von Willebrand Factor–Cleaving Protease in Thrombotic Thrombocytopenic Purpura and the Hemolytic–Uremic Syndrome

Abstract

Thrombotic thrombocytopenic purpura and the hemolytic–uremic syndrome are severe microvascular disorders of platelet clumping with similar signs and symptoms. Unusually large multimers of von Willebrand factor, capable of agglutinating circulating platelets under high shear stress, occur in the two conditions. We investigated the prevalence of von Willebrand factor–cleaving protease deficiency in patients with familial and nonfamilial forms of these disorders. Plasma samples were obtained from 53 patients with thrombotic thrombocytopenic purpura or hemolytic–uremic syndrome. Von Willebrand factor–cleaving protease was assayed in diluted plasma samples with purified normal von Willebrand factor as the substrate. The extent of the degradation of von Willebrand factor was assessed by electrophoresis in sodium dodecyl sulfate–agarose gels and immunoblotting. To determine whether an inhibitor of von Willebrand factor–cleaving protease was present, we measured the protease activity in normal plasma after incubation with plasma from the patients. We examined 30 patients with thrombotic thrombocytopenic purpura and 23 patients with the hemolytic–uremic syndrome. Of 24 patients with nonfamilial thrombotic thrombocytopenic purpura, 20 had severe and 4 had moderate protease deficiency during an acute event. An inhibitor found in 20 of these patients was shown to be IgG in five of five tested plasma samples. Of 13 patients with nonfamilial hemolytic–uremic syndrome, 11 had normal levels of activity of von Willebrand factor–cleaving protease during the acute episode, whereas in 2 patients, the activity was slightly decreased. All 6 patients with familial thrombotic thrombocytopenic purpura lacked von Willebrand factor–cleaving protease activity but had no inhibitor, whereas all 10 patients with familial hemolytic–uremic syndrome had normal protease activity. In vitro proteolytic degradation of von Willebrand factor by the protease was studied in 5 patients with familial and 7 patients with nonfamilial hemolytic–uremic syndrome and was normal in all 12 patients. Nonfamilial thrombotic thrombocytopenic purpura is due to an inhibitor of von Willebrand factor–cleaving protease, whereas the familial form seems to be caused by a constitutional deficiency of the protease. Patients with the hemolytic–uremic syndrome do not have a deficiency of von Willebrand factor–cleaving protease or a defect in von Willebrand factor that leads to its resistance to protease.