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)lb-V-IX complex is the receptor for von Willebrand factor and is composed of four membrane-spanning p olypeptides: GPIb alpha, GPIb beta, GPIX, and GPV. A qualitative or qu antitative deficiency in the GPIb-V-IX complex on the platelet membran e is the cause of the congenital platelet disorder Bernard-Soulier syn drome (BSS). We describe the molecular basis of a novel variant BSS in a patient in which GPIb alpha was absent from the platelet surface bu t present in a soluble form in the plasma, DNA sequence analysis showe d a homozygous dinucleotide deletion in the codon for Tyr 508 (T<(AT)u nder bar>) in GPIb alpha. This mutation (GPIb alpha Delta<(AT)under ba r>) causes a frame shift that alters the amino acid sequence of GPIb a lpha within its transmembrane region. The hydrophobic nature of the pr edicted transmembrane region and the cytoplasmic tail at the COOH term inal are altered before reaching a new premature stop codon 38 amino a cids short of the wild-type peptide. Although GPIb alpha Delta<(AT)und er bar> was not detectable on the platelet surface, immunoprecipitatio n of plasma with specific monoclonal antibodies (MoAbs) identified cir culating GPIb alpha. Transient expression of recombinant GPIb alpha De lta<(AT)under bar> in 293T cells also generated a soluble form of the protein. Moreover, when a plasmid encoding GPIb alpha Delta<(AT)under bar> was transiently transfected into Chinese hamster ovary (CHO) cell s stably expressing the GP beta-IX complex, it failed to be expressed on the cell surface. Thus, a dinucleotide deletion in the codon for Ty r 508 causes a frameshift that alters the amino acid sequence of GPIb alpha starting within its transmembrane region, changes the hydrophobi city 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 pla telets and causes the observed BSS phenotype with circulating soluble GPIb alpha. (C) 1997 by The American Society of Hematology.