Defects in synthesis, phosphorylation, and maturation of acid alpha-glucosidase in glycogenosis type II

Glycogenosis type II is an inherited lysosomal storage disease with acid alpha-glucosidase deficiency as the primary defect. Using cultured skin fibroblasts, we have studied the biosynthesis of acid alpha-glucosidase in clinically different forms of this disease. Three unrelated patients were identified (one with an infantile, one with a juvenile, and one with an adult form of the disease) producing normal quantities of the 110-kDa precursor form of acid alpha-glucosidase. However, post-translational modification to mature 76-kDa enzyme protein was either completely deficient or extremely inefficient. No abnormalities were observed in glycosylation of the mutant precursors, as measured by the incorporation of [3H]mannose, but phosphorylation was only detectable for the precursor synthesized by fibroblasts from the juvenile patient. In three other patients (one with a juvenile and two with adult forms of glycogenosis type II) apparently reduced synthesis of precursor protein was observed, but the processing to mature enzyme seemed to be undisturbed. Finally, neither precursor nor mature forms of acid alpha-glucosidase were detectable in one particular case of infantile glycogenosis type II. The studies reveal an unexpected degree of genetic heterogeneity in this disease and identify various mutants which could be of importance to further elucidate the biosynthetic events during lysosomal enzyme formation.