Interaction between hyperhomocysteinemia and inherited thrombophilic factors in venous thromboembolism.

Hyperhomocysteinemia is an established risk factor for deep vein thrombosis. Factor V Leiden has been reported to potentiate the thrombotic risk related with severe hyperhomocysteinemia, being more represented in thrombotic patients with homocystinuria as compared with patients without a history of thrombosis. The results concerning the interaction between moderate hyperhomocysteinemia and inherited thrombophilic factors such as Factor V Leiden or the prothrombin G20210A mutation are contradictory. The relative risk for venous thrombosis has been reported to be increased 10- to 50-fold in patients carrying both hyperhomocysteinemia and inherited thrombophilia in comparison with normal controls, suggesting a synergistic interaction, yet other studies failed to confirm such conclusion. The heterogeneity of these findings is in part due to the small number of individuals with double defects, leading to statistically unreliable results. Genotyping for mutations that are possible causes of moderate hyperhomocysteinemia, such as the thermolabile variant (C677T) of methylenetetrahydrofolate reductase (MTHFR), does not seem useful to identify individuals at higher risk for venous thromboembolism. In fact, in most of the studies the presence of the C677T MTHFR homozygous genotype does not increase the thrombotic risk associated with Factor V Leiden or the prothrombin mutation.