A dinucleotide deletion results in defective membrane anchoring and circulating soluble glycoprotein Ib alpha in a novel form of Bernard-Soulier syndrome.

The platelet membrane glycoprotein (GP)Ib-V-IX complex is the receptor for von Willebrand factor and is composed of four membrane-spanning polypeptides: GPIb alpha, GPIb beta, GPIX, and GPV. A qualitative or quantitative deficiency in the GPIb-V-IX complex on the platelet membrane is the cause of the congenital platelet disorder Bernard-Soulier syndrome (BSS). We describe the molecular basis of a novel variant BSS in a patient in which GPIb alpha was absent from the platelet surface but present in a soluble form in the plasma. DNA sequence analysis showed a homozygous dinucleotide deletion in the codon for Tyr 508 (TAT) in GPIb alpha. This mutation (GPIb alpha deltaAT) causes a frame shift that alters the amino acid sequence of GPIb alpha within its transmembrane region. The hydrophobic nature of the predicted transmembrane region and the cytoplasmic tail at the COOH terminal are altered before reaching a new premature stop codon 38 amino acids short of the wild-type peptide. Although GPIb alpha deltaAT was not detectable on the platelet surface, immunoprecipitation of plasma with specific monoclonal antibodies (MoAbs) identified circulating GPIb alpha. Transient expression of recombinant GPIb alpha deltaAT in 293T cells also generated a soluble form of the protein. Moreover, when a plasmid encoding GPIb alpha deltaAT was transiently transfected into Chinese hamster ovary (CHO) cells stably expressing the GP beta-IX complex, it failed to be expressed on the cell surface. Thus, a dinucleotide deletion in the codon for Tyr 508 causes a frameshift that alters the amino acid sequence of GPIb alpha starting within its transmembrane region, changes the hydrophobicity of the normal transmembrane region, and truncates the cytoplasmic domain affecting binding to the cytoskeleton and cytoplasmic proteins. This mutation affects anchoring of the GPIb alpha polypeptide in platelets and causes the observed BSS phenotype with circulating soluble GPIb alpha.