About glycogen storage disease iii

What is glycogen storage disease iii?

Forbes disease (GSD-III) is one of several glycogen storage disorders (GSD) that are inherited as autosomal recessive traits. Symptoms are caused by a lack of the enzyme amylo-1,6 glucosidase (debrancher enzyme). This enzyme deficiency causes excess amounts of an abnormal glycogen (the stored form of energy that comes from carbohydrates) to be deposited in the liver, muscles and, in some cases, the heart.

There are two forms of this disorder. GSD-IIIA affects about 85% of patients with Forbes disease and involves both the liver and the muscles. GSD-IIIB affects only the liver.

What are the symptoms for glycogen storage disease iii?

The median age at the first clinical presentations is in the first year of life. Most common presenting symptoms are enlarged liver (hepatomegaly) (98%), low blood sugar (hypoglycemia) (53%), failure to thrive (49%) and recurrent illness and/or infections (17%). Symptoms and signs of GSD-III, at least during the first 4 to 6 years of life, may be indistinguishable from GSD type I. The amount of glycogen in the liver and muscles is abnormally high, the liver is enlarged, and the abdomen protrudes. The muscles tend to be flaccid or weak.

A typical child with GSD-III has short stature, low blood sugar after fasting that does not respond to the hormone glucagon, and an elevated level of fatty substances in the blood, known as hyperlipidemia. Hypoglycemia is usually associated with increased ketone bodies, and ketonemia can precede hypoglycemia, reflecting activation of burning fat stores. Patients with GSD-III may also have difficulty fighting infections, and may experience unusually frequent nosebleeds. Enlarged heart muscle (cardiac hypertrophy) is common in individuals with GSD-IIIa and can already appear in early childhood. However, in most children, heart function remains within normal limits. Children with GSD-III often grow slowly during childhood and puberty may be delayed, but their adult height is usually normal. Most signs and symptoms improve significantly with adequate dietary management.

In adulthood, the liver manifestations of the disease usually subside, but progression to liver scarring (cirrhosis) and malignancy (carcinoma) may occur. Despite dietary management, muscle disease can get worse. As the cohort of adult GSD-III patients is still relatively young and small, the course of the disease over time is incompletely described.

Some affected individuals may have virtually no symptoms (asymptomatic) other than a protruding abdomen and an enlarged liver in childhood. These patients tend to lose these few symptoms during adolescence when their liver decreases progressively in size.


There are four subtypes of GSD-III:

GSD-IIIa is the most common type, affecting 85%, and affects both the liver and (cardiac and/or skeletal) muscles.

GSD-IIIb affects about 15% of individuals and only affects the liver. AGL molecular testing can display mutations specific to GSD-IIIb.

GSD-IIIc is extremely rare and believed to be caused by loss of activity of the glucosidase active site of the glycogen debranching enzyme.

GSD-IIId is extremely rare and believed to be caused by loss of activity of the transferase active site of the glycogen debranching enzyme.

What are the causes for glycogen storage disease iii?

GSD-III is an inborn error of metabolism caused by mutations in the AGL gene that is located on chromosome 1p21. The AGL gene is responsible for the production of the debranching enzyme.

Glycogen is stored in the liver and muscles for future energy needs. Glycogen can then be converted into sugar (glucose). Glucose is used as a readily available source of energy during fasting or exercise. The debranching enzyme has two active (catalytic) sites called amylo-1,6-glucosidase and 4-alpha-glucanotransferase. Both sites on the enzyme are together with the phosphorylase and phosphorylase kinase enzymes (impaired in GSD types VI and IX, respectively) responsible for breaking down glycogen to raise the blood sugar concentration. Without normal debranching enzyme function, two changes take place. If glycogen can only be broken down partially, an insufficient amount of energy/glucose can be produced. The structure that is left, resembling a molecule called a “limit dextrin”, is excessively stored in liver, and (skeletal and cardiac) muscle tissues.


GSD-III is a genetic disorder characterized by variable liver, cardiac muscle and skeletal muscle abnormalities. Symptoms are associated with abnormalities in the AGL gene, causing deficiency of the glycogen debranching enzyme. GSD-III is inherited as an autosomal recessive trait.

Recessive genetic disorders occur when an individual inherits two copies of an altered gene for the same trait, one from each parent. If an individual inherits one normal gene and one gene for the disease, the person will be a carrier for the disease but usually will not show symptoms. The risk for two carrier parents to both pass the altered gene and have an affected child is 25% with each pregnancy. The risk to have a child who is a carrier like the parents is 50% with each pregnancy. The chance for a child to receive normal genes from both parents is 25%. The risk is the same for males and females.

All individuals carry mutations/variants in ± 4-5 genes. Parents who are close relatives (consanguineous) or who originate from closed communities have a higher chance than unrelated parents to both carry the same abnormal gene, which increases the risk to have children with a recessive genetic disorder.

What are the treatments for glycogen storage disease iii?

Dietary management is the cornerstone.

  • Infants and children with GSD-III are treated with a high-protein diet every 3-4 hours. The recommended daily amount of protein is ± 3-4 grams per kg bodyweight per day and should be well divided during the day. Cornstarch may already be introduced in the first year of life. This is a dietary complex starch like glycogen and the dose/frequency of supplementation is titrated to maintain normoglycemia. Although fructose and galactose can be metabolized, the (extend of) restrictions of so-called simple/fast carbohydrates is a matter of debate. These simple sugars include glucose, galactose (dairy sugar), lactose (galactose + glucose), fructose (fruit sugar), sucrose (fructose + glucose) and maltodextrin. The latter is frequently used as a food additive and typically a mixture of 3-17 glucose units. Special formulas are not required. Fasting should be avoided and for the overnight fast, (a combination of) a bedtime snack, frequent feeds, cornstarch, and/or continuous nocturnal gastric drip feeding may be needed.
  • Adolescents and adults have lower basic carbohydrate requirements. The recommended daily amount of protein is ± 25 % of the total caloric intake. A bedtime snack or an overnight high protein formula may be prescribed for patients with myopathy.
  • Good dietary control includes at home monitoring of blood glucose and ketones. Based on clinical observations, it is believed that the diet can prevent or resolve heart and/or muscle disease.
  • The role of and indications for ketogenic diets (and variations, including Atkins diet) and medium chain triglycerides (MCT) oil are debatable and deserve further, systematic research.

Liver transplantation is indicated only for patients with severe hepatic cirrhosis, liver dysfunction and /or liver cancer (hepatocellular carcinoma).

Clinical Testing and Follow-Up

Emergency letters should be provided and shared care with local physicians should be organized. Liver ultrasound and baseline heart tests (electrocardiogram and echocardiograms) are usually recommended to determine the medical needs for individual patients based on the severity of the condition.

Genetic counseling is recommended for families of children with glycogen storage diseases.

What are the risk factors for glycogen storage disease iii?

Glycogen storage disease type 3 is also known as Forbes-Cori disease. The disease is caused by mutations to the AGL gene that instruct the synthesis of glycogen debranching enzyme. Debranching is one of the initial steps of glycogenolysis that is intended to release glucose from stored glycogen during fasting. Due to the enzyme deficiency, glycogen continues to deposit in the liver, cardiac and skeletal muscles. The affected infants start to present the symptoms from the age of 3 months. The disease causes severe complications such as renal failure, liver cirrhosis, and coronary disorders.

Risk factors

  • Inheritance: The pathogenic variants of the gene AGL are inherited in an autosomal recessive manner. Positive familial history for the mutant gene is a risk factor.
  • Obesity and hepatitis: Obesity and Hepatitis B and C increase the risk of worsening the symptoms of the disease. Obesity adds to hyperlipidemia. Hence, all the factors that enhance obesity, indirectly contribute to the progression of the symptoms. Hepatitis increases the burden on the liver.
  • Descendance from Faroe Island: One in every 3600 children born with ancestors hailing from Faroe island is diagnosed with the disease. Backed by the statistical evidence, the ancestral history of Faroe Island can be considered a risk factor.

Muscle Cramps,Full Cheeks,Mental disability,Swollen Belly,Weak Muscles,Heat Intolerance
Enlarged Liver,Slow Growth,Low Blood Sugar Levels,Short Stature,Immune Deficiency

Is there a cure/medications for glycogen storage disease iii?

A particular form of glucose/ sugar/ carbohydrate in the human body is called glycogen. Usually, carbohydrates are the energy source of the body. But just like excess sugar levels give diabetes, glycogen stores in muscles and liver give birth to Glycogen Storage Disease iii.

It is an inherited birth disease in a baby in the form of an autosomal recessive manner. During infancy, it starts to be deposited in blood as the extra fat and hoisted blood levels of liver enzymes.

In the case of Glycogen Storage Disease iii, infants show poor growth in terms of physical as well as mental development. It is one type of glycogen storage also called Cori’s Disease. There are seven to nine other types of this disease with different storage errors of glycogen.


With the level of severity in symptoms, doctors suspect about GSD iii possibility. To confirm those suspects, they use the following methods:

  • Gene Testing: Errors in chromosomes and gene copies.
  • Biopsy of Tissues: Glycogen presence in muscle or liver tissue.
  • Blood tests: Glucose level in blood.
  • Ultrasound: Enlarged liver test.


Treatment of GSD iii depends on the symptoms baby is showing:

Maintain Blood Sugars:

  • Diet changes
  • Prescribed Medications
  • Eliminate Fructose and Lactose from foods
  • Uncooked Corn starch

Avoid Fatigue/ Cramps:

  • High Protein Diet
  • Exercise
  • Glucose intake

GSD iii can get severe in the case of children and infants. Low or negligible symptoms in the case of adults.

Muscle Cramps,Full Cheeks,Mental disability,Swollen Belly,Weak Muscles,Heat Intolerance
Enlarged Liver,Slow Growth,Low Blood Sugar Levels,Short Stature,Immune Deficiency

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