Dopamine transporter gene (DAT1) VNTR polymorphism in major psychiatric disorders: family-based association study in the Bulgarian population

OBJECTIVE: A 40-bp variable number tandem repeat in the 3'-UTR of dopamine transporter gene (DAT1) has been examined for association with major psychiatric disorders in several case-control studies. No significant results have been found. We used a new collection of parent-offspring trios to test for association with schizophrenia (SZ), bipolar 1 disorder (BPI) and schizoaffective (SA) disorder. METHOD: We genotyped trios from Bulgarian origin where the proband had SZ (178 trios), BPI (77 trios) and SA (29 trios). Alleles ranging from 5 to 11 repeats were observed. The results were analysed with the extended TDT (ETDT). RESULTS: No preferential transmission of alleles was observed for any diagnostic group. The presence of allele DAT*10 was associated with the severity and frequency of auditory hallucinations, however, this result is not significant if corrected for multiple testing. CONCLUSION: Our results are in agreement with previous reports of a lack of association between this polymorphism and major psychiatric disorders.     

Dopamine transporter (DAT) genotype (VNTR) and phenotype in extrapyramidal symptoms induced by antipsychotics

INTRODUCTION: Impaired dopamine transporter (DAT) function may be involved in antipsychotic (AP)-induced extrapyramidal symptoms (EPS). A polymorphism involving a variable number of tandem repeats (VNTR) has been described in the DAT gene (SLC6A3). OBJECTIVE: We studied whether the SLC6A3 VNTR polymorphism is a risk or protection factor for AP-induced EPS. We also investigated the relationship between the polymorphism and DAT availability in the schizophrenic patient's brain. METHODS: Sixty-one patients receiving AP therapy participated in the EPS study. Of these, thirty-two cases presented EPS (Simpson-Angus >3) and twenty-nine without EPS (Simpson-Angus < or =3). The DAT expression was studied in fifteen AP-naive patients by [(123)I] FP-CIT SPECT. RESULTS: No significant differences were observed for the more common alleles ((*)9R and (*)10R) or for genotype frequencies between patients with EPS and those without EPS. The frequency of the (*)9R and (*)10R alleles was similar to that described in other European populations. There were no significant differences in striatal DAT binding among the three major VNTR genotype groups. CONCLUSIONS: Our results suggest that the VNTR polymorphism did not influence AP-induced EPS and did not affect DAT gene expression or protein function.     

Lack of association between VNTR polymorphism of DAT gene and schizophrenia

The substantial importance of genetic factors in the etiology of schizophrenia was demonstrated in many studies. The complex model of disease inheritance seems to be the most probable, involving epistatic interaction of many genes. Association studies are based on the analysis of the so-called candidate genes, which are coding for neurotransmitter receptors, neurotransmitter transporters and enzymes involved in their metabolism. In the present work the results of an association study of VNTR polymorphism of dopamine transporter gene (DAT) in schizophrenia are presented. This polymorphism is characterised by a different number of tandem repeats (VNTR) within the 3'-untranslated region of gene. In Caucasians the 40 base pair motif has 3 to 11 repeats, the most common (about 90%) are alleles containing 9 and 10 repeats. In the present study no association between the polymorphism studied and schizophrenia was found.     

Identification of a novel polymorphism of the human dopamine transporter (DAT1) gene and the significant association with alcoholism

Human dopamine transporter gene (DAT1) has a variable number of tandem repeats (VNTR) in its 3'-untranslated region (UTR). The association between the VNTR polymorphism and neuropsychiatric disorders has been studied, but their relationship is still unclear. Here we identified a novel polymorphism in the 3'-UTR of the DAT1 gene, G2319A, and a significant association between the polymorphism and alcoholism was observed in both genotypic and allelic frequencies (P = 0.040 and 0.019, extended Fisher's exact test, respectively). There was a significant gene dose effect on the risk for alcoholism associated with the 2319-A allele (chi2 = 6.16, df = 2, P = 0.046, linearity tendency test: Cochranq-Armitage analysis). Moreover, in the haplotype analysis with G2319A- and VNTR-polymorphisms, a positive gene dose efffect on the risk with the A10 allele (P = 0.044, linearity tendency test) and a negative gene dose effect with the G10 allele (P = 0.010, linearity tendency test) for alcoholism were significantly detected. Odds ratio for alcoholism with the A10 and G10 alleles were 1.76 (1.12-2.76) and 0.53 (0.32-0.79), respectively. These results indicate that the DAT1 gene may confer vulnerability to alcoholism.     

Association of the dopamine transporter gene (DAT1) with poor methylphenidate response

OBJECTIVE: This study attempted to relate the alleles of the D2 (DRD2), D4 (DRD4), and dopamine transporter (DAT1) genes to the behavioral outcome of methylphenidate therapy. METHOD: African-American children with attention-deficit hyperactivity disorder were treated with methylphenidate in doses not in excess of 60 mg/day. The dosage was increased until behavioral change was achieved, using a decrement in scores of less than or equal to 1 on a commonly used rating scale or until the maximum tolerated dose was achieved. Blood samples were obtained at that point, and genotypes for polymorphism at the respective genes were identified. RESULTS: Genotypes were then tested by chi 2 to assess the significance of any association with drug response. Only the dopamine transporter gene was found to be significant. Homozygosity of the 10-repeat allele was found to characterize nonresponse to methylphenidate therapy (p = .008). CONCLUSIONS: While the results suggest that alleles of the dopamine transporter gene play a role in methylphenidate response, replication in additional studies is needed.     

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.     

The effect of manganese on dopamine toxicity and dopamine transporter (DAT) in control and DAT transfected HEK cells

Chronic exposure to Mn results in the development of a neurological disorder known as manganism characterized by neurological deficits resembling that seen in Parkinsonism. Although dopaminergic neurons within the nigrostriatal pathway appear intact, Mn-induced irregularities in DA transmission have been observed including decreased amphetamine-induced DA release and loss of the dopamine transporter (DAT). Results of studies to evaluate the effect of Mn and DA on cell viability in control and DAT-transfected HEK cells reveal that Mn is equally toxic to both cell lines whereas DA was only toxic to cells containing DAT. DA toxicity was saturable suggesting that transport may be rate limiting. When Mn and DA were added simultaneously to the media, cell toxicity was similar to that produced by Mn alone suggesting that Mn may suppress DA uptake in the DAT containing cells. Preincubation of DA prior to the addition of Mn resulted in cell death which was essentially additive with that produced independently by the two agents. Mn was also shown to decrease DA uptake and amphetamine-induced DA efflux in DAT containing cells. Time-lapsed confocal microscopy indicates that Mn can promote trafficking of cell surface DAT into intracellular compartments which may account for the decrease in DA uptake and DA efflux in these cells. Mn-induced internalization of DAT may provide an explanation for disruption in DA transmission previously reported in the striatum.     

Lack of association between VNTR polymorphism of dopamine transporter gene (SLC6A3) and schizophrenia in a Brazilian sample

A role of dopaminergic dysfunction has been postulated in the aetiology of schizophrenia. We hypothesized that variations in the dopamine transporter gene (SLC6A3) may be associated with schizophrenia. We conducted case-control and family based analysis on the polymorphic SLC6A3 variable number tandem repeat (VNTR) in a sample of 220 schizophrenic patients, 226 gender and ethnic matched controls, and 49 additional case-parent trios. No differences were found in allelic or genotypic distributions between cases and controls and no significant transmission distortions from heterozygous parents to schizophrenic offspring were detected. Thus, our results do not support an association of the SLC6A3 VNTR with schizophrenia in our sample.     

Oestrogens prevent loss of dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) in substantia nigra of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine mice

Previous results from our laboratory have shown that 17beta-oestradiol prevents 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) striatal dopamine depletion. 17beta-oestradiol, oestriol and oestrone are the naturally occurring oestogens in humans. Using various dopamine markers, the present study investigated whether oestrone and oestriol such as 17beta-oestradiol have neuroprotective activity in MPTP-treated mice. Male mice were treated with 17beta-oestradiol, oestriol or oestrone for 5 days before and after MPTP administration, and were compared with nonlesioned mice receiving the same treatment. Striatal concentrations of dopamine and its metabolites, 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA), were assayed by high-performance liquid chromatography. Dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) specific binding were measured by autoradiography. DAT, VMAT2 and tyrosine hydroxylase mRNA levels were measured by in situ hybridisation. MPTP induced a loss of DAT and VMAT2 specific binding in the striatum and substantia nigra, as well as a decrease of VMAT2 mRNA in the substantia nigra. 17beta-oestradiol treatment prevented the loss of these dopaminergic markers, as well as striatal concentrations of dopamine, DOPAC and HVA. Mice receiving oestriol and oestrone showed catecholamine concentrations comparable to MPTP mice. Oestriol treatment had no effect on dopaminergic markers in MPTP mice whereas oestrone prevented striatal DAT loss and the decrease of VMAT2 mRNA in the substantia nigra. In nonlesioned mice, 17beta-oestradiol, oestriol or oestrone had no effect on all the dopaminergic markers investigated. In conclusion, a weak or a lack of effect of oestriol and oestrone was observed compared to 17beta-oestradiol in MPTP mice and none of these steroids had an effect in nonlesioned mice. A DAT and VMAT2 specific binding decrease after MPTP in the striatum and substantia nigra, as well as a decrease of substantia nigra VMAT2 mRNA, was observed and could be prevented by oestradiol.     

Rapid and differential losses of in vivo dopamine transporter (DAT) and vesicular monoamine transporter (VMAT2) radioligand binding in MPTP-treated mice

The dose- and time-dependent changes of in vivo radioligand binding to the neuronal membrane dopamine transporter (DAT) and vesicular monoamine transporter type 2 (VMAT2) were examined in mouse brain after MPTP (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine) administrations. Regional brain distribution studies were done in male C57BL/6 mice using simultaneous injections of d-threo-[(3)H]methylphenidate (DAT) and (+)-alpha-[(11)C]dihydrotetrabenazine (VMAT2). Single (55 mg/kg i.p. ) or multiple (4 x 10 mg/kg i.p., 1-hour intervals) administration of MPTP caused significant reductions in [(3)H]methylphenidate and [(11)C]dihydrotetrabenazine specific striatal binding, measured 14 days later. The single high dose of MPTP produced greater losses of [(11)C]dihydrotetrabenazine binding than did the multiple MPTP dosing regimen. Using the single high dose of MPTP, changes of in vivo binding of the two radioligands were determined at 1, 3, and 14 days after neurotoxin injection. At 1 day, there are large losses of [(3)H]methylphenidate binding (DAT) but no changes in [(11)C]dihydrotetrabenazine binding to the VMAT2 site in the striatum. At 3 and 14 days, there were >50% losses of binding of both bot radioligands, but significantly (P < 0.001) greater losses of VMAT2 binding of [(11)C]dihydrotetrabenazine. These studies indicate that the losses of the neuronal membrane and vesicular transporters are not always equal, and do not occur in the same time frame, after administration of the neurotoxin MPTP.     

Systematic screening for DNA sequence variation in the coding region of the human dopamine transporter gene (DAT1)

The dopamine transporter (DAT) plays a central role in dopaminergic neurotransmission in the human brain. Genetic association studies have used a variable number of tandem repeat (VNTR) polymorphism in the 3'-flanking region of the dopamine transporter gene (DAT1) to implicate the DAT in the development of various neuropsychiatric disorders. In this study, we have examined the possibility that a mutation exists in the coding region of the DAT1 gene which through linkage disequilibrium accounts for the observed associations. The complete coding region, as well as exon-intron boundaries, was screened in 91 unrelated individuals including 45 patients with bipolar affective disorder and 46 healthy control individuals by the means of single strand conformation analysis. Our findings suggest that the DAT1 gene is highly conserved since we detected only two rare missense substitutions (Ala559Val, Glu602Gly) and three silent mutations (242C/T, 1342A/G, and 1859C/T) in the whole coding region. Five sequence variants were observed in intronic sequences but none affects known splice sites. The lack of frequent variants of possible functional relevance indicates that genetic variation in the coding region of the DAT1 gene is not responsible for the previously observed associations with neuropsychiatric disorders. The two rare missense substitutions were found in single bipolar patients but not in controls. Investigation of the patients' families revealed independent segregation between the Ala559Val variant and affective disorder. The Glu602Gly variant was inherited by the proband from an affected father. It therefore remains possible that Glu602Gly may be a rare cause of bipolar affective disorder.     

A 4-year dopamine transporter (DAT) imaging study in neuroleptic-naive first episode schizophrenia patients

Alterations in the dopaminergic system have long been implicated in schizophrenia. A key component in dopaminergic neurotransmission is the striatal dopamine transporter (DAT). To date, there have been no longitudinal studies evaluating the course of DAT in schizophrenia. A 4-year follow-up study was therefore conducted in which single photon emission computed tomography was used to measure DAT binding in 14 patients and 7 controls. We compared the difference over time in [¹²³I] FP-CIT striatal/occipital uptake ratios (SOUR) between patients and controls and the relationship between this difference and both symptomatology and functional outcome at follow-up. We also calculated the relationship between baseline SOUR, symptoms and functional outcome at follow-up. There were no statistically significant differences between patients' SOUR changes over time and those of controls. A significant negative correlation was observed between patients' SOUR changes over time and negative symptomatology at follow-up. A significant negative correlation was also found between baseline SOUR in patients and negative symptomatology, and there was a significant association between lower SOUR at baseline and poor outcome. Although the study found no overall differences in DAT binding during follow-up between schizophrenia patients and controls, it demonstrated that differences in DAT binding relate to patients' characteristics at follow-up.     

Functional effects of the DAT1 polymorphism on EEG measures in ADHD

OBJECTIVE: This paper examines whether dopamine transporter gene (DAT1) allele status mediates medication-related change in cognitive and neurophysiological measures among children with attention-deficiency/hyperactivity disorder (ADHD). METHOD: A single 10-mg dose of methylphenidate was given in a double-blind, placebo-controlled fashion to children with ADHD who were seen for cognitive testing and EEG recording. Buccal samples were obtained and genotyped for the DAT1 polymorphism. RESULTS: DAT1 allele status was associated with performance on a sustained attention task and medication-related EEG changes. Compared with those with one or more copies of the DAT1 9-repeat allele (9R), children with two copies of the 10-repeat allele (10R) exhibited poorer performance on the vigilance task. In addition, children with 10R exhibited medication-related EEG changes of increased central and parietal beta power, decreased right frontal theta power, and lower theta/beta ratios; 9R carriers showed the opposite pattern. CONCLUSIONS: The data suggest that the DAT1 polymorphism mediates medication-related changes in cortical activity among children with ADHD.     

DAT1 gene polymorphism in alcoholism: a family-based association study.

BACKGROUND: The present study tests the hypothesis that the 9-repeat allele of the dopamine transporter gene (DAT1; SLC6A3) is more frequent in alcohol-dependent probands--and in particular those with severe withdrawal symptoms (seizures and/or delirium)--compared to nonalcoholics. METHODS: To avoid stratification effects, the family-based association approach of Falk and Rubinstein was used in our sample of 87 alcohol-dependent probands and their biological parents. RESULTS: By applying a family-based association approach, we were not able to detect significant association between allele 9 at DAT1 (SLC6A3) and alcoholism as well as between patients with or without severe withdrawal symptoms. CONCLUSIONS: Based on our data, the impact of the 9-repeat allele of the dopamine transporter gene in alcoholism and the severity of alcohol withdrawal symptoms is putatively not substantial.     

Serotonin transporter gene promoter (5-HTTLPR) and intron 2 (VNTR) polymorphisms in Croatian suicide victims.

BACKGROUND: Disturbances in serotonin (5HT) transmission are the most frequently reported neurobiological substrates of suicidal behavior. Because 5HT transporter plays a central role in the regulation of 5HT synaptic function and its gene contains two functional polymorphisms (5-HTTLPR in the promoter region and VNTR in the second intron), it represents an interesting candidate for association studies in suicidal behavior. METHODS: In this study, a possible association of 5-HTTLPR and intron 2-VNTR polymorphisms of the 5HT transporter gene with suicidal behavior was investigated in a sample of 135 suicide victims and 299 healthy control subjects of Croatian/southern Slavic origin. RESULTS: There were no significant differences in 5-HTTLPR and intron 2-VNTR genotype- and allele- frequency distributions between suicide victims and healthy control subjects; however, a tendency toward an increase of 5-HTTLPR allele L and VNTR-allele 10 were observed in suicide group. Analysis of distribution of estimated haplotype frequencies revealed differences between suicide victims and control subjects, with an excess of haplotype L10 among suicide victims (p =.0112). CONCLUSIONS: Our results provide modest evidence for a possible association of the 5HT transporter gene with a completed suicide. Further studies are needed to determine whether alterations in 5HTt gene expression are involved in suicidal behavior.     

Multimodal dopamine transporter (DAT) imaging and magnetic resonance imaging (MRI) to characterise early Parkinson's disease

Idiopathic Parkinson's disease (PD), the second most common neurodegenerative disorder, is characterised by the progressive loss of dopaminergic nigrostriatal terminals. Currently, in early idiopathic PD, dopamine transporter (DAT)-specific imaging assesses the extent of striatal dopaminergic deficits, and conventional magnetic resonance imaging (MRI) of the brain excludes the presence of significant ischaemic load in the basal ganglia as well as signs indicative of other forms of Parkinsonism.In this article, we discuss the use of multimodal DAT-specific and MRI protocols for insight into the early pathological features of idiopathic PD, including: structural MRI, diffusion tensor imaging, nigrosomal iron imaging and neuromelanin-sensitive MRI sequences. These measures may be acquired serially or simultaneously in a hybrid scanner.From current evidence, it appears that both nigrosomal iron imaging and neuromelanin-sensitive MRI combined with DAT-specific imaging are useful to assist clinicians in diagnosing PD, while conventional structural MRI and diffusion tensor imaging protocols are better suited to a research context focused on characterising early PD pathology. We believe that in the future multimodal imaging will be able to characterise prodromal PD and stratify the clinical stages of PD progression.