Glycogen storage disease due to liver phosphorylase kinase deficiency

Glycogen storage disease due to liver phosphorylase kinase deficiency (also known as GSD type IX, or PhK deficiency of the liver) is a group of inherited metabolic disorders in which the enzyme phosphorylase kinase is deficient or dysfunctional in the liver. Phosphorylase kinase plays a critical role in glycogen breakdown (glycogenolysis) by activating glycogen phosphorylase. When this enzyme is deficient, glycogen accumulates abnormally in the liver, leading to hepatomegaly (enlarged liver), growth retardation, and elevated liver transaminases. Affected children typically present in early childhood with a protuberant abdomen due to hepatomegaly, short stature, fasting hypoglycemia, and hyperlipidemia. Fasting ketosis may also occur. The condition is generally considered one of the milder glycogen storage diseases, and symptoms often improve with age, with many patients experiencing resolution of hepatomegaly and normalization of growth by adulthood. The most common form of liver phosphorylase kinase deficiency is X-linked (GSD IXa), caused by mutations in the PHKA2 gene, which encodes the liver-specific alpha subunit of phosphorylase kinase. Less common autosomal recessive forms exist, caused by mutations in the PHKB gene (encoding the beta subunit, GSD IXb) or the PHKG2 gene (encoding the liver-specific gamma subunit, GSD IXc). The PHKG2-related form tends to be more severe and may progress to liver fibrosis or cirrhosis in some cases. Diagnosis is based on clinical findings, biochemical testing (including enzyme activity assays in liver tissue or blood cells), and molecular genetic testing. Treatment is primarily supportive and dietary. Frequent meals rich in complex carbohydrates, avoidance of prolonged fasting, and in some cases cornstarch supplementation are used to maintain blood glucose levels and support normal growth. Protein supplementation may also be beneficial. Prognosis is generally favorable, particularly for the X-linked form, with most patients leading normal lives in adulthood. Regular monitoring of liver function and growth is recommended throughout childhood.

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