Gene-gene and gene-environment interactions determine risk of thrombosis in families with inherited antithrombin deficiency.

To analyze inherited antithrombin deficiency as a risk factor for venous thromboembolism in various conditions with regard to the presence or absence of additional genetic or acquired risk factors, we compared 48 antithrombin-deficient individuals with 44 nondeficient individuals of 14 selected families with inherited antithrombin deficiency. The incidence of venous thromboembolism for antithrombin deficient individuals was 20 times higher than among nondeficient individuals (1.1% v 0.05% per year). At the age of 50 years, greater than 50% of antithrombin-deficient individuals had experienced thrombosis compared with 5% of nondeficient individuals. Additional genetic risk factors, Factor V Leiden and PT20210A, were found in more than half of these selected families. The effect of exposure to 2 genetic defects was a 5-fold increased incidence (4.6% per year; 95% confidence interval [CI], 1.9% to 11.1%). Acquired risk factors were often present, determining the onset of thrombosis. The incidence among those with exposure to antithrombin deficiency and an acquired risk factor was increased 20-fold (20.3% per year; 95% CI, 12.0% to 34.3%). In conclusion, in these thrombophilia families, the genetic and environmental factors interact to bring about venous thrombosis. Inherited antithrombin deficiency proves to be a prominent risk factor for venous thromboembolism. The increased risks among those with exposure to acquired risk factors should be considered and adequate prophylactic anticoagulant therapy in high-risk situations seems indicated in selected families with inherited antithrombin deficiency.