Schizophrenia is a complex genetic disorder characterized by profound disturbances of cognition, emotion and social functioning. The median lifetime prevalence of SZ is 0.7%–0.8% 1. The public health importance of SZ is clear. Numerous family, twin, and adoption studies showed conclusively that the risk of schizophrenia was increased among the relatives of affected individuals and that it was the result largely of genes rather than shared environment 2. It's believed that the disease tends to run in families, with an estimated heritability of 80–85% 3. In the children and siblings of individuals with schizophrenia, the increase in risk is approximately 10-fold. Recent decades, many candidate genes have been implicated in the susceptibility of SZ with independent replicative evidences from multiple populations 245.

Following the searching of genetic basis for the mechanism of SZ, some lines of evidences have also emerged from other kinds of biomarkers, such as level of gene expression. Recent advances have facilitated the use of circulating blood to conduct genomic analyses of human disease 67. Vawter et al. 8 identified nine genes that were differentially expressed between SZ patients and controls. Thereafter, by analyzing the blood-derived RNA from 74 samples, linear and non-linear combinations of eight putative biomarker genes (APOBEC3B, ADSS, ATM, CLC, CTBP1, DIDO1, CXCL1, and S100A9) were able to discriminate among schizophrenia, bipolar disorder, and control samples 9, with an overall accuracy of 95%–97%. None of these genes have yet been investigated for their association or linkage disequilibrium with SZ.

Among them, ADSS is down-regulated in the SZ patients. The de novo biosynthesis of AMP from IMP involves two steps, with the first step catalyzed by ADSS followed by adenylosuccinate lyase catalyzing the second step 10. The gene encoding ADSS maps to 1cen-q12, the chromosomal loci previously linked to schizophrenia in meta-analysis 1112. The sequence of ADSS gene is 44 kb in length, with 13 extrons and 12 introns.

However, it is unknown whether the alteration in ADSS expression is due to defect of the gene, or secondary to other disease-related factors. In order to address this issue, and to test the hypothesis that sequence variations of ADSS gene influence the risk for the disease, we conducted a case-control association study on 6 SNPs (rs3102460, rs3127459, rs3127460, rs3127465, rs3006001, and rs3003211) within the gene in a Chinese Han Population.

Although genome-wide scan for linkage in multiply-affected families with schizophrenia has consumed lots of money and effort, no forms of schizophrenia following Mendelian inheritance patterns have yet been discovered 2. Numerous evidences show that multiple genes of small effect contribute to increasing liability to SZ, which seems to have a cumulative effect, and putatively, threshold effect. On the other hand, the study on schizophrenia has confirmed the vital role of genes in its etiology, but has not so far identified the relationship between observed genetic risks and specific DNA variants, protein alterations, or biological processes 18, nor was diagnostic neuropathology, or even unequivocally replicated associations with the same alleles or haplotype within each gene identified 4.

Meta-analyses of SZ linkage studies have provided evidence of susceptibility in chromosomes 2p, 5q, 3p, 11q, 6p, 1q, 22q, 8p, 20q, 22q, and 14p 212, some of which are strong enough to withstand rigorous correction for whole genome analysis (e.g. 1q, 6p, 6q, 15q) 19. Recently, many polymorphisms located in regions of chromosome 1 were reported as susceptibility loci for SZ 520, including risk markers in 1q44 21 1q43 22, both of which lie adjacent to ADSS.

Although whole genome linkage scan and functional studies have implied several very promising positional candidates, such as NRG1 23, DTNBP1 24, COMT 4, DISC1 25, DAOA 2627, RGS4 28 and so on 52328293031, there may be several genes that play subtle or weak roles in the pathogenesis of SZ, making it difficult to investigate by traditional approaches. To tackle this problem, another approach to finding potential candidate genes is using microarray technology to examine RNA of a gene which shows differential expression between patients and controls, by which ADSS gene was suggested as one of candidates for SZ.

Functionally, the ADSS catalyzes the key step in the synthesis of AMP, which influences the energy metabolism through the purine nucleotide cycle (PNC) and the AMPK (AMP-activated protein kinase) pathway 32. The reaction is as follows: IMP+L-aspartate+GTP ↔ adenylosuccinate+GDP+phosphate. Genetically, ADSS gene resides in the susceptibility loci and the blood-derived RNA of ADSS was among the eight genes with putatively differential diagnostic power for SZ. Hereto, there is no genetic association study of ADSS with SZ, and the mechanism of hypo-expression of ADSS is elusive, nevertheless, it suggested that ADSS gene was possibly involved in the genetic architecture of SZ, for genetic variation or polymorphisms can dominate gene expression as well as confer disease risk to human beings.

Our study is an exploration study to seek the trait's genetic basis following a clue from RNA alterations in SZ. However, the data did not display any statistically significant difference for both the allelic and genotypic distribution between cases and controls. Although haplotype analysis is a powerful tool for identifying candidate genes for complex disease, there was no positive finding in current study. Therefore, our result does not support the association of ADSS gene with SZ. It implies that down-regulation of ADSS in patients may not result from the gene's polymorphisms.

One common issue in the study of complex diseases is the limited sample size, resulting in inadequate power to detect association. With regard to our data, assuming the frequency of risk allele in controls to be 0.5, our sample of 480 cases and 502 controls is able to detect an odds ratio (OR) of 1.43 or above with 80% power 33. It is possible that false-negative results may arise from lack of power. Another issue of researches in multifactor diseases is the underlying high degree of genetic heterogeneity of perplexing phenotype. SZ is not simply defined by several major genes but rather evolves from addition or potentiation of a specific cluster of genes, which subsequently determine the genetic vulnerability of an individual. Thus, there are likely different genes or sets of genes associated with schizophrenia disorders in different populations 34. It is necessary to validate or replicate our association results using independent samples especially from other ethnic populations.

It's been suggested that the promoter and 5' flanking region of murine ADSS gene possessed regulatory elements that could influence its expression 35. In addition, ADSS has several splicing variants. Therefore, the examinations of SNPs within 5' flanking region and those potentially affecting splicing may be valuable in further study.

In conclusion, our data did not support ADSS gene as a susceptibility gene for SZ in Chinese Han population. Further study on genetic regulation, neural network, or epigenetics will facilitate to elucidate the role of ADSS in SZ.

APOBEC3B: apolipoprotein B mRNA editing enzyme, catalytic polypeptide-like 3B; ATM: ataxia telangiectasia mutated; CLC: Charcot-Leyden crystal protein; COMT: catechol-O-methyltransferase; CTBP1: C-terminal binding protein 1; CXCL1: chemokine (C-X-C motif) ligand 1; DAOA: D-amino acid oxidase activator; DIDO1: death inducer-obliterator 1; DISC1: disrupted in schizophrenia 1; DTNBP1: dystrobrevin binding protein 1; NRG1: neuregulin 1; RGS4: regulator of G-protein signaling 4; S100A9: S100 calcium binding protein A9.

The authors declare that they have no competing interests.

SPC designed and coordinated the study. ZFQ, XY, and SYQ drafted the manuscript. FH, SHC and HXZ participate in data management and statistical analysis. LPZ participated in the supervision of the project. All authors read and approved the final manuscript.