Novel Homozygous Mutation (c.175delG) in Platelet GlycoproteinITGA2BGene as Cause of Glanzmann's Thrombasthenia Type I

Molecular characterization of Glanzmann's thrombasthenia in Iran: identification of three novel mutations

: Quantitative and/or qualitative defects of the platelet membrane glycoprotein IIb/IIIa complex lead to the clinical entity of Glanzmann's thrombasthenia. A large variety of mutations and polymorphisms are responsible for the aberrant expression and defective activity of this heterodimeric complex. The current study aimed to determine the pattern of mutations among Iranian population with Glanzmann's thrombasthenia. A total of 20 patients with Glanzmann's thrombasthenia have been evaluated. All exons and splice sites of ITGA2B and ITGB3 genes were amplified using touchdown PCR. Mutation screening was analyzed using conformation sensitive gel electrophoresis heteroduplex PCR, and DNA sequencing. In addition to finding one previously identified mutation and polymorphism, the experimenters explored 3 and 2 novel mutations and polymorphisms, respectively. One substitution mutation, two deletions of a single nucleotide, one insertion of a single nucleotide, two synonymous polymorphisms, and one missense polymorphism were found using Sanger sequencing method. All detected mutations were homozygous which will most likely contribute to the pathogenesis of Glanzmann's thrombasthenia. Furthermore, it suggested ITGB3 as the mainly affected gene impaired in the patients with Glanzmann's thrombasthenia. As expected, the molecular results were consistent with the phenotypic findings so that GPIIb/IIIa complex was disrupted due to mutations in all type-I Glanzmann's thrombasthenia patients. It is concluded that intronic alterations or epigenetic regulations could be responsible for aberrant expression and/or defective activity of GPIIb/IIIa complex among other patients.

Alterations of the platelet proteome in type I Glanzmann thrombasthenia caused by different homozygous delG frameshift mutations in ITGA2B

Glanzmann thrombasthenia (GT) is one of the best characterised inherited platelet function disorders but global platelet proteome has not been determined in these patients. We investigated the proteome and function of platelets from two patients with type I GT, caused by different homozygous ITGA2b mutations, from family members and unrelated controls. The global proteome of highly purified washed platelets was quantified by liquid chromatography-mass spectrometry (LC-MS) and targeted MS-methods. Platelet function was analysed by flow cytometry, light transmission aggregometry and flow-based assays. Platelets from GT patients showed less than 5 % relative levels of the integrin subunit α_IIb and 5-9 % fibrinogen compared to controls. These patients demonstrated loss of α_IIbβ₃-dependent platelet function, but normal platelet granule secretion induced by physiological agonists. Platelets from heterozygous family members of a patient expressed 50-60 % of control α_IIb levels which were sufficient for normal α_IIbβ₃-dependent platelet function. Studying type I GT as model disease we established quantitative LC-MS to detect and clearly distinguish normal platelets, platelets from GT heterozygotes and platelets from GT patients. Diminished levels of factor XIIIB chain, plasminogen and carboxypeptidase 2B were identified in thrombasthenic platelets. Additionally, GT platelets showed up to 2.5-fold increased levels of FcγRIIA and laminin-α4 chain. Elevated levels of platelet FcγRIIA was associated with increased CD63-surface expression after FcγRIIA-crosslinking in one GT-patient which might present a compensatory mechanism of platelet activation in GT. We demonstrate that quantitative LC-MS based proteomics is suitable to validate known but also to identify previously unknown protein level changes of dysfunctional platelets.

Functional comparison of induced pluripotent stem cell- and blood-derived GPIIbIIIa deficient platelets

Human induced pluripotent stem cells (hiPSCs) represent a versatile tool to model genetic diseases and are a potential source for cell transfusion therapies. However, it remains elusive to which extent patient-specific hiPSC-derived cells functionally resemble their native counterparts. Here, we generated a hiPSC model of the primary platelet disease Glanzmann thrombasthenia (GT), characterized by dysfunction of the integrin receptor GPIIbIIIa, and compared side-by-side healthy and diseased hiPSC-derived platelets with peripheral blood platelets. Both GT-hiPSC-derived platelets and their peripheral blood equivalents showed absence of membrane expression of GPIIbIIIa, a reduction of PAC-1 binding, surface spreading and adherence to fibrinogen. We demonstrated that GT-hiPSC-derived platelets recapitulate molecular and functional aspects of the disease and show comparable behavior to their native counterparts encouraging the further use of hiPSC-based disease models as well as the transition towards a clinical application.

Characterisation of patients with Glanzmann thrombasthenia and identification of 17 novel mutations

Glanzmann thrombasthenia (GT) is an autosomal recessive bleeding disorder characterised by quantitative and/or qualitative defects of the platelet glycoprotein (GP) IIb/IIIa complex, also called integrin αIIbβ3. αIIbβ3 is well known as a platelet fibrinogen receptor and mediates platelet aggregation, firm adhesion, and spreading. This study describes the molecular genetic analyses of 19 patients with GT who were diagnosed on the basis of clinical parameters and platelet analyses. The patients' bleeding signs include epistaxis, mucocutaneous bleeding, haematomas, petechiae, gastrointestinal bleeding, and menorrhagia. Homozygous or compound heterozygous mutations in ITGA2B or ITGB3 were identified as causing GT by sequencing of genomic DNA. All exons including exon/intron boundaries of both genes were analysed. In a patient with an intronic mutation, splicing of mRNA was analysed using reverse transcriptase (RT)-PCR of platelet-derived RNA. In short, 16 of 19 patients revealed 27 different mutations (ITGA2B: n=17, ITGB3: n=10). Seventeen of these mutations have not been published to date. Mutations in ITGA2B or ITGB3 were identified as causing GT in 16 patients. We detected a total of 27 mutations in ITGA2B and ITGB3 including 17 novel missense, nonsense, frameshift and splice site mutations. In addition, three patients revealed no molecular genetic anomalies in ITGA2B or ITGB3 that could explain the suspected diagnosis of GT. We assume that these patients may harbour defects in a regulatory element affecting the transcription of these genes, or other proteins may exist that are important for activating the αIIbβ3 complex that may be affected.