Association between polymorphisms of microsomal epoxide hydrolase and COPD: results from meta-analyses.

Background and objective:  COPD is a complex polygenic disease in which gene–environment interactions are very important. The gene encoding microsomal epoxide hydrolase (EPHX1) is one of several candidate loci for COPD pathogenesis and is highly polymorphic. Based χ on the polymorphisms of EPHX1 gene (tyrosine/histidine 113, histidine/arginine 139), the population can be classified into four groups of putative EPHX1 phenotypes (fast, normal, slow and very slow). A number of studies have investigated the association between the genotypes and phenotypes of EPHX1 and COPD susceptibility in different populations, with inconsistent results. A systematic review and meta-analysis of the published data was performed to gain a clearer understanding of this association. Methods:  The MEDLINE database was searched for case–control studies published from 1966 to August 2007. Data were extracted and pooled odds ratios (OR) with 95% confidence intervals (CI) were calculated. Results:  Sixteen eligible studies, comprising 1847 patients with COPD and 2455 controls, were included in the meta-analysis. The pooled result showed that the EPHX1 113 mutant homozygote was significantly associated with an increased risk of COPD (OR 1.59, 95% CI: 1.14–2.21). Subgroup analysis supported the result in the Asian population, but not in the Caucasian population. When the analysis was limited to only the larger-sample-size studies, studies in which controls were in Hardy–Weinberg equilibrium and studies in which controls were smokers/ex-smokers, the pooled results supported the conclusion. The EPHX1 139 heterozygote protected against the development of COPD in the Asian population, but not in the Caucasian population. The other gene types of EPHX1 113 and EPHX1 139 were not associated with an increased risk of COPD. The slow activity phenotype of EPHX1 was associated with an increased risk of COPD. The fast activity phenotype of EPHX1 was a protective factor for developing COPD in the Asian population, but not in the Caucasian population. However, the very slow activity phenotype of EPHX1 was a risk for developing COPD in the Caucasian population, but not in the Asian population. Conclusions:  The polymorphisms of EPHX1 113 and EPHX1 139 are genetic contributors to COPD susceptibility in Asian populations. The phenotypes of EPHX1 were contributors to overall COPD susceptibility.