EAAT2 regulation and splicing: relevance to psychiatric and neurological disorders

The excitatory amino acid transporter 2 (EAAT2) is responsible for the majority of glutamate uptake in the brain and its dysregulation has been associated with multiple psychiatric and neurological disorders. However, investigation of this molecule has been complicated by its complex pattern of alternative splicing, including three coding isoforms and multiple 5'- and 3'-UTRs that may have a regulatory function. It is likely that these sequences permit modulation of EAAT2 expression with spatial, temporal and or activity-dependent specificity; however, few studies have attempted to delineate the function of these sequences. Additionally, there are problems with the use of antibodies to study protein localization, possibly due to posttranslational modification of critical amino acid residues. This review describes what is currently known about the regulation of EAAT2 mRNA and protein isoforms and concludes with a summary of studies showing dysregulation of EAAT2 in psychiatric and neurological disorders. EAAT2 has been either primarily or secondarily implicated in a multitude of neuropsychiatric diseases in addition to the normal physiology of learning and memory. Thus, this molecule represents an intriguing therapeutic target once we improve our understanding of how it is regulated under normal conditions.

Quantitative analysis of glutamate transporter mRNA expression in prefrontal and primary visual cortex in normal and schizophrenic brain

Abnormalities of the glutamatergic system in schizophrenia have been identified in numerous studies, but little is known about the role of glutamate transporters and their messenger RNA (mRNA) expression. In addition, the abundances of the two major isoforms of human excitatory amino acid transporter 2 (EAAT2) or its rat ortholog, glutamate transporter 1, have never been compared in a quantitative manner. Using quantitative reverse transcription-polymerase chain reaction, we established that the expression of the EAAT1, EAAT2a, EAAT2b, and EAAT3 transcripts was not different in the dorsolateral prefrontal and primary visual cortices of persons with schizophrenia relative to matched controls. EAAT2a expression was about 25-fold and 10-fold higher than EAAT2b in human and rat brain, respectively. The data provided no evidence of an effect of antipsychotic medications on the mRNA expression of the glutamate transporters. However, because most of the schizophrenic subjects in the cohort had been treated with antipsychotics for many years, it is still possible that changes in transporter expression were masked by medication effects.

Fine-scale mapping of a locus for severe bipolar mood disorder on chromosome 18p11.3 in the Costa Rican population

We have searched for genes predisposing to bipolar disorder (BP) by studying individuals with the most extreme form of the affected phenotype, BP-I, ascertained from the genetically isolated population of the Central Valley of Costa Rica (CVCR). The results of a previous linkage analysis on two extended CVCR BP-I pedigrees, CR001 and CR004, and of linkage disequilibrium (LD) analyses of a CVCR population sample of BP-I patients implicated a candidate region on 18p11.3. We further investigated this region by creating a physical map and developing 4 new microsatellite and 26 single-nucleotide polymorphism markers for typing in the pedigree and population samples. We report the results of fine-scale association analyses in the population sample, as well as evaluation of haplotypes in pedigree CR001. Our results suggest a candidate region containing six genes but also highlight the complexities of LD mapping of common disorders.

Genome screening for linkage disequilibrium in a Costa Rican sample of patients with bipolar-I disorder: a follow-up study on chromosome 18

Linkage disequilibrium (LD) methods offer great promise for mapping complex traits, but have thus far been applied sparingly. In this paper we describe an LD mapping study of severe bipolar disorder (BP-I) in the genetically isolated population of the Central Valley of Costa Rica. This study provides the first complete screen of a chromosome for a complex trait using LD mapping and presents the first application of a new LD mapping statistic (ancestral haplotype reconstruction (AHR)) that evaluates haplotype sharing among affected individuals. The results of this chromosome-wide analysis are instructive for genome-wide LD mapping in isolated populations. Furthermore, the analysis continues to support a possible BP-I locus on 18pter, suggested by previous analyses in this population. Evidence for a possible BP-I locus on 18q12.2 is also described.

Assessing the feasibility of linkage disequilibrium methods for mapping complex traits: an initial screen for bipolar disorder loci on chromosome 18

Linkage disequilibrium (LD) analysis has been promoted as a method of mapping disease genes, particularly in isolated populations, but has not yet been used for genome-screening studies of complex disorders. We present results of a study to investigate the feasibility of LD methods for genome screening using a sample of individuals affected with severe bipolar mood disorder (BP-I), from an isolated population of the Costa Rican central valley. Forty-eight patients with BP-I were genotyped for markers spaced at approximately 6-cM intervals across chromosome 18. Chromosome 18 was chosen because a previous genome-screening linkage study of two Costa Rican families had suggested a BP-I locus on this chromosome. Results of the current study suggest that LD methods will be useful for mapping BP-I in a larger sample. The results also support previously reported possible localizations (obtained from a separate collection of patients) of BP-I-susceptibility genes at two distinct sites on this chromosome. Current limitations of LD screening for identifying loci for complex traits are discussed, and recommendations are made for future research with these methods.

Mapping genes for psychiatric disorders and behavioral traits

In the past year, findings from genetic studies in non-human organisms have yielded a number of exciting insights regarding the genetic basis of complex behaviors. Although there were encouraging developments in the genetic study of human behavioral traits, particularly those involved with cognitive function, there was relatively little progress in genetic mapping of the most common psychiatric phenotypes.

A complete genome screen for genes predisposing to severe bipolar disorder in two Costa Rican pedigrees

Bipolar mood disorder (BP) is a debilitating syndrome characterized by episodes of mania and depression. We designed a multistage study to detect all major loci predisposing to severe BP (termed BP-I) in two pedigrees drawn from the Central Valley of Costa Rica, where the population is largely descended from a few founders in the 16th-18th centuries. We considered only individuals with BP-I as affected and screened the genome for linkage with 473 microsatellite markers. We used a model for linkage analysis that incorporated a high phenocopy rate and a conservative estimate of penetrance. Our goal in this study was not to establish definitive linkage but rather to detect all regions possibly harboring major genes for BP-I in these pedigrees. To facilitate this aim, we evaluated the degree to which markers that were informative in our data set provided coverage of each genome region; we estimate that at least 94% of the genome has been covered, at a predesignated threshold determined through prior linkage simulation analyses. We report here the results of our genome screen for BP-I loci and indicate several regions that merit further study, including segments in 18q, 18p, and 11p, in which suggestive lod scores were observed for two or more contiguous markers. Isolated lod scores that exceeded our thresholds in one or both families also occurred on chromosomes 1, 2, 3, 4, 5, 7, 13, 15, 16, and 17. Interesting regions highlighted in this genome screen will be followed up using linkage disequilibrium (LD) methods.

Genetic mapping using haplotype, association and linkage methods suggests a locus for severe bipolar disorder (BPI) at 18q22-q23

Manic depressive illness, or bipolar disorder (BP), is characterized by episodes of elevated mood (mania) and depression. We designed a multistage study in the genetically isolated population of the Central Valley of Costa Rica to identify genes that promote susceptibility to severe BP (termed BPI), and screened the genome ot two Costa Rican BPI pedigrees (McInnes et al., submitted). We considered only individuals who fulfilled very stringent diagnostic criteria for BPI to be affected. The strongest evidence for a BPI locus was observed in 18q22-q23. We tested 16 additional markers in this region and seven yielded peak lod scores over 1.0. These suggestive lod scores were obtained over a far greater chromosomal length (about 40 cM) than in any other genome region. This localization is supported by marker haplotypes shared by 23 of 26 BPI affected individuals studied. Additionally, marker allele frequencies over portions of this region are significantly different in the patient sample from those of the general Costa Rican population. Finally, we performed an analysis which made use of both the evidence for linkage and for association in 18q23, and we observed significant lod scores for two markers in this region.

Mapping genes for psychiatric disorders and behavioral traits

In the past year, some of the most exciting findings in the genetic investigation of mammalian behavior have been obtained through mapping and through gene manipulation studies in the mouse system. These include the localization of a gene for circadian periodicity in the mouse, gene knockouts of serotonin receptors, and the development of a transgenic model of Alzheimer's disease. The recent development of genetic maps covering the entire human genome and the implementation of new approaches to genetic analysis may now facilitate elucidation of complex behaviors in humans, particularly psychiatric disorders.