von Willebrand factor (VWF) is a central protein in hemostasis. Abnormalities in VWF result in von Willebrand disease (VWD), the most common inherited bleeding disorder in humans. During the program's last funding period, we characterized a number of human VWD mutations and identified a major modifier gene of plasma VWF levels in a mouse model system. The aims of the current proposal are to extend our previous observations on the molecular basis of VWD, with a new emphasis on the identification of novel genetic modifiers for the variable expressivity and incomplete penetrance of VWD. Additional studies will be performed to further characterize the molecular basis for the altered GALGT2 expression associated with the A/vvvfallele that leads to low plasma VWF in the RIIIS/J mouse. We will test the hypothesis that accelerated clearance of GALGT2-modified VWF is mediated via the asialoglycoprotein receptor (ASGPR). We will also analyze mutant and transgenic mouse strains to identify other endothelial cell-dependent proteins subject to genetic modification by Mvwf. Molecular studies will characterize the specific as-acting element within the Galgt2 gene responsible for the endothelial cell-specific expression program associated with the Mvwf allele. Further genetic studies will be performed in inbred mouse strains with extreme differences in plasma VWF level in an attempt to identify additional modifier genes. A genome scan will also be performed in a sib cohort in an attempt to map modifiers of plasma VWF levels in the general human population. Human homologues of candidate VWF modifiers identified in the mouse will also be tested by linkage analysis. In summary, this proposal will apply the rapidly advancing methodologies of mouse and human genetics to identify the genetic modifier genes which are responsible for the wide variation in plasma VWF levels in the general population, and likely also determine the severity of bleeding in type 1VWD patients. Through the interaction of VWF with factor VIII, these modifier genes may also function as important genetic factors contributing to thrombosis predisposition. The results of these studies should offer new insight into the biology of endothelial cell function and VWF biosynthesis and may also lead to improved diagnosis and therapy for VWD and thrombophilia.