Infants with Tay-Sachs disease seem to develop as usual during the first few months of their lives.
Defining Tay-Sachs Disease
Tay-Sachs disease is a form of fatal genetic lipid storage disorder where harmful amounts of a fatty substance known as, 'ganglioside GM2,' build up in a person's nerve cells and tissues in their brain. The condition is caused by a lack of sufficient activity on the part of an enzyme known as, 'beta-hexosaminidase A,' which catalyzes the biodegration of acidic fatty materials such as gangliosides. Gangliosides are both made and biodegraded quickly in early life as the person's brain develops.
Infants with Tay-Sachs disease seem to develop as usual during the first few months of their lives. However, as their nerve cells become distended with fatty material - a process of deterioration involving their mental and physical abilities happens. Children with the disease become deaf, blind, and lose the ability to swallow. Their muscle start to atrophy and paralysis sets in. Additional neurological symptoms experienced by children with Tay-Sachs disease include seizures, dementia, and an increased startle response to noise. There is a much rarer form of Tay-Sachs disease which occurs in people who are in their twenties to early thirties that is characterized by progressive neurological deterioration and an unsteady gait. People with the disease also experience red spots in their eyes.
The occurrence of Tay-Sachs disease is particularly high among persons of Eastern European and Askhenazi Jewish descent. People with the disease, as well as carriers, may be identified through a simple blood test which measures beta-hexosaminidase A activity. Both of a person's parents must carry the mutated gene for them to be affected. When this occurs, there is a twenty-five percent chance with each pregnancy that the child these parents have will be affected by Tay-Sachs disease.
Causes of Tay-Sachs Disease
Tay-Sachs disease results from defects in a particular gene on chromosome fifteen, which codes for the production of enzyme Hex-A. People have two copies of this particular gene; if either or both Hex-A genes are active, a person's body produces enough of the enzyme to prevent buildup of the GM2 ganglioside lipid. People who carry Tay-Sachs disease have one copy of the inactive gene, as well as one copy of the inactive gene; they remain healthy. Carriers do not have the disease, although they have the ability to pass the active gene to children.
People who are carriers of Tay-Sachs disease have a fifty-percent chance of passing the active gene for the disease to their children. Children who inherit one inactive gene for the disease are carriers, like their parents. Should both parents of a child be carriers, and their child inherits the active gene from each of them, the child will have Tay-Sachs disease.
Anyone can be a carrier of Tay-Sachs disease, although the incidence of the disease is much higher among people of Eastern European Jewish descent. About one in every twenty-seven Jews in America is a carrier of the Tay-Sachs disease gene. Non-Jewish French Canadians who live near to the St. Lawrence River, or in the Cajun community of Louisiana, also experience a higher incidence of the disease. Among the general population, approximately one in two-hundred and fifty people carry the gene for Tay-Sachs disease.
Symptoms of Tay-Sachs Disease
There are three forms of Tay-Sachs disease, which are categorized according to the types of symptoms they present, as well as the age at which the person first experiences the symptoms. The most common form of the disease presents symptoms when a child is only three to six months old. The disease progresses from there, rapidly proceeding to death by the time the person reaches the age of four or five years. When the infant is born they seem healthy and develop in a regular fashion for the first few months of their life. As the buildup of ganglioside GM2 starts to affect their nerves, the symptoms associated with Tay-Sachs start to appear. These symptoms can initially include:
Decreased eye contact
Increased startle reaction
Delayed mental and social skills
Slow body growth with increasing head size
The infant stops smiling, crawling or rolling over and loses the ability to grasp or reach out
As the disease continues to progress, the symptoms the child experiences become more dominant. The child then begins experiencing symptoms that include:
Loss of motor skills
Abnormal body tone
Loss of intellectual skills
Rarer forms of Tay-Sachs disease that develop later in a persons life are due to low levels of the Hex-A enzyme, instead of a complete deficiency of Hex-A such as in the infantile form of the disease. Children with juvenile Hex-A deficiency often develop symptoms between the ages of two and five years, commonly dying before reaching age five. A milder form of Hex-A deficiency may develop in people between the ages of five and early age thirties. The symptoms can include an unsteady gait, tremors, slurred speech, and mental illness.
Late-Onset Tay-Sachs disease, which is a much rarer form of the disease, affects adults and causes intellectual and neurological impairment. This form of Tay-Sachs disease has only recently been identified. The disease has not been extensively descried.
Diagnosing Tay-Sachs Disease
Tay-Sachs disease can be identified through a simple blood test. A person's blood samples can be analyzed for DNA studies or enzyme assay. An enzyme assay is a biochemical test that measures the level of Hex-A in the person's blood. People who are carriers of the Tay-Sachs disease have less Hex-A in their blood and cells than people who do not carry the disease.
Carrier testing that is DNA-based looks for particular mutations, or changes in the person's gene that codes for Hex-A. The Hex-A gene was isolated in the year 1985, and since then more than fifty different mutations in the gene have been identified. There are some mutations that remain unknown, however. Current tests have the ability to detect approximately ninety-five percent of people who are carriers among the Jewish population, and approximately sixty-percent of people who are carriers among those in the general population. Genetic testing is available for parents who are carriers of Tay-Sachs disease.
Treatment of Tay-Sachs Disease
Medical science currently has not created a cure for Tay-Sachs disease, or an effective form of treatment. Researchers are pursuing a number of approaches towards finding a cure for the disease. One of these involves enzyme replacement therapy to provide the Hex-A that babies with Tay-Sachs disease lack. Bone marrow transplantation has been attempted, although it has not been successful to date in either reversing or slowing down the damages to a baby's nervous system. An additional path of research being pursued involves gene transfer into cells to replace an abnormal gene. The approach presents promise for people who experience Tay-Sachs disease.