C282Y and H63D Polymorphisms in Hemochromatosis Gene and Risk of Parkinson’s Disease: A Meta-Analysis

The available articles published in English (up to January 2015) were identified by extended computer-based searches from the following databases: (1) PubMed and (2) Web of Science. The following key words were used: (“HFE” or “hemochromatosis” or “C282Y”or “H63D”) and (“polymorphism” or “mutation” or “genes”) and (“Parkinson’s disease” or “Parkinson” or “PD”). We also reviewed the references cited in the studies and review articles to identify additional studies not captured by our database searches.

The inclusion criteria were as follows: (1) case–control or cohort study published as original study to evaluate the association between C282Y and H63D polymorphisms in the HFE gene and risk of PD; (2) numbers for the genotype were reported in case and control groups for case–control studies, exposed and unexposed groups for cohort studies for each genotype, or data provided to calculate them; and (3) participants in each study should come from the same ethnicity and period. The most recent and complete articles were chosen if data from the same population had been published more than once. Two investigators carefully reviewed all identified studies independently to determine whether an individual study was eligible for inclusion in this meta-analysis.

Two investigators collected the data independently and reached a consensus on all items. The following basic information was extracted from the eligible studies: first author, journal, year of publication, country, ethnicity of studied population, sample size, mean age, and distributions of allele and genotype. When it came to conflicting evaluations, it was resolved by the third reviewer.

Departure from Hardy-Weinberg equilibrium (HWE) for the C282Y and H63D and genotype distribution of HFE gene in cases and controls was tested by χ² analysis with exact probability (HWE: P > .05). Odds ratio (OR) with 95% confidence interval (CI) was used to assess the strength of the association of the C282Y and H63D polymorphisms in HFE gene with the risk of PD. We conducted analysis for 2 polymorphisms considering dominant (YY + CY vs CC) and (DD + HD vs HH), recessive (YY vs CY + CC) and (DD vs HD + HH), and codominant (Y vs C) and (D vs H) models for C282Y and H63D, respectively. The I² of Higgins and Thompson⁸ and Q test were used to assess heterogeneity among studies. The DerSimonian and Laird random effect model (REM) was adopted as the pooling method if substantial heterogeneity is present (I²> 50%)⁹; otherwise, the fixed-effect model (FEM) was used as the pooling method. Influence analysis¹⁰ was conducted to describe how robust the pooled estimator is to the removal of individual studies. If the main estimate of an individual study’s omitted analysis lies outside the 95% CI of the combined analysis, it is suspected of excessive influence. Egger’s test¹¹ was used to estimate the publication bias. Besides, both theoretical and empirical evidence suggest that general genetic variants causally associated with common diseases will have small effects (risk ratios mostly <3.0),^12,13 considering the fact that original studies with relatively small participants might be underpowered to detect the effect. Thus, for sensitivity analysis, we further excluded the studies with OR > 3.0 as the criteria to control the impact within each single study on the pooled effect. STATA version 10.0 (Stata Corporation, College Station, Texas) was used to perform statistical analyses. All reported probabilities (P values) were 2 sided, with P value less than .05 considered statistically significant.

We identified 8 articles^14-21 with 9 eligible outcomes for this meta-analysis, including 9 outcomes for the C282Y polymorphism and 8 outcomes for the H63D polymorphism. All included articles were case–control designs. General characteristics and genotype distributions of the above-mentioned polymorphisms are summarized in Tables 1 and 2. Figure 1 presents the flowchart for exclusion/inclusion process.

The results of the pooled analysis are summarized in Table 3.

For the C282Y polymorphism, overall, no significant association was found between the Y allele and the risk of PD in dominant (FEM: OR = 0.87, 95% CI = 0.70-1.09), recessive (FEM: OR = 1.58, 95% CI = 0.61-4.10), and codominant (FEM: OR = 0.88, 95% CI = 0.72-1.09) models. Considering 8 of the 9 studies come from Europe, the subgroup analysis for European population was conducted. The Y allele also found no association with risk of PD in dominant (FEM: OR = 1.04, 95% CI = 0.80-1.36), recessive (FEM: OR = 2.37, 95% CI = 0.83-6.82), and codominant (FEM: OR = 1.05, 95% CI = 0.82-1.35) models. All included articles were in HWE for cases and controls. The associations were not altered significantly after excluding articles with OR > 3.0.^16,18Figure 2 presents the forest plot of OR for PD in dominant model of the C282Y polymorphism in the HFE gene.

For the H63D polymorphism, this meta-analysis showed no significant effect of the D allele on risk of PD in dominant (FEM: OR = 1.04, 95% CI = 0.87-1.24), recessive (REM: OR = 1.23, 95% CI = 0.70-2.18), and codominant (FEM: OR = 1.04, 95% CI = 0.89-1.22) models. The included studies about the H63D polymorphism were all conducted in Europe, so the subgroup analysis about H63D was not further investigated. All included articles were in HWE for cases and controls, and all included articles were with OR < 3.0. Figure 3 presents the forest plot of OR for PD in codominant model of the H63D polymorphism in the HFE gene.

If the frequency of mutation homozygous was 0 in the case group, then it was not sufficient to calculate pooled OR for recessive model. For this reason, 4 articles^14,17-19 about C282Y polymorphism were not sufficient to calculate the pooled OR in the recessive model. For the overall risk of PD, after excluding the articles with OR > 3.0,^16,18 the risk effects of the Y allele showed no significant effect in dominant (FEM: OR = 0.82, 95% CI = 0.65-1.02), recessive (REM: OR = 0.77, 95% CI = 0.23-2.56), and codominant (FEM: OR = 0.83, 95% CI = 0.67-1.03) models. No studies with OR > 3.0 existed in the H63D polymorphism.

For the 2 polymorphisms, moderate evidence of heterogeneity (I² < 50%) was found in 3 genetic models. For the C282Y polymorphism, only 1 study was conducted in Europe, hence subgroup analyses were performed only on European population and the heterogeneity was effectively decreased or removed after the subgroup analyses.

Before and after exclusion of articles with OR > 3.0, no individual study was found having excessive influence on the pooled effect in any of the dominant, recessive, and codominant models considering all the polymorphisms.

Egger’s test was used to assess the publication bias. No significant publication bias was detected in any of the above-mentioned inherited models considering the 2 polymorphisms. Figure 4 presents the result of the publication bias for PD in recessive model of the H63D polymorphism in the HFE gene.