump, as well as some other medications still in preclinical trials, seem to be the most promising.

Neurons Are GIRKed in GNB1 Encephalopathy: Unraveling Pathogenic Mechanisms in a Complex Neurodevelopmental Disorder

Epilepsy in a Mouse Model of GNB1 Encephalopathy Arises From Altered Potassium (GIRK) Channel Signaling and Is Alleviated by a GIRK Inhibitor Colombo A, Reddy HP, Petri S, Williams DJ, Shalomov B, Dhindsa RS, Gelfman S, Krizay D, Bera AK, Yang M, Peng Y, Makinson CD, Boland MJ, Frankel WN, Goldstein DB, Dascal N. Front Cell Neurosci. 2023;17:1175895. doi:10.3389/fncel.2023.1175895 De novo mutations in GNB1 , encoding the Gβ1 subunit of G proteins, cause a neurodevelopmental disorder with global developmental delay and epilepsy, GNB1 encephalopathy. Here, we show that mice carrying a pathogenic mutation, K78R, recapitulate aspects of the disorder, including developmental delay and generalized seizures. Cultured mutant cortical neurons also display aberrant bursting activity on multi-electrode arrays. Strikingly, the antiepileptic drug ethosuximide (ETX) restores normal neuronal network behavior in vitro and suppresses spike-and-wave discharges (SWD) in vivo . ETX is a known blocker of T-type voltage-gated Ca2+ channels and G protein-coupled potassium (GIRK) channels. Accordingly, we present evidence that K78R results in a gain-of-function (GoF) effect by increasing the activation of GIRK channels in cultured neurons and a heterologous model (Xenopus oocytes)—an effect we show can be potently inhibited by ETX. This work implicates a GoF mechanism for GIRK channels in epilepsy, identifies a new mechanism of action for ETX in preventing seizures, and establishes this mouse model as a pre-clinical tool for translational research with predicative value for GNB1 encephalopathy.

Case report: Suspecting guanine nucleotide-binding protein beta 1 mutation in dyskinetic cerebral palsy is important

Herein, we describe the case of a 43-month-old girl who presented with clinical manifestations of dyskinetic cerebral palsy (CP), classified as the Gross Motor Function Classification System (GMFCS) V. The patient had no family history of neurological or perinatal disorders. Despite early rehabilitation, serial assessments using the Gross Motor Function Measure (GMFM) showed no significant improvements in gross motor function. Brain magnetic resonance imaging showed nonspecific findings that could not account for developmental delay or dystonia. Whole-genome sequencing identified a heterozygous NM_002074.5(GNB1):c.239T>C (p.Ile80Thr) mutation in guanine nucleotide-binding protein beta 1 (GNB1) gene. Considering this case and previous studies, genetic testing for the etiology of dyskinetic CP is recommended for children without relevant or with nonspecific brain lesions.

Atypical clinical course in two patients with GNB1 variants who developed acute encephalopathy

INTRODUCTION

Variants in the GNB1 gene, which encodes the β1 subunit of a trimeric G protein, can cause moderate to severe psychomotor retardation. Acute encephalopathies have also been observed in patients with central nervous system abnormalities; however, severe neurological sequelae have not previously been reported.

CASE PRESENTATIONS

Patient 1 was a Japanese female with a de novo GNB1 variant (c.284 T > C). At 8 months old she contracted influenza A and developed generalized convulsions. In the acute phase, brain magnetic resonance imaging (MRI) findings indicated acute encephalopathy; diffuse cerebral atrophy was present 1 month later. Although multidisciplinary treatment was administered, she had severe neurological sequelae including spastic tetraplegia, severe intellectual disabilities, and refractory epilepsy. Patient 2 was a Japanese male with a de novo GNB1 variant (c.239 T > C). He experienced an unexplained respiratory arrest aged 17 years; refractory convulsions developed. Brain MRI at 1 month showed bilateral basal ganglia high intensities; at 3 months, diffuse cerebral cortex and white matter atrophy was observed. Despite multidisciplinary treatment, he developed severe spastic tetraplegia and mental regression.

DISCUSSION

We report two patients with GNB1 variants who had acute lesions on brain MRI and unexpected disease courses. In such patients with acute neurological deterioration, multidisciplinary treatment is required; patients should also be carefully observed for progression to acute encephalopathy.

Epilepsy in a mouse model of GNB1 encephalopathy arises from altered potassium (GIRK) channel signaling and is alleviated by a GIRK inhibitor

De novo mutations in GNB1, encoding the Gβ1 subunit of G proteins, cause a neurodevelopmental disorder with global developmental delay and epilepsy, GNB1 encephalopathy. Here, we show that mice carrying a pathogenic mutation, K78R, recapitulate aspects of the disorder, including developmental delay and generalized seizures. Cultured mutant cortical neurons also display aberrant bursting activity on multi-electrode arrays. Strikingly, the antiepileptic drug ethosuximide (ETX) restores normal neuronal network behavior in vitro and suppresses spike-and-wave discharges (SWD) in vivo. ETX is a known blocker of T-type voltage-gated Ca2+ channels and G protein-coupled potassium (GIRK) channels. Accordingly, we present evidence that K78R results in a gain-of-function (GoF) effect by increasing the activation of GIRK channels in cultured neurons and a heterologous model (Xenopus oocytes)-an effect we show can be potently inhibited by ETX. This work implicates a GoF mechanism for GIRK channels in epilepsy, identifies a new mechanism of action for ETX in preventing seizures, and establishes this mouse model as a pre-clinical tool for translational research with predicative value for GNB1 encephalopathy.