TYPE 2M VON-WILLEBRAND-DISEASE - F606I AND I662F MUTATIONS IN THE GLYCOPROTEIN IB BINDING DOMAIN SELECTIVELY IMPAIR RISTOCETIN - BUT NOT BOTROCETIN-MEDIATED BINDING OF VON-WILLEBRAND-FACTOR TO PLATELETS

von Willebrand disease (vWD) is a common, autosomally inherited, bleed ing disorder caused by quantitative and/or qualitative deficiency of v on Willebrand factor (VWF). We describe two families with a variant fo rm of vWD where affected members of both families have borderline or l ow VWF antigen levels, normal vWF multimer patterns, disproportionatel y low ristocetin cofactor activity, and significant bleeding symptoms. Whereas ristocetin-induced binding of plasma vWF from affected member s of both families to fixed platelets was reduced, botrocetin-induced platelet binding was normal. The sequencing of genomic DNA identified unique missense mutations in each family in the vWF exon 28. In Family A, a missense mutation at nucleotide 4105T --> A resulted in a Phe606 Ile amino acid substitution (F606I) and in Family B, a missense mutati on at nucleotide 4273A --> T resulted in an Ile662Phe amino acid subst itution (I662F). Both mutations are within the large disulfide loop be tween Cys509 and Cys695 in the Al domain that mediates vWF interaction with platelet glycoprotein Ib. Expression of recombinant vWF containi ng either F606I or I662F mutations resulted in mutant recombinant vWF with decreased ristocetin-induced platelet binding, but normal multime r structure, botrocetin-induced platelet binding, collagen binding, an d binding to the conformation-sensitive monoclonal antibody, AvW-3. Bo th mutations are phenotypically distinct from the previously reported variant type 2M(Milwaukee-1) because of the presence of normal botroce tin-induced platelet binding, collagen binding, and AvW-3 binding, as well as the greater frequency and intensity of clinical bleeding. When the reported type 2M mutations are mapped on the predicted three-dime nsional structure of the Al loop of vWF, the mutations cluster in one region that is distinct from the region in which the type 2B mutations cluster. (C) 1998 by The American Society of Hematology.