Molecular Dynamics Analysis of a Novel β3 Pro189Ser Mutation in a Patient with Glanzmann Thrombasthenia Differentially Affecting αIIbβ3 and αvβ3 Expression

Mutations in ITGA2B and ITGB3 cause Glanzmann thrombasthenia, an inherited bleeding disorder in which platelets fail to aggregate when stimulated. Whereas an absence of expression or qualitative defects of αIIbβ3 mainly affect platelets and megakaryocytes, αvβ3 has a widespread tissue distribution. Little is known of how amino acid substitutions of β3 comparatively affect the expression and structure of both integrins. We now report computer modelling including molecular dynamics simulations of extracellular head domains of αIIbβ3 and αvβ3 to determine the role of a novel β3 Pro189Ser (P163S in the mature protein) substitution that abrogates αIIbβ3 expression in platelets while allowing synthesis of αvβ3. Transfection of wild-type and mutated integrins in CHO cells confirmed that only αvβ3 surface expression was maintained. Modeling initially confirmed that replacement of αIIb by αv in the dimer results in a significant decrease in surface contacts at the subunit interface. For αIIbβ3, the presence of β3S163 specifically displaces an α-helix starting at position 259 and interacting with β3R261 while there is a moderate 11% increase in intra-subunit H-bonds and a very weak decrease in the global H-bond network. In contrast, for αvβ3, S163 has different effects with β3R261 coming deeper into the propeller with a 43% increase in intra-subunit H-bonds but with little effect on the global H-bond network. Compared to the WT integrins, the P163S mutation induces a small increase in the inter-subunit fluctuations for αIIbβ3 but a more rigid structure for αvβ3. Overall, this mutation stabilizes αvβ3 despite preventing αIIbβ3 expression.