RETROVIRAL TRANSFER OF ACID ALPHA-GLUCOSIDASE CDNA TO ENZYME-DEFICIENT MYOBLASTS RESULTS IN PHENOTYPIC SPREAD OF THE GENOTYPIC CORRECTION BY BOTH SECRETION AND FUSION

Myoblasts have properties that make them suitable vehicles for gene re placement therapy, and lysosomal storage diseases are attractive targe ts for such therapy, Type II Glycogen Storage Disease, a deficiency of acid alpha-glucosidase (GAA), results in the abnormal accumulation of glycogen in skeletal and cardiac muscle lysosomes, The varied manifes tations of the enzyme deficiency in affected patients are ultimately l ethal, We used a retroviral vector carrying the cDNA encoding for GAA to replace the enzyme in deficient myoblasts and fibroblasts and analy zed the properties of the transduced cells, The transferred gene was e fficiently expressed, and the de novo-synthesized enzyme reached lysos omes where it digested glycogen, In enzyme-deficient myoblasts after t ransduction, enzyme activity rose to more than 30-fold higher than in normal myoblasts and increased about five-fold more when the cells wer e allowed to differentiate into myotubes, The transduced cells secrete d GAA that was endocytosed via the mannose-6-phosphate receptor into l ysosomes of deficient cells and digested glycogen, Moreover, the trans duced myoblasts were able to fuse with and provide enzyme for GAA-defi cient fusion partners, Thus, the gene-corrected cells, which appear ot herwise normal, may ultimately provide phenotypic correction to neighb oring GAA-deficient cells by fusion and to distant cells by secretion and uptake mechanisms.