Platelet glycoprotein Ib alpha polymorphisms affect the interaction with von Willebrand factor under flow conditions

Interaction of platelet glycoprotein (GP) Ibα with von Willebrand factor (VWF) is essential for thrombus formation, particularly under high shear conditions. Previous case-control studies indicated that two GPIbα polymorphisms, ¹⁴⁵Thr/Met and/or variable number (1-4) tandem repeats of 13 amino-acid sequences, are associated with arterial thrombosis. The ¹⁴⁵Met-allele and the 3R- or 4R-allele is associated with increased risk. However, there is little clear experimental data to support this association. To elucidate the functional effects of these polymorphisms, we prepared recombinant GPIbα fragments and tested them in vitro. The dissociation constants of ristocetin-induced ¹²⁵I-labelled VWF binding to two forms of soluble recombinant GPIbα [¹His- ³⁰²Ala, either ¹⁴⁵Thr (145T) or ¹⁴⁵Met (145M)] were not different. Four types of Chinese hamster ovary cells expressing full-length GPIbαβ/IX, 145T with one repeat (T1R), 145M with one repeat (M1R), 145T with four repeats (T4R), and 145M with four repeats (M4R), were prepared, and cell interactions with immobilized-VWF were examined under various shear conditions. The cell rolling velocity of M4R under a shear condition of 114/s was significantly slower than that of T1R. Intermediate values were obtained with M1R and T4R. The results suggest that M4R interacts more strongly with VWF under flow conditions.