Hypertension, the leading risk factor for cardiovascular diseases and premature death, has become a severe public health challenge worldwide. In China, the estimated prevalence of hypertension among adults has increased from 7.5% in 1979 to 25.2% in 2012 [1
]. However, the exact pathophysiology of hypertension remains unclear, and multiple factors are thought to contribute to the development and progression of the disorder. Established environmental determinants of blood pressure levels include age, sex, tobacco use, sodium intake, alcohol consumption, dietary habits, being overweight/obesity, and dyslipidemia [2
]. In addition to environmental factors, genetic factors also have been identified as modulators of hypertension. Hundreds of genes and polymorphisms have been hypothesized to be involved in the pathogenesis of hypertension, and it is estimated that 30%–40% of the variation in blood pressure could be attributed to heritability [5
]. A recent genome-wide association study has linked genetic variation at eight loci with high blood pressure, including the gene encoding methylenetetrahydrofolate reductase (MTHFR
), an enzyme catalyzing the reduction of 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate [6
]. The enzyme resides at an important metabolic branch point directing the distribution of folate derivatives to meet requirements for homocysteine and DNA methylation or for DNA and RNA biosynthesis. C677T is a common polymorphism in the MTHFR
gene. The incidence of this mutation in the homozygous and heterozygous state is 43.9% and 23.2% in the Chinese population, respectively, which is higher compared with many other populations worldwide [7
]. As a result of this polymorphism, the 677TT carriers have 30% and 677CT carriers have 60% of the wild-type (677CC) enzymatic activity, causing impaired remethylation of homocysteine to methionine and subsequent hyperhomocysteinemia, especially under conditions of low dietary folate [8
]. Hyperhomocysteinemia has been linked to hypertension as it may induce arteriolar constriction, renal dysfunction, increased sodium reabsorption, and increase arterial stiffness and oxidative stress [9
]. The 677T allele could also cause DNA hypomethylation [10
], which has been involved in the pathogenesis of hypertension [11
]. Therefore, the MTHFR
C677T polymorphism is expected to be potentially associated with hypertension.
Previously, numerous epidemiological studies have explored the relationship of the MTHFR
C677T polymorphism with hypertension, but the results were inconsistent [12
]. Our previous large meta-analysis including 114 studies found that the polymorphism was significantly related to hypertension among East Asians and Caucasians, but not among Latinos, Black Africans, Indians, and Sri Lankans [12
]. Several genome-wide association studies, mostly among Europeans, have identified more than 30 genomic loci linked to blood pressure levels [6
]; however, subsequent genome-wide association studies among East Asians showed inconsistent results [14
]. Cumulative effect of the genetic loci identified through these genome-wide association studies explained <2.5% of systolic and diastolic blood pressure variance despite estimates of heritability of blood pressure of 30%–40% [5
]. These inconsistent observations may be explained by a number of hypotheses, with gene-environment interaction emerging as a strong candidate because genetic effects on blood pressure can be altered by environmental exposures via multiple biological pathways.
In recent years, greater emphasis has been placed on the joint effects of genetic and environmental factors on complex disease traits, including hypertension. Two recent studies respectively investigated gene-smoking and gene-alcohol interaction effects on blood pressure using genome-wide data from the Framingham Heart Study and identified several novel blood pressure loci [4
]. Xi et al.
investigated the effect of obesity on relationship of six genetic loci recently identified by genome-wide association studies with hypertension among Chinese children and found five loci were significantly associated with hypertension only in obese individuals [17
]. A study in Brazil revealed that angiotensinogen Met235Thr and endothelial nitric oxide synthase Glu298Asp polymorphisms had interactions with smoking, sedentary lifestyle, elevated total cholesterol, and older age towards the development of hypertension [18
]. For the MTHFR
C677T polymorphism, its interaction with B-vitamins on cardiovascular diseases, hypertension, and blood pressure lowering has also been extensively investigated [19
]. For example, Klerk et al.
found that the 677TT genotype was associated with an increased risk of coronary heart disease in Europeans but not in North Americans, which was likely to be driven by differences in exposure to B-vitamins, especially folate intake [19
]. In addition, Wilson and colleagues demonstrated that riboflavin supplementation is effective in reducing blood pressure specifically in individuals with the 677TT genotype [21
]. However, the influence of other potential risk factors (such as age, being overweight/obesity, alcohol drinking, and cigarette smoking) on association between the MTHFR
C677T polymorphism and the risk of hypertension received little attention, especially considering that these factors have been shown to, independently or in combination with the MTHFR
C677T polymorphism, affect homocysteine levels [22
]. Hence, we performed a cross-sectional study to explore the associations of some environmental factors, the MTHFR
C677T polymorphism, and their interactions with hypertension predisposition in a Northern Chinese Han population.
In this study we systematically explored the effects of the MTHFR C677T polymorphism, several environmental factors, and their interactions on the development of hypertension among the Northern Chinese Han population. Participants with older age, high BMI, smoking habits, drinking habits, or carrying the 677T allele were at increased risk of hypertension. Furthermore, we observed some indications for interactions of the MTHFR C677T mutant genotype with older age and being overweight/obesity.
Numerous prior epidemiological studies have investigated the association of the MTHFR
C677T polymorphism with hypertension, but the results were controversial [12
]. A recent meta-analysis by Niu et al.
included nine studies conducted among the Chinese population, four of which reported a significant association between the C677T polymorphism and essential hypertension, but the remaining five did not [28
]. Our research group also demonstrated differences in hypertension risk in relation to the polymorphism among different populations worldwide [12
]. In the present study, we observed that the 677T allele was significantly associated with an increased risk of hypertension in the Northern Chinese Han population. Many factors may contribute to the phenomenon that the MTHFR
C677T polymorphism is related to hypertension in one population, but not in another population. Among them, gene-environment interaction could not be neglected as our current understanding of pathogenesis of hypertension indicates a multifactorial and multistep process including various genetic and environmental factors.
We further explored possible interaction effects of the MTHFR C677T polymorphism with several environmental factors, and found that the joint effects of the “at risk” genotype with older age and being overweight/obesity were greater than the sum of their individual effects. The RERIs for the interaction of older age and being overweight/obesity with the “at risk” genotype were 1.26 (95% CI = −0.33–2.85) and 1.33 (95% CI = −0.15–2.80), respectively, indicating an OR that is 1.26 and 1.33 times higher, respectively, as a result of the interaction. Notably, the calculated lower confidence limits for both RERIs were less than, but close, to zero, meaning that the interactions were marginally significant. One possible explanation for the marginally significant interaction might be the relatively small sample size of the present study, especially in some subgroups.
The precise mechanisms by which age and the MTHFR
C677T polymorphism interact to influence the development of hypertension remain unclear. However, both older age and the 677T allele can cause hyperhomocysteinemia and aberrant DNA methylation [10
], which are significantly associated with hypertension [9
]. Furthermore, an interaction between age and the MTHFR
C677T polymorphism on plasma homocysteine concentrations has been reported [32
]. Therefore, the synergistic effect of the MTHFR
C677T polymorphism and age on hypertension may be partially due to their interactive effects on homocysteine concentrations and DNA status. The age-gene synergism observed in this study may be biologically plausible and expected.
Consistent with our findings on the potential interaction between the polymorphism and being overweight/obesity, a large population-based case-control study among Chinese population by Xi et al.
observed that the MTHFR
C677T polymorphism was significantly associated with hypertension only in obese children [17
]. Overweight/obesity can increase the risk of hypertension by activating the sympathetic nervous system and renin-angiotensin system, inducing insulin resistance, and impairing endothelial function [33
]. Some of these pathophysiological mechanisms such as insulin resistance and endothelial dysfunction are shared by hyperhomocysteinemia in causing hypertension [9
]. Additionally, several epidemiological studies observed that overweight/obesity subjects had higher homocysteine and/or lower folate levels than normal weight ones [34
]. Furthermore, a body of evidence has suggested that elevated homocysteine levels might cause the development of being overweight/obesity via epigenetic control gene expression in the regulation of body fat storage [36
]. Furthermore, a recent study by Yin et al.
showed that the MTHFR
C677T polymorphism interacted with overweight/obesity to modulate serum lipid levels [38
]. In the present study, we also evaluated possible joint effects of the polymorphism and being overweight/obesity on serum lipid levels and found that subjects carrying the “at risk” genotype and being overweight/obesity have significantly higher TC and TG levels and lower HDL levels than other subjects (data not shown). Dyslipidemia has a causal relationship with hypertension. So we inferred that the interaction of the MTHFR
C677T polymorphism and overweight/obesity on dyslipidemia may be a partial reason for their joint effects on hypertension found in this study. In spite of this, the biological mechanisms by which overweight/obesity modifies the relationship between the MTHFR
C677T polymorphism and hypertension risk remain unclear and just can be speculated. Further studies are therefore still warranted to investigate the precise mechanism of the interaction in the pathogenesis of hypertension.
The relationship between cigarette smoking and hypertension is controversial. Some investigators reported that cigarette smoking can increase the risk of hypertension, while others reported that it was associated with reduced hypertension risk, and some found no direct association between them [39
]. However, cigarette smoking was demonstrated to play a role in modifying the effects of some genes on hypertension [42
]. For example, Yin et al.
observed no association between cigarette smoking and hypertension in a Guangxi Bai Ku Yao population; however, they found that cigarette smoking showed interaction with the MTHFR
C677T polymorphism to affect blood pressure levels [42
]. In contrast, our results showed no interaction between cigarette smoking and the polymorphism. Alcohol consumption (especially excessive alcohol consumption) is a known risk factor for hypertension, which was also confirmed in our study. The intermediate metabolites of alcohol can change genes directly and modulate their expression via epigenetic mechanisms [43
]. Therefore, interaction between alcohol drinking and genes may be biologically plausible. Another analysis by Yin et al.
found interactive effects of alcohol consumption with the MTHFR
C677T polymorphism on hypertension [43
], which was different from our findings. The discrepancies between our findings and those reported by Yin et al.
] can be caused by many factors, including differences in ethnic background (Bai Ku Yao [42
Han (the present study)), sample size, dietary habits, and other environmental exposures.
In interpreting the findings of our study, several limitations need to be considered. Firstly, this study used a cross-sectional design, which precludes causal inference. Secondly, the study subjects were recruited from only one hospital, which compromises the representation of the general population and our findings may not be generalizable to the whole Chinese population, although the genotype distribution of the polymorphism in our study subjects complied with the Hardy-Weinberg equilibrium. Thirdly, selection bias is possible because the majority of the study participants were workers in the Dagang Oil Field, and there were significantly more male subjects than females. The significant gender difference in the number of study subjects also makes it difficult to draw conclusions regarding gender. Fourthly, because we used a questionnaire to collect personal exposure information (such as smoking and drinking), the responses might be influenced by the participant’s memory, so recall bias cannot be excluded. Fifthly, some information on relevant potential confounders, such as menopause status, dietary habits (especially folate and riboflavin intake), physical activity, and other genes were not included and controlled in this study. This could have affected the accuracy of our estimates. Despite these limitations, our study still has several apparent advantages. First, the study was carried out in a northern region where 677TT genotype frequency was high according to our previous findings [7
], which guarantees sufficient number of 677TT genotype carriers to enhance the statistical power and to save the research costs. Second, all participants were of Han nationality, which reduces the potential effects of population stratification and improves the validity of statistical analysis. Third, to the best of our knowledge, this is the first study to explore and report the joint effects of the MTHFR
C677T polymorphism with age, being overweight/obesity, gender, alcohol drinking, and cigarette smoking on hypertension predisposition.