Competition between normal [674C] and mutant [674R] subunits: role of the molecular chaperone BiP in the processing of GPIIb-IIIa complexes

Flow cytometric analysis of platelet surface glycoproteins in the diagnosis of thirty-two Turkish patients with Glanzmann thrombasthenia: a multicenter experience

Background/aim

Glanzmann thrombasthenia (GT) is a rare autosomal recessively inherited bleeding disorder characterized by the quantitative (type 1 and type 2) or qualitative (type 3) deficiency in platelet membrane glycoprotein (GP) IIb/IIIa (CD41a/CD61) fibrinogen receptors. In type 1, 2, and 3, CD41a/CD61 expression is 5%, 5%–20% and above 20%, respectively. In this study, diagnosis of GT was confirmed and subgroups were identified in 32 Turkish patients by flow cytometry analysis.

Materials and methods

CD41a/CD61 expression levels in platelet-rich plasma (PRP) obtained from peripheral venous EDTA blood samples were analyzed with a BD FACSCanto II flow cytometer (Becton Dickinson, Franklin Lakes, NJ, USA). GT subgroup analysis was performed by counting 50,000 events in the BD FACSDiva Software v6.1.3 program of the instrument.

Results

In the present study, in blood samples of 32 patients from 23 families with GT and 22 healthy controls, co-expression levels of CD41a and CD61 in PRP was analyzed. 12 out of 23 families were consistent with type 1 GT (52.2%), 4 were consistent with type 2 GT (17.4%), and 7 were consistent with type 3 GT (30.4%).

Conclusion

Especially due to consanguineous marriages, GT with various glycoprotein levels may be detected. As a result of the flow cytometry analysis of the present study with the highest GT patient population in Turkey, type 1 GT patients were the most common subgroup. In the determination of the GT subgroups; especially in the detection of type 3 GT, flow cytometry is the most sensitive glycoprotein analysis method. In addition to light transmission aggregometry, CD41a/CD61 study by flow cytometer confirms diagnosis when mutation analysis cannot be performed.

Profiling the Genetic and Molecular Characteristics of Glanzmann Thrombasthenia: Can It Guide Current and Future Therapies?

Glanzmann thrombasthenia (GT) is the most widely studied inherited disease of platelet function. Platelets fail to aggregate due to a defect in platelet-to-platelet attachment. The hemostatic plug fails to form and a moderate to severe bleeding diathesis results. Classically of autosomal recessive inheritance, GT is caused by defects within the ITGA2B and ITGB3 genes that encode the αIIbβ3 integrin expressed at high density on the platelet surface and also in intracellular pools. Activated αIIbβ3 acts as a receptor for fibrinogen and other adhesive proteins that hold platelets together in a thrombus. Over 50 years of careful clinical and biological investigation have provided important advances that have improved not only the quality of life of the patients but which have also contributed to an understanding of how αIIbβ3 functions. Despite major improvements in our knowledge of GT and its genetic causes, extensive biological and clinical variability with respect to the severity and intensity of bleeding remains poorly understood. I now scan the repertoire of ITGA2B and ITGB3 gene defects and highlight the wide genetic and biological heterogeneity within the type II and variant subgroups especially with regard to bleeding, clot retraction, the internal platelet Fg storage pool and the nature of the mutations causing the disease. I underline the continued importance of gene profiling and biological studies and emphasize the multifactorial etiology of the clinical expression of the disease. This is done in a manner to provide guidelines for future studies and future treatments of a disease that has not only aided research on rare diseases but also contributed to advances in antithrombotic therapy.

Expanding the Mutation Spectrum Affecting αIIbβ3 Integrin in Glanzmann Thrombasthenia: Screening of the ITGA2B and ITGB3 Genes in a Large International Cohort

We report the largest international study on Glanzmann thrombasthenia (GT), an inherited bleeding disorder where defects of the ITGA2B and ITGB3 genes cause quantitative or qualitative defects of the αIIbβ3 integrin, a key mediator of platelet aggregation. Sequencing of the coding regions and splice sites of both genes in members of 76 affected families identified 78 genetic variants (55 novel) suspected to cause GT. Four large deletions or duplications were found by quantitative real-time PCR. Families with mutations in either gene were indistinguishable in terms of bleeding severity that varied even among siblings. Families were grouped into type I and the rarer type II or variant forms with residual αIIbβ3 expression. Variant forms helped identify genes encoding proteins mediating integrin activation. Splicing defects and stop codons were common for both ITGA2B and ITGB3 and essentially led to a reduced or absent αIIbβ3 expression; included was a heterozygous c.1440-13_c.1440-1del in intron 14 of ITGA2B causing exon skipping in seven unrelated families. Molecular modeling revealed how many missense mutations induced subtle changes in αIIb and β3 domain structure across both subunits, thereby interfering with integrin maturation and/or function. Our study extends knowledge of GT and the pathophysiology of an integrin.

Glanzmann thrombasthenia: a review of ITGA2B and ITGB3 defects with emphasis on variants, phenotypic variability, and mouse models

Characterized by mucocutaneous bleeding arising from a lack of platelet aggregation to physiologic stimuli, Glanzmann thrombasthenia (GT) is the archetype-inherited disorder of platelets. Transmitted by autosomal recessive inheritance, platelets in GT have quantitative or qualitative deficiencies of the fibrinogen receptor, αIIbβ3, an integrin coded by the ITGA2B and ITGB3 genes. Despite advances in our understanding of the disease, extensive phenotypic variability with respect to severity and intensity of bleeding remains poorly understood. Importantly, genetic defects of ITGB3 also potentially affect other tissues, for β3 has a wide tissue distribution when present as αvβ3 (the vitronectin receptor). We now look at the repertoire of ITGA2B and ITGB3 gene defects, reexamine the relationship between phenotype and genotype, and review integrin structure in the many variant forms. Evidence for modifications in platelet production is assessed, as is the multifactorial etiology of the clinical expression of the disease. Reports of cardiovascular disease and deep vein thrombosis, cancer, brain disease, bone disorders, and pregnancy defects in GT are discussed in the context of the results obtained for mouse models where nonhemostatic defects of β3-deficiency or nonfunction are being increasingly described.

Variations in platelet protein associated with arterial thrombosis

INTRODUCTION

Hyperactivity of platelets has been associated with thrombotic episodes by molecular mechanisms not yet elucidated. The present work aimed at identifying whether the platelet protein content from patients who had suffered an arterial thrombosis episode differed from that of platelets obtained from normal healthy donors.

METHODS

Differential platelet protein profiles were determined by 2-dimensional (2-D) gel electrophoresis and Western blot analysis of total platelet lysates. Identification of differentially expressed proteins was carried out by mass spectrometry (MALDI-TOF).

RESULTS

We found a decreased platelet content of three protein spots in patients of arterial thrombosis: integrin linked kinase (ILK), fructose bisphosphate aldolase (aldolase) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) whereas the content of four other protein spots was increased: actin binding protein, coronine like (p57), non-muscle myosin heavy chain (NMMHC-A), pyruvate kinase M2 isoenzyme (PK) and phosphoglycerate kinase (PGK). The variations in ILK, GAPDH and PK were validated by Western blot analysis. The proteins showing a decreased platelet content in arterial thrombosis patients are associated with the cytoskeletal insoluble fraction and the detected increase in some proteins seems to be due to the generation of peptides caused by a limited proteolysis. Differences in the protein profiles of circulating platelets from arterial thrombosis were maintained months after the acute thrombotic event and disappear in the long term.

CONCLUSIONS

The observed variations in some platelet proteins suggest the existence of a perturbation in the cytoskeletal organization and increased proteolysis, both indicative of a platelet pro-active state, persistent after the thrombotic event.

Disruption of the beta3 663-687 disulfide bridge confers constitutive activity to beta3 integrins

The platelet fibrinogen receptor, integrin alphaIIbbeta3, is a noncovalent heterodimer of glycoproteins IIb and IIIa. This work was aimed at elucidating the role played by the carboxy-terminal extracellular, trans-membrane, and cytoplasmic regions of the glycoprotein beta3 in the formation of functional complexes with alpha subunits. Progressive carboxy-terminal deletions of beta3 revealed that surface exposure of alphaIIbbeta3 or alphavbeta3 could not occur in the absence of the transmembrane domain of beta3. In contrast, internal deletions 616 to 690 of the carboxy-terminal regions of the beta3 ectodomain led to surface exposure of constitu tive active receptors in CHO cells, as indicated by the enhanced rate of cell adhesion to immobilized ligands and spontaneous binding to soluble fibrinogen or activation-dependent antibody PAC-1. The functional analysis of cysteine mutations within the 616 to 690 region of beta3 or chimeric beta3-beta7 subunits revealed that disruption of the C663-C687 disulfide bridge endows constitutive activity to the alphaIIbbeta3 receptor. It is concluded that the carboxy-terminal tail of the beta3 ectodomain, so-called beta tail domain (betaTD), is not essential for cell surface expression of beta3 receptors. However, a basal, nonactivated, low ligand-affinity state of the beta3 integrins demands a normal conformation of this domain.

A novel homozygous splice junction mutation in GPIIb associated with alternative splicing, nonsense-mediated decay of GPIIb-mRNA, and type II Glanzmann's thrombasthenia

This work reports the study of a patient suffering a bleeding disorder clinically diagnosed as Glanzmann's thrombasthenia (GT). Immunoblotting and flow cytometric analysis showed a low ( Collapse

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MESH Headings

• Alternative Splicing/genetics
• Codon, Nonsense
• Conserved Sequence/genetics
• Family Health
• Female
• Homozygote
• Humans
• Infant
• Mutation, Missense
• Platelet Glycoprotein GPIb-IX Complex/genetics
• Platelet Membrane Glycoproteins
• Point Mutation
• RNA Splice Sites/genetics
• RNA Stability/genetics
• RNA, Messenger/analysis
• Thrombasthenia/genetics

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Affiliation(s)

C González-Manchón Department of Pathophysiology and Human Molecular Genetics, Centro de Investigaciones Biológicas (CSIC), Madrid, Spain.
E G Arias-Salgado
N Butta
G Martín
R B Rodríguez
I Elalamy
R Parrilla
R Favier

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Agonist-induced aggregation of Chinese hamster ovary cells coexpressing the human receptors for fibrinogen (integrin alphaIIbbeta3) and the platelet-activating factor: dissociation between adhesion and aggregation

This work reports the establishment of a Chinese hamster ovary (CHO) cell line stably coexpressing the human alphaIIbbeta3 integrin and the platelet-activating factor receptor (PAFR). These cells aggregate in response to PAF in a Ca(++), alphaIIbbeta3, and soluble fibrinogen (Fg)-dependent manner that is prevented by PAF antagonists or alphaIIbbeta3 blockade. The aggregating response is accompanied by enhanced binding of fibrinogen and the activation-dependent IgM PAC1. This model has permitted us to identify, for the first time, intracellular signals distinctly associated with either alphaIIbbeta3-mediated adhesion or aggregation. Nonreceptor activation of protein kinase C (PKC) by phorbol ester produced cellular adhesion and spreading onto immobilized Fg, but it was not a sufficient signal to provoke cellular aggregation. Moreover, inhibition of PKC impeded the PAF stimulation of cellular adhesion, whereas the aggregation was not prevented. The PAF-induced cellular aggregation was distinctly associated with signaling events arising from the liganded Fg receptor and the agonist-induced stimulation of a calcium/calmodulin-dependent signaling pathway. Sustained tyrosine phosphorylation of both mitogen-activated protein kinase (MAPK) and an approximately 100-kd protein was associated with the PAF-induced aggregation, whereas phosphorylation of focal adhesion kinase (FAK) was preferably associated with cellular adherence and spreading onto immobilized Fg.