Liver Glycogen Phosphorylase Deficiency Leads to Profibrogenic Phenotype in a Murine Model of Glycogen Storage Disease Type VI

Mutations in the liver glycogen phosphorylase (Pygl) gene are associated with the diagnosis of glycogen storage disease type VI (GSD-VI). To understand the pathogenesis of GSD-VI, we generated a mouse model with Pygl deficiency (Pygl(-/-)). Pygl(-/-) mice exhibit hepatomegaly, excessive hepatic glycogen accumulation, and low hepatic free glucose along with lower fasting blood glucose levels and elevated blood ketone bodies. Hepatic glycogen accumulation in Pygl(-/-) mice increases with age. Masson's trichrome and picrosirius red staining revealed minimal to mild collagen deposition in periportal, subcapsular, and/or perisinusoidal areas in the livers of old Pygl(-/-) mice (>40 weeks). Consistently, immunohistochemical analysis showed the number of cells positive for alpha smooth muscle actin (alpha-SMA), a marker of activated hepatic stellate cells, was increased in the livers of old Pygl(-/-) mice compared with those of age-matched wild-type (WT) mice. Furthermore, old Pygl(-/-) mice had inflammatory infiltrates associated with hepatic vessels in their livers along with up-regulated hepatic messenger RNA levels of C-C chemokine ligand 5 (Ccl5/Rantes) and monocyte chemoattractant protein 1 (Mcp-1), indicating inflammation, while age-matched WT mice did not. Serum levels of aspartate aminotransferase and alanine aminotransferase were elevated in old Pygl(-/-) mice, indicating liver damage. Conclusion: Pygl deficiency results in progressive accumulation of hepatic glycogen with age and liver damage, inflammation, and collagen deposition, which can increase the risk of liver fibrosis. Collectively, the Pygl-deficient mouse recapitulates clinical features in patients with GSD-VI and provides a model to elucidate the mechanisms underlying hepatic complications associated with defective glycogen metabolism.