In all, we included 81 data points from 80 studies in this meta-analysis (; ), with a total of 26 000 cases and 31 813 controls. We identified 24 studies that we believe fitted the earlier reviews' inclusion criteria,5,11
but which were not included in either of the original meta-analyses either because they were not identified by the original search strategies or because they have been published since this meta-analysis. We found an overall random effect odds ratio for coronary artery disease comparing TT with CC genotypes of 1.14 (95% confidence interval 1.05 to 1.24). Five studies showed evidence to suggest (P < 0.05) that genotypes were not in Hardy-Weinberg equilibrium in controls. However, exclusion of these studies made very little difference to the overall result (odds ratio 1.15, 1.06 to 1.26). We also found evidence that effect estimates were related to study size, suggesting that small study bias, such as publication bias, may have distorted the findings (Egger test P = 0.03).
Summary findings by geographical region: odds ratios for coronary heart disease comparing TT with CC genotypes, and difference in geometric mean serum homocysteine concentrations
Results of search strategy
We found strong evidence of between study heterogeneity (P < 0.001, I2 = 38.4%). Stratified analysis by geographical region removed much of the heterogeneity within Europe, North America, Australia, and the Middle East; the random effect odds ratio were 1.08 (0.99 to 1.18; Pheterogeneity = 0.19, I2 = 16.1%) among the 42 European studies, 0.93 (0.80 to 1.10; Pheterogeneity = 0.64, I2 = 0%) among the 15 North American studies, 1.04 (0.73 to1.49; Pheterogeneity = 0.97, I2 = 0%) among the three Australian studies, and 2.61 (1.81 to 3.75; Pheterogeneity = 0.57, I2 = 0%) among the five Middle Eastern studies (). Evidence of heterogeneity remained between Asian studies, but this was only apparent between Japanese studies and others and within studies done in Japan. The random effects odds ratio for Japanese studies was 1.71 (1.23 to 2.37; Pheterogeneity = 0.01, I2 = 62.5%); studies done in China, Taiwan, and Korea produced homogeneous results, with an overall odds ratio of 0.81 (0.60 to 1.10; Pheterogeneity = 0.41, I2 = 3.0%).
Association between MTHFR C677T polymorphism and coronary heart disease (TT versus CC genotype). *Unpublished data taken from Klerk et al, 2002 meta-analysis
To minimise the effect of inter-laboratory variation in measurement of homocysteine concentrations, we used differences in homocysteine concentrations between TT and CC genotypes rather than absolute homocysteine concentrations as a measure of geographical differences in dietary folate, as differences in homocysteine concentrations by MTHFR C677T genotype have been shown to be greater at lower levels of folate intake and are reduced after folate supplementation.14
We examined whether these differences were associated with the size of effect of the MTHFR genotype on coronary heart disease risk. Of the 20 studies that gave mean homocysteine concentrations and standard deviations by genotype, the differences ranged from -0.8 to 11 μmol/l (). A random effects meta-analysis of mean difference in homocysteine concentrations between CC and TT genotypes found an overall difference of 2.24 μmol/l (95% confidence interval 1.55 to 2.94). In a meta-regression analysis we found evidence to suggest that differences in serum homocysteine concentrations by genotype were associated with the effect of genotype on coronary artery disease risk (β = 0.103, P = 0.02), although this does not take into account the uncertainty in homocysteine differences by genotype. This equates to an increase in the odds ratio of exponential 0.103 (0.019 to 0.186) or 1.11 (1.01 to 1.20) per 1 μmol/l increase in the difference in homocysteine concentrations between CC and TT genotypes.
Studies reporting serum homocysteine concentrations (μmol/l) according to MTHFR genotype: homozygotes for the mutant allele (TT), heterozygotes (CT), and wild type homozygotes (CC)
We calculated the predicted odd ratios for coronary heart disease risk for given changes in homocysteine concentrations by region () and compared them with the Wald et al5
meta-analysis (). Findings were broadly similar for increased odds of coronary heart disease associated with the CC genotype and the effect of a 5 μmol/l increase or a 3 μmol/l decrease in homocysteine concentration between our meta-analysis and Wald's.
Summary results from three meta-analyses of MTHFR C677T and coronary heart disease