ORIGINAL ARTICLE Functional effects of dopamine transporter gene genotypes on in vivo dopamine transporter functioning: a meta-analysis SV Faraone 1 , TJ Spencer 2 , BK Madras 3,4 , Y Zhang-James 1 and J Biederman 2 Much psychiatric genetic research has focused on a 40-base pair variable number of tandem repeats (VNTR) polymorphism located in the 3 0 -untranslated region (3 0 UTR) of the dopamine active transporter (DAT) gene (SLC6A3). This variant produces two common alleles with 9- and 10-repeats (9R and 10R). Studies associating this variant with in vivo DAT activity in humans have had mixed results. We searched for studies using positron emission tomography (PET) or single-photon emission computed tomography (SPECT) to evaluate this association. Random effects meta-analyses assessed the association of the 3 0 UTR variant with DAT activity. We also evaluated heterogeneity among studies and evidence for publication bias. We found twelve studies comprising 511 subjects, 125 from PET studies and 386 from SPECT studies. The PET studies provided highly significant evidence that the 9R allele was associated with increased DAT activity in human adults. The SPECT studies were highly heterogeneous. As a group, they suggested no association between the 3 0 UTR polymorphism and DAT activity. When the analysis was limited to the most commonly used ligand, [123I]b-CIT, stratification by affection status dramatically reduced heterogeneity and revealed a significant association of the 9R allele with increased DAT activity for healthy subjects. In humans, the 9R allele of the 3 0 UTR polymorphism of SLC6A3 regulates dopamine activity in the striatal brain regions independent of the presence of neuropsychiatric illness. Differences in study methodology account for the heterogeneous results across individual studies. Molecular Psychiatry (2014) 19, 880–889; doi:10.1038/mp.2013.126; published online 24 September 2013 Keywords: ADHD; dopamine transporter; genetics; meta-analysis; PET; SPECT INTRODUCTION The dopamine active transporter (DAT) is a key regulator of the dopamine system and the gene that encodes it (SLC6A3) has been the focus of much research in biological psychiatry, having been implicated in several disorders including attention deficit hyper- activity disorder (ADHD), 1–3 pediatric bipolar disorder, 4 Tourette syndrome 5 and alcoholism, 6 but not schizophrenia. 7 Much research in this area has focused on the DAT gene (SLC6A3), especially a 40-base pair variable number of tandem repeats (VNTR) polymorphism located in the 3 0 -untranslated region (3 0 UTR) of the gene, which has a regulatory role during transcription. This variant produces two common alleles with 9- and 10-repeats (9R and 10R). In humans, meta-analyses suggest the 10R allele of this polymorphism is associated with ADHD in youth 8 whereas the 9R allele is associated with ADHD in adults. 9 Meta-analysis also associates the 9R allele with alcoholism, 6 a common comorbidity of ADHD in adults. The DAT was initially implicated in ADHD by the stimulant drugs, which are efficacious for the disorder and block the DAT, thereby increasing the concentration of dopamine in the synaptic cleft. These effects are most pronounced in the nucleus accum- bens and dorsal striatum due to the high density of DATs in these regions. 10,11 Positron emission tomography (PET) studies in humans show that both methylphenidate 12 and amphetamine 13 increase extracellular dopamine levels in the striatum. Single- photon emission tomography (SPECT) and PET studies also show that methylphenidate treatment blocks the DAT. 14 Consistent with this, methylphenidate normalizes elevated DAT densities in a rat model of ADHD. 3 Based on a meta-analysis of nine in vivo SPECT and PET studies, Fusar-Poli et al. 15 concluded that DAT activity was 14% higher in ADHD patients compared with controls and that, among ADHD patients, DAT activity was higher among patients with a history of medication (although this latter conclusion has been questioned due to incorrect coding of medication status. 16 ) Functional in vitro studies have shown mixed results as to whether it is the 9R or 10R allele that increases DAT gene expression. 17–21 These results varied in the reporter gene designs and cell types used. A few studies measured in vivo striatal DAT gene expression using postmortem brains, and the results were also inconsistent. 22–25 MRI and magnetic resonance spectroscopy studies have also produced heterogeneous results. 26–28 A review of neuropsychological studies found little evidence supporting the idea that the SLC6A3 3 0 UTR is associated with deficits in cognition, 29 with the possible exception of functions mediated by the striatum. 27 PET and SPECT neuroimaging studies have examined the association of the 3 0 UTR polymorphism with in vivo striatal DAT binding in humans. Such studies are particularly compelling because they directly measure the protein produced by the gene rather than measuring mRNA level, or downstream effects of brain activation or cognition. DAT binding may be an intermediate phenotype that mediates the effects of DAT gene variants on dopamine-regulated brain functions and, ultimately, a wide array of behavior; including information processing, inhibition, emotion, movement, salience and reward. Advances in molecular imaging and the development of highly specific DAT binding ligands allow 1 Departments of Psychiatry and of Neuroscience and Physiology, SUNY Upstate Medical University, Syracuse, NY, USA; 2 Pediatric Psychopharmacology Unit, Psychiatry Service, Massachusetts General Hospital, Department of Psychiatry, Harvard Medical School, Boston, MA, USA; 3 Division of Neuroscience, New England Primate Research Center, Southborough, MA, USA and 4 Harvard Medical School, Boston, MA, USA. Correspondence: Dr SV Faraone, SUNY Upstate Medical University, 750 East Adams Street, Syracuse, NY 13210, USA. E-mail: sfaraone@childpsychresearch.org Received 20 February 2013; revised 15 August 2013; accepted 16 August 2013; published online 24 September 2013 Molecular Psychiatry (2014) 19, 880–889 & 2014 Macmillan Publishers Limited All rights reserved 1359-4184/14 www.nature.com/mp