We have studied a patient with a congenital
bleeding disorder and phenotypic manifestations typical of
Bernard-Soulier syndrome, including giant platelets with absent
ristocetin-induced
von Willebrand factor binding. Two
monoclonal antibodies reacting with distinct
epitopes in the amino-terminal domain of the alpha-chain of
glycoprotein (GP) Ib were used to estimate the number of GP Ib molecules on the platelet membrane. In the patient, binding of one antibody (LJ-Ib10) was approximately 50% of normal, while binding of the other (LJ-Ib1) was absent. Binding of both
antibodies was reduced to approximately 50% of normal in the mother and one sister of the propositus, and their platelets exhibited approximately 70% of normal
von Willebrand factor binding. Immunoblotting studies confirmed the presence of GP Ib alpha, as well as GP IX, in patient platelets. Antibody LJ-Ib10, but not LJ-Ib1, could immunoprecipitate the patient's GP Ib alpha from surface-labeled
proteins. Thus, platelets from the propositus contained a structurally and functionally altered GP Ib-IX complex lacking a specific antibody
epitope and the ability to bind
von Willebrand factor. In contrast, the binding of human
alpha-thrombin to the patient's platelets was normal, and three classes of binding sites with high, intermediate, and low affinity could be detected. These studies define a distinct variant form of
Bernard-Soulier syndrome and provide evidence, based on a naturally occurring mutant molecule, that the amino-terminal region of GP Ib alpha contains a
von Willebrand factor-binding domain distinct from the high affinity
thrombin-binding site. Use of different
monoclonal antibodies with distinct
epitope specificities appears to be essential for a correct identification of variant
Bernard-Soulier syndrome.