Chapter 5 of 5 - Glycogenesis Course
Connecting glycogen biochemistry to the clinic: inherited defects produce predictable patterns of hypoglycemia, organomegaly, myopathy, or cirrhosis depending on which step of synthesis or breakdown is impaired.
Glycogen storage diseases (GSDs) are a group of inherited metabolic disorders caused by deficiencies in enzymes (or, for some subtypes, transporters) involved in glycogen synthesis, cytosolic breakdown, or lysosomal degradation. They are traditionally classified by type number (I, II, III, and so on) and by the affected tissue phenotype (hepatic, muscular, or mixed).
The majority of classic GSDs follow autosomal recessive inheritance. Clinical severity ranges from life-threatening hypoglycemia in infancy to exercise intolerance in adulthood, depending on the enzyme step and residual activity.
The table summarizes high-yield types tested in biochemistry and medicine courses. Many additional GSDs and variant subtypes exist; always correlate with enzymatic and molecular diagnosis.
| Type | Deficient enzyme / protein | Key clinical features |
|---|---|---|
| 0 | Glycogen synthase deficiency | Reduced ability to synthesize glycogen; fasting hypoglycemia; hepatic and muscle forms described |
| I (Von Gierke) | Glucose-6-phosphatase system (including transporter subtypes Ia, Ib, etc.) | Severe fasting hypoglycemia, hepatomegaly, lactic acidosis, hyperuricemia, hyperlipidemia; enlarged kidneys common |
| II (Pompe) | Acid alpha-glucosidase (acid maltase) | Lysosomal glycogen accumulation; infantile form with cardiomegaly and hypotonia; later-onset forms with progressive skeletal muscle weakness |
| III (Cori / Forbes) | Glycogen debranching enzyme (amylo-1,6-glucosidase / transferase) | Hepatomegaly, hypoglycemia, short outer chains limit phosphorylase action; generally milder course than type I in many patients |
| IV (Anderson) | Glycogen branching enzyme | Abnormal poorly branched glycogen (polyglucosan / amylopectin-like); progressive hepatic disease with fibrosis or cirrhosis; neuromuscular variants |
| V (McArdle) | Muscle glycogen phosphorylase | Exercise intolerance, muscle cramps, myoglobinuria after strenuous activity; second-wind phenomenon with oral glucose |
Quick Check
Which glycogen storage disease is caused by a deficiency of the branching enzyme?
Hepatic GSDs (notably types I, III, IV, and VI) typically present in infancy or childhood with hepatomegaly and, when gluconeogenic export is impaired or glycogenolysis is limited, fasting hypoglycemia. Type VI (Hers disease) reflects liver glycogen phosphorylase deficiency - less severe hypoglycemia than type I in many cases, but still a hepatic storage pattern.
Muscle GSDs such as type V (McArdle) and type VII (Tarui disease, phosphofructokinase deficiency) primarily cause exercise intolerance, muscle cramps, and sometimes rhabdomyolysis or myoglobinuria because skeletal muscle cannot mobilize glycogen adequately for anaerobic glycolysis.
Pompe disease (type II) is distinctive: deficiency of lysosomal acid alpha-glucosidase leads to glycogen accumulation in lysosomes of cardiac and skeletal muscle (and other tissues), overlapping clinically with both storage myopathies and infantile cardiomyopathy in severe cases.
Diagnosis rests on clinical phenotype, biochemical markers (lactate, ketones, liver enzymes, CK), enzyme activity assays in blood, fibroblasts, or muscle, genetic testing, and sometimes liver or muscle biopsy with electron microscopy and special stains.
Treatment is disease-specific: frequent feeds and uncooked cornstarch to prolong euglycemia in hepatic GSDs; enzyme replacement therapy for Pompe disease; liver transplantation in selected severe hepatic GSDs; avoidance of high-intensity anaerobic exercise and tailored activity plans in muscle GSDs. Multidisciplinary metabolic and neuromuscular care improves outcomes.
Fill in the Blank
Type I glycogen storage disease (Von Gierke disease) is caused by a deficiency of ________ which prevents the liver from releasing free glucose, leading to severe fasting hypoglycemia.
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