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What is Tay-Sachs disease?
Tay-Sachs disease (TSD) is an inherited (genetic), incurable disease of the central nervous system. In the classical form, symptoms first appear at about the age of 6 months, when an apparently healthy baby stops smiling, crawling or turning over, loses their ability to grasp or reach out, and gradually becomes blind and paralysed. There is no cure for the disease and death usually occurs before the age of five years. Babies with TSD do not have the correct genetic information to tell his/her body to produce an enzyme called B-Hexosaminidase A (HexA), which is necessary for breaking down certain substances in the brain and nerve cells. These substances then accumulate in the nerve cells causing irreversible damage.
Who gets Tay-Sachs disease?
Genetic diseases are often more common in specific community groups. TSD is most common amongst descendants of Central and Eastern European (Ashkenazi) Jews and some French Canadians.
What causes Tay-Sachs disease?
TSD is caused by inheriting a 'double-dose' of a fault in a gene which codes for the HexA enzyme. Every person has two copies of this gene in each cell of their body. A person will only be affected by TSD if s/he has a fault (mutation) in both copies of the HEXA gene. This can only happen if one faulty copy is inherited from each parent. About one in every 26 Ashkenazi Jews and 1 in every 40 French Canadians carries one copy of the faulty gene and one regular copy of the gene. In the general population, however, only about one in 300 people carry a faulty copy of the HEXA gene. Carriers themselves are not affected by the disease.
How is the disease transmitted?
When two unaffected carriers of the HEXA gene mutation become parents, then for each pregnancy:
There is a 1-in-4 (25%) chance that the child will inherit the regular HEXA gene from both parents and will be completely free of the HEXA gene mutation and will not be affected by TSD There is a 2-in-4 (50%) chance that the child will inherit both a regular copy and a faulty copy of the HEXA gene and be a carrier of the faulty gene, unaffected by TSD, just like his/her parents There is a 1-in-4 (25%) chance that the child will inherit the faulty HEXA gene from both parents. This child will be affected by Tay-Sachs disease.
Does it affect one's health to be a carrier of
the HEXA genetic mutation?
No. Carriers of the HEXA gene mutation are completely unaffected by TSD.
Does Tay-Sachs disease affect only males?
No. Tay-Sachs disease can affect both males and females.
Is it possible to test to see if a person is a
carrier of the faulty HEXA gene?
Yes. Although there is no cure or effective treatment for TSD, genetic carrier testing is available. Extensive genetics carrier testing in various regions of the world has led to the virtual elimination of TSD in these areas.
Why is a person's ancestry relevant
to genetics carrier testing?
There are different faults (mutations) found in genes. Some faults are specific to particular ethnic groups or regions in the world. The various laboratory tests being used have each been designed to detect specific genetic faults that are most common among individuals who are of particular ancestries, origin and descent. Our staff will need to find out whether you are of Ashkenazi or non-Ashkenazi descent, whether you were born Jewish or whether you are from a non-Jewish background.
For whom is genetics carrier testing for TSD relevant?
Testing is available for all men and women over the age of 16. However, it is particularly relevant for Jewish men and women.
How is the testing performed?
A small blood sample is taken and the level of the HexA enzyme is measured. Carriers of the faulty HEXA gene have only half the level of this enzyme compared to non-carriers. The laboratory also examines the actual DNA structure of the HEXA gene in any difficult cases.
Genetic carriers of Sandhoff disease, a disease with a similar presentation to TSD, are also detected incidentally when testing for TSD since the same enzyme test is involved. We will inform you if we think that further tests are required.
What confidence can I have in the test results?
The testing laboratory has been accredited by the International Tay-Sachs disease Testing Quality Control and Data Collection Center. This accreditation is updated each year.
If I do not have a family history of Tay-Sachs
disease do I still need to be tested?
Yes. Over 95% of the couples with children affected by TSD have no family history of the disease because the HEXA faulty gene is silently passed down through the generations. However, if there is a family history, then the risk is much greater. People who have a known family history of TSD should seek further information through genetics counselling.
Is genetic carrier testing available for other conditions?
Yes. The laboratory can test for genetic carriers of some other inherited disorders at the same time as testing for TSD. Testing is already available for cystic fibrosis, Canavan disease and Fanconi Anaemia all of which are genetic conditions inherited in a similar manner to TSD. Over the next few years, testing will become available for other inherited conditions. You will be asked to provide written consent if you want us to test for these additional conditions in the future.
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