Glycogen Storage Disease Type IX
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NORD gratefully acknowledges Priya S. Kishnani, MD and Jennifer Goldstein, PhD, CGC, Division of Medical Genetics, Department of Pediatrics, Duke University Health System, for assistance in the preparation of this report.
Synonyms of Glycogen Storage Disease Type IX
- PhK deficiency
- phosphorylase kinase deficiency
- glycogen storage disease type Ixa
- glycogen storage disease type Ixb
- glycogen storage disease type Ixc
- glycogen storage disease type Ixd
Glycogen storage disease type IX (GSD-IX) is a group of at least four disorders characterized by a deficiency of the enzyme phosphorylase kinase. This enzyme is necessary to break down (metabolize) a type of complex sugar known as glycogen. Normally, glycogen is metabolized into a simple sugar known as glucose. Glucose is one of the main sources of energy for the body. When there is excess glycogen, it is stored in the body, primarily in the liver and muscles and, when the body needs more energy, is eventually converted into glucose. Because individuals with GSD-IX cannot properly break down glycogen, excess amounts accumulate in the liver, muscles, or both. GSD-IX is sometimes categorized into a liver form (caused by phosphorylase kinase deficiency in the liver, or liver and muscle) and muscle form, which is rare and is caused by phosphorylase kinase deficiency in the muscle only.
Common symptoms of the liver form include abnormal enlargement of the liver (hepatomegaly), unusually low levels of blood glucose (hypoglycemia) during fasting, and growth delays. The specific symptoms that develop and the overall severity of GSD-IX can vary greatly from one individual to another, even among individuals with the same subtype. The liver form of GSD-IX is inherited in either an X-linked or autosomal recessive manner and can be caused by a mutation in one of three different genes. Mutations in only one gene have been found in individuals with the muscle form. This form is rare and is inherited in an X-linked manner.
GSD-IX is part of a larger group of disorders in which the body cannot metabolize glycogen into glucose (glycogen storage diseases). The underlying cause is different for each glycogen storage disease. GSD-IX was first described in the medical literature in 1966 by Dr. Hug, et al. They reported on a young girl with phosphorylase kinase deficiency of the liver that was consistent with autosomal recessive inheritance. Later on, similar individuals were described in the medical literature whose cases were more consistent with X-linked inheritance. This second group of individuals was originally classified as having glycogen storage disease type VIII. However, the X-linked form is now classified as a subtype of GSD-IX since the disorder involves the same enzyme complex as the autosomal recessive forms. The classification of GSD-VIII is no longer used (obsolete).
Glycogen Storage Disease Type IX Resources
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