Association of the dopamine transporter gene with Parkinson's disease in Korean patients

Psychosis in Parkinson's Disease: A Lesson from Genetics

Psychosis in Parkinson's disease (PDP) represents a common and debilitating condition that complicates Parkinson's disease (PD), mainly in the later stages. The spectrum of psychotic symptoms are heterogeneous, ranging from minor phenomena of mild illusions, passage hallucinations and sense of presence to severe psychosis consisting of visual hallucinations (and rarely, auditory and tactile or gustatory) and paranoid delusions. PDP is associated with increased caregiver stress, poorer quality of life for patients and carers, reduced survival and risk of institutionalization with a significant burden on the healthcare system. Although several risk factors for PDP development have been identified, such as aging, sleep disturbances, long history of PD, cognitive impairment, depression and visual disorders, the pathophysiology of psychosis in PD is complex and still insufficiently clarified. Additionally, several drugs used to treat PD can aggravate or even precipitate PDP. Herein, we reviewed and critically analyzed recent studies exploring the genetic architecture of psychosis in PD in order to further understand the pathophysiology of PDP, the risk factors as well as the most suitable therapeutic strategies.

The 10-Repeat 3'-UTR VNTR Polymorphism in the SLC6A3 Gene May Confer Protection Against Parkinson's Disease: A Meta-analysis

The dopamine transporter (DAT) is encoded by the SLC6A3 gene and plays an important role in the regulation of the neurotransmitter dopamine. The SLC6A3 gene contains several repetition alleles (3-11 repeats) of a 40-base pair variable number of tandem repeats (VNTR) in the 3'-untranslated region (3'-UTR), which may affect DAT expression levels. The 10-repeat (10R) allele could play a protective role against PD. However, inconsistent findings have been reported. Methods: A comprehensive meta-analysis was performed to accurately estimate the association between the 10R allele of the 3'-UTR VNTR in SLC6A3 and PD among four different genetic models. Results: This meta-analysis included a total of 3,142 patients and 3,496 controls. We observed a significant difference between patients and controls for the allele model (10R vs. all others: OR = 0.860, 95% CI: 0.771-0.958, P = 0.006), pseudodominant model (10R/10R + 10R/9R vs. all others: OR = 0.781, 95% CI: 0.641-0.952, P = 0.014) and pseudorecessive model (10R/10R vs. all others: OR = 0.858, 95% CI: 0.760-0.969, P = 0.013) using a fixed effects model. No significant differences were observed under the pseudocodominant model (10R/9R vs. all others: OR = 1.079, 95% CI: 0.945-1.233, P = 0.262). By subgroup analysis, the 10R, 10R/10R and 10R/9R genotypes were found to be significantly different from PD in Asian populations. Conclusion: Our findings suggest that the SLC6A3 10R may be a protective factor in susceptibility to PD.

Allelic variant in SLC6A3 rs393795 affects cerebral regional homogeneity and gait dysfunction in patients with Parkinson's disease

Aims

We sought to explore the role of the SLC6A3rs393795 allelic variant in cerebral spontaneous activity and clinical features in Parkinson’s disease (PD) via imaging genetic approach.

Methods

Our study recruited 50 PD and 45 healthy control (HC) participants to provide clinical, genetic, and resting state functional magnetic resonance imaging (rs-fMRI) data. All subjects were separated into 16 PD-AA, 34 PD-CA/CC, 14 HC-AA, and 31 HC-CA/CC four subgroups according to SLC6A3rs393795 genotyping. Afterwards, main effects and interactions of groups (PD versus HC) and genotypes (AA versus CA/CC) on cerebral function reflected by regional homogeneity (ReHo) were explored using two-way analysis of covariance (ANCOVA) after controlling age and gender. Finally, Spearman’ s correlations were employed to investigate the relationships between significantly interactive brain regions and clinical manifestations in PD subgroups.

Results

Compared with HC subjects, PD patients exhibited increased ReHo signals in left middle temporal gyrus and decreased ReHo signals in left pallidum. Compared with CA/CC carriers, AA genotype individuals showed abnormal increased ReHo signals in right inferior frontal gyrus (IFG) and supplementary motor area (SMA). Moreover, significant interactions (affected by both disease factor and allelic variation) were detected in right inferior temporal gyrus (ITG). Furthermore, aberrant increased ReHo signals in right ITG were observed in PD-AA in comparison with PD-CA/CC. Notably, ReHo values in right ITG were negatively associated with Tinetti Mobility Test (TMT) gait subscale scores and positively related to Freezing of Gait Questionnaire (FOG-Q) scores in PD-AA subgroup.

Conclusions

Our findings suggested that SLC6A3rs393795 allelic variation might have a trend to aggravate the severity of gait disorders in PD patients by altering right SMA and IFG function, and ultimately result in compensatory activation of right ITG. It could provide us with a new perspective for exploring deeply genetic mechanisms of gait disturbances in PD.

The influence of SLC6A3 and DRD2 polymorphisms on levodopa-therapy in patients with sporadic Parkinson's disease

OBJECTIVES

The aim of this study was to evaluate a possible relationship between DRD2/ANKK1 (rs1800497) and SLC6A3/DAT1 (rs28363170) gene polymorphisms with the response to levodopa (L-DOPA)-therapy in patients with Parkinson's disease (PD).

METHODS

One hundred and ninety-five patients with idiopathic PD were investigated. Patients were genotyped for rs1800497 and rs28363170 polymorphisms using PCR-RFLP. Logistic regression was performed to assess the association of polymorphisms with the occurrence of the chronic complications of L-DOPA therapy.

KEY FINDINGS

Our results showed association between the occurrence of dyskinesia with an increased greater disease severity (P = 0.007), higher L-DOPA dose (P = 0.007) and use of dopamine agonist (P = 0.020). Moreover, there were significant protective effects for age (P = 0.004) and male subjects (P = 0.006).

CONCLUSIONS

Clinical and demographic characteristics of Brazilian PD patients and differences in DRD2 and DAT1 genes may to determine individual variations in the therapeutic response to L-DOPA in the Brazilian PD patients.

Protective effects of Ndfip1 on MPP(+)-induced apoptosis in MES23.5 cells and its underlying mechanisms

Apoptosis has been implicated as one of the important mechanisms involved in the degeneration of dopaminergic neurons in Parkinson's disease (PD). Increasing evidence suggests that Ndfip1 is a neuroprotective protein, and Ndfip1-mediated protein ubiquitination might be a possible survival strategy in neuronal injury. The aim of the present study is to investigate the neuroprotective effect of Ndfip1 on 1-methyl-4-phenylpyridinium (MPP(+))-treated MES23.5 cells and the underlying mechanisms. Results showed that overexpression of Ndfip1 could significantly attenuate MPP(+)-induced cell loss and nuclear condensation. Further experiments demonstrated that Ndfip1 could increase Bcl-2/Bax ratio, suppress cytochrome c release from the mitochondria to cytoplasm and decrease caspase-3 activation induced by MPP(+). These results suggested that Ndfip1 protected MES23.5 cells against MPP(+) by its anti-apoptotic effect. In addition, we found that Ndfip1 overexpression could decrease the protein level of dopamine transporter (DAT). In parallel, proteasome inhibitor MG132 could markedly reverse Ndfip1-induced degradation of DAT. These data suggest that Ndfip1 exerts its inhibitory effect on DAT by modulating DAT degradation, in which ubiquitin-proteasome system activation might be involved. Collectively, our study indicated that the ability to decrease the DAT of Ndfip1 might be one of the mechanisms underlying its protective effect on MPP(+)-induced cell damage in MES23.5 cells.

SLC6A3 is a risk factor for Parkinson's disease: a meta-analysis of sixteen years' studies

The human dopamine transporter gene (gene symbol: SLC6A3) is considered as a candidate risk factor for Parkinson's disease because dopamine transporter accumulates cytotoxic dopamine or other toxins in the dopamine neurons. However, findings from numerous association studies in different populations have been inconsistent with each other. In this study, we performed a combined analysis of published case-control genetic association data between SLC6A3 and Parkinson's disease. The results indicate that SLC6A3 confers a modest but significant risk for Parkinson's disease in various populations. Allele 10-repeat of the 40-base pair variable number tandem repeat, a well studied polymorphism in the 3' untranslated region of SLC6A3, confers neuroprotection in East Asian (OR: 0.78, 95% CI: 0.65, 0.94 and p=0.009) but not in Caucasian populations. Genotype GG and allele G of the promoter single nucleotide polymorphism rs2652510 is associated with a risk in Caucasians (allelic G, OR: 1.26, 95% CI: 1.04-1.54, and p=0.018; genotypic GG OR: 1.37, 95% CI: 1.03-1.84 and p=0.032). Such information implies a population-dependent involvement of SLC6A3 in the etiology of Parkinson's disease.

Decreased vesicular monoamine transporter 2 (VMAT2) and dopamine transporter (DAT) function in knockout mice affects aging of dopaminergic systems

Dopamine (DA) is accumulated and compartmentalized by the dopamine transporter (DAT; SLC3A6) and the vesicular monoamine transporter 2 (VMAT2; SLC18A2). These transporters work at the plasma and vesicular membranes of dopaminergic neurons, respectively, and thus regulate levels of DA in neuronal compartments that include the extravesicular cytoplasmic compartment. DA in this compartment has been hypothesized to contribute to oxidative damage that can reduce the function of dopaminergic neurons in aging brains and may contribute to reductions in dopaminergic neurochemical markers, locomotor behavior and responses to dopaminergic drugs that are found in aged animals. The studies reported here examined aged mice with heterozygous deletions of VMAT2 or of DAT, which each reduce transporter expression to about 50% of levels found in wild-type (WT) mice. Aged mice displayed reduced locomotor responses under a variety of circumstances, including in response to locomotor stimulants, as well as changes in monoamine levels and metabolites in a regionally dependent manner. Several effects of aging were more pronounced in heterozygous VMAT2 knockout (KO) mice, including aging induced reductions in locomotion and reduced locomotor responses to cocaine. By contrast, some effects of aging were reduced or not observed in heterozygous DAT KO mice. These findings support the idea that altered DAT and VMAT2 expression affect age-related changes in dopaminergic function. These effects are most likely mediated by alterations in DA compartmentalization, and might be hypothesized to be exacerbated by other factors that affect the metabolism of cytosolic DA. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Membrane transporters and drug development: relevance to pharmacogenomics, nutrigenomics, epigenetics, and systems biology

The study of membrane transporters may result in breakthroughs in the discovery of new drugs and the development of safer drugs. Membrane transporters are essential for fundamental cellular functions and normal physiological processes. These molecules influence drug absorption and distribution and play key roles in drug therapeutic effects. A primary goal of current research in drug discovery and development is to fully understand the interactions between transporters and drugs at both the system levels in the human body and the individual level for personalized therapy. Systematic studies of membrane transporters will help in not only better understanding of diseases from the systems biology point of view but also better drug design and development. The exploration of both pharmacogenomics and systems biology in transporters is necessary to connect individuals' genetic profiles with systematic drug responses in the human body. Understanding of gene-diet interactions and the effects of epigenetic changes on transporter gene expression may help improve clinical drug efficacy. The integration of pharmacogenomics, nutrigenomics, epigenetics, and systems biology may enable us to move from disease treatment to disease prevention and optimal health. The key issues in such integrative understanding include the correlations between structure and function, genotype and phenotype, and systematic interactions among transporters, other proteins, nutrients, drugs, and the environment. The exploration in these key issues may ultimately contribute to personalized medicine with high efficacy but less toxicity.

A novel approach to the detection of genomic approximate tandem repeats in the Levenshtein metric

An efficient algorithm for detecting approximate tandem repeats in genomic sequences is presented. The algorithm is based on innovative statistical criteria to detect candidate regions which may include tandem repeats; these regions are subsequently verified by alignments based on dynamic programming. No prior information about the period size or pattern is needed. Also, the algorithm is virtually capable of detecting repeats with any period. An implementation of the algorithm is compared with the two state-of-the-art tandem repeats detection tools to demonstrate its effectiveness both on natural and synthetic data. The algorithm is available at www.cs.brown.edu/people/domanic/tandem/.

Molecular genetics of monoamine transporters: relevance to brain disorders

We have demonstrated in both the human serotonin transporter gene (5HTT) and the dopamine transporter gene (DAT1) that specific polymorphic variants termed Variable Number Tandem Repeats (VNTRs), which correlate with predisposition to a number of neurological and psychiatric disorders, act as transcriptional regulatory domains. We have demonstrated that these domains can act as both tissue-specific and stimulus-inducible regulators of gene expression. As such they can act to be mechanistically associated with the progression or initiation of a behavioural disorder by altering the level of transporter mRNA, which in turn regulates the concentration of transporter in specific cells or in response to a challenge; chemical, environmental or physiological. The synergistic actions of such transcriptional domains will modulate gene expression. Our hypothesis is that these VNTR variants are one mechanism by which nurture can modify concentrations of neurotransmitters in a differential manner.

5′ and 3′ region variability in the dopamine transporter gene ( SLC6A3 ), pesticide exposure and Parkinson's disease risk: a hypothesis-generating study

The dopamine transporter gene (SLC6A3) is a candidate gene for Parkinson's disease (PD) on the basis of its critical role in dopaminergic neurotransmission. Previously, we identified 22 SNPs in the 5' region of SLC6A3, which segregate as eight haplotypes that differ in transcriptional activity when transfected in rat dopamine-producing cells. In the present work from a case-control study size of 293 cases and 395 controls, we employed a cladistic approach to examine gene-disease association. First, we found strong evidence of balancing selection in this region, as determined by a Tajima's D statistic of 2.97 (P<0.001). Second, we found that the eight haplotypes fit into two main clades and that diplotypes of these clades were marginally associated with PD. Then, after we classified cases and controls by the number of risk alleles, accounting for the well-known 3' region VNTR polymorphism, we found that having two or more risk alleles resulted in a modest but significant increase in PD risk [odds ratio=1.58; 95% confidence interval (CI): 1.03-2.40]. Finally, we detected a significant interaction between occupational pesticide exposure in men and the number of risk alleles. Among pesticide-exposed subjects, the odds ratio for having two or more risk alleles was 5.66 (95% CI: 1.73-18.53). Thus, allelic variants in SLC6A3, which affect gene expression, are associated with PD in this population and may interact with occupational pesticide exposure to increase PD risk.

Dopamine transporter (SLC6A3) 5′ region haplotypes significantly affect transcriptional activity in vitro but are not associated with Parkinson's disease

The dopamine transporter (DAT) plays a critical role in dopaminergic neurotransmission and is also the major site of action for some drugs of abuse. The coding region of the DAT gene, SLC6A3, is well conserved, but non-coding regions are more variable, most notably a variable number of tandem repeats (VNTR) polymorphism in the 3' untranslated region, which has been studied in a number of dopamine-related neurological disorders, including Parkinson's disease (PD). We aimed to characterize variation in the 5' region of SLC6A3 because little is known about the extent of variation in this region and potential consequences of such variation on gene expression. We identified multiple single nucleotide polymorphisms (SNPs) covering approximately 5000 bp 5' of exon 1 through the start of exon 2 (+2106). These SNPs segregated as eight haplotypes, six of which were common. These haplotypes differed significantly in activity in a reporter gene activity assay. However, we did not observe associations between common SNPs or haplotypes and PD in a case-control study of 261 incident cases and 376 age- and gender-matched unrelated controls. By contrast, we did observe a modest association of the 3' VNTR 9-repeat allele with PD (odds ratio=1.45; 95% confidence interval=1.04-2.03). This association was limited to subjects 60 years of age and greater versus those less than 60 years of age. We conclude that although DAT 5' region SNPs haplotypes significantly alter in vitro transcriptional activity, they are not related to PD risk. In addition, our findings provide further evidence supporting an association of PD with the VNTR polymorphism.

The effect of monoamine oxidase B (MAOB) and catechol-O-methyltransferase (COMT) polymorphisms on levodopa therapy in patients with sporadic Parkinson's disease

OBJECTIVES

The etiology of sporadic idiopathic Parkinson's disease (PD) is considered multifactorial with both genetic and environmental factors modifying the disease expression. Recent studies suggest that polymorphism in monoamine oxidase B (MAOB) and catechol-O-methyltransferase (COMT) might influence the risk and treatment of PD. The aim of the study was to evaluate the effect of MAOB and COMT genetic polymorphism on effective daily dose of levodopa applied during the first 5 years of treatment, and to find out if a relationship exists between MAOB and COMT haplotypes and motor disturbances onset in PD patients treated with levodopa preparations.

MATERIALS AND METHODS

A total of 95 patients (40 females and 55 males) of Polish origin diagnosed with sporadic PD were enrolled into the study, and were divided into two groups. Group 1 - patients treated with doses of levodopa below 500 mg/day during the first 5 years of treatment. Group 2 - patients requiring levodopa doses exceeding 500 mg/24 h during the first 5 years of treatment. Low activity alleles of MAOB and COMT, i.e. MAOB allele A and COMT(L) as well as high activity ones, i.e. MAOB allele G and COMT(H), were determined using PCR-RFLP method.

RESULTS

No statistically significant differences were found in MAOB and COMT allele distribution in the two groups. However, the frequency of COMT(L/L) homozygotes was higher in the group treated with low doses of levodopa when compared with the second group. MAOB and COMT AG-HH haplotype predominated in the group of females treated with high daily doses of levodopa when compared with AG-LL haplotype in the group of females treated with low daily doses of levodopa (<500 mg/24 h).

CONCLUSION

The results of the study suggest that patients with COMT(L/L) genotype and possibly MAOB genotype A may benefit from more efficient and safer levodopa therapy.

Polymorphism in candidate genes: implications for the risk and treatment of idiopathic Parkinson's disease

Idiopathic Parkinson's disease (IPD) is a progressive neurodegenerative disorder for which no restorative or neuroprotective therapy is available. Interest has recently been directed to association studies on polymorphisms of various genes, mainly those related to dopamine metabolism and transport, and their effect on response to PD, which includes primarily levodopa and dopaminomimetics. Approximately 15-20% of patients with PD do not respond to levodopa, and the majority of those who do respond develop adverse fluctuations in motor response, primarily levodopa-induced dyskinesias. This review summarizes the influence of polymorphisms in various genes on the relative risk of IPD and on levodopa efficacy. It focuses on the importance of well-designed polymorphism studies that include large samples of patients with IPD and tightly matched controls and use identical methodologies. Valid data on such polymorphisms might increase the efficacy of levodopa, decrease its side effects, and reduce the occurrence of levodopa-induced dyskinesias. They might also provide a novel diagnostic tool for PD.

Dopamine transporter: basic science and human variation of a key molecule for dopaminergic function, locomotion, and parkinsonism

We review the basic science of the dopamine transporter (DAT), a key neurotransmitter for locomotor control and reward systems, including those lost or deranged in Parkinson's disease (PD). Physiology, pharmaceutical features, expression, cDNA, protein structure/function relationships, and phosphorylation and regulation are discussed. The localization of DAT provides the best marker for the integrity of just the pre-synaptic dopaminergic systems that are most affected in PD. Its function is key for the actions of several toxins that provide some of the best current models for idiopathic parkinsonism, and its variation can clearly alter movement. The wealth of information about this interesting molecule that has been developed over the last 12 years has led to increased interest in DAT among workers interested in both normal and abnormal movement.

Polymorphisms of dopamine receptor and transporter genes and hallucinations in Parkinson's disease

The authors conducted a case-control study of Parkinson's disease patients with and without visual hallucinations to investigate associations of the polymorphisms of the dopamine receptors D2 32806 C>T (Taq1A), D3 Ser9Gly and Msp1, D5 978T>C and dopamine transporter 3'-UTR 40 bp VNTR with visual hallucinations in Parkinson's disease. No significant differences were found between hallucinators and non-hallucinators in either the genotypic or allelic distributions. Our data suggest that the loci investigated here are not associated with the visual hallucinogenesis in Parkinson's disease.

The homozygote 10-copy genotype of variable number tandem repeat dopamine transporter gene may confer protection against Parkinson's disease for male, but not to female patients

We investigated the role of variable number tandem repeat (VNTR) polymorphism of the dopamine transporter gene (DAT) in the pathogenesis of Parkinson's disease (PD) in Taiwanese. A case-control study was carried out to examine the association of the VNTR polymorphism within the DAT between 193 sporadic PD patients and 254 controls, matched by age and sex. Six alleles of VNTR polymorphism in the DAT, consisting of 6, 7, 8, 9, 10 and 11 copies of the 40-base-pair (bp) repeat sequence, were detected in the study. There were no differences of allele frequency (chi(2)=5.239, p=0.387) and genotype polymorphism of the DAT VNTR (chi(2)=11.873, p=0.157) in PD patients from the controls. Further analysis stratified by sex and age at onset did not show associations. However, PD patients carrying homozygote 10-copy genotype of the DAT VNTR polymorphism were 0.67 times fewer than controls (chi(2)=4.569, odds radio (OR)=0.67, 95% confidence interval (CI)=0.45-0.97, p=0.033). The reduced risk of the homozygosity with PD genotype was only in male PD patients (chi(2)=2.923, OR=0.48, 95% CI=0.25-0.93, p=0.026), but not in female PD patients (chi(2)=0.002, OR=1.02, 95% CI=0.49-2.11, p=0.966). In conclusion, the results of our study show that homozygote 10-copy genotype of the VNTR polymorphism within the DAT may confer a protective factor for male PD patients, but not for female PD patients.

Effects of green tea polyphenols on dopamine uptake and on MPP+ -induced dopamine neuron injury

As antioxidants, polyphenols are considered to be potentially useful in preventing chronic diseases in man, including Parkinson's disease (PD), a disease involving dopamine (DA) neurons. Our studies have demonstrated that polyphenols extracted from green tea (GT) can inhibit the uptake of 3H-dopamine (3H-DA) and 1-methyl-4-phenylpyridinium (MPP(+)) by DA transporters (DAT) and partially protect embryonic rat mesencephalic dopaminergic (DAergic) neurons from MPP(+)-induced injury. The inhibitory effects of GT polyphenols on 3H-DA uptake were determined in DAT-pCDNA3-transfected Chinese Hamster Ovary (DAT-CHO) cells and in striatal synaptosomes of C57BL/6 mice in vitro and in vivo. The inhibitory effects on 3H-MPP(+) uptake were determined in primary cultures of embryonic rat mesencephalic DAergic cells. Inhibition of uptake for both 3H-DA and 3H-MPP(+) was dose-dependent in the presence of polyphenols. Incubation with 50 microM MPP(+) resulted in a significant loss of tyrosine-hydroxylase (TH)-positive cells in the primary embryonic mesencephalic cultures, while pretreatment with polyphenols (10 to 30 microg/ml) or mazindol (10 microM), a classical DAT inhibitor, significantly attenuated MPP(+)-induced loss of TH-positive cells. These results suggest that GT polyphenols have inhibitory effects on DAT, through which they block MPP(+) uptake and protect DAergic neurons against MPP(+)-induced injury.

Association between dopamine transporter gene polymorphism and susceptibility to Parkinson's disease in Japan

We studied a polymorphism in the dopamine transporter (DAT) gene in 236 Japanese patients with Parkinson's disease (PD) and compared the results with 220 controls. The 1215A/G genotype of the DAT gene was significantly different between PD patients and controls, suggesting a possible involvement of DAT in genetic susceptibility to PD.

Cocaine, reward, movement and monoamine transporters

Recent evidence enriches our understanding of the molecular sites of action of cocaine reward and locomotor stimulation. Dopamine transporter blockade by cocaine appears a sufficient explanation for cocaine-induced locomotion. Variation in DAT appears to cause differences in locomotion without drug stimulation. However, previously-held views that DAT blockade was the sole site for cocaine reward have been replaced by a richer picture of multitransporter involvement with the rewarding and aversive actions of cocaine. These new insights, derived from studies of knockout mice with simultaneous deletions and/or blockade of multiple transporters, provide a novel model for the rewarding action of this heavily-abused substance and implicate multiple monoamine systems in cocaine's hedonic activities.