Risk of venous thrombosis in carriers of a common mutation in the homocysteine regulatory enzyme methylenetetrahydrofolate reductase

Background: Elevated levels of homocysteine are an independent risk factor for venous thrombosis. A common mutation in methylenetetrahydrofolate reductase (MTHFR), an enzyme required for efficient homocysteine metabolism, creates a thermolabile (tl-) enzyme with reduced activity that may predispose to hyperhomo-cysteinemia. Methods and Results: To assess whether this common mutation is a risk factor for venous thromboembolism, a polymerase chain reaction-based genotyping assay was used to compare the prevalence of this mutation in a group with thrombosis versus several jcontrol groups. Of the 331 thrombosis subjects, 47% were heterozygous and 11% homozygous for tl-MTHFR. In comparison, heterozygotes constituted 42-47% and homozygotes 15-16% of each of three control groups (totaling 593 subjects). There was no significant difference in the tl-MTHFR homozygote frequency or allele frequency between the thrombosis and control study groups. Although the prevalence of the factor V R506Q (Leiden) mutation causing activated protein C resistance was significantly higher in the thrombosis (19%) than in the control groups (4-9%), the concomitant presence of tl-MTHFR with factor V R506Q did not contribute to any excess thrombotic risk. Conclusions: Although the tl-MTHFR mutation may predispose to hyperhomocysteinemia, a known risk factor for venous thrombosis, this common genotype is not a direct genetic risk factor for venous thrombosis, either alone or in combination with the factor V R506Q mutation.