Glycogen storage disease type 3

Glycogen debranching enzyme (GDE) deficiency, or glycogen storage disease type III (GSD III), is a form of glycogen storage disease characterized by severe muscle weakness and hepatopathy.

The disease is caused by mutations in the AGL gene (1p21), leading to a deficiency in the GDE that works with the glycogen phosphorylase to catabolize glycogen. The deficiency may occur in the liver and muscle (GSD IIIa) or only in the liver (GSD IIIb).

Transmission is autosomal recessive. Estimated prevalence is approximately 1/100,000 births (it may be higher among North Africans).

GSD III commonly occurs in early childhood. Children present with hepatomegaly, growth retardation and occasional seizures related to hypoglycemia. Hepatomegaly may disappear with adulthood. Muscle weakness is slowly progressive. Other frequently associated signs include muscular hypotonia and hypertrophic cardiomyopathy. Symptoms often improve at puberty, except in the few cases where cirrhosis or myopathy appears. Biological findings include hypoglycemia without acidosis, hypertriglyceridemia, and hypertransaminasemia during childhood.

The diagnosis is based on the evidence of enzymatic deficiency in fresh leukocytes, fibroblasts, or on a liver or a muscle biopsy. Unlike GSD type I, there is a response to glucagon after meals.

Prenatal diagnosis is possible by enzyme assay and/or DNA analysis.

Treatment is based on a specific diet, with enteral nasogastric drip feeding at night in case of hypoglycemia, frequent meals, and uncooked starch supplements. For patients with myopathy, a high protein diet is also recommended.

Rarely, patients may develop complications such as hepatic failure or hepatocellular carcinoma.

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