Inheritance of c.628-6G>A GNB5 hypomorphic allele uncovers another challenge in the pathogenic prediction of genomic variants.

We studied a patient with a severe phenotype carrying two GNB5 variants: c.514delT from the unaffected heterozygous mother and c.628-6G>A from the unaffected homozygous father. Functional genomics studies showed that parents express 50% (nonsense-mediated decay, NMD) of the RNA/protein while the patient does not produce enough protein for normal development.

Injection of IK1 through dynamic clamp can make all the difference in patch-clamp studies on hiPSC-derived cardiomyocytes.

Human-induced stem cell-derived cardiomyocytes (hiPSC-CMs) are a valuable tool for studying development, pharmacology, and (inherited) arrhythmias. Unfortunately, hiPSC-CMs are depolarized and spontaneously active, even the working cardiomyocyte subtypes such as atrial- and ventricular-like hiPSC-CMs, in contrast to the situation in the atria and ventricles of adult human hearts. Great efforts have been made, using many different strategies, to generate more mature, quiescent hiPSC-CMs with more close-to-physiological resting membrane potentials, but despite promising results, it is still difficult to obtain hiPSC-CMs with such properties. The dynamic clamp technique allows to inject a current with characteristics of the inward rectifier potassium current (IK1), computed in real time according to the actual membrane potential, into patch-clamped hiPSC-CMs during action potential measurements. This results in quiescent hiPSC-CMs with a close-to-physiological resting membrane potential. As a result, action potential measurements can be performed with normal ion channel availability, which is particularly important for the physiological functioning of the cardiac SCN5A-encoded fast sodium current (INa). We performed in vitro and in silico experiments to assess the beneficial effects of the dynamic clamp technique in dissecting the functional consequences of the SCN5A-1795insD+/- mutation. In two separate sets of patch-clamp experiments on control hiPSC-CMs and on hiPSC-CMs with mutations in ACADVL and GNB5, we assessed the value of dynamic clamp in detecting delayed afterdepolarizations and in investigating factors that modulate the resting membrane potential. We conclude that the dynamic clamp technique has highly beneficial effects in all of the aforementioned settings and should be widely used in patch-clamp studies on hiPSC-CMs while waiting for the ultimate fully mature hiPSC-CMs.

Extended Phenotyping and Functional Validation Facilitate Diagnosis of a Complex Patient Harboring Genetic Variants in MCCC1 and GNB5 Causing Overlapping Phenotypes.

Identifying multiple ultra-rare genetic syndromes with overlapping phenotypes is a diagnostic conundrum in clinical genetics. This study investigated the pathogenicity of a homozygous missense variant in GNB5 (GNB5L; NM_016194.4: c.920T > G (p. Leu307Arg); GNB5S; NM_006578.4: c.794T > G (p. Leu265Arg)) identified through exome sequencing in a female child who also had 3-methylcrotonyl-CoA carboxylase (3-MCC) deficiency (newborn screening positive) and hemoglobin E trait. The proband presented with early-onset intellectual disability, the severity of which was more in keeping with GNB5-related disorder than 3-MCC deficiency. She later developed bradycardia and cardiac arrest, and upon re-phenotyping showed cone photo-transduction recovery deficit, all known only to GNB5-related disorders. Patient-derived fibroblast assays showed preserved GNB5S expression, but bioluminescence resonance energy transfer assay showed abolished function of the variant reconstituted Gß5S containing RGS complexes for deactivation of D2 dopamine receptor activity, confirming variant pathogenicity. This study highlights the need for precise phenotyping and functional assays to facilitate variant classification and clinical diagnosis in patients with complex medical conditions.

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

BACKGROUND: Pathogenic variants of GNB5 encoding the ß5 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.

[Intellectual developmental disorder with cardiac arrhythmia syndrome in a family caused by GNB5 variation and literature review].

Objective: To explore the clinical characteristics of intellectual developmental disorder with cardiac arrhythmia syndrome (IDDCA) in a family caused by GNB5 gene variation and to review the literature. Methods: The clinical and genetic data of an infant with IDDCA, who visited Shenzhen Children's Hospital in September 2018, were collected and analyzed. His parents' and brother's gene analysis was also done by the next-generation sequencing and confirmed by Sanger sequencing. Related literature up to March 2020 was searched in Online Mendelian Inheritance in Man (OMIM), PubMed, CNKI and Wanfang databases with "GNB5" "IDDCA" "LADCI" "intellectual developmental disorder with cardial arrhythmia" "language delay and attention deficit-hyperactivity disorder or cognitive impairment with or without cardiac arrhythmia" as the key words. The related papers were retrieved and analyzed to summarize the clinical and genetic characteristics of this disorder. Results: The proband was an 11-month-old boy who presented with mental and motor developmental retardation, accompanied with convulsion and muscle weakness. Sinus arrest was also detected. His electroencephalogram (EEG) and flash visual evoked potential (FVEP) were both abnormal. Genetic analysis identified the homozygous frameshift variation of GNB5 gene (c.136delG, p.Glu46Argfs*8) in this infant and heterozygous variation in his parents, confirmed the diagnosis of IDDCA. The same GNB5 variation was identified in his brother, who was 4 years and 8 months old and had developed the similar clinical manifestations after birth. There were only 7 papers reporting this disease in the literature review, with a total of 27 patients from 14 families. Including these 2 cases, there were 29 patients in total, whose age of diagnosis ranged from 5.5 months to 23 years. Among all the patients, 20 cases (69%) were diagnosed as IDDCA, while 8 cases (28%) as LADCI; and 11 (38%) were males while 18 (62%) females. Regarding the clinical features, 66% (19/29) had mental retardation, 41% (12/29) had seizures, 79% (23/29) developed language delay and 62%（18/29） had sinus node dysfunction. Genetic tests showed that 4 patients from 3 families had complex heterozygous variation, and 25 patients (86%) from 12 families had homozygous variation. Seventeen patients from 8 families were consanguineous. Among the total 12 variations, there were 4 nonsense, 3 frameshift, 2 missense and 2 shear mutations, and 1 shear disorder caused by synonymous mutation. Conclusions: IDDCA caused by GNB5 gene variations mainly manifests as general developmental delay or severe mental retardation, and sinus node dysfunction. GNB5 associated syndromes have phenotypic heterogeneity and are inherited in an autosomal recessive manner.

Severe Phenotype in a Patient With Homozygous 15q21.2 Microdeletion Involving BCL2L10, GNB5, and MYO5C Genes, Resembling Infantile Developmental Disorder With Cardiac Arrhythmias (IDDCA).

Homozygous and compound heterozygous mutations in GNB5 gene have been associated with a wide spectrum of clinical presentations, ranging from neurodevelopmental issues with or without cardiac arrhythmia (LADCI) to severe developmental delay with epileptic encephalopathy, retinal dystrophy, and heart rhythm abnormalities (IDDCA). While missense or missense/non-sense mutations usually lead to milder form, the biallelic loss of function of GNB5 gene causes the severe multisystemic IDDCA phenotype. So far, only 27 patients have been described with GNB5-associated disease. We report the first case of a patient carrying a homozygous 15q21.2 microdeletion, encompassing GNB5 and the two contiguous genes BCL2L10 and MYO5C. The clinical features of the child are consistent with the severe IDDCA phenotype, thus confirming the GNB5 loss-of-function mechanism in determining such presentation of the disease.

Genetic characterization and long-term management of severely affected siblings with intellectual developmental disorder with cardiac arrhythmia syndrome.

We report two brothers with severe global cognitive and motor delay, cortical visual impairment and sick sinus syndrome who were born to consanguineous parents. Standard genetic evaluations did not reveal the cause of their mental retardation. As expected, chromosomal microarray (CMA) revealed extensive regions of homozygosity. Exome sequencing revealed that both affected boys were homozygous for a nonsense mutation in the G-protein ß5 (GNB5) gene (NM_016194.3:c.1032C > G; Tyr344Ter), and that the parents were carriers of this mutation. No other DNA variants that were explanatory for the sick sinus or the developmental delay/intellectual disability were identified, and no other clinical parameters are likely to have contributed to this unusual combination of phenotypes. The neurologic features of our patients are more severe than those of most of the other patients previously reported with GNB5 variants, probably because of the homozygous, complete loss-of-function (nonsense/stop-gain) nature of their variant, and their clinical course has been monitored for longer duration.

The epileptology of GNB5 encephalopathy.

Pathogenic variants in GNB5 cause an autosomal recessive neurodevelopmental disorder with neonatal sinus bradycardia. Seizures or epilepsy occurred in 10 of 22 previously reported cases, including 6 children from one family. We delineate the epileptology of GNB5 encephalopathy. Our nine patients, including five new patients, were from seven families. Epileptic spasms were the most frequent seizure type, occurring in eight of nine patients, and began at a median age of 3 months (2 months to 3 years). Focal seizures preceded spasms in three children, with onset at 7 days, 11 days, and 4 months. One child presented with convulsive status epilepticus at 6 months. Three children had burst suppression on electroencephalography (EEG), three had hypsarrhythmia, and one evolved from burst suppression to hypsarrhythmia. Background slowing was present in all after age 3 years. Magnetic resonance imaging (MRI) showed cerebral atrophy in one child and cerebellar atrophy in another. All nine had abnormal development prior to seizure onset and ultimately had profound impairment without regression. Hypotonia was present in all, with contractures developing in two older patients. All individuals had biallelic pathogenic variants in GNB5, predicted by in silico tools to result in protein truncation and loss-of-function. GNB5 developmental and epileptic encephalopathy is characterized by epileptic spasms, focal seizures, and profound impairment.

Increased store-operated Ca2+ entry mediated by GNB5 and STIM1.

Recent human genetic studies have shown that Gß5 is related to various clinical symptoms, such as sinus bradycardia, cognitive disability, and attention deficit hyperactivity disorder. Although the calcium signaling cascade is closely associated with a heterotrimeric G-protein, the function of Gß5 in calcium signaling and its relevance to clinical symptoms remain unknown. In this study, we investigated the in vitro changes of store-operated calcium entry (SOCE) with exogenous expression of Gß5. The cells expressing Gß5 had enhanced SOCE after depletion of calcium ion inside the endoplasmic reticulum. Gß5 also augmented Stim1- and Orai1-dependent SOCE. An ORAI1 loss-of-function mutant did not show inhibition of Gß5-induced SOCE, and a STIM1-ERM truncation mutant showed no enhancement of SOCE. These results suggested a novel role of GNB5 and Stim1, and provided insight into the regulatory mechanism of SOCE.

Intellectual developmental disorder with cardiac arrhythmia syndrome in a child with compound heterozygous GNB5 variants.

Identification of a novel compound heterozygous of GNB5 in a patient with intellectual developmental disorder with cardiac arrhytmia (IDDCA), from non-consaguineous family. Three-dimensional modelling and in silico predictions suggest that GNB5 variants are causative of the phenotype, extending the number of IDDCA patients so far identified.

A NGS-Targeted Autism/ID Panel Reveals Compound Heterozygous GNB5 Variants in a Novel Patient.

Homozygous and compound heterozygous pathogenic variants in GNB5 have been recently associated with a spectrum of clinical presentations varying from a severe multisystem form of the disorder including intellectual disability, early infantile developmental and epileptic encephalopathy, retinal abnormalities and cardiac arrhythmias (IDDCA) to a milder form with language delay, attention-deficit/hyperactivity disorder, cognitive impairment, with or without cardiac arrhythmia (LADCI). Approximately twenty patients have been described so far; here we report a novel case of a 2.5-year-old female who is a compound heterozygote for a frameshift and a missense variant in the GNB5 gene. Her clinical presentation is consistent with a moderate phenotype, corroborating the direct correlation between the type and pathogenic mechanism of the GNB5 genetic variant and the severity of related phenotype.

Increased store-operated Ca²⁺ entry mediated by GNB5 and STIM1

Recent human genetic studies have shown that Gβ5 is related to various clinical symptoms, such as sinus bradycardia, cognitive disability, and attention deficit hyperactivity disorder. Although the calcium signaling cascade is closely associated with a heterotrimeric G-protein, the function of Gβ5 in calcium signaling and its relevance to clinical symptoms remain unknown. In this study, we investigated the in vitro changes of store-operated calcium entry (SOCE) with exogenous expression of Gβ5. The cells expressing Gβ5 had enhanced SOCE after depletion of calcium ion inside the endoplasmic reticulum. Gβ5 also augmented Stim1- and Orai1-dependent SOCE. An ORAI1 loss-of-function mutant did not show inhibition of Gβ5-induced SOCE, and a STIM1-ERM truncation mutant showed no enhancement of SOCE. These results suggested a novel role of GNB5 and Stim1, and provided insight into the regulatory mechanism of SOCE.