Natural Progression of Canine Glycogen Storage Disease Type IIIa

Glycogen storage disease type IIIa (GSD IIIa) is caused by a deficiency of glycogen debranching enzyme activity. Hepatomegaly, muscle degeneration, and hypoglycemia occur in human patients at an early age. Long-term complications include liver cirrhosis, hepatic adenomas, and generalized myopathy. A naturally occurring canine model of GSD IIIa that mimics the human disease has been described, with progressive liver disease and skeletal muscle damage likely due to excess glycogen deposition. In the current study, long-term follow-up of previously described GSD IIIa dogs until 32 mo of age (n = 4) and of family-owned GSD IIIa dogs until 11 to 12 y of age (n = 2) revealed that elevated concentrations of liver and muscle enzyme (AST, ALT, ALP, and creatine phosphokinase) decreased over time, consistent with hepatic cirrhosis and muscle fibrosis. Glycogen deposition in many skeletal muscles; the tongue, diaphragm, and heart; and the phrenic and sciatic nerves occurred also. Furthermore, the urinary biomarker Glc₄, which has been described in many types of GSD, was first elevated and then decreased later in life. This urinary biomarker demonstrated a similar trend as AST and ALT in GSD IIIa dogs, indicating that Glc₄ might be a less invasive biomarker of hepatocellular disease. Finally, the current study further demonstrates that the canine GSD IIIa model adheres to the clinical course in human patients with this disorder and is an appropriate model for developing novel therapies.Abbreviations: CCR, curly-coated retriever; CPK, creatine phosphokinase; GSD IIIa, glycogen storage disease type IIIa; Glc₄, Glcα1-6Glcα1-4Glcα1-4GlcGlycogen storage disease type IIIa (GSD IIIa; OMIM, 232400) is an autosomal recessive disorder caused by mutations in the glycogen debranching enzyme gene (AGL), leading to various clinical signs. The tissues mainly affected are liver, heart, and skeletal muscle. Clinical manifestations include hypoglycemia, elevated serum concentrations of liver and muscle enzymes, hepatomegaly, growth retardation, muscle weakness, cardiac hypertrophy with arrhythmia risk, polycystic ovaries and neuropathy.^15,17,29 Current treatments are mainly symptomatic and are not curative. The most frequently used therapies are dietary, such as providing uncooked corn starch to prevent hypoglycemia at young ages and high-protein diets, which have been shown to reverse the extent of cardiomyopathy associated with GSD IIIa.^7,8,30,37 In addition, the use of medium-chain triglycerides has shown positive therapeutic effects in patients with GSD Ia and GSD IIIa.^11,22 However, dietary therapies do not prevent the long-term complications of GSD IIIa, including hepatic cirrhosis, hepatocellular adenoma, hepatocellular carcinoma, cardiomyopathy, neuropathy, and myopathy.³¹An appropriate animal model is necessary to test novel therapies and address the long-term effects of GSD IIIa. Recently a mouse model for GSD III has been described that may prove beneficial in testing new therapies.¹⁹ However, the limitations of mouse models include a short lifespan that curtails the study of the long-term effects of novel treatments. In addition, a large animal model often mimics human disease more closely than do mouse models, as occurs in GSD type Ia dog models, which exhibit lactic acidosis similar to human patients, a characteristic that mouse models of GSD Ia lack.¹⁶ Therefore a naturally occurring large animal model for GSD IIIa may be more effective in terms of the development of new treatments than are available mouse models.GSD IIIa (OMIA, 001577) has been reported to occur in curly-coated retriever dogs (CCR) and is caused by a naturally occurring homozygous frameshift mutation in exon 32 that leads to the deletion of 126 amino acids at the C-terminus of glycogen debranching enzyme.^12,40 The dogs in these previous studies proved to have abnormalities similar to those seen in humans affected with the disorder, namely progressive glycogen accumulation in muscle and liver, elevated liver and muscle enzymes (ALP, AST, creatine phosphokinase [CPK], and ALT), and eventual liver fibrosis. However, these animals were not followed beyond 16 mo of age in the earlier studies.^12,40 The goal of the current study is to provide biochemical follow-up on these animals and analyze more extensively other tissues and organs involved in GSD IIIa in the dog model. A brief analysis of the naturally high protein diets of GSD IIIa dogs, as well as the effects of an increased protein diet in 2 dogs for the last few months of life, is included.We also include the analysis of a urinary biomarker, Glcα1–6Glcα1– 4Glcα1–4Glc (Glc₄), which is a breakdown product of glycogen by α-amylase and neutral α-1,4-glucosidase.³² Elevated levels of Glc₄ have been found in urine from patients with GSD types II, III, IV, VI, and IX and may correlate with disease advancement.^1,18,24,32 To our knowledge, Glc₄ has not been evaluated previously in dogs; we therefore here evaluated the utility of Glc₄ as a biomarker of canine GSD IIIa. A correlation of Glc₄ levels with liver enzyme concentrations in blood might indicate a role of Glc₄ as a less-invasive biomarker for determining the advancement of liver disease in human and canine patients.