A novel amino acid substitution of integrin αIIb in Glanzmann thrombasthenia confirms that the N-terminal region of the receptor plays a role in maintaining β-propeller structure

A novel heterozygous mutation flanking the fourth calcium-binding domain of the ITGA2B gene induces severe bleeding complications: a case report and literature review

Glanzmann thrombasthenia is a rare autosomal recessive genetic disease characterized by platelet aggregation dysfunction caused by a congenital defect of platelet membrane glycoprotein IIb/IIIa (integrin αIIbβ3). Integrin αIIbβ3, a calcium-dependent heterodimer, plays a critical role in platelet aggregation. We described a boy who was hospitalized with serious epistaxis at 10 months of age who had a history of repeated petechiae and spontaneous epistaxis since birth. Flow cytometry showed normal surface expression of platelet antigens. Genetic analysis and sequencing revealed the novel missense mutation c.G1252>T (p.Gly418Cys) in ITGA2B. This heterozygous amino acid mutation flanked the fourth calcium-binding domain and may interfere with integrin biogenesis via mechanisms other than merely altering cell surface expression. We discuss the heterogeneity of the genotype and phenotype with this atypical case and review the relevant literature on mutations adjacent to or within the calcium-binding domains in Glanzmann thrombasthenia.

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.

In silico analysis of Glanzmann variants of Calf-1 domain of αIIbβ3 integrin revealed dynamic allosteric effect

Integrin α_IIbβ₃ mediates platelet aggregation and thrombus formation. In a rare hereditary bleeding disorder, Glanzmann thrombasthenia (GT), α_IIbβ₃ expression / function are impaired. The impact of deleterious missense mutations on the complex structure remains unclear. Long independent molecular dynamics (MD) simulations were performed for 7 GT variants and reference structure of the Calf-1 domain of α_IIb. Simulations were analysed using a structural alphabet to describe local protein conformations. Common and flexible regions as well as deformable zones were observed in all the structures. The most flexible region of Calf-1 (with highest B-factor) is rather a rigid region encompassed into two deformable zones. Each mutated structure barely showed any modifications at the mutation sites while distant conformational changes were observed. These unexpected results question the relationship between molecular dynamics and allostery; and the role of these long-range effects in the impaired α_IIbβ₃ expression. This method is aimed at studying all α_IIbβ₃ sub-domains and impact of missense mutations at local and global structural level.

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.

Natural and artificial mutations in αIIb integrin lead to a structural deformation of a calcium-binding site

The platelet integrin αIIbβ3 is widely accepted as a structural and a functional model of the broad integrin protein family. The four calcium-binding sites in the αIIb subunit contribute to biogenesis and stability of the protein. Mansour et al. (J Thromb Haemost 9:192-200, 2011) showed that the natural Asn2Asp mutation causing Glanzmann thrombasthenia, prevented surface expression of αIIbβ3, whereas the artificial Asn2Gln mutation only decreased its level. Molecular dynamics simulations and EDTA chelation assay were used here to explore the mechanism of these structural deformations. We show a considerable expansion of the calcium-binding site 3 in Asn2Asp mutation, whereas the Asn2Gln toggles between normal and expanded conformations. The αIIbβ3 surface expression level correlates to the relative spending time in the expanded conformation. By a comparison to other calcium-binding sites of αIIb and of other α integrins we show that the size of a calcium-binding loop is conserved. EDTA chelation assay shows a sensitivity to calcium removal, which correlates with the reduction in αIIbβ3 surface expression and with the calcium binding site expansion, thus verifying the simulation data. Here we indicate that Asn2 mutation affects a calcium-binding site 3 of αIIb, which structural deformation is proposed to deprive calcium binding and interfere with an integrin intracellular trafficking and its surface expression.