The dopamine transporter gene and Parkinson's disease in a Chinese population

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.

Variants in the 3' End of SLC6A3 in Northwest Han Population with Parkinson's

Parkinson's disease (PD) is one of the most common neurodegenerative disorders in neurology. It is possible that multifactorial and genetic factors are related to its pathogenesis. Recently, there have been reports of SLC6A3 genetic variants leading to PD. However, the role of 3′ end of SLC6A3 in PD is less studied in different ethnic groups. To explore the roles of 3′ end of SLC6A3 in PD development, 17 SNP sites in 3′ end of SLC6A3 were analyzed in 360 PD patients and 392 normal controls of Han population residing in northwest of China. The significant difference of gene type and allele frequencies between the PD and control groups was detected only in rs40184 (P = 0.013 and 0.004, respectively; odds ratio 2.529, 95% confidence interval 1.325–4.827). The genotype and allele frequencies of the other 16 SNP sites were not found to be different between the PD group and the control group. rs2550936, rs3776510, and rs429699 were selected to construct the haplotypes; no significant difference was found in a frequency of 5 haplotypes between the PD group and the control group. These results suggest that the SLC6A3 variant in rs40184 A allele may increase the risk of PD in northwest Han population and may be a biomarker of 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.

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.

SPECT and PET of the dopamine transporter in attention-deficit/hyperactivity disorder

Abnormalities of frontostriatal circuits, which are modulated by dopamine, have been found by brain imaging studies in patients with attention-deficit/hyperactivity disorder (ADHD). With special radiolabeled ligands selective imaging of the dopamine transporter (DAT), which has a key function in dopamine metabolism, can be performed by SPECT and PET. Most of the studies showed a higher DAT availability in untreated patients with ADHD compared with controls. The relationship between DAT availability and a polymorphism of DAT1 gene in patients with ADHD is not clear and the results are controversial. It has been shown that methylphenidate lowers DAT availability very effectively in normal people and in patients with ADHD. First results seem to indicate that nonresponders to methylphenidate among ADHD patients have a low primary DAT availability, whereas patients with a good response to the drug have high DAT. Nicotine seems to lower DAT availability such as stimulant medication; this may explain the high percentage of smokers among patients with ADHD. Zinc is a DAT inhibitor and seems to have a positive therapeutic effect on ADHD symptoms. This article reviews the function and structure of the DAT, the results of DAT imaging with SPECT and PET, the relations between DAT availability and the DAT1 gene polymorphism, the influence of stimulants on DAT and the significance of DAT for therapeutic response, nicotine, zinc and psychotic symptoms in patients with ADHD.

Genetics and Parkinson's disease

Idiopathic Parkinson's disease is a neurodegenerative disorder that affects 1-2% of the population over the age of 65 years. Its aetiology is most likely a combination of complex genetic and environmental factors. Although Mendelian inheritance is seen in less than 5% of cases, recent studies have identified three genes mutations causing Parkinson's disease with a Mendelian inheritance pattern: autosomal dominantly inherited mutations of the alpha-synuclein gene on chromosome 4q21-q23, autosomal recessively inherited mutations of the parkin gene on chromosome 6q25.2-q27 and an autosomal dominantly inherited mutation of the Ubiquitin C-terminal hydrolase L1 (UCH-L1) gene on chromosome 4p14-15.1. A number of other candidate gene polymorphisms including cytochrome P450 2D6, N-acetyltransferase 2, monoamine oxidase-B and glutathione-s-transferase M1 are implicated in sporadic and familial cases and may also play a minor role in the aetiology of Parkinson's disease.

Advances in the genetics of Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disorder affecting a significant proportion of the ageing population. The etiology is unknown and it is likely due to a multifactorial interaction of genes and the environment on the background of ageing. Findings in the last decade suggest that the contribution of genetics to familial forms of PD is much greater than previously appreciated. Twelve loci are now associated with highly penetrant autosomal dominant or recessive PD, and causative mutations have been identified in eight genes with mutation carriers often characterized by a phenotype indistinguishable from idiopathic disease. To date, PD pharmacotherapy is symptomatic only and does not slow disease progression. Understanding how genetic mutations cause familial PD is likely to clarify molecular mechanisms underlying PD in general and will provide a guide for the development of novel therapies, both preventative and palliative, applicable to all forms of parkinsonism. This review outlines the advances in the study of the genetic background of PD and their possible clinical implications.

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.

Gene-environment interactions in sporadic Parkinson's disease

Much has been learned in recent years about the genetics of familial Parkinson's disease. However, far less is known about those malfunctioning genes which contribute to the emergence and/or progression of the vast majority of cases, the 'sporadic Parkinson's disease', which is the focus of our current review. Drastic differences in the reported prevalence of Parkinson's disease in different continents and countries suggest ethnic and/or environmental-associated multigenic contributions to this disease. Numerous association studies showing variable involvement of multiple tested genes in these distinct locations support this notion. Also, variable increases in the risk of Parkinson's disease due to exposure to agricultural insecticides indicate complex gene-environment interactions, especially when genes involved in protection from oxidative stress are explored. Further consideration of the brain regions damaged in Parkinson's disease points at the age-vulnerable cholinergic-dopaminergic balance as being involved in the emergence of sporadic Parkinson's disease in general and in the exposure-induced risks in particular. More specifically, the chromosome 7 ACHE/PON1 locus emerges as a key region controlling this sensitive balance, and animal model experiments are compatible with this concept. Future progress in the understanding of the genetics of sporadic Parkinson's disease depends on globally coordinated, multileveled studies of gene-environment interactions.

Lack of association between polymorphic microsatellites of the VMAT2 gene and Parkinson's disease in Japan

The etiology of Parkinson's disease (PD) remains unclear; however, generation of reactive oxygen species during oxidation of dopamine (DA) could be one of the factors leading to selective loss of nigral dopaminergic neurons in PD. Vesicular monoamine transporter type 2 (VMAT2) proteins in nerve terminals uptake and partition DA from neuronal cytoplasm into synaptic vesicles. Therefore, alterations of VMAT2 function may cause cytoplasmic accumulation of free DA, toxic to dopaminergic neurons. Upstream of a putative promoter region of the VMAT2 gene, there exist polymorphic sequences consisting of two microsatellites, (CA)n and (GA)n. We performed a case-control study of this polymorphic region to determine whether the VMAT2 gene is related to PD. We found six genotypes; however, there was no significant difference in the allele frequencies between patients with PD and control subjects. Our data suggest that the polymorphic region of the VMAT2 gene studied here is not closely related to PD.

Study of familial Parkinson's disease in Russia, Uzbekistan, and Zambia

OBJECTIVE

The aims of this study were (A) to determine inheritance patterns of familial Parkinson's disease in three different geographical areas (Russia, Uzbekistan, and Zambia); (B) compare clinical characteristics of familial with sporadic Parkinson's disease; and (C) assess whether there were ethnic differences in clinical manifestations of the disease.

METHODS

Fifty two index cases of familial Parkinson's disease in Moscow, 55 in Tashkent, and 27 in Lusaka were selected on the basis of the typical clinical features of Parkinson's disease with a familial history. The sex ratio, transmission patterns, and segregation ratio were determined by pedigree analysis.

RESULTS

Familial Parkinson's disease was found in all three countries (30 families in Russia, 12 in Uzbekistan, and seven in Zambia), and appeared more common in Russia. Both autosomal dominant and autosomal recessive patterns of inheritance were seen, but autosomal dominance was more common in all countries.

CONCLUSIONS

In all three countries men have a higher risk of developing Parkinson's disease than women and there are ethnic differences in clinical manifestations of the disease. The onset of both familial and sporadic Parkinson's disease in Zambian patients occurs at a younger age and is associated with slow progression and a benign course, and generally responds well to levodopa treatment.

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.

Prevalence of Parkinson's disease in a Chinese population

BACKGROUND

Ethnic differences in the prevalence of Parkinson's disease have been observed, but may be due to differences in screening instruments and diagnostic criteria.

AIMS

To compare the prevalence of Parkinson's disease in Hong Kong Chinese and Australians in Sydney.

METHODS

A door-to-door community survey in the Hong Kong Chinese population was carried out using a two-staged procedure (screening questionnaire followed by physical examination), which had been validated for use in the community in a survey among Australians, to determine the prevalence in Chinese and to compare with that in Australians.

RESULTS

A total of 1080 households were contacted, the non-response rate was 27%. The prevalence among those aged 55 years in the community and above was 0.5%, and 0.186% for the overall population assuming no subjects were below 55 years. These figures are lower than the 3.6% for Australians, aged >55 years.

CONCLUSION

There appears to be a difference in prevalence of Parkinson's disease between Hong Kong Chinese and Australians in Sydney. Such differences may be due to documented differences in prevalence of genetic polymorphisms associated with Parkinson's disease between Chinese and Australians, or to differences in environmental factors.

The dopamine transporter and neuroimaging in attention deficit hyperactivity disorder

There is evidence that abnormalities within the dopamine system in the brain play a major role in the pathophysiology of attention deficit hyperactivity disorder (ADHD). For instance, dopaminergic psychostimulants, the drugs of first choice in ADHD, interact directly with the dopamine transporter (DAT). Molecular genetic studies suggest involvement of a polymorphism of the DAT gene in ADHD. More recent imaging studies show abnormalities in various brain structures, but particularly in striatal regions. In the current paper we review recent studies in this area. First in vivo measurements of DAT with single photon emission computed tomography (SPECT) in ADHD patients revealed an elevation of striatal DAT density. No differences in DAT density between the left and right side and between putamen and caudate nucleus have been found in [99mTc]TRODAT-1 SPECT of ADHD patients. Patients with ADHD and with a history of nicotine abuse both displayed lower values of DAT density in [99mTc]TRODAT-1 SPECT than non-smokers with ADHD. DAT seem to be elevated in non-smoking ADHD patients suffering from the purely inattentive subtype of ADHD as well as in those with the combined or purely hyperactive/impulsive subtype.

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.

Genetic and environmental factors in the cause of Parkinson's disease

Despite being the subject of intense study, the pathogenesis of Parkinson's disease still remains unclear. In recent years, however, there has been increasing evidence to support a role for genetic factors in its cause. This has come from twin and family studies, the mapping and cloning of PARK genes that are associated with the development of PD, and analysis of potential susceptibility genes. There is also evidence indicating that environmental factors may play a role in the disease process. It is likely that for most cases, there is a complex interplay between these genetic and environmental influences in the causation of Parkinson's disease. This article reviews the evidence in support of genetic and environmental factors in the cause of PD.

Case-control study of dopamine transporter-1, monoamine oxidase-B, and catechol-O-methyl transferase polymorphisms in Parkinson's disease

We investigated the association of Parkinson's disease (PD) with dopamine transporter-1 (DAT1), monoamine oxidase-B (MAO-B), and catechol-O-methyltransferase (COMT) gene polymorphisms. Overall, we observed no significant association of PD with the DAT1-3'-variable numbers of tandem repeats, the MAO-B-(GT)(n), and the COMT-Val108Met gene polymorphisms in a sample of 319 unrelated PD cases and 196 control subjects. Analyses stratified by sex, age at examination, family history of PD, and ethnic origin also yielded negative findings, with three exceptions. We found statistically significant associations of PD with MAO-B polymorphisms in older patients and with a COMT polymorphism in younger subjects and in women. These significant differences at the two-tailed alpha level of 0.05 and restricted to subgroup analyses may have a biological basis or may be chance findings.

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.

Spinocerebellar ataxia type 2 with parkinsonism in ethnic Chinese

OBJECTIVE

To describe the clinical and molecular genetic analysis of a large family of northern Chinese descent with a mutation at the SCA2 locus causing carbidopa-levodopa-responsive parkinsonism.

BACKGROUND

Most causes of parkinsonism remain unknown. However, molecular genetic analysis of families with parkinsonism has recently identified five distinct loci and pathogenic mutations in four of those. Additionally, some of the spinocerebellar ataxia syndromes (SCA), particularly Machado-Joseph syndrome (SCA3), are known to cause parkinsonism. Spinocerebellar ataxia type 2 (SCA2) has not previously been described as causing a typical dopamine-responsive asymmetric PD phenotype.

METHODS

A large family was evaluated clinically and molecularly for apparent autosomal dominant parkinsonism.

RESULTS

The phenotype includes presentation consistent with typical dopamine-responsive parkinsonism. Other presentations in this family include a parkinsonism/ataxia phenotype, which is classic for SCA2 and parkinsonism, resembling progressive supranuclear palsy.

CONCLUSIONS

Patients presenting with a family history of parkinsonism, including familial progressive supranuclear palsy and PD, should be tested for the spinocerebellar ataxia type 2 expansion.

Human dopamine transporter gene: coding region conservation among normal, Tourette's disorder, alcohol dependence and attention-deficit hyperactivity disorder populations

The dopamine transporter (DAT) provides major regulation of the synaptic levels of dopamine and is a principal target of psychostimulant drugs. Associations between DAT gene polymorphisms and human disorders with possible links to dopaminergic neurotransmission, including attention-deficit/hyperactivity disorder (ADHD) and consequences of cocaine and alcohol administration, have been reported. We now report approximately 60000 bp of genomic sequence containing the entire DAT gene. This sequence was used to amplify each of the 15 DAT gene exons and several introns and analyze these amplification products by single-stranded sequence conformation (SSCP) and/or direct sequencing. These results define silent allelic single nucleotide sequence variants in DAT gene exons 2, 6, 9 and 15. Rare conservative mutations are identified in amino acids encoded by DAT exons 2 and 8. Analyses of the common nucleotide variants and the previously reported VNTR in the non-coding region of exon 15 define the pattern of linkage disequilibrium across the DAT locus. These comprehensive analyses, however, fail to identify any common protein coding DAT sequence variant in more than 150 unrelated individuals free of neuropsychiatric disease, 109 individuals meeting City of Hope criteria for Tourette's syndrome, 64 individuals with DSM-IV diagnoses of ethanol dependence, or 15 individuals with ADHD. These data are consistent with substantial evolutionary conservation of the DAT protein sequence. They suggest that gene variants that alter levels of DAT expression provide the best current candidate mechanism for reported associations between DAT gene markers, ADHD and other more tentatively associated neuropsychiatric disorders.

Low-density lipoprotein receptor-related protein (LRP) gene 766T polymorphism and Parkinson's disease

The C766T polymorphism in exon 3 of the low-density lipoprotein receptor-related protein (LRP) gene is underrepresented in Alzheimer's disease (AD) compared with normal subjects. We examined this polymorphism in 186 patients with Parkinson's disease (PD) and 187 age-matched normal Chinese subjects in addition to 227 newborns representing the general population. The fraction of individuals with 766T was 12.8% in normal subjects and 11.3% in patients with PD, not a significant difference (p = 0.77). The odds ratio was 0.86 with a 95% confidence interval of 0.44-1.69, thus the LRP C766T polymorphism does not play a major role in risk for PD, although the possibility cannot be excluded that it plays a minor role or is a significant risk factor in other ethnic groups.

Global variation of a 40-bp VNTR in the 3'-untranslated region of the dopamine transporter gene (SLC6A3)

BACKGROUND

The dopamine transporter (DAT) is the primary mechanism for dopamine clearance from the synapse in midbrain dopaminergic neurons, and the target of psychostimulant and neurotoxic drugs such as cocaine, amphetamine, and MPTP. Consequently, the gene for DAT (SLC6A3) has been the focus of many population-based case-control association studies using a 40-bp VNTR in the 3'-untranslated region. Results have differed depending on the population studied, suggesting allele frequency effects are involved. For this reason, a global survey of allele frequencies for this VNTR polymorphism was performed.

METHODS

Individuals (n = 1528) from 30 populations around the world were typed for this VNTR using PCR and agarose gel electrophoresis.

RESULTS

As with previous studies, the ten-repeat allele is most common, except for a Middle Eastern population in which the nine-repeat allele is most frequent. Frequencies of the nine- and ten-repeat alleles vary widely even among European populations.

CONCLUSIONS

Many previous association studies have used "white" or "black" U.S. populations. However, many different ethnic groups have contributed to these populations. The large variation in allele frequencies observed in this study emphasizes the inadequacy of most past studies using the case-control design and the importance of matching patient and control populations in future association studies.

Lack of allelic association of dopamine D4 receptor gene polymorphisms with Parkinson's disease in a Chinese population

Parkinson's disease (PD) is a neurodegenerative disease caused by a multitude of environmental, neurochemical, and genetic factors. The gene for human dopamine D4 receptor (DRD4) has been considered as a plausible candidate for the pathogenesis of PD. Different dopamine D4 receptor allelic forms have variable affinity toward certain neuroleptics such as clozapine, suggesting a role for dopamine D4 receptors in neurologic disorders. To test the hypothesis that the DRD4 polymorphism is associated with the susceptibility to Parkinson's disease, we have examined differences in allele frequencies of different DRD4 polymorphisms in 101 Chinese patients with PD and in 105 age-matched control subjects in Hong Kong. The DRD4 gene was analyzed by a non-radioactive polymerase chain reaction-based Southern hybridization with chemiluminescence detection. The number of polymorphic 48 base pair tandem repeats in exon 3 was identified in each study subject. The DRD4 alleles with high frequencies in the control subjects are 4-repeat allele (72.4%), 2-repeat allele (21.4%), and 7-repeat allele (3.8%) which accounted for over 97% of the total alleles in the elderly Chinese population. The most prevalent genotype in the control subjects is the 4/4 (47.6%), followed by 4/2 (38.6), 4/7 (7.6%), and 2/2 (3.0%). None of the variable number tandem repeat polymorphism showed evidence for genetic association with Parkinson's disease.

Genetic epidemiology of Parkinson's disease

The cause of Parkinson's disease (PD) is unknown. The major risk factors identified to date are family history, age, and elements of rural living. Nearly one-third of all PD cases are familial, a small subset of which appears autosomal dominant; however, the majority exhibit no clear inheritance pattern. Autosomal dominant PD is genetically heterogeneous: two PD genes have been mapped to chromosomes 2 and 4 and there may be additional as yet unidentified genes. The common forms of PD-both familial and sporadic cases-appear to involve a complex interplay of genetic susceptibility and environmental exposure. The observations that rural residence and pesticide exposure increase the risk of developing PD, and that a synthetic drug, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, can cause parkinsonism, suggest that at least a subset of PD may be caused by a toxin. Furthermore, modest but significant associations have been reported between PD susceptibility and genes that regulate metabolism of drugs and neurotoxins. There is also evidence for mitochondrial dysfunction in PD, a finding that was recently traced to anomalies in mitochondrial DNA. At the present time, the genetics of PD appear to be complex, involving multiple nuclear genes and possibly mitochondrial genes as well.