Multimeric composition of plasma von Willebrand factor in chronic myeloproliferative disorders

In chronic myeloproliferative disorders (CMPD) thrombohaemorrhagic complications occur occasionally in association with thrombocytosis. We studied the multimeric composition of plasma von Willebrand factor (vWf) in 15 patients with polycythaemia vera (PV), 12 with essential thrombocythaemia (ET) and eight with primary myelofibrosis (PMF). The relative content of large (multimer band greater than or equal to 11) multimers calculated by densitometer scan following SDS-agarose gel electrophoresis was 18.5 +/- 4.4% (mean +/- SD) in normal controls, 8.3 +/- 7.9% in PV, 8.1 +/- 4.6% in ET and 19.6 +/- 6.7% in PMF. The patients with PV and ET but not PMF had a significantly lower percentage of large multimers than normal controls (P less than 0.001). The relative content of large multimers was negatively correlated with WBC and platelet count (P less than 0.02 each) in PV. It was negatively correlated with platelet count (P less than 0.005) and was positively correlated with a ratio of ristocetin cofactor/vWf antigen (RCof/vWf:Ag) (P less than 0.01) in ET. These results indicate that acquired defects of vWf are quite common in PV and ET but not in PMF. In addition, some CMPD patients with high platelet counts completely lacked large multimers. The negative correlation of the relative content of large multimers with platelet count suggests that large multimers may be preferentially consumed during thrombocytosis or degraded by protease(s) from increased blood cells.