Glycogen storage disease due to muscle phosphorylase kinase (PhK) deficiency is a benign inborn error of glycogen metabolism characterized by exercise intolerance.
The disease is very rare with less than 30 patients reported in the literature.
The disease starts generally in adolescence or adulthood. Patients may present with exercise intolerance with myalgia, cramps, fatigue, and sometimes myoglobinuria. In some cases, patients may present with progressive muscle weakness. Symptoms are usually mild, and myopathy may be asymptomatic. A neonatal form with generalized muscular hypotonia and respiratory insufficiency has also been described.
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. Muscle-specific isoforms of the alpha and gamma subunits are encoded by the PHKA1 gene and the PHKG1 gene respectively, but until now mutations have been only identified in the PHKA1 gene and the transmission is X-linked.
Biochemical diagnosis of the muscle form can be made by measuring phosphorylase kinase activity in a muscle biopsy. Genetic testing is useful to confirm or establish the diagnosis.
Differential diagnoses include GSD due to myophosphorylase deficiency (GSD type V), and other GSDs affecting the muscle (GSD types XI, XII, XIII and XIV) (see these terms).
Most patients do not require any specific treatment.
Prognosis is generally good.
Last update: May 2011
- Dr Roseline FROISSART
- Pr Philippe LABRUNE
- Dr Pascal LAFORET