Background
Prevention of chronic kidney disease (CKD) is a major public health issue. Similarly to the survey reporting the prevalence of CKD in the United States [
1], analysis of data, sampled from over half a million individuals in the general Japanese population, predicted that about 13% of the Japanese adult population had CKD in 2005 [
2]. Considering that major outcomes of CKD are loss of kidney function and cardiovascular disease [
3], primary prevention for CKD is required. Therefore, it is crucial to detect the modifiable risk factors of CKD using epidemiological approaches.
There have been several epidemiological studies on the association between alcohol consumption and CKD. A case–control study did not show an increased risk of CKD associated with alcohol consumption [
4]. Another case–control study reported that consumption of more than two alcoholic drinks per day was associated with the risk of end-stage renal disease [
5]. Both cross-sectional and longitudinal analyses in a population-based cohort showed an independent association between heavy alcohol consumption and CKD [
6]. However, a large prospective cohort study suggested an inverse relationship between moderate alcohol consumption and the risk of renal dysfunction [
7]. A large-scale cross-sectional study also showed an inverse association between frequency of alcohol consumption and CKD in healthy Japanese men [
8]. These epidemiological studies did not assess genetic factors.
Mitochondrial DNA cytosine/adenine (Mt5178 C/A) polymorphism, which is also recognized as NADH dehydrogenase subunit-2 237 leucine/methionine (ND2-237 Leu/Met) polymorphism, is associated with longevity in Japanese [
9]. The frequency of the Mt5178A genotype is significantly higher in Japanese centenarians than in the general population. Moreover, Japanese individuals with Mt5178A are more resistant to adult-onset diseases, such as hypertension [
10], diabetes [
11], myocardial infarction [
12,
13] and cerebrovascular disorders [
14], than those with Mt5178C. This polymorphism modifies the effects of alcohol consumption on blood pressure [
15], risk of hypertension [
10], serum triglyceride levels [
16], risk of hyper-LDL cholesterolemia [
17] and serum uric acid levels [
18]. In addition to these previous reports [
10,
15‐
18], several molecular epidemiological studies on CKD and kidney function [
19‐
25] have encouraged us to examine the joint effects of Mt5178 C/A polymorphism and alcohol consumption on renal function.
In this study, we investigated whether longevity-associated Mt5178 C/A polymorphism modifies the effects of habitual alcohol consumption on renal function in male Japanese health check-up examinees.
Results
No significant differences in age, BMI, systolic blood pressure, diastolic blood pressure, serum total cholesterol levels, serum HDL cholesterol levels, serum triglyceride levels, fasting plasma glucose levels, serum uric acid levels or blood urea nitrogen levels were observed between the Mt5178C and Mt5178A genotypes (Table
1). There were no outliers in eGFR for either genotype; eGFR in Mt5178C genotypic men ranged from 50 to 119 ml/min/1.73 m
2 and that in Mt5178A genotypic men ranged from 52 to 112 ml/min/1.73 m
2. Among men with Mt5178C, the frequency of daily drinkers was 46.4%, that of occasional drinkers was 35.2% and that of non-drinkers was 18.4%. Among those with Mt5178A, the frequency of daily drinkers was 47.7%, that of occasional drinkers was 38.7% and that of non-drinkers was 13.6%. Chi-squared test showed no significant differences in alcohol consumption between the Mt5178 C/A genotypes (
P = 0.426).
Table 1
Clinical characteristics of study subjects by Mt5178 C/A genotype
Age (years) | 54.3 ± 7.8 | 53.2 ± 7.8 | 0.171 |
Body mass index (kg/m2) | 23.3 ± 2.8 | 23.5 ± 2.6 | 0.461 |
Systolic blood pressure (mmHg) | 125.8 ± 15.9 | 125.7 ± 14.1 | 0.940 |
Diastolic blood pressure (mmHg) | 74.0 ± 10.7 | 73.8 ± 9.1 | 0.823 |
Serum total cholesterol (mg/dl) | 203.5 ± 34.3 | 202.1 ± 31.8 | 0.672 |
Serum HDL cholesterol (mg/dl) | 54.6 ± 13.6 | 56.3 ± 16.2 | 0.269 |
Serum triglyceride (mg/dl) | 136.7 ± 91.1 | 139.5 ± 90.8 | 0.766 |
Fasting plasma glucose (mg/dl) | 97.2 ± 9.4 | 97.6 ± 9.7 | 0.672 |
Serum uric acid (mg/dl) | 5.98 ± 1.21 | 5.93 ± 1.21 | 0.673 |
Blood urea nitrogen (mg/dl) | 15.8 ± 3.7 | 15.2 ± 3.4 | 0.143 |
Serum creatinine (mg/dl) | 0.817 ± 0.117 | 0.797 ± 0.111 | 0.090 |
eGFR (ml/min/1.73 m2) | 79.1 ± 13.1 | 81.5 ± 12.4 | 0.066 |
Renal function (eGFR ≥ 90/90 > eGFR ≥ 60/eGFR < 60) (%) | 17.6/77.8/4.6 | 25.2/72.2/2.6 | 0.133 |
Antihypertensive medications (%) | 18.8 | 12.9 | 0.122 |
Alcohol consumption (daily drinkers/occasional drinkers/non-drinkers) (%) | 46.4/35.2/18.4 | 47.7/38.7/13.6 | 0.426 |
Smoking status (never- or ex-/1–20 cigarettes per day/>20 cigarettes per day) (%) | 59.4/29.7/10.9 | 59.4/25.8/14.8 | 0.429 |
Coffee consumption (≤1 cup per day/2–3 cups per day/≥4 cups per day) (%) | 61.1/29.7/9.2 | 55.5/32.9/11.6 | 0.510 |
For Mt5178A genotypic men, the frequency of alcohol consumption was positively and significantly associated with eGFR (
P for trend = 0.003) (Table
2). After adjustment, this positive association between alcohol consumption and eGFR remained significant. Moreover, eGFR was significantly higher in daily drinkers than in non-drinkers (
P = 0.005). After adjustment for age and BMI or for age, BMI, habitual smoking, coffee consumption and use of antihypertensive medication, eGFR was also significantly higher in daily drinkers than in non-drinkers (
P = 0.025 and
P = 0.026, respectively). On the other hand, although eGFR was significantly higher in occasional drinkers with Mt5178C than in non-drinkers with Mt5178C (
P = 0.043), the association between Mt5178C genotype and eGFR did not appear to depend on the frequency of alcohol intake.
Table 2
Estimated glomerular filtration rates by alcohol consumption and Mt5178 C/A genotype
Mt5178C (N = 239) | | | |
Non-drinkers (N = 44) | 75.8 ± 2.0 | 76.5 ± 1.9 | 74.5 ± 2.3 |
Occasional drinkers (N = 84) | 81.8 ± 1.4* | 81.1 ± 1.3 | 79.1 ± 1.9 |
Daily drinkers (N = 111) | 78.4 ± 1.2 | 78.6 ± 1.2 | 76.8 ± 1.6 |
|
P for trend = 0.667 |
P for trend = 0.693 |
P for trend = 0.612 |
Mt5178A (N = 155) | | | |
Non-drinkers (N = 21) | 74.3 ± 2.6 | 75.8 ± 2.6 | 75.8 ± 3.0 |
Occasional drinkers (N = 60) | 81.3 ± 1.6 | 81.0 ± 1.5 | 80.8 ± 2.1 |
Daily drinkers (N = 74) | 83.8 ± 1.4** | 83.6 ± 1.4* | 83.7 ± 2.0* |
|
P for trend = 0.003 |
P for trend = 0.009 |
P for trend = 0.008 |
For subjects with Mt5178A, the risk of decreased eGFR may depend on frequency of alcohol consumption (
P for trend = 0.003) (Table
3). After adjustment, the negative association between increasing frequency of alcohol consumption and the risk of decreased eGFR remained significant. The crude OR for decreased eGFR was significantly lower in daily drinkers than in non-drinkers (OR = 0.092, 95% confidence interval (CI): 0.012–0.727,
P = 0.024). After adjustment for age and BMI or for age, BMI, habitual alcohol consumption, coffee consumption and use of antihypertensive medication, a significant OR remained (adjusted OR = 0.098, 95% CI: 0.012–0.784,
P = 0.029 and adjusted OR = 0.091, 95% CI: 0.011–0.747,
P = 0.026, respectively). On the other hand, the association between Mt5178C genotype and the risk of decreased eGFR does not appear to depend on alcohol consumption.
Table 3
Odds ratios (ORs) and 95% confidence intervals (CIs) for decreased eGFR (< 90 ml/min/1.73 m
2
) by Mt5178 C/A genotype and alcohol consumption
Mt5178C (N = 239) | | | | | |
Non-drinkers | 6 (13.6) | 38 (86.4) | 1 (reference) | 1 (reference) | 1 (reference) |
Occasional drinkers | 20 (23.8) | 64 (76.2) | 0.505 (0.186–1.369) | 0.633 (0.222–1.810) | 0.786 (0.261–2.372) |
Daily drinkers | 16 (14.4) | 95 (85.6) | 0.938 (0.341–2.576) | 1.038 (0.359–2.999) | 1.024 (0.339–3.092) |
| | |
P for trend = 0.690 |
P for trend = 0.587 |
P for trend = 0.587 |
Mt5178A (N = 155) | | | | | |
Non-drinkers | 1 (4.8) | 20 (95.2) | 1 (reference) | 1 (reference) | 1 (reference) |
Occasional drinkers | 12 (20.0) | 48 (80.0) | 0.200 (0.024–1.642) | 0.276 (0.032–2.395) | 0.280 (0.031–2.505) |
Daily drinkers | 26 (35.1) | 48 (64.9) | 0.092 (0.012–0.727)* | 0.098 (0.012–0.784)* | 0.091 (0.011–0.747)* |
| | |
P for trend = 0.003 |
P for trend = 0.004 |
P for trend = 0.004 |
Discussion
In the present study, we observed that Mt5178 C/A polymorphism apparently modifies the effects of habitual alcohol drinking on eGFR in Japanese men. This is a new gene-environment interaction on renal function. For men with Mt5178A, habitual alcohol consumption may reduce the risk of mildly decreased eGFR. On the other hand, for those with Mt5178C, alcohol consumption does not appear to influence eGFR.
Schaeffner et al. reported an inverse association between moderate alcohol consumption and the subsequent risk of renal dysfunction in large cohort of apparently healthy men [
7]. They used eGFR calculated by the Cockcroft-Gault equation, and reported that men who consumed at least seven drinks per week had an approximately 25% lower risk of reduced eGFR (<55 ml/min) in a 14-year period than those who consumed one or fewer drinks per week. Funakoshi et al. also revealed an inverse relationship between frequency of alcohol consumption and CKD in apparently healthy men [
8]. They used eGFR calculated using the three-variable Japanese equation and found that everyday drinkers had an approximately 40% lower risk of CKD (eGFR <60 ml/min/1.73 m
2) than non-drinkers. Judging from both investigations [
7,
8], alcohol consumption was observed to have a desirable effect on the risk of decreased eGFR. However, our observations suggest that genetic information is required to assess the relationship between alcohol intake and renal function.
From the viewpoint of preventing mildly decreased eGFR, habitual drinking appears to be beneficial for Mt5178A genotypic men. Moreover, for men with Mt5178A, alcohol consumption decreases serum triglyceride levels [
16]. However, it is uncertain whether alcohol consumption is beneficial overall for Mt5178A genotypic men. In contrast, for Mt5178C genotypic men, daily alcohol consumption may increase the risks of hypertension [
10] and hyperuricemia [
18], but may also reduce the risk of hyper-LDL cholesterolemia [
17]. Thus, it is unclear whether alcohol consumption is detrimental for Mt5178C genotypic men. Although further genetic epidemiological research is necessary, genotyping of Mt5178 C/A is thought to be practical for personalized prevention of lifestyle-related diseases, including CKD.
The mechanisms underlying the joint effects of Mt5178 C/A polymorphism and alcohol consumption on renal function have not been elucidated. This gene-environment interaction is presumed to result from differences in the biophysical and biochemical properties of ND2-237 Leu/Met. NADH dehydrogenase is regarded as the major physiological and pathological site of reactive oxygen species (ROS) generation in mitochondria, and as a target of assault by ROS [
28]. Mouse mitochondrial DNA 4738 C/A (Mt4738 C/A) polymorphism leads to a leucine to methionine substitution in NADH dehydrogenase subunit 2. ROS production by NADH dehydrogenase is significantly lower in mice with Mt4738A than in those with Mt4738C [
29]. The results in experimental mice indicate that ND2-237Met suppresses ROS production in humans. Moreover, as methionine residues play an antioxidant role in scavenging ROS [
30], ND2-237Met may also protect NADH dehydrogenase itself from ROS attack.
Ethanol metabolism is directly involved in the production of ROS [
31]. There may be biophysical and biochemical differences in the protection against ethanol-induced ROS or the reduction of ethanol-induced ROS generation between ND2-237Leu and ND2-237Met. These apparent disparities are thought to result in the combined effects of Mt5178 C/A polymorphism and habitual drinking on eGFR. Recently, using male C57BL/6 mice, Yuan et al. demonstrated that moderate ethanol exposure can provide protection for kidneys against ischemia/reperfusion-induced renal injury by enhancing antioxidant capacity characterized by higher activity of superoxide dismutase, which is a critical enzyme responsible for detoxifying ROS [
32]. However, to determine the mechanisms responsible for interaction between ND2-237 Leu/Met genotypes and alcohol consumption on renal function, further biochemical and pharmacological studies are necessary.
In the Japanese population, several genetic polymorphisms have been reported to be associated with CKD, defined as eGFR <50 ml/min/1.73 m
2[
21,
22] or <60 ml/min/1.73 m
2[
23,
24]. In addition, promoter polymorphism of the endotoxin receptor [
19], single nucleotide polymorphism rs1287637 in the nephronophthisis 4 gene is associated with mildly decreased eGFR (<90 ml/min/1.73 m
2) [
20]. Therefore, these molecular epidemiological studies, as well as our results, indicate gene-gene or gene-gene-environment interactions on renal function.
In accordance with K/DOQI CKD classification [
3], eGFR of <90 ml/min/1.73 m
2 was defined as reduced eGFR in this cross-sectional study. In the present subjects, the prevalence of eGFR of <90 ml/min/1.73 m
2 was 82.4% in men with Mt5178C and was 74.8% in those with Mt5178A. A large-scale population-based epidemiological study showed that the prevalence of eGFR of <90 ml/min/1.73 m
2 was 80.6% in Japanese men aged 30–69 years [
2]. Therefore, whether the validity of the definition of reduced eGFR adopted in our study is worthy of further consideration.
There are several important limitations in this study. First, as compared with other genetic epidemiological studies on renal function [
19‐
25], the study sample was too small. Second, selection bias is likely due to the recruiting of subjects from those visiting the hospital for regular medical check-ups, and the prevalence of moderately decreased eGFR of <60 ml/min/1.73 m
2, recognized as CKD, was low among the study subjects. Third, this study was a cross-sectional study, and although the study design can suggest causal links, it cannot establish valid causality. To overcome these limitations, a large-scale population-based follow-up study is necessary. Fourth, because of lack of data on the amount of alcohol intake, the evaluation of habitual alcohol intake was based on the frequency of alcohol consumption. Although we have also used this evaluation method in previous studies [
10,
15‐
18], the existence of joint effects between Mt5178 C/A polymorphism and volume of alcohol intake on eGFR warrants further investigation. Finally, we lacked information on proteinuria, which is an early and sensitive marker of kidney damage in various types of CKD [
3].
Competing interests
The authors declare that they have no competing interests.
Authors’ contributions
AK designed the study, carried out the epidemiological survey, carried out the genotyping, analyzed the data, and drafted the manuscript; MI collected the samples; NM assisted with genotyping; KK and MY carried out the epidemiological survey; NS, TO, TS, HO and HH assisted in data analysis and interpretation; YT designed the study and carried out the epidemiological survey. All authors have read and approved the final manuscript.