Fibronectin binding to von Willebrand factor occurs via the A1 domain.

BACKGROUND: Collagen interactions with von Willebrand factor (VWF) perform an important role in initiation of hemostasis. OBJECTIVES: We hypothesized that in addition to collagen, other extracellular matrix (ECM) proteins such as fibronectin can bind VWF. METHODS: Fibronectin-VWF interactions were measured by ELISA using both plasma-derived and recombinant VWF-containing variants in specific domains. Inhibition was measured by antibody competition using antibodies directed against both VWF and fibronectin. Binding affinities were measured by the Octet Biosensor for fibronectin and collagen IV. RESULTS: Fibronectin was able to bind both plasma-derived and recombinant wild-type VWF. This interaction was inhibited by both anti-VWF antibodies and collagen types III and IV. Several VWF A1 domain variants in the region of the collagen IV binding site also demonstrated absent fibronectin binding, as did variants with defects in high-molecular-weight multimers. Binding affinity testing showed fibronectin has a strong affinity for VWF, in a range similar to that of collagen IV. Fibronectin binds VWF via a restricted region of the A1 domain. This interaction requires high-molecular-weight multimers and is similar to that seen with vascular collagens. CONCLUSIONS: Therefore, VWF would appear to be the common factor linking platelet adhesion to various ECM proteins and facilitating hemostasis under conditions of ECM exposure.

Common VWF sequence variants associated with higher VWF and FVIII are less frequent in subjects diagnosed with type 1 VWD.

BACKGROUND: Genetic variation in the VWF gene is associated with von Willebrand factor (VWF) and factor VIII (FVIII) levels in healthy individuals. OBJECTIVES: We hypothesized that VWF sequence variants associated with higher VWF or FVIII could impact the diagnosis of type 1 von Willebrand disease (VWD). METHODS: We examined VWF antigen (VWF:Ag), VWF ristocetin cofactor activity (VWF:RCo), VWF propeptide (VWFpp), and FVIII levels along with VWF gene sequencing in 256 healthy control and 97 type 1 VWD subjects as part of a cross-sectional study. RESULTS: We found several VWF sequence variants (VWF c.2880G>A and VWF c.2365A>G(;)c.2385T>C, found in linkage disequilibrium) associated with higher VWF and FVIII levels in healthy controls (P < .001 for both variants). In addition, these variants were significantly more common in controls than in subjects diagnosed with type 1 VWD and VWF:Ag <30 (P < .005). The decreased variant frequencies in type 1 VWD was not seen in other VWD types. VWF:Ag, VWF:RCo, and FVIII were not statistically different in type 1 VWD subjects who had these VWF variants compared to type 1 VWD patients without them. There was no difference in ABO blood group, VWF propeptide levels (excluding subjects with known VWF clearance defects), or bleeding score using the ISTH bleeding assessment tool. CONCLUSIONS: These data suggest that certain VWF sequence variants associated with elevated FVIII and VWF levels may protect against reduced VWF levels. These findings were independent of other pathogenic sequence variants in VWF, suggesting a possible independent effect of c.2880G>A and c.2365A>G(;)c.2385T>C on VWF levels.