Methods and kits for measuring von Willebrand factor

1. A method for determining von Willebrand factor (VWF) activity in a biological sample from a patient, wherein the sample is mixed with purified glycoprotein Ib heavy chain (GPIbα) protein to give an assay mix, comprising the steps of:

a) providing an isolated GPIbα protein or functional fragment thereof associated with a particulate solid phase, wherein the isolated GPIbα protein comprises a combination of mutations selected from G233V, D235Y and M239V relative to SEQ ID NO:2, wherein neither ristocetin nor botrocetin is added to the assay mix;

and

b) determining the VWF activity in the assay mix.

2. The method of claim 1 , wherein no ristocetin or botrocetin equivalent substance is added to the assay mix.

3. The method of claim 1 , wherein the amino acids at position 233 and 239 are both valine.

4. The method of claim 1 , wherein the amino acid at position 239 is valine.

5. The method of claim 1 , wherein the GPIbα protein is a recombinant or synthetic protein.

6. The method of claim 1 , wherein the GPIbα protein is fused at the C terminus to at least one affinity tag.

7. The method of claim 6 , wherein the at least one affinity tag is selected from the group consisting of a His tag, a Flag tag, and a c-Myc tag.

8. The method of claim 7 , wherein the GPIbα protein is associated with the particulate solid phase via an antibody.

9. The method of claim 8 , wherein the GPIbα protein is associated with the particulate solid phase via an anti-GPIbα antibody.

10. The method of claim 8 , wherein the GPIbα protein is associated with the particulate solid phase via an anti-affinity tag antibody.

11. The method of claim 1 , wherein the particulate solid phase comprises latex.

12. The method of claim 1 , wherein the VWF activity is determined on the basis of GPIbα-mediated agglutination of the particulate solid phase.

13. The method of claim 12 , wherein the GPIbα protein is fused at the C terminus to at least one Flag tag, His tag, or c-Myc tag, and is associated with the particulate solid phase via an anti-Flag antibody, an anti-His antibody, or an anti-c-Myc antibody.

14. The method of claim 1 , wherein the sample is from an individual having or suspected of having von Willebrand disease.

15. A method of measuring von Willebrand factor (VWF) without using a platelet agglutination agonist, the method comprising the steps of:

providing a solid-phase surface comprising immobilized purified platelet glycoprotein Ibα (GPIbα) or a functional fragment thereof, wherein the immobilized GPIbα or functional fragment thereof comprises a combination of mutations selected from G233V, D235Y and M239V relative to SEQ ID NO:2;

contacting a biological sample from a patient with the surface, wherein the contacting is done without a platelet aggregation agonist; and

detecting a complex of VWF and GPIbα.

16. In a method of assessing Von Willebrand factor (VWF) comprising performing an ELISA assay on a biological sample from a patient to detect binding of GPIbα to VWF, the improvement wherein the assay uses a purified GPIbα having a combination of GPIbα mutations selected from G233V, D235Y, and M239V and the assay is performed in the absence of added ristocetin or botrocetin.

17. The method of claim 16 , wherein the surface is a solid-phase surface selected from the group consisting of agarose, glass, latex and plastic.

18. The method of claim 17 , wherein the solid-phase surface comprises an anti-GPIbα antibody that binds the GPIbα or functional fragment thereof.

19. The method of claim 16 , wherein the sample is plasma.

20. The method of claim 16 , wherein the combination of mutations is selected from the group consisting of D235Y/G233V, D235Y/M239V and G233V/M239V.

21. The method of claim 16 , wherein the combination of mutations is D235Y/G233V/M239V.

22. The method of claim 16 , wherein a labeled anti-VWF antibody is used to detect the complex of VWF and GPIbα.