The relationships between plasma total homocysteine and selected atherosclerotic risk factors according to the C677T methylenetetrahydrofolate reductase gene in Japanese

Introduction

Several epidemiological studies have reported that elevated plasma tHcy is an independent risk factor for atherosclerotic thrombotic vascular diseases [1]. A mutation of the 5,10-methylenetetrahydrofolate reductase (C677T MTHFR) gene is well known to increase plasma tHcy levels [1, 2]. Notably, this mutation modifies the relationships between plasma tHcy and several modifiable factors such as serum folate, vitamin B₁₂ [1, 2], and uric acid concentrations [3], indicating a so-called gene–environment interaction. Since these factors are also related to other factors (e.g., alcohol drinking, smoking, and hematological variables) [4], we analyzed the relationships of selected atherosclerotic risk factors (age, sex, uric acid, hematocrit, hemoglobin, and creatinine) with plasma tHcy concentration considering the potential modification effects of the C677T MTHFR polymorphism.

Methods and materials

The participants (213 men and 242 women) were randomly selected from all residents aged 40–69 years in Shiso county, Japan in 2000. The assessment of lifestyle factors, blood collection, biochemical analyses, and MTHFR genotyping were previously described [2]. A written informed consent was obtained from all participants and the study was approved by the ethics review committees of National Institute of Health and Nutrition (NIHN), and Tokyo Medical and Dental University, Japan.

Results

Mean ± SD of age was 53.7 ± 8.6 years. The medium value of plasma tHcy was 11.1 μmol/l and those were significantly higher in men than in women (12.9 and 9.5 μmol/l, respectively; P < 0.0001 by U-test). The frequency of the mutant allele (V) in men was 0.42, and in women it was 0.38. The frequencies of the MTHFR AA, AV, and VV genotypes were 31.9, 52.1, and 16.0% in men, respectively, whereas it was 36.3, 50.8, and 12.9% in women, respectively, and the genotype distributions were in Hardy–Weinberg equilibrium.

Serum uric acid, hematocrit, hemoglobin, and serum creatinine were positively associated with plasma tHcy independently of age and sex (Table 1). Of these, the positive relationships between serum uric acid and plasma tHcy were 2.5- and 4-fold steeper in those with AV (standardized regression coefficient ± SE = 0.15 ± 0.06) and VV (0.25 ± 0.10) genotypes than in those with AA genotype (0.06 ± 0.05), respectively, even after multivariate adjustment for known confounding factors [age, sex, serum folate, serum vitamin B₁₂, body mass index (BMI), and serum creatinine], suggesting a so-called gene–environment interaction, but the overall statistical test of interaction was not significant (P = 0.11). The positive associations of hematocrit, hemoglobin, and serum creatinine with plasma tHcy were observed in each genotype, but no differences for these associations among the three genotypes were found.

Discussion

Our study showed a significant association between serum uric acid and plasma tHcy levels in all subjects. The association was only significant and strongest in the homozygous mutant (VV) genotype after adjustment for known confounding factors. This result was approximately compatible with Motti et al. [3] who reported a significantly stronger relationship between uric acid and plasma tHcy level in those with VV genotype than in those with AA and AV genotypes. Our result was the first report for adult healthy subjects in a Japanese population. Although the underlying mechanism is still obscured, adenosine originating from S-adenosyl-homocysteine could represent a link between the homocysteine and uric acid metabolic pathway, and the MTHFR mutation could involve this mechanism such as the de novo synthesis of purines [3, 4]. A reduced intake of purinerich foods could prevent hyperuricemia and hyperhomocysteinemia.

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Table 1

Associations between blood biochemical variables and plasma tHcy level

Variables	Plasma tHcy (μmol/l) Geometric LSM, Geometric SE#
Uric acid (log mg/dl) †
≤ 1.46	10.8, 1.02
> 1.46 and ≤ 1.72	10.9, 1.02
> 1.72	11.7, 1.02
P for homogeneity	0.019
P for trend	0.0015
Hemoglobin, g/dl †
≤ 13.7	10.4, 1.02
> 13.7 and ≤ 15	11.0, 1.02
> 15	12.1, 1.02
P for homogeneity	< 0.0001
P for trend	≤ 0.0001
Hematocrit (%) †
≤ 40.2	10.2, 1.02
> 40.2 and ≤ 43.7	11.0, 1.02
> 43.7	12.2, 1.02
P for homogeneity	< 0.0001
P for trend	< 0.0001
Serum creatinine mg/dl †
≤ 0.6	10.7, 1.02
> 0.6 and ≤0.7	11.5, 1.02
> 0.7	12.0, 1.03
P for homogeneity	0.0007
P for trend	0.0001

^# Values are geometric least square mean (Geometric LSM), and geometric standard error (Geometric SE) of plasma tHcy adjusted for age and sex by analysis of covariance. P for homogeneity: P value for the overall test of equality of the plasma tHcy distribution among the tertile groups.

The hematocrit, hemoglobin, and serum creatinine were significantly associated with plasma tHcy in each genotype. Although such findings have not previously been reported in Japanese, the results are compatible with the other study [4]. The mechanisms involved are still not known. Smoking status was encountered in the determination of hemoglobin and hematocrit levels, and renal homocysteine excretion was also important for the determination of tHcy concentration. The plasma tHcy was strongly significantly associated with sex (men). It may be explained by BMI, smoking status, and lifestyle-related factors (serum folate and vitamin B12 levels) [1, 2], and smoking status in men was significantly associated with plasma tHcy (r = 0.20, P = 0.0041). We should emphasize on monitoring the recent preventive measure for smoking and body weight control.

In conclusion, men, serum uric acid, serum creatinine, blood hemoglobin, and hematocrit were determinants of plasma tHcy levels. In addition, the interaction of serum uric acid and the MTHFR mutation may influence the variation of plasma tHcy in this population. Our findings need further investigation.