Three novel β-propeller mutations causing Glanzmann thrombasthenia result in production of normally stable pro-αIIb, but variably impaired progression of pro-αIIbβ3 from endoplasmic reticulum to Golgi

Abstract

Background: Glanzmann thrombasthenia (GT) is an autosomal recessive bleeding disorder characterized by lack of platelet aggregation in response to most physiological agonists and caused by either a lack or dysfunction of the platelet integrin αIIbβ3 (glycoprotein IIb/IIIa). Objectives: To determine the molecular basis of GT and characterize the mutations by in vitro expression studies. Patients: We studied three unrelated patients from southern India whose diagnosis was consistent with GT. Results: Immunoprecipitation of the cell lysates and immunoblotting showed no detectable mature αIIb in the G128S mutant, in contrast to 6% and 33% of the normal amount of mature αIIb in the S287L and G357S mutants, respectively. Pulse-chase analysis demonstrated pro-αIIb in the mutants comparable with the normal pro-αIIb, but no conversion to mature αIIb in the G128S mutant, and only trace conversion to mature αIIb in the S287L and G357S mutants. The disappearance of pro-αIIb in the three mutants was similar to that in cells expressing normal αIIbβ3 or αIIb only. All three mutants demonstrated pro-αIIbβ3 complexes and co-localized with an ER marker by immunofluorescence. The G128S mutant showed no co-localization with a Golgi marker, and the other two mutants showed minimal and moderate co-localization with the Golgi marker. Conclusions: These three β-propeller mutations do not affect the production of pro- αIIb, its ability to complex with β3, or its stability, but do cause variable defects in transport of pro- αIIbβ3 complexes from the endoplasmic reticulum to the Golgi. © 2005 International Society on Thrombosis and Haemostasis.