Glycogen Storage Disorders (GSDs)

Introduction

Glycogen Metabolism - Brief Review (Basis for Understanding GSDs)

1. Glucose is phosphorylated to glucose-6-phosphate (by glucokinase in liver, hexokinase in muscle)
2. Phosphoglucomutase converts glucose-6-phosphate → glucose-1-phosphate
3. Glucose-1-phosphate + UTP → UDP-glucose (the activated glucose donor)
4. Glycogenin (autoglycosylating protein) forms the primer chain
5. Glycogen synthase adds glucose via α-1,4 glycosidic bonds (linear chains)
6. Branching enzyme creates α-1,6 branch points

1. Phosphorylase kinase activates phosphorylase
2. Glycogen phosphorylase releases glucose-1-phosphate from α-1,4 linkages (until 4 residues from a branch point)
3. Debranching enzyme (with two activities: 4-α-glucanotransferase and α-1,6-glucosidase) removes the branch residues, releasing free glucose
4. A small amount of glycogen is catabolized in lysosomes by acid maltase (α-1,4-glucosidase)

Classification

Major Types of Glycogen Storage Diseases

Type I - Von Gierke Disease (Hepatorenal GSD)

• Deficient enzyme: Glucose-6-phosphatase (Type Ia) or glucose-6-phosphate translocase (Type Ib)
• Affected tissue: Liver, kidneys, intestine
• Pathophysiology: Glucose-6-phosphate cannot be converted to free glucose → unable to maintain blood glucose during fasting. Glucose-6-phosphate is shunted to glycolysis → excess lactate, fatty acid synthesis → hyperlipidemia
• Clinical features:
  • Severe fasting hypoglycemia (most dangerous feature)
  • Hepatomegaly (massive, due to glycogen and fat accumulation)
  • Lactic acidosis
  • Hyperuricemia (gout in older patients)
  • Hyperlipidemia (hypertriglyceridemia, hypercholesterolemia)
  • Doll-like facies with round cheeks and thin extremities
  • Short stature
  • Nephromegaly
  • Type Ib additionally shows neutropenia, recurrent bacterial infections, and inflammatory bowel disease
• Complications: Hepatic adenomas (risk of malignant transformation), renal failure, pulmonary hypertension
• Lab findings: Low blood glucose, high lactate, elevated uric acid, elevated triglycerides
• Treatment: Frequent feeds; uncooked cornstarch (provides sustained glucose release); avoidance of fructose and galactose; liver transplant in severe cases

Type II - Pompe Disease (Glycogenosis Type II / Lysosomal GSD)

• Deficient enzyme: Acid α-glucosidase (acid maltase) - a lysosomal enzyme
• Affected tissue: All tissues (heart, muscle, liver, CNS)
• Pathophysiology: Glycogen accumulates within lysosomes. Pompe disease is unique as it is also a lysosomal storage disease
• Clinical forms:
  • Classic infantile form: Presents within the first few months of life; massive cardiomegaly (hypertrophic cardiomyopathy), hypotonia ("floppy baby"), respiratory failure, and hepatomegaly. Death usually occurs within the first year from cardiorespiratory failure
  • Late-onset / adult form: Progressive proximal muscle weakness, respiratory muscle weakness, no cardiomegaly; often mistaken for limb-girdle muscular dystrophy
• Lab findings: Elevated CK, vacuolar myopathy on biopsy; confirmed by acid alpha-glucosidase enzyme assay (dried blood spot or leukocytes) and GAA gene mutation analysis
• Treatment: Enzyme replacement therapy (ERT) with alglucosidase alfa (myozyme) - a major therapeutic advance; gene therapy is emerging

Type III - Cori Disease / Forbes Disease (Debrancher Deficiency)

• Deficient enzyme: Glycogen debranching enzyme (amylo-1,6-glucosidase)
• Subtypes: Type IIIa (liver + muscle), Type IIIb (liver only)
• Pathophysiology: Debranching enzyme cannot remove branch points → abnormal glycogen with short outer chains accumulates (limit dextrin)
• Clinical features: Hepatomegaly, fasting hypoglycemia (milder than Type I), growth retardation, muscle weakness and wasting (in IIIa), hyperlipidemia. Unlike Type I, lactic acidosis and hyperuricemia are absent/mild
• Complications: Progressive liver fibrosis, hepatic adenomas; cardiomyopathy and peripheral neuropathy in adulthood
• High-risk populations: Faroe Islanders, Native Canadians (Inuit)

Type IV - Andersen Disease (Brancher Deficiency)

• Deficient enzyme: Glycogen branching enzyme (α-1,4 to α-1,6 glucan transferase)
• Pathophysiology: Abnormal glycogen with very long unbranched chains (amylopectin-like / polyglucosan bodies) accumulates. The abnormal glycogen is immunogenic and triggers a foreign-body reaction
• Clinical features:
  • Hepatic form (classic childhood): Failure to thrive, progressive hepatosplenomegaly, liver cirrhosis, liver failure - usually fatal before age 5 years without liver transplant
  • Adult form (Adult Polyglucosan Body Disease / APBD): After age 40 years; upper and lower motor neuron involvement, neurogenic bladder, lower limb spasticity and weakness, peripheral neuropathy, cognitive impairment and dementia
• High-risk populations: Ashkenazi Jews (for APBD)

Type V - McArdle Disease (Muscle Phosphorylase Deficiency)

• Deficient enzyme: Muscle glycogen phosphorylase (myophosphorylase)
• Affected tissue: Skeletal muscle only (liver phosphorylase is normal)
• Pathophysiology: Muscle cannot break down glycogen to generate energy during exercise
• Clinical features: Presents in young adults
  • Exercise intolerance, painful muscle cramps during exercise (particularly intense or isometric exercise)
  • Myoglobinuria (reddish-brown urine after exercise) due to rhabdomyolysis
  • "Second wind" phenomenon - characteristic feature: after brief rest, patient can resume moderate exercise as the body switches to fatty acid and blood glucose utilization
  • Fixed proximal muscle weakness develops in later life
• Lab findings: No rise in venous lactate with ischemic forearm exercise test (because muscle cannot break down glycogen); elevated CK; myoglobinuria
• Treatment: Avoid intense exercise; adequate warm-up; sucrose intake before exercise

Type VI - Hers Disease (Liver Phosphorylase Deficiency)

• Deficient enzyme: Hepatic glycogen phosphorylase
• Clinical features: Mild hepatomegaly, mild fasting hypoglycemia, elevated liver enzymes. Generally benign prognosis; symptoms resolve with age
• Treatment: Frequent feeds

Type VII - Tarui Disease (Phosphofructokinase Deficiency)

• Deficient enzyme: Muscle phosphofructokinase (PFK)
• Clinical features: Similar to McArdle disease - exercise intolerance, cramps, myoglobinuria; additionally associated with hemolytic anemia (as PFK is also present in red blood cells)
• "Out of wind" phenomenon - unlike McArdle disease, sucrose intake before exercise worsens symptoms (because glucose bypasses the PFK block but inhibits fatty acid mobilization)

Type IX - Phosphorylase Kinase Deficiency

• Deficient enzyme: Phosphorylase kinase
• X-linked (PHKA2) - most common form, affecting the liver
• Clinical features: Hepatomegaly, mild fasting hypoglycemia. Generally benign; most children improve spontaneously with age

Summary Table of Key Glycogen Storage Diseases

Diagnosis

1. Clinical suspicion based on symptoms (age of onset, organ involvement)
2. Biochemical tests: Blood glucose (low), lactate (elevated in Type I), uric acid, liver function tests, CK (elevated in muscle types)
3. Ischemic forearm exercise test: No rise in lactate = muscle glycogenolysis defect (Type V, VII)
4. Enzyme assay: Specific enzyme activity in leukocytes, erythrocytes, liver/muscle biopsy
5. Genetic/molecular testing: Mutation analysis of disease-specific genes (gold standard and increasingly first-line)
6. Liver/muscle biopsy: Histology showing glycogen accumulation (PAS positive); electron microscopy for lysosomal glycogen (Type II)
7. Newborn screening: Dried blood spot enzyme assay available for Pompe disease

Treatment (General Principles)

Conclusion