Glycogen

Schematic two-dimensional cross-sectional view of glycogen: A core protein of glycogenin is surrounded by branches of glucose units. The entire globular granule may contain around 30,000 glucose units.<ref>{{{last}}}, William D. McArdle, Frank I. Katch, Victor L. Katch, Exercise physiology: energy, nutrition, and human performance. online version, 6 edition, Lippincott Williams & Wilkins, 2006, ISBN 978-0-7817-4990-9,</ref>

A view of the atomic structure of a single branched strand of glucose units in a glycogen molecule.

Glycogen (black granules) in spermatozoa of a flatworm; transmission electron microscopy, scale: 0.3 µm

Glycogen is a multibranched polysaccharide of glucose that serves as a form of energy storage.

Glycogen is a complex, multibranched polysaccharide of glucose that plays a crucial role in the body's energy storage. Its structure enables rapid glucose release when needed for energy production.<ref>Berg, Jeremy M.,

 Biochemistry, 
 , 
 2007,</ref> Disruptions in the formation and utilization of glycogen can lead to a group of metabolic disorders known as Glycogen storage diseases (GSDs).

Glycogen Storage Diseases (GSDs)

Glycogen storage diseases encompass a variety of conditions triggered by the body's inability to produce or breakdown glycogen effectively. There are numerous types of GSDs, each associated with specific gene mutations that affect various aspects of glycogen metabolism.

GSD Type 0

One such type is GSD Type 0, which has two forms: muscle GSD 0 and liver GSD 0. In muscle GSD 0, glycogen synthesis in muscle tissues is impaired, while in liver GSD 0, glycogen synthesis in the liver is compromised.<ref>Kishnani, PS,

 Glycogen Storage Diseases, 
 Neurologist, 
 2007,
 Vol. 13(Issue: 1),
 pp. 15–24,
 DOI: 10.1097/01.nrl.0000252947.02611.8c,</ref>

Signs and Symptoms

Muscle GSD 0

Individuals affected by muscle GSD 0 usually manifest signs and symptoms in early childhood. Symptoms may include muscle pain and weakness, fainting episodes, long QT syndrome (a heart rhythm condition), and an elevated risk of sudden cardiac arrest, especially after physical activity. Sudden death from cardiac arrest can occur in childhood or adolescence.<ref>Orho, M,

 Mutations in the liver glycogen synthase gene in children with hypoglycemia of infancy, 
 Nature Genetics, 
 1996,
 Vol. 13(Issue: 4),
 pp. 427–430,
 DOI: 10.1038/ng0896-427,</ref>

Liver GSD 0

In contrast, signs and symptoms of liver GSD 0 typically emerge in infancy. These may include hypoglycemia, lethargy, pallor, and nausea during periods of fasting, and ketosis. Over time, liver GSD 0 may also result in developmental delays and growth failure.<ref>Newgard, CB,

 Glycogen Storage Disease Type 0, 
 GeneReviews, 
 2003,
 
 
 
 
 
 
 Full text,</ref>

Prevalence

The prevalence of GSD 0 is currently unknown, with fewer than 10 reported cases of muscle GSD 0 and fewer than 30 reported cases of liver GSD 0 in the scientific literature. As many cases might be undiagnosed due to sudden early death or mild symptoms, GSD 0 is thought to be underdiagnosed.<ref>Bali, DS,

 Glycogen storage diseases: new perspectives, 
 World J Gastroenterol, 
 2012,
 Vol. 18(Issue: 18),
 pp. 2147–2151,
 DOI: 10.3748/wjg.v18.i18.2147,</ref>

Causes and Pathophysiology

Muscle GSD 0

Muscle GSD 0 is caused by mutations in the GYS1 gene, which encodes the enzyme glycogen synthase, crucial for glycogen formation. This enzyme is abundant in cardiac and skeletal muscles. During cardiac muscle contractions or rapid/sustained skeletal muscle movement, glycogen stored in muscle cells is broken down to supply the cells with energy. Mutations in the GYS1 gene disrupt this process, leading to a lack of functional glycogen synthase and a consequent deficiency in glycogen storage.<ref>Burwinkel, B,

 Fatal congenital heart glycogenosis caused by a recurrent activating R531Q mutation in the gamma 2-subunit of AMP-activated protein kinase (PRKAG2), not by phosphorylase kinase deficiency, 
 American Journal of Human Genetics, 
 2005,
 Vol. 76(Issue: 6),
 pp. 1034–1049,
 DOI: 10.1086/430843,</ref>

Liver GSD 0

Liver GSD 0 is caused by mutations in the GYS2 gene. This gene encodes liver glycogen synthase, responsible for glycogen storage in the liver. Liver glycogen is rapidly broken down when glucose is required to maintain normal blood sugar levels between meals. GYS2 gene mutations result in a lack of functional liver glycogen synthase, impeding the production of glycogen from glucose. Consequently, liver cells are devoid of a vital source of stored energy, leading to the signs and symptoms associated with GSD 0.<ref>Orho, M,

 Mutations in the liver glycogen synthase gene in children with hypoglycemia of infancy, 
 Nature Genetics, 
 1996,
 Vol. 13(Issue: 4),
 pp. 427–430,
 DOI: 10.1038/ng0896-427,</ref>

Treatment and Management

Currently, there is no cure for GSD 0. Management strategies primarily aim at controlling symptoms and preventing complications. For instance, regular meals and snacks can help prevent hypoglycemia in liver GSD 0, while physical activity moderation can reduce symptoms in muscle GSD 0. More research is needed to explore potential treatment options and improve the quality of life for individuals with GSD 0.<ref>Kishnani, PS,

 Glycogen Storage Diseases, 
 Neurologist, 
 2007,
 Vol. 13(Issue: 1),
 pp. 15–24,
 DOI: 10.1097/01.nrl.0000252947.02611.8c,</ref>

Prognosis

The prognosis for individuals with GSD 0 varies depending on the severity and specific subtype of the disease. For some, the signs and symptoms are mild and may go unnoticed for years. These individuals can live relatively normal lives with proper management of their blood sugar levels. For those with severe symptoms, particularly individuals with muscle GSD 0, the disease can be life-threatening due to the risk of cardiac arrest and sudden death. Research into the natural history of the disease is ongoing to better understand the progression and long-term outcomes of GSD 0.<ref>Labrune, P.,

 Glycogen storage disease type 0, 
 Orphanet Journal of Rare Diseases, 
 2012,
 Vol. 7,
 pp. 13,
 DOI: 10.1186/1750-1172-7-13,</ref>

See Also

• Glycogen storage disease
• Glucose
• Glycogen synthase
• Glycogenolysis
• Glycogenesis

External Links

• Orphanet: Glycogen storage disease due to glycogen synthase deficiency
• GHR: Glycogen storage disease type 0
• NORD: Glycogen storage disease type 0

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