Molecular bases of type III and type IIAvon Willebrand disease.

Abstract

von Willebrand factor (vWF) is a multimeric plasma glycoprotein that serves critical functions in hemostasis. von Willebrand disease (vWD) results from quantitative (type I/III) or qualitative (type II) defects in vWF. We have characterized the molecular defect responsible for type III vWD, the most severe quantitative form, using an adaptation of the polymerase chain reaction (PCR). Using DNA sequence polymorphisms located within exons of the vWF gene, expression from the two vWF alleles can be distinguished by RNA PCR analysis from peripheral blood platelets. By this approach, we have demonstrated loss of expression from a vWF allele(s) in a pedigree with type III vWD. These data indicate that the quantitative vWF defect in this type III family is due to a mutation(s) within the vWF gene that disrupts vWF mRNA transcription or processing. In other studies, we have examined the molecular basis of type IIA vWD. The type IIA variant is the most common qualitative form, is inherited in autosomal dominant fashion, and is characterized by a reduction of the high molecular weight (HMW) plasma vWF multimeric species. We have identified 7 single base missense mutations, accounting for 9/11 families studied. Expression by transient transfection revealed two classes of mutations, Group I and II, defined by different molecular mechanisms. Four mutations, termed Group I, impaired transport of vWF between the endoplasmic reticulum and the Golgi complex, with more profound effects upon the HMW species. Three mutations, termed Group II, did not appear to affect synthesis and secretion, suggesting that in this group, loss of HMW multimers occurs after secretion in vivo, most likely through proteolysis in plasma. Further studies of Group I mutations demonstrated defective storage as well as secretion in a regulated cell-line, Rin5F. In addition, Group I mutations acted as trans-dominant mutations, consistent with the inheritance pattern of type IIA vWD. Further insights into the pathogenesis of vWD could help in the rational design of future therapies.