Comprehensive association analysis of 27 genes from the GABAergic system in Japanese individuals affected with schizophrenia
Introduction
Schizophrenia is a complex psychiatric disorder, manifesting heterogeneous behavioral and cognitive deficits, and afflicting approximately one percent of the global population (Owen et al., 2016). Overwhelming evidence supports neurodevelopmental abnormalities caused by genetic and environmental factors, in schizophrenia pathogenesis (Hashimoto et al., 2008b, Lewis and Levitt, 2002, Schmidt and Mirnics, 2015). In addition to the progressive symptoms seen in individuals affected with schizophrenia, observations of reduced cortical thickness, enlarged ventricles, reductions in gray matter volume, whole-brain volume, white matter anisotropy and a decreased neurogenic: gliogenic competence ratio further underscore neurodevelopmental dysfunction in disease pathology (Bakhshi and Chance, 2015, Harrison, 1999, Lewis and Lieberman, 2000, Toyoshima et al., 2016). In keeping with the neurodevelopment hypothesis, changes in neurodevelopment precede the onset of disease, affecting normal maturation, which in turn influences neuroplastic processes during development (Bakhshi and Chance, 2015).
The gamma-aminobutyric acid (GABA)-ergic system plays a significant role in neurodevelopment, by regulating neural proliferation, migration, differentiation, neuronal connectivity and synaptic activity (Deidda et al., 2014). Gamma-aminobutyric acid is the major inhibitory neurotransmitter in adult brains, and is synthesized from glutamate, by glutamate decarboxylases (GAD65 and GAD67) (Soghomonian and Martin, 1998). Its inhibitory function is mediated through the GABAA receptor, a heteropentameric ligand-gated chloride channel (Jacob et al., 2008). In addition, several receptor associated proteins aid in trafficking, tethering and lateral movement of GABAA receptors on the neuronal surface, thereby regulating GABAergic activity (Luscher et al., 2011).
The disruption of GABAergic function has been implicated in psychiatric diseases, like schizophrenia, for a long time (Gonzalez-Burgos et al., 2011, Nakazawa et al., 2012). Post-mortem studies have consistently showed abnormalities in parvalbumin-positive GABAergic interneurons, as well as altered expression of GABA related genes in the prefrontal cortex of individuals suffering from schizophrenia (Fung et al., 2010, Hoftman et al., 2015, Lewis et al., 2012, Lewis et al., 2005). In addition, impaired working memory, a characteristic feature of schizophrenia, has been attributed to aberrant gamma oscillations stemming from abnormal GABAergic interneuron activity in the prefrontal cortex of individuals affected with schizophrenia (Haenschel et al., 2009). Furthermore, development and maturation of the GABAergic system is also thought to be a convergent point for genetic and environmental susceptibility factors in schizophrenia (Schmidt and Mirnics, 2015).
Although post-mortem and animal studies showed GABAergic deficits in schizophrenia, human genetics data is limited to candidate gene association analysis and hampered by discrepant results (Cherlyn et al., 2010). A recent study showed for the first time that copy number variations (CNVs) are enriched for genes involved in GABAergic neurotransmission in schizophrenia cases (Pocklington et al., 2015). Moreover, results from our previous genome-wide association study (GWAS) in Japanese individuals affected with schizophrenia have shown association signals on the GABAA receptor subunit gene cluster at chromosome 5q34 (Yamada et al., 2011). In this study, we aimed to systematically investigate the role of common genetic variants from the GABAergic system in determining predisposition to schizophrenia. To this end, we performed a comprehensive case-control genetic association study of GABAA receptor subunit genes, and GABA related genes (glutamate decarboxylase genes, GABAergic-marker gene, genes involved in GABA receptor trafficking and scaffolding) in a large cohort of Japanese individuals affected with schizophrenia.
Section snippets
Subjects
The study examined 2926 unrelated Japanese case-control samples, consisting of 1415 individuals with schizophrenia (771 males, 644 females, mean ageĀ Ā±Ā SDĀ =Ā 51.19Ā Ā±Ā 13.68Ā years) and 1511 unrelated healthy controls (514 males, 997 females, mean ageĀ Ā±Ā SDĀ =Ā 44.11Ā Ā±Ā 14.04Ā years). The cohort also included samples from our previous GWAS study (120 probands from Japanese schizophrenia trio samples) (Yamada et al., 2011). Diagnosis of schizophrenia was based on Diagnosis and Statistical Manual of Mental Disorders IV
Allelic and genotypic analyses of 27 genes
A total of 375 SNPs were successfully genotyped after applying quality control (QC) criteria (SNP call rateĀ >Ā 90%, sample call rateĀ >Ā 95%) from the 406 SNPs selected from 27 GABA receptors genes and genes involved in the GABAergic signaling (Supplementary Table 1). One SNP (rs187269) was excluded due to deviation from Hardy Weinberg equilibrium (PĀ <Ā 0.001) and two SNPs in GAD2 gene (rs1330580 and rs11015010) were monomorphic, bringing the total number of SNPs studied to 372 (Supplementary Table 1).
Discussion
Accumulating evidence from post-mortem studies, imaging studies and animal experiments link the GABAergic system to schizophrenia pathogenesis, through altered neurodevelopmental events (Schmidt and Mirnics, 2015). Although several genes associated with GABAergic function are implicated in this process, the contribution of common genetic variants in the GABAergic system to schizophrenia susceptibility is still undefined. In this study, we performed genetic association analysis of common
Funding body agreements and policies
This study was supported in part by the Grant-in-Aid for Scientific Research (S.B.; 26860954, K.Y.; 26293267 and Tak. H.; 25293247) from Japan Society for the Promotion of Science (JSPS), the Grant-in-Aid for Scientific Research on Innovative Areas (Unraveling the microendophenotypes of psychiatric disorders at the molecular, cellular, and circuit levels) (Tak. H.: 15H01280 and T.Y.: 24116002), and the Strategic Research Program for Brain Sciences from Japan Agency for Medical Research and
Contributors
S.B., Ka. Y., Tak. H. and T.Y. designed the experiments. S.B., Ka. Y., C.S. and M.M. analyzed the data. S.B. and T.Y. wrote the manuscript. Y.I. performed the experiments. T.T., S.T., T.W., Y.K., D.K., Ko. Y., M.K., Tas. H. and N.K. coordinated sample recruitment and phenotype assessment. T.Y. supervised all phases of the project. All authors discussed the results and implications and approved the final manuscript.
Conflict of interest
All authors declare that they have no conflict of interest.
Acknowledgments
We thank all the participants of this study. We also thank Professors Mori, Minabe and Iyo, and Dr. Hattori, for their help in recruiting individuals with schizophrenia.
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These authors contributed equally to this study.