Glanzmann Thrombasthenia - Novel Mutations In the Gpiib- and GPIIIa-Genes.

Abstract 3692

The GPIIb/IIIa (αIIbβ3, CD41/CD61) is a calcium-dependent, non-covalently associated heterodimer on the platelet surface. It belongs to the integrin superfamily and, along with the vitronectin receptor αVβ3, forms the cytoadhesion class. The GPIIb/IIIa is expressed exclusively in platelets and is the most abundant integrin on the surface of platelets. In case of vessel wall injury platelets become activated and also GPIIb/IIIa changes its conformation and becomes a high affinity receptor for its ligands fibrinogen, fibronectin and von Willebrand factor. During haemostasis GPIIb/IIIa mediates the aggregation of platelets via fibrinogen and adhesion to subendothelial matrix proteins. Patients with Glanzmann thrombasthenia (GT) with a diminished, total lack or dysfunction of GPIIb/IIIa often suffer from severe bleeding complications. In our study we have performed a molecular and functional characterization of 25 GT patients, belonging to 12 unrelated families, in order to collect information about the biosynthesis and structure-function relationships of the αIIbβ3 complex. The investigated patients have different genetic backgrounds (Caucasian: n=13; Asian: n=9; African: n=3). The expression of the platelet integrin αIIbβ3 was performed by flow cytometry using monoclonal antibodies. Mutations were identified by sequencing of cDNA and genomic DNA. In addition, platelet functions studies (including PAC-1-binding, aggregations) were implemented. In the present study 10 novel and 2 previously described gene alterations, that are known to cause GT, were identified in 12 unrelated patients and their first-degree relatives. We have found 9 different mutations of the αIIb gene in 20 patients and 4 mutations of the β3 gene in 5 patients. 5 missense mutations, 4 splicing site mutations, 2 insertions of a single nucleotide, 1 deletion of a single nucleotide, and a deletion of 2 nucleotides were found. Overall, 9 patients resulted to be homozygous for a mutation in the αIIb gene (n = 7) or in the β3 gene (n = 2), whereas two patients were found to be compound heterozygous. In 14 patients only a mutation in heterozygous state was found and no additional gene defect was identified. We came to know that the patients with splicing site defects suffer from mild to moderate bleeding, whereas point mutations and insertions/deletions tendencially result in moderate to severe bleeding. Furthermore, our data indicate that mutations occurring in the β3 subunit seem to be associated with complete failure of receptor expression. In general, all patients with less than 25% of present αIIbβ3 have a medical history of bleeding. In conclusion, our investigation of several patients, and in some cases including their first degree relatives, revealed a number of different mutations within the αIIb and β3 subunits associated with causing GT. This vast number of different mutations indicates the genetic heterogeneity in the studied group, respectively in GT.