CiRA researchers use patient iPS cells to study the development of Pompe disease in muscle cells.

Pompe disease is a metabolic disorder in which a defective enzyme prevents the conversion of glycogen into glucose in lysosomes, which are organelles found in all cells in the body. As a result, cells suffer from accumulated glycogen that especially weakens and eventually kills muscle cells. Of the two types of Pompe disease, children with infantile-onset Pompe disease (IOPD) will rarely reach their second birthday. Using iPS cell technology, CiRA researchers have reprogrammed the skin cells of three IOPD patients into muscle cells and found a new drug target for the disease.

"Pompe disease is caused by a defect in lysosomal acid alpha-glucosidase (GAA). The defect causes an accumulation of glycogen in lysosomes in skeletal and heart muscles," explained CiRA Associate Professor Hidetoshi Sakurai, who led the study.

Enzyme replacement therapy with recombinant GAA is standard treatment. Recombinant GAA improves symptoms and extends lifespans, but for unknown reasons skeletal muscle is less responsive to the therapy than heart muscle, such that patients still develop a need for artificial respiration.

Because the muscle cells in Pompe disease patients are already extensively damaged by the time of diagnosis, animal models have been used to test experimental drugs, but according to Sakurai none adequately replicate IOPD.

"The symptoms in mouse models are more like late-onset Pompe disease (LOPD). The onset of muscle weakness is slower, and the animals live longer," he said.

Noting that human cells are preferred, the Sakurai lab compared muscle cells they prepared from the iPS cells of IOPD patients and healthy donors. The patient iPSC-muscle cells showed characteristics consistent of the disease. The cells had depleted GAA, and lysosomes were enlarged and carried high glycogen content even in low glucose conditions. The conversion of glycogen to glucose could be obtained when recombinant GAA was applied to the cells, suggesting patient iPSC-muscle cells behave like patient muscle cells and can be used to test new drug candidates.

Additional study confirmed that the disease particularly afflicts the mitochondria to change the metabolism of muscle cells. Further, the scientists found that the signaling of mTORC1, a molecule that has a role in cell growth and cell metabolism, was affected. mTORC1 is indispensable and unlikely to make a suitable drug target, since changes its activity can have a deleterious effect on other organs. However, Sakurai noted other molecules regulated by mTORC1 could make promising targets and that these molecules could be found with patient iPSC-muscle cells.

"The relationship between mTORC1 signaling and the patho-mechanism of Pompe disease has not been fully elucidated. Further study could help us find new targets to replace or improve enzyme replacement therapy," he said.