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HEXA
hexosaminidase subunit alpha
The HEXA gene provides instructions for synthesising the alpha subunit of the beta-hexosaminidase A enzyme, which is crucial for breaking down specific fatty substances in the central nervous system. The HEXA gene is essential for proper brain and spinal cord function, directing the production of a key enzyme component.
HEXA is located on the long (q) arm of chromosome 15, at band 15q23. Arm ratio per GRCh38 - banding schematic.
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Overview
The HEXA gene encodes the alpha subunit of beta-hexosaminidase A, an enzyme integral to cellular waste breakdown processes, particularly within the brain and spinal cord. This enzyme functions within lysosomes, which are cellular compartments responsible for recycling and degrading waste products.
Disruptions to HEXA gene function can lead to the enzyme's impaired activity, resulting in the accumulation of fatty substances called GM2 gangliosides. This build-up is toxic to nerve cells, causing progressive neurological damage and is characteristic of conditions such as Tay-Sachs disease.
What the gene does
The HEXA gene provides the genetic blueprint for the alpha subunit of the beta-hexosaminidase A enzyme. This alpha subunit combines with a beta subunit, produced from the HEXB gene, to form the fully functional beta-hexosaminidase A enzyme. This complex enzyme is strategically located within lysosomes, which are vital organelles within cells responsible for the breakdown and recycling of various molecules.
A primary role of beta-hexosaminidase A in the central nervous system is the degradation of GM2 gangliosides. These are fatty substances that are components of cell membranes. The enzyme breaks down GM2 gangliosides into simpler molecules, preventing their accumulation. When the HEXA gene is altered, the resulting alpha subunit may be dysfunctional or absent, impairing the enzyme's ability to clear GM2 gangliosides. This leads to a detrimental build-up of these lipids, particularly within nerve cells, causing cellular dysfunction and death.
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Chromosome location
The HEXA gene is situated on chromosome 15 at position 15q23. This chromosomal location indicates its specific address within the human genome. The gene contains multiple exons, which are the coding segments that are translated into the protein sequence.
Protein structure
The HEXA gene encodes a protein composed of 529 amino acids, which forms the alpha subunit of the beta-hexosaminidase A enzyme. This protein's structural integrity is crucial for its enzymatic function. A specific region, encompassing amino acids 423-424, is critical for the hydrolysis of GM2 gangliosides, highlighting its importance in the enzyme's catalytic activity.
Key variants
Variants within the HEXA gene can lead to a range of effects on the resulting alpha subunit protein, from complete loss of function to reduced enzymatic activity. Over 210 different types of variants have been identified in the HEXA gene that are associated with disease. These genetic changes can affect various aspects of protein production, including its stability, proper folding, or its ability to interact correctly with the beta subunit to form the active enzyme. The specific location and nature of a variant often dictate the severity and onset of the associated condition.
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.1014del | p.Phe338fs | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1043_1046del | p.Phe348fs | Pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1048C>T | p.Gln350Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1069C>T | p.Gln357Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1073+1G>A | - | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1088dup | p.Ser364fs | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1098T>A | p.Tyr366Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1098T>G | p.Tyr366Ter | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1114del | p.Val372fs | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
c.1123del | p.Glu375fs | Pathogenic/Likely pathogenic | ★★☆☆ | Tay-Sachs disease |
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
Variants in the HEXA gene are primarily associated with Tay-Sachs disease, an autosomal recessive neurodegenerative disorder. This condition arises when both copies of the HEXA gene carry pathogenic variants, leading to a significant deficiency in beta-hexosaminidase A enzyme activity. The reduced enzyme function results in the accumulation of GM2 gangliosides, which are toxic to nerve cells in the brain and spinal cord, causing progressive neurological deterioration.
Inheritance pattern
Conditions caused by pathogenic HEXA 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 HEXA gene is included in several NHS Genomic Medicine Service national test panels, indicating its recognised clinical significance within the UK. It features on panels such as 'Tay-Sachs disease' (R286) and 'Lysosomal storage disorder' (R276), reflecting its role in these inherited metabolic conditions. Furthermore, HEXA is relevant for broader panels covering 'Intellectual disability', 'Early onset or syndromic epilepsy', and 'Leukodystrophy, adult onset' (R62), highlighting its impact across various neurological presentations.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Research into inherited metabolic conditions, including those related to the HEXA gene, generally focuses on understanding disease mechanisms rather than specific lifestyle interventions to prevent their onset. For individuals affected by or carrying variants in genes like HEXA, maintaining a balanced diet and a healthy lifestyle is a general recommendation for overall well-being. However, there is no conclusive evidence suggesting that particular lifestyle choices can prevent the development or alter the progression of conditions caused by HEXA gene variants.
Supplement considerations
Currently, there is no scientific evidence to suggest that specific dietary supplements can prevent or treat conditions associated with variants in the HEXA gene, such as Tay-Sachs disease. The primary issue in these conditions is an enzymatic deficiency leading to toxic accumulation of specific molecules within cells. Individuals considering any supplements should always consult with a healthcare professional to discuss potential benefits, risks, and interactions with other treatments.
Frequently asked questions
What is the primary function of the HEXA gene?
The HEXA gene provides instructions for making the alpha subunit of the beta-hexosaminidase A enzyme, which is critical for breaking down GM2 gangliosides in the central nervous system within cellular lysosomes.
What happens if the HEXA gene has a pathogenic variant?
Pathogenic variants in the HEXA gene can lead to a deficiency in the beta-hexosaminidase A enzyme. This deficiency results in the accumulation of GM2 gangliosides, which are toxic to nerve cells and can cause conditions like Tay-Sachs disease.
Is Tay-Sachs disease inherited?
Yes, Tay-Sachs disease is inherited in an autosomal recessive pattern. This means an individual must inherit two copies of the altered HEXA gene (one from each parent) to develop the condition.
Can lifestyle changes prevent Tay-Sachs disease?
No, Tay-Sachs disease is a genetic condition caused by specific variants in the HEXA gene. There is no evidence that lifestyle changes can prevent its onset or alter its progression.
How does the HEXA gene relate to lysosomes?
The enzyme produced by the HEXA gene, beta-hexosaminidase A, functions primarily within lysosomes. Lysosomes are cellular organelles responsible for breaking down waste materials and cellular debris, including GM2 gangliosides.
References
- Lemieux MJ, Mark BL, Cherney MM. Crystallographic structure of human beta-hexosaminidase A: interpretation of Tay-Sachs mutations and loss of GM2 ganglioside hydrolysis. Journal of molecular biology. 2006. PMID: 16698036
- Montalvo AL, Filocamo M, Vlahovicek K. Molecular analysis of the HEXA gene in Italian patients with infantile and late onset Tay-Sachs disease: detection of fourteen novel alleles. Human mutation. 2005. PMID: 16088929
- Matsuzawa F, Aikawa S, Sakuraba H. Structural basis of the GM2 gangliosidosis B variant. Journal of human genetics. 2003. PMID: 14577003
- Myerowitz R, Lawson D, Mizukami H. Molecular pathophysiology in Tay-Sachs and Sandhoff diseases as revealed by gene expression profiling. Human molecular genetics. 2002. PMID: 12019216
- Adam MP, Bick S, Mirzaa GM. HEXA Disorders. 1993. PMID: 20301397