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Glycogen Storage Disease, Type Ia
Updated: Nov 5, 2009
Introduction
Background
A glycogen storage disease (GSD) is the result of an enzyme defect. These enzymes normally catalyze reactions that ultimately convert glycogen compounds to glucose. Enzyme deficiency results in glycogen accumulation in tissues. In many cases, the defect has systemic consequences, but in some cases, the defect is limited to specific tissues. Most patients experience muscle symptoms, such as weakness and cramps, although certain GSDs manifest as specific syndromes, such as hypoglycemic seizures or cardiomegaly. (See image below and Image 1.)
Although at least 14 unique GSDs are discussed in the literature, the 4 that cause clinically significant muscle weakness are Pompe disease (GSD type II, acid maltase deficiency), Cori disease (GSD type III, debranching enzyme deficiency), McArdle disease (GSD type V, myophosphorylase deficiency), and Tarui disease (GSD type VII, phosphofructokinase deficiency). One form, von Gierke disease (GSD type Ia, glucose-6-phosphatase [G-6-P] deficiency), causes clinically significant end-organ disease with significant morbidity. The remaining GSDs are not benign but are less clinically significant; therefore, the physician should consider the aforementioned GSDs when initially entertaining the diagnosis of a GSD. Interestingly, GSD type 0, which is due to defective glycogen synthase, also is recognized.
These inherited enzyme defects usually present in childhood, although some, such as McArdle disease and Pompe disease, have separate adult-onset forms. In general, GSDs are inherited as autosomal-recessive conditions. Several different mutations have been reported for each disorder.
Unfortunately, no specific treatment or cure exists, although diet therapy may be highly effective at reducing clinical manifestations. In some cases, liver transplantation may abolish biochemical abnormalities. Active research continues.
Diagnosis depends on patient history, physical examination, muscle biopsy, electromyelography, ischemic forearm test, and creatine kinase levels. Biochemical assay for enzyme activity is the method of definitive diagnosis.
G-6-P deficiency is the specific enzyme deficiency in von Gierke disease. GSD type Ib is a similar condition with the defect in the G-6-P transporter protein. A newly described form, GSD type Ic, does not appear to be related to mutations within the transporter protein.
Pathophysiology
With an enzyme defect, carbohydrate metabolic pathways are blocked and excess glycogen accumulates in affected tissues. Each GSD represents a specific enzyme defect, and each enzyme is in specific, or most, body tissues. As noted above, G-6-P, which is found in the liver and kidney, is the specific enzyme that is deficient in von Gierke disease. Glucose-6-phosphate is an intermediate in the glycogen pathway.
Von Gierke disease is an autosomal-recessive condition. Von Gierke disease may be explained by mutations of the phosphohydrolase catalytic unit gene of the G-6-P complex, unlike GSD type Ib and GSD type Ic.
Deficiency of G-6-P blocks the final steps of glycogenolysis and gluconeogenesis.1 This results in severe hypoglycemia. Glucose production increases with age, making hypoglycemia less of an issue.
Because glucose cannot leave the hepatocyte phosphorylated, an increase in glycolytic pathway metabolites occurs. These intermediates are metabolized into lactate. Lactate may provide the brain with a ready-to-use energy source. By competing with uric acid, lactate decreases renal clearance, resulting in hyperuricemia. Glucose also is shunted into making more triglycerides, causing an increase in low-density and very low-density lipoproteins.2
Frequency
International
Herling and colleagues studied the incidence and frequency of inherited metabolic conditions in British Columbia. GSDs are found in 2.3 children per 100,000 births per year.
Mortality/Morbidity
Immediate morbidity arises from hypoglycemic seizures. Serious long-term complications resulting in morbidity and mortality include nephropathy and hepatic adenoma.
Sex
GSDs are autosomal-recessive conditions, with an equal number of males and females being affected.
Age
In general, GSDs present in childhood. Later onset correlates with a less severe form.
Clinical
History
- Initial presentation may be active seizures, specifically hypoglycemic seizures.
- Global muscle weakness is not a uniform feature of von Gierke disease. Schwahn and colleagues found height, weight, bone mass and grip force decreased in one group of GSD 1a patients.3
- Patients may give a history of kidney stones or gout.
- Patients may have had pancreatitis.
Physical
- Physical examination may reveal hepatomegaly. Because many causes of hepatic injury exist, suspicion of glycogen storage disease (GSD) must be high.
- Hypotonia is found in infants.
- Hypoglycemia is concerning and may lead to hypoglycemic seizures.
- Xanthomas may be present on the buttocks or extensor surfaces.
- Acute manifestations of gout may be observed.
- Hypertension or other manifestations of renal failure may be present.
- Short stature may be seen in the untreated patient.
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References
Jones JG, Garcia P, Barosa C, et al. Hepatic anaplerotic outflow fluxes are redirected from gluconeogenesis to lactate synthesis in patients with Type 1a glycogen storage disease. Metab Eng. May 2009;11(3):155-62. [Medline].
Bandsma RH, Prinsen BH, van Der Velden Mde S, et al. Increased de novo lipogenesis and delayed conversion of large VLDL into intermediate density lipoprotein particles contribute to hyperlipidemia in glycogen storage disease type 1a. Pediatr Res. Jun 2008;63(6):702-7. [Medline].
Schwahn B, Rauch F, Wendel U, Schönau E. Low bone mass in glycogen storage disease type 1 is associated with reduced muscle force and poor metabolic control. J Pediatr. Sep 2002;141(3):350-6. [Medline].
Kalkan Ucar S, Coker M, et al. A monocentric pilot study of an antioxidative defense and hsCRP in pediatric patients with glycogen storage disease type IA and III. Nutr Metab Cardiovasc Dis. Jul 2009;19(6):383-90. [Medline].
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Fernandes J, Smit G. The Glycogen Storage Diseases. In: Fernandes J, Saudubray JM, Van Den Berghe G, eds. Inborn Metabolic Diseases: Diagnosis and Treatment. 3rd ed. New York, NY: Springer-Verlag; 2000:87-101.
Geberhiwot T, Alger S, McKiernan P, Packard C, Caslake M, Elias E. Serum lipid and lipoprotein profile of patients with glycogen storage disease types I, III and IX. J Inherit Metab Dis. Jun 2007;30(3):406. [Medline].
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Hou DC, Kure S, Suzuki Y. Glycogen storage disease type Ib: structural and mutational analysis of the microsomal glucose-6-phosphate transporter gene. Am J Med Genet. Sep 17 1999;86(3):253-7. [Medline].
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Further Reading
Clinical guidelines:
AASLD practice guidelines: evaluation of the patient for liver transplantation. American Association for the Study of Liver Diseases - Private Nonprofit Research Organization. 2000 Jan (revised 2005 Jun). 26 pages. NGC:004333 :
Clinical trials:
Growth and Development Study of Myozyme
Pompe Disease Registry
Pompe Lactation Sub-Registry
Pompe Pregnancy Sub-Registry
Pompe Prevalence Study in Patients With Muscle Weakness Without Diagnosis (POPS)
Keywords
glycogen storage disease, von Gierke's disease, gluconeogenesis, glucose 6 phosphatase, glucose 6 phosphatase deficiency, G6P, von Gierke disease, glycogen storage disease type Ia, GSD type Ia, hepatonephromegaly glycogenica


Overview: Glycogen Storage Disease, Type Ia