Abnormalities of Plasma von Willebrand Factor in Patients with Chronic Myeloproliferative Disorders

In chronic myeloproliferative disorders (CMPD) hemorrhagic complications are relatively common, in association with thrombocytosis. We studied the multimeric composition of plasma von Willebrand factor (vWf) in CMPD and investigated the possibilities of in vivo or ex vivo proteolysis. The relative amount of large multimers in patients with CMPD was significantly decreased in relation to platelet count as compared with normal subjects. It correlated negatively with platelet count, and positively with the ristocetin cofactor/vWf antigen ratio. All patients with primary myelofibrosis who had normal or subnormal platelet counts showed an almost normal multimeric pattern. The vWf abnormalities normalized with decreasing platelet counts after starting chemotherapy or when blast crisis occurred with decreased platelet counts. These abnormalities would be caused by in vivo proteolysis, because they were not corrected even when blood was obtained in the presence of protease inhibitors. The relative amount of large multimers of vWf did not correlate with plasma concentration of plasmin or elastase. Thus neither plasmin nor elastase appear to be the enzymes responsible for in vivo proteolysis