Glycogen Storage Disease Type VI

What else is it called?

GSD type VI



Hepatic glycogen phosphorylase deficiency

Hers disease

Liver phosphorylase deficiency syndrome

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What causes it?

Glycogen Storage Disease VI (GSD6) is caused by a problem in PYGL gene. The PYGL gene provides instructions for making an enzyme called ‘liver glycogen phosphorylase’. The liver glycogen phosphorylase enzyme is found in your liver cells and breaks down a substance called glycogen into a type of sugar called glucose-1-phosphate, this in turn is converted into glucose. Glucose is a major source of stored energy in your body and is what your body partly uses for fuel.

Problems in the PYGL gene prevent the liver glycogen phosphorylase enzyme from break down glycogen into glucose. Because your liver cells cannot break down glycogen into glucose, patients with GSD6 will use fats for energy, resulting in ketosis. Ketosis is a normal process that happens when your body doesn’t have enough carbs to burn for energy and so instead it burns fat and makes substances called ketones, which are used to fuel your body. Problems in the PYGL gene lead to an accumulation of glycogen in liver cells which causes the cells to become enlarged and dysfunctional.

How common is it?

The exact prevalence of GSD6 is unknown, however at least 11 cases have been reported worldwide. It is thought that due to sometimes mild symptoms, GSD6 is underdiagnoses, therefore the prevalence rate could be higher than what is currently known. It is believed that GSD6 is more common in the Old Order *Mennonite population, affecting 1 in 1,000 individuals.

*Mennonite groups of Swiss German and south German heritage who practice a lifestyle without some elements of modern technology,

What are the signs and symptoms?

Childhood symptoms can include;

  • A swollen abdomen due to enlarged liver cells.
  • A history of poor growth or short stature.
  • Always feeling hungry and a need to feed frequently
  • Low blood sugar (especially when fasting)
  • Fatigue following activity.

Children with GSD6 tend to grow slower but often achieve normal height during adulthood. Some children may experience mild delays in the development of motor skills such as sitting, standing or walking.

Severe (potential) symptoms in later life, can include;

  • Scarring of the liver.

It is recommended that pregnant women with GSD6 are closely monitored.

How is it diagnosed?

Individuals with GSD6 are usually diagnose via physical examinations, your doctor will look for signs such as an enlarged liver and low blood sugar levels. Final diagnosis is made via specialised tests (genetic testing) which look for mutations of the PYGL gene. A liver biopsy may also be performed, if the genetic tests are unclear. The biopsy tests the function of the liver glycogen phosphorylase enzyme.

Can it be treated?

Patients with GSD6 should avoid prolonged periods of time without eating, eating frequent meals can avoid the need for your body to break down glycogen. Blood sugar levels should also be monitored regularly. In some cases, no other treatment is necessary for GSD6 and patients can manage and control their symptoms via diet.

If you have been diagnosed with GSD6 you will be referred to a specialist dietician and metabolic team, who will provide you with a specialised diet. It is important you stick to the diet and instructions the team have given you to prevent you from becoming unwell. It also important that you do not change your diet without consulting your metabolic dietician and specialist first.

At present there are no clinical trials taking place, relating to the treatment of GSD6.

If you have been diagnosed with GSD6 you may want to consider genetic counselling, you will need to speak to your GP or metabolic specialist for a referral to a genetic counsellor. If you would like more information about genetic counselling, a toolkit can be found on the Metabolic Support UK website.

Do my family need to be tested?

GSD 6 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 6 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 VI are carriers, which means both parents of the patient each carry a copy of the mutated PYGL gene. Despite both parents carrying the gene, typically they will not show signs or symptoms.

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 the Old Order *Mennonite community get tested.

*Mennonite groups of Swiss German and south German heritage who practice a lifestyle without some elements of modern technology.

Relevant Organisations

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


References are available on request. Please contact Helen Morris by phoning 0845 241 2173 or emailing [Resource Library No: AAP002].

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