Pharmacogenetic Analysis of Functional Glutamate System Gene Variants and Clinical Response to Clozapine

Abstract

Altered glutamate neurotransmission is implicated in the etiology of schizophrenia (SCZ) and the pharmacogenetics of response to clozapine (CLZ), which is the drug of choice for treatment-resistant SCZ. Response to antipsychotic therapy is highly variable, although twin studies suggest a genetic component. We investigated the association of 10 glutamate system gene variants with CLZ response using standard genotyping procedures. GRM2 (rs4067 and rs2518461), SLC1A2 (rs4354668, rs4534557, and rs2901534), SLC6A9 (rs12037805, rs1978195, and rs16831558), GRIA1 (rs2195450), and GAD1 (rs3749034) were typed in 163 European SCZ/schizoaffective disorder patients deemed resistant or intolerant to previous pharmacotherapy. Response was assessed following 6 months of CLZ monotherapy using change in Brief Psychiatric Rating Scale (BPRS) scores. Categorical and continuous response variables were analyzed using χ² tests and analysis of covariance, respectively. We report no significant associations following correction for multiple testing. Prior to correction, nominally significant associations were observed for SLC6A9, SLC1A2, GRM2, and GRIA1. Most notably, CC homozygotes of rs16831558 located in the glycine transporter 1 gene (SLC6A9) exhibited an allele dose-dependent improvement in positive symptoms compared to T allele carriers (p_uncorrected = 0.008, p_corrected = 0.08). To clarify the role of SLC6A9 in clinical response to antipsychotic medication, and CLZ in particular, this finding warrants further investigation in larger well-characterized samples.

Keywords: Clozapine; Glutamate; Glycine transporter 1 (SLC6A9); Pharmacogenetics; Schizophrenia.

Fig. 1

HaploReg database results for 6 putatively functional variants included in this study [55]. To be considered for study inclusion, variants were required to have a minor allele frequency of ≥5% and to satisfy ≥3 of 7 functionality criteria. Chr = Chromosome; pos = position; LD = linkage disequilibrium; r² = correlation coefficient; D′ = Hedrick's multiallelic D′; Ref = reference allele; Alt = alternate allele; AFR/AMR/ASN/EUR freq = African/admixed American/Asian/European minor allele frequency; GERP/SiPhy cons = genomic evolutionary rate profiling/SiPhy mammalian conservation algorithms; DNAse = DNAse I hypersensitivity site; eQTL = expression quantitative trait loci; dbSNP func annot = dbSNP functional annotation.

Fig. 2

Chromosome location, size, and schematic representation of the GRM2, SLC1A2, SLC6A9, GRIA1, and GAD1 genes. The SNPs genotyped in this study are shown. Dark boxes represent coding exons and light boxes represent 5′ and 3′ untranslated regions. The gene diagrams were constructed using the NCBI Reference Sequence from Homo sapiens, transcript variant 1. Alternatively spliced variants exist for all 5 genes and can be viewed on NCBI AceView [64].

Fig. 3

GRM2, SLC6A9, and SLC1A2 LD plots (Haploview v4.2). Haplotype blocks defined using Solid Spine of LD. The confidence bounds color scheme is used. Dark gray: strong evidence of LD; light gray: uninformative; white: strong evidence of recombination. Correlation coefficient values as percentages (r²) are displayed within each box.

Fig. 4

C allele carriers of the SLC6A9 variant rs16831558 experienced an allele dose-dependent reduction in BPOS scores following 6 months of CLZ therapy (p_uncorrected = 0.008, p_corrected = 0.08, assuming 9.36 independent tests); however, this finding did not remain significant following correction for multiple testing.