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HEXA
hexosaminidase subunit alpha
The HEXA gene provides instructions for making a vital enzyme component, beta-hexosaminidase A, which is crucial for nerve cell function in the brain and spinal cord. HEXA encodes the alpha subunit of beta-hexosaminidase A, an enzyme found within lysosomes that plays a key role in breaking down fatty substances in the central nervous system.
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, also known as hexosaminidase subunit alpha, contains the genetic blueprint for producing the alpha subunit of the enzyme beta-hexosaminidase A. This enzyme is essential for normal neurological function, particularly within the central nervous system, which includes the brain and spinal cord. Its primary role involves breaking down specific fatty molecules, called GM2 gangliosides, within cellular compartments known as lysosomes. Proper functioning of the HEXA gene is therefore critical for maintaining cellular health and preventing the accumulation of toxic substances.
Disruptions to the HEXA gene's function can lead to the build-up of GM2 gangliosides, causing cellular damage and severe health consequences. Conditions linked to HEXA gene variants are typically inherited in an autosomal recessive pattern, meaning an individual must inherit two altered copies of the gene (one from each parent) to develop the condition. Tay-Sachs disease is the most well-known condition associated with pathogenic HEXA variants.
What the gene does
The HEXA gene provides instructions for synthesising the alpha subunit of the beta-hexosaminidase A enzyme. This alpha subunit combines with a beta subunit, produced by the HEXB gene, to form the complete and functional beta-hexosaminidase A enzyme. This complex enzyme is strategically located within lysosomes, which are cellular organelles acting as the cell's recycling centres, responsible for breaking down waste materials and toxic substances.
A crucial function of beta-hexosaminidase A involves the catabolism of GM2 ganglioside, a complex fatty molecule found in cell membranes, particularly abundant in nerve cells. Within the lysosome, the enzyme facilitates the breakdown of GM2 ganglioside into simpler components, preventing its accumulation. This process is vital for maintaining the health and proper function of neurons in the central nervous system. When the HEXA gene is altered, the resulting enzyme may be non-functional or have reduced activity, leading to the build-up of GM2 gangliosides. This accumulation becomes toxic to nerve cells, resulting in progressive neurological damage characteristic of associated conditions.
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Chromosome location
The HEXA gene is situated on the long arm (q) of chromosome 15, specifically at position 15q23. Chromosomes are organised structures of DNA and proteins found within cells, carrying genetic information. Humans typically have 23 pairs of chromosomes. The designation '15q23' precisely indicates the gene's physical address on chromosome 15, helping researchers and clinicians locate and study it. The HEXA gene spans a region of approximately 35,000 base pairs on this chromosome and contains 14 exons.
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 alpha subunit combines with a beta subunit to create the active enzyme. The enzyme contains a specific region critical for its function in hydrolysing GM2 gangliosides, located at amino acids 423-424. This region is vital for the enzyme's ability to break down the fatty GM2 ganglioside molecules within lysosomes.
Key variants
Variants within the HEXA gene can disrupt the normal production or function of the alpha subunit of the beta-hexosaminidase A enzyme. These genetic changes can range from single nucleotide substitutions to larger deletions or insertions within the gene's sequence. Over 210 distinct variants have been identified that are associated with inherited conditions. The clinical severity and presentation of conditions related to HEXA variants often depend on the specific type of variant and its impact on enzyme activity, with some variants leading to complete loss of function and severe disease, while others may result in residual enzyme activity and milder phenotypes.
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. This is an autosomal recessive neurodegenerative disorder characterised by the progressive death of nerve cells in the brain and spinal cord. Individuals affected by Tay-Sachs disease experience a range of neurological symptoms, including movement difficulties, intellectual and developmental disabilities, and other severe neurological problems due to the toxic accumulation of GM2 gangliosides in neurons.
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 recognised in the UK for its association with several inherited conditions, as indicated by its inclusion in multiple NHS Genomic Medicine Service national test directories. It is listed on panels for conditions such as Ataxia and cerebellar anomalies, Dystonia, chorea or related movement disorder, Early onset or syndromic epilepsy, Hereditary ataxia, Leukodystrophy, and Lysosomal storage disorder. Notably, HEXA is specifically included for Tay-Sachs disease (R286) and for conditions classified as a Likely inborn error of metabolism (R98) and Undiagnosed metabolic disorders, underscoring its importance in diagnosing a range of neurological and metabolic conditions within the NHS.
Sources: NHS GMS PanelApp · Genomics England PanelApp · NHS National Genomic Test Directory
Diet & lifestyle considerations
Maintaining a healthy lifestyle, including a balanced diet and regular physical activity, generally supports overall well-being. However, there is no conclusive research indicating that specific lifestyle changes can prevent or treat conditions directly caused by HEXA gene variants, such as Tay-Sachs disease, as these are genetic in origin. General recommendations for individuals with genetic conditions often focus on supportive care and symptom management. Consult a healthcare professional for personalised advice.
Supplement considerations
Currently, there is no scientific evidence to suggest that specific dietary supplements can prevent, treat, or alleviate the symptoms of conditions caused by HEXA gene variants. Genetic conditions like Tay-Sachs disease result from underlying genetic alterations that affect enzyme function, which supplements cannot correct. Always consult a healthcare professional before taking any new supplements, especially if managing a genetic condition.
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. This enzyme is crucial for breaking down GM2 gangliosides, a type of fatty molecule, within the lysosomes of nerve cells.
What happens if the HEXA gene has a pathogenic variant?
Pathogenic variants in the HEXA gene can lead to reduced or absent beta-hexosaminidase A enzyme activity. This causes GM2 gangliosides to accumulate to toxic levels in nerve cells, resulting in cellular damage and neurological problems.
Which condition is most commonly associated with HEXA gene variants?
The most commonly associated condition is Tay-Sachs disease, an autosomal recessive neurodegenerative disorder characterised by progressive neurological decline due to the build-up of GM2 gangliosides.
How is Tay-Sachs disease inherited?
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 or supplements treat conditions related to HEXA gene variants?
No, there is currently no scientific evidence that specific lifestyle changes or dietary supplements can prevent, treat, or cure conditions caused by HEXA gene variants. Management typically focuses on supportive care for symptoms.
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