A point mutation in the cysteine-rich domain of glycoprotein (GP) IIIa results in the expression of a GPIIb-IIIa (alpha IIb beta 3) integrin receptorlocked in a high-affinity state and a Glanzmann thrombasthenia-like phenotype

This article reports a Glanzmann thrombasthenia (GT) patient, N.M., with a point mutation in the third cysteine-rich repeat of beta3-integrin or plate let glycoprotein (GP) IIIa, leading to the expression of a constitutively a ctivated fibrinogen receptor. The diagnosis of GT was based on a severely r educed platelet-aggregation response to a series of agonists and approximat ely 20% of surface-expressed GPIIb-IIIa. The patient's GPIIb-IIIa constitut ively expressed epitopes recognized by antibodies to ligand-induced binding sites (LIBS) and also spontaneously bound the fibrinogen-mimetic antibody, PAC-1. Furthermore, significant amounts of bound fibrinogen were detected on his platelets ex vivo. No signs of platelet activation were observed on sections of unstimulated platelets from N.M. by electron microscopy. Immuno gold labeling highlighted the presence of surface-bound fibrinogen but reve aled platelet heterogeneity with regard to the surface density. When the pa tient's platelets were stimulated by thrombin-receptor activating peptide, amounts of surface-expressed GPIIb-IIIa increased and the aggregation respo nse improved, although it failed to normalize. Platelets from N.M. were abl e to adhere and spread on immobilized fibrinogen. Sequence analysis of geno mic DNA from N.M. revealed a homozygous g1776T>C mutation in GPIIIa, leadin g to a Cys560Arg amino acid substitution. A stable Chinese hamster ovary (C HO) cell line was prepared expressing surface GPIIb-Arg560IIIa. Like platel ets from the patient, GPIIb-Arg560IIIa-transfected CHO cells constitutively bound LIBS antibodies and PAC-1. They also showed an enhanced ability to a dhere on surface-bound fibrinogen. Overall, these data demonstrate that a g ain-of-function mutation can still be associated with a thrombasthenic phen otype even though platelets show spontaneous fibrinogen binding. (C) 2001 b y The American Society of Hematology.