Glycogen storage disease type 9
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Glycogenosis type IX is an inherited metabolic disorder caused by a deficiency in enzyme activity phosphorylase kinase, which plays a fundamental role in the breakdown of glycogen.
Phosphorylase kinase is an enzyme composed of four subunits encoded by different genes: α subunit (PHKA2; Xp22.13), β (PHKB: 16q12.1), γ (PHKG2; 16p11.2), and δ (CALM1; 14q32.11), in liver isoform.
Mutations in PHKA2, PHKG2 and PHKB genes, produce glycogenosis IXa, glycogenosis IXb and glycogenosis IXc. The GSD IXa is the most common form and is transmitted as an X-linked recessive, while the other variants have an autosomal recessive mode of inheritance.
Investigators have identified two types of disorders that comprise phosphorylase kinase deficiency: liver phosphorylase kinase (hepatomegaly children and stunting, and sometimes fasting hypoglycemia and ketosis) and kinase muscle phosphorylase deficiency (it is less frequent; its symptoms usually comprise myalgia, myoglobinuria, cramps, and intolerance to physical activity, among others.
Glycogen storage disease IX due to liver phosphorylase kinase is associated with mutations in genes, that encodes isoforms of the alpha subunits liver (PHKA2) and gamma (PHKG2), of that enzyme. These changes are transmitted under an autosomal recessive mode of inheritance and X-linked.
In patients with glycogen storage disease IX due to muscle phosphorylase kinase, have been reported only PHKA1 (Xq13.1-q13.2) mutations, which encodes the α subunit of the muscle isoform. The mutations are inherited in a pattern of X-linked inheritance
See more in glycogenosis.