A cluster of three GABAA receptor subunit genes is deleted in a neurological mutant of the mouse p locus

Next Generation Sequencing and Cytogenetic Based Evaluation of Indian Pierre Robin Sequence Families Reveals CNV Regions of Modest Effect and a Novel LOXL3 Mutation

BACKGROUND

Pierre Robin Sequence (PRS) affects approximately 1 per 8500 to 14000 new-borns worldwide. Although the clinical entity is well defined, the pathogenesis of PRS is debated. The present study aims to understand the contribution of genomic imbalances and genetic variants in patients clinically diagnosed of PRS.

METHODOLOGY

A total of 7 independent patients with nonsyndromic PRS thoroughly evaluated by a medical geneticist at a tertiary care hospital, were included in the study. Blood samples were collected from these patients and their family members. Array CGH was performed on all 7 patients and their respective family members for detection of underlying cytogenetic defects. Whole exome sequencing (WES) was performed for 5 families to capture single nucleotide variants or small indels.

RESULTS

Cytogenetic analyses did not detect any previously reported gross chromosomal aberrations for PRS in the patient cohort. However, copy number variations (CNVs) of size <1 Mb were detected in patients which may have implications in PRS. The present study provided evidence for the occurrence of de novo deletions at 7p14.1 locus in PRS patients: further validating the candidate loci susceptibility in oral clefts. WES data identified LOXL3 as candidate gene, carrying novel deleterious variant, which is suggestive of the role of point mutations in the pathogenesis of PRS.

CONCLUSION

The present study offered considerable insight into the contribution of cytogenetic defects and novel point mutation in the etiology of nonsyndromic PRS. Studies comprising large number of cases are required to fully elucidate the genetic mechanisms underlying the PRS phenotype.

GABAA receptors in GtoPdb v.2021.3

The GABA_A receptor is a ligand-gated ion channel of the Cys-loop family that includes the nicotinic acetylcholine, 5-HT₃ and strychnine-sensitive glycine receptors. GABA_A receptor-mediated inhibition within the CNS occurs by fast synaptic transmission, sustained tonic inhibition and temporally intermediate events that have been termed 'GABA_A, slow' [45]. GABA_A receptors exist as pentamers of 4TM subunits that form an intrinsic anion selective channel. Sequences of six α, three β, three γ, one δ, three ρ, one ε, one π and one θ GABA_A receptor subunits have been reported in mammals [278, 235, 236, 283]. The π-subunit is restricted to reproductive tissue. Alternatively spliced versions of many subunits exist (e.g. α4- and α6- (both not functional) α5-, β2-, β3- and γ2), along with RNA editing of the α3 subunit [71]. The three ρ-subunits, (ρ1-3) function as either homo- or hetero-oligomeric assemblies [359, 50]. Receptors formed from ρ-subunits, because of their distinctive pharmacology that includes insensitivity to bicuculline, benzodiazepines and barbiturates, have sometimes been termed GABA_C receptors [359], but they are classified as GABA _A receptors by NC-IUPHAR on the basis of structural and functional criteria [16, 235, 236]. Many GABA_A receptor subtypes contain α-, β- and γ-subunits with the likely stoichiometry 2α.2β.1γ [168, 235]. It is thought that the majority of GABA_A receptors harbour a single type of α- and β - subunit variant. The α1β2γ2 hetero-oligomer constitutes the largest population of GABA_A receptors in the CNS, followed by the α2β3γ2 and α3β3γ2 isoforms. Receptors that incorporate the α4- α5-or α 6-subunit, or the β1-, γ1-, γ3-, δ-, ε- and θ-subunits, are less numerous, but they may nonetheless serve important functions. For example, extrasynaptically located receptors that contain α6- and δ-subunits in cerebellar granule cells, or an α4- and δ-subunit in dentate gyrus granule cells and thalamic neurones, mediate a tonic current that is important for neuronal excitability in response to ambient concentrations of GABA [209, 272, 83, 19, 288]. GABA binding occurs at the β+/α- subunit interface and the homologous γ+/α- subunits interface creates the benzodiazepine site. A second site for benzodiazepine binding has recently been postulated to occur at the α+/β- interface ([254]; reviewed by [282]). The particular α-and γ-subunit isoforms exhibit marked effects on recognition and/or efficacy at the benzodiazepine site. Thus, receptors incorporating either α4- or α6-subunits are not recognised by 'classical' benzodiazepines, such as flunitrazepam (but see [356]). The trafficking, cell surface expression, internalisation and function of GABA_A receptors and their subunits are discussed in detail in several recent reviews [52, 140, 188, 316] but one point worthy of note is that receptors incorporating the γ2 subunit (except when associated with α5) cluster at the postsynaptic membrane (but may distribute dynamically between synaptic and extrasynaptic locations), whereas as those incorporating the δ subunit appear to be exclusively extrasynaptic. NC-IUPHAR [16, 235, 3, 2] class the GABA_A receptors according to their subunit structure, pharmacology and receptor function. Currently, eleven native GABA_A receptors are classed as conclusively identified (i.e., α1β2γ2, α1βγ2, α3βγ2, α4βγ2, α4β2δ, α4β3δ, α5βγ2, α6βγ2, α6β2δ, α6β3δ and ρ) with further receptor isoforms occurring with high probability, or only tentatively [235, 236]. It is beyond the scope of this Guide to discuss the pharmacology of individual GABA_A receptor isoforms in detail; such information can be gleaned in the reviews [16, 95, 168, 173, 143, 278, 216, 235, 236] and [9, 10]. Agents that discriminate between α-subunit isoforms are noted in the table and additional agents that demonstrate selectivity between receptor isoforms, for example via β-subunit selectivity, are indicated in the text below. The distinctive agonist and antagonist pharmacology of ρ receptors is summarised in the table and additional aspects are reviewed in [359, 50, 145, 223]. Several high-resolution cryo-electron microscopy structures have been described in which the full-length human α1β3γ2L GABA_A receptor in lipid nanodiscs is bound to the channel-blocker picrotoxin, the competitive antagonist bicuculline, the agonist GABA (γ-aminobutyric acid), and the classical benzodiazepines alprazolam and diazepam [198].

Abnormal sleep physiology in children with 15q11.2-13.1 duplication (Dup15q) syndrome

BACKGROUND

Sleep disturbances in autism spectrum disorder (ASD) represent a common and vexing comorbidity. Clinical heterogeneity amongst these warrants studies of the mechanisms associated with specific genetic etiologies. Duplications of 15q11.2-13.1 (Dup15q syndrome) are highly penetrant for neurodevelopmental disorders (NDDs) such as intellectual disability and ASD, as well as sleep disturbances. Genes in the 15q region, particularly UBE3A and a cluster of GABA_A receptor genes, are critical for neural development, synaptic protein synthesis and degradation, and inhibitory neurotransmission. During awake electroencephalography (EEG), children with Dup15q syndrome demonstrate increased beta band oscillations (12-30 Hz) that likely reflect aberrant GABAergic neurotransmission. Healthy sleep rhythms, necessary for robust cognitive development, are also highly dependent on GABAergic neurotransmission. We therefore hypothesized that sleep physiology would be abnormal in children with Dup15q syndrome.

METHODS

To test the hypothesis that elevated beta oscillations persist in sleep in Dup15q syndrome and that NREM sleep rhythms would be disrupted, we computed: (1) beta power, (2) spindle density, and (3) percentage of slow-wave sleep (SWS) in overnight sleep EEG recordings from a cohort of children with Dup15q syndrome (n = 15) and compared them to age-matched neurotypical children (n = 12).

RESULTS

Children with Dup15q syndrome showed abnormal sleep physiology with elevated beta power, reduced spindle density, and reduced or absent SWS compared to age-matched neurotypical controls.

LIMITATIONS

This study relied on clinical EEG where sleep staging was not available. However, considering that clinical polysomnograms are challenging to collect in this population, the ability to quantify these biomarkers on clinical EEG-routinely ordered for epilepsy monitoring-opens the door for larger-scale studies. While comparable to other human studies in rare genetic disorders, a larger sample would allow for examination of the role of seizure severity, medications, and developmental age that may impact sleep physiology.

CONCLUSIONS

We have identified three quantitative EEG biomarkers of sleep disruption in Dup15q syndrome, a genetic condition highly penetrant for ASD. Insights from this study not only promote a greater mechanistic understanding of the pathophysiology defining Dup15q syndrome, but also lay the foundation for studies that investigate the association between sleep and cognition. Abnormal sleep physiology may undermine healthy cognitive development and may serve as a quantifiable and modifiable target for behavioral and pharmacological interventions.

Spectrum of epilepsy and electroencephalogram patterns in idic (15) syndrome

Previous reports summarized the seizure types occurring in patients with idic(15) syndrome. To better define this issue, we retrospectively analyzed the evolution of electroencephalogram findings and seizures in 35 patients with confirmed idic(15). Epilepsy occurred in 28 patients (80%), with a median age of onset of 3 years 3 months. The initial seizures were infantile spasms associated with a hypsarrhythmic electroencephalogram (nine patients), focal/generalized tonic (seven patients), or atypical absences (eight patients). High doses of oral steroids were given in all nine children with infantile spasms, with remission of seizures and resolution of electroencephalogram abnormalities. Among them, three were seizure free at the time of evaluation, but six later developed Lennox-Gastaut syndrome or Lennox-Gastaut-like syndrome. The eight patients with atypical absences developed Lennox-Gastaut syndrome or Lennox-Gastaut-like syndrome. Epilepsy was well controlled in 32% of the patients; satisfactorily controlled (seizures reduced >75%) in 21.4%; partially controlled (seizures reduced <50%) in 10.7%; and uncontrolled in 32%. One patient was not taking any anti-epileptic drugs by his parents' choice. Fourteen percent were on monotherapy; whereas the other 82% were on polytherapy. Seizures stopped at a median age of 5 years 5 months. The interictal electroencephalogram showed slow/sharp waves, and/or biphasic spikes-polyspikes, spike/wave complexes, and an excess of fast activity mainly over the fronto-temporal areas. Epilepsy is a major clinical challenge in patients with idic(15), associated with a poor prognosis in 55%. Frontal lobe seizures are a novel finding. © 2016 Wiley Periodicals, Inc.

Discovery of a substrate selectivity motif in amino acid decarboxylases unveils a taurine biosynthesis pathway in prokaryotes

Taurine, the most abundant free amino acid in mammals, with many critical roles such as neuronal development, had so far only been reported to be synthetized in eukaryotes. Taurine is the major product of cysteine metabolism in mammals, and its biosynthetic pathway consists of cysteine dioxygenase and cysteine sulfinic acid decarboxylase (hCSAD). Sequence, structural, and mutational analyses of the structurally and sequentially related hCSAD and human glutamic acid decarboxylase (hGAD) enzymes revealed a three residue substrate recognition motif (X1aa19X2aaX3), within the active site that is responsible for coordinating their respective preferred amino acid substrates. Introduction of the cysteine sulfinic acid (CSA) motif into hGAD (hGAD-S192F/N212S/F214Y) resulted in an enzyme with a >700 fold switch in selectivity toward the decarboxylation of CSA over its preferred substrate, l-glutamic acid. Surprisingly, we found this CSA recognition motif in the genome sequences of several marine bacteria, prompting us to evaluate the catalytic properties of bacterial amino acid decarboxylases that were predicted by sequence motif to decarboxylate CSA but had been annotated as GAD enzymes. We show that CSAD from Synechococcus sp. PCC 7335 specifically decarboxylated CSA and that the bacteria accumulated intracellular taurine. The fact that CSAD homologues exist in certain bacteria and are frequently found in operons containing the recently discovered bacterial cysteine dioxygenases that oxidize l-cysteine to CSA supports the idea that a bona fide bacterial taurine biosynthetic pathway exists in prokaryotes.

Structural characterization of the mechanism through which human glutamic acid decarboxylase auto-activates

Imbalances in GABA (γ-aminobutyric acid) homoeostasis underlie psychiatric and movement disorders. The ability of the 65 kDa isoform of GAD (glutamic acid decarboxylase), GAD65, to control synaptic GABA levels is influenced through its capacity to auto-inactivate. In contrast, the GAD67 isoform is constitutively active. Previous structural insights suggest that flexibility in the GAD65 catalytic loop drives enzyme inactivation. To test this idea, we constructed a panel of GAD65/67 chimaeras and compared the ability of these molecules to auto-inactivate. Together, our data reveal the important finding that the C-terminal domain of GAD plays a key role in controlling GAD65 auto-inactivation. In support of these findings, we determined the X-ray crystal structure of a GAD65/67 chimaera that reveals that the conformation of the catalytic loop is intimately linked to the C-terminal domain.

A case of isodicentric chromosome 15 presented with epilepsy and developmental delay

We report a case of isodicentric chromosome 15 (idic(15) chromosome), the presence of which resulted in uncontrolled seizures, including epileptic spasms, tonic seizures, and global developmental delay. A 10-month-old female infant was referred to our pediatric neurology clinic because of uncontrolled seizures and global developmental delay. She had generalized tonic-clonic seizures since 7 months of age. At referral, she could not control her head and presented with generalized hypotonia. Her brain magnetic resonance imaging scans and metabolic evaluation results were normal. Routine karyotyping indicated the presence of a supernumerary marker chromosome of unknown origin (47, XX +mar). An array-comparative genomic hybridization (CGH) analysis revealed amplification from 15q11.1 to 15q13.1. Subsequent fluorescence in situ hybridization analysis confirmed a idic(15) chromosome. Array-CGH analysis has the advantage in determining the unknown origin of a supernumerary marker chromosome, and could be a useful method for the genetic diagnosis of epilepsy syndromes associated with various chromosomal aberrations.

Mouse mutagenesis and disease models for neuropsychiatric disorders

In this chapter, mutant mouse resources which have been developed by classical genetics as well as by modern large-scale mutagenesis projects are summarized. Various spontaneous and induced mouse mutations have been archived since the rediscovery of Mendel's genetics in 1900. Moreover, genome-wide, large-scale mutagenesis efforts have recently been expanding the available mutant mouse resources. Forward genetics projects using ENU mutagenesis in the mouse were started in the mid-1990s. The widespread adoption of reverse genetics, using knockouts and conditional mutagenesis based on gene-targeting technology, followed. ENU mutagenesis has now evolved to provide a further resource for reverse genetics, with multiple point mutations in a single gene and this new approach is described. Researchers now have various options to obtain mutant mice: point mutations, transgenic mouse strains, and constitutional or conditional knockout mice. The established mutant strains have already contributed to modeling human diseases by elucidating the underlying molecular mechanisms as well as by providing preclinical applications. Examples of mutant mice, focusing on neurological and behavioral models for human diseases, are reviewed. Human diseases caused by a single gene or a small number of major genes have been well modeled by corresponding mutant mice. Current evidence suggests that quantitative traits based on polygenes are likely to be associated with a range of psychiatric diseases, and these are now coming within the range of modeling by mouse mutagenesis.

The inv dup (15) or idic (15) syndrome (Tetrasomy 15q)

The inv dup(15) or idic(15) syndrome displays distinctive clinical findings represented by early central hypotonia, developmental delay and intellectual disability, epilepsy, and autistic behaviour. Incidence at birth is estimated at 1 in 30,000 with a sex ratio of almost 1:1. Developmental delay and intellectual disability affect all individuals with inv dup(15) and are usually moderate to profound. Expressive language is absent or very poor and often echolalic. Comprehension is very limited and contextual. Intention to communicate is absent or very limited. The distinct behavioral disorder shown by children and adolescents has been widely described as autistic or autistic-like. Epilepsy with a wide variety of seizure types can occur in these individuals, with onset between 6 months and 9 years. Various EEG abnormalities have been described. Muscle hypotonia is observed in almost all individuals, associated, in most of them, with joint hyperextensibility and drooling. Facial dysmorphic features are absent or subtle, and major malformations are rare. Feeding difficulties are reported in the newborn period.

Chromosome region 15q11q13, known for its instability, is highly susceptible to clinically relevant genomic rearrangements, such as supernumerary marker chromosomes formed by the inverted duplication of proximal chromosome 15. Inv dup(15) results in tetrasomy 15p and partial tetrasomy 15q. The large rearrangements, containing the Prader-Willi/Angelman syndrome critical region (PWS/ASCR), are responsible for the inv dup(15) or idic(15) syndrome. Diagnosis is achieved by standard cytogenetics and FISH analysis, using probes both from proximal chromosome 15 and from the PWS/ASCR. Microsatellite analysis on parental DNA or methylation analysis on the proband DNA, are also needed to detect the parent-of-origin of the inv dup(15) chromosome. Array CGH has been shown to provide a powerful approach for identifying and detecting the extent of the duplication. The possible occurrence of double supernumerary isodicentric chromosomes derived from chromosome 15, resulting in partial hexasomy of the maternally inherited PWS/ASCR, should be considered in the differential diagnosis. Large idic(15) are nearly always sporadic. Antenatal diagnosis is possible. Management of inv dup(15) includes a comprehensive neurophysiologic and developmental evaluation. Survival is not significantly reduced.

The inv dup(15) or idic(15) syndrome can also be termed "tetrasomy 15q". About 160 patients have been reported in the medical literature [1-5].

Association of the GABRB3 Gene with Nonsyndromic Oral Clefts

Objective:

Nonsyndromic oral clefts are common craniofacial anomalies classified into two subgroups: cleft lip with or without cleft palate and isolated cleft palate. Nonsyndromic oral clefts are multifactorial diseases, with both genetic and environmental factors involved in their pathogenesis. The inhibitory neurotransmitter, γ-aminobutyric acid plays a role in normal embryonic, and particularly facial, development and γ-aminobutyric acid receptor type A β-3 subunit (GABRB3) knockout mice have been shown to have cleft palate. The GABRB3 gene is therefore a strong candidate gene for nonsyndromic oral clefts. We investigated here whether genetic variations of the GABRB3 gene affect the risk for nonsyndromic oral clefts.

Method:

In this case-control study, a total of 178 Japanese patients with cleft lip with or without cleft palate and 374 unrelated controls were recruited and were genotyped for six single nucleotide polymorphisms and a dinucleotide repeat marker of the GABRB3 gene.

Results:

None of the single nucleotide polymorphisms showed complete linkage disequilibrium with other single nucleotide polymorphisms. In a case-control association study with the six-locus haplotype of the gene, TGTGCT haplotype frequency in patients with cleft lip with or without cleft palate was significantly higher than in the controls (corrected p value = .029). None of the alleles of the dinucleotide repeat marker showed significant association with cleft lip with or without cleft palate.

Conclusions:

Our data suggest that the GABRB3 gene is involved in the pathogenesis of cleft lip with or without cleft palate in the Japanese population.

GABA production by glutamic acid decarboxylase is regulated by a dynamic catalytic loop

Gamma-aminobutyric acid (GABA) is synthesized by two isoforms of the pyridoxal 5'-phosphate-dependent enzyme glutamic acid decarboxylase (GAD65 and GAD67). GAD67 is constitutively active and is responsible for basal GABA production. In contrast, GAD65, an autoantigen in type I diabetes, is transiently activated in response to the demand for extra GABA in neurotransmission, and cycles between an active holo form and an inactive apo form. We have determined the crystal structures of N-terminal truncations of both GAD isoforms. The structure of GAD67 shows a tethered loop covering the active site, providing a catalytic environment that sustains GABA production. In contrast, the same catalytic loop is inherently mobile in GAD65. Kinetic studies suggest that mobility in the catalytic loop promotes a side reaction that results in cofactor release and GAD65 autoinactivation. These data reveal the molecular basis for regulation of GABA homeostasis.

The inv dup(15) or idic(15) syndrome: a clinically recognisable neurogenetic disorder

The chromosome region 15q11q13 is known for its instability, and many rearrangements may occur in this imprinted segment: deletions associated either with Angelman syndrome (AS) or with Prader-Willi syndrome (PWS), according to parental origin; translocations; inversions; and supernumerary marker chromosomes formed by the inverted duplication of proximal chromosome 15. Inv dup(15) constitute the most common of the heterogeneous group of the extra structurally abnormal chromosomes, and their presence results in tetrasomy 15p and partial tetrasomy 15q. Inv dup(15), containing the Prader-Willi/Angelman syndrome region, are associated with altered behaviour, developmental delay/mental retardation, and seizures/epilepsy. Clinicians should suspect this syndrome in any infant/child with early central hypotonia, minor dysmorphic features, developmental delay, autism or autistic-like behaviour, and who subsequently develops hard to control seizures/epilepsy. Diagnosis is confirmed by standard cytogenetic techniques and FISH analysis. Although, about 100 cases have been reported to date, limited data are available on the natural history. To obtain better information on diagnosis and outcome in a clinical setting, we reviewed the available literature on clinical and behavioural phenotype of inv dup(15) syndrome.

GABAA receptor beta3 subunit gene-deficient heterozygous mice show parent-of-origin and gender-related differences in beta3 subunit levels, EEG, and behavior

The homozygous knockout mouse for the beta3 subunit of the GABAA receptor has been proposed as a model for the neurodevelopmental disorder, Angelman syndrome, based on phenotypic similarities of craniofacial abnormalities, cognitive defects, hyperactivity, motor incoordination, disturbed rest-activity cycles, and epilepsy. Since most children with Angelman syndrome are autosomal heterozygotes of maternal origin, apparently through genomic imprinting, we used gabrb3-deficient heterozygote mice of defined parental origin to investigate whether this phenotype is also maternally imprinted in mouse. Whole brain extracts showed greatly reduced beta3 subunit levels in male mice of maternal origin but not in male mice of paternal origin. Females of both parental origin showed greatly reduced beta3 subunit levels. Heterozygotes did not exhibit hyperactive circling behavior, convulsions, or electrographically recorded seizures. EEGs showed qualitative differences among heterozygotes, with male mice of maternal origin demonstrating more abnormalities including increased theta activity. Ethosuximide inhibited theta bursts, suggesting an alteration in the thalamocortical relay. Carbamazepine induced EEG slowing in males and EEG acceleration in females, with a larger effect in paternal-origin heterozygotes. Evidence thus suggests both parent-of-origin and gender-related components in developmental regulation of beta3 expression, in particular, that the maternally-derived male heterozygote may carry a developmental modification resulting in less beta3 protein, which may reflect partial genomic imprinting of the gabrb3 gene in mice.

Conserved function of medaka pink-eyed dilution in melanin synthesis and its divergent transcriptional regulation in gonads among vertebrates

Medaka is emerging as a model organism for the study of vertebrate development and genetics, and its effectiveness in forward genetics should prove equal to that of zebrafish. Here, we identify by positional cloning a gene responsible for the medaka i-3 albino mutant. i-3 larvae have weakly tyrosinase-positive cells but lack strongly positive and dendritic cells, suggesting loss of fully differentiated melanophores. The region surrounding the i-3 locus is syntenic to human 19p13, but a BAC clone covering the i-3 locus contained orthologs located at 15q11-13, including OCA2 (P). Medaka P consists of 842 amino acids and shares approximately 65% identity with mammalian P proteins. The i-3 mutation is a four-base deletion in exon 13, which causes a frameshift and truncation of the protein. We detected medaka P transcripts in melanin-producing eyeballs and (putative) skin melanophores on embryos and an alternatively spliced form in the non-melanin-producing ovary or oocytes. The mouse p is similarly expressed in gonads, but not alternatively spliced. This is the first isolation of nonmammalian P, the functional mechanism of action of which has not yet been elucidated, even in mammals. Further investigation of the functions of P proteins and the regulation of their expression will provide new insight into body color determination and gene evolution.

GABAA receptors: building the bridge between subunit mRNAs, their promoters, and cognate transcription factors

The type A gamma-aminobutyric acid (GABA(A)) receptors mediate the majority of fast inhibitory neurotransmission in the CNS, and alterations in GABA(A) receptor function is believed to be involved in the pathology of several neurological and psychiatric illnesses, such as epilepsy, anxiety, Alzheimer's disease, and schizophrenia. GABA(A) receptors can be assembled from eight distinct subunit families defined by sequence similarity: alpha(1-6), beta(1-3), gamma(1-3), delta, pi, theta, and rho(1-3). The regulation of GABA(A) receptor function in the brain is a highly compensating system, influencing both the number and the composition of receptors at the cell surface. While transcriptional and translational points of control operate in parallel, it is becoming increasingly evident that many functional changes in GABA(A) receptors reflect the differential gene regulation of its subunits. The fact that certain GABA(A) receptor subunit genes are transcribed in distinct cell types during specific periods of development strongly suggests that genetic control plays a major role in the choice of subunit variants available for receptor assembly. This review focuses on the physiological conditions that alter subunit mRNA levels, the promoters that may control such levels, and the use of a conceptual framework created by bioinformatics to study coordinate and independent GABA(A) receptor subunit gene regulation. As this exciting field moves closer to identifying the language hidden inside the chromatin of GABA(A) receptor subunit gene clusters, future experiments will be aimed at testing models generated by computational analysis with biologically relevant in vivo and in vitro assays. It is hoped that through this functional genomic approach there will be the identification of new targets for therapeutic intervention.

Angelman syndrome: etiology, clinical features, diagnosis, and management of symptoms

It is estimated that Angelman syndrome (AS) accounts for up to 6% of all children presenting with severe mental retardation and epilepsy. The main clinical features of AS may not be apparent early in life. Clinical findings present in all patients include developmental delay, which becomes apparent by 6-12 months of age, severely impaired expressive language, ataxic gait, tremulousness of limbs, and a typical behavioral profile, including a happy demeanor, hypermotoric behavior, and low attention span. Seizures, abnormal electroencephalography, microcephaly, and scoliosis are observed in >80% of patients. Approximately 70% of patients show a deletion involving the maternally inherited chromosome 15q11-q13, encompassing a cluster of gamma-aminobutyric acid receptor subunit genes, 3% show chromosome 15 paternal uniparental disomy (UPD), 1% harbor a mutation in the imprinting center (a transcriptional regulatory element), and 6% harbor intragenic mutations of the ubiquitin-protein ligase E3A (UBE3A) gene. Twenty percent of patients have no detectable genetic abnormality. Rare cases of familial recurrence of AS show either imprinting center (IC) or UBE3A mutations. Approximately 75% of cases are detected through the methylation test, which allows the detection of AS due to deletions, UPD and IC mutations. Mutation analysis of the UBE3A gene should be performed when the methylation test is negative. Individuals with chromosome 15q11-q13 deletions have a more severe clinical picture and are more prone to develop severe epilepsy. Epilepsy has typical features, including absence and myoclonic seizures, and insidious episodes of nonconvulsive or subtle myoclonic status which are easily overlooked as children appear apathetic or in a state of neurologic regression. Tremulousness, present in all patients even when seizures are well controlled or absent, is related to distal cortical myoclonus. Valproic acid (sodium valproate), benzodiazepines, and ethosuximide, in various combinations, are quite effective in treating the typical seizure types. Piracetam may help in reducing distal myoclonus. Carbamazepine and vigabatrin may seriously aggravate absence and myoclonic seizures and should be avoided. Cognitive, language, and orthopedic problems must be addressed with vigorous rehabilitation programs, including early physical therapy, which may help to develop communicative skills and prevent severe scoliosis and subsequent immobility. Where these treatment strategies are applied, individuals with AS may reach an appreciable level of integration, self care, and have a normal life span.

ANALYSIS OFGABAARECEPTORFUNCTION ANDDISSECTION OF THEPHARMACOLOGY OFBENZODIAZEPINES ANDGENERALANESTHETICSTHROUGHMOUSEGENETICS

GABAA receptors are molecular substrates for the regulation of vigilance, anxiety, muscle tension, epileptogenic activity, and memory functions, and the enhancement of GABAA receptor-mediated fast synaptic inhibition is the basis for the pharmacotherapy of various neurological and psychiatric disorders. Two kinds of GABAA receptor-targeted mutant mice have been generated: (a) knockout mice that lack individual GABAA receptor subunits (alpha1, alpha5, alpha6, beta2, beta3, gamma2, delta, and rho1) and (b) knockin mice that carry point mutations affecting the action of modulatory drugs [alpha1(H101R), alpha2(H101R), alpha3(H126R), alpha5(H105R), and beta3(N265M)]. Whereas the knockout mice have provided information primarily with respect to the regulation of subunit gene transcription, receptor assembly, and some physiological functions of individual receptor subtypes, the point-mutated knockin mice in which specific GABAA receptor subtypes are insensitive to diazepam or some general anesthetics have revealed the specific contribution of individual receptor subtypes to the pharmacological spectrum of diazepam and general anesthetics.

Nonneuronal expression of the GABA(A) beta3 subunit gene is required for normal palate development in mice

Cleft palate is one of the most common birth defects in humans, in which both genetic and environmental factors are involved. In mice, loss of the GABA(A) receptor beta3 subunit gene (Gabrb3) or the targeted mutagenesis of the GABA synthetic enzyme (Gad1) leads to cleft palate. These observations indicate that a GABAergic system is important in normal palate development. To determine what cell types, neuronal or nonneuronal, are critical for GABA signaling in palate development, we used the neuron-specific enolase promoter to express the beta3 subunit in Gabrb3 mutant mice. Expression of this construct was able to rescue the neurological phenotype, but not the cleft palate phenotype. Combined with the previous observation demonstrating that ubiquitous expression of the beta3 subunit rescued the cleft palate phenotype, a nonneuronal GABAergic system is implicated in palate development. Using immunohistochemistry, we detected GABA in the developing palate, initially in the nasal aspect of palatal epithelium of the vertical shelves; later in the medial edge epithelium of the horizontally oriented palatal shelves and in the epithelial seam during fusion. Based on these observations, we propose that GABA, synthesized by the palatal epithelium, acts as a signaling molecule during orientation and fusion of the palate shelves.

Peripheral markers of the gamma-aminobutyric acid (GABA)ergic system in Angelman's syndrome

It has recently been demonstrated that patients with Angelman's syndrome who exhibited a deletion on cytogenetic tests show more severe clinical pictures with drug-resistant epilepsy than patients with Angelman's syndrome not carrying the deletion. To verify if this difference in clinical severity can be attributed to genes for the three gamma-aminobutyric acid (GABA)A receptor subunits (GABRB3, GABRA5, GABRG3) located in the deleted region, a possible modification of peripheral markers of the GABAergic system was investigated in 12 subjects with Angelman's syndrome and 20 age-matched subjects (8 with idiopathic epilepsy and 12 not affected by neurologic diseases). The results confirmed a more severe clinical picture, and epilepsy syndrome in particular, in Angelman's syndrome patients with deletions versus patients without deletions. In contrast, biochemical study (based on dosage of plasma levels of GABA and diazepam binding inhibitor, an endogenous ligand of GABAA and peripheral benzodiazepine receptors, showed contradictory results: patients with Angelman's syndrome showed significantly higher levels of GABA and diazepam binding inhibitor than patients without neurologic impairment but significantly lower levels than epileptic controls.

Mild generalized epilepsy and developmental disorder associated with large inv dup(15)

PURPOSE

Several studies attempted to clarify the genotype-phenotype correlations in patients with inverted duplication of chromosome 15 [inv dup(15)], which is usually characterized by severe mental retardation and epilepsy in individuals with large duplications including the Prader-Willi/Angelman region. We report two patients with inv dup(15) who, in spite of a large duplication, had a mild phenotype including adult-onset epilepsy. This report may help to define the milder spectrum of the syndrome.

METHODS

A 25-year-old girl with mild mental retardation had a 6-year history of absence seizures, with occasional head drop. Interictal EEG revealed diffuse spike-wave complexes. Epilepsy was well controlled by a combination of lamotrigine (LTG) and valproate (VPA). The other patient, a 27-year-old man with mild mental retardation, had a 5-year history of rare generalized tonic-clonic seizure during sleep, and frequent episodes of unresponsiveness, which appeared to be atypical absence seizures on video-EEG recordings. A combination of VPA and LTG led to a remarkable improvement, although no complete control.

RESULTS

Molecular analysis revealed a large inv dup15 in both patients.

CONCLUSIONS

The discrepancy between the mild phenotype and the severe chromosomal abnormality detected in these two patients further supports the notion that the site of breakpoint might be contributory to the inv dup(15) phenotype. Inv dup(15) should be considered in atypical cases of generalized epilepsy of adult onset without clear-cut etiology.

GABA and GABA receptors in the central nervous system and other organs

Gamma-aminobutyrate (GABA) is a major inhibitory neurotransmitter in the adult mammalian brain. GABA is also considered to be a multifunctional molecule that has different situational functions in the central nervous system, the peripheral nervous system, and in some nonneuronal tissues. GABA is synthesized primarily from glutamate by glutamate decarboxylase (GAD), but alternative pathways may be important under certain situations. Two types of GAD appear to have significant physiological roles. GABA functions appear to be triggered by binding of GABA to its ionotropic receptors, GABA(A) and GABA(C), which are ligand-gated chloride channels, and its metabotropic receptor, GABA(B). The physiological, pharmacological, and molecular characteristics of GABA(A) receptors are well documented, and diversity in the pharmacologic properties of the receptor subtypes is important clinically. In addition to its role in neural development, GABA appears to be involved in a wide variety of physiological functions in tissues and organs outside the brain.

Chromosomal abnormalities and epilepsy: a review for clinicians and gene hunters

PURPOSE

We analyzed databases on chromosomal anomalies and epilepsy to identify chromosomal regions where abnormalities are associated with clinically recognizable epilepsy syndromes. The expectation was that these regions could then be offered as targets in the search for epilepsy genes.

METHODS

The cytogenetic program of the Oxford Medical Database, and the PubMed database were used to identify chromosomal aberrations associated with seizures and/or EEG abnormalities. The literature on selected small anomalies thus identified was reviewed from a clinical and electroencephalographic viewpoint, to classify the seizures and syndromes according to the current International League Against Epilepsy (ILAE) classification.

RESULTS

There were 400 different chromosomal imbalances described with seizures or EEG abnormalities. Eight chromosomal disorders had a high association with epilepsy. These comprised: the Wolf-Hirschhorn (4p-) syndrome, Miller-Dieker syndrome (del 17p13.3), Angelman syndrome (del 15q11-q13), the inversion duplication 15 syndrome, terminal deletions of chromosome 1q and 1p, and ring chromosomes 14 and 20. Many other segments had a weaker association with seizures. The poor quality of description of the epileptology in many reports thwarted an attempt to make precise karyotype-phenotype correlations.

CONCLUSIONS

We identified certain chromosomal regions where aberrations had an evident association with seizures, and these regions may be useful targets for gene hunters. New correlations with specific epilepsy syndromes were not revealed. Clinicians should continue to search for small chromosomal abnormalities associated with specific epilepsy syndromes that could provide important clues for finding epilepsy genes, and the epileptology should be rigorously characterized.

Pervasive developmental disorders and GABAergic system in patients with inverted duplicated chromosome 15

Pervasive developmental disorders are characterized by severe, pervasive impairment in several areas of development, with distorted communication skills and stereotypical behavior. Pervasive developmental disorders have a heterogeneous etiology related to brain damage, familial affective psychopathology, chromosomal abnormalities, or dysfunction of neuromodulators. Recently, it has been suggested that the GABRB3 gene, located within chromosome 15q11-13, is a candidate for pervasive developmental disorder. In inverted duplicated chromosome 15 syndrome, in which there is a small marker chromosome derived from inversion and duplication of the chromosome 15q11-q13 region, all patients present with pervasive developmental disorder. To further investigate a possible involvement of the gamma-aminobutyric acid (GABA)ergic system in the inverted duplicated chromosome 15 syndrome, we evaluated plasma levels of GABA and diazepam binding inhibitor in 6 patients with inverted duplicated chromosome 15 and in 8 subjects not affected by neurologic disease. Our findings do not seem to support this hypothesis as no significant differences were found in the GABA and diazepam binding inhibitor plasma levels between patients with inverted duplicated chromosome 15 and controls, but we must consider the possibility that a genetic abnormality of the GABA(A) receptor could be present in patients with inverted duplicated chromosome 15 and still not be reflected in an alteration in either GABA or diazepam binding inhibitor levels in plasma.

Ribozyme-mediated reduction of the GABA(A) receptor alpha1 subunit

As an approach to understanding the role of the alpha1 subunit of the GABA(A) receptor, ribozymes were designed to reduce expression of this subunit protein by hydrolysis of alpha1 subunit message and antisense inactivation. The ribozyme cleavage sites were selected through homology comparison of all known murine GABA(A) receptor subunits at the amino acid and nucleotide sequence level. Two ribozymes were designed and synthesized: one against the extracellular domain and the other against the cytoplasmic domain. These ribozymes were cloned in a mammalian expression plasmid, pZeoSV2 (+). Cleavage of both extracellular and cytoplasmic domain transcripts by the respective ribozymes was observed when each ribozyme was tested against in vitro transcribed mRNA. The stable cell line, 122, expressing recombinant human GABA(A) alpha1, beta2 and gamma2S subunits of receptor was stably transfected with the cytoplasmic domain ribozyme (cy) alone and with both the cytoplasmic (cy) and extracellular domain (ex) ribozyme expression plasmids. Northern analysis showed a 55-60% reduction of alpha1 mRNA in clones of cells transfected with either the single ribozyme (Cy) or with both ribozymes (EC). The alpha1 protein level was reduced 75% in a stable Cy clone and more than 90% in a stable EC clone when compared with alpha1 expression in 122 cells and the vector transfected (Zeo) cells. Electrophysiological analysis revealed that the GABA(A) receptor properties were very similar in 122 cells and in stable clones in which the subunit protein expression had been greatly reduced. No significant difference was detected in the potentiation of the receptor response by either bretazenil or zolpidem. These data demonstrate the efficacy of the ribozyme approach in dramatically reducing GABA(A) subunit protein levels in transfected cells and identify those elements that will be important to the application of similar ribozymes to knock-down transmitter receptor subunit proteins under inducible promoters in transgenic mice.

Ion channel diseases of the central nervous system

In the last decade, advances in molecular genetics and cellular electrophysiology have increased our understanding of ion channel function. A number of diseases termed "channelopathies" have been discovered that are caused by ion channel dysfunction. Channelopathies can be caused by autoimmune, iatrogenic, toxic or genetic mechanisms. Mutations in genes encoding ion channel proteins that disrupt channel function are now the most commonly identified cause of channelopathies, perhaps because gene disruption is readily detected by the methods of molecular genetics. Ion channels are abundant in the central nervous system (CNS), but CNS channelopathies are rare; however, they overlap with some important neurological disorders, such as epilepsy, ataxia, migraine, schizophrenia, Alzheimer's disease and other neurodegenerative diseases. It is possible that more CNS channelopathies will be discovered when additional ion channels are characterized and the complex mechanisms of brain function are better understood. At present, increased knowledge of the identity, structure and function of ion channels is facilitating diagnosis and treatment of many channelopathies.

Aberrant pH of melanosomes in pink-eyed dilution (p) mutant melanocytes

In past studies, we cloned the mouse p gene and its human homolog P, which is associated with oculocutaneous albinism type 2. Both mouse and human genes are expressed in melanocytes and encode proteins predicted to have 12 membrane-spanning domains with structural homology to known ion transporters. We have also demonstrated that the p protein is localized to the melanosomal membrane and does not function as a tyrosine transporter. In this study, immunohistochemistry and confocal microscopy were used to show that the p protein plays an important role in the generation or maintenance of melanosomal pH. Melanosomes (and their precursor compartments) were defined by antiserum directed against the melanosomal marker tyrosinase related protein 1. Acidic vesicles were identified by 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine incorporation, visualized with anti-dinitrophenol. In C57BL/6+/+ (wild-type) melanocytes, 94.2% of vesicles demonstrated colocalization of tyrosinase related protein 1 and 3-(2, 4-dinitroanilino)-3'-amino-N-methyldipropylamine, indicating that almost all melanosomes or their precursors were acidic. By contrast, only 7%-8% of the staining vesicles in p mutant cell lines (pJ/pJ and pcp/p6H) showed colocalization of tyrosinase related protein 1 and 3-(2,4-dinitroanilino)-3'-amino-N-methyldipropylamine. Thus, without a functional p protein, most melanosomes and their precursors are not acidic. As mammalian tyrosinase activity in situ is apparently dependent on low pH, we postulate that in the absence of a low pH environment brought about by ionic transport mediated by the p protein, tyrosinase activity is severely impaired, leading to the minimal production of melanin that is characteristic of p mutants. Additionally (or alternatively), an abnormal pH may also impair the assembly of the normal melanogenic complex.

Domain-restricted expression of two glutamic acid decarboxylase genes in midgestation mouse embryos

Glutamic acid decarboxylase (GAD) is the biosynthetic enzyme for gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in the central nervous system (CNS) of vertebrates. In addition to the adult CNS, GABA and GAD also have been detected in embryos, although their precise localization and specific functions in embryonic development have not been elucidated. In this paper, the authors studied the cellular distribution of two GAD isoforms, GAD65 and GAD67, in midgestation mouse embryos by in situ hybridization histochemistry. With few exceptions, it was found that GAD65 and GAD67 mRNAs are localized in overlapping cellular domains of the embryonic CNS that later develop into regions with a strong GABAergic contribution. The GAD-expressing cells are situated in the differentiating zone of the embryonic day 10.5 (E10.5) through E11.5 CNS and in the subventricular zone and the mantle zone of the E12.5 CNS, which suggests that they are committed neuronal precursors. By using a specific serum for GABA, a similar pattern of distribution was obtained, indicating that GAD mRNAs are translated efficiently into enzymatically active GAD, which produces embryonic GABA. The expression domains of GAD overlap with those of genes that are known to be involved in the patterning of the embryonic CNS. The two GAD mRNAs also are detected outside of the embryonic CNS in various cell types, mainly those of placodal and neural crest origin. This pattern of expression is consistent with the notion that GAD and its product, GABA, play a signaling role during development.

Sox6 is a candidate gene for p100H myopathy, heart block, and sudden neonatal death

The mouse p locus encodes a gene that functions in normal pigmentation. We have characterized a radiation-induced mutant allele of the mouse p locus that is associated with a failure-to-thrive syndrome, in addition to diminished pigmentation. Mice homozygous for this mutant allele, p(100H), show delayed growth and die within 2 wk after birth. We have discovered that the mutant mice develop progressive atrioventricular heart block and significant ultrastructural changes in both cardiac and skeletal muscle cells. These observations are common characteristics described in human myopathies. The karyotype of p(100H) chromosomes indicated that the mutation is associated with a chromosome 7 inversion. We demonstrate here that the p(100H) chromosomal inversion disrupts both the p gene and the Sox6 gene. Normal Sox6 gene expression has been examined by Northern blot analysis and was found most abundantly expressed in skeletal muscle in adult mouse tissues, suggesting an involvement of Sox6 in muscle maintenance. The p(100H) mutant is thus a useful animal model in the elucidation of myopathies at the molecular level.

New insights into the mechanism of action of hypnotics

Between 1987 and 1989, the different protein subunits that make up the receptor for the inhibitory neurotransmitter gamma-aminobutyric acid (GABA) were identified. These make up the alpha, beta, gamma and delta families, for each of which exist several subtypes. This receptor is the molecular target of modern hypnotic drugs (i.e. benzodiazepines, zopiclone, zolpidem and zaleplon). In the 10 years that have followed this milestone, significant progress has been made in exploring the molecular mechanisms of hypnotic drug action. Receptor subtype specificity of hypnotics has been explained in terms of differential affinity for receptors containing different alpha subunits, which are expressed in different brain regions. Zolpidem and zaleplon bind preferentially to alpha1-containing receptors, whereas benzodiazepines and zopiclone are aspecific. Different sets of subunits are encoded in contiguous 'cassettes' on the genome, and the transcription of each set appears to be regulated coherently. The predominant GABA(A) receptor composition found in the brain is alpha1beta2gamma2, which are all encoded on human chromosome 5. Targeted gene disruption has provided clues to the physiological functions served by GABA(A) receptors containing different subunits. Receptors containing gamma2 appear to have a vital role in maintaining appropriate central inhibition, beta3-containing receptors may also be important determinants of excitability in certain brain regions, whereas a clear role for alpha5-, alpha6- and gamma3-containing receptors has not yet been established by these techniques. Site-directed mutagenesis has indicated that benzodiazepines bind to a cleft on the GABA(A) receptor surface at the interface between the alpha and gamma subunits. Other drugs (flumazenil, zopiclone, zolpidem) also bind to the a subunit, but interact with amino acids in different binding domains to the benzodiazepines. The molecular mechanism of hypnotic dependence has been explored, and seems to involve downregulation of transcription of the normally prevalent alpha1, beta2 and gamma2 subunits, and the reciprocal upregulation of the expression of rarer subunits. Chronic treatment with hypnotic drugs that may have less dependence potential, such as zopiclone and zolpidem, appears to produce more limited change in GABA(A) receptor subunit expression. These ideas will be important both for designing new hypnotic drugs with a better safety/efficacy profile, and for evaluating more appropriate ways of using the drugs available today.

Possible association between childhood absence epilepsy and the gene encoding GABRB3

BACKGROUND

Childhood Absence Epilepsy (CAE) is considered to have a predominantly, perhaps exclusively, genetic background. To date, genes responsible for susceptibility to CAE have not been identified. The object of the present study was to test association between CAE and the genes encoding the gamma-aminobutyric acid (GABA) type-A receptor subunits alpha 5 (GABRA5) and beta 3 (GABRB3) located on the long arm of chromosome 15 (15q11-q13).

METHODS

A family-based candidate gene approach was applied: 50 Austrian nuclear families ascertained for the presence of an affected child were investigated. GABRA5 and GABRB3 subunit genes were genotyped using DNA gained from peripheral blood samples by Polymerase Chain Reactions (PCR). Genetic association was tested using a Monte Carlo Version of the multi-allele Transmission-Disequilibrium Test (TDT).

RESULTS

The TDT displayed significant overall association with GABRB3 (p = .0118).

CONCLUSIONS

The present data suggest that the tested polymorphism may be either directly involved in the etiology of CAE or in linkage disequilibrium with disease-predisposing sites.

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 (γ-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 GABAAreceptor 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 GABAA receptor subunits α2 (Gabra2), β1 (Gabrb1), and γ1 (Gabrg1) on mouse chromosome 5. Rat cDNA probes and specific sequence probes for all three GABAA 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, theGabrg1–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 GABAA gene cluster on mouse chromosome 5 and the corresponding region on human chromosome 4.

The sequence data described in this paper have been submitted to the GenBank/GSS data libraries under accession nos.AF156490 and AQ589406–AQ589436.

An update on GABAA receptors

Recent advances in molecular biology and complementary information derived from neuropharmacology, biochemistry and behavior have dramatically increased our understanding of various aspects of GABAA receptors. These studies have revealed that the GABAA receptor is derived from various subunits such as alpha1-alpha6, beta1-beta3, gamma1-gamma3, delta, epsilon, pi, and rho1-3. Furthermore, two additional subunits (beta4, gamma4) of GABAA receptors in chick brain, and five isoforms of the rho-subunit in the retina of white perch (Roccus americana) have been identified. Various techniques such as mutation, gene knockout and inhibition of GABAA receptor subunits by antisense oligodeoxynucleotides have been used to establish the physiological/pharmacological significance of the GABAA receptor subunits and their native receptor assemblies in vivo. Radioligand binding to the immunoprecipitated receptors, co-localization studies using immunoaffinity chromatography and immunocytochemistry techniques have been utilized to establish the composition and pharmacology of native GABAA receptor assemblies. Partial agonists of GABAA receptors are being developed as anxiolytics which have fewer and less severe side effects as compared to conventional benzodiazepines because of their lower efficacy and better selectivity for the GABAA receptor subtypes. The subunit requirement of various drugs such as anxiolytics, anticonvulsants, general anesthetics, barbiturates, ethanol and neurosteroids, which are known to elicit at least some of their pharmacological effects via the GABAA receptors, have been investigated during the last few years so as to understand their exact mechanism of action. Furthermore, the molecular determinants of clinically important drug-targets have been investigated. These aspects of GABAA receptors have been discussed in detail in this review article.

D2 dopamine receptor and GABA(A) receptor beta3 subunit genes and alcoholism

As the dopaminergic and GABAergic systems have been implicated in alcohol-related behaviors, variants of the D2 dopamine receptor (DRD2) and GABA(A) receptor beta3 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 x 10(-5)) and frequency (P = 1.6 x 10(-5)) of the DRD2 minor (A1) allele. Moreover, a significant progressive increase was observed in A1 allelic prevalence (P = 3.1 x 10(-6)) and frequency (P = 2.7 x 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 x 10(-3)) and frequency (P = 2.7 x 10(-2)) of the GABRB3 major (G1) allele. Furthermore, a significant progressive decrease was noted in G1 allelic prevalence (P = 2.4 x 10(-3)) and frequency (P = 1.9 x 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.

Expression of alpha3, beta3 and gamma1 GABA(A) receptor subunit messenger RNAs in visual cortex and lateral geniculate nucleus of normal and monocularly deprived monkeys

Complementary RNA probes derived from complementary DNA specifically subcloned from monkey tissue were used to localize, by in situ hybridization histochemistry, the relatively rare alpha3, beta3 and gamma1 subunit transcripts of the GABA(A) receptor in visual cortex and lateral geniculate nucleus of normal monkeys and in monkeys that had been deprived of vision in one eye. Overall, levels of alpha3, beta3 and gamma1 subunit transcripts were very low. In the primary visual cortex (area 17) they were concentrated in layers II and VI and in a stratum of white matter subjacent to layer VI. The localization and density of the three messenger RNAs closely resembled those of other rare (alpha2, alpha5 and beta1) transcripts but their distribution also overlapped that of the predominant alpha1, beta2 and gamma2 subunit transcripts. In area 18, alpha3 and beta3 transcript distribution resembled that in area 17, with the addition of a third band of hybridization in layer IV for beta3. Gamma1 subunit transcript localization in area 18 differed significantly from that in area 17, with increased expression restricted to layer IV. In the dorsal lateral geniculate nucleus, beta3 and gamma1 transcripts were expressed at low levels across all layers while alpha3 transcripts were restricted to the magnocellular layers. Following 15 and 18 day periods of monocular deprivation, induced by intravitreal injections of tetrodotoxin, levels of alpha3 receptor subunit transcripts showed modest reductions in layer VI of area 17 and in deprived geniculate laminae of adult animals. Reductions in alpha3 transcript levels were much more pronounced in layer IVCbeta of a five-month-old monkey deprived for the same time. Levels of beta3 and gamma1 transcripts were unaffected by monocular deprivation in cortex and geniculate at any age. Taken together with studies of other GABA(A) receptor transcripts, these results demonstrate the heterogeneity of GABA(A) receptor messenger RNA expression in the monkey geniculo-striate pathway and the varied response to reduced neuronal activity.

Rat and human hippocampal alpha5 subunit-containing gamma-aminobutyric AcidA receptors have alpha5 beta3 gamma2 pharmacological characteristics

The gamma-aminobutyric acid (GABA)A receptor is a hetero-oligomer consisting of five subunits, the combination of which confers unique pharmacological properties to the receptor. To understand the physiological role of native GABAA receptors, it is critical to determine their subunit compositions. The pharmacological characteristics of human alpha5 beta3 gamma2 and alpha5beta3gamma3 GABAA receptors stably expressed in L(tk-) cells were characterized with the alpha5-selective ligand [3H]L-655,708 and compared with the pharmacological characteristics of [3H]L-655,708 binding sites from rat and human hippocampus. Saturation analyses revealed a 9-fold selective affinity of [3H]L-655,708 for alpha5 beta3 gamma2 receptors (Kd = 1.7 +/- 0.4 nM), compared with alpha5 beta3 gamma3 receptors (Kd = 15 +/- 3 nM). Rat and human hippocampal [3H]L-655,708 binding sites had affinities of 2.2 +/- 0.6 and 1.0 +/- 0.2 nM, respectively, comparable to the affinity of alpha5 beta3 gamma2 receptors. Pharmacological analysis of [3H]L-655,708 binding sites in rat and human hippocampi revealed a strong correlation with the affinities of seven benzodiazepine site ligands for alpha5 beta3 gamma2 but not alpha5 beta3 gamma3 receptors. Immunoprecipitation of [3H]L-655,708 binding sites from rat hippocampus with a gamma2-selective antibody yielded 19 +/- 4% of total benzodiazepine binding sites measured using [3H]Ro15-1788, whereas no specific binding was measured after immunoprecipitation with an anti-gamma3 antibody. Combinatorial immunoprecipitations of [3H]muscimol binding sites with anti-alpha5 and anti-gamma2 or anti-alpha5 and anti-gamma3 antibodies established the preferential expression of alpha5 gamma2 receptors, accounting for 22 +/- 2% of total rat hippocampal GABAA receptors. These observations provide pharmacological and structural evidence for the prevalence of alpha5 beta3 gamma2 GABAA receptors in rat hippocampus, despite the clustering of alpha5 and gamma3 loci on the same chromosome.

GABA(A)-receptor assembly in vivo: lessons from subunit mutant mice

The rules governing the assembly of GABA(A) receptors in vivo were assessed in subunit mutant mice. The transcription of individual subunit genes was regulated independently. The lack of a particular subunit did not result in a molecular rescue by an enhanced transcription of other subunits. In addition, the availability of an alpha- and beta-subunit was essential for receptor formation. Finally, highly selective recognition processes directed the subcellular targeting of receptors. The loss of a particular receptor subtype (alpha5) did not lead to a subcellular redistribution of the remaining subtype (alpha2) present in the same cell.

GABAA-receptor alpha-subunit is an essential prerequisite for receptor formation in vivo

The mechanisms governing the assembly of alpha-, beta- and gamma-subunits to form GABAA-receptors are poorly understood. Here, we report that the alpha-subunit is essential for receptor assembly. In mice homozygous for a deletion on chromosome 7 spanning the alpha 5- and gamma 3-subunit genes, zolpidem-insensitive benzodiazepine binding sites, corresponding to GABAA-receptors containing the alpha 5-subunit, were absent in the hippocampus. This loss of alpha 5-GABAA-receptor binding was also apparent as a 21% decrease in the total number of benzodiazepine binding sites in the hippocampus. In addition, immunoreactivity for the beta 2,3- and gamma 2-subunit was decreased exclusively in neurons which normally express the alpha 5-subunit, such as olfactory bulb granule cells and hippocampal pyramidal cells. In other brain regions of the mutants, the beta 2,3- and gamma 2-subunit staining was unaffected. Controls included two lines of mice homozygous for a shorter chromosomal deletion, that either included or excluded the gamma 3-subunit gene. These two lines were indistinguishable with regard to numbers of benzodiazepine binding sites and levels alpha 5-, beta 2,3- and gamma 2-subunit immunoreactivity, indicating that the lack of gamma 3-subunit gene did not contribute to the observed deficit in receptor formation. These results demonstrate that the absence of the alpha 5-subunit gene prevents the formation of the entire respective receptor complex in adult mouse brain. Thus, the alpha-subunit, unlike the gamma 2-subunit, might play a major role in the assembly or targeting of GABAA-receptor complexes.

Mice devoid of gamma-aminobutyrate type A receptor beta3 subunit have epilepsy, cleft palate, and hypersensitive behavior

gamma-Aminobutyric acid type A receptors (GABA(A)-Rs) mediate the bulk of rapid inhibitory synaptic transmission in the central nervous system. The beta3 subunit is an essential component of the GABA(A)-R in many brain regions, especially during development, and is implicated in several pathophysiologic processes. We examined mice harboring a beta3 gene inactivated by gene targeting. GABA(A)-R density is approximately halved in brain of beta3-deficient mice, and GABA(A)-R function is severely impaired. Most beta3-deficient mice die as neonates; some neonatal mortality, but not all, is accompanied by cleft palate. beta3-deficient mice that survive are runted until weaning but achieve normal body size by adulthood, although with reduced life span. These mice are fertile but mothers fail to nurture offspring. Brain morphology is grossly normal, but a number of behaviors are abnormal, consistent with the widespread location of the beta3 subunit. The mice are very hyperactive and hyperresponsive to human contact and other sensory stimuli, and often run continuously in tight circles. When held by the tail, they hold all paws in like a ball, which is frequently a sign of neurological impairment. They have difficulty swimming, walking on grids, and fall off platforms and rotarods, although they do not have a jerky gait. beta3-deficient mice display frequent myoclonus and occasional epileptic seizures, documented by electroencephalographic recording. Hyperactivity, lack of coordination, and seizures are consistent with reduced presynaptic inhibition in spinal cord and impaired inhibition in higher cortical centers and/or pleiotropic developmental defects.

The GABAA receptor alpha 6 subunit gene (Gabra6) is tightly linked to the alpha 1-gamma 2 subunit cluster on mouse chromosome 11

We have established that the GABAA receptor alpha 6 (Gabra6) and alpha 1 (Gabra 1) subunit genes are tightly linked on mouse chromosome 11 by analysing the strain distribution patterns of RFLPs for the two genes and microsatellite markers flanking these genes in 26 BXD recombinant inbred strains. These results further demonstrate clustering of the GABAA receptor subunit genes on mouse chromosomes and the synteny for these clusters between the mouse and human genomes.

Structure and organization of GABRB3 and GABRA5

The genes encoding the gamma-aminobutyric acid (GABA) type-A receptor subunits beta 3 (GABRB3), alpha 5 (GABRA5), and gamma 3 (GABRG3) map to chromosome 15q11-q13. The three genes are contained within roughly 800 kb of the distal part of the imprinted Prader-Willi and Angelman syndrome region. A 570-kb contig encompassing GABRB3 and GABRA5 has been constructed in P1, lambda phage, and PAC clones. GABRB3 spans 250 kb of DNA and is organized into 9 exons that range from 68 to 504 bp, while GABRA5 is encoded by 11 exons (65 to 924 bp in length) within 86 kb. The exon/intron borders for both genes have been characterized and, primers have been designed to amplify each of the individual exons. Two reference STR markers have been positioned in the contig. The reference STR for GABRB3 is in fact located at least 60 kb beyond the 3' terminus of GABRB3, while D15S97 is contained within intron 4 of GABRB3. The detailed physical map of this GABAA receptor subunit gene cluster should not only be useful in genetic studies of the 15q11-q13 region, but will also be important for investigating the evolution and expression of the GABAA receptor gene superfamily.

Albinism: an update

Albinism connotes a large group of genetic disorders that are characterized by diminished ocular and oftentimes cutaneous pigmentation. These disorders are generally subclassified as oculocutaneous albinism (OCA) or ocular albinism (OA) based on the extent of their effects on the pigmentation of the skin and hair. Sometimes, different mutations in the same gene can cause OCA or OA.

Genetic approaches to CNS disorders with particular reference to GABAA-receptor mutations

The tools of molecular biology will bring the field of human genetics into a new era by permitting the analysis of the genetic contribution to disease. Most single gene disorders, inherited in a Mendelian fashion, will be molecularly diagnosed. In addition, the genetic susceptibility of common, complex diseases such a schizophrenia can be clarified, even though the conditions are not inherited as Mendelian characteristics. The mapping of the human genome will increase the rate at which new disease genes are identified and isolated. Finally, the development of genetically engineered animal models will help to dissect the steps involved in physiological and pathophysiological processes and thereby enhance our understanding of complex biological systems.

Genes for Prader Willi syndrome/Angelman syndrome and fragile X syndrome are homologous, with genetic imprinting and unstable trinucleotide repeats causing mental retardation, autism and aggression

Genes for Prader Willi syndrome/Angelman syndrome are homologous to genes for fragile X syndrome. Genetic imprinting and expanded trinucleotide repeats cause mental retardation, autism and aggression.

Cortical myoclonus in Angelman syndrome

Angelman syndrome (AS) results from lack of genetic contribution from maternal chromosome 15q11-13. This region encompasses three GABAA receptor subunit genes (beta3, alpha5, and gamma3). The characteristic phenotype of AS is severe mental retardation, ataxic gait, tremulousness, and jerky movements. We studied the movement disorder in 11 AS patients, aged 3 to 28 years. Two patients had paternal uniparental disomy for chromosome 15, 8 had a >3 Mb deletion, and 1 had a microdeletion involving loci D15S10, D15S113, and GABRB3. All patients exhibited quasicontinuous rhythmic myoclonus mainly involving hands and face, accompanied by rhythmic 5- to 10-Hz electroencephalographic (EEG) activity. Electromyographic bursts lasted 35 +/- 13 msec and had a frequency of 11 +/- 2.4 Hz. Burst-locked EEG averaging in 5 patients, generated a premyoclonus transient preceding the burst by 19 +/- 5 msec. A cortical spread pattern of myoclonic cortical activity was observed. Seven patients also demonstrated myoclonic seizures. No giant somatosensory evoked potentials or C-reflex were observed. The silent period following motor evoked potentials was shortened by 70%, indicating motor cortex hyperexcitability. Treatment with piracetam in 5 patients significantly improved myoclonus. We conclude that spontaneous, rhythmic, fast-bursting cortical myoclonus is a prominent feature of AS.

Localisation of two candidate genes for mental retardation using a YAC physical map of the Xq21.1-21.2 subbands

Genetic studies in families with X linked mental retardation have suggested the location of several MR genes in the human q21 region. Since the establishment of cloned resources is an essential step towards the cloning of genes involved in inherited diseases, we built a yeast artificial chromosome (YAC) contig and an STS map of this part of the X chromosome. The contig, which extends from PGK1 in Xq13.3 to DXS1002 in Xq21.2, consists of 30 YACs mapped with 21 markers and spans about 6 Mb. The YAC contig was used as a framework to localise several previously known genes and CEPH/Genethon polymorphic markers, as well as to construct a physical map of the region surrounding one of these genes. We recently localised a presumed MR locus to the region flanked by DXS233 (proximal) and CHM (distal). In the present work, the zinc finger gene, ZNF6, has been shown to lie within this region and to be highly expressed in brain, making it a good candidate MR gene. Similarly the VDAC1 gene has been mapped between DXS986 and DXS72 and its candidate gene status for the Allan-Herndon-Dudley syndrome is discussed.

Chronic treatment with diazepam or abecarnil differently affects the expression of GABAA receptor subunit mRNAs in the rat cortex

Diazepam and abecarnil produce their overt effects by interaction with the GABAA receptor. Chronic treatment with abecarnil, however, does not induce diazepam-like tolerance. This study investigates the effects of chronic diazepam and abecarnil treatment on expression of GABAA receptor alpha 1-6 beta 1-3 and gamma 1-3 subunit isoform mRNAs in rat cortex. Male Sprague-Dawley rats were injected subcutaneously once daily for 7 or 14 days with 15 mg/kg diazepam or 6 mg/kg abecarnil in sesame-oil vehicle, and steady-state levels of GABAA receptor subunit mRNAs were quantified by solution hybridization. The levels of alpha 4- and alpha-, beta 1- and gamma 3-subunit mRNAs were significantly increased after 7 days of diazepam treatment, and this effect was maintained at 14 days. A significant increase in alpha 3-subunit mRNA was apparent only after 14 days of diazepam treatment and a significant decrease in beta 2-subunit mRNA was seen only after 14 days of abecarnil treatment. Gamma 2-Subunit mRNA was significantly decreased after 14 days of either diazepam or abecarnil exposure. A degree of association between a particular drug treatment and changes in the levels of mRNAs arising from a given gene cluster was noted. Our results are consistent with a model of diazepam dependence based on GABAA receptor subunit isoform switching.