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Glycogen storage disease due to liver and muscle phosphorylase kinase deficiency
A benign inborn error of glycogen metabolism. It is the mildest form of GSD due to PhK deficiency.
ORPHA:79240
Classification level: Disorder
- GSD due to liver and muscle phosphorylase kinase deficiency
- GSD type 9B
- GSD type IXb
- Glycogen storage disease type 9B
- Glycogen storage disease type IXb
- Glycogenosis due to liver and muscle phosphorylase kinase deficiency
- Glycogenosis type 9B
- Glycogenosis type IXb
Prevalence: Unknown
Inheritance: Autosomal recessive
Age of onset: Childhood
Prevalence is unknown.
The disease manifests in childhood. Patients have marked hepatomegaly and mild muscular hypotonia. Hypoglycemia may occur only after prolonged fasting. These symptoms improve with age and adults are generally asymptomatic.
Phosphorylase kinase (PhK) is an enzyme which plays a key role in the regulation of glycogenolysis as it is required for glycogen phosphorylase activation. It consists of four copies of each four subunits (alpha, beta, gamma and calmoduline) encoded by different genes on different chromosomes and differentially expressed in various tissues. GSD due to liver and muscle PhK deficiency is due to mutations in the PHKB gene (16q12-q13) which encodes the beta subunit.
Biochemical diagnosis can be made by measuring phosphorylase kinase activity in blood cells or in a liver or muscle biopsy. Serum transaminase levels may also be elevated. Genetic testing is useful to confirm or establish the diagnosis.
Differential diagnoses include other glycogen storage diseases such as GSD due to liver phosphorylase deficiency (GSD type VI), GSD due to glycogen debranching enzyme deficiency (GSD type III), and GSD due to glucose-6-phosphatase (GSD type I).
Transmission is autosomal recessive.
Most patients do not require any specific treatment; a few patients may need to use cornstarch snacks.
Prognosis is good.
Last update: May 2011 - Expert reviewer(s): Dr Roseline FROISSART - Pr Philippe LABRUNE
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