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Introduction

Parkinson’s disease (PD, OMIM: 168600) is the second most common neurodegenerative disorder that increases the social and economic burden on societies. PD affects ~2% of the population aged ≥65 years (1,2). The clinical features of PD are resting tremor, muscular rigidity, bradykinesia and postural instability (3). PD can cause pain (4), depression (5,6), visual hallucinations (7), dementia (8) and other non-motor symptoms (9–12).

Environmental and genetic factors may be involved in the pathogenesis of PD. The environmental hypothesis was dominant for much of the 20th century (3). Environmental factors such as encephalitis (13), oxidative stress (14), smoking and coffee (15) and environmental toxins (16) were found to be significantly associated with PD. Recent genetic studies have identified a few genetic markers of PD (17–19), however, the etiology of PD remains unclear.

Findings of previous studies suggested that the degeneration of dopamine neurons in the substantia nigra contributes to the pathogenesis of PD (20,21). Dopamine is a major modulatory neurotransmitter in the central nervous system (CNS) and thus affects neuroendocrine secretion (20), which was shown to be associated with smoking, a well-known risk factor of PD (22–24). There are five subtypes of G-protein-coupled dopamine receptors (DRD1–5) comprising D-1-like (DRD1 and DRD5) and D-2-like (DRD2–4). Among these subtypes, DRD2 (25–36) located on chromosome 11q23 and DRD3 (33,35–40) located on chromosome 3q13.3 are the most studied in the association tests of PD.

Previous case-control studies showed inconsistent results between PD and the two dopamine receptor genes (DRD2 and DRD3). These discrepancies may be due to different ethnic background, or sample size difference (41), or the uncorrected physiological status among various association studies (42). Meta-analysis is a robust method for application to enhance statistical power and to draw a more convincing conclusion through the pooling of data from each of the involved studies (43). The aim of this meta-analysis was to evaluate the contribution of DRD2 and DRD3 gene polymorphisms to PD and to determine the causes of the inconsistent results among various case-control association studies.

Materials and methods

Data collection

Studies were selected through a search of PubMed by using the combinations, including ‘Parkinson DRD2 association or Parkinson DRD2 polymorphism’ and ‘Parkinson DRD3 association or Parkinson DRD3 polymorphism’. Studies were included if they met the following criteria: i) It was an original case-control study with an assessment of the association of DRD2 and DRD3 genes with PD in humans; ii) it contained sufficient information to infer the odd ratios (ORs) and 95% confidence intervals (CIs); and iii) the genotype distribution of each polymorphism in controls met the Hardy-Weinberg equilibrium (HWE). We extracted and calculated the following information from each study: Genetic locus, first author’s name, year of publication, country, ethnicity, number of cases and controls, HWE for controls, reported association results and power of each case-control study.

Statistical analysis

The Arlequin program was used to test HWE (44). Statistical heterogeneity across studies included in the meta-analysis was assessed by Cochran’s Q statistic and I2 test (45). In our meta-analysis, the fixed-effect model was used for the studies with minimal to moderate heterogeneity (I2<50%) and the random-effect model was used for the studies with significant heterogeneity (I2≥50%). Funnel plots were also drawn to observe the potential publication bias. Statistical analyses of meta-analyses were carried out by Review Manager 5 (46). The power of each study was calculated by the Power and Sample Size Calculation program (47).

Results

As shown in Fig. 1, 33 relevant studies of DRD2 and 19 studies of DRD3 were included from PubMed. In addition, we also retrieved two articles on DRD2 gene from the Chinese Wangfang database and one article on DRD3 gene from the China National Knowledge Internet. Following removal of three duplicates, 16 articles for the current meta-analysis (Table I) (25–40). Moreover, nine stages (36) (Table I) were excluded with significant deviation from HWE (P<0.05). Altogether there were 4,279 PD patients and 5,661 controls in the current meta-analyses of 9 polymorphisms (Tables II and III).

Figure 1 Flowchart of the selection process in the meta-analyses.

Table I Characteristics of the case-control studies in the current meta-analyses.

Table I

Characteristics of the case-control studies in the current meta-analyses.

Genetic locus	First author	Year	Ethnicity	Cases/controls	HWE	Resultsa	Power	(Refs.)
DRD2 rs1800497	Costa-Mallen et al	2000	Europeans	125/202	Yes	NS	0.154	(27)
DRD2 rs1800497	Grevle et al	2000	Europeans	72/81	Yes	S	0.085	(26)
DRD2 rs1800497	Oliveri et al	2000	Europeans	135/202	Yes	NS	0.139	(25)
DRD2 rs1800497	Tan et al	2003	Asians	209/216	Yes	NS	0.258	(29)
DRD2 rs1800497	Chen	2006	Asians	180/387	Yes	NS	0.293	(30)
DRD2 rs1800497	Singh et al	2008	Asians	70/100	Yes	NS	0.125	(31)
DRD2 rs1800497	Lee et al	2009	Asians	402/558	Yes	NS	0.496	(33)
DRD2 rs1800497	Li	2009	Asians	166/170	Yes	NS	0.218	(32)
DRD2 rs1800497	Kiyohara et al	2011	Asians	238/369	Yes	NS	0.335	(34)
DRD2 rs1800497	Lee et al	2011	Asians	500/559	Yes	NS	0.545	(35)
DRD2 rs1800497	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	NS	0.708	(36)
DRD2 rs1800497	McGuire et al 2	2011	Europeans	141/164	Yes	NS	0.181	(36)
DRD2 rs1800497	McGuire et al 3	2011	Asians	120/226	No	NS	0.201	(36)
DRD2 rs1800497	McGuire et al 4	2011	Africans	29/66	Yes	S	0.089	(36)
DRD2 rs1079597	Oliveri et al	2000	Europeans	135/202	Yes	S	0.119	(25)
DRD2 rs1079597	Costa-Mallen et al	2000	Europeans	152/231	Yes	NS	0.146	(27)
DRD2 rs1079597	Kelada et al	2002	Europeans	179/290	Yes	NS	0.174	(28)
DRD2 rs1079597	Tan et al	2003	Asians	204/216	Yes	NS	0.26	(29)
DRD2 rs1079597	Chen	2006	Asians	180/387	Yes	NS	0.297	(30)
DRD2 rs1079597	Singh et al	2008	Asians	70/100	Yes	NS	0.121	(31)
DRD2 rs1079597	Li	2009	Asians	166/170	Yes	NS	0.217	(32)
DRD2 rs1076563	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	NS	0.844	(36)
DRD2 rs1076563	McGuire et al 2	2011	Europeans	141/164	Yes	NS	0.2	(36)
DRD2 rs1076563	McGuire et al 3	2011	Asians	120/226	Yes	NS	0.119	(36)
DRD2 rs1076563	McGuire et al 4	2011	Africans	29/66	Yes	NS	0.089	(36)
DRD2 rs6279	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	NS	0.797	(36)
DRD2 rs6279	McGuire et al 2	2011	Europeans	141/164	Yes	NS	0.191	(36)
DRD2 rs6279	McGuire et al 3	2011	Asians	120/226	No	NS	0.207	(36)
DRD2 rs6279	McGuire et al 4	2011	Africans	29/66	No	NS	0.089	(36)
DRD2 rs6278	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	NS	0.628	(36)
DRD2 rs6278	McGuire et al 2	2011	Europeans	141/164	No	NS	0.153	(36)
DRD2 rs6278	McGuire et al 3	2011	Asians	120/226	No	NS	0.201	(36)
DRD2 rs6278	McGuire et al 4	2011	Africans	29/66	Yes	NS	0.071	(36)
DRD2 rs273482	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	NS	0.798	(36)
DRD2 rs273482	McGuire et al 2	2011	Europeans	141/164	Yes	NS	0.196	(36)
DRD2 rs273482	McGuire et al 3	2011	Asians	120/226	No	NS	0.207	(36)
DRD2 rs273482	McGuire et al 4	2011	Africans	29/66	No	NS	0.089	(36)
DRD2 rs1799732	McGuire et al 1	2011	Europeans	1,035/1,279	No	NS	0.463	(36)
DRD2 rs1799732	McGuire et al 2	2011	Europeans	141/164	Yes	NS	0.147	(36)
DRD2 rs1799732	McGuire et al 3	2011	Asians	120/226	Yes	NS	0.134	(36)
DRD2 rs1799732	McGuire et al 4	2011	Africans	29/66	Yes	NS	0.084	(36)
DRD3 rs6280	Nanko et al	1993	Europeans	60/90	Yes	NS	0.111	(37)
DRD3 rs6280	Nanko et al	1994	Asians	70/81	Yes	NS	0.104	(38)
DRD3 rs6280	Higuchi et al	1995	Asians	70/70	Yes	NS	0.439	(39)
DRD3 rs6280	Lee et al	2009	Asians	398/558	Yes	NS	0.485	(33)
DRD3 rs6280	Wang et al	2000	Asians	122/104	Yes	NS	0.141	(40)
DRD3 rs6280	Lee et al	2011	Asians	497/558	Yes	NS	0.835	(35)
DRD3 rs6280	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	NS	0.201	(36)
DRD3 rs6280	McGuire et al 2	2011	Europeans	141/164	Yes	S	0.178	(36)
DRD3 rs6280	McGuire et al 3	2011	Asians	120/226	Yes	NS	0.074	(36)
DRD3 rs6280	McGuire et al 4	2011	Africans	29/66	Yes	NS	0.11	(36)
DRD3 rs2134655	McGuire et al 1	2011	Europeans	1,035/1,279	Yes	S	0.791	(36)
DRD3 rs2134655	McGuire et al 2	2011	Europeans	141/164	Yes	NS	0.157	(36)
DRD3 rs2134655	McGuire et al 3	2011	Asians	120/226	No	NS	0.189	(36)
DRD3 rs2134655	McGuire et al 4	2011	Africans	29/66	Yes	NS	0.07	(36)

a The association between DRD2 and DRD3 genes and Parkinson’s disease.

{ label (or @symbol) needed for fn[@id='tfn2-br-02-02-0275'] } HWE, Hardy-Weinberg equilibrium; DRD2, dopamine receptor D2; NS, not significant; S, significant; DRD3, dopamine receptor D3.

Table II Meta-analyses of the eight polymorphisms DRD2: rs1079597, rs6278, rs6279, rs273482, rs1799732 and rs1076563; DRD3: rs6280 and rs2134655 with Parkinson’s disease.

Table II

Meta-analyses of the eight polymorphisms DRD2: rs1079597, rs6278, rs6279, rs273482, rs1799732 and rs1076563; DRD3: rs6280 and rs2134655 with Parkinson’s disease.

Genetic locus	Cases/controls	Amount of stages	OR (95% CI)	P-value	I2 (%)	Power
DRD2 rs1079597 (G vs. A)	1,086/1,506	7	1.11 (0.97–1.25)	0.12	25.00	0.855
DRD3 rs6280 (G vs. A)	2,542/3,192	10	1.04 (0.96–1.13)	0.33	0.00	0.996
DRD3 rs2134655 (A vs. G)	1,205/1,509	3	1.17 (1.03–1.32)	0.01	0.00	0.847
DRD2 rs6278 (T vs. G)	1,064/1,345	2	0.45 (0.04–4.94)	0.51	68.00	0.637
DRD2 rs6279 (G vs. C)	1,176/1,443	2	1.03 (0.92–1.16)	0.57	0.00	0.851
DRD2 rs273482 (G vs. C)	1,064/1,345	2	0.99 (0.88–1.22)	0.91	46.00	0.825
DRD2 rs1799732 (D vs. C)	290/456	3	0.94 (0.72–1.24)	0.67	0.00	0.313
DRD2 rs1076563 (G vs. T)	1,325/1,735	4	0.96 (0.86–1.07)	0.44	0.00	0.941

[i] DRD2, dopamine receptor D2; DRD3, dopamine receptor D3; OR, odds ratio; CI, confidence interval; NA, not applicable.

Table III Meta-analyses of DRD2 rs1800497 polymorphism with Parkinson’s disease.

Table III

Meta-analyses of DRD2 rs1800497 polymorphism with Parkinson’s disease.

Genetic locus	Cases/controls	Ethnicity	Amount of stages	OR (95% CI)	P-value	I2 (%)	Power
DRD2 rs1800497 (T vs. C)	3,297/4,353	Overall	13	0.97 (0.84–1.13)	0.73	73.00	0.999
	1,508/1,928	Europeans	5	1.13 (1.00–1.27)	0.05	31.00	0.862
	1,789/2,425	Asians	8	0.87 (0.71–1.06)	0.16	76.00	0.983

[i] DRD2, dopamine receptor D2; OR, odd ratio; CI, confidence interval.

As shown in Tables II and III, no significant associations were observed in the meta-analyses of five DRD2 polymorphisms. A further subgroup study by ethnicity showed a borderline association between DRD2 rs1800497 polymorphism and PD in Europeans (P=0.05, OR=1.13, 95% CI: 1.00–1.27; Table III and Fig. 2). Significant results was found in the meta-analysis of DRD3 rs2134655 polymorphism (P=0.01, OR=1.17, 95% CI: 1.03–1.32; Table II and Fig. 2). No significant results were observed in other meta-analyses. There was no publication bias for any of the meta-analyses (Figs. 2 and 3).

Figure 2 Forest plots and funnel plots of DRD2 rs1800497 and DRD3 rs2134655 polymorphisms with Parkinson’s disease (PD). (A) Forest plot of DRD2 rs1800497 polymorphism with PD. (B) Forest plot of DRD3 rs2134655 polymorphism with PD. (C) Funnel plot of DRD2 rs1800497 polymorphism with PD. (D) Funnel plot of DRD3 rs2134655 polymorphism with PD.

Figure 3 (A) Funnel plot of DRD2 rs1800497 polymorphism with Parkinson’s disease (PD). (B) Funnel plot of DRD2 rs1079597 polymorphism with PD. (C) Funnel plot of DRD3 rs6280 polymorphism with PD. (D) Funnel plot of DRD2 rs6278 polymorphism with PD. (E) Funnel plot of DRD2 rs6279 polymorphism with PD; (F) Funnel plot of DRD2 rs273482 polymorphism with PD; (G) Funnel plot of DRD2 rs1799732 polymorphism with PD; (H) Funnel plot of DRD2 rs1076563 polymorphism with PD.

As shown in Tables II and III, there was no statistically significant heterogeneity (I2<50%) in the meta-analyses of five DRD2 polymorphisms (rs1079597, rs6279, rs273482, rs1799732 and rs1076563) and two DRD3 polymorphisms (rs6280 and rs2134655). Significant heterogeneity was observed in DRD2 rs1800497 (I2=73%) and DRD2 rs6278 (I2=68%). The allele frequency of rs1800497-G in Asians (HapMap-HCB) is 0.512 which is much lower to the 0.805 in Europeans (HapMap-CEU). A further analysis of the rs1800497 polymorphism showed a moderate ethnic difference between Asians and Europeans (Fst=0.1). Since there existed different genotypic distribution of rs1800497 polymorphism between Asians and Europeans, we performed subgroup ethnic meta-analyses for the two populations. A significant heterogeneity was found in Asians (I2=76%, Table III), which may be explained by various types of allele frequency in Asians (HapMap-CHB=0.223, HapMap-JPT=0.349 and HapMap-CHD=0.310).

Power analyses showed that sufficient power (power>0.8) for the meta-analyses of five DRD2 polymorphisms (rs1800497, rs1079597, rs6279, rs273482 and rs1076563) and two DRD3 polymorphisms (rs6280 and rs2134655). The results also indicated that there was a lack of power for the meta-analyses of DRD2 rs6278 (power=0.637) and rs1799732 (power=0.313) polymorphisms.

Discussion

In the current meta-analysis, we summarized the associations of DRD2 (n=7) and DRD3 (n=2) gene variants with PD from 16 studies (46 stages) among 4,279 cases and 5,661 controls. Our meta-analyses demonstrated a borderline significant association with PD for DRD2 rs1800497 in Europeans and a significant association with PD for DRD3 rs2134655. Previous studies have shown that the DRD2 rs1800497-T allele reduced DRD2 density in the postmortem brain (48) and decreased dopamine activity in the CNS (49), which may play an important role in the pathogenesis of PD. DRD3 rs2134655 was a C to T transition that showed significant associations with neurological diseases such as schizophrenia (50).

Our study was the only meta-analysis for DRD2 and DRD3 gene polymorphisms in PD with strict selection criteria. Although power analyses showed our meta-analyses have much larger power than that of each of the individual studies, we did not exclude the possibility of false-negative results in the meta-analyses for DRD2 rs6278 (power=0.637) and rs1799732 (power=0.313). We excluded 9 stages that failed to meet HWE in their control samples (36). We also performed subgroup analyses by ethnicity for DRD2 rs1800497, rs1079597 and DRD3 rs6280.

Our study has several limitations that need to be carefully considered. Firstly, although the power in most meta-analyses is sufficient, the number of involved stages in the Europeans was limited for polymorphisms (including rs2134655 of DRD3 gene; and rs1800497, rs1076563, rs1799732, rs6278, rs6279 and rs273482 of the DRD2 gene). Studies with a larger sample size are therefore needed. Secondly, factors such as gender, age and other different physiological status of PD patients may have influenced the result of our study and partially explain the discrepancies in the involved case-control studies. Future case-control studies may need to provide detailed information to better estimate the contribution of these factors to PD susceptibility. Thirdly, we have performed multiple association tests for the 9 polymorphisms, however, we did not provide correction of P-values that cause false-positive results in our findings. Additionally, genetic heterogeneity may exist for the polymorphisms of DRD2 and DRD3 genes. There are 134 polymorphisms in DRD2 and 88 polymorphisms in DRD3. Our meta-analyses only focused on 9 polymorphisms that might not fully represent the overall contribution of DRD2 and DRD3 gene polymorphisms. Analyses of other polymorphisms in DRD2 and DRD3 genes are required in future investigations to evaluate their contribution to PD.

In conclusion, our meta-analysis observed a borderline significant association of DRD2 rs1800497 polymorphism with PD in Europeans and a significant association of DRD3 rs2134655 with PD. Large-scale well-designed studies are required in future to evaluate the polymorphisms of DRD2 and DRD3 genes that might contribute to the risk of PD.

Acknowledgements

The study was supported by grants from the National Natural Science Foundation of China (nos. 31100919 and 81371469), Natural Science Foundation of Zhejiang Province (no. LR13H020003), K.C. Wong Magna Fund in Ningbo University and Ningbo Social Development Research Projects (no. 2012C50032).

References