Identification and functional characterization of G6PC2 coding variants influencing glycemic traits define an effector transcript at the G6PC2-ABCB11 locus.

Genome wide association studies (GWAS) for fasting glucose (FG) and insulin (FI) have identified common variant signals which explain 4.8% and 1.2% of trait variance, respectively. It is hypothesized that low-frequency and rare variants could contribute substantially to unexplained genetic variance. To test this, we analyzed exome-array data from up to 33,231 non-diabetic individuals of European ancestry. We found exome-wide significant (P<5×10-7) evidence for two loci not previously highlighted by common variant GWAS: GLP1R (p.Ala316Thr, minor allele frequency (MAF)=1.5%) influencing FG levels, and URB2 (p.Glu594Val, MAF = 0.1%) influencing FI levels. Coding variant associations can highlight potential effector genes at (non-coding) GWAS signals. At the G6PC2/ABCB11 locus, we identified multiple coding variants in G6PC2 (p.Val219Leu, p.His177Tyr, and p.Tyr207Ser) influencing FG levels, conditionally independent of each other and the non-coding GWAS signal. In vitro assays demonstrate that these associated coding alleles result in reduced protein abundance via proteasomal degradation, establishing G6PC2 as an effector gene at this locus. Reconciliation of single-variant associations and functional effects was only possible when haplotype phase was considered. In contrast to earlier reports suggesting that, paradoxically, glucose-raising alleles at this locus are protective against type 2 diabetes (T2D), the p.Val219Leu G6PC2 variant displayed a modest but directionally consistent association with T2D risk. Coding variant associations for glycemic traits in GWAS signals highlight PCSK1, RREB1, and ZHX3 as likely effector transcripts. These coding variant association signals do not have a major impact on the trait variance explained, but they do provide valuable biological insights.

A Multiparent Advanced Generation Inter-Cross to fine-map quantitative traits in Arabidopsis thaliana.

Identifying natural allelic variation that underlies quantitative trait variation remains a fundamental problem in genetics. Most studies have employed either simple synthetic populations with restricted allelic variation or performed association mapping on a sample of naturally occurring haplotypes. Both of these approaches have some limitations, therefore alternative resources for the genetic dissection of complex traits continue to be sought. Here we describe one such alternative, the Multiparent Advanced Generation Inter-Cross (MAGIC). This approach is expected to improve the precision with which QTL can be mapped, improving the outlook for QTL cloning. Here, we present the first panel of MAGIC lines developed: a set of 527 recombinant inbred lines (RILs) descended from a heterogeneous stock of 19 intermated accessions of the plant Arabidopsis thaliana. These lines and the 19 founders were genotyped with 1,260 single nucleotide polymorphisms and phenotyped for development-related traits. Analytical methods were developed to fine-map quantitative trait loci (QTL) in the MAGIC lines by reconstructing the genome of each line as a mosaic of the founders. We show by simulation that QTL explaining 10% of the phenotypic variance will be detected in most situations with an average mapping error of about 300 kb, and that if the number of lines were doubled the mapping error would be under 200 kb. We also show how the power to detect a QTL and the mapping accuracy vary, depending on QTL location. We demonstrate the utility of this new mapping population by mapping several known QTL with high precision and by finding novel QTL for germination data and bolting time. Our results provide strong support for similar ongoing efforts to produce MAGIC lines in other organisms.

Association analyses of the DAOA/G30 and D-amino-acid oxidase genes in schizophrenia: further evidence for a role in schizophrenia.

A number of linkage studies have previously implicated the region of chromosome 13q34 in schizophrenia. Chumakov and colleagues (2002) identified a gene complex called G72 (now termed D-amino acid oxidase activator: DAOA)/G30 in this region and performed association analyses of the DAOA/G30 as well as the D-amino-acid oxidase (DAAO) gene with schizophrenia. DAAO oxidizes D-serine, a potent activator of the N-methyl-D-aspartate (NMDA) type glutamate receptor in the human brain whereas the DAOA protein is considered an activator of DAAO. The interaction of these two genes has thus been implicated in the NMDA receptor regulation pathway in schizophrenia. To date, several studies have shown a relatively consistent positive association between schizophrenia and DAOA/G30, but not with DAAO. The aim of our study was to further evaluate the contributions of these genes to the susceptibility to schizophrenia using two different sample sets. Our sample consisted of 168 matched case-control pairs as well as a family-based sample (n=113) for transmission disequilibrium test. Significant associations between the DAOA/G30 M-7 and M-18 polymorphisms and schizophrenia were observed in our case-control sample whereas no associations were observed for DAAO markers. We also observed significant or suggestive transmission disequilibrium for DAOA/G30 M-7, M-23, and M-24 to probands with schizophrenia in our family-based sample. Subsequent analysis of haplotypes made up of four DAOA/G30 markers, one marker selected from each of two linkage disequilibrium blocks that were observed in our sample as well as both ends (M-7 and M-25), were also associated with schizophrenia. Our data provide further evidence that the DAOA/G30 locus may play a role in the pathophysiology of schizophrenia. Although no direct link to genetic polymorphism in these genes and NMDA receptor function has been revealed, the present findings support previous reports implicating DAOA/G30 as susceptibility genes for schizophrenia. Further research is warranted to determine the functional variation underlying these findings and to relate this to the pathophysiology of schizophrenia.

Genetic association analysis of the glutathione peroxidase (GPX1) gene polymorphism (Pro197Leu) with tardive dyskinesia.

A possible involvement of oxidative stress in the pathophysiology of tardive dyskinesia (TD) has previously been proposed (reviewed in [Andreassen, O.A., Jorgensen, H.A., 2000. Neurotoxicity associated with neuroleptic-induced oral dyskinesias in rats. Implications for tardive dyskinesia? Progress in Neurobiology 61, 525-541.]). Long-term administration of neuroleptics alters dopaminergic turnover, which results in increased formation of reactive oxygen species (ROS). This is hypothesized to lead to TD through neuronal toxicity as a consequence of oxidative stress. In the present study, the relationship between TD and a possible functional polymorphism of the human glutathione peroxidase (GPX1) gene (an important antioxidant enzyme) was studied in 68 chronic treatment-refractory patients with schizophrenia. A proline (Pro) to leucine (Leu) substitution at codon 197 (Pro197Leu) in the GPX1 gene was genotyped. No significant difference in total Abnormal Involuntary Movements Scale (AIMS) scores was observed among patients in the three genotype groups. Moreover, no significant differences in genotype or allele frequencies were observed between subjects with and without TD. Our results suggest that the GPX1 gene polymorphism does not confer increased susceptibility to TD, although further studies are warranted before a conclusion can be drawn.

Genetic and post-mortem mRNA analysis of the 14-3-3 genes that encode phosphoserine/threonine-binding regulatory proteins in schizophrenia and bipolar disorder.

BACKGROUND: Previous work with animal models of psychosis, human genetic studies, and human post-mortem gene expression studies implicate the 14-3-3 family of genes in schizophrenia. The 14-3-3 genes code for a family of proteins that bind to and regulate other proteins, and they modulate neurodevelopment, cell-division, signal transduction and gene transcription. OBJECTIVE: To explore the role of five 14-3-3 isoforms (beta, gamma, epsilon, zeta, and eta) in schizophrenia by: (1) comparing mRNA levels in post-mortem brain from schizophrenic, bipolar and control subjects and (2) assessing genetic association with schizophrenia in both case-control and nuclear family samples. METHODS: Quantitative PCR (q-PCR) was used to determine relative mRNA levels in dorsolateral prefrontal cortex (Brodmann's area 46) samples donated by the Stanley Medical Research Institute (SMRI). Selected SNPs were genotyped in all five isoforms for association analysis in both family and case-control samples. RESULTS: No significant differences in 14-3-3 mRNA expression levels between the diagnostic groups were found. A significant genetic association with schizophrenia was found for the 14-3-3zeta isoform in a subset of nuclear families of British ancestry (TDT: chi(2)=7.2; df=1; p=0.0073), in the case-control sample overall (p=0.011), and in a subset of the case-control sample. CONCLUSION: The results, in combination with other published evidence, suggest that further work is necessary to clarify what role the 14-3-3 genes may play in the etiology and pathogenesis of schizophrenia.

Human p53 tumor suppressor gene (TP53) and schizophrenia: case-control and family studies.

The human p53 tumor suppressor gene (TP53) is considered as a candidate susceptibility gene for schizophrenia because of its functions in neurodevelopment. To test for an association between TP53 and schizophrenia, both the case-control study and the transmission disequilibrium test (TDT) were performed on genotype data from eight polymorphisms in TP53. Our samples included 286 Toronto schizophrenia cases and 264 controls, and 163 Portuguese nuclear families. In the Toronto case-control study significant differences of allele frequencies of the CAA Ins/Del (p=0.027) and the 16bp Ins/Del (p=0.022) were detected. In TDT analysis we found significant differences for transmission of the CAA Ins/Del (p=0.017) in Portuguese schizophrenia families. Haplotype analysis also showed a significant association between TP53 and schizophrenia. These results provide further evidence that TP53 may play a role in the pathogenesis of schizophrenia.

Linkage disequilibrium between dopamine D1 receptor gene (DRD1) and bipolar disorder.

BACKGROUND: Based on the dopamine hypothesis, the dopamine D1 receptor gene (DRD1) is considered to be a good candidate gene for bipolar disorder (BP). METHODS: In our study, three polymorphisms of the DRD1 gene, -800T/C, -48A/G, and 1403T/C, were analyzed in 286 BP trios. Both the transmission disequilibrium test (TDT) and haplotype TDT were performed on the genotype data to test for the presence of linkage disequilibrium between DRD1 and bipolar disorder. With the extended transmission disequilibrium test (ETDT), we also calculated the maternal transmission and paternal transmission for each allele. RESULTS: Although no association was found for each individual polymorphism, there is a significant association between DRD1 and BP for haplotype TDT analysis (chi(2) = 16.068, df = 3, p =.0011). CONCLUSIONS: These results indicate that DRD1 may play a role in the etiology of bipolar disorder.

Association between schizophrenia and the syntaxin 1A gene.

BACKGROUND: Both microarray and candidate molecule studies have demonstrated that protein and mRNA expression of syntaxin and other genes involved in synaptic function are altered in the cerebral cortex of patients with schizophrenia. METHODS: Genetic association between polymorphic markers in the syntaxin 1A gene and schizophrenia was assessed in a matched case-control sample of 192 pairs, and in an independent sample of 238 nuclear families. RESULTS: In the family-based sample, a significant genetic association was found between schizophrenia and one of the four single nucleotide polymorphisms (SNPs) tested: an intron 7 SNP (transmission disequilibrium test [TDT] chi(2) = 5.898; df = 1; p =.015, family-based association test [FBAT] z = 2.280, p =.023). When the results for the TDT and case-control analyses were combined, the association was stronger (n = 430; z(c) = 2.859; p =.004). Haplotype analysis supported the association with several significant values that appear to be driven by the intron 7 SNP. CONCLUSIONS: The results should be treated with caution until replicated, but this is the first report of a genetic association between syntaxin 1A and schizophrenia.

Family-based association study of the serotonin-2A receptor gene (5-HT2A) and bipolar disorder.

OBJECTIVES: The serotonin 2A receptor gene (5-HT2A) is of great interest for research in neuropsychiatric disorders based on the observation that various neuroleptic agents and antidepressants bind with relatively high affinity at 5-HT2A receptors, and the fact that the receptor density in platelets tends to increase in depression. To test for the presence of association between 5-HT2A and bipolar disorder (BP), we studied a large number of triad families having probands affected with DSM-IV bipolar I (BPI), bipolar II (BPII) or schizoaffective disorder, bipolar type. METHODS: Two polymorphisms of 5-HT2A, 102T/C, and His452Tyr were analyzed in the 274 bipolar triad families. Both the transmission disequilibrium test (TDT) and haplotype TDT were performed on the genotype data. We also calculated the maternal transmission and paternal transmission for each allele and compared the mean ages of onset across probands grouped by genotype at each of the two markers. RESULTS: No significant transmission disequilibrium between the alleles of 5-HT2A and BP was found. Separate studies of the sub-phenotypes also failed to demonstrate significant association. However, we found a trend towards transmission disequilibrium with the haplotype 102C.His452 (p = 0.0504). This trend may become more significant with a larger sample size. SIGNIFICANCE: At present, results of this study suggest that the 5-HT2A is unlikely to play a major role in the genetic susceptibility to BP. Future studies will be directed towards increasing sample size, focusing on subtypes of BP or biochemical measures as phenotypes, and investigating other polymorphisms of 5-HT2A to provide more information at the DNA level.

No association between the Pro197Leu polymorphism in the glutathione peroxidase (GPX1) gene and schizophrenia.

OBJECTIVE: Oxidative stress such as free radical-mediated neuronal dysfunction may be involved in the pathophysiology of schizophrenia. The human glutathione peroxidase (GPX1) is a selenium-dependent enzyme, which plays an important role in the detoxification of free radicals. We therefore hypothesized that the GPX1 gene, which is located on chromosome 3p21.3, may be involved in the pathophysiology of schizophrenia. The aim of this study is to examine whether a potentially functional polymorphism, a proline (Pro) to leucine (Leu) substitution at codon 197 (Pro197Leu) of the human GPX1 gene, is associated with susceptibility to schizophrenia. METHODS: We genotyped the Pro197Leu polymorphism in a total of 113 nuclear families that had a proband with schizophrenia. Genetic association was tested using the transmission disequilibrium test (TDT), the sib transmission disequilibrium test (STDT), and the family-based association test (FBAT). RESULTS: The minor allele (Leu) frequency was calculated to be 0.282. We could not find significant transmission disequilibrium of the alleles for the Pro197Leu polymorphism in the GPX1 gene in association with the presence of schizophrenia in our family sample (TDT, chi2=0.03, degrees of freedom=1, P=0.86; combined TDT-STDT, Z'=-0.052, P=0.47; FBAT, Z=0.000, P=1.000). CONCLUSION: The results of this study suggest that the GPX1 polymorphism is unlikely to be associated with susceptibility to schizophrenia.

DNA methylation patterns in cord blood DNA and body size in childhood.

BACKGROUND: Epigenetic markings acquired in early life may have phenotypic consequences later in development through their role in transcriptional regulation with relevance to the developmental origins of diseases including obesity. The goal of this study was to investigate whether DNA methylation levels at birth are associated with body size later in childhood. PRINCIPAL FINDINGS: A study design involving two birth cohorts was used to conduct transcription profiling followed by DNA methylation analysis in peripheral blood. Gene expression analysis was undertaken in 24 individuals whose biological samples and clinical data were collected at a mean ± standard deviation (SD) age of 12.35 (0.95) years, the upper and lower tertiles of body mass index (BMI) were compared with a mean (SD) BMI difference of 9.86 (2.37) kg/m(2). This generated a panel of differentially expressed genes for DNA methylation analysis which was then undertaken in cord blood DNA in 178 individuals with body composition data prospectively collected at a mean (SD) age of 9.83 (0.23) years. Twenty-nine differentially expressed genes (>1.2-fold and p<10(-4)) were analysed to determine DNA methylation levels at 1-3 sites per gene. Five genes were unmethylated and DNA methylation in the remaining 24 genes was analysed using linear regression with bootstrapping. Methylation in 9 of the 24 (37.5%) genes studied was associated with at least one index of body composition (BMI, fat mass, lean mass, height) at age 9 years, although only one of these associations remained after correction for multiple testing (ALPL with height, p(Corrected) = 0.017). CONCLUSIONS: DNA methylation patterns in cord blood show some association with altered gene expression, body size and composition in childhood. The observed relationship is correlative and despite suggestion of a mechanistic epigenetic link between in utero life and later phenotype, further investigation is required to establish causality.

N-methyl-D-aspartate receptor NR1 subunit gene (GRIN1) in schizophrenia: TDT and case-control analyses.

The N-methyl-d-aspartate glutamate receptors (NMDAR) act in the CNS as regulators of the release of neurotransmitters such as dopamine, noradrenaline, acetylcholine, and GABA. It has been suggested that a weakened glutamatergic tone increases the risk of sensory overload and of exaggerated responses in the monoaminergic system, which is consistent with the symptomatology of schizophrenia. We studied two silent polymorphisms in GRIN1. GRIN1/1 is a G/C substitution localized on the 5' untranslated region; GRIN1/10 is an A/G substitution localized in exon 6 of GRIN1. Minor allele frequencies in our sample were calculated to be 0.05 and 0.2 respectively. We genotyped 86 nuclear families and 91 ethnically matched case-control pairs. Both samples were collected from the Toronto area. We tested the hypothesis that GRIN1 polymorphisms were associated with schizophrenia using the transmission disequilibrium test (TDT) and comparing allele frequencies between cases and controls. The results are as follows: GRIN1/1: chi(2) = 2.19, P = 0.14; GRIN1/10: chi(2) = 1.5, P = 0.22. For the case-control sample: GRIN1/1: chi(2) = 0.013, P = 0.908; GRIN1/10: chi(2) = 0.544, P = 0.461. No significant results were obtained. Haplotype analyses showed a borderline significant result for the 2,1 haplotype (chi(2) = 3.86, P-value = 0.049). An analysis of variance (ANOVA) to evaluate the association between genetic makeup and age at onset was performed, with no significant results: GRIN1/1, F[df = 2] = 0.42, P-value = 0.659; GRIN1/10, F[df = 2] = 0.16, P-value = 0.853. We are currently collecting additional samples to increase the power of the analyses.