Development of Platforms to Phenotype Variants of Uncertain Significance in VWF

Platelet sequestration at sites of vascular injuries and factor VIII (FVIII) stabilization critically relies on von Willebrand factor (VWF). Specific domains within VWF coordinate these functions and may harbor mutations that result in von Willebrand disease (VWD). To survey all possible mutations that may lead to type 2 VWD, we developed prokaryotic and eukaryotic systems to display VWF fragments that are screened for a specific function and identified by DNA sequencing. We constructed an M13 filamentous phage display library consisting of ~2.8x10⁶ independent clones that express random VWF fragments. Following a single round of selection for platelet binding, bound phage were eluted and analyzed by next generation DNA sequencing, revealing an optimal fragment spanning the A1 domain and a second region of weaker enrichment over the D4 domain. All A1 fragments encompassed C1272-C1458, indicating that intramolecular disulfide bridging of these cysteines optimizes platelet binding. Competitive binding assays between phage displaying a fragment of the D4 domain vs. a control phage resulted in enrichment of the former, suggesting a potential, novel role for the VWF D4 domain. Analogous to our previously reported method for analyzing VWF proteolysis (Kretz CA et al. Proc Natl Acad Sci USA. 2015), we mutagenized and displayed the VWF A1 domain to determine the spectrum of mutations that may alter the interaction between VWF and its platelet receptor, GPIbα. The mutant A1 library comprised ~5x10⁶ independent clones, sufficient to represent all 3,933 single substitutions. Screening this library against platelets in the presence of excess control phage resulted in an enrichment of phage displaying an A1 fragment. DNA sequencing identified mutations located outside the VWF A1/platelet GPIbα interface, indicating allosteric control of this interaction. Screening phage displayed VWF fragments for FVIII binding, however, failed to identify critical residues; thus, we adapted our approach for eukaryotes. When displayed on mammalian cells, the VWF D'D3 domains retained their capacity to bind FVIII. Introduction of the VWD subtype 2N mutation, R816W, into the displayed VWF D'D3 domains abolished FVIII binding, demonstrating feasibility for mutational analyses. Together, these results demonstrate the utility of phage and mammalian display for structure-function analyses and that these platforms provide an efficient method for phenotyping variants of uncertain significance.