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GAA
alpha glucosidase
The GAA gene encodes the acid alpha-glucosidase enzyme, essential for glycogen breakdown within cellular lysosomes, with variants associated with Pompe disease. The GAA gene provides the genetic blueprint for acid alpha-glucosidase, an enzyme critical for cellular metabolism.
GAA is located on the long (q) arm of chromosome 17, at band 17q25.3. Arm ratio per GRCh38 - banding schematic.
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Overview
The GAA gene is responsible for producing an enzyme known as acid alpha-glucosidase, sometimes called acid maltase. This enzyme plays a vital role in the body's metabolic processes, specifically in the breakdown of complex sugars. Its function is primarily carried out within lysosomes, which are cellular organelles acting as recycling centres.
Within these lysosomes, acid alpha-glucosidase breaks down glycogen, a stored form of glucose, into simpler glucose molecules that cells can use for energy. Disruptions to this process, often caused by pathogenic variants in the GAA gene, can lead to the accumulation of glycogen within cells, resulting in various health implications.
What the gene does
The GAA gene provides instructions for synthesising acid alpha-glucosidase, an enzyme located within lysosomes. Lysosomes are specialised compartments within cells responsible for degrading and recycling waste products and cellular debris. The acid alpha-glucosidase enzyme is crucial for the catabolism of glycogen, a complex carbohydrate stored in cells, particularly in the liver and muscles, as an energy reserve.
Specifically, this enzyme breaks down glycogen into glucose, which is the primary energy source for most cellular functions. This process ensures that glycogen does not accumulate to harmful levels within the lysosomes. If the activity of acid alpha-glucosidase is reduced or absent due to genetic variants, glycogen can build up in lysosomes, impairing normal cellular function and leading to conditions such as Pompe disease.
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Chromosome location
The GAA gene is situated on chromosome 17, specifically at position 17q25.3. Chromosomes are highly organised structures found within the nucleus of eukaryotic cells, carrying genetic information in the form of DNA. The designation 'q' refers to the long arm of the chromosome, and '25.3' indicates the specific band and sub-band location on this arm. This precise genomic address helps in identifying and studying the gene within the human genome.
Protein structure
The acid alpha-glucosidase protein, encoded by the GAA gene, consists of 952 amino acids. Its structural organisation is critical for its enzymatic function. Notably, the protein features a Disordered (Region) spanning amino acids 47-82, indicating a segment that lacks a fixed three-dimensional structure under physiological conditions. Additionally, a P-type (Domain) is identified from amino acids 80-131, which is important for the protein's overall folding and activity. These specific regions contribute to the enzyme's ability to bind to and break down glycogen within lysosomes.
Key variants
Variants in the GAA gene can alter the function of the acid alpha-glucosidase enzyme. Over 200 distinct genetic changes have been identified in individuals with conditions linked to GAA. These variants can involve changes to single DNA building blocks, which may result in altered amino acids in the protein. Other types of variants include insertions or deletions of genetic material within the GAA gene. Such changes can significantly reduce the enzyme's activity or lead to its complete absence, thereby preventing the effective breakdown of glycogen.
Sample of pathogenic variants
10 pathogenic / likely-pathogenic variants from ClinVar, ranked by review status (expert-panel-reviewed first). This is a sample; recurrent founder variants in a specific population may not appear here - see the full ClinVar listing via the link above.
| Variant (HGVS) | Protein change | Classification | Evidence | Associated condition |
|---|---|---|---|---|
c.1051del | p.Val351fs | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1057del | p.Gln353fs | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1062C>A | p.Tyr354Ter | Pathogenic | ★★★☆ | not provided |
c.1064T>C | p.Leu355Pro | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1075G>A | p.Gly359Arg | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1075G>T | p.Gly359Ter | Pathogenic | ★★★☆ | not provided |
c.1076-1G>C | - | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1076-22T>G | - | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1241del | p.Phe414fs | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
c.1551+3_1551+6del | - | Pathogenic | ★★★☆ | Glycogen storage disease, type II |
Evidence stars indicate ClinVar review status. Individual variant interpretation should always be performed by a qualified clinical laboratory - many variants remain classified as Variants of Uncertain Significance (VUS) pending more research.
Associated conditions
Pathogenic variants in the GAA gene are primarily associated with Pompe disease. This is an autosomal recessive inherited condition, meaning an individual must inherit two copies of a pathogenic variant (one from each parent) to develop the condition. Pompe disease is characterised by the accumulation of glycogen within lysosomes, particularly in muscle cells, leading to muscle weakness and other systemic issues.
Inheritance pattern
Conditions caused by pathogenic GAA variants typically follow autosomal recessive inheritance.
When both parents are carriers, each child has a 25% chance of being affected, 50% of being a carrier, and 25% of being unaffected.
UK clinical status
The GAA gene is recognised in the UK's NHS Genomic Medicine Service due to its clinical significance. It is included on several PanelApp panels, indicating its relevance in diagnostic testing for a range of conditions. These include panels for Acute rhabdomyolysis, Foetal anomalies, Glycogen storage disease, and Lysosomal storage disorder, among others. Its presence on these panels highlights its established role in various inherited metabolic and muscle disorders.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Research suggests that while lifestyle choices cannot prevent or cure conditions associated with GAA gene variants, managing overall health can support wellbeing. For individuals with glycogen storage disorders like Pompe disease, a balanced diet and appropriate physical activity, as recommended by healthcare professionals, may help manage symptoms and improve quality of life. Maintaining a healthy weight and avoiding excessive sugar intake could also be beneficial for general health, though this does not directly impact the underlying genetic cause. Engaging in regular, moderate exercise, tailored to individual capabilities, is generally encouraged for muscle health.
Supplement considerations
There is currently no conclusive evidence that specific dietary supplements can prevent or cure conditions linked to GAA gene variants. While some individuals may explore various supplements, it is crucial to understand that they cannot address the fundamental genetic defect. Patients considering any supplements should always consult with their healthcare provider or a registered dietitian. This is important to ensure safety, avoid potential interactions with medications, and confirm that the supplement is appropriate for their specific health needs.
Frequently asked questions
What does the GAA gene do?
The GAA gene provides instructions for creating acid alpha-glucosidase, an enzyme that breaks down glycogen into glucose within cellular lysosomes. This process is crucial for providing cells with energy and preventing glycogen accumulation.
What is Pompe disease?
Pompe disease is an inherited disorder caused by pathogenic variants in the GAA gene. It leads to the build-up of glycogen in lysosomes, primarily affecting muscle tissue and causing progressive muscle weakness and other health problems.
How is Pompe disease inherited?
Pompe disease follows an autosomal recessive inheritance pattern. This means an individual must inherit two pathogenic copies of the GAA gene, one from each parent, to develop the condition.
Can lifestyle changes prevent Pompe disease?
No, lifestyle changes cannot prevent Pompe disease as it is a genetic condition caused by specific variants in the GAA gene. However, a healthy lifestyle, under medical guidance, can support overall well-being and symptom management.
Is there a cure for conditions associated with GAA gene variants?
Currently, there is no cure for conditions caused by GAA gene variants, such as Pompe disease. However, treatments like enzyme replacement therapy are available to manage symptoms and improve quality of life for affected individuals.
References
- Oba-Shinjo SM, da Silva R, Andrade FG. Pompe disease in a Brazilian series: clinical and molecular analyses with identification of nine new mutations. Journal of neurology. 2009. PMID: 19588081
- van der Ploeg AT, Reuser AJ. Pompe's disease. Lancet (London, England). 2008. PMID: 18929906
- Fukuda T, Roberts A, Plotz PH. Acid alpha-glucosidase deficiency (Pompe disease). Current neurology and neuroscience reports. 2007. PMID: 17217857
- Montalvo AL, Bembi B, Donnarumma M. Mutation profile of the GAA gene in 40 Italian patients with late onset glycogen storage disease type II. Human mutation. 2006. PMID: 16917947
- Moreland RJ, Jin X, Zhang XK. Lysosomal acid alpha-glucosidase consists of four different peptides processed from a single chain precursor. The Journal of biological chemistry. 2005. PMID: 15520017
- Yan B, Raben N, Plotz P. The human acid alpha-glucosidase gene is a novel target of the Notch-1/Hes-1 signaling pathway. The Journal of biological chemistry. 2002. PMID: 12065598
- Hoefsloot LH, Hoogeveen-Westerveld M, Reuser AJ. Characterization of the human lysosomal alpha-glucosidase gene. The Biochemical journal. 1990. PMID: 2268276