Etiology of TTP Thrombotic thrombocytopenic purpura (TTP) is caused by Congenital or acquired deficient activity of the plasma enzyme ADAMTS13 The ADAMTS13 enzyme is a plasma protease that cleaves von Willebrand factor into smaller sizes and thereby eliminates unusually large von Willebrand factor (VWF) multimers that would otherwise accumulate on endothelial cells where they can cause platelet thrombi.