This article reports a Glanzmann thrombasthenia (GT) patient, N.M., with a point mutation in the third cysteine-rich repeat of ␤3-integrin or platelet glycoprotein (GP) IIIa, leading to the expression of a constitutively activated fibrinogen receptor. The diagnosis of GT was based on a severely reduced platelet-aggregation response to a series of agonists and approximately 20% of surface-expressed GPIIb-IIIa. The patient's GPIIb-IIIa constitutively expressed epitopes recognized by antibodies to ligandinduced 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. Immunogold labeling highlighted the presence of surface-bound fibrinogen but revealed platelet heterogeneity with regard to the surface density. When the patient's platelets were stimulated by thrombin-receptor activating peptide, amounts of surface-expressed GPIIb-IIIa increased and the aggregation response improved, although it failed to normalize. Platelets from N.M. were able to adhere and spread on immobilized fibrinogen. Sequence analysis of genomic DNA from N.M. revealed a homozygous g1776T>C mutation in GPIIIa, leading to a Cys560Arg amino acid substitution. A stable Chinese hamster ovary (CHO) cell line was prepared expressing surface GPIIbArg560IIIa. Like platelets from the patient, GPIIb-Arg560IIIa-transfected CHO cells constitutively bound LIBS antibodies and PAC-1. They also showed an enhanced ability to adhere on surface-bound fibrinogen. Overall, these data demonstrate that a gain-of-function mutation can still be associated with a thrombasthenic phenotype even though platelets show spontaneous fibrinogen binding. IntroductionIntegrins are heterodimeric transmembrane proteins that mediate cell adhesion and cell migration. [1][2][3] They are involved in numerous fundamental biologic processes, including development, homing, inflammation, angiogenesis, and wound healing, all processes in which integrin function is subjected to a highly sophisticated regulation. [4][5][6][7][8] The platelet fibrinogen receptor, the ␣ IIb ␤ 3 integrin (glycoprotein [GP] IIb-IIIa), has constituted an ideal model for the study of an integrin's role in cell contact interactions. [9][10][11][12][13] This receptor has the capacity to shift through conformational changes from a low-affinity state unable to bind macromolecular ligands to a ligand-competent state on activated platelets. This process of affinity modulation is an essential part of integrin function. Furthermore, ligand binding to GPIIb-IIIa itself modifies the conformation of this integrin, exposing neoepitopes known as ligand-induced binding sites (LIBS) and leading to postreceptor occupancy events such as clustering of GPIIb-IIIa complexes, generation of secondary signals, and cytoskeletal rearrangement, all of which contribute to maximal platelet aggregatio...