Determinants of bleeding severity in von Willebrand disease.

von Willebrand disease (VWD) is one of the most common bleeding disorders. It is caused by abnormalities in the von Willebrand factor (VWF) protein, and is characterized by incomplete penetrance and variable expressivity. VWF levels vary widely in the population. The best-characterized human genetic modifier of VWF is the ABO blood group. Patients with VWD show considerable variation in bleeding tendency even within the same family, independently of VWF levels. It is possible that several modifier genes influence the phenotype. Variants of genes that encode for platelet receptors as well as those that encode for clotting factor levels have been proposed as modifiers. It is hoped that new clinical-genetic studies will shed light on these issues and help practitioners to determine the population at risk for bleeding.