A recent meta-analysis of genome-wide association studies identified a variant, rs671, in mitochondrial aldehyde dehydrogenase 2 (ALDH2) associated with variations in blood pressure (BP) in East Asians 1. Furthermore, the wild-type ALDH2*1 allele, which has been shown to be a risk factor for elevated BP, was actually associated with a reduced risk of coronary artery disease (CAD). These associations are believed to be largely mediated by alcohol intake, because this variant determines an individual’s tolerance to alcohol by altering the ALDH2 enzymatic activity 2. Accordingly, the authors interpreted the deleterious effect of the ALDH2*1 allele on BP to be balanced by the protective effects on the lipid profile, thus resulting in a net reduction in the risk of CAD.
The pathophysiological effects of the inactive ALDH2*2 gene product would be mediated by the accumulation of acetaldehyde and other reactive aldehydes, and thus it can protect against alcoholism but may also predispose the organism to more severe damage induced by a presteady state burst of arterial acetaldehyde following alcohol intake 3. On the basis of this information, we hypothesized that the ALDH2*2 allele is not only a protective factor against hypertension by affecting alcohol drinking behavior but also a risk for hypertension through acetaldehyde-induced oxidative stress, especially in individuals harboring an additional dysfunctional variant of a mitochondrial antioxidant enzyme, superoxide dismutase 2 (SOD2) 4.
This study included 444 Japanese participants (310 men and 134 women, 53.3±9.3 years old) in a health-screening program at the Japanese Red Cross Kumamoto Hospital Health Center. This protocol was approved by the institutional ethics committees of the Faculty of Life Sciences, Kumamoto University, and the Japanese Red Cross Kumamoto Hospital Health Center. The study was carried out in accordance with the Declaration of Helsinki, and all participants provided written informed consent for their participation.
All participants completed a standardized health-screening program. Blood samples were collected after overnight fasting.
The BP was measured three times after rest in a sitting position and the mean value was used. Hypertension was defined as a systolic BP at least 140 mmHg, a diastolic BP at least 90 mmHg, and/or a history of hypertension. Information on smoking habits and alcohol consumption was obtained by face-to-face interviews with medical staff members using a structured questionnaire.
Genomic DNA was extracted from whole blood using a DNA purification kit (Flexi Gene DNA Kit; Qiagen, Hilden, Germany). The genotypes of ALDH2 were determined by PCR restriction amplifications using the oligonucleotide primers 5′-CAAATTACAGGGTCAACTGCT-3′ (forward) and 5′-AGACCCTCAAGCCCCAACAG-3′ (reverse). The resulting amplicon of 176 bp was digested by Eco57I (Fermentas International Inc., Burlington, Ontario, Canada) into fragments of 133 and 43 bp for the wild-type *1 allele and remained undigested for the variant *2 allele. The genotypes of SOD2 Val-9Ala were determined using a previously reported method 5.
The categorical variables were compared using Fisher’s exact test, and continuous variables were compared using Student’s t-test and a one-way ANOVA. The interaction between alcohol consumption and the genotype on the risk of hypertension was assessed using a generalized linear model. The associations among hypertension; the ALDH2 or SOD2 genotypes, or their combinations; and alcohol consumption were examined using a multiple logistic regression analysis with calculation of the adjusted odds ratios (ORs) and 95% confidence intervals (95% CI), stratified by alcohol consumption (0, 20, 30, or 40 g/day). The relationship between alcohol consumption and BP was assessed using a multiple linear regression analysis. A P value of less than 0.05 was considered to be statistically significant. All statistical analyses were carried out using the SPSS software package (version 17.0; SPSS Inc., Chicago, Illinois, USA).
One hundred and twenty-three individuals were diagnosed with hypertension (95 men and 28 women). The incidence of hypertension was higher in drinkers (79/253, 31.2%) than in nondrinkers (44/191, 23.0%), but this difference was not statistically significant (P=0.068). The frequencies of the ALDH2*1/*1, *1/*2, and *2/*2 genotypes were 55.2, 36.5, and 8.3%, respectively. Those of the SOD2 Val/Val, Val/Ala, and Ala/Ala genotypes were 75.2, 23.0, and 1.8%, respectively. In addition, both genotypes were in Hardy–Weinberg equilibrium.
Neither the genotype alone nor a combination of the genotypes had any effect on the incidence of hypertension in the overall subjects, whereas interactive effects were observed between alcohol intake and the SOD2 genotype or the combination of the SOD2 and ALDH2 genotypes on the risk of hypertension (P=0.009 and 0.003, respectively). The risk of hypertension was significantly higher in drinkers (≥20 g/day) than in nondrinkers among individuals with the ALDH2*2 allele (OR, 2.50; P=0.032) and the SOD2 Val/Val genotype (OR, 2.19; P=0.014), and the increased risk in drinkers was more pronounced among individuals with both of these genotypes (OR, 6.22; P<0.001) (Table 1). However, the risk was not affected by alcohol consumption of up to more than 40 g/day among the individuals with the ALDH2*1/*1 genotype and the SOD2Ala allele (data not shown).
As the amount of alcohol consumption increased (0, <20, ≥20 g/day), the systolic and diastolic BP of individuals harboring both the ALDH2*2 allele and the SOD2 Val/Val genotype increased significantly (113.3/71.1, 121.8/75.6, and 125.5/77.2 mmHg, respectively). These associations were observed among individuals with either of the ALDH2*2 allele or the SOD2 Val/Val genotype (details are shown in Supplementary table, Supplementary digital content 1, http://links.lww.com/FPC/A538). The systolic/diastolic BP increased by 0.24/0.14 mmHg (P<0.001/P=0.003) in individuals harboring both the ALDH2*2 allele and the SOD2 Val/Val genotype and by 0.11/0.10 mmHg (P=0.012/P=0.001) in those with the other genotype combinations for each 1g/day increase in alcohol consumption.
To our knowledge, this is the first report showing that the combination of a low enzyme-activity ALDH2*2 allele and the SOD2 Val/Val genotype increased the risk of hypertension, and an elevated systolic and diastolic BP, in a dose-dependent manner in relation to alcohol intake. The results of this study suggested that these polymorphisms synergistically influenced the risk of elevations in alcohol-related BP.
A SOD2 polymorphism (Val-9Ala, V16A, or rs4880) has been identified in the mitochondrial targeting sequence. The substitution of Ala for Val causes conformational change and impaired mitochondrial import, resulting in a decrease in the SOD2 activity in mitochondria 6. The low-activity Val allele was found to increase the oxidized LDL-induced apoptosis of macrophages and the risk of CAD 7. SOD2 has been implicated as a key regulator of oxidative stress and endothelial dysfunction. SOD2 heterozygous knockout (SOD2+/−) mice have been reported to develop hypertension with age 4. Dikalova et al.8 showed mitochondrial superoxide to play a critical role in endothelial dysfunction and hypertension, by either depleting or overexpressing SOD2 in cultured endothelial cells and transgenic mice with Ang II-induced hypertension.
A cooperative role of SOD2 and ALDH2 in maintaining the vascular function has emerged as a new aspect of the molecular mechanisms of nitrate tolerance 9. ALDH2 bioactivates glyceryl trinitrate (GTN), and the GTN tolerance may be associated with GTN-driven mitochondrial and vascular reactive oxygen species formation, which inhibits ALDH2 9. The SOD2+/− mice developed aggravated tolerance in response to GTN, thus correlating closely with an increased reactive oxygen species formation and with a marked decrease in the ALDH2 activity in comparison with the wild-type mice 9. These facts suggest that the disruption of the cooperative function of SOD2 and ALDH2 caused by genetically low enzyme activities and alcohol consumption may be a possible mechanism underlying our findings 4,8,9.
Alcohol intake has been known to exert a marked effect on BP and the risk of hypertension 1,2,10. Excess alcohol increases reactive aldehydes and oxidative stress through several pathways, and the BP was shown to increase significantly with the consumption of more than two drinks (20–30 g) per day in all race groups 10. Itoh et al.10 have reported a significantly positive correlation between the prevalence of hypertension and daily alcohol consumption among the individuals with visible flushing, a symptom of alcohol intoxication, but not among those without such flushing, at up to more than 27 g/day. In the present study, the combination of the ALDH2*2 allele and the SOD2 Val/Val genotype significantly increased the risk of hypertension, even in individuals who consumed less than 20 g/day of alcohol. Among them, the increase in the systolic/diastolic BP for each 1g/day increase in alcohol consumption was also significant and greater than the other genotype combinations.
The limitations of this study include the small sample size and the fact that there were no data on the pattern of drinking, other lifestyle-related factors, such as physical exercise, dietary regimens, mental stress, or sleep and work schedules, or any information on the medication history, which all also affect both BP and hypertension. Therefore, the results from this pilot study should be confirmed in a larger prospective study.
We have shown that alcohol intake, even less than 20 g/day, could increase the risk of hypertension in Japanese individuals with a combination of the SOD2 Val/Val genotype and the ALDH2*2 allele (∼1/3 of the Japanese population). This information about the genetic predisposition to alcohol-related diseases can be useful for recommendations of lifestyle modifications for high-risk individuals.
This work was supported by KAKENHI (No. 23510348) and in part by a grant from the Smoking Research Foundation.
Conflicts of interest
There are no conflicts of interest.
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