THE MOLECULAR-BIOLOGY OF VONWILLEBRAND-FACTOR

Human von Willebrand factor (vWF) is a plasma glycoprotein that is synthesized by endothelial cells and megakaryocytes, and perhaps by syncytiotrophoblast of placenta. The biosynthesis of vWF is very complex, involving proteolytic processing, glycosyla-tion, disulfide bond formation, and sulfation. Mature vWF consists of a single subunit of ∼ 250,000 daltons that is assembled into multimer ranging from dimers to species of over 10 million daltons. vWF performs its essential hemostatic function through several binding interactions, forming a bridge between specific receptors on the platelet surface and components of damaged vascular subendothelial connective tissue. Inherited deficiency of vWF, or von Willebrand disease (vWD), is the most common genetically transmitted bleeding disorder worldwide. The last two years has been a time of very rapid progress in understanding the molecular biology of vWF. Four research groups have independently isolated and sequenced the 9 kilobase full-length vWF cDNA. The predicted protein sequence has provided a foundation for understanding the biosynthetic processing of vWF, and has clarified the relationship between vWF and a 75-100 kilodalton plasma protein of unknown function, von Willebrand antigen II (vWAgll)/ vWAgll is co-distributed with vWF in endothelial cells and platelets, and is deficient in patients with vWD. The cDNA sequence of vWF shows that vWAgll is a rather large pro-peptide for vWF, explaining the biochemical and genetic association between the two proteins. vWF has a complex evolutionary history marked by many separate gene segment duplications. The primary structure of the protein contains four distinct types of repeated domains present in two to four copies each. Repeated domains account for over 90 percent of the protein sequence. This sequence provides a framework for ordering the functional domains that have been defined by protein chemistry methods. A tryptic peptide from the amino-terminus of vWF that overlaps domain D3 binds to factor VIII and also appears to bind to heparin. Peptides that include domain A1 bind to collagens, to heparin, and to platelet glycoprotein Ib. A second collagen binding site appears to lie within domain A3. The vWF cDNA has been expressed in heterologous cells to produce small amounts of functionally and structurally normal vWF, indicating that endothelial cells are not unique in their ability to process and assemble vWF multimers. Site-directed mutagenesis has been used to show that deletion of the propeptide of vWF prevents the formation of multimers. Cloned cDNA probes have been employed to isolate vWF genomic DNA from cosmid and λ-phage libraries, and the size of the vWF gene appears to be ∼ 150 kilobases. The vWF locus has been localized to human chromosome 12p12—pter. Several intragenic RFLPs have been characterized. With them, vWF has been placed on the human genetic linkage map as the most telomeric marker currently available for the short arm of chromosome 12. A second apparently homologous locus has been identified on chromosome 22, but the relationship of this locus to the authentic vWF gene is not yet known. The mechanism of vWD has been studied by Southern blotting of genomic DNA with cDNA probes in a few patients. Three unrelated pedigrees have been shown to have total deletions of the vWF gene as the cause of severe vWD (type III). This form of gene deletion appears to predispose to the development of inhibitory alloantibodies to vWF during therapy with cryoprecipitate. During the next several years recombinant DNA methods will continue to contribute our understanding of the evolution, biosynthesis, and structure-function relationships of vWF, as well as the mechanism of additional variants of vWD at the level of gene structure.