Functional Assay of VWF Measured by Its Interaction with Platelet GPIb without the Use of Ristocetin.

Abstract

The clinical laboratory assessment of von Willebrand factor (VWF) includes measurement of the amount of VWF protein (VWF:Ag) and the functional interaction of VWF with platelets. The universally accepted measure of VWF function utilizes the ability of VWF to bind to platelets (or platelet GPIb) in the presence of ristocetin resulting in VWF-dependent agglutination of platelets measured quantitatively as ristocetin cofactor activity (VWF:RCo) by platelet aggregometry or turbidometry. An international reference standard has been assigned a biologic activity in international units by the WHO and the SSC of the ISTH. VWF:RCo has a recognized wide variability and reproducibility using different assays or when performed in different clinical laboratories. Alternative assays with improved reproducibility have recently been suggested that use recombinant GPIb in an ELISA assay of VWF binding, but this binding still requires ristocetin. Patients have been reported that have reduced interaction with ristocetin such that the VWF:RCo is markedly reduced (<0.12 IU/dL) yet patients have no bleeding symptoms even with major surgical challenge. Furthermore, recent studies done on normals entered into the ZPMCB-VWD have demonstrated a systematic problem with the measurement of VWF:RCo in African Americans in whom there VWF:RCo is significantly reduced compared with their VWF:Ag. Since this difference does not appear to have a clinical phenotype, a functional assay of VWF in the absence of ristocetin might be desirable. Such an assay should be sensitive to the measurement of the more-functional large VWF multimers, correlate with VWF:Ag in patients with reduced VWF function, and remain unaffected by mutations that affect VWF binding of ristocetin but do not have a bleeding phenotype. Since, platelet-type VWD variants of GPIb bind normal plasma VWF in the absence of added ristocetin and preferentially bind large VWF multimers, we explored an assay using combinations of the three GPIb mutations, G233V, D235Y, M239V, either individually or together to measure VWF function in the absence of ristocetin. Initially cell-based assays using mutant GPIB expressed in HEK293T cells were used in flow cytometry assays in which test VWF binding to mutant GPIb was measured using a labeled anti-VWF monoclonal antibody. Results were expressed in VWF:IbCo units. A normal curve was developed using serial dilutions of reference plasma previously standardized against both the ISTH and WHO VWF standards based on the VWF:Ag international standard. Patient samples included 41 normal, 16 type 2M, 5 type 2B, and 5 type 2A VWD patient plasmas. Samples included those with apparent type 2M VWD but without clinical symptoms and African American samples with a reduced VWF:RCo/VWF:Ag (RCo/Ag) ratio. Of the 16 type 2M VWD samples, 7 had markedly reduced VWF:IbCo (consistent with the VWF:RCo assay) and 9 had normal VWF:IbCo. African Americans with the SNPs associated with reduced RCo/Ag ratios had VWF:IbCo assays that correlated with their VWF:Ag in contrast to the abnormal RCo/Ag ratios identified by standard assays. Type 2A patients exhibited reduced VWF:IbCo assays and multimer size seemed to correlate with VWF:IbCo activity. Thus, measurement of VWF function using the VWF:IbCo assay may more directly correlate with VWF function and avoid some of the pitfalls and functional variability of VWF:RCo assays.