Tay-Sachs Disease

Tay-Sachs disease is a rare inherited disorder that results in slow destruction of the central nervous system (brain and spinal cord).

Tay-Sachs disease is a rare metabolic * disorder with severe neurologic (nervous system) symptoms. The term metabolic refers to the body's chemical processes that produce protein and other substances and break down nutrients to release energy. Tay-Sachs disease is a metabolic disorder because it is caused by the absence of the enzyme * (a type of protein) hexosaminidase A (Hex-A). Hex-A is necessary for breaking down fatty substances called lipids. Without Hex-A, these lipids build up in, and eventually destroy, the nerve cells in the brain. Ultimately, the nervous system stops functioning properly.

The most common form of Tay-Sachs disease (classical Tay-Sachs) affects children and is usually fatal. It is caused by a complete lack of Hex-A. Destruction of nerve cells begins before birth, but affected babies do not begin to lose nerve function until they are about six months old. By age two, the child may have seizures * and begins to lose skills such as crawling, sitting, turning over, and reaching for objects. Eventually, the child will be blind, paralyzed, and intellectually disabled. Children with this form of Tay-Sachs disease do not live past five years of age.

A variation of this scenario occurs when children develop symptoms between the ages of two and five years rather than as an infant. The same symptoms emerge, but the disease progresses more slowly. Children with this form usually die by age 15.

Late-onset Tay-Sachs disease (LOTS) is less common than the infantile form. It affects teenagers and adults in their 20s and 30s by causing a gradual loss of nerve function. People with LOTS have low levels of Hex-A rather than a complete lack of it. As LOTS develops, people affected by it may grow clumsy, uncoordinated, and moody. They may experience muscle weakness, twitching, slurred speech, and intellectual impairment. The symptoms vary in type and severity from person to person. Because this form develops so gradually, life expectancy of affected people seems to be similar to that of unaffected people.

Tay-Sachs disease is caused by a mutation (abnormal change) in the gene that codes for Hex-A. It is a recessive trait, which means that people will have the disease if they have two copies of the defective gene, but they will not have the disease if they have at least one unaffected copy. People with one normal copy and one defective copy are called carriers because they can pass the disease on to their children.

Just about anyone can be a carrier of the gene for Tay-Sachs disease. In the general population, about 1 in 250 people carries the gene. However, some populations of people include more carriers than others. For example, 1 in 27 people of eastern European Jewish (Ashkenazi) descent in the United States is a carrier. People of French-Canadian ancestry from one part of Quebec and the Cajun population in Louisiana also have a higher than usual risk of carrying the Tay-Sachs gene.

Although researchers are looking for a way to prevent or treat Tay-Sachs disease, as of 2015, no treatment or cure was known. However, tests have been developed that allow people to find out if they carry the defective gene. Blood tests can determine the level of Hex-A in people's blood (carriers have about half as much as noncarriers), and DNA tests may find evidence of mutations in the Hex-A gene. Testing is particularly useful for people who have had relatives with Tay-Sachs disease and for people in high-risk populations. Finding out about risk before having a baby can prevent an afflicted child from suffering and prevent the parental anguish of watching a child develop and then die from TaySachs disease.

Prenatal tests also exist for women who are already pregnant. The amniotic fluid (the fluid in which the fetus develops) or the chorionic villus (structures inside the mother's uterus) both contain fluid from the developing baby that can be sampled and tested for the presence of HexA. If Hex-A is present, that means that the fetus does not have Tay-Sachs disease.

See also Genetic Diseases: Overview • Metabolic Disease

* metabolic (meh-tuh-BALL-ik) pertains to the process in the body (metabolism) that converts food into energy and waste products.

* enzyme (EN-zime) is a protein that helps speed up a chemical reaction in cells or organisms.

* seizures (SEE-zhurs) are sudden bursts of disorganized electrical activity that interrupt the normal functioning of the brain, often leading to uncontrolled movements in the body and sometimes a temporary change in consciousness.