Elsevier

Atherosclerosis

Volume 132, Issue 1, 11 July 1997, Pages 105-113
Atherosclerosis

The 677C→T mutation in the methylenetetrahydrofolate reductase gene: associations with plasma total homocysteine levels and risk of coronary atherosclerotic disease

https://doi.org/10.1016/S0021-9150(97)00084-1Get rights and content

Abstract

Homozygosity for a 677C→T mutation at the locus that codes for 5,10-methylenetetrahydrofolate reductase (MTHFR), a folate-dependent crucial enzyme in homocysteine metabolism, may render the enzyme thermolabile and less active and has been associated with increased levels of plasma total homocysteine (tHcy). We assessed whether this mutation was associated with increased risk of coronary atherosclerosis and plasma levels of tHcy and furthermore studied whether folate status would modify the associations. Data were collected from subjects with substantial coronary atherosclerosis (≥90% occlusion in one and ≥40% occlusion in a second coronary artery, referred to as cases, n=131) or virtually no coronary narrowing (referred to as coronary controls, n=87) and from a population-based control group (n=100), all residing in the Rotterdam area, The Netherlands. Both males and females, aged 25–65 years were studied. The frequency of homozygosity for the mutation (+/+) in cases (10.0%) did not significantly differ statistically from that observed in coronary controls (11.5%, P=0.71), population-based controls (7.0%, P=0.43), or combined control groups (9.1%, P=0.80). In the overall group (as well as in the three subgroups), plasma tHcy levels, fasting and to a lesser extent after a methionine-loading test, were higher in +/+ subjects than in homozygous normal subjects (−/−), whereas heterozygous subjects (+/−) had intermediate levels (Ptrend=0.001). The +/+ subjects with erythrocyte folate levels <790 nmol/l (population median) had a 77% (95% CI, 27–144%) higher geometric mean fasting tHcy (21.4, μmol/l) than those with higher erythrocyte folate (12.1 μmol/l). The odds ratio (OR) of coronary atherosclerosis for +/+ subjects, with +/− and −/− subjects as the reference group, in analyses with combined control groups, was 1.1 (95% CI, 0.5–2.4). The ORs were 2.2 (95% CI, 0.7–6.8) and 0.6 (95% CI, 0.2–1.7) among subjects with low and high folate levels, respectively. Our study indicates that homozygosity for the 677C→T MTHFR mutation, especially in combination with low folate status, predisposes to high plasma levels of fasting tHcy. However, homozygosity for this mutation, whether or not in combination with low folate status, was not associated with increased risk of coronary artery disease.

Introduction

Elevated plasma total homocysteine (tHcy), an independent risk factor for atherosclerotic vascular disease 1, 2, is partly genetically determined 3, 4. Homocysteine is formed from methionine and is either catabolized in the vitamin B6-dependent transsulfuration pathway or remethylated to methionine. This latter reaction is catalyzed by the enzyme methionine synthase, which requires 5-methyltetrahydrofolate (5-methyl-THF) as substrate and vitamin B12 as cofactor. 5-Methyl-THF is formed by the reduction of 5,10-methylene-THF by 5,10-methylene-THF reductase (MTHFR), which is a regulating enzyme in homocysteine metabolism [5]. Consequently, deficiencies of MTHFR may result in elevation of plasma tHcy.

In 1988, Kang et al. 6, 7discovered a variant of the MTHFR enzyme, characterized by a specific enzyme activity of approximately 50% of the normal activity. The enzyme appeared to be thermolabile, providing an opportunity to distinguish between this variant and the normal enzyme. Furthermore, they and others described that thermolability of MTHFR was common in the normal population (5%), associated with raised tHcy levels, and increased risk of coronary artery disease 8, 9, 10. Recently, a 677C→T mutation was detected in the MTHFR gene, and homozygosity for this mutation was associated with decreased specific enzyme activity, increased thermolability, and elevated tHcy [11], mainly in subjects with low levels of plasma folate 12, 13. Several investigations have now studied whether homozygosity for the MTHFR 677C→T mutation is a risk factor for cardiovascular disease. Three studies found a two- to threefold increased risk for atherosclerotic vascular disease 14, 15, 16, one study found a twofold increased risk for thrombotic vascular disease [17], but seven other studies observed virtually no association or even a slight inverse association between homozygosity for the mutation and risk of cardiovascular disease 18, 24.

In the present study, we investigated the frequency of 677C→T genotype and its association with tHcy levels before and after methionine loading in subjects with severely occluded coronaries and normal coronaries, and in controls from the general population. Since an adequate folate status may counterbalance the defective production of 5-methyl-THF in subjects with thermolabile MTHFR, we also studied the interactive effect of genotype and erythrocyte folate status on plasma tHcy level and on risk of coronary atherosclerosis.

Section snippets

Study population

A case-control study was conducted from June 1992 to June 1994. Cases and one control group were selected from patients aged 25–65 years, who underwent coronary angiography in the Zuiderziekenhuis Hospital in Rotterdam, the Netherlands. Subjects with either severe coronary occlusions (referred to as cases) or without substantial coronary occlusions (referred to as coronary controls) were included. A second control group was drawn from the general population and comprised subjects with no

Characteristics and tHcy

Age, gender, coronary risk factors and plasma tHcy levels of the cases and the two control groups are shown in Table 1. Except for age and gender, all other risk factors were adjusted for age and gender. In comparison with each of the control groups, cases had a statistically significant higher mean age, proportion of hypertensive subjects, mean level of total/HDL cholesterol, mean serum level of triglycerides and mean pack years of smoking. In addition, cases had a statistically significant

Discussion

In the present investigation, we showed that homozygosity for the 677C→T mutation in the MTHFR gene was associated with raised plasma tHcy levels, particularly when measured in the fasting state. However, +/+ genotype was not associated with increased risk of severe coronary atherosclerosis. Elevation of tHcy in +/+ subjects was limited to those with erythrocyte folate levels below the median of the study population. However, we have not been able to prove the hypothesis that the combination of

Acknowledgements

The study was supported by a grant from the Netherlands Organization for Scientific Research (project 904-61-047). The authors would like to express their gratitude to Drs A.A.A. Bak, S.C. Balduw, G.J. van Beek, M.P. Freericks, F.M.A. Harms, R. van Mechelen, W.M. Muijs van de Moer and R. Wardeh for their support in selecting the participants. We thank Dr D.E. Grobbee and Dr J.C.M. Witteman for their help in designing and conducting of the study. We are very grateful to Annelies Legters, the

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