Multimeric composition of plasma von Willebrand factor in chronic myelocytic leukaemia

Bleeding signs due to acquired von Willebrand syndrome at diagnosis of chronic myeloid leukaemia in children

A considerable proportion of patients with chronic myeloid leukaemia (CML) may present at diagnosis with high platelet counts. This may result in thrombosis or bleeding complications due to binding of von Willebrand factor (VWF) multimers to platelets. Paediatric CML is very rare and no systematic investigation on clinical complications of elevated platelets has been reported. Data on platelet count and associated haemostaseological complications were retrospectively analysed in a cohort of 156 children with CML. Fifty-one percent (81/156) patients presented with thrombocytosis (platelet count> 500 × 10⁹ /l), and were extreme (>1 000 × 10⁹ /l) in 23/156 (16%). There were no cases of thrombosis but mild bleeding signs were present in 12% (n = 9) children with thrombocytosis. Bleeding occurred without correlation to elevated platelet counts and was associated with reduced large VWF multimers, indicating a diagnosis of acquired von Willebrand syndrome (AVWS), which resolved after initiation of CML treatment. Patients with paediatric CML frequently exhibit high platelet counts not resulting in thrombosis. In patients with thrombocytosis mild bleeding signs due to a low percentage of large VWF multimers can be demonstrated. AVWS may be underdiagnosed in paediatric CML (Clinical-Trials.gov NCT00445822, 9 March 2007).

Acquired von Willebrand's disease

von Willebrand's disease (vWD) is the commonest inherited bleeding disorder in man with an estimated incidence of 1 per thousand of the population. Acquired von Willebrand's disease (AvWD) is rare with less than 70 cases reported. AvWD is usually associated with autoimmune or clonal proliferation disorders and whilst the precise mechanism of acquired deficiency of von Willebrand factor (vWF) is poorly understood, the most likely candidate mechanism(s) are; antibodies inactivate or form a complex with immunologic or functional sites on vWF, or vWF multimers are selectively absorbed by malignant cells. Unlike hereditary vWD, the acquired form of the disease can be exceedingly difficult to manage. We report 4 cases of AvWD diagnosed at our centre over the past 3 yr. There was no evidence of a previous personal or family history of bleeding in any of the patients and AvWD was confirmed by laboratory testing. All 4 patients had a recognised primary medical condition known to be associated with AvWD (Waldenstrom's Macroglobulinaemia in 2 patients, hypothyroidism in 1 patient and monoclonal gammopathy of unknown significance (MGUS) in 1 patient). The acquired haemostatic defect corrected following treatment of the primary condition in 3 patients with the other patient requiring on demand von Willebrand Factor replacement to control spontaneous and surgery induced bleeding.

Acquired von Willebrand disease in myeloproliferative disorders

The laboratory features of acquired von Willebrand defect (AvWD) in myeloproliferative disorders (MPD) are characterized by a very high platelet count, a normal or prolonged bleeding time, a normal factor VIII and von Willebrand factor (vWF) antigen level, but a decreased vWF-ristocetine cofactor activity and collagen binding activity with a decrease or absence of large vWF-multimers simulating a type II von Willebrand factor deficiency. Although the pathogenesis of type II AvWD in MPD remains unclear, evidence is accumulating that the increased number of platelets in the circulating blood seems to be directly responsible for the observed decrease of large vWF-multimers in plasma leading to a spontaneous bleeding tendency at very high platelet counts. This observation may have clinical implications for the use of platelet anti-aggregants such as aspirin in MPD at platelet counts in excess of 1000 to 2000 x 10(9)/L.

Granulocyte proteases do not process endothelial cell-derived unusually large von Willebrand factor multimers to plasma vWF in vivo

The unusually large von Willebrand factor (ULvWF) multimers present within endothelial cells and platelets are larger than the vWF multimers normally found in adult human plasma. Furthermore, ULvWF multimers are cleared rapidly from the circulation if they are released by intense endothelial cell stimulation. The mechanisms by which the ULvWF multimers are processed to large plasma vWF multimers are not known. It has been demonstrated that granulocyte proteases are capable of decreasing vWF multimer size in vitro, and that some patients with myeloproliferative syndromes have a relative absence of large plasma vWF multimers in sodium citrate-anticoagulated plasma samples. In order to assess the influence of granulocyte proteases on vWF multimer size, we evaluated the vWF multimeric patterns in 94 plasma samples from 60 patients with neutrophil counts that were either considerably elevated or extremely reduced. In 83 of 94 plasma samples, the vWF multimeric patterns were normal. No patients with very low neutrophil counts had ULvWF multimers present. These observations suggest that granulocyte proteases are not likely to be involved in vivo in the processing of ULvWF multimers from endothelial cells to the smaller vWF forms in circulation.

Plasma von Willebrand factor proteolysis in patients with chronic myeloproliferative disorders: no possibility of ex vivo degradation by calcium-dependent proteases

Patients with chronic myeloproliferative disorders (CMPD) frequently have abnormalities of plasma von Willebrand factor (vWf) multimers. The pathogenesis of this phenomenon is still unknown. In order to evaluate the possibility of ex vivo degradation of vWf during blood processing, we compared vWf antigen, ristocetin cofactor and the multimeric composition of vWf in plasmas obtained in the presence of trisodium citrate with or without calcium-dependent protease inhibitors (leupeptin, N-ethylmaleimide and Na2EDTA). The subjects included 20 patients with CMPD, 11 with other diseases and 8 normal subjects. In patients with CMPD and normal subjects, the values of vWf antigen, ristocetin cofactor, ristocetin cofactor/vWf antigen ratio and the relative amount of large multimers of vWf did not significantly differ from each other in plasma samples with and without protease inhibitors. In other diseases, especially in a patient with disseminated intravascular coagulation, a somewhat higher amount of large multimers were found in plasma with protease inhibitors than without inhibitors. These findings indicate that the ex vivo proteolysis during blood processing is negligible in patients with CMPD, and that the observed abnormalities in vWf is an in vivo phenomenon.