Major psychiatric disorders and the serotonin transporter gene (SLC6A4): family-based association studies

Association between the SLC6A4 gene and schizophrenia: an updated meta-analysis

BACKGROUND

In order to explore the association between the SLC6A4 gene and the risk of schizophrenia, an updated meta-analysis was conducted using a total of 46 scientific articles.

METHODS

Through a literature search, papers studied included 35 articles on serotonin-transporter-linked polymorphic region (5-HTTLPR) with 8,752 cases and 10,610 controls, 17 articles on second intron variable number of tandem repeats with 7,284 cases and 8,544 controls, four studies on rs1042173 with 1,351 cases and 2,101 controls, and four studies on rs140700 with 1,770 cases and 2,386 controls. Pooled, subgroup, and sensitivity analyses were performed, and the results were visualized by forest and funnel plots.

RESULTS

An association between 5-HTTLPR and the risk of schizophrenia was not found, except for an Indian subgroup analysis (P_z =0.014, OR =1.749, 95% CI =1.120-2.731). A 10 repeats/12 repeats (10R/12R) genotype was a protective factor against schizophrenia (P_z =0.020, OR =0.789, 95% CI =0.646-0.963), but a 12R/12R genotype was a risk factor for schizophrenia (P_z =0.004, OR =1.936, 95% CI =1.238-3.029) in the pooled analyses. In Caucasians, a GG genotype of rs1042173 may be a risk factor for schizophrenia (P_z =0.006, OR =1.299, 95% CI =1.079-1.565). No association was found between rs140700 and the risk for schizophrenia.

CONCLUSION

Through meta-analysis, we were able to gain insight into previously reported associations between SLC6A4 polymorphism and schizophrenia.

Meta-analysis reveals association between serotonin transporter gene STin2 VNTR polymorphism and schizophrenia

The serotonin transporter gene (SLC6A4) is a candidate gene for schizophrenia based on serotonin transporter's crucial role in serotonergic neurotransmission. However, association studies have produced conflicting results regarding the association between two common SLC6A4 gene polymorphisms, the promoter insertion/deletion (5-HTTLPR) and the intron 2 VNTR (STin2 VNTR) polymorphisms, and schizophrenia susceptibility. To further elucidate the putative association between the two SLC6A4 gene polymorphisms and schizophrenia susceptibility, we performed a meta-analysis based on all original published association studies between schizophrenia and the 5-HTTLPR and STin2 VNTR polymorphisms published before April 2004. Our analyses showed no statistically significant evidence for the association between the Short allele of the 5-HTTLPR polymorphism and schizophrenia (random-effects pooled odds ratio (OR)=0.99, 95% Confidence Interval (CI)=0.92-1.07, Z=-0.23, P=0.82) from 19 population-based association studies consisting of 2990 case and 3875 control subjects. However, highly significant evidence for association between the STin2.12 allele of the STin2 VNTR polymorphism (random-effects pooled OR=1.24, 95% CI=1.11-1.38, Z=3.82, P=0.00014) and schizophrenia was found from 12 population-based association studies consisting of 2177 cases and 2369 control subjects. Our meta-analysis suggests that the STin2.12 allele of the STin2 VNTR polymorphism is likely a risk factor for schizophrenia susceptibility. Our data imply that following completion of the International HapMap Project, a comprehensive evaluation of a set of markers that fully characterize the linkage disequilibrium relationships at the SLC6A4 gene should be tested in large well-characterized clinical samples in order to understand the role of this gene in schizophrenia susceptibility.

Population-based and family-based studies on the serotonin transporter gene polymorphisms and bipolar disorder: a systematic review and meta-analysis

The serotonin transporter (5-HTT) is a candidate gene for bipolar disorder (BPD). It has been investigated for association with the illness in a series of studies, but overall results have been inconsistent and its role in the disorder remains controversial. Systematic reviews using meta-analytical techniques are a useful method for objectively and reproducibly assessing individual studies and generating combined results. We performed two meta-analyses of published studies--both population-based and family-based studies--investigating the association between BPD and the 5-HTT gene-linked polymorphic region (5-HTTLPR) and the intron 2 variable numbers of tandem repeats (VNTR) polymorphisms. The literature was searched using Medline and Embase to identify studies for inclusion. We statistically joined population-based and family-based studies into a single meta-analysis. For both polymorphisms, our review revealed significant pooled odds ratios (ORs): 1.12 (95% CI 1.03-1.21) for the 5-HTTLPR and 1.12 (95% CI 1.02-1.22) for the intron 2 VNTR. Meta-regression showed that neither the study type (population-based vs family-based; P=0.41 for the 5-HTTLPR and P=0.91 for the intron 2 VNTR) nor the sample ethnicity (Caucasian vs non-Caucasian; P=0.35 for the 5-HTTLPR and P=0.66 for the intron 2 VNTR) significantly contributed to the heterogeneity of the meta-analyses. The observed ORs could be regarded simply as a very small but detectable effect of the 5-HTT, which has an additive effect when combined with other susceptibility loci. Alternative hypotheses on this finding were also discussed: a stronger effect of the haplotypes involving the two polymorphisms or other SNP markers; a more direct effect of these polymorphisms on specific phenotypes of BPD; and the presence of gene-environment interaction as a mediator of the genetic effects of 5-HTT.

Recent findings on the genetic basis of bipolar disorder

What began as a search for a specific gene for bipolar disorder has now become a search for multiple susceptibility genes as it has be-come clear that the genetic basis of bipolar disorder probably involves multiple genes interacting with each other and with environmental components in as-yet mysterious ways. This article reviews the most recent findings and the emerging picture in the genetics of bipolar disorder.

Intergenomic consensus in multifactorial inheritance loci: the case of multiple sclerosis

Genetic linkage and association studies define chromosomal regions, quantitative trait loci (QTLs), which influence the phenotype of polygenic diseases. Here, we describe a global approach to determine intergenomic consensus of those regions in order to fine map QTLs and select particularly promising candidate genes for disease susceptibility or other polygenic traits. Exemplarily, human multiple sclerosis (MS) susceptibility regions were compared for sequence similarity with mouse and rat QTLs in its animal model experimental allergic encephalomyelitis (EAE). The number of intergenomic MS/EAE consensus genes (295) is significantly higher than expected if the animal model was unrelated to the human disease. Hence, this approach contributes to the empirical evaluation of animal models for their applicability to the study of human diseases.

Meta-analysis of serotonin transporter polymorphisms and affective disorders

Polymorphisms in the promoter region (5-HTTLPR) of the serotonin transporter and a variable number of tandem repeats polymorphism in the second intron have been widely studied. However, the results of association studies examining unipolar depression (MDD) or bipolar disorder depression (BPD) have been mixed. To precisely ascertain small associations with both polymorphisms, a meta-analysis was performed involving several thousand subjects, using random-effects modeling. For MDD, the effect of the 5-HTTLPR genotype was significant (chi2=6.1, P<0.05), with 21% of MDD subjects and 17% of controls homozygous for the short (S) allele (odds ratio, 1.16). Similar findings were noted in BPD, with a higher frequency of S/S genotypes in affected patients, although the results did not reach statistical significance. Results of transmission disequilibrium tests trended in a similar direction but also did not reach statistical significance. No consistent effect of the variable number of tandem repeats polymorphism was revealed for either MDD or BPD. The results suggest that the S allele, or a neighboring allele in linkage disequilibrium, is recessive for MDD and possibly BPD. Notably, the association is very small. With these small associations, confounding issues such as population stratification require addressing. Significant heterogeneity between studies was also evident, possibly reflecting differences in diagnosis, different control populations, and different ethnic populations. These factors should Influence the interpretation of the association found in this analysis.

Association study of serotonin transporter gene VNTR polymorphism and mood disorders, onset age and suicide attempts in a Chinese sample

The human serotonin transporter (5-HTT) gene is an important candidate for the pathogenesis of mood disorders. Associations have been reported between a variable-number tandem-repeat polymorphism in intron 2 of the serotonin transporter gene (5-HTTVNTR) and mood disorders in a number of studies of Western and Chinese populations. However, no such relationships have been determined in other analogous research. To replicate these positive findings in a Chinese population and to determine the association between onset age of bipolar disorder and 5-HTTVNTR, we investigated the prevalence of this polymorphism in an independent Chinese population (83 bipolar disorder patients, 77 major depressive disorder patients and 169 controls), demonstrating no significant association between the 5-HTTVNTR polymorphism and mood disorders or age at onset. Further, no association was demonstrated between this polymorphism and suicidal history in mood disorder patients. These negative findings suggest that 5-HTTVNTR does not play a major role in the pathogenesis of mood disorder in Chinese populations.