Tay-Sachs disease

Tay-Sachs disease (TSD) is a genetic condition that affects the nervous system. It is caused by an alteration in a gene on chromosome 15. TSD is more commonly seen in people of Ashkenazi Jewish or French-Canadian descent, but it can occur in people from any background.

What happens in Tay-Sachs disease#

Tay-Sachs disease is a degenerative condition, which means symptoms become worse over time. The nerve cells in the brain and spinal cord are gradually destroyed, leading to paralysis. Symptoms usually first appear at around six months of age in a baby who had previously seemed healthy.

The life expectancy for children with TSD is around five years of age, and there is currently no effective treatment. Males and females are equally affected.

Why the nervous system is affected#

The cells of the nervous system (neurones), including those in the brain and spinal cord, need an enzyme called beta-hexosaminidase A (HexA) to regularly break down a fatty substance called GM2 ganglioside.

In children with TSD, the gene that tells the neurones to produce HexA is altered, so the neurones do not make the enzyme. As a result, the fatty substance builds up in the brain and spinal cord, causing cell damage and death.

How Tay-Sachs disease is inherited#

The genes in our cells come in pairs. We inherit one copy of each gene from each of our parents. TSD follows an autosomal recessive inheritance pattern, which means both copies of the gene need to be altered for a person to have the condition.

If a person inherits a working HEXA gene from one parent and an altered HEXA gene from the other, they are a genetic carrier. Carriers do not develop TSD.

To develop TSD, a child must inherit an altered gene copy from both parents. This means both parents must be genetic carriers. When two carriers conceive a child, there is:

• a one-in-four chance the child will have TSD
• a two-in-four chance the child will be a genetic carrier of TSD
• a one-in-four chance the child will be completely unaffected and not a carrier

If only one parent carries the altered gene, none of their children will develop the condition, but each child has a 50 per cent chance of being a genetic carrier.

Symptoms of Tay-Sachs disease#

The symptoms of TSD in a young baby can include:

• movement problems, such as losing the ability to smile, reach out, hold objects, turn over or sit up
• seizures
• vision and hearing impairment
• exaggerated reactions to loud noises

Testing for Tay-Sachs disease#

If TSD runs in your family, start by discussing testing options with your doctor. There are two main ways to test whether a person is a genetic carrier of TSD.

The first is to check the level of the HexA enzyme in the person’s blood. Because a carrier has only one working copy of the gene, they have about half the usual amount of HexA in their bloodstream. This is still enough for normal brain function. If levels are found to be around 50 per cent, the result can be confirmed by examining the HEXA gene itself.

The second method examines the person’s DNA directly for an altered HEXA gene. The DNA is usually collected by sampling the cells lining the inside of the cheek. In the “mouthwash test,” the person swills a mouthful of water and spits it into a container. DNA can also be collected by wiping the inside of the cheek with a cotton swab. Examining a person’s DNA is the more accurate test.

Counselling and support#

If your child or a family member has been diagnosed with TSD, or if it runs in your family, it can help to speak with a genetic counsellor. Genetic counsellors are health professionals qualified in both counselling and genetics.

As well as providing emotional support, they can help you understand TSD, what causes it, how it is inherited, and what a diagnosis means for your child’s health and development. They are trained to provide information and support that is sensitive to your family circumstances, culture and beliefs.

A genetic counsellor can explain what genetic testing options are available to you and other family members. You may choose to see one if you are planning a family, to understand your risk of passing the condition on, or to arrange prenatal tests. If testing shows that you and your partner are both carriers, a genetic counsellor can advise you about your reproductive options.

Being a TSD genetic carrier does not pose any health risk to you. One working gene copy is all that is needed to produce enough HexA to regularly break down the fatty substance in the brain and spinal cord.

Key points#

• Tay-Sachs disease is caused by an alteration in a gene on chromosome 15
• it is more commonly seen in people of Ashkenazi Jewish or French-Canadian descent
• it is a degenerative condition, meaning symptoms become worse over time
• symptoms usually first appear at around six months of age in a previously healthy baby
• life expectancy for children with TSD is around five years of age, and there is currently no effective treatment

Where to get help#