Mucolipidosis II (I-cell disease): studies of muscle biopsy and muscle cultures.

Muscle cultures from a patient with I-cell disease showed the characteristic morphologic and biochemical abnormalities previously observed in cultured fibroblasts. At early stages of myoblast growth, there were numerous inclusions. Biochemically, the intracellular activities of beta-galactosidase, beta-hexosaminidase, and alpha-mannosidase were reduced to 1, 14, and 5%, respectively, of control values, and this was accompanied by elevated levels of beta-hexosaminidase and alpha-mannosidase in the culture medium. Cultures from the patient did not fuse as well as controls; however, when well-developed myotubes possessing distinct cross-striations were present, these no longer had inclusions. In the muscle biopsy from this patient, only beta-galactosidase was decreased to approximately 50% of the mean control value whereas beta-hexosaminidase and alpha-mannosidase activities were increased as compared to controls. These data suggest that the I-cell mutation is expressed during early myogenesis but not in well-differentiated myotubes or mature muscle fibers.