Crucial role for the VWF A1 domain in binding to type IV collagen

Abstract

• Collagen 4 binds to the VWF A1 domain, and this binding is reduced or abrogated by select VWF A1 domain sequence variations.• Platelet binding to collagen 4 under flow conditions is dependent on the presence of VWF.Von Willebrand factor (VWF) contains binding sites for platelets and for vascular collagens to facilitate clot formation at sites of injury. Although previous work has shown that VWF can bind type IV collagen (collagen 4), little characterization of this interaction has been performed. We examined the binding of VWF to collagen 4 in vitro and extended this characterization to a murine model of defective VWF-collagen 4 interactions. The interactions of VWF and collagen 4 were further studied using plasma samples from a large study of both healthy controls and subjects with different types of von Willebrand disease (VWD). Our results show that collagen 4 appears to bind VWF exclusively via the VWF A1 domain, and that specific sequence variations identified through VWF patient samples and through site-directed mutagenesis in the VWF A1 domain can decrease or abrogate this interaction. In addition, VWF-dependent platelet binding to collagen 4 under flow conditions requires an intact VWF A1 domain. We observed that decreased binding to collagen 4 was associated with select VWF A1 domain sequence variations in type 1 and type 2M VWD. This suggests an additional mechanism through which VWF variants may alter hemostasis.

Clinical and laboratory variability in a cohort of patients diagnosed with type 1 VWD in the United States

“…Differences in collagen binding were observed as previously reported. 19 At the time of initiation of this study, type 2M did not routinely include collagen-binding variants, so these subjects remained in the type 1 cohort for this study.…”

“…Our data showed that subjects with laboratory findings otherwise consistent with type 1 VWD but with low VWF:CB4 had increased bleeding symptoms as measured by BS. 19 Although only 12 subjects were affected, this represented 4% of type 1 subjects. This raises the possibility that these subjects may be better classified as type 2M VWD on the basis of a functional defect in the VWF protein, even though the VWF:RCo/VWF:Ag ratio was normal.…”

“…19 The median BS for the subjects with low VWF:CB4 group was 10.5, as compared with a median BS of 5 for the remainder of the type 1 subjects with VWF:CB4/VWF:Ag .0.5 or median BS of 6 when comparison type 1 subjects were matched for comparable VWF:Ag, age, gender, race, and ethnicity. This difference was not statistically significant, but when subjects ,18 were excluded (due to having less time to generate significant bleeding symptoms), the median BS was 13.5 for those with low VWF:CB4/VWF:Ag ratios compared with 7 for those with normal ratios (P , .01).…”

“…17 Multimer distribution was assayed by quantitative gel electrophoresis. 18 Ligand binding assays were performed as previously described, including VWF binding to type III collagen (VWF:CB3), 16 VWF binding to type IV collagen (VWF:CB4), 19 and VWF binding to mutant platelet GPIb (VWF:GPIbM). 20,21 VWF propeptide (VWFpp) was measured to evaluate for VWF clearance defects.…”

Molecular diagnosis of von Willebrand disease

“…In this case, the majority of mutations are located in the A3 domain, and exons 29-32 should be investigated. A small proportion of A1 domain mutations result from collagen types IV or VI binding defects [34].…”