Phenotype-genotype correlations in patients with GNB1 gene variants, including the first three reported Japanese patients to exhibit spastic diplegia, dyskinetic quadriplegia, and infantile spasms

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.

Clinical Characteristics Suggestive of a Genetic Cause in Cerebral Palsy: A Systematic Review

BACKGROUND

Cerebral palsy (CP) is a clinical diagnosis and was long categorized as an acquired disorder, but more and more genetic etiologies are being identified. This review aims to identify the clinical characteristics that are associated with genetic CP to aid clinicians in selecting candidates for genetic testing.

METHODS

The PubMed database was systematically searched to identify genes associated with CP. The clinical characteristics accompanying these genetic forms of CP were compared with published data of large CP populations resulting in the identification of potential indicators of genetic CP.

RESULLTS

Of 1930 articles retrieved, 134 were included. In these, 55 CP genes (described in two or more cases, n = 272) and 79 candidate genes (described in only one case) were reported. The most frequently CP-associated genes were PLP1 (21 cases), ARG1 (17 cases), and CTNNB1 (13 cases). Dyskinesia and the absence of spasticity were identified as strong potential indicators of genetic CP. Presence of intellectual disability, no preterm birth, and no unilateral distribution of symptoms were classified as moderate genetic indicators.

CONCLUSIONS

Genetic causes of CP are increasingly identified. The clinical characteristics associated with genetic CP can aid clinicians regarding to which individual with CP to offer genetic testing. The identified potential genetic indicators need to be validated in large CP cohorts but can provide the first step toward a diagnostic algorithm for genetic CP.

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.

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.

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.

Dystonia with myoclonus and vertical supranuclear gaze palsy associated with a rare GNB1 variant

GNB1 encephalopathy (OMIM: 616973), caused by pathogenic variants in the GNB1 gene, is a rare neurodevelopmental syndrome characterized by global developmental delay (GDD) variably co-occurring with movement disorders. For the latter, dystonia, although the most frequent, remains uncommon. Other phenomenologies including myoclonus, tics, chorea, and ataxia, as well as oculomotor abnormalities are rare [1]. Most pathogenic variants in GNBI occur in exons 6 and 7, which are considered to be mutational hotspots [2]. Here, we report a case of GNB1 encephalopathy arising from a de novo mutation in a gene region with few reported pathogenic variants (i.e., exon 11) presenting with a unique phenotype consisting of dystonia with myoclonus and vertical supranuclear gaze palsy.

Motor, epileptic, and developmental phenotypes in genetic disorders affecting G protein coupled receptors-cAMP signaling

Over the last years, a constantly increasing number of genetic diseases associated with epilepsy and movement disorders have been recognized. An emerging group of conditions in this field is represented by genetic disorders affecting G-protein-coupled receptors (GPCRs)–cAMP signaling. This group of postsynaptic disorders includes genes encoding for proteins highly expressed in the central nervous system and involved in GPCR signal transduction and cAMP production (e.g., GNAO1, GNB1, ADCY5, GNAL, PDE2A, PDE10A, and HPCA genes). While the clinical phenotype associated with ADCY5 and GNAL is characterized by movement disorder in the absence of epilepsy, GNAO1, GNB1, PDE2A, PDE10A, and HPCA have a broader clinical phenotype, encompassing movement disorder, epilepsy, and neurodevelopmental disorders. We aimed to provide a comprehensive phenotypical characterization of genetic disorders affecting the cAMP signaling pathway, presenting with both movement disorders and epilepsy. Thus, we reviewed clinical features and genetic data of 203 patients from the literature with GNAO1, GNB1, PDE2A, PDE10A, and HPCA deficiencies. Furthermore, we delineated genotype–phenotype correlation in GNAO1 and GNB1 deficiency. This group of disorders presents with a highly recognizable clinical phenotype combining distinctive motor, epileptic, and neurodevelopmental features. A severe hyperkinetic movement disorder with potential life-threatening exacerbations and high susceptibility to a wide range of triggers is the clinical signature of the whole group of disorders. The existence of a distinctive clinical phenotype prompting diagnostic suspicion and early detection has relevant implications for clinical and therapeutic management. Studies are ongoing to clarify the pathophysiology of these rare postsynaptic disorders and start to design disease-specific treatments.

Whole-exome sequencing and adrenocorticotropic hormone therapy in individuals with infantile spasms

AIM

To identify additional genes associated with infantile spasms using a cohort with defined infantile spasms.

METHOD

Whole-exome sequencing (WES) was performed on 21 consented individuals with infantile spasms and their unaffected parents (a trio-based study). Clinical history and imaging were reviewed. Potentially deleterious exonic variants were identified and segregated. To refine potential candidates, variants were further prioritized on the basis of evidence for relevance to disease phenotype or known associations with infantile spasms, epilepsy, or neurological disease.

RESULTS

Likely pathogenic de novo variants were identified in NR2F1, GNB1, NEUROD2, GABRA2, and NDUFAF5. Suggestive dominant and recessive candidate variants were identified in PEMT, DYNC1I1, ASXL1, RALGAPB, and STRADA; further confirmation is required to support their relevance to disease etiology.

INTERPRETATION

This study supports the utility of WES in uncovering the genetic etiology in undiagnosed individuals with infantile spasms with an overall yield of five out of 21. High-priority candidates were identified in an additional five individuals. WES provides additional support for previously described disease-associated genes and expands their already broad mutational and phenotypic spectrum.

Inhibition of G-protein signalling in cardiac dysfunction of intellectual developmental disorder with cardiac arrhythmia (IDDCA) syndrome

BACKGROUND

Pathogenic variants of GNB5 encoding the β5 subunit of the guanine nucleotide-binding protein cause IDDCA syndrome, an autosomal recessive neurodevelopmental disorder associated with cognitive disability and cardiac arrhythmia, particularly severe bradycardia.

METHODS

We used echocardiography and telemetric ECG recordings to investigate consequences of Gnb5 loss in mouse.

RESULTS

We delineated a key role of Gnb5 in heart sinus conduction and showed that Gnb5-inhibitory signalling is essential for parasympathetic control of heart rate (HR) and maintenance of the sympathovagal balance. Gnb5-/- mice were smaller and had a smaller heart than Gnb5+/+ and Gnb5+/- , but exhibited better cardiac function. Lower autonomic nervous system modulation through diminished parasympathetic control and greater sympathetic regulation resulted in a higher baseline HR in Gnb5-/- mice. In contrast, Gnb5-/- mice exhibited profound bradycardia on treatment with carbachol, while sympathetic modulation of the cardiac stimulation was not altered. Concordantly, transcriptome study pinpointed altered expression of genes involved in cardiac muscle contractility in atria and ventricles of knocked-out mice. Homozygous Gnb5 loss resulted in significantly higher frequencies of sinus arrhythmias. Moreover, we described 13 affected individuals, increasing the IDDCA cohort to 44 patients.

CONCLUSIONS

Our data demonstrate that loss of negative regulation of the inhibitory G-protein signalling causes HR perturbations in Gnb5-/- mice, an effect mainly driven by impaired parasympathetic activity. We anticipate that unravelling the mechanism of Gnb5 signalling in the autonomic control of the heart will pave the way for future drug screening.

Case Report: A Novel GNB1 Mutation Causes Global Developmental Delay With Intellectual Disability and Behavioral Disorders

Diseases of neurodevelopment mostly exhibit neurological and psychiatric symptoms that go from very mild to extremely severe. While the etiology of most cases of neurodevelopmental disease is still unknown, the discovery of underlying genetic causes is rapidly increasing, with hundreds of genes being currently implicated as disease-causing. Here, we report a clinical case of a patient with a previously undiagnosed syndrome comprising severe global developmental delay, intellectual disability, and behavioral disorders (such as attention-deficit/hyperactivity disorder, autism spectrum disorder and recurrent bouts of aggressive behavior). After genetic testing, a pathogenic variant was detected in the GNB1 gene, which codes for the G-protein subunit β1. The detected variant (c.217G>A, p.A73T) has not been previously reported in any of the 58 published cases of GNB1 encephalopathy. However, it localizes to the mutational hotspot in exons 6 and 7 in which 88% of all missense mutations occur. An in silico model predicts that this mutation is likely to disrupt the WD40 domain of the GNB1 protein, which is required for its interaction with other G-proteins and, consequently, for downstream signal transduction. In conclusion, we reported an additional GNB1 encephalopathy patient, bearing a novel mutation, taking another step toward a better understanding of its clinical presentation and prospective development of treatments for the disease.

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

Mutations in the GNB1 gene, encoding the Gβ₁ subunit of heterotrimeric G proteins, cause GNB1 Encephalopathy. Patients experience seizures, pointing to abnormal activity of ion channels or neurotransmitter receptors. We studied three Gβ₁ mutations (K78R, I80N and I80T) using computational and functional approaches. In heterologous expression models, these mutations did not alter the coupling between G protein-coupled receptors to G_i/o, or the Gβγ regulation of the neuronal voltage-gated Ca²⁺ channel Ca_V2.2. However, the mutations profoundly affected the Gβγ regulation of the G protein-gated inwardly rectifying potassium channels (GIRK, or Kir3). Changes were observed in Gβ₁ protein expression levels, Gβγ binding to cytosolic segments of GIRK subunits, and in Gβγ function, and included gain-of-function for K78R or loss-of-function for I80T/N, which were GIRK subunit-specific. Our findings offer new insights into subunit-dependent gating of GIRKs by Gβγ, and indicate diverse etiology of GNB1 Encephalopathy cases, bearing a potential for personalized treatment.

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GIRK channels are key players affected by GNB1 mutations under study (K78R and I80N/T)

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Effects of mutations (LoF or GoF) are channel subunit composition-specific

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The findings help to understand the GNB1 encephalopathy and to devise treatments

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The results yield new insights into mechanisms of Gβγ regulation of GIRKs

Subtype-dependent regulation of Gβγ signalling

G protein-coupled receptors (GPCRs) transmit information to the cell interior by transducing external signals to heterotrimeric G protein subunits, Gα and Gβγ subunits, localized on the inner leaflet of the plasma membrane. Though the initial focus was mainly on Gα-mediated events, Gβγ subunits were later identified as major contributors to GPCR-G protein signalling. A broad functional array of Gβγ signalling has recently been attributed to Gβ and Gγ subtype diversity, comprising 5 Gβ and 12 Gγ subtypes, respectively. In addition to displaying selectivity towards each other to form the Gβγ dimer, numerous studies have identified preferences of distinct Gβγ combinations for specific GPCRs, Gα subtypes and effector molecules. Importantly, Gβ and Gγ subtype-dependent regulation of downstream effectors, representing a diverse range of signalling pathways and physiological functions have been found. Here, we review the literature on the repercussions of Gβ and Gγ subtype diversity on direct and indirect regulation of GPCR/G protein signalling events and their physiological outcomes. Our discussion additionally provides perspective in understanding the intricacies underlying molecular regulation of subtype-specific roles of Gβγ signalling and associated diseases.

Second patient with GNB2-related neurodevelopmental disease: Further evidence for a gene-disease association

G-proteins are ubiquitously expressed heterotrimeric proteins consisting of α, β and γ subunits and mediate G-protein coupled receptor signalling cascades. The β subunit is encoded by one of five highly similar paralogs (GNB1-GNB5, accordingly). The developmental importance of G-proteins is highlighted by the clinical relevance of variants in genes such as GNB1, which cause severe neurodevelopmental disease (NDD). Recently the candidacy of GNB2 was raised in association with NDD in an individual with a de novo variant affecting a codon conserved across paralogs and recurrently mutated in GNB1-related disease, c.229G>A p.(Gly77Arg), in association with global developmental delay, intellectual disability and dysmorphic features. Here, we report a patient with strikingly similar facial features and NDD in association with a de novo GNB2 variant affecting the same codon, c.229G>T p.(Gly77Trp). In addition, this individual has epilepsy and overgrowth. Our report is the second to implicate a de novo GNB2 variant with a severe yet variable NDD.

Proteomic differences in the hippocampus and cortex of epilepsy brain tissue

Epilepsy is a common neurological disorder affecting over 70 million people worldwide, with a high rate of pharmaco-resistance, diverse comorbidities including progressive cognitive and behavioural disorders, and increased mortality from direct (e.g. sudden unexpected death in epilepsy, accidents, drowning) or indirect effects of seizures and therapies. Extensive research with animal models and human studies provides limited insights into the mechanisms underlying seizures and epileptogenesis, and these have not translated into significant reductions in pharmaco-resistance, morbidities or mortality. To help define changes in molecular signalling networks associated with seizures in epilepsy with a broad range of aetiologies, we examined the proteome of brain samples from epilepsy and control cases. Label-free quantitative mass spectrometry was performed on the hippocampal cornu ammonis 1-3 region (CA1-3), frontal cortex and dentate gyrus microdissected from epilepsy and control cases (n = 14/group). Epilepsy cases had significant differences in the expression of 777 proteins in the hippocampal CA1 - 3 region, 296 proteins in the frontal cortex and 49 proteins in the dentate gyrus in comparison to control cases. Network analysis showed that proteins involved in protein synthesis, mitochondrial function, G-protein signalling and synaptic plasticity were particularly altered in epilepsy. While protein differences were most pronounced in the hippocampus, similar changes were observed in other brain regions indicating broad proteomic abnormalities in epilepsy. Among the most significantly altered proteins, G-protein subunit beta 1 (GNB1) was one of the most significantly decreased proteins in epilepsy in all regions studied, highlighting the importance of G-protein subunit signalling and G-protein-coupled receptors in epilepsy. Our results provide insights into common molecular mechanisms underlying epilepsy across various aetiologies, which may allow for novel targeted therapeutic strategies.

Haploinsufficiency as a disease mechanism in GNB1-associated neurodevelopmental disorder

BACKGROUND

GNB1 encodes a subunit of a heterotrimeric G-protein complex that transduces intracellular signaling cascades. Disruptions to the gene have previously been shown to be embryonic lethal in knockout mice and to cause complex neurodevelopmental disorders in humans. To date, the majority of variants associated with disease in humans have been missense variants in exons 5-7.

METHODS

Genetic sequencing was performed on two patients presenting with complex neurological phenotypes including intellectual disability, hypotonia, and in one patient seizures. Reported variants were assessed using RNA sequencing and functional BRET/BiFC assays.

RESULTS

A splice variant reported in patient 1 was confirmed to cause usage of a cryptic splice site leading to a truncated protein product. Patient 2 was reported to have a truncating variant. BRET and BiFC assays of both patient variants confirmed both were deficient in inducing GPCR-induced G protein activation due to lack of dimer formation with the Gγ subunit.

CONCLUSION

Here, we report two patients with functionally confirmed loss of function variants in GNB1 and neurodevelopmental phenotypes including intellectual disability, hypotonia, and seizures in one patient. These results suggest haploinsufficiency of GNB1 is a mechanism for neurodevelopmental disorders in humans.

The Emerging Role of Gβ Subunits in Human Genetic Diseases

Environmental stimuli are perceived and transduced inside the cell through the activation of signaling pathways. One common type of cell signaling transduction network is initiated by G-proteins. G-proteins are activated by G-protein-coupled receptors (GPCRs) and transmit signals from hormones, neurotransmitters, and other signaling factors, thus controlling a number of biological processes that include synaptic transmission, visual photoreception, hormone and growth factors release, regulation of cell contraction and migration, as well as cell growth and differentiation. G-proteins mainly act as heterotrimeric complexes, composed of alpha, beta, and gamma subunits. In the last few years, whole exome sequencing and biochemical studies have shown causality of disease-causing variants in genes encoding G-proteins and human genetic diseases. This review focuses on the G-protein β subunits and their emerging role in the etiology of genetically inherited rare diseases in humans.