Introduction

Hypertension affects nearly one-third of the adult population worldwide and is associated with stroke and cardiovascular disease, which are major contributors to the global health burden.1 Although lifestyle factors (for example, excess sodium intake and lack of physical activity) can contribute to elevated blood pressure (BP) and increased hypertension risks,2 genetic factors have also been implicated in the pathogenesis of these diseases.3, 4, 5 Genome-wide association studies have identified many genetic variants that may be associated with hypertension.6 These exciting new results elucidating hypertension's underlying molecular mechanisms have greatly expanded our knowledge and provided new insights into the etiology of hypertension.7

The cytochrome P450 family 3 subfamily A polypeptide 5 (CYP3A5) gene encodes a member of the cytochrome P450 super family of enzymes. The CYP3A5 gene is part of a cluster of cytochrome P450 genes on chromosome 7q21.1.8 Cytochrome P450 proteins are monooxygenases that have an important role in the metabolism of both endogenous substances (for example, hormones) and exogenous substances (for example, drugs).9, 10 Cytochrome P450 proteins are expressed in an organ-specific manner, with CYP3A5 predominantly expressed in the kidney.11 As a glucocorticoid 6β-hydroxylase, the CYP3A5 enzyme converts cortisol or corticosterone to 6β-corticosterone, which may lead to increased sodium and water retention.12, 13 The CYP3A5 gene has a common polymorphism (6986G>A, rs776746) in the third intron. The G allele generates an aberrant RNA splicing site and results in a premature stop codon, which causes truncation of the CYP3A5 protein.14 In genetic epidemiology studies, CYP3A5*1 refers to the A allele (functional allele) and CYP3A5*3 refers to the G allele (nonfunctional allele).

Recently, several studies have suggested a possible relationship between CYP3A5 gene polymorphisms and BP/hypertension. However, their conclusions have been inconsistent. This inconsistency could be due to many factors, including insufficient statistical power, varying recruitment procedures of the study population and differences in the genetic and environmental backgrounds. In this study, we performed a meta-analysis to assess the association between the CYP3A5 rs776746 polymorphism and BP/hypertension.

Methods

Literature and search strategy

We searched literature databases, including PubMed (1950 to 2010), the ISI Web of Science (1975 to 2010) and Embase (1966 to 2010). The search strategy to identify all possible studies involved the use of the following key words: ‘cytochrome P-450 3A5’ (or ‘CYP3A5’) and ‘blood pressure’ (or ‘hypertension’). The reference lists of retrieved articles were then manually searched. If more than one article was published using the same case series, only the study with largest sample size was included. The literature search was updated on 5 December 2010.

Inclusion criteria and data extraction

The studies included in the meta-analysis necessarily met all of the following inclusion criteria: (1) evaluating the association of the CYP3A5 polymorphism with hypertension or BP; (2) a case–control design; (3) sufficient data for calculation of effect size (odds ratio (OR) or mean difference) with a corresponding confidence interval (CI); and (4) subjects without serious diseases. The following information was extracted from each study: (1) the name of the first author; (2) the year of publication; (3) the ethnicity of the study population; (4) the gender ratio and mean age (or age range) of the study population; (5) the number of subjects with the CYP3A5 polymorphism in hypertension cases and controls; and (6) the mean and s.d. of BP among carriers and non-carriers with CYP3A5*1. The articles were independently assessed for compliance with the inclusion/exclusion criteria, and all disagreements were resolved to give a consistent decision for each article.

Statistical analysis

The association of the CYP3A5 polymorphism with hypertension was estimated by calculating the pooled OR and the 95% CI. The association with BP was estimated by calculating the pooled mean differences and the 95% CI. The significance of the pooled effect size was determined by a Z-test (P<0.05 was considered statistically significant). A Q-test was performed to evaluate whether the variation was due to heterogeneity or by chance. A random (DerSimonian–Laird method15)- or fixed (Mantel–Haenszel method16)-effects model was used to calculate the pooled effect estimates in the presence (P<=0.10) or absence (p>0.10) of heterogeneity, respectively. Begg's funnel plot, a scatter plot of effect against a measure of study size, was generated as a visual aid to detect bias or systematic heterogeneity.17 Publication bias was assessed by Egger's test18(P<0.05 was considered statistically significant). Subgroup analysis by ethnicity was also performed. To evaluate the stability of the results, sensitivity analysis was performed by removing one study at a time. Data analysis was performed using STATA version 11 (StataCorp LP, College Station, TX, USA) as described previously.19, 20, 21, 22, 23

Results

Characteristics of the studies

The literature search identified a total of 52 potentially relevant papers. Of these, 35 were excluded because of obvious irrelevance ascertained by reading the titles and abstracts. Some studies were excluded, because the subjects were pregnant women,24 renal transplant patients,25 coronary angioplasty patients26 or coronary artery disease patients (INVEST-GENES cohort).27 The subjects of two papers28, 29 were from the same cohort; therefore, only one paper was included.29 One paper was excluded, because its study population contained only one CYP3A5*3/*3 allele carrier.30 Therefore, 12 papers met the inclusion criteria and were included in the meta-analysis.27, 29, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40 Individuals carrying at least one CYP3A5*1 allele were combined as CYP3A5*1 carriers in one group (high expression), and individuals without a CYP3A5*1 allele were combined as CYP3A5*1 non-carriers in the other group (low expression). The characteristics of the included studies are listed in Tables 1 and 2.

Table 1 Characteristics of studies for association between CYP3A5 polymorphism and hypertension
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Table 2 Characteristics of studies for association between CYP3A5 polymorphism and blood pressure
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Meta-analysis for determining the association with hypertension

Ten sub-studies with 2799 cases and 6794 controls were included to determine the association with hypertension.27, 33, 34, 35, 38 Overall, there was evidence of heterogeneity between studies (P=0.064, I-squared=44.2%). Under a random-effects model, the meta-analysis result showed no association between the CYP3A5 rs776746 polymorphism and hypertension (OR=1.123, 95% CI 0.906 to 1.394, P=0.290). In the subgroup analysis of groups stratified by ethnicity, the heterogeneity was not significant in the white subgroup (p>0.10), but it remained in black populations (P<0.10). The subgroup meta-analysis results showed that there was no association between the CYP3A5 rs776746 polymorphism and hypertension in either white (OR=1.016, 95% CI 0.880 to 1.172, P=0.831) or black populations (OR=1.278, 95% CI 0.554 to 2.948, P=0.566; Figure 1 and Table 3). Sensitivity analysis was performed by excluding a single study at a time and similar results were obtained (data not shown).

Figure 1
figure 1

Forest plot of the meta-analysis of the association between the cytochrome P450 family 3 subfamily A polypeptide 5 (CYP3A5) polymorphism (CYP3A5*1 carriers vs. CYP3A5*1 non-carriers) and hypertension risk. A full color version of this figure is available at the Hypertension Research journal online.

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Table 3 Subgroup analysis of the association of CYP3A5 polymorphism (CYP3A5*1 carriers vs. CYP3A5*1 non-carriers) with hypertension and blood pressure by ethnicity
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Meta-analysis to determine the association with BP

Twelve sub-studies with 9076 subjects were included to determine the association with BP.27, 29, 31, 32, 35, 36, 37, 38, 39, 40 As shown in Figures 2 and 3, there was evidence of heterogeneity between studies (systolic BP (SBP): P=0.056, I-squared=43.0%; diastolic BP (DBP): P=0.029, I-squared=48.8%). Overall, the meta-analysis results showed no association between the CYP3A5 rs776746 polymorphism with either SBP or DBP (for SBP: mean difference=−0.242, 95% CI −1.576 to 1.093, P=0.722; for DBP: mean difference=−0.207, 95% CI −1.173 to 0.759, P=0.675). After stratification by ethnicity, the heterogeneity was not significant in white populations (P>0.10). The subgroup meta-analysis showed that, in white populations, CYP3A5*1 carriers had significantly lower SBP compared with non-carriers (mean difference=−1.322, 95% CI −2.401 to −0.242 mm Hg, P=0.016; Figure 4 and Table 3). The stability of these results was also confirmed by sensitivity analysis (data not shown).

Figure 2
figure 2

Forest plot of the meta-analysis of the association between the cytochrome P450 family 3 subfamily A polypeptide 5 (CYP3A5) polymorphism (CYP3A5*1 carriers vs. CYP3A5*1 non-carriers) and systolic blood pressure. A full color version of this figure is available at the Hypertension Research journal online.

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Figure 3
figure 3

Forest plot of the meta-analysis of the association between the cytochrome P450 family 3 subfamily A polypeptide 5 (CYP3A5) polymorphism (CYP3A5*1 carriers vs. CYP3A5*1 non-carriers) and diastolic blood pressure. A full color version of this figure is available at the Hypertension Research journal online.

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Figure 4
figure 4

Forest plot of the meta-analysis of the association between the cytochrome P450 family 3 subfamily A polypeptide 5 (CYP3A5) polymorphism (CYP3A5*1 carriers vs. CYP3A5*1 non-carriers) and systolic blood pressure in white populations. A full color version of this figure is available at the Hypertension Research journal online.

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Publication bias

Begg's funnel plots were generated to assess potential publication bias (figures not shown). No publication bias was detected (Egger's test, P=0.243, 0.179 and 0.516 for the associations with hypertension, SBP and DBP, respectively).

Discussion

In this study, we performed a meta-analysis to investigate the association between the CYP3A5 rs776746 polymorphism and BP/hypertension. Overall, no significant association was observed either for BP or hypertension. However, significant heterogeneity was found in the studies. In meta-analyses, it is well accepted that potential heterogeneity between studies may interfere with the conclusions. Given the large interethnic differences in allele frequencies,41 a subgroup analysis based on ethnicity was performed. Except for hypertension risks in blacks, heterogeneity was not significant within each ethnic group. We found a significant association between the CYP3A5 rs776746 polymorphism and SBP in whites. The sensitivity analysis results confirmed these conclusions.

Currently, data on the association of CYP3A5 polymorphisms with BP or hypertension are relatively limited. Givens et al.31 first reported that the CYP3A5*1 allele was associated with increased SBP, but not DBP in 25 African Americans. Since then, several studies have been performed to investigate the association in different ethnic populations; however, the results have been inconsistent. For example, a large population-based study (n=6777, age range: 28–75 years) by Kreutz et al.35 showed that the CYP3A5*1 allele was associated with lower SBP only in women. In a study of an elderly Finnish Caucasian population (n=373, age: ⩾75 years), CYP3A5*1 carriers had a significantly greater hypertension risk.34 In a Japanese population, the CYP3A5*1 allele was associated with DBP, but not SBP, and further analysis showed that the significant association was observed only among *3/*3 carriers with high salt intake. This indicated that the CYP3A5 rs776746 polymorphism might be a risk factor for salt sensitivity.40

The underlying biological mechanism by which CYP3A5 exerts its effects on the pathogenesis of hypertension is still not known. As a potential intermediate phenotype, cortisol may have an important role in the regulation of BP levels and the development of hypertension.42 Some previous studies suggest that excessive intrarenal conversion of cortisol to 6β-hydroxycoricosterone through increased renal CYP3A activity could enhance post-renal proximal tubular sodium reabsorption and thus result in elevated BP levels.12, 13 However, CYP3A could also protect the expression of corticosterone-induced active sodium transport in kidney cells by mineralocorticoid receptor-mediated mechanisms, which could then lead to decreased BP.43 This may be the underlying mechanism by which the CYP3A5*1 allele resulted in a lower SBP.

The current meta-analysis has advantages over individual studies; however, it does have some limitations. First, it was based primarily on unadjusted effect estimates and confidence intervals. Second, the methods for measuring BP were not uniform for the included studies (office BP vs. ambulatory BP monitoring). Third, there may also be an age effect, because in some populations, the CYP3A5 genotype was associated with a different profile of BP increases with age. Fourth, the effect of gene–gene and gene–environment interactions (salt intake is an example of the latter) was not addressed in this meta-analysis. Finally, if the various populations are considered separately, the results of the subgroup analysis should be interpreted with caution because of limited statistical power. We anticipate that these issues will be addressed in future studies.

In summary, our meta-analysis suggested a modestly significant association between the CYP3A5*1 genotype and a lower SBP in white populations. Given the strong interethnic differences in CYP3A5*1 allele frequency, it is possible that the effects of CYP3A5 may vary depending on the genetic context and physiological background. As the CYP3A5 gene has been implicated in BP regulation, additional epidemiological, biological and clinical studies are required to further investigate the role of CYP3A5 in the pathogenesis of hypertension.