Homocysteine and coronary heart disease: meta-analysis of MTHFR case-control studies, avoiding publication bias.

Background: Moderately elevated blood levels of homocysteine are weakly correlated with coronary heart disease (CHD)risk, but causality remains uncertain. When folate levels are low, the TT genotype of the common C677T polymorphism(rs1801133) of the methylene tetrahydrofolate reductase gene (MTHFR) appreciably increases homocysteine levels, so‘‘Mendelian randomization’’ studies using this variant as an instrumental variable could help test causality.Methods and Findings: Nineteen unpublished datasets were obtained (total 48,175 CHD cases and 67,961 controls) inwhich multiple genetic variants had been measured, including MTHFR C677T. These datasets did not include measurementsof blood homocysteine, but homocysteine levels would be expected to be about 20% higher with TT than with CCgenotype in the populations studied. In meta-analyses of these unpublished datasets, the case-control CHD odds ratio (OR)and 95% CI comparing TT versus CC homozygotes was 1.02 (0.98–1.07; p = 0.28) overall, and 1.01 (0.95–1.07) inunsupplemented low-folate populations. By contrast, in a slightly updated meta-analysis of the 86 published studies (28,617CHD cases and 41,857 controls), the OR was 1.15 (1.09–1.21), significantly discrepant (p = 0.001) with the OR in theunpublished datasets. Within the meta-analysis of published studies, the OR was 1.12 (1.04–1.21) in the 14 larger studies(those with variance of log OR,0.05; total 13,119 cases) and 1.18 (1.09–1.28) in the 72 smaller ones (total 15,498 cases).Conclusions: The CI for the overall result from large unpublished datasets shows lifelong moderate homocysteine elevationhas little or no effect on CHD. The discrepant overall result from previously published studies reflects publication bias ormethodological problems.