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 α_IIbβ₃ (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 α_IIb in the G128S mutant, in contrast to 6% and 33% of the normal amount of mature α_IIb in the S287L and G357S mutants, respectively. Pulse-chase analysis demonstrated pro-IIb in the mutants comparable with the normal pro-α_IIb, but no conversion to mature α_IIb in the G128S mutant, and only trace conversion to mature α_IIb in the S287L and G357S mutants. The disappearance of pro-IIb in the three mutants was similar to that in cells expressing normal α_IIbβ₃ or α_IIb only. All three mutants demonstrated pro-α_IIbβ₃ 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-α_IIb, its ability to complex with β₃, or its stability, but do cause variable defects in transport of pro-α_IIbβ₃ complexes from the endoplasmic reticulum to the Golgi.