title: Population-based and family-based studies on the serotonin transporter gene polymorphisms and bipolar disorder: a systematic review and meta-analysis.
abstract: 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.
authors.affiliation: Institute of Psychiatry, King's College London, London, UK. h.cho@iop.kcl.ac.uk
authors.forename: H J
authors.initials: HJ
authors.lastname: Cho
authors.forename: I
authors.initials: I
authors.lastname: Meira-Lima
authors.forename: Q
authors.initials: Q
authors.lastname: Cordeiro
authors.forename: L
authors.initials: L
authors.lastname: Michelon
authors.forename: P
authors.initials: P
authors.lastname: Sham
authors.forename: H
authors.initials: H
authors.lastname: Vallada
authors.forename: D A
authors.initials: DA
authors.lastname: Collier
mesh_terms: D001714:Bipolar Disorder / Q000209:etiology / Q000235:genetics*; D004777:Environment; D005190:Family; D005260:Female; D006801:Humans; D008297:Male; D008562:Membrane Glycoproteins / Q000235:genetics*; D026901:Membrane Transport Proteins / Q000235:genetics*; D009419:Nerve Tissue Proteins / Q000235:genetics*; D011110:Polymorphism, Genetic; D050486:Serotonin Plasma Membrane Transport Proteins
chemical_list: D008562:Membrane Glycoproteins; D026901:Membrane Transport Proteins; D009419:Nerve Tissue Proteins; C493230:SLC6A4 protein, human; D050486:Serotonin Plasma Membrane Transport Proteins