Effect modification by population dietary folate on the association between MTHFR genotype, homocysteine, and stroke risk: a meta-analysis of genetic studies and randomised trials

Abstract

Background: The MTHFR 677C→T polymorphism has been associated with raised homocysteine concentration and increased risk of stroke. A previous overview showed that the effects were greatest in regions with low dietary folate consumption, but differentiation between the effect of folate and small-study bias was difficult. A meta-analysis of randomised trials of homocysteine-lowering interventions showed no reduction in coronary heart disease events or stroke, but the trials were generally set in populations with high folate consumption. We aimed to reduce the effect of small-study bias and investigate whether folate status modifies the association between MTHFR 677C→T and stroke in a genetic analysis and meta-analysis of randomised controlled trials.

Methods: We established a collaboration of genetic studies consisting of 237 datasets including 59,995 individuals with data for homocysteine and 20,885 stroke events. We compared the genetic findings with a meta-analysis of 13 randomised trials of homocysteine-lowering treatments and stroke risk (45,549 individuals, 2314 stroke events, 269 transient ischaemic attacks).

Findings: The effect of the MTHFR 677C→T variant on homocysteine concentration was larger in low folate regions (Asia; difference between individuals with TT versus CC genotype, 3·12 μmol/L, 95% CI 2·23 to 4·01) than in areas with folate fortification (America, Australia, and New Zealand, high; 0·13 μmol/L, -0·85 to 1·11). The odds ratio (OR) for stroke was also higher in Asia (1·68, 95% CI 1·44 to 1·97) than in America, Australia, and New Zealand, high (1·03, 0·84 to 1·25). Most randomised trials took place in regions with high or increasing population folate concentrations. The summary relative risk (RR) of stroke in trials of homocysteine-lowering interventions (0·94, 95% CI 0·85 to 1·04) was similar to that predicted for the same extent of homocysteine reduction in large genetic studies in populations with similar folate status (predicted RR 1·00, 95% CI 0·90 to 1·11). Although the predicted effect of homocysteine reduction from large genetic studies in low folate regions (Asia) was larger (RR 0·78, 95% CI 0·68 to 0·90), no trial has evaluated the effect of lowering of homocysteine on stroke risk exclusively in a low folate region.

Interpretation: In regions with increasing levels or established policies of population folate supplementation, evidence from genetic studies and randomised trials is concordant in suggesting an absence of benefit from lowering of homocysteine for prevention of stroke. Further large-scale genetic studies of the association between MTHFR 677C→T and stroke in low folate settings are needed to distinguish effect modification by folate from small-study bias. If future randomised trials of homocysteine-lowering interventions for stroke prevention are undertaken, they should take place in regions with low folate consumption.

Funding: Full funding sources listed at end of paper (see Acknowledgments).

Figure 1

Difference in homocysteine concentration in individuals without cardiovascular disease according to MTHFR 677C→T genotype, by probable folate status category Large studies are those with more than 500 individuals. AANZ=America, Australia, and New Zealand.

Figure 2

Odds ratio of stroke according to MTHFR 677C→T variant, by probable folate status category Left panel compares individuals homozygous for T allele with CC participants. Right panel compares heterozygous with CC individuals. AANZ=America, Australia, and New Zealand.

Figure 3

Odds ratio of stroke in large studies (≥400 stroke events) according to MTHFR 677C→T genotype, by probable folate status category Differences in homocysteine concentration (derived from large studies; figure 1) are shown in category subheadings.

Figure 4

Pooled relative risk of stroke from randomised clinical trials of homocysteine-lowering interventions in the context of genetic studies of the MTFHR 677C→T variant RCT=randomised controlled trials. AANZ=America, Australia, and New Zealand.

Figure 5

Sensitivity analyses of effect of homocysteine-lowering interventions from randomised clinical trials on risk of stroke All analyses are random effects (DerSimonian and Laird). Number of clinical events reports stroke only as default and stroke plus TIA when specified in row title. RCT=randomised controlled trial. TIA=transient ischaemic attack. *Studies with fewer than 1000 participants were Wrone, Zoungas, Liem, Righetti, and House. †See webappendix p 24 for classification of risk of bias per study. ‡Studies reporting stroke and TIA separately were Righetti, Wrone, and HOPE-2. §Studies that used B vitamins (other than folic acid) as one of the comparator groups were Ebbing and Bonaa; studies that used low-dose folic acid as comparator group (and not placebo or standard care) were Wrone and Toole.