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Normal Function
The HEXB gene provides instructions for making a protein that is one part (the beta subunit) of two related enzymes, beta-hexosaminidase A and beta-hexosaminidase B. Each of these enzymes is made up of two subunits. Beta-hexosaminidase A includes one alpha subunit (produced from the HEXA gene) and one beta subunit. Beta-hexosaminidase B is composed of two beta subunits.
Beta-hexosaminidase A and beta-hexosaminidase B play a critical role in nerve cells (neurons) in the brain and spinal cord (central nervous system). In neurons, these enzymes are found in lysosomes, which are structures in cells that break down toxic substances and act as recycling centers. Within lysosomes, the beta-hexosaminidase A and B enzymes break down fatty compounds called sphingolipids, complex sugars called oligosaccharides, and molecules that are linked to sugars (such as glycoproteins). In particular, beta-hexosaminidase A forms part of a complex that breaks down a fatty substance called GM2 ganglioside.
Health Conditions Related to Genetic Changes
Sandhoff disease
Sandhoff disease is caused by variants (also known as mutations) in the HEXB gene. These variants reduce or eliminate the activity of both beta-hexosaminidase A and beta-hexosaminidase B. A reduction in enzyme activity results in an inability to properly break down GM2 ganglioside and other molecules, which allows these compounds to accumulate within cells. Increased levels of GM2 ganglioside are particularly toxic to neurons in the central nervous system. Excess GM2 ganglioside leads to the progressive destruction of these cells, which causes many of the characteristic features of Sandhoff disease.
Most of the known variants in the HEXB gene cause the severe form of Sandhoff disease, which becomes apparent in infancy. These variants prevent cells from making any beta-hexosaminidase A or beta-hexosaminidase B, or lead to the production of completely nonfunctional versions of these enzymes. The most common variant deletes a large segment of DNA near the beginning of the HEXB gene, which results in a total loss of enzyme activity. Other variants reduce but do not eliminate the activity of the enzymes; these genetic changes are responsible for the less severe forms of Sandhoff disease, which appear later in life.
More About This Health ConditionOther Names for This Gene
- beta-N-acetylhexosaminidase B
- ENC-1AS
- Hex B
- HEXB_HUMAN
- hexosaminidase B
- hexosaminidase B (beta polypeptide)
Additional Information & Resources
Tests Listed in the Genetic Testing Registry
Scientific Articles on PubMed
Catalog of Genes and Diseases from OMIM
References
- Mahdieh N, Mikaeeli S, Tavasoli AR, Rezaei Z, Maleki M, Rabbani B. Genotype, phenotype and in silico pathogenicity analysis of HEXB mutations: Panel based sequencing for differential diagnosis of gangliosidosis. Clin Neurol Neurosurg. 2018 Apr;167:43-53. doi: 10.1016/j.clineuro.2018.02.011. Epub 2018 Feb 8. Citation on PubMed
- Maier T, Strater N, Schuette CG, Klingenstein R, Sandhoff K, Saenger W. The X-ray crystal structure of human beta-hexosaminidase B provides new insights into Sandhoff disease. J Mol Biol. 2003 May 2;328(3):669-81. doi: 10.1016/s0022-2836(03)00311-5. Citation on PubMed
- Mark BL, Mahuran DJ, Cherney MM, Zhao D, Knapp S, James MN. Crystal structure of human beta-hexosaminidase B: understanding the molecular basis of Sandhoff and Tay-Sachs disease. J Mol Biol. 2003 Apr 11;327(5):1093-109. doi: 10.1016/s0022-2836(03)00216-x. Citation on PubMed or Free article on PubMed Central
- Myerowitz R, Lawson D, Mizukami H, Mi Y, Tifft CJ, Proia RL. Molecular pathophysiology in Tay-Sachs and Sandhoff diseases as revealed by gene expression profiling. Hum Mol Genet. 2002 May 15;11(11):1343-50. doi: 10.1093/hmg/11.11.1343. Citation on PubMed
- Tavasoli AR, Parvaneh N, Ashrafi MR, Rezaei Z, Zschocke J, Rostami P. Clinical presentation and outcome in infantile Sandhoff disease: a case series of 25 patients from Iranian neurometabolic bioregistry with five novel mutations. Orphanet J Rare Dis. 2018 Aug 3;13(1):130. doi: 10.1186/s13023-018-0876-5. Citation on PubMed
- Tim-Aroon T, Wichajarn K, Katanyuwong K, Tanpaiboon P, Vatanavicharn N, Sakpichaisakul K, Kongkrapan A, Eu-Ahsunthornwattana J, Thongpradit S, Moolsuwan K, Satproedprai N, Mahasirimongkol S, Lerksuthirat T, Suktitipat B, Jinawath N, Wattanasirichaigoon D. Infantile onset Sandhoff disease: clinical manifestation and a novel common mutation in Thai patients. BMC Pediatr. 2021 Jan 7;21(1):22. doi: 10.1186/s12887-020-02481-3. Citation on PubMed
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