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The scope of GSD VI now also includes glycogen storage disease type VIII, [2] IX [2] (caused by phosphorylase b kinase deficiency) and X [2] (deficiency protein kinase A). The incidence of GSD VI is approximately 1 case per 65,000–85,000 births, [2] representing approximately 30% all cases of glycogen storage disease.
Without this process, the body is unable to liberate glycogen from the liver and convert it into blood glucose, leading to an accumulation of stored glycogen in the liver. Hepatomegaly from the accumulation of stored glycogen in the liver is considered a form of non-alcoholic fatty liver disease.
A glycogen storage disease (GSD, also glycogenosis and dextrinosis) is a metabolic disorder caused by a deficiency of an enzyme or transport protein affecting glycogen synthesis, glycogen breakdown, or glucose breakdown, typically in muscles and/or liver cells.
The long unbranched molecules have low solubility, leading to glycogen precipitation in the liver. These deposits subsequently build up in the body tissue, especially the heart and liver. The inability to break down glycogen in muscle cells causes muscle weakness. The probable result is cirrhosis and death within five years.
The two organs most commonly affected are the liver and the skeletal muscle. Glycogen storage diseases that affect the liver typically cause hepatomegaly and hypoglycemia; those that affect skeletal muscle cause exercise intolerance, progressive weakness and cramping. [1] Glucose-6-phosphate isomerase deficiency affects step 2 of glycolysis.
Glycogenesis is the process of glycogen synthesis or the process of converting glucose into glycogen in which glucose molecules are added to chains of glycogen for storage. This process is activated during rest periods following the Cori cycle , in the liver , and also activated by insulin in response to high glucose levels .
Clinical manifestations of glycogen storage disease type III are divided into four classes: [3] GSD IIIa, is the most common, (along with GSD IIIb) and which clinically includes muscle and liver involvement; GSD IIIb, which clinically has liver involvement but no muscle involvement; GSD IIIc which clinically affects liver and muscle.
The different functions of glycogen in muscle or liver make the regulation mechanisms of its metabolism differ in each tissue. [7] These mechanisms are based mainly on the differences on structure and on the regulation of the enzymes that catalyze synthesis, glycogen synthase (GS), and degradation, glycogen phosphorylase (GF).