A Val193Met mutation in GPIIIa results in a GPIIb/IIIa receptor with a constitutively high affinity for a small ligand

We have identified a patient designated as (GTa) with Glanzmann's Thrombast henia (GT) diagnosed on the basis of a prolonged bleeding time and failure of the patient's platelets to aggregate. The number of glycoprotein (GP,)II b/IIIa receptors on the platelet surface was 37% of normal and those recept ors displayed a defect in soluble fibrinogen binding. Nevertheless, GTa pla telets showed increased adhesion to solid-phase fibrinogen and binding affi nity for the RGD-mimetic H-3-SC52012, a non-peptide GPIIb/IIIa antagonist. Dithiothreitol (DIT) and ADP enhanced the affinity for [H-3]-SC52012 in nor mal platelets, but had little effect in GTa platelets. These findings sugge sted that GTa platelets were locked in an altered affinity state. Genetic a nalysis showed that GTa was a compound heterozygote for the GPIIIa gene, On e allele showed a deletion at the 3' end of exon 3 resulting in a premature stop codon. The second GPIIIa allele had a G to A transition at nucleotide 577, resulting in a Val193Met substitution. HEK 293T cells transfected wit h mutant GPIIb/IIIaV193M bound [H-3]-SC52012 with a higher affinity than wi ld-type GPIIb/IIIa, and this was not increased by DTT. The mutant receptor distinguishes between platelet adhesion and aggregation, and demonstrates t he phenotype that may be expected when platelet aggregation alone is inhibi ted.