Enzyme replacement therapy (ERT) became a reality for patients with Pompe disease, a fatal cardiomyopathy and skeletal muscle myopathy caused by a deficiency of glycogen-degrading lysosomal enzyme acid alpha-glucosidase (GAA). The therapy, which relies on receptor-mediated endocytosis of recombinant human GAA (rhGAA), appears to be effective in cardiac muscle, but less so in skeletal muscle. We have previously shown a profound disturbance of the lysosomal degradative pathway (autophagy) in therapy-resistant muscle of GAA knockout mice (KO). Our findings here demonstrate a progressive age-dependent autophagic buildup in addition to enlargement of glycogen-filled lysosomes in multiple muscle groups in the KO. Trafficking and processing of the therapeutic enzyme along the endocytic pathway appear to be affected by the autophagy. Confocal microscopy of live single muscle fibers exposed to fluorescently labeled rhGAA indicates that a significant portion of the endocytosed enzyme in the KO was trapped as a partially processed form in the autophagic areas instead of reaching its target--the lysosomes. A fluid-phase endocytic marker was similarly mistargeted and accumulated in vesicular structures within the autophagic areas. These findings may explain why ERT often falls short of reversing the disease process and point toward new avenues for the development of pharmacological intervention.

Autophagy is a major pathway for delivery of proteins and organelles to lysosomes where they are degraded and recycled. We have previously shown excessive autophagy in a mouse model of Pompe disease (glycogen storage disease type II), a devastating myopathy caused by a deficiency of the glycogen-degrading lysosomal enzyme acid a-glucosidase. The autophagic buildup constituted a major pathological component in skeletal muscle and interfered with delivery of the therapeutic enzyme. To assess the role of autophagy in the pathogenesis of the human disease, we have analyzed vesicles of the lysosomal-degradative pathway in isolated single muscle fibers from Pompe patients. Human myofibers showed abundant autophagosome formation and areas of autophagic buildup of a wide range of sizes. In patients, as in the mouse model, the enormous autophagic buildup causes greater skeletal muscle damage than the enlarged, glycogenfilled lysosomes outside the autophagic regions. Clearing or preventing autophagic buildup seems, therefore, a necessary target of Pompe disease therapy.

In Pompe disease, a deficiency of lysosomal acid alpha-glucosidase, intralysosomal glycogen accumulates in multiple tissues, with skeletal and cardiac muscle most severely affected.(1) Complete enzyme deficiency results in rapidly progressive infantile cardiomyopathy and skeletal muscle myopathy that is fatal within the first two years of life. Patients with partial enzyme deficiency suffer from skeletal muscle myopathy and experience shortened lifespan due to respiratory failure. The major advance has been the development of enzyme replacement therapy, which recently became available for Pompe patients. However, the effective clearance of skeletal muscle glycogen, as shown by both clinical and preclinical studies, has proven more difficult than anticipated.(2-4) Our recent work published in Annals of Neurology(5) was designed to cast light on the problem, and was an attempt to look beyond the lysosomes by analyzing the downstream events affected by the accumulation of undigested substrate in lysosomes. We have found that the cellular pathology in Pompe disease spreads to affect both endocytic (the route of the therapeutic enzyme) and autophagic (the route of glycogen) pathways, leading to excessive autophagic buildup in therapy-resistant skeletal muscle fibers of the knockout mice.