Tay–Sachs Disease

Tay–Sachs disease is a rare genetic disorder that causes the destruction of nerve cells in the brain and spinal cord. The most common form is infantile Tay–Sachs, which typically manifests around the age of three to six months.

This is characterised by loss of motor skills, followed by seizures, hearing loss, and eventually death by age three to five. Juvenile and late-onset forms are less severe but still result in significant morbidity and mortality.

Cherry-red spot as seen in the retina in Tay–Sachs disease.

Tay–Sachs is caused by mutations in the HEXA gene on chromosome 15, which leads to a deficiency in the enzyme hexosaminidase A. This enzyme deficiency results in the accumulation of GM2 gangliosides in neurons, causing cellular toxicity.

Overview

Signs and Symptoms

Infantile

Infants appear normal for the first six months before showing symptoms such as decreased ability to turn over, sit, or crawl. This is followed by progressive deterioration, including blindness, deafness, and paralysis. Death usually occurs before the age of four.

Juvenile

This form appears in children aged between two and ten years, characterised by cognitive and motor skill deterioration, dysarthria, dysphagia, ataxia, and spasticity. Death typically occurs between the ages of five and fifteen.

Late-Onset

Late-onset Tay–Sachs usually presents in adolescence or early adulthood and is characterised by unsteadiness of gait, speech, and swallowing difficulties, cognitive decline, and psychiatric symptoms such as schizophrenia-like psychosis. This form is often misdiagnosed and may result in wheelchair dependence but is usually not fatal.

Diagnosis

Diagnosis typically involves measuring hexosaminidase A levels in blood, fibroblasts, or leukocytes. Genetic testing can confirm mutations in the HEXA gene. A distinctive "cherry-red" spot on the retina is often observed, which is due to ganglioside accumulation in retinal ganglion cells.

Pathophysiology

Tay–Sachs disease is caused by insufficient activity of the enzyme hexosaminidase A, which is essential for breaking down sphingolipids in lysosomes. The lack of this enzyme leads to the accumulation of GM2 gangliosides, primarily in neurons, causing cellular dysfunction and death.

Tay–Sachs disease is inherited in an autosomal recessive pattern.

The HEXA gene is located on the long (q) arm of human chromosome 15, between positions 23 and 24.

Genetics

The disease is inherited in an autosomal recessive pattern, meaning both parents must be carriers. The most prevalent mutation in the Ashkenazi Jewish population is a four base pair insertion in exon 11 of the HEXA gene. Compound heterozygosity, involving two different mutations in the HEXA gene, can result in variable disease severity, including late-onset forms.

Prevention

Prevention strategies include prenatal diagnosis through chorionic villus sampling or amniocentesis, preimplantation genetic diagnosis in IVF, and pre-marriage screening programmes, especially in high-risk populations like Ashkenazi Jews.

Treatment and Management

Currently, there is no cure for Tay–Sachs disease. Treatment is supportive, aiming to ease symptoms and prevent complications. This includes feeding tubes for infants who cannot swallow and medications to manage psychiatric symptoms in late-onset forms.

Research Directions

Enzyme Replacement Therapy

Efforts are being made to replace the nonfunctional enzyme using methods such as intrathecal enzyme delivery, although crossing the blood-brain barrier remains a significant hurdle.

Gene Therapy

Recent advancements include using adeno-associated viruses to deliver functional HEXA genes to brain cells, showing promise in initial trials.

Substrate Reduction Therapy

Research is looking at the use of alternative enzymes and drugs like miglustat to reduce GM2 ganglioside accumulation.

Animal Models

The Jacob sheep model, which has a similar HEXA gene mutation, offers a promising avenue for gene therapy trials.

Founder effects occur when a small number of individuals from a larger population establish a new population.

Epidemiology

Tay–Sachs disease is more common in certain populations, such as Ashkenazi Jews, French Canadians, and Cajuns. In the general population, the carrier frequency is about 1 in 300. The founder effect and heterozygote advantage theories have been proposed to explain the high prevalence in these groups.

Self-assessment MCQs (single best answer)

What is the main cause of Tay–Sachs disease?

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At what age do symptoms of infantile Tay–Sachs typically begin to manifest?

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What genetic pattern does Tay–Sachs disease follow?

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Which population has a higher prevalence of Tay–Sachs disease due to a specific mutation?

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What is a distinctive clinical sign observed in the retina of individuals with Tay–Sachs disease?

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What method is commonly used for prenatal diagnosis of Tay–Sachs disease?

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What is the most common form of Tay–Sachs disease?

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Which treatment option is currently available for managing Tay–Sachs disease?

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What is the consequence of the mutation in the HEXA gene?

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What animal model is being used to trial gene therapy for Tay–Sachs disease?

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