Pathogenesis and Differences from Aglycogenoses
The etiology of glycogenoses involves the congenital absence or reduced activity of enzymes responsible for the mobilization (breakdown) of stored carbohydrate. As a result, glycogen accumulates in excess within target organs. Depending on the specific mutation, the structure of the polymer itself may remain completely normal or be pathologically altered.
The main metabolic disturbance in this group of diseases is hypoglycemia, a drop in blood glucose levels. This occurs because blocked enzymatic pathways prevent tissues from releasing energy substrates into the systemic circulation.
There is also a diametrically opposite group of disorders known as aglycogenoses. Their cause is a defect in glycogen synthesis (for example, due to hepatic glycogen synthase deficiency). In this case, tissue glycogen levels are drastically reduced, but the primary metabolic disturbance remains the same—patients suffer from severe hypoglycemia.
Classification of Glycogen Storage Diseases
Different types of glycogenoses are differentiated by their biochemical defect and the localization of the deficient enzyme. The most notable variants are presented in the table below:
| Disease | Defective Enzyme | Localization | Manifestations and Glycogen Structure |
|---|---|---|---|
| Von Gierke disease | Glucose-6-phosphatase | Liver, kidneys | Normal glycogen structure accumulation, hypoglycemia, acidosis, hyperuricemia |
| Cori disease | Amylo-1,6-glucosidase | Liver, muscles | Accumulation of polymer with short outer branches |
| Andersen disease | Amylo-1,6-glucosidase (branching enzyme) | Liver, spleen | Glycogen with long outer branches and sparse branching points |
| McArdle disease | Glycogen phosphorylase | Skeletal muscles | Accumulation of muscle glycogen with normal structure |
| Hers disease | Glycogen phosphorylase | Liver | Accumulation of hepatic glycogen with normal structure |
Additionally, there are disorders involving impaired cAMP-dependent protein kinase or liver glycogen phosphorylase kinase. The clinical picture in these cases is similar to Hers disease.
Von Gierke Disease: Detailed Analysis (Type I Glycogenosis)
Von Gierke disease is caused by an inherited defect of the enzyme glucose-6-phosphatase, which is localized in the liver and kidneys. Its absence critically impairs the final step of glucose release into the blood.
Metabolic alterations in Von Gierke disease follow this sequence:
- Postprandial state: Glycogen synthesis proceeds without hindrance and is even enhanced due to abundant substrates. The polymer massively accumulates in hepatocytes, leading to organ enlargement (hepatomegaly).
- Mobilization of reserves: Breakdown starts normally. Glycogen phosphorylase yields glucose-1-phosphate, which phosphoglucomutase converts to glucose-6-phosphate. However, the chain blocks at this point.
- Fasting: In the postabsorptive period, free glucose release into the blood is absent or severely reduced because defective glucose-6-phosphatase cannot cleave the phosphate group. Severe hypoglycemia develops.
Simultaneously, lactate accumulates, causing lactic acidosis, and uric acid levels rise (hyperuricemia).
Differences in Carbohydrate Metabolism: Liver vs. Muscles
Understanding the pathogenesis of glycogenoses requires knowing the difference between hepatic and muscular metabolic pathways. Normally, the enzyme glucose-6-phosphatase is absent in skeletal muscles.
This means muscle glycogen is never used to maintain systemic blood glucose levels. It is consumed exclusively for local needs—providing energy for muscle contraction. Conversely, the liver serves as the primary reservoir tasked with supplying the systemic circulation and protecting the brain from hypoglycemia between meals. This is why a defect in hepatic glucose-6-phosphatase has such catastrophic systemic consequences.