Heterozygous Prothrombin Mutation-Associated Thrombophilia.

Background:  Venous thromboembolism (VTE) is predisposed by thrombotic mutations in patients with hereditary thrombophilia. Although prothrombin deficiencies caused by homozygous or compound heterozygous mutations are associated with bleeding diathesis, rare cases have shown a correlation between heterozygous prothrombin mutations and thrombosis.

Materials and methods:  We surveyed genetic variants involved in thrombosis and hemostasis in 347 patients with unprovoked VTE or having a positive family history of thrombosis. For patients identified with heterozygous prothrombin mutations, we conducted family investigations and performed a thrombin generation test (TGT) to elucidate the thrombotic risk. Novel mutants were expressed and subjected to functional assays to clarify the underlying thrombotic mechanisms.

Results:  Heterozygous prothrombin mutations were identified in 3.5% of patients (12/347), including three novel mutations Phe382Ser, Phe382Leu, and Asp597Tyr found in one patient each, as well as previously reported Arg541Trp mutation in four patients and Arg596Gln mutation in five patients. A total of 42 mutation carriers were identified within the 12 pedigrees, among whom 64.3% (27/42) had experienced thrombotic events. TGT results demonstrated hypercoagulability for carriers of the five mutations, with Arg596Gln showing the highest thrombin generation potential followed by Arg541Trp. The Phe382-associated mutations severely impaired thrombomodulin-binding ability of thrombin, resulting in obviously reduced protein C (PC) activation. The Asp597Tyr mutation exhibited a mild reduction in both inactivation by antithrombin and PC activation reactions.

Conclusion: The presence of heterozygous prothrombin mutations represents a potential genetic predisposition for VTE. All thrombosis-associated mutations potentiate coagulation activity by either conferring antithrombin resistance and/or impairing PC pathway activity.