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Glycogen Storage Disease Type VI (GSD VI)

Name of disease:

Glycogen Storage Disease Type VI (GSD VI)

ICD-10 diagnosis code:

E74.09

Causes:

  • Mutations in the PYGL gene cause GSDVI.
  • The PYGL gene provides instructions for making an enzyme called liver glycogen phosphorylase. This enzyme is found only in liver cells where it breaks down glycogen into a type of sugar called glucose-1-phosphate.
  • Additional steps convert glucose-1-phosphate into glucose, a simple sugar that is the main energy source for most cells in the body.
  • PYGL gene mutations prevent liver glycogen phosphorylase from breaking down glycogen effectively. Because liver cells cannot break down glycogen into glucose, individuals with GSDVI can have hypoglycaemia and may use fats for energy, resulting in ketosis. Glycogen accumulates within liver cells, causing these cells to become enlarged and dysfunctional.

Mode of inheritance:

  • This condition is inherited in an autosomal recessive pattern, which means both copies of the gene in each cell have mutations.
  • The parents of an individual with an autosomal recessive condition each carry one copy of the mutated gene, but they typically do not show signs and symptoms of the condition.

Prevalence:

Approximately 1:65,000 - 85,000

Diagnosis:

The diagnosis of GSDVI is established in a proband with typical clinical findings and/or biallelic pathogenic variants in PYGL identified by molecular genetic testing.

Age of onset:

Infancy to early childhood

Common signs and symptoms:

  • An enlarged liver (hepatomegaly)
  • Low blood sugar (hypoglycaemia)
  • Elevated levels of ketones in the blood (ketosis)
  • Children with GSDVI tend to grow slower than their peers but they often achieve normal height as adults
  • Some affected children have mild delays in the development of motor skills such as sitting, standing or walking.
  • The signs and symptoms of GSDVI tend to improve with age.
  • Most adults with this condition do not have any related health problems.

Available treatment (medicinal and non-medicinal):

  • Some individuals with GSDVI may not require any treatment, but treatment with cornstarch and protein improves growth and stamina and ameliorates biochemical abnormalities, including hypoglycaemia and ketosis.
  • Even in those with no hypoglycaemia, a bedtime dose of cornstarch improves energy and prevents ketosis.
  • Monitoring of blood glucose and blood ketone levels at least several times per month, and particularly during times of stress, including illness, intense activity, period of rapid growth, or any time at which intake of food is reduced.
  • Annual liver ultrasound examination should start at the age of five.
  • Bone density examination is recommended when puberty is complete.

Disease management tips:

  • Excessive amounts of simple sugars to prevent excessive hepatic glycogen deposition
  • Glucagon administration as a rescue therapy for hypoglycaemia because blood glucose concentrations will not increase
  • Growth hormone for short stature because it usually exacerbates ketosis and may increase the risk of complications
  • Contact sports when hepatomegaly is present (or use appropriate cautions)

References:

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