P47 Inherited Familial Thrombocytopenia Due to an Autosomal Dominant Mutation in the Cytoplasmic Domain of Platelet Β3‐integrin That is Associated with the Expression of Activation‐Dependent Epitopes

Background Glanzmann's Thrombasthenia (GT) and Bernard Soulier Syndrome (BSS) are the two most common inherited forms of thrombasthenia. They are caused by alterations in expression or function of integrin alphaIIb/β3 or glycoprotein (GP) Ib/IX/V respectively. We identified an individual with chronic mild macrothrombocytopenia (range 59–100 × 109 L−1) but no overt bleeding diathesis. The propositus was found to have atypical binding of monoclonal antibodies (mAb) to β3 integrin and increased expression of GPIbVIX.Methods and Results The exons and flanking intronic sequences of ITGB3, ITGA2B GPIBA, GPIBB, GP9 and GP5 genes were sequenced. Heterozygous, non‐synonymous single nucleotide polymorphisms (SNP) were identified in the ITGB3 and the GPIBA genes. These result in the substitution of Histidine for Aspartate at residue 723 (D723H) in ITGB3 and Leucine for Proline at residue 69 (P69L) in GPIBA. Asp723 in ITGB3 is known to form a critical salt‐bridge with ITGA2B and charge reversal at residue 723 causes constitutive activation of the integrin in vitro. In agreement with this, mutant alphaIIb/β3‐His723 expressing CHO cell lines showed an increased expression of activation dependent epitopes relative to wild‐type as measured by mAb PAC‐1 binding. Increased PAC‐1 binding was also observed on resting platelets of affected individuals compared with non‐affected family members. Further investigation of three generations of the propositus’ family identified several individuals with both SNPs each of whom had an identical platelet phenotype. However, in the one family member identified who carried the GPIBA mutation alone, normal platelet size and count were observed suggesting that the ITGB3 D723H substitution is the causative mutation. Genotyping a panel of 1127 unrelated controls for the two SNPs identified five individuals with the GPIBA mutation and none with the ITGB3 mutation.Conclusions We report a novel autosomal dominant cause of a mild form of macrothrombocytopenia caused by a D723H substitution in the beta3 integrin. We postulate that the low platelet count is due to increased consumption of platelets due to enhanced levels of surface expressed, activated GPIIbIIIa.