Glycogen Storage Disease Type IX

What else is it called?

  • GSD IX
  • PhK deficiency
  • Phosphorylase b kinase deficiency
  • Phosphorylase kinase deficiency

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What causes it?

Glycogen Storage Disease Type X1 (GSDIX) is caused by a problem in the PHKA1, PHKA2, PHKB or PHKG2 genes. These genes provide instructions for making pieces of an enzyme called phosphorylase b kinase. Phosphorylase b kinase plays an important role in providing energy to our cells and our body uses it to ‘activate’ enzymes to break 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.  An enzyme is made up of 16 subunits and at least two different versions of phosphorylase b kinase are formed from these subunits. One version is most prominent in the liver and the second is most prominent in other muscle cells.

Problems in the PHKA1, PHKA2, PHKB or PHKG2 genes reduce the ‘activity’ of phosphorylase b kinase in liver, muscle and blood cells. A reduction in this activity results in a build up of glycogen which causes damage to cells and a reduction in energy. A build of glycogen can damage organs and tissues throughout the body, particularly affecting the liver and muscles. This then leads to the signs and symptoms of GSDIX, outlined below.

There are four types of GSDIX; glycogen storage disease type IXA, glycogen storage disease type IXB, glycogen storage disease type IXC, glycogen storage disease type IXD.

How common is it?

GSD1X affects 1 in 100,00 people. The forms of disease that affect muscles or muscles and liver are much less common. The X-linked form of the disease is believed to be more prevalent in males, however females have been known to suffer the same symptoms.

What are the signs and symptoms?

Symptoms usually begin during early childhood

  • Enlarged liver
  • Slow growth (affected children are often shorter than normal)
  • Low blood sugar (during long periods of fasting)
  • Delayed development of motor skills such as sitting, standing, walking.
  • Mild muscle weakness
  • Delayed puberty
  • Liver disease (some cases)
  • Fatigue
  • Muscle pain
  • Cramps
  • Red or brown urine

Typically, children diagnosed with GSDIX catch up developmentally and adults reach normal height.

How is it diagnosed?

GSD1X is typically diagnosed via a physical examination, review of patient history and specialised enzyme and genetic testing. These tests will measure the activity of phosphorylase kinase in your red blood cells, liver tissue or muscle tissue. Sometimes molecular testing is used, this test enables specialists to detect mutations in the PHKA1, PHKA2, PHKB or PHKG2 genes.

Prenatal diagnosis for at-risk pregnancies is sometimes conducted, for families with affected individuals.

If you suspect you may have GSDIX, we recommend you speak to your doctor in the first instance, GSDIX is difficult to diagnose and will require further specialised testing.

Can it be treated?

Patients with GSDIX will be referred to a Metabolic Specialist and will usually be treated by a multidisciplinary health care team. Treatment is usually directed towards specific symptoms which vary in each patient. Health care professionals usually involved in the care and treatments of patients with GSDIX include; Paediatricians, liver specialists, gastroenterologists, nutritionists, physical therapists and other healthcare professionals.

There are no dietary restrictions for patients with GSD1X, however a high-protein and complex carbohydrate diet is often recommended. It is also recommended that prolonged fasting is avoided. A metabolic specialist or specialist dietician will assess your signs and symptoms and recommend a suitable diet.

It is also recommended that patients with GSD1X avoid vigorous exercise and any drugs that can damage muscle tissue, such as statins. It is important to speak to your consult before taking any medication that has not been prescribed to you.

Patients with GSDIX should receive regular blood tests and an abdominal ultrasound or MRI every 6-12 months, the frequency of these tests will vary depending on the severity of your symptoms. Prognosis is good for patients with GSDIX with most living to adulthood. At present there is no clinical treatment or research taking place into GSDIX.

If you or someone you know has been affected by GSDIX, 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 IX can have different inheritance patterns, depending on the genetic cause.

When the disease is caused by problems with the PHKA1 or PHKA2 gene, the disease is inherited in an X-linked recessive pattern. This means the genes are located on the X chromosomes. In males (who only have one X chromosome) one problematic gene in each cell can cause the condition. In females (who have two X chromosomes) a problem would have to occur in both copies of the gene to cause the disorder. This inheritance pattern indicates that males are affected by X-linked recessive disorders much more frequently than females. However, fathers cannot pass X-linked traits to their sons.

When the disorder is caused by problems in the PHKB or PHKG2 gene, the disorder is inherited in autosomal recessive pattern. This means that both parents of the patient each carry a copy of the mutated genes. Despite both parents carrying the mutated gene, typically they will not show signs or symptoms. Please see the diagram below for more information regarding both inheritance patterns.

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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