Glycogen Storage Disease Type V

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Glycogen Storage Disease V (GSD V) is caused by problems in the PYGM gene. The PYGM gene provides instructions for making an enzyme called ‘myophosphorylase’. Myophosphorylase is found in muscle cells and breaks down a substance called glycogen. Glycogen is a major source of stored energy in your body and is what your body partly uses for fuel. The myophosphorylase enzyme breaks glycogen down into a simpler sugar called glucose-1-phosphate, which is then converted into glucose and used as the main energy source for most cells.

Problems in the MYGM gene prevent the myophosphorylase enzyme from breaking down glycogen. This results in muscle cells not producing enough energy and so the muscles become fatigued.

The global prevalence of GSDV is currently unknown. However, it is believed to be most prevalent in the Dallas-Fort Worth area of Texas and Spain, affecting 1 in 100,000 people in Texas and 1 in 170,000 in Spain.

Symptoms typically begin during teenage years or early twenties but can appear during infancy. The severity of the condition varies per patient, some patients will experience a deterioration in muscle weakness over time, whereas some will find muscle weakness remains stable. Some patients will experience mild symptoms whereas others do not experience any symptoms at all.

• Fatigue, muscle pain and cramps (during first few minutes of exercise)
• Prolonged or intense exercise can cause muscle damage.
• Muscle breakdown
• Red or brown urine
• Kidney failure (in extreme cases)

Patients with GSD V are typically diagnosed via muscle biopsy, usually from the biceps or quadriceps. More recently, DNA studies have been used to look for the mutated PYGM gene.

If you suspect you may have GSD V, we recommend you speak to your doctor in the first instance. GSD V can be difficult to diagnose and will require specialised testing.

At present there is no cure for GSD V, however several different therapeutic approaches are being utilised.

• Nutritional Supplements/Drugs
• Exercise interventions
• Gene therapy (investigational therapy)

GSD V has autosomal recessive inheritance pattern. Humans have chromosomes made up of DNA. Genes are pieces of DNA that carry the genetic information. Each chromosome may have several thousand genes. We inherit chromosomes from the egg of the mother and sperm of the father. The genes on those chromosomes carry the instructions that determine a person’s characteristics, which are a combination of the parents.  

The pattern of inheritance of GSD V is autosomal recessive. This means that carriers of the condition do not have the disorder because the other gene of this pair is working normally. Parents of children with glycogen storage disease type V are carriers.   

When both parents are carriers, the risk to the baby in each pregnancy is  

• 25% chance (1 in 4) of developing the condition  
•  50% chance (1 in 2) for the baby to be a carrier of the condition  
•  25% chance (1 in 4) for the baby to have two working genes and neither have the condition nor be a carrier  

Genetic counselling can be requested to get a full explanation.  

It is recommended that individuals from more frequently affected populations, get tested.

For more information, please visit Association for Glycogen Storage Disease UK.

References are available on request. Please contact us on 0800 652 3181 or by emailing contact@metabolicsupport.org.

Please note calls to 0800 numbers are free from UK landlines and mobiles. Calls from business lines may be charged – please check with your provider.

This information about metabolic diseases is provided by Metabolic Support UK and is intended for educational purposes only. It should not be used for diagnostic or treatment purposes.

Should you require more detailed information please contact Metabolic Support UK by email (contact@metabolicsupport.org) or by telephone ( 0800 652 3181). For specific medical information regarding a disorder or individual please contact your GP or Paediatrician. Metabolic Support UK accepts no responsibility for any errors or omissions nor does Metabolic Support UK assume any liability of any kind for the content of any information contained within this summary or any use that you may make of it.