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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

Summary.  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.