GABRB3 Research Articles

GABA and epileptogenesis: comparing gabrb3 gene-deficient mice with Angelman syndrome in man

The GABAergic system has long been implicated in epilepsy with defects in GABA neurotransmission being linked to epilepsy in both experimental animal models and human syndromes ( Olsen and Avoli, 1997). However, to date no human epileptic syndrome has been directly attributed to an altered GABAergic system. The observed defects in GABA neurotransmission in human epileptic syndromes may be the indirect result of a brain besieged by seizures. The use of animal models of epilepsy has sought to address these matters. The advent of gene targeting methodologies in mice now allows for a more direct assessment of GABA’s involvement in epileptogenesis. To date several genes associated with the GABAergic system have been disrupted. These include the genes for glutamic acid decarboxylase, both the 65- and 67-kDa isoforms (GAD65 and GAD67), the tissue non-specific alkaline phosphatase gene (TNAP) and genes for the GABA A receptor subunits α 6, β 3, γ 2, and δ (gabra6, gabrb3, gabrg2, and gabrd respectively). Gene disruptions of either GAD67 or gabrg2 result in neonatal lethality, while others, GAD65, TNAP, and gabrb3 exhibit increased mortality and spontaneous seizures. GABA receptor expression has been found to be both regionally and developmentally regulated. Thus in addition to their obvious role in controlling excitability in adult brain, a deficit in GABAergic function during development could be expected to elicit pleiotropic neurodevelopmental abnormalities perhaps including epilepsy. The GABA A receptor β 3 subunit gene, gabrb3/GABRB3 (mouse/human), is of particular interest because of its expression early in development and its possible role in the neurodevelopmental disorder Angelman syndrome. Individuals with this syndrome exhibit severe mental retardation and epilepsy. Mice with the gabrb3 gene disrupted likewise exhibit electroencephalograph (EEG) abnormalities, seizures, and behavioral characteristics typically associated with Angelman syndrome. These gabrb3 gene knockout mice provide direct evidence that a reduction of a specific subunit of the GABA A receptor system can result in epilepsy and support a GABAergic role in the pathophysiology of Angelman syndrome.

Emotion‐Induced Myoclonic Absence‐Like Seizures in a Patient with Inv‐Dup(15) Syndrome: A Clinical, EEG, and Molecular Genetic Study

We have described a clinical EEG and molecular genetic study of a 9-year-old boy with inv-dup(15) syndrome in whom seizures were induced by emotionally gratifying stimuli. The reflex seizures began 5-20 s after the onset of repeated cheek-kissing from his mother or after viewing of pleasant or funny events. They were characterized by bilateral discharges involving mainly the temporal regions and evolving into myoclonic absence-like seizures. Nonemotional stimuli, such as a pinch, sucking or rubbing his cheeks, or the sound of the kiss alone, failed to provoke seizures. The seizures were resistant to antiepileptic (AED) treatments. Molecular genetic investigations revealed a correct methylation pattern of the chromosomes 15, and three copies (two maternal and one paternal) of the segment 15q11-q13, including the GABRb3 gene. We hypothesize that an overexpression of cerebral gamma-aminobutyric acid (GABA)-mediated inhibition accounts for the severe epilepsy that we observed in this patient.

A Sequence-Ready BAC Contig of the GABAA Receptor Gene Cluster Gabrg1–Gabra2–Gabrb1 on Mouse Chromosome 5

The type-A receptors for the neurotransmitter GABA (gamma-aminobutyric acid) are ligand-gated chloride channels that mediate postsynaptic inhibition. The functional diversity of these receptors comes from the use of a large repertoire of subunits encoded by separate genes, as well as from differences in subunit composition of individual receptors. In mammals, a majority of GABA(A) receptor subunit genes are located in gene clusters that may be important for their regulated expression and function. We have established a high-resolution physical map of the cluster of genes encoding GABA(A) receptor subunits alpha2 (Gabra2), beta1 (Gabrb1), and gamma(1) (Gabrg1) on mouse chromosome 5. Rat cDNA probes and specific sequence probes for all three GABA(A) receptor subunit genes have been used to initiate the construction of a sequence-ready contig of bacterial artificial chromosomes (BACs) encompassing this cluster. In the process of contig construction clones from 129/Sv and C57BL/6J BAC libraries were isolated. The assembled 1.3-Mb contig, consisting of 45 BACs, gives five- to sixfold coverage over the gene cluster and provides an average resolution of one marker every 32 kb. A number of BAC insert ends were sequenced, generating 30 new sequence tag sites (STS) in addition to 6 Gabr gene-based and 3 expressed sequence tag (EST)-based markers. STSs from, and surrounding, the Gabrg1-Gabra2-Gabrb1 gene cluster were mapped in the T31 mouse radiation hybrid panel. The integration of the BAC contig with a map of loci ordered by radiation hybrid mapping suggested the most likely genomic orientation of this cluster on mouse chromosome 5: cen-D5Mit151-Gabrg1-Gabra2-Gabrb1-D5Mit58- tel. This established contig will serve as a template for genomic sequencing and for functional analysis of the GABA(A) gene cluster on mouse chromosome 5 and the corresponding region on human chromosome 4.

D 2 dopamine receptor and GABA A receptor β3 subunit genes and alcoholism

As the dopaminergic and GABAergic systems have been implicated in alcohol-related behaviors, variants of the D 2 dopamine receptor (DRD2) and GABA A receptor β3 subunit (GABRB3) genes were determined in a population-based association study of Caucasian non-alcoholic and alcoholic subjects. In severe alcoholics, compared to non-alcoholics, a significant increase was found in the prevalence ( P=1.7×10 −5) and frequency ( P=1.6×10 −5) of the DRD2 minor (A1) allele. Moreover, a significant progressive increase was observed in A1 allelic prevalence ( P=3.1×10 −6) and frequency ( P=2.7×10 −6) in the order of non-alcoholics, less severe and severe alcoholics. In severe alcoholics, compared to non-alcoholics, a significant decrease was found in the prevalence ( P=4.5×10 −3) and frequency ( P=2.7×10 −2) of the GABRB3 major (G1) allele. Furthermore, a significant progressive decrease was noted in G1 allelic prevalence ( P=2.4×10 −3) and frequency ( P=1.9×10 −2) in non-alcoholics, less severe and severe alcoholics, respectively. In sum, in the same population of non-alcoholics and alcoholics studied, variants of both the DRD2 and GABRB3 genes independently contribute to the risk for alcoholism, with the DRD2 variants revealing a stronger effect than the GABRB3 variants. However, when the DRD2 and the GABRB3 variants are combined, the risk for alcoholism is more robust than when these variants are considered separately.

Mice Lacking the β3Subunit of the GABAAReceptor Have the Epilepsy Phenotype and Many of the Behavioral Characteristics of Angelman Syndrome

Angelman syndrome (AS) is a severe neurodevelopmental disorder resulting from a deletion/mutation in maternal chromosome 15q11-13. The genes in 15q11-13 contributing to the full array of the clinical phenotype are not fully identified. This study examines whether a loss or reduction in the GABAA receptor beta3 subunit (GABRB3) gene, contained within the AS deletion region, may contribute to the overall severity of AS. Disrupting the gabrb3 gene in mice produces electroencephalographic abnormalities, seizures, and behavior that parallel those seen in AS. The seizures that are observed in these mice showed a pharmacological response profile to antiepileptic medications similar to that observed in AS. Additionally, these mice exhibited learning and memory deficits, poor motor skills on a repetitive task, hyperactivity, and a disturbed rest-activity cycle, features all common to AS. The loss of the single gene, gabrb3, in these mice is sufficient to cause phenotypic traits that have marked similarities to the clinical features of AS, indicating that impaired expression of the GABRB3 gene in humans probably contributes to the overall phenotype of Angelman syndrome. At least one other gene, the E6-associated protein ubiquitin-protein ligase (UBE3A) gene, has been implicated in AS, so the relative contribution of the GABRB3 gene alone or in combination with other genes remains to be established.

Linkage-Disequilibrium Mapping of Autistic Disorder, with 15q11-13 Markers

Autistic disorder is a complex genetic disease. Because of previous reports of individuals with autistic disorder with duplications of the Prader-Willi/Angelman syndrome critical region, we screened several markers across the 15q11-13 region, for linkage disequilibrium. One hundred forty families, consisting predominantly of a child with autistic disorder and both parents, were studied. Genotyping was performed by use of multiplex PCR and capillary electrophoresis. Two children were identified who had interstitial chromosome 15 duplications and were excluded from further linkage-disequilibrium analysis. Use of the multiallelic transmission-disequilibrium test (MTDT), for nine loci on 15q11-13, revealed linkage disequilibrium between autistic disorder and a marker in the gamma-aminobutyric acidA receptor subunit gene, GABRB3 155CA-2 (MTDT 28.63, 10 df, P=.0014). No evidence was found for parent-of-origin effects on allelic transmission. The convergence of GABRB3 as a positional and functional candidate along with the linkage-disequilibrium data suggests the need for further investigation of the role of GABRB3 or adjacent genes in autistic disorder.

Evidence for uniparental, paternal expression of the human GABAA receptor subunit genes, using microcell-mediated chromosome transfer.

We have constructed mouse A9 hybrids containing a single normal human chromosome 15, via microcell-mediated chromosome transfer. Cytogenetic and DNA-polymorphic analyses identified mouse A9 hybrids that contained either a paternal or maternal human chromosome 15. Paternal specific expression of the known imprinted genes SNRPN (small nuclear ribonucleoprotein-associated polypeptide N gene) and IPW (imprinted gene in the Prader-Willi syndrome region) was maintained in the A9 hybrids. Using this system, we first demonstrated that human GABAAreceptor subunit genes, GABRB3 , GABRA5 and GABRG3 , were expressed exclusively from the paternal allele and that E6-AP (E6-associated protein or UBE3A ) was biallelically expressed. Moreover, the 5' portion of the GABRB3 gene was found to be hypermethylated on the paternal allele. Our data imply that GABAAreceptor subunit genes are imprinted and are possible candidates for Prader-Willi syndrome, and that this human monochromosomal hybrid system enables the efficient analysis of imprinted loci.

Lack of association between juvenile myoclonic epilepsy and GABRA5 and GABRB3 genes

Lack of association between juvenile myoclonic epilepsy and GABRA5 and GABRB3 genes

Lack of association between juvenile myoclonic epilepsy and GABRA5 and GABRB3 genes

Alpha 5 and beta 3 GABAA receptor genes are major candidates for epilepsy, as they code for subunits of the most important human inhibitory neurotransmitter. Moreover, they are located within a region of the human genome previously implicated in disorders including epilepsy. We carried out an association study between dinucleotide repeat polymorphisms in these two genes and juvenile myoclonic epilepsy (JME). JME is the most common idiopathic epilepsy and is characterized by a complex mode of inheritance. We did not find significant differences between controls and patients for allele or genotype frequencies.

Relationship of the GABRB3 gene to adult ADHD and personality traits in Caucasian and African-American samples

Relationship of the GABRB3 gene to adult ADHD and personality traits in Caucasian and African-American samples

Phenotypic consequences of deletion of the gamma 3, alpha 5, or beta 3 subunit of the type A gamma-aminobutyric acid receptor in mice.

Three genes (Gabrg3, Gabra5, and Gabrb3) encoding the gamma 3, alpha 5, and beta 3 subunits of the type A gamma-aminobutyric acid receptor, respectively, are known to map near the pink-eyed dilution (p) locus in mouse chromosome 7. This region shares homology with a segment of human chromosome 15 that is implicated in Angelman syndrome, an inherited neurobehavioral disorder. By mapping Gabrg3 on a panel of p-locus deletions, we have determined that the order of genes within this cluster is centromere-p(D15S12h)-Gabrg3-Gabra5-Gabrb3-telom ere. Like Gabrb3, neither the Gabra5 nor Gabrg3 gene is functionally imprinted in adult mouse brain. Mice deleted for all three subunits die at birth with a cleft palate, although there are rare survivors (approximately 5%) that do not have a cleft palate but do exhibit a neurological abnormality characterized by tremor, jerky gait, and runtiness. We have previously suggested that deficiency of the beta 3 subunit may be responsible for the clefting defect. Most notably, however, in this report we describe mice carrying two overlapping, complementing p deletions that fail to express the gamma 3 transcript, as well as mice from another line that express neither the gamma 3 nor alpha 5 transcripts. Surprisingly, mice from both of these lines are phenotypically normal and do not exhibit any of the neurological symptoms characteristic of the rare survivors that are deleted for all three (gamma 3, alpha 5, and beta 3) subunits. These mice therefore provide a whole-organism type A gamma-aminobutyric-acid receptor background that is devoid of any receptor subtypes that normally contain the gamma 3 and/or alpha 5 subunits. The absence of an overt neurological phenotype in mice lacking the gamma 3 and/or alpha 5 subunits also suggests that mutations in these genes are unlikely to provide useful animal models for Angelman syndrome in humans.

Evaluation of potential models for imprinted and nonimprinted components of human chromosome 15q11-q13 syndromes by fine-structure homology mapping in the mouse.

Prader-Willi and Angelman syndromes are complex neurobehavioral contiguous gene syndromes whose expression depends on the unmasking of genomic imprinting for different genetic loci in human chromosome 15q11-q13. The homologous chromosomal region in the mouse genome has been fine-mapped by using interspecific (Mus spretus) crosses and overlapping, radiation-induced deletions to evaluate potential animal models for both imprinted and nonimprinted components of these syndromes. Four evolutionarily conserved sequences from human 15q11-q13, including two cDNAs from fetal brain (DN10, D15S12h; DN34, D15S9h-1), a microdissected clone (MN7; D15F37S1h) expressed in mouse brain, and the gene for the beta 3 subunit of the gamma-aminobutyric acid type A receptor (Gabrb3), were mapped in mouse chromosome 7 by analysis of deletions at the pink-eyed dilution (p) locus. Three of these loci are deleted in pre- and postnatally lethal p-locus mutations, which extend up to 5.5 +/- 1.7 centimorgans (cM) proximal to p; D15S9h-1, which maps 1.1 +/- 0.8 cM distal to p and is the mouse homolog of the human gene D15S9 (which shows a DNA methylation imprint), is not deleted in any of the p-locus deletion series. A transcript from the Gabrb3 gene, but not the transcript detected by MN7 at the D15F37S1h locus, is expressed in mice homozygous for the p6H deletion, which have an abnormal neurological phenotype. Furthermore, the Gabrb3 transcript is expressed equally well from the maternal or paternal chromosome 7 and, therefore, its expression is not imprinted in mouse brain. Deletions at the mouse p locus should serve as intermediate genetic reagents and models with which to analyze the genetics and etiology of individual components of human 15q11-q13 disorders.

Molecular study of chromosome 15 in 22 patients with Angelman syndrome

DNA studies in 22 families with Angelman syndrome (AS) were performed using the chromosome 15 marker loci D15S9, D15S10, D15S11, D15S12, D15S13, D15S18, D15S24, D15S86, the alpha-actin gene and the GABA beta 3 receptor gene (GABRB3). Uniparental disomy of chromosome 15 was excluded in all patients. Eighteen AS patients (82%) showed a molecular deletion of chromosome 15q11-q13 with one or more of these markers. No duplications of junction fragments, bridging deletions or duplication breakpoints were observed. The GABRB3 gene was deleted in all deletion-positive patients tested. Analysis of maternal DNA indicated that each deletion was a de novo event. All deletions were of maternal origin; this is in agreement with genomic imprinting in AS.