Breaking down Tay-Sachs

Tay-Sachs disease is a heritable metabolic condition that affects the neurons in the brain. The disease is more common in infants and young children as well as people of Ashkenazi Jewish descent, although it can occur in any ethnicity. Symptoms of the disease most commonly manifest themselves in children around six months of age. However, it is possible to develop symptoms from five years old to the teenage years. There are three different forms of the disease, each appearing at different stages of life: infantile, juvenile, and adult. The adult form is much rarer and non-fatal but can still cause neuron dysfunction and psychosis. Early symptoms of the disease include mobility issues such as difficulty crawling, and as the disease progresses, the child may suffer from seizures, vision, and hearing loss. In the classic infantile form, the disease is fatal within the first few years of life or by three to five years old. In infants, infection and respiratory complications, such as pneumonia, are the most common cause of death.

Being categorised as an autosomal recessive disease means that in order to display the phenotype, two copies of the mutated HEXA gene must be present in an individual. This HEXA gene is located on chromosome 15 and is responsible for producing enzymes that affect the nerve cells. The carrier frequency of Tay-Sachs is highly dependent on ethnic backgrounds, with carrier frequency being 1 in 30 for those of Ashkenazi Jewish descent and 1 in 300 for others. The chance of developing the disease early or late is predicated on the specific type of HEXA mutation that is inherited within the family. Meaning, if one child in a family possesses the infantile form, all other members of the family will also possess the infantile form (if they express the phenotype). When both parents are carriers of the Tay-Sachs gene mutation, there is a 25% chance with each pregnancy that the child will inherit two mutated copies of the HEXA gene and thus be affected by the disease. Also, there is a 50% chance the child will be a carrier like the parents and a 25% chance the child will inherit two normal copies of the gene and be unaffected. Furthermore, this particular type of gene mutation results in the disease being commonly labelled as a hexosaminidase A deficiency.

The HEXA gene’s significance in the disease is further highlighted due to its ability to code for specific alpha subunits in the enzyme β-hexosaminidase A. This enzyme is involved in breaking down molecules that can be recycled in a cell through the use of lysosomes. This key cellular function helps a cell undergo apoptosis (programmed cell death) or help evade bacteria that can damage a cell. However, in individuals with this HEXA gene mutation, less of the enzyme β-hexosaminidase A is produced, which results in less degradation of GM2 ganglioside. GM2 ganglioside is a lipid involved in a host of processes such as membrane organisation, neuronal differentiation, and signal transduction. In addition, due to its lack of degradation, it accumulates inside the body. The rate at which the lipid accumulates inside the cell ultimately determines the form of Tay-Sachs an individual will possess. It is worth noting that this GM2 ganglioside pathology also includes other diseases, such as Sandhoff disease and the AB variant, which have similar disease prognoses. Furthermore, the disease specifically targets the brain as gangliosides are the main lipids that compose neuronal plasma membranes. Their expression is specific to brain regions, impacting key neurodevelopmental processes like neural tube formation and synaptogenesis.

Furthermore, ganglioside synthesis is a highly regulated process facilitated by glycosyltransferases during transcription and post-transcription. They also modulate ion channels and receptor signalling, which are crucial for neurotransmission, memory, and learning. The exact mechanism of how this ganglioside accumulation due to HEXA malfunction leads to neuronal death remains unclear. Figure 1 illustrates the dysfunction of the alpha subunit in HEXA as it cannot break down GM2 gangliosides. This results in an accumulation of GM2 within the liposome, contrasting with its concentration in the external environment. This accumulation of GM2 causes lysosomal dysfunction and eventually cell damage, which leads to the symptoms commonly associated with Tay-Sachs.

Mouse models have been created to understand this GM2 pathway in greater detail to develop treatments. However, this is quite limited as mice do not have the same pathway of breaking down GM2 as humans. Also, since the disease may be prevalent before birth, it is hard to establish the damage done to a baby inside the womb, making reversing this disease in infants very challenging. However, the later onset types of Tay-Sachs disease might respond to treatment. Implementing ganglioside synthesis inhibitors in combination with existing DNA and enzymatic screening programs holds promise for eventually managing and controlling this condition.

Parents can undergo genetic screening to assess their risk of carrying the Tay-Sachs gene, which is done by doing a simple blood test that examines the DNA for mutations in the HEXA gene. Genetic screening is particularly important for couples who have a family history of Tay-Sachs disease or who belong to ethnic groups with a higher prevalence of the condition. Early detection through genetic screening allows couples to make informed reproductive decisions, such as pursuing in vitro fertilisation with preimplantation genetic testing or opting for prenatal testing during pregnancy to determine if the foetus has inherited the mutated gene. Utilising the acronym SHADES as a mnemonic to recognise potential signs of Tay-Sachs disease in their child can help parents get a prompt medical evaluation if any symptoms arise.

SHADES:

Startle response

Hearing loss

Affecting vision

Developmental delay

Epileptic seizures

Swallowing difficulties

Written by Hamran Shah

REFERENCES

Center, N. (2015). Tay-Sachs disease. Nih.gov. Available at: https://www.ncbi.nlm.nih.gov/books/NBK22250/.

Leal, A.F., Benincore-Flórez, E., Solano-Galarza, D., Garzón Jaramillo, R.G., Echeverri-Peña, O.Y., Suarez, D.A., Alméciga-Díaz, C.J. and Espejo-Mojica, A.J. (2020). GM2 Gangliosidoses: Clinical Features, Pathophysiological Aspects, and Current Therapies. International Journal of Molecular Sciences, 21(17), p.6213. doi:https://doi.org/10.3390/ijms21176213.

Ramani, P.K. and Parayil Sankaran, B. (2022). Tay-Sachs Disease. PubMed. Available at: https://www.ncbi.nlm.nih.gov/books/NBK564432/.