Association study of polymorphisms in synaptic vesicle-associated genes, SYN2 and CPLX2, with schizophrenia

Schizophrenia is a severe, chronic mental illness affecting 0.5–1.5% of the general population worldwide [1]. The contribution of genetic factors to the vulnerability to schizophrenia has been well established by family, twin, and adoption studies that have suggested a significant heritability of approximately 50–70% [2]. Many studies have attempted to identify the allelic variants that confer susceptibility to the illness, but no single genes have been identified that produce a major effect on the vulnerability [3].

Recently the synaptic hypothesis of schizophrenia has gained attention by attributing the fundamental pathology of schizophrenia to the dysfunction of synaptic transmission involving various molecules [4]. Synapsins, a family of synaptic vesicle-associated phosphoproteins, play a crucial role in the regulation of neurotransmission, synaptogenesis, and neuronal plasticity [5]. Three human synapsin genes have been identified (SYN1, 2, and 3; OMIM 313440, 600755, and 602705) [6]. Complexin 1 and complexin 2, which are encoded by CPLX1 (OMIM 605032) and CPLX2 (OMIM 605033), respectively, and are also called synaphins, are pre-synaptic membrane proteins that preferentially bind to syntaxin within the SNARE (soluble N-ethylmaleimide-sensitive fusion attachment protein receptors) complex. These proteins are important regulators of transmitter release immediately preceding vesicle fusion [7]. Previous studies have demonstrated that the concentrations of synapsins and complexins are reduced in the brains of schizophrenics [8, 9]. The expression levels of both synapsins were significantly decreased in the hippocampal tissue of schizophrenic patients [10]. The levels of synapsin 2 and complexin 2 mRNA were also significantly reduced in the prefrontal cortex, cerebellum, and hippocampus of schizophrenics [11–14].

SYN2 was mapped to chromosome 3p25 [15], and CPLX2 is located on chromosome 5q35.3 (OMIM 605033). These loci were identified as potential regions conferring susceptibility to schizophrenia in diverse populations [16–18]. Based on their localization, well-established neurobiological roles, and expression patterns in schizophrenic patients, we selected SYN2 and CPLX2 as candidate genes for conferring susceptibility to schizophrenia. In this report, we present an association study of SYN2 and CPLX2 with schizophrenia using 12 polymorphisms in the Korean population.

In this study, we observed significant, pairwise haplotype associations with schizophrenia for two pairs of SNPs in SYN2 (SYN2-1 – SYN2-2 and SYN2-2 – SYN2-4; P _corr = 9.35 × 10^-5 and P _corr = 0.019, respectively) and one pair of SNPs in CPLX2 (CPLX2-1 – CPLX2-2, P _corr = 0.009) (Table 3, 5). The three-way haplotype (SYN2-1, SYN2-2, and SYN2-4) also showed a significant association with schizophrenia (P _corr = 1.1 × 10^-5). The SYN2-1 and CPLX2-1 SNPs are located in the respective promoter regions, -98 and -156. SYN2-1 was located within the GC box motif and CPLX2-1 within the C/EBP motif in a database search http://www.cbrc.jp/research/db/TFSEARCH. The positive haplotype associations seem to be based on an increase of LD in the schizophrenia group compared to the control group because the D' values of the schizophrenia group were higher than those of the controls [(SYN2-1 – SYN2-2, 0.935 vs. 0.531 (schizophrenics vs. controls)), (SYN2-2 – SYN2-4, 0.750 vs. 0.453)] (Table 3). A similar situation was also observed with the positive association of the haplotype in CPLX2 with schizophrenia [CPLX2-1 – CPLX2-2, 0.852 vs. 0.412 (schizophrenics vs. controls)] (Table 5).

Chen et al. [23] recently reported an association study of four SNPs in SYN2 using Han Chinese samples. They found significant associations of SNP rs795009 and a haplotype constructed by the four SNPs with schizophrenia. Chen et al. [23] and our study examined two SNPs (rs2308169 and rs308963) in common, and their genotypic and allelic frequencies were similar in both studies. Although Chen et al. [23] did not mention the pairwise haplotype association study that we performed, they did report a significant difference in the overall four-way haplotype frequencies between schizophrenics and controls. Since two independent studies have reported a significant haplotype association of SYN2 with schizophrenia, this gene is probably involved in the pathogenesis of schizophrenia.

Several studies have suggested that the decreased expression of synaptic genes is characteristic of schizophrenia. In the hippocampus of schizophrenic patients, several studies have shown a consistent pattern of decreases in presynaptic proteins and their encoding mRNAs, such as synapsin 2, synaptophysin, and synaptosomal-associated protein-25 (SNAP-25) [8–10, 24]. Furthermore, a reduction in the synapsin 2 mRNA levels was observed in the prefrontal cortex of schizophrenic patients [14], but controvertible results have also been reported [25]. The altered expression levels of other presynaptic proteins, complexin 1 and complexin 2, have been reported in schizophrenic patients [11–13]. Interestingly, complexin 1 is enriched in axosomatic regions, inhibitor neurons, and their synapses, while complexin 2 is enriched in the axodendritic terminals [9, 26]. The differential expression of complexins 1 and 2 implies their involvement in the excitatory synapse in the hippocampus of schizophrenic patients [11]. These observations suggest that abnormal expression of SYN2 and CPLX2 may cause the vulnerability to schizophrenia by altering neurotransmitter release and neuroplasticity.

We found significant differences in the haplotype frequencies in both SYN2 and CPLX2 polymorphisms between schizophrenia and control groups. In addition, the haplotype constructed from three polymorphisms (SYN2-1, SYN2-2, and SYN2-4) showed a significant association with schizophrenia. Our results suggest that both SYN2 and CPLX2 polymorphisms may contribute susceptibility to schizophrenia in the Korean population.