Decreased content and surface expression of α-granule membrane protein GMP-140 in one of two types of platelet αδ storage pool deficiency

Abstract

To determine whether α-granule membranes are present in platelets of patients with storage pool deficiencies of both alpha and dense granules (αδ-SPD), we examined the content and surface expression of the α-granule membrane protein GMP-140 in one patient (J.C.) with a severe α-granule deficiency and in three members of a family (family C) with milder α-granule deficiencies. Surface expression of GMP-140 in stimulated platelets, assessed by flow cytometric measurements of the binding of two anti-GMP-140 monoclonal antibodies, was 24-38% of normal values in platelets from patient J.C., vs. 60-95% of normal values in family C. Total platelet content of GMP-140, determined in platelet lysates by antigen-capture ELISA, was 49% of normal in patient J.C., but normal in the members of family C. Platelets of patient J.C. were found to be heterogeneous with respect to GMP-140 content and surface expression by both flow cytometry and immunogold electron microscopy. Approximately 80% of her platelets expressed little or no GMP-140 after stimulation, whereas the remaining 20% expressed normal amounts of GMP-140 and showed extensive immunogold labeling of typical α-granules and clear vacuoles. No such heterogeneity was found in platelets from family C. These findings in the severe αδ-SPD patient are in clear contrast to the observations of normal GMP-140 content in the three other αδ-SPD patients, and in patients with the gray platelet syndrome, reported previously by others. These results illustrate the phenotypic heterogeneity of α-granule deficiencies in human platelets, and suggest that a defect in granule formation in the megakaryocytes may account for the α-granule defect in at least one form of αδ-SPD.