Naturally Occurring Mutations in Glycoprotein Ibα That Result in Defective Ligand Binding and Synthesis of a Truncated Protein

Abstract

The platelet GPIb-V-IX complex is the receptor for the initial binding of von Willebrand factor (vWF) mediating platelet adhesion. The complex is composed of four membrane-spanning glycoproteins (GP): GPIbα, GPIbβ, GPIX, and GPV. Bernard-Soulier syndrome results from a qualitative or quantitative defect in one or more components of the platelet membrane GPIb-V-IX complex. We describe the molecular basis of a novel Bernard-Soulier syndrome variant in two siblings in whom GPIbα was not detected on the platelet surface but that was present in a soluble form in plasma. DNA sequence analysis showed that the affected individuals were compound heterozygotes for two mutations. One, inherited from a maternal allele, a T777 → C point mutation in GPIbα converting Cys65 → Arg within the second leucine rich repeat, the other, a single nucleotide substitution (G2078 → A) for the tryptophan codon (TGG) causing a nonsense codon (TGA) at residue 498 within the transmembrane region of GPIbα, inherited from a mutant paternal allele. The Bernard-Soulier phenotype was observed in siblings who were compound heterozygotes for these two mutations. Although GPIbα was not detected on the surface of the patient's platelets, soluble GPIbα could be immunoprecipitated from plasma. When plasmids encoding GPIbα containing the Cys65 → Arg mutation were transiently transfected into Chinese hamster ovary (CHO) cells stably expressing the GPβ-IX complex (CHOβIX), the expression of GPIbα was similar to the wild-type (WT) GPIbα, but did not bind vWF. When plasmids encoding GPIbα containing the Trp498 → stop were transiently transfected into CHOβIX, the surface expression of GPIbα was barely detectable compared with the WT GPIbα. Thus, this newly described compound heterozygous defect produces Bernard-Soulier syndrome by a combination of synthesis of a nonfunctional protein and of a truncated protein that fails to insert into the platelet membrane and is found circulating in plasma.