Glycogen and Glycogen Storage Diseases
Glycogen is the storage form of glucose (sugar) in the body. It is a complex material made of individual glucose molecules linked together in long chains with many branches off the chains (just like a tree). Glycogen is mainly stored in the liver and muscle cells, but the kidneys and intestines also store some limited amounts of glycogen.
Metabolism is the process by which our body breaks down the food we eat and converts it to energy. To briefly review metabolism: a simple form of sugar, called glucose, is our bodies' main source of energy. After we eat, we have too much glucose in our blood, so our bodies store the extra glucose in the form of glycogen (much like we deposit our extra money in a bank). When our bodies need more energy, certain enzymes convert the glycogen back to glucose and withdraw it from the liver and the muscles (just like we withdraw spending money from the bank).
Glycogen Storage Disease (GSD)
The underlying problem in all of the Glycogen Storage Diseases is the use and storage of glycogen. Sometimes GSDs are also referred to as glycogenoses because they are caused by difficulty in glycogen metabolism.
All of the Glycogen Storage Diseases are considered inherited metabolic disorders. A metabolic disorder is a disease that disrupts metabolism. Therefore, a person who has a metabolic disorder has a difficult time breaking down certain foods and creating energy. A metabolic disease is most frequently caused by an absence or deficiency in an enzyme (or protein). An enzyme can act to help the body break down food into energy. There are many enzymes in the body and each act like a machine on an assembly line. When one of the enzymes is not working properly, the process of breaking down of specific foods can go more slowly or shut down completely.
A person with a glycogen storage disease (GSD) has an absence or deficiency of one of the enzymes responsible for making or breaking down glycogen in the body. This is called an enzyme deficiency. The enzyme deficiency causes either abnormal tissue concentrations of glycogen (too much or too little) or incorrectly or abnormally formed glycogen (shaped wrong). Depending on the type of GSD a person has, their enzyme deficiency may be important in all parts of the body, or only in some parts of the body, like the liver or muscle. Typically, the forms of GSD are described by the part of the body that has trouble because of the enzyme deficiency. The categories most often are: the liver only, the muscles only, or both the liver and the muscles. Other systems that may be involved include blood cells (red blood cells, white blood cells, and platelets), heart, and kidneys amongst others.
All types of GSD cause the body to either not be able to make enough glucose, or not be able to use glucose as a form of energy. Determining what type of GSD a person has (diagnosis) depends on an individual's symptoms. Typically a doctor will do a physical examination and some blood and urine testing. Occasionally, a muscle and/or liver biopsy will be needed to measure the amount of a certain enzyme in that part of the body.
There are about eleven known types of GSD, which are classified by a number, by the name of the defective enzyme, or by the name of the doctor who first described the condition. For example, Glycogen Storage Disease Type Ia, caused by a defect in the enzyme glucose-6-phosphatase, was originally known as “von Gierke's Disease” (after Edgar von Gierke, the doctor who discovered it) but is also referred to as “Glucose-6-Phosphatase Deficiency Glycogen Storage Disease.”
The GSDs are genetic disorders. This means that they are caused by a change in a part of an individual’s genetic information. Our genetic information is stored on genes. Genes serve as the instruction manual for our bodies. They tell our bodies how to grow and function. They also determine our physical features, such as hair color and eye color. We have around 30,000 genes in every cell of our body. We get two sets of every gene, one set from our mother and one set from our father. This is why we appear to be a combination of our parents. Our parents have no control over which genes they pass on to us. The genes we inherit from our parents happen purely by chance.
If there is a change in the genetic information contained on one of these genes, our bodies are unable to read its instructions. Therefore, it may cause a difference in the way our body functions. This is similar to having a page missing out of an instruction manual for putting an appliance together. Without that page, we would be unable to properly assemble the appliance and it would not be able to work. Almost all forms of GSD occur when a child inherits an incorrect genetic instruction from both their mother and their father (autosomal recessive inheritance). Some forms of GSD are caused by a genetic change that is passed from mother to son (sex or X-linked inheritance).