Studies of multimerin in patients with von Willebrand disease and platelet von Willebrand factor deficiency

In normal platelet alpha-granules von Willebrand factor (VWF) is stored with multimerin and factor V in an eccentric electron-lucent zone. Because the platelet stores of VWF are deficient in 'platelet low' type 1 and type 3 von Willebrand disease (VWD), we investigated their electron-lucent zone proteins. The patients with VWD had partial to complete deficiencies of plasma and platelet VWF but normal alpha-granular multimerin and factor V, and normal alpha-granular fibrinogen, thrombospondin-1, fibronectin, osteonectin and P-selectin. In type 3 VWD platelets, alpha-granular electron-lucent zones lacking VWF-associated tubules were identified and multimerin was found in its normal alpha-granular location, These findings indicate that the formation of the electron-lucent zone and the sorting of multimerin to this region occur independent of VWF. The isolated abnormalities in VWF suggests a VWF gene mutation is the cause of 'platelet low' type 1 VWD.