Classification of glycogenoses

Glycogen is a branched homopolymer of glucose with a "tree-like" structure. Glucose residues are linked by an alpha (1-"4)-glycosidic bond, and at the branching points - by an alpha (1-,6)-glycosidic bond. Glycogen is stored in the liver and muscles. Liver glycogen serves primarily to maintain the level of glucose in the blood, while in muscles it is a source of hexose units used during glycolysis in the organ itself. There are 12 known forms of glycogenoses, the most common of them in children are types I, II, III, IX, and in adults - type V. The total frequency of glycogenoses is 1:20,000 live newborns. Glycogenoses are divided into two large groups - with predominant liver damage and with predominant muscle tissue damage. According to the accepted classification, each of the glycogenoses is assigned a number reflecting the sequence of their description.

Liver forms include glycogenosis I, III, IV, VI, liver variant IX and 0. Most of them are characterized by hepatomegaly, decreased blood glucose levels, and growth retardation. Glycogenosis type I is the most severe form of this group, since it disrupts not only the breakdown of glycogen, but also the synthesis of glucose from other sources (gluconeogenesis). Glycogenosis type VI and liver form type IX are the mildest forms of the disease, in which a slight decrease in glucose levels is observed, liver size returns to normal with age, and the quality of life of patients is practically unaffected.

Hepatic forms of glycogenoses

Glycogenosis type I - synonyms: von Gierke disease, glycogen storage disease type I (GSD I). Glycogenosis type I is a hereditary disease that manifests itself at an early age. Glycogenosis type I is divided into two forms - 1a and lb, which are caused by mutations in different genes and differ somewhat clinically.

Glycogenosis type III - synonyms: amylo-1,6-glucosidase deficiency, Cori disease, Forbes disease, glycogen storage disease type HI (GSD III). There are two clinical forms of the disease. Most patients have generalized enzyme deficiency (its activity is reduced in the liver, muscle tissue, leukocytes and skin fibroblasts) - glycogenosis type IIIa; clinically it manifests itself as myopathy and cardiomyopathy. In approximately 15%, clinical manifestations are limited to liver damage - glycogenosis IIIb.

Glycogenosis type IV - synonyms: glycogen branching enzyme deficiency, amylo-1,4:1,6-glucantransferase deficiency, Andersen's disease, amylopectinosis, polyglucose body disease, glycogen storage disease type IV (GSD IV). Several forms are known, differing in age of onset and clinical manifestations.

Glycogenosis type VI - synonyms: glycogen phosphorylase deficiency, Hers disease, glycogen storage disease type VI (GSD VI).

Glycogenosis type IX - synonyms: phosphorylase kinase deficiency, glycogen storage disease type IX (GSD IX). Glycogenosis type IX is a group of hereditary disorders of glycogen storage with different types of inheritance. According to the type of inheritance and clinical manifestations, six subtypes of the disease are distinguished:

• X-linked liver variant (XLG or GSD IXa) is the most common;
• combined liver and muscle variant (GSD IXb);
• autosomal recessive hepatic variant (GSD IXc);
• X-linked muscular variant (GSD IXd);
• autosomal recessive muscular variant (GSD IXe);
• with heart muscle disease (GSD IXf), the type of inheritance of which has not been established.

Glycogenosis type 0 - synonyms: glycogen synthase deficiency, glycogen storage disease type 0 (GSD 0). In this disease, no accumulation of glycogen in the liver is observed, but since the symptoms of the disease are similar to other glycogenoses, it is classified in this group, although this is a disorder not of the breakdown, but of the synthesis of glycogen.

Muscle forms of glycogenosis

At rest, fatty acids are the primary energy source in muscle tissue. During exercise, glucose is also used, most of which comes from liver glycogen. During intense exercise, the primary energy source is anaerobic glycolysis, which results from the breakdown of muscle glycogen. Defects in the enzymes involved in these metabolic pathways affect muscle function.

Glycogenosis type II - synonyms: Pompe disease, acid a-D-glucosidase deficiency, acid maltase deficiency, glycogen storage disease type II (GSD II). Glycogenosis type I is an autosomal recessive disease associated with impaired glycogen metabolism, related to muscle forms of glycogenoses, as well as lysosomal storage diseases. This disease can be classified as a neuromuscular pathology, metabolic myopathy or glycogenosis (Pompe disease is the only glycogenosis in which impaired glycogen breakdown is associated with a defect in lysosome function), and the infantile form of the disease is also classified as a cardiological pathology due to pronounced changes in the heart.

Glycogenosis type V - synonyms: myophosphorylase deficiency, McArdle disease, glycogen storage disease type V (GSD V).

Glycogenosis type VII - synonyms: phosphofructokinase deficiency (PFK), Tarui disease, glycogen storage disease type VII (GSD VII).

Rare forms of muscle glycogenosis

Glycogenosis type IIb - synonyms: Danon disease. glycogen storage disease type lib (GSD lib). pseudo-Pompe disease.

Phosphoglycerate kinase deficiency.

Glycogenosis type XI - synonyms: glycogen storage disease type XI (GSD XI), lactate dehydrogenase deficiency.

Glycogenosis type X - synonyms: phosphoglycerate mutase deficiency (PGAM), glycogen storage disease type X (GSD X).

Glycogenosis type XII - synonyms: glycogen storage disease type XII (GSD XII), aldolase A deficiency.

Glycogenosis type XIII - synonyms: glycogen storage disease type XIII (GSD XIII), deficiency (Z-enolase.

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