Association Between ALDH-2 rs671 and Essential Hypertension Risk or Blood Pressure Levels: A Systematic Review and Meta-Analysis

Background: The association between Aldehyde dehydrogenase II (ALDH-2) rs671 polymorphism and essential hypertension (EH) risk or blood pressure (BP) levels remains unclear. Objective: To systematically review the influence of the aldehyde dehydrogenase II rs671 polymorphism on essential hypertension risk and blood pressure levels. Methods: The PubMed, EMbase, Web of Science, Cochrane Library, CNKI and CBM databases were electronically searched to identify case-control or cohort studies published prior to July 2019 that examined the association between the rs671 polymorphism and the risk of essential hypertension or blood pressure levels. A meta-analysis was conducted with Stata 15.1 software. Results: Twenty-two articles were included. Among these articles, 20 incorporated 30 individual studies evaluating the association between the rs671 polymorphism and EH (11,051 hypertensive patients and 15,926 normotensive controls), and 8 incorporated 12 individual studies evaluating the association between the rs671 polymorphism and BP (20,512 subjects). The results of the meta-analysis showed that the mutation of the rs671 polymorphism was associated with a significantly decreased risk of EH in all models: allelic model (OR = 0.80, 95% CI: 0.73–0.87), homozygous model (OR = 0.71, 95% CI: 0.63–0.80), heterozygous model (OR = 0.79, 95% CI: 0.72–0.87), dominant model (OR = 0.79, 95% CI: 0.71–0.87), and recessive model (OR = 0.76, 95% CI: 0.68–0.85). In the stratified analyses, significant associations were found for males, drinkers and population-based studies. Simultaneously, the A carriers had lower SBP (WMD = −1.78, 95% CI: −3.02 to −0.53) and DBP (WMD = −1.09, 95% CI: −1.58 to −0.61) levels than individuals with the GG homozygote. Conclusion: The collective findings of this meta-analysis suggested that the ALDH-2 rs671 polymorphism represented an important genetic marker in the development of hypertension. Considering the overall quality of evidence and the relatively small pooled sample size, more well-conducted high-quality studies are required to verify the above conclusion. Systematic Review Registration Number: PROSPERO (CRD42019129746).

Background: The association between Aldehyde dehydrogenase II (ALDH-2) rs671 polymorphism and essential hypertension (EH) risk or blood pressure (BP) levels remains unclear.
Objective: To systematically review the influence of the aldehyde dehydrogenase II rs671 polymorphism on essential hypertension risk and blood pressure levels.
Methods: The PubMed, EMbase, Web of Science, Cochrane Library, CNKI and CBM databases were electronically searched to identify case-control or cohort studies published prior to July 2019 that examined the association between the rs671 polymorphism and the risk of essential hypertension or blood pressure levels. A meta-analysis was conducted with Stata 15.1 software.

INTRODUCTION
Hypertension is one of the most common chronic non-infectious diseases and is recognized as a major causal risk factor for cardiovascular diseases. Elevated blood pressure is the leading cause of death worldwide, and the burden of hypertension is expected to increase globally. In 2012, hypertension affected 270 million individuals in China and had a prevalence of 25.2% , which significantly increased with age. In 2013, hypertension alone accounted for 6.61% of the 3.1869 trillion RMB spent on healthcare in China (Chen et al., 2017). In Japan, the mean blood pressure has steadily declined over the past 50 years, but hypertension remains one of the biggest risk factors for non-communicable diseases, particularly cardiovascular disease (Ikeda et al., 2011;Lim et al., 2012). Hypertension is a disease whose pathophysiological mechanism involves hundreds of genes (Hwang et al., 2012). Numerous epidemiological studies have elucidated some risk factors, such as sex, age, and drinking alcohol (Kario, 2015). Previous studies have investigated the association between gene polymorphisms and hypertension (Ma et al., 2016;Niu et al., 2019). The aldehyde dehydrogenase (ALDH) super family includes key enzymes in the major pathway of alcohol metabolism. Aldehyde dehydrogenase 2 (ALDH-2) has a critical role in mediating the conversion of aldehydes into much less reactive chemical species . Several studies have shown that ALDH-2 deletion is a susceptibility factor for blood pressure levels and could increase oxidative stress (Ohsawa et al., 2003). The mutation in exon 12 in which G is changed to A (rs671, Glu504Lys) resulted in decreased enzyme activity (Perez-Miller et al., 2010), thereby affecting blood acetaldehyde concentrations after alcohol intake (Eriksson, 2001). Recently, numerous published studies have confirmed that the ALDH-2 rs671 polymorphism is associated with hypertension (Ota et al., 2016;Du, 2018). However, the evidence remains inconclusive. Other studies have found that ALDH-2 rs671 polymorphism is not associated with hypertension (Wu et al., 2013;Zhang et al., 2016). Whether there is a relationship between the ALDH-2 rs671 polymorphism and hypertension remains unclear. Even previous meta-analysis studies on this issue have drawn different conclusions, and only focused on the association between ALDH-2 and essential hypertension risk (Jia et al., 2015;Li et al., 2017;Wu et al., 2017). In order to conclude the influence of the ALDH-2 rs671 polymorphism on hypertension, we performed a more comprehensive systematic review and meta-analysis. Furthermore, we not only estimate the association between the ALDH-2 rs671 polymorphism and essential hypertension risk, but also estimate the association between the ALDH-2
Languages were not restricted during the searching process.
Published articles listed in the references of the keyword index results were also screened carefully to avoid possible omissions.

Inclusion and Exclusion Criteria
Studies were included if they fulfilled all of the following criteria: (i) case-control design or cohort study design; (ii) studies that examined the association between the ALDH-2 rs671 polymorphism and the risk of essential hypertension or blood pressure levels; (iii) diagnosis of hypertension defined as either systolic blood pressure ≥140 mmHg or diastolic blood pressure ≥90 mmHg (continuously or more than 3 times in a sitting position, on three different days), or taking antihypertensive medication. The exclusive criteria were as follows: (i) repeated publication of literature or reported duplicate data; (ii) research that is not available in full text; and (iii) reports with incomplete data or no usable data.

Data Extraction
A data-extraction table was designed in Excel 2016 by all the researchers. The following information was included: name of the FIGURE 2 | Forest graphs for the association between the ALDH-2 rs671 polymorphism and essential hypertension risk under the allelic model.
Frontiers in Genetics | www.frontiersin.org first author, year of publication, ethnicity of study population, study design, source of population, genotyping methodology, sample size, average age, gender, alcohol consumption, SBP and DBP. Two researchers independently extracted the necessary data, and all disagreements were resolved through discussion with a third researcher.

Quality Assessment
Two researchers evaluated the quality of each included study using the Newcastle-Ottawa Scale (NOS) (Stang, 2010) independently. Three major aspects of study quality were scored: (i) selection of the study groups (0-4 scores); (ii) determination of the exposure of interest in the studies (0-3 scores); and (iii) the quality of the adjustment for confounding variables (0-2 scores). Scores ranged from zero to nine stars, and a score of six or above was considered a high-quality study.

Statistical Analysis
The χ 2 test was used to assess whether the genotype distributions in the control group of each study were in Hardy-Weinberg Equilibrium (HWE). Heterogeneity among the same category was evaluated using the χ 2 test and Cochran's Q statistic, and the I 2 statistic was used to quantify the percentage variability of the heterogeneity (Higgins et al., 2003). If P > 0.1 and I 2 ≤ 50%, the fixed-effect model was selected; otherwise, the randomeffect model was adopted (DerSimonian and Laird, 1986). The risk of EH was estimated by the pooled odds ratio (OR) along with the 95% confidence interval (CI), and the levels of BP was signified as the weighted mean difference (WMD) with its 95% CI. The pooled ORs and WMDs were measured using the Z-test, and a P-value of < 0.05 was considered statistically significant. A sensitivity analysis was performed to detect the individual effect of each study on the pooled ORs or WMDs by omitting one individual inter-study at a time. Publication bias was evaluated with Begg's test, Egger's test and the trim-and-fill method. All statistical analyses were performed with Stata 15.1 software.

Characteristics of Included Studies
Two researchers independently sifted through the literature, extracted the data and cross-checked them. In case of FIGURE 3 | Forest graphs for the association between the ALDH-2 rs671 polymorphism and essential hypertension risk under the homozygous model.
Frontiers in Genetics | www.frontiersin.org disagreement, the decision was made through discussion or arbitration by the third researcher. A total of 353 articles were found after searching the existing literature electronic databases. After removing duplicates, 281 articles were retained. A total of 212 articles with irrelevant data were excluded after a further review of titles and abstracts. The full texts of the remaining articles were screened carefully, and another 47 articles were excluded. Finally, 22 articles were included. The selection process for the qualified publications is presented in Figure 1. Finally, 22 articles (Takagi et al., 2001;Amamoto et al., 2002;Saito et al., 2003;Iwai et al., 2004;Hui et al., 2007;Hasi et al., 2011;Feng et al., 2012;Lv et al., 2013;Nakagawa et al., 2013;Wang et al., 2013;Wu et al., 2013Wu et al., , 2017Yokoyama et al., 2013;Zhang et al., 2013Zhang et al., , 2016Zhang et al., , 2018Isomura et al., 2015;Jing et al., 2015;Ota et al., 2016;Li et al., 2017;Ma et al., 2017;Du, 2018) met the preset inclusion criteria. Among these articles, 20 (Takagi et al., 2001;Amamoto et al., 2002;Saito et al., 2003;Iwai et al., 2004;Hui et al., 2007;Hasi et al., 2011;Feng et al., 2012;Lv et al., 2013;Nakagawa et al., 2013;Wang et al., 2013;Wu et al., 2013Wu et al., , 2017Yokoyama et al., 2013;Jing et al., 2015;Ota et al., 2016;Zhang et al., 2016Zhang et al., , 2018Li et al., 2017;Ma et al., 2017;Du, 2018) assessed the association between the rs671 polymorphism and EH risk in 30 individual studies (11,051 hypertensive patients and 15,926 normotensive controls), and 8 (Takagi et al., 2001;Amamoto et al., 2002;Saito et al., 2003;Wang et al., 2013;Zhang et al., 2013;Isomura et al., 2015;Ota et al., 2016;Li et al., 2017) assessed the association between the rs671 polymorphism and BP levels in 12 individual studies (20,512 subjects). The baseline characteristics of all included articles are summarized in Table 1 (for EH) and Table 2 (for BP). For the EH association articles, the genotype distribution in the control groups was not in line with HWE in two articles (Yokoyama et al., 2013;Du, 2018), and in another two articles (Nakagawa et al., 2013;Ota et al., 2016) HWE could not be tested. The NOS scores of all studies were six or higher, indicating that they were highquality studies. For the BP association articles, we extracted the  blood pressure data of rs671A variant carriers. The NOS scores of all studies were higher than six, indicating that they were high-quality studies.

Meta-Analysis for the Risk of Essential Hypertension: Integral Analyses
The risk prediction of the rs671 polymorphism for essential hypertension was investigated separately under the allelic, homozygous, heterozygous, dominant and recessive models. The detailed results of the ORs and 95% CIs for different comparisons are shown in Table 3.

Meta-Analysis for the Risk of Essential Hypertension: Stratified Analyses
Because the heterogeneity in the integral analyses was significant, a string of stratified analyses were implemented to determine the potential reasons for the between-study heterogeneity from other methodological aspects. In the stratified analyses, 20 articles were stratified by ethnicity, gender, alcohol consumption, source of control, study design, sample size, and genotyping methodology under the allelic, homozygous, heterozygous, dominant and recessive models ( Table 3).
When all studies were stratified by ethnicity, there was a significant association between the rs671 polymorphism and EH risk in populations of Japanese descent under all models, and the same result was found in populations of Chinese descent. When stratified by gender, there was a significant association between the rs671 polymorphism and EH risk in men under all models; however, there was no significant association in women. When stratified by alcohol consumption, there was a significant association between the rs671 polymorphism and EH risk in drinkers under the allelic, heterozygous and dominant models; however, there was no significant association in non-drinkers. In the subgroup analysis stratified by the source of control, the EH risk prediction was significant in population-based studies for all models; however, the EH risk prediction was not significant in hospital-based studies for all models. When stratified by study design, the EH risk prediction was significant in casecontrol studies for all models; the EH risk prediction was only not significant in cohort studies for the recessive model. When stratified by sample size, the EH risk prediction was significant in studies with a sample size ≥ 1,000 for all models, and the EH risk prediction was significant in studies with a sample size <1,000 for the heterozygous and dominant models.

Meta-Analysis for Blood Pressure Levels
Because of the low frequency of AA homozygotes and to avoid deviations from sample size, the association between the rs671 polymorphism and blood pressure levels was only investigated under the dominant model (A carriers vs. GG carriers). The detailed results of the WMDs and 95% CIs for different comparisons are shown in Table 4.

Sensitivity Analysis
The sensitivity analysis was performed by sequentially dropping one inter-study at a time to detect the influence of each interstudy on the summary OR and WMD. The outcomes of our meta-analysis were not altered greatly when each individual study was omitted, suggesting that the overall results were stable and robust.
FIGURE 7 | Forest graphs for the association between the ALDH-2 rs671 polymorphism and systolic blood pressure levels.
Frontiers in Genetics | www.frontiersin.org FIGURE 8 | Forest graphs for the association between the ALDH-2 rs671 polymorphism and diastolic blood pressure levels.

Publication Bias
Publication bias of the included studies was assessed using Begg's test, Egger's test and the trim-and-fill method. First, we applied Begg's test and Egger's test to evaluate publication bias. All pvalues more than 0.05 was considered to have no evidence of publication bias ( Table 5). The results of the trim-and-fill method were k = 0. The shape of the funnel plots of the trimand-fill method in all comparisons did not show any obvious asymmetrical evidence (Figures 9-15), which revealed that there was little evidence of publication bias in the overall analysis.

DISCUSSION
Previous studies exploring the association between the ALDH-2 rs671 polymorphism and hypertension risk and blood pressure levels have provided controversial results, and the sample sizes in most of these studies were relatively small; thus, it was difficult to obtain credible genetic effects. Meta-analyses have been considered as one of the most important tools to precisely define the association between selected genetic polymorphisms and the risk for a morbid state. Based on this situation, we performed this study. Our meta-analysis indicated that the ALDH-2 rs671 polymorphism is not only a major protective factor against the development of hypertension, particularly in males and drinkers, but it is also a critical factor in decreasing blood pressure. Based on the following aspects, we believe that our findings are more comprehensive and convincing. First, our meta-analysis incorporated more eligible studies, thus providing sufficient statistical power. Second, the association between the rs671 polymorphism and essential hypertension was investigated extensively with five genetic models. Third, we performed a series of more comprehensive subgroup analyses by factors addressed across different studies, which may influence the reliability.
Fourth, the sensitivity analysis indicated that the results are stable and reliable. Finally, little evidence of publication bias was found in the overall analysis.
For essential hypertension association studies, a statistically significant association between the rs671 polymorphism and EH risk was observed under all models. Li et al. did a casecontrol study and meta-analysis, and shown that ALDH2 rs671 polymorphism may not associate with EH (Li et al., 2017). We have made a more comprehensive systematic analysis, and included the latest studies that they did not include. Therefore, we endorse our result even more. There was a common polymorphism of ALDH2 (rs671 G→ A) in East Asians. When FIGURE 9 | Filled funnel plots for the association between the ALDH-2 rs671 polymorphism and essential hypertension risk under the allelic model. stratified by ethnicity, significant associations were observed in both the Japanese and Chinese subgroups, suggesting that the association between the ALDH-2 rs671 polymorphism and the risk of essential hypertension did not differ between Chinese and Japanese populations. In the stratification analysis by gender, signification associations were found in males, although no significant associations were found in females. This may be due to physiological differences between males and females. Studies found that the female heart has elevated phosphorylation and ALDH2 activity (Lagranha et al., 2010). In the stratification analysis by alcohol consumption, signification associations were found in drinkers, although no significant associations were found in non-drinkers. This finding was not consistent with previous meta-analysis results (Fan et al., 2018). Zhang et al. concluded that the rs671 polymorphism may influence the risk of EH independent of alcohol consumption (Zhang et al., 2014). Compared with him, we added a 2018 study on the association between ALDH2 rs671 gene polymorphism and essential hypertension in non-drinkers (Zhang et al., 2018). We concluded that the rs671 polymorphism may affect the risk of essential hypertension in drinkers. Zhang et al. found that the hypertensive effect of alcohol was attributed to ethanol rather than acetaldehyde . The rs671 polymorphism, G→ A, decreases the activity of alcohol-metabolizing enzymes. As a result, the rs671 polymorphism drinkers had less ethanol, which protected them from hypertension. In the subgroup analysis stratified by the source of control, the EH risk prediction was significant in population-based studies for all models, whereas the EH risk prediction was not significant in hospitalbased studies for all models. We trusted that studies whose control groups were from populations accurately reflected the relationship between the rs671 polymorphism and EH risk.   Finally, the study design and sample size did not alter the overall result.
For blood pressure association studies, a significant variation in blood pressure between A carriers and GG homozygote carriers was observed. This result was consistent with the results of the association between the rs671 polymorphism and EH risk. In the stratification analysis by gender, signification associations were found in the male subgroup for SBP and DBP levels, although no significant associations were found in females. In the stratification analysis by alcohol consumption, signification associations were found in both drinkers and non-drinkers' DBP levels, although no significant associations were found in SBP levels. Notably, there were relatively few studies based on gender differences and alcohol consumption differences; thus, it was difficult to obtain credible genetic effects.  However, there were several limitations in our study. First, only articles published in English and Chinese were incorporated, which led to a potential selection bias. Second, because of the lack of uniform background data for studies in metaanalyses, the data were not further stratified by other factors that may affect blood pressure such as salt consumption and smoking. Third, The ALDH2 polymorphism is observed to be associated with increased risk for diseases such as coronary artery disease and diabetics. However, due to the limitation of included articles, further stratification of the subjects according to these accompanied diseases could not be implemented in this study. Forth, As medications could affect the BP levels. However, due to the limitation of included articles, we were unable to analyze the use of drugs. Fifth, significant heterogeneity was detected even we performed subgroup analyses. Finally, the meta-analysis was limited by the FIGURE 15 | Filled funnel plots for the association between the ALDH-2 rs671 polymorphism and diastolic blood pressure levels.
inadequate sample size, particularly in the alcohol consumption subgroup analysis.

CONCLUSIONS
The collective findings of this meta-analysis demonstrated that the mutation of the ALDH-2 rs671 polymorphism was significantly associated not only with a decreased predisposition toward essential hypertension but also with lowering blood pressure, suggesting that the ALDH-2 rs671 polymorphism might represent an important genetic marker of hypertension. These findings potentially further our understanding of the contributing role of the ALDH-2 rs671 polymorphism in blood pressure regulation and in the pathogenesis of hypertension.

DATA AVAILABILITY STATEMENT
All datasets generated for this study are included in the article/supplementary material.