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Glycogen storage disease type Ia
GSD Ia is a genetic disorder impacting how the body uses sugar, specifically causing difficulties in maintaining normal blood glucose levels. It can lead to severe low blood sugar and liver enlargement, typically becoming apparent in infancy.
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Overview
Glycogen Storage Disease Type Ia (GSD Ia) is an inherited metabolic disorder characterised by the body's inability to convert stored glycogen into glucose, the main sugar used for energy. This occurs due to a deficiency in a specific enzyme called glucose-6-phosphatase. Glycogen is the primary way the body stores glucose, mainly in the liver and muscles. When this enzyme is not working correctly, glycogen builds up in organs like the liver and kidneys, while the body struggles to produce enough glucose, particularly between meals [PMID:31194462].
Individuals with GSD Ia often experience episodes of very low blood sugar (hypoglycaemia), especially during fasting, as their bodies cannot release glucose from glycogen stores. The condition typically presents in infancy, often within the first year of life. Careful management, primarily involving dietary adjustments, is crucial to prevent complications and support healthy development.
Symptoms & clinical features
The symptoms of GSD Ia typically appear in infancy, often around 3-4 months of age when infants start sleeping through the night and have longer periods without feeding. A key symptom is severe low blood sugar (hypoglycaemia), which can lead to weakness, irritability, sweating, and, if severe, seizures or loss of consciousness [PMID:31194462].
Other common signs include an enlarged liver (hepatomegaly) due to the accumulation of glycogen, and a distended abdomen. Individuals may also have delayed growth and short stature. The kidneys can also be affected, sometimes leading to kidney stones or kidney enlargement. Blood tests often show elevated levels of fats (lipids), uric acid, and lactate in the blood.
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Affected organs
GSD Ia primarily affects the liver, where glycogen stores accumulate, leading to its enlargement (hepatomegaly). This impaired liver function also directly impacts the body's ability to maintain stable blood glucose levels. The kidneys are also commonly affected, with glycogen accumulation sometimes leading to kidney enlargement and an increased risk of kidney stones. Other organs, such as the intestine and blood cells, can also store glycogen, though the most significant clinical impacts are observed in the liver and kidneys.
Risks & severity
GSD Ia has a variable severity, but without appropriate management, it can lead to significant health complications. Persistent or severe low blood sugar can cause brain damage and developmental delays. Long-term liver complications can include the development of benign liver tumours (adenomas), which in rare cases may become cancerous (hepatocellular carcinoma) in later life [PMID:24653301].
Kidney involvement can progress to chronic kidney disease in some individuals. Other potential long-term issues include osteoporosis (weak bones), anaemia, and an increased risk of infections due to impaired white blood cell function. The exact prevalence of GSD Ia is not well established, but it is estimated to affect approximately 1 in 100,000 births.
Genetic causes
Glycogen Storage Disease Type Ia is caused by pathogenic variants in the G6PC gene. This gene provides instructions for making an enzyme called glucose-6-phosphatase catalytic subunit 1, which is found primarily in the liver, kidneys, and small intestine. This enzyme is essential for the final step in converting stored glycogen into glucose, a process called gluconeogenesis.
When pathogenic variants occur in the G6PC gene, the glucose-6-phosphatase enzyme either does not work properly or is completely absent. This prevents the body from effectively releasing glucose from its glycogen stores, leading to glycogen accumulation in the affected organs and recurrent episodes of low blood sugar. These variants disrupt the metabolic pathway crucial for maintaining stable blood glucose levels, particularly during periods without food intake.
- G6PC glucose-6-phosphatase catalytic subunit 1
Inheritance pattern
GSD Ia is inherited in an autosomal recessive pattern. This means that a person must inherit two altered copies of the G6PC gene - one from each parent - to develop the condition. Individuals who inherit only one altered copy of the G6PC gene are known as carriers. Carriers typically do not show any symptoms of GSD Ia because their one working copy of the gene is sufficient to produce enough functional enzyme.
If both parents are carriers of a pathogenic variant in the G6PC gene, there is a 25% chance with each pregnancy that their child will inherit two altered copies and develop GSD Ia. There is a 50% chance the child will be a carrier, and a 25% chance the child will inherit two working copies of the gene and not be a carrier.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
Diagnosis & testing
A diagnosis of GSD Ia is often suspected based on clinical symptoms such as recurrent low blood sugar, an enlarged liver, and characteristic blood test abnormalities (e.g., high lactate, uric acid, and lipids). The diagnosis is typically confirmed through genetic testing, which looks for pathogenic variants in the G6PC gene [PMID:31194462].
In the UK, if GSD Ia is suspected, a specialist clinician, such as a paediatrician or metabolic consultant, would typically refer an individual for genetic testing. This testing would usually be performed as part of the NHS Genomic Medicine Service, often guided by specific R-codes for inherited metabolic disorders. Previously, a liver biopsy could be used to measure enzyme activity, but genetic testing is now the primary confirmatory method.
Management & lifestyle
Management of GSD Ia focuses on preventing low blood sugar and its associated complications through dietary interventions. This usually involves frequent, small meals throughout the day and the consistent intake of uncooked cornstarch (or similar slow-releasing carbohydrates) between meals and overnight. The cornstarch slowly releases glucose, helping to maintain stable blood sugar levels. Infants may require continuous feeding via a nasogastric tube during the night.
Regular monitoring of blood glucose, lactate, uric acid, and lipid levels is essential to adjust the dietary regimen as needed. Individuals are typically managed by a multidisciplinary team, including metabolic specialists, dietitians, and nurses, within the NHS framework. Genetic counsellors can provide support and information regarding the genetic aspects of the condition for affected individuals and their families. While there is no cure, careful management can significantly improve health outcomes and quality of life.
UK care pathway
In the UK, individuals with suspected GSD Ia would typically be referred by their GP or a specialist to a clinical genetics service or a metabolic specialist centre. These centres are part of the NHS Genomic Medicine Service (GMS) and can offer genetic testing and diagnosis. Genetic counselling is also available to help families understand the condition, its inheritance pattern, and implications for other family members. The NHS uses specific R-codes for genetic tests for inherited metabolic disorders, which guide the diagnostic pathway.
Frequently asked questions
What is glycogen and why is it important?
Glycogen is the main way your body stores glucose (sugar) for energy, primarily in the liver and muscles. It acts like a reserve fuel tank, providing glucose to keep your blood sugar levels stable, especially between meals or during physical activity.
Can GSD Ia be cured?
Currently, there is no cure for GSD Ia. However, with careful and consistent dietary management, including frequent meals and uncooked cornstarch, individuals can live full and active lives. This management helps prevent the severe symptoms and long-term complications associated with the condition.
What is the typical age for GSD Ia symptoms to appear?
Symptoms of GSD Ia typically become noticeable in infancy, often between 3 to 4 months of age. This is when babies start having longer periods between feeds, leading to difficulties in maintaining stable blood sugar levels due to the enzyme deficiency.
How does GSD Ia affect family planning?
Because GSD Ia is an autosomal recessive condition, if you are a carrier or have a family history, you might consider genetic counselling. A genetic counsellor can explain the risks for future pregnancies, discuss options like prenatal diagnosis, and provide support for family planning decisions.
Are there specific foods to avoid with GSD Ia?
Individuals with GSD Ia generally need to avoid foods containing fructose and galactose, as the body cannot properly metabolise these sugars without the correct enzyme function, which can worsen metabolic issues. A specialised dietitian will provide tailored dietary advice.
References
- Derks TGJ, Rodriguez-Buritica DF, Ahmad A. Glycogen Storage Disease Type Ia: Current Management Options, Burden and Unmet Needs. Nutrients. 2021. PMID: 34836082
- Weinstein DA, Derks TG, Rodriguez-Buritica DF. Safety and Efficacy of DTX401, an AAV8-Mediated Liver-Directed Gene Therapy, in Adults With Glycogen Storage Disease Type I a (GSDIa). Journal of inherited metabolic disease. 2025. PMID: 40064185
- Rossi A, Ruoppolo M, Formisano P. Insulin-resistance in glycogen storage disease type Ia: linking carbohydrates and mitochondria? Journal of inherited metabolic disease. 2018. PMID: 29435782
- Tenente J, Campos T, Vasconcelos C. Neonatal Glycogen Storage Disease Type IA: A Rare Presentation. Endocrine, metabolic & immune disorders drug targets. 2023. PMID: 37859321
- Du J, Dou LM, Jin YH. Glycogen storage disease type Ia misdiagnosed as multiple acyl-coenzyme A dehydrogenase deficiency by mass spectrometry. Frontiers in pediatrics. 2022. PMID: 36452356
- Steg Saban O, Pode-Shakked B, Abu-Libdeh B. Glycogen Storage Disease type IA refractory to cornstarch: Can next generation sequencing offer a solution? European journal of medical genetics. 2022. PMID: 35550444
- Chou JY, Kim GY, Cho JH. Recent development and gene therapy for glycogen storage disease type Ia. Liver research (Beijing, China). 2017. PMID: 29576889
- van Schaftingen E, Gerin I. The glucose-6-phosphatase system. The Biochemical journal. 2002. PMID: 11879177