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.     

αIIbβ3 integrin: new allelic variants in Glanzmann thrombasthenia,effects on ITGA2B and ITGB3 mRNA splicing,expression, and structure–function

Glanzmann thrombasthenia (GT) is an autosomal recessive inherited bleeding disorder characterized by an impaired platelet aggregation due to defects in integrin αIIbβ3 (ITGA2B, ITGB3), a fibrinogen receptor. Mutations from 24 GT patients and two carriers of various origins, Caucasian, North‐African and Asian were characterized. Promoter and exon sequences of αIIb and β3 genes were amplified and directly sequenced. Among 29 identified mutations, 17 new allelic variants resulting from nonsense, missense and deletion/insertion mutations were described. RNA alterations were evaluated by using Web servers. The αIIb p.S926L, p.V903F, and β3 p.C38Y, p.M118R, p.G221D substitutions prevented complex expression at the surface of COS‐7 cells by altering the αIIb or the β3 subunit structure. As shown by free energy analyses applied on the resolved structure of αIIbβ3 and structural modeling of the mutant, the p.K253M substitution of β3 helped to define a key role of the K253 in the interaction of the αIIb β‐propeller and the β3 β‐I domains. finally, the αIIb p.Q595H substitution allowed cell surface expression of the complex but its corresponding c.2800G>T mutation is predicted to alter normal RNA splicing. In conclusion, our study yielded the discovery of 17 new GT allelic variants, revealed the key role of K253 of αIIb for the αIIbβ3 complex formation and provides an additional example of an apparently missense mutation causing a splicing defect. Hum Mutat 30:1–10, 2010. © 2010 Wiley‐Liss, Inc.