Mitochondrial genome variations and functional characterization in Han Chinese families with schizophrenia
Introduction
Schizophrenia is a world-wide prevalent psychiatric disorder showing almost 80% heritability (Cannon et al., 1998). There are many genome-wide association studies (GWAS) aiming to investigate the risk genes for schizophrenia, and a very recent meta-analysis of the previous GWAS identified a total of 108 susceptibility loci for schizophrenia (Schizophrenia Working Group of the Psychiatric Genomics Consortium, 2014). However, the GWAS-identified common variants could only account for 10–30% of the variability of schizophrenia (Gunter, 2009), and the genetic risk factors for schizophrenia remain largely unknown.
The role of mitochondrial dysfunction and mitochondrial DNA (mtDNA) variants in schizophrenia has been strongly debated (Shao et al., 2008, Verge et al., 2010). Over the past decade, many studies have reported mitochondrial dysfunction in schizophrenia patients (Arvindakshan et al., 2003, Ben-Shachar and Karry, 2008, Dror et al., 2002, Karry et al., 2004, Kung and Roberts, 1999, Maurer et al., 2001, Munakata et al., 2005, Prabakaran et al., 2004, Washizuka et al., 2009). The types of dysfunction have included OXPHOS deficiency and altered expression of mitochondria related genes. This has led to the hypothesis that mitochondrial dysfunction has an important role in the causation of schizophrenia.
Since mtDNA is maternally inherited and an excess of maternal transmission of the disease has been observed in previous studies of schizophrenia (DeLisi et al., 2000, Gottesman and Bertelsen, 1989, Ichikawa et al., 2012, Verge et al., 2012), it is appropriate to conduct a comprehensive evaluation of the influence of mtDNA variants on schizophrenia. There are already several reports showing mtDNA sequence variations in patients with schizophrenia (Bamne et al., 2008, Bandelt et al., 2005, Lindholm et al., 1997, Mamdani et al., 2014, Marchbanks et al., 2003, Martorell et al., 2006, Rollins et al., 2009, Sequeira et al., 2012, Sequeira et al., 2015, Ueno et al., 2009), but no consistent results have been found and most of these studies were focused on sporadic patients rather than on maternally inherited pedigrees. Furthermore, functional characterization of the mtDNA variants in patients with schizophrenia was rarely considered, partly because the genetic code of mtDNA is different from the universal code and we lack a robust system to fulfill the aim. The allotopic expression system has been used to investigate the function of mtDNA variants in previous studies (Ellouze et al., 2008, Kaltimbacher et al., 2006), in which the mtDNA gene was converted to a nuclear-encoded version, and protein was first expressed in the cytosol and then was imported into mitochondria under the control of mitochondrial leader peptides.
In order to test the effect of mtDNA variants in schizophrenia, we conducted a comprehensive mtDNA analysis of the probands of 11 families with a maternal transmission appearance of schizophrenia. We identified 11 nonsynonymous private variants, one rRNA private variant and one tRNA private variant in 5 of the 11 probands. Web-based searches and in silico program affiliated analysis showed that some of these private variants were very likely to be pathogenic. Functional characterization further confirmed the potential adverse functioning of these variants.
Section snippets
Subjects
Subjects from eleven pedigrees (ten from Hunan Province and one from Anhui Province) were recruited in this study. Schizophrenia was regarded as being maternally inherited in the pedigree based on two criteria: (1) each family had at least three maternally related patients with schizophrenia; (2) none of the non-maternal members had schizophrenia or other psychiatric disease (Supplementary Table S1). The schizophrenia patients were diagnosed independently by two psychiatrists according to
mtDNA genome sequence variations in Han Chinese patients with schizophrenia
In our recent studies to identify schizophrenia patients from Hunan and other provinces in China (Ma et al., 2014, Zhang et al., 2014), we were able to recruit eleven families, each of which had at least three maternally related members affected by schizophrenia. Analysis of the complete mtDNA sequences for the 11 probands from these families showed that they belonged to 10 different haplogroups, all common in East Asians (Kong et al., 2006) (Fig. 1). There was no overall statistically
Discussion
Schizophrenia is a major psychosis showing almost 80% heritability (Gottesman and Bertelsen, 1989). Genetic association studies based on common polymorphisms, even using a high throughput genotyping platform, only account for a little of the heritability (Gunter, 2009) and the missing heritability of schizophrenia has not been well explained (Crow, 2011, Girard et al., 2011, Ma et al., 2013). mtDNA may be a good target for consideration because the brain is the biggest energy consumer of human
Role of the funding source
The work was supported by the National Natural Science Foundation of China (31171225 and 31000557) and the Strategic Priority Research Program (B) of the Chinese Academy of Sciences (XDB02020003).
Contributors
Rui Bi, Wen Zhang and Shi-Yi Chen carried out the molecular genetic studies, participated in the sequence alignment and performed the statistical analysis. Rui Bi, Dandan Yu and Xiao Li carried out the functional assays. Jinsong Tang and Xiaogang Chen collected the samples. Rui Bi, Wen Zhang, Yong-Gang Yao and Xiaogang Chen drafted the manuscript. Yong-Gang Yao and Xiaogang Chen participated in the design of the study. All authors read and approved the final manuscript.
Conflict of interest
The authors declare that there is no conflict of interest.
Acknowledgments
We are grateful for the participants in this study. We thank Dr. Corral-Debrinski for their kind donation of plasmid pCMV-Tag 4A with the ATP6 gene and Mr. Ian Logan for language editing. We thank Dr. Ling Xu, Dr. Yu Fan, Mr. Jia-Qi Feng, Miss Hui-Zhen Wang, Miss Qiu-Xiang Hu and Dr. Hao Guo for technical assistance. The work was supported by the National Natural Science Foundation of China (31171225 and 31000557) and the Strategic Priority Research Program (B) of the Chinese Academy of
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