Deficient Activity of von Willebrand Factor–Cleaving Protease in Chronic Relapsing Thrombotic Thrombocytopenic Purpura

TLDR

Thrombotic thrombocytopenic purpura is associated with unusually large von Willebrand factor multimers that arise when the protease that cleaves vWF is deficient. The study examined vWF‑cleaving protease activity in four patients with chronic relapsing TTP. Plasma samples were incubated with purified vWF under conditions favoring protease activity, and degradation was assessed by SDS‑agarose gel electrophoresis and immunoblotting. All four patients showed markedly reduced or absent vWF‑cleaving protease activity, with no detectable inhibitor or antibody, indicating that protease deficiency—potentially autosomal recessive—causes the large multimers and may serve as a diagnostic marker for latent TTP.

Abstract

Abstract In patients with thrombotic thrombocytopenic purpura (TTP), excessive intravascular platelet aggregation has been associated with appearance in plasma of unusually large von Willebrand factor (vWF ) multimers. These extremely adhesive vWF multimers may arise due to deficiency of a “depolymerase” cleaving vWF to smaller molecular forms, either by reducing the interdimeric disulfide bridges or by proteolytic degradation. We studied the activity of a recently described vWF-cleaving protease in four patients with chronic relapsing TTP. Diluted plasma samples of TTP patients were incubated with purified normal human vWF in the presence of a serine protease inhibitor, at low ionic strength, and in the presence of urea and barium ions. The extent of vWF degradation was assayed by electrophoresis in sodium dodecyl sulfate-agarose gels and immunoblotting. Four patients, that included two brothers, with chronic relapsing TTP displayed either substantially reduced levels or a complete absence of vWF-cleaving protease activity. In none of these patient plasmas was an inhibitor of or an antibody against the vWF-cleaving protease established. Our data suggest that the unusually large vWF multimers found in TTP patients may be caused by deficient vWF-cleaving protease activity. Deficiency of this protease may be inherited in an autosomal recessive manner and seems to predispose to chronic relapsing TTP. The assay of the vWF-cleaving protease activity may be used as a sensitive diagnostic tool for identification of subjects with a latent TTP tendency.