In silico analysis of structural modifications in and around the integrin alphaIIb genu caused by ITGA2B variants in human platelets with emphasis on Glanzmann thrombasthenia.

BACKGROUND: Studies on the inherited bleeding disorder, Glanzmann thrombasthenia (GT), have helped define the role of the alphaIIbbeta3 integrin in platelet aggregation. Stable bent alphaIIbbeta3 undergoes conformation changes on activation allowing fibrinogen binding and its taking an extended form. The alphaIIb genu assures the fulcrum of the bent state. Our goal was to determine how structural changes induced by missense mutations in the alphaIIb genu define GT phenotype. METHODS: Sanger sequencing of ITGA2B and ITGB3 in the index case followed by in silico modeling of all known GT-causing missense mutations extending from the lower part of the beta-propeller, and through the thigh and upper calf-1 domains. RESULTS: A homozygous c.1772A>C transversion in exon 18 of ITGA2B coding for a p.Asp591Ala substitution in an interconnecting loop of the lower thigh domain of alphaIIb in a patient with platelets lacking alphaIIbbeta3 led us to extend our in silico modeling to all 16 published disease-causing missense variants potentially affecting the alphaIIb genu. Modifications of structuring H-bonding were the major cause in the thigh domain although one mutation gave mRNA decay. In contrast, short-range changes induced in calf-1 appeared minor suggesting long-range effects. All result in severe to total loss of alphaIIbbeta3 in platelets. The absence of mutations within a key Ca2+-binding loop in the genu led us to scan public databases; three potential single allele variants giving major structural changes were identiffied suggesting that this key region is not protected from genetic variation. CONCLUSIONS: It appears that the alphaIIb genu is the object of stringent quality control to prevent platelets from circulating with activated and extended integrin.