Searching for susceptibility genes of schizophrenia: positional cloning strategy
Date Issued
2012
Date
2012
Author(s)
Liu, Chih-Min
Abstract
Schizophrenia is a chronic debilitating neuropsychiatric illness. The causes of schizophrenia are still unclear, but family, twin, and adoption studies indicate that it has a strong genetic component and high heritability. It is a complex disorder. The methods for searching the susceptibility genes are positional cloning strategies, including genetic linkage study, candidate gene association study, positional candidate gene association study, and genome-wide association study. This thesis summarized the research results using positional cloning to search for the susceptibility genes of schizophrenia in Taiwanese family samples, and commented on the strength and weakness of different strategies and proposed future perspectives.
As for the genetic linkage study, we took the linkage study on chromosome 8p22-21 in Taiwanese schizophrenia families for example.This study genotyped eleven microsatellite markers on chromosome 8p22-21 in 52 Taiwanese schizophrenic families with at least two affected siblings. Maximum non-parametric linkage scores (NPL score) of 2.45 (p = 0.008) were obtained for the marker D8S1222. The marker D8S1222 was about 400 kbp distal to the exon 1 of glial growth factor 2 (GGF2), an isoform of Neuregulin 1 gene (NRG1), which has been highly suggested to be a candidate gene for schizophrenia. The results provide suggestive linkage evidence of schizophrenia to loci near NRG1 on chromosome 8p21 in an ethnically distinct Taiwanese sample. Further exploration of the candidate gene and nearby chromosome regions is warranted.
As for the candidate gene association study, we took the association study between Dystrobrevin-binding protein 1 (DTNBP1) and schizophrenia in Taiwan. The aim was to examine the association evidence of the candidate gene in 693 Taiwanese families with at least two affected siblings of schizophrenia. We genotyped nine SNPs of this gene with average intermarker distance of 17 kb. Single locus and haplotype association analyses were performed with TRANSMIT program. We found no significant association between schizophrenia and DTNBP1 either through single locus or haplotype analyses. We failed to replicate the association evidence between DTNBP1 and schizophrenia and this gene may not play a major role in the etiology of schizophrenia in this Taiwanese family sample.
As for the positional candidate gene association study, the genome wide linkage study of 557 family samples in Taiwan has revealed linkage evidence of chromosome 10q22 to schizophrenia. We further performed fine mapping study to identify the susceptibility genes in this region. The study sample comes from the original family sample, including 476 Han Taiwanese families having at least two siblings affected with schizophrenia and at least one of the affected siblings has received the Continuous Performance Test (CPT) and Wisconsin Card Sorting Test (WCST).
First, based on the results of the previous linkage analysis, we identified the region from D10S1432 (93.97 cM) to D10S1239 (121.81 cM) as the region with a 95% confidence interval of linkage. Then we selected 18 additional microsatellite markers in this region for further genotyping. The maximum NPL score was 2.79 on D10S195 (94.72cM) using MERLIN. We clustered families into four different subgroups by performances of CPT and WCST of affected siblings using hierarchical clustering method. We found the maximum NPL score was 3.70 (p = 0.00008) on D10S195 in the family cluster with attention deficit and execution deficit (ADED).
Second, we genotyped 79 haplotype tagSNPs between D10S1432 (93.97 cM) and D10S580 (95.52 cM) in 90 ADED families. Single-point association analysis using FBAT and TRANSMIT indicated significant transmission distortion for nine SNPs. Using the longest significance run (LSR) method, we identified a 427-kb genomic region that spanned the four SNPs (rs4492736, rs7087762, rs12644, rs4746136) as a significant candidate region (p = 0.0048). This genomic region encompasses nine genes.
Third, we studied the relative mRNA transcript levels of the nine genes in the EBV-transformed lymphocytes of 95 schizophrenic patients and 36 normal controls. In schizophrenic patients, there was significantly lower expression of annexin A7 (ANXA7), protein phosphatase 3 (formerly 2B), catalytic subunit, beta isoform (PPP3CB), and DnaJ (Hsp40) homolog, subfamily C, member 9 (DNAJC9) and significantly higher expression of zinc finger, MYND-type containing 17 (ZMYND17). Expression of ANXA7 was significantly lower in patients with the risk allele of the tagSNP (rs12258241) than those without this allele and the controls. The expression levels of ANXA7 and PPP3CB are significantly positively correlated with the performance of WCST of the schizophrenic patients.
We have identified ANXA7, PPP3CB, DNAJC9, and ZMYND17 as potential candidate genes for schizophrenia, especially in patients with deficits in sustained attention and executive function. Considering the biological function and the relationship with the neurocognitive deficits of these four genes, ANXA7 and PPP3CB are the most likely susceptibility genes.
In summary, it is a better strategy to combine positional candidate gene association study and endophenotype approach using neurobiological deficits, and further to integrate the genetic association and gene expression results to identify the most potential candidate genes. In the future, genome-wide association study, copy number variations study, exome sequencing study and genetic and neurobiological studies using induced pluripotent stem cells may cast much insight for understanding the pathophysiological mechanism of this disorder.
Subjects
schizophrenia
susceptibility genes
positional cloning
genetic linkage
genetic association
endophenotype
gene expression
Type
thesis
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