Tay-Sachs is a serious and rare genetic disease that mainly affects children and causes the progressive destruction of their nervous system.
- Classic Tay-Sachs disease, the infantile one, begins to manifest itself between three and six months of life of the newborn.
- Between two and ten years, the juvenile one.
- Subsequently we have the adult / chronic one.
It is caused by the absence of a vital enzyme called hexosaminidase-A (Hex-A). Without the Hex-A. A type of fat called GM2 ganglioside accumulates abnormally inside cells, especially in neurons in the brain.
This accumulation causes serious damage to the cells themselves.
In the infant form. this destructive process already begins in the fetus stage, during pregnancy. However, the affected child does not have symptoms until his development slows down at about 3 to 6 months of age. There is no cure for the infantile form. Its treatment is about managing your symptoms as best you can.
The juvenile form affects mostly children about the age of two: fits of convulsions, and decreases in their mental faculties.
Infants – that is, children up to adolescence – regress and lose the ability to crawl, turn, sit or stretch.
Eventually the affected child goes blind, has severe cognitive problems, is paralysed, is unresponsive.
By the time the child is about four he is so ill that his death usually occurs before the age of five.
In the adult form – a rarer, later-onset form of Tay-Sachs disease – it affects adults causing severe neurological and intellectual problems.
This disease has only recently been identified and is not yet fully described.
Its causes, and how it is passed on from parents
Tay-Sachs disease is due to a defect in a gene located on chromosome 15 which codes for the production of an enzyme called Hex-A. We all have two copies of this gene.
If both, or even just one of these genes is active, the body produces enough of the enzyme to prevent the accumulation of GM2 ganglioside.
Carriers of Tay-Sachs – that is, people who have only one copy of the gene active in two – are healthy.
- They don’t have the disease, but they could be passing the faulty gene to their babies.
- Carriers have a 50% chance of passing the defective gene to their offspring.
The child who inherits the inactive gene, like the parent, becomes a carrier of Tay-Sachs in turn.
If both parents are carriers, and their child inherits both defective genes, this child will be affected.
When both parents are carriers, there is a 25% chance that their child is ill, and a 50% chance that the child is a carrier.
The state of research on treatments
There is no cure for Tay-Sachs disease. Scientists go many ways to find a valid treatment.
Some are using an approach called enzyme replacement therapy to replace the missing enzyme in sick children.
Bone marrow transplantation has also been attempted, but has not been successful in reversing the disease or slowing it down.
Another approach is that of gene therapy (Gene Therapy in English) an approach in which one would like to transfer a normal gene into diseased cells to replace the non-functioning one.
The latter is probably the most promising approach.
How it is identified
A simple blood test can identify carriers of Tay-Sachs disease.
Blood samples can be examined by an enzymatic analysis or by DNA.
- An enzyme test is a biochemical test that measures the level of Hex-A in a person’s blood. Compared to non-carriers, carriers of the disease have a lower level of Hex-A in body fluids and cells.
- DNA-based carrier screening identifies specific mutations or changes in the gene that codes for Hex-A. Since 1985 when the Hex-A gene was isolated, over 50 different mutations in this gene have been identified. However some mutations are not yet known.
If both parents are carriers, they may want to see a genetic counselor to help them decide whether to conceive or have the fetus tested for Tay-Sachs.
Today, most cases of Tay-Sachs disease occur in populations not thought to be at high risk.
Prenatal testing for Tay-Sachs
Prenatal testing for Tay-Sachs disease can be done around the 11th week of pregnancy using CVS.
During CVS, a small piece of placenta is removed.
Alternatively, the fetus can be tested via an amniocentesis around the 16th week of pregnancy. With this procedure, a needle is used to remove and test a sample of the fluid surrounding the baby.
Medically assisted procreation (MAP)
Medically assisted reproductive therapy is an option for carrier couples who do not want to risk giving birth to a child with Tay Sachs.
This new technique – used in conjunction with in vitro fertilization (fertilization done in the laboratory, in which the already fertilized embryos are then transferred into the mother’s uterus) – allows parents with Tay-Sachs to give birth to healthy children.
Embryos created in vitro are tested for Tay-Sachs genetic mutations before being implanted in the mother, thus allowing only healthy embryos to be selected.
In different countries, this matter is regulated differently.
In Italy, medically assisted procreation (MAP) is regulated by law 40 of 2004, which governs that set of treatments for reproductive processes.
Its article 1 establishes that “In order to favor the solution of reproductive problems deriving from human sterility or infertility, recourse to medically assisted procreation is permitted, under the conditions and according to the modalities established by this law, which ensures the rights of all subjects involved, including the conceived “.
This law, which has been the subject of rather heated political and cultural debates and also of various interventions by the Constitutional Court, sets numerous and stringent limits to protect some reasons of a superior nature.
Katherine Johnson, M.D., is a board-certified obstetrician-gynecologist with clinical expertise in general obstetrics and gynecology, family planning, women’s health, and gynecology.
She is affiliated with the Obstetrics and Gynecology division at an undisclosed healthcare institution and the online platform, Maternicity.com.