Characterization of a novel mutation in the von Willebrand factor propeptide in a distinct subtype of recessive von Willebrand disease

Summary vonWillebrand factor (VWF) is a plasma protein that consists of a series of multimers of which the high-molecular-weight VWF multimers are the most potent in platelet adhesion and aggre-gation. The propeptide of theVWF (VWFpp) is known to be essential in the process of multimer assembly. Genetic studies were performed in a patient with a phenotype of vonWillebrand disease (VWD) characterized by very low plasma factorVIII and VWF levels and a VWF consisting of only a dimeric band and total absence of all multimers in plasma. The patient was found to be homozygous for the novel C570S mutation, caused by a 1709G>C transition in exon 14 of theVWF gene coding for the propeptide. Three asymptomatic relatives were found to be heterozygous. In-vitro mutagenesis and expression in COS-7 cells confirmed the detrimental effect of the mutation on VWF multimerization. Our findings show that the C570S mutation in the VWFpp abolishes multimerization of VWF. The mutation probably disrupts the normal configuration of the VWFpp, which is essential for correct orientation of the protomers and ultimately multimerization. The mutant amino acid is located in a region that is highly conserved across several species which underlines its critical role. This variant constitutes a distinct subtype of recessive 2AVWD with the exclusive presence of the dimeric form of VWF in plasma.