A paradoxical thrombogenic mutation in factor II at the target site of arthropod bleeding toxin

Loss-of-function mutations in coagulation cascade proteins lead to bleeding diasthesis. In contrast, gain-of-function mutations in these proteins, which are exceptionally rare, lead to hereditary thrombosis. This is best exemplified by Factor V (i.e., Factor V Leiden) and Factor II (i.e., p.Arg596Leu). Here, we report a family with hereditary thrombosis. The proposita presented with cerebral venous thrombosis accompanied by infarction at the age of 12 years. Despite anticoagulation therapy with oral warfarin, she later developed deep venous thrombosis in her hepatic portal veins at the age of 27 years. A medical exome analysis identified a de novo heterozygous mutation p.Tyr434His in Factor II, which was segregated within the family. A retrospective molecular diagnosis was made using a preserved surgical specimen from the proposita's mother, who had died 10 years earlier. The p.Tyr434 residue, which was substituted in the presently reported family, was located in "exosite I" of thrombin, a critical recognition site for fibrinogen, protein C, and thrombin aptamer. Therefore, the mutant thrombin likely exerted its thrombophilic effect by altering the affinity of thrombin to downstream substrates. Furthermore, the "exosite I" domain of thrombin was inhibited by the arthropod bleeding toxin Triabin (a protein discovered from the saliva of the blood-sucking triatomine bug Triatoma pallidipennis). Our observation that patients with a p.Tyr434His mutation exhibited recurrent thrombosis provides the first proof-of-concept in humans that a pharmacologic agent targeting "exosite I" could be an effective means of specifically modulating the thrombogenic-side of the coagulation system via the inhibition of factor II.