Variation in GRM3 affects cognition, prefrontal glutamate, and risk for schizophrenia

Abstract

GRM3, a metabotropic glutamate receptor-modulating synaptic glutamate, is a promising schizophrenia candidate gene. In a familybased association study, a common GRM3 haplotype was strongly associated with schizophrenia (P ‫؍‬ 0.0001). Within this haplotype, the A allele of single-nucleotide polymorphism (SNP) 4 (hCV11245618) in intron 2 was slightly overtransmitted to probands (P ‫؍‬ 0.02). We studied the effects of this SNP on neurobiological traits related to risk for schizophrenia and glutamate neurotransmission. The SNP4 A allele was associated with poorer performance on several cognitive tests of prefrontal and hippocampal function. The physiological basis of this effect was assessed with functional MRI, which showed relatively deleterious activation patterns in both cortical regions in control subjects homozygous for the SNP4 A allele. We next looked at SNP4's effects on two indirect measures of prefrontal glutamate neurotransmission. Prefrontal N-acetylaspartate, an in vivo MRI measure related to synaptic activity and closely correlated with tissue glutamate, was lower in SNP4 AA homozygotes. In postmortem human prefrontal cortex, AA homozygotes had lower mRNA levels of the glial glutamate transporter EAAT2, a protein regulated by GRM3 that critically modulates synaptic glutamate. Effects of SNP4 on prefrontal GRM3 mRNA and protein levels were marginal. Resequencing revealed no missense or splice-site SNPs, suggesting that the intronic SNP4 or related haplotypes may exert subtle regulatory effects on GRM3 transcription. These convergent data point to a specific molecular pathway by which GRM3 genotype alters glutamate neurotransmission, prefrontal and hippocampal physiology and cognition, and thereby increased risk for schizophrenia.

Expression of a GRM3 Splice Variant is Increased in the Dorsolateral Prefrontal Cortex of Individuals Carrying a Schizophrenia Risk SNP

“…We analyzed SNPs that had been previously associated with schizophrenia (rs6465084, rs2228595, and rs1468412), an intron 4 SNP (rs7804100) that is part of the high-risk haplotype in our prior study (Egan et al, 2004), and four additional SNPs that were selected from HapMap as tag SNPs for haplotypes in the 5 0 regulatory region of the gene (rs1527768, rs906415, rs802457, and rs187993; Table 2). DNA was extracted from cerebellar tissue using a protocol by PUREGENE (Gentra Systems, Minneapolis, MN, USA).…”

“…DNA was extracted from cerebellar tissue using a protocol by PUREGENE (Gentra Systems, Minneapolis, MN, USA). Genotyping was performed using the Taqman 5 0 -exonuclease allelic discrimination assay, as described elsewhere (Egan et al, 2004). Reproducibility was routinely assessed by re-genotyping all samples for selected SNPs and was generally 499%.…”

“…The infrequent 'T' allele of SNP 6 (exon 3 Ala293Ala; rs2228595) was previously associated with risk for schizophrenia (Egan et al, 2004;Marti et al, 2002). In the CBDB/NIMH DLPFC collection, individuals carrying the 'T' allele (n ¼ 9, all heterozygotes) had B21% higher expression of GRM3D4 compared to C/C individuals (n ¼ 80) (Mann-Whitney U-test, Z ¼ À2.58, p ¼ 0.008, ANCOVA: F ¼ 5.67, p ¼ 0.008; Figure 3a).…”

“…SNPs or haplotypes in GRM3 have been associated with schizophrenia in several independent samples (Chen et al, 2005;Egan et al, 2004;Fujii et al, 2003;Marti et al, 2002;Schwab et al, 2006), although negative studies have been reported as well (Marti et al, 2002;Norton et al, 2005;Tochigi et al, 2006). Recently, polymorphisms in GRM3 have also been associated with bipolar disorder (Fallin et al, 2005;Green et al, 2006), which shares points of genetic vulnerability with schizophrenia.…”

“…Recently, polymorphisms in GRM3 have also been associated with bipolar disorder (Fallin et al, 2005;Green et al, 2006), which shares points of genetic vulnerability with schizophrenia. Finally, SNPs in GRM3 are associated with cognitive performance in normal control subjects (de Quervain and Papassotiropoulos, 2006;Egan et al, 2004). However, the mechanisms underlying these genetic associations are unclear, since all known common SNPs within GRM3 are either noncoding or synonymous, and thus do not alter the amino-acid sequence of the translated receptor.…”