What is a glycogen storage disease?

A glycogen storage disease is a genetic defect that causes a metabolic enzyme to become inactive. It inhibits a cell's ability to either generate  or utilise glycogen.

How do we get  glycogen storage diseases ?

Glycogen storage diseases are inherited from parents who carry copies of a defective gene.

What are the general symptoms of glycogen storage diseases?

Symptoms vary among the types of glycogen storage diseases, but common symptoms include low blood sugar, enlarged liver, retarded growth, and abnormal blood biochemistry.

Are all glycogen storage diseases fatal?

No. Depending on the type of disease, the ethnic background and lifestyle of the patient, some glycogen storage diseases may be so mild that they go unnoticed.

How are glycogen storage diseases diagnosed?

A definite diagnosis can be made by testing a liver biopsy sample for glycogen concentration, and by assaying the sample for enzyme activity. Distinguishing symptoms of glycogen storage diseases are type-specific.

In a suspected glycogen storage disease, what information could be gained from a liver biopsy?

A liver biopsy could determine the activity of one of several enzymes in question. It might be used to make a firm diagnosis in suspected cases of von Gierke's Disease or Pompe's Disease. However, a liver biopsy is an invasive procedure and should only be performed when a definite diagnosis cannot be made by other methods.

How might a carrier for a glycogen storage disease be diagnosed?

An allele-specific oligonucleotide hybridization test, if positive, would be conclusive.One could also use full-length human cDNA that has the gene sequence for the enzyme in question (e.g. glycogen phosphorylase for McArdle's Disease) as a probe for restriction fragment length polymorphism (RFLP).

What is the approximate frequency of occurrence of Pompe's Disease in live births?

1 in 100,000

Why do patients with McArdle's Disease often excrete burgundy-colored urine?

Their urine contains myoglobin, which is burgundy-colored.

Are glycogen storage diseases dominant or recessive?

All glycogen storage diseases are recessive.

In order for a child to have a glycogen storage disease, must both parents have a defective gene?

Yes. Both parents must have at least one defective copy of the gene, but neither parent must express the disease.

When is the  mother  described as a "carrier" for a glycogen storage disease.?

This means she has two copies of a gene that codes for an enzyme involved in glycogen storage; one copy is healthy, the other is defective. She can pass along either copy to her offspring with equal probability. (If the father is also a carrier, their offspring has a 25% chance of inheriting the disease, and a 50% chance of becoming a carrier.)

Which organs and tissues are most directly affected by glycogen storage diseases?

The liver is the body's primary repository of glycogen, so most glycogen storage diseases have a direct adverse effect of the liver. These diseases also affect muscle tissue, which must comply with sudden demands for energy.

Which enzyme is deficient in McArdle's Disease?

Glycogen phosphorylase, the enzyme that releases glucose from the non-reducing end of a glycogen polymer.

 Why do patients with McArdle's Disease experience muscle cramps and fatigue?

Their muscles lack the glucose necessary for ATP production(energy source). Note: since little glucose is available to start with, glycolysis does not form lactate in large quantity. Therefore muscle fatigue is not the result of lactic acid buildup. This is a common misconception about McArdle's Disease.

What is an alternate energy source for patients with McArdle's Disease?

Creatine kinase can convert phosphocreatine into creatine, meanwhile phosphorylating ADP to make ATP. However, this process provides energy for only about four seconds of physical exertion.

Which enzyme is deficient in Pompe's Disease?

Lysosomal-1,4-glucosidase, also known as acid maltase, which is normally responsible for digesting glucose in lysosomes.

Why does Pompe's Disease affect every organ in the body?

Pompe's Disease affects the functions of lysosomes, which are found in all types of tissue.

Why is Pompe's Disease sometimes called Acid Maltase Deficiency?

The interior of a lysosome is more acidic than the cytosol, and lysosomal enzymes are more active at low pH than at neutral pH.

Which enzyme is deficient in von Gierke's Disease?

Glucose-6-phosphatase, the enzyme that converts glucose-6-phosphate to glucose as it passes from the cytosol into the endoplasmic reticulum in the liver.

What reaction is catalyzed by glucose-6-phosphatase?

Glucose-6-phosphatase catalyzes the conversion of glucose-6-phosphate to glucose.

Where in the body is glucose-6-phosphatase located? Where is it not located?

Glucose-6-phosphatase is located in liver and kidney cells. It is not located in muscle or brain cells.

Why can't glucose-6-phosphate cross the endoplasmic reticulum membrane?

Glucose-6-phosphate is negatively charged. Only uncharged molecules can cross the membrane.

What treatments exist for glycogen storage diseases?

There are no cures for any of the glycogen storage diseases. The less severe diseases can be treated with dietary restrictions, usually high-protein, low-carbohydrate diets. Palliative care may be the only option in more severe cases.

How long must dietary restrictions continue?

Dietary restrictions must continue throughout life.


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