Tay-Sachs disease (TSD) is hereditary. A mutation in the Hex-A gene causes the body to have no or very low levels of the Hex-A enzyme. Without Hex-A, cells (especially nerve cells in the brain) are unable to break down fatty waste products. The waste products begin to build up inside the cells causing it to swell and eventually die.
Since the Hex-A gene was isolated in 1985 it is believed there are between 75 and 100 different mutations that can cause TSD. The various mutations can result in Infantile (most common), Juvenile or Late On-Set. The Infantile and Juvenile forms of the disease have never been treatable and have always resulted in death. The Late On-Set form works at a slower pace, but continues to progress as cells clog with waste product.
The pace of the disease is directly related to the amount of Hex-A the affected person has. Infantile cases generally have no Hex-A present and therefore the disease progresses very quickly – often resulting in death by age 3. The damage caused by the gene mutation actually begins to occur in the fetal stages of development, but symptoms are often not evident for months after birth. Most TSD sufferers appear totally normal at birth and for some time after that.
A person can only be affected by Tay-Sachs Disease when two carrier parents pass a Hex-A gene mutation to their child. When two-carrier parents have children, three outcomes are possible.
- Both parents do not pass the gene mutation to the child – child will be normal.
- One parent pass the gene mutation to the child but the other does not – child will not suffer from TSD but will be a carrier of the Tay-Sachs gene.
- Both parents pass the gene mutation to the child. Child will suffer from TSD and depending upon the mutations passed will likely die at a very young age.
As we look at these four possible outcome – there is a 25% chance both parents do not pass the gene mutation to the child; a 50% chance one of the parents passes the gene mutations; and a 25% chance both parents pass the gene mutation. See the chart below:
If only one of the parents is a carrier of the Tay-Sachs gene mutation then only two outcomes are possible:
- That one parent passes the gene mutation to the child and the child is a carrier of the Tay-Sachs gene
- The parent does not pass the gene mutation and the child is normal and not a carrier
A one-carrier family has a 50% chance that no gene mutation is passed and the children are not carriers and a 50% chance the gene mutation is passed and the child is a carrier. A one-carrier family cannot have an affected child. See chart below: