Bernard-Soulier syndrome Karlstad : Trp 498-->Stop mutation resulting in a truncated glycoprotein Ib alpha that contains part of the transmembranous domain

In Bernard-Soulier syndrome, a hereditary bleeding disorder, the platelets are deficient in the glycoprotein (GP) Ib-IX-V complex, a major receptor for the von Willebrand factor. The components of the complex are encoded by separate genes. Patients with this syndrome have a variable expression level of the receptor protein on platelets depending on the specific genetic abnormality. We describe a patient with life-long bleeding symptoms, who is homozygous for a unique stop mutation. Trp 498-->Stop in the GPIb alpha gene, resulting in a truncated GPIb alpha polypeptide chain. In contrast to previously reported truncated forms of GPIb alpha, this form contains a portion of the transmembranous domain as well as the juxtamembranous cysteines engaged in a disulphide bond with GPIb beta. Flow cytometry with GPIb alpha antibodies demonstrated the presence of GPIb on the patient's platelets, although in reduced amounts compared to normal controls. GPIX was similarly detectable. Immunoblotting demonstrated that the patient synthesized a truncated GPIb alpha of the expected size of 130 K, which was, however, sensitive to proteolysis. These studies show that GPIb alpha lacking the intracytoplasmic tail can be expressed at the platelet surface provided elements of the transmembranous domain are present. (Less)