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Molecular pathology and enzyme processing in various phenotypes of acid maltase deficiency

Objectives: To examine at molecular, biochemical, and muscle pathology level the striking clinical heterogeneity resulting from acid α-glucosidase deficiency. Methods: We investigated 23 patients with infantile-onset or late-onset glycogen storage disease type II by enzyme activity, protein expression by immunoblotting, GAA gene mutations, and muscle pathology including immunolabeling for Golgi and sarcolemmal proteins. Results: The enzyme activity was absent or minimal in infantile-onset cases and variably reduced in late-onset patients. Genotype-phenotype correlation (seven novel mutations were found) showed that most late-onset patients had the heterozygous IVS1 leaky splicing mutation (one patient was homozygous), but the course of the disease was often difficult to predict on the basis of the mutations alone. All patients showed an abnormal pattern of enzyme protein processing, with increased amounts of the inactive forms and very low or absent amounts of the mature forms. The molecular weight of the mature and the intermediate forms appeared higher in patients’ samples than in the control muscle. We observed a Golgi proliferation in muscle fibers possibly caused by the retention of inactive forms of enzyme protein that cannot be correctly targeted from Golgi to lysosomes. The vacuolar membranes expressed sarcolemmal proteins in late-onset but not in infantile-onset patients, suggesting an extensive autophagy and vacuolar membrane remodeling in late-onset patients. Conclusions: The different protein molecular weight between patients and controls could be due to an excessive sialylation of mutant enzyme: this might be possibly caused by a delayed transport and longer transit of the inactive proteins in the Golgi apparatus.