Structural Biochemistry/Tay-Sachs

Overview
Tay-Sachs is a genetically inherited, autosomal recessive condition that is caused a mutation on chromosome 15; the result is the lack of an essential enzyme within the body. In a normal person, there is the presence of an enzyme called hexosaminidase A (HEX A), which is found in the brain. Its primary function is to metabolize a fatty substance in the brain called GN12 ganglioside, which ironically, is needed for properly brain function when present in small amounts. Individuals with Tay-Sachs do not produce this enzyme because the gene that's responsible for making it is mutated. Consequently, these gangliosides, particularly ganglioside GM2, accumulate in cells, especially targeting the ones in the brain.

The Tay-Sachs gene is the most prevalent among the Ashkenazi Jewish population, where approximately 4% of the population carry this lethal gene.

Mode of Inheritance
Tay-Sach is inherited when both parents have mutated copies of the gene and both are passed down to the offspring. Normal chromosomes are usually present in pairs, known as alleles, each with identical genes and functions that could dominate just in case of a mutation on the other chromosome. Individuals who have wild-type alleles on both chromosomes have no chance of passing it out to their children. However, if one of the parents has the one copy of the Tay-Sachs gene, he/she becomes a carrier for the disease, but since he/she possesses a normal copy, the disease will not be expressed. However, if both parents are carries of the disease, children who inherit both mutate copies will express the disease to fullest extent. The figure to the right shows inheritance of Tay-Sachs. Because this disease is inherited through one of the 22 autosomal pairs and is only expressed with two copies of the recessive gene have been inherited, it is known as an autosomal, recessive disease.

Diagnosis/Symptoms
Tay-Sachs can be diagnosed using blood tests to determine the levels of HEX A in the blood or conducting an eye exam to determine if the cherry red spot, a landmark of Tay-Sachs, is present. There are three forms of Tay-Sachs: infantile, juvenile, and late-onset.

Infants with Tay-Sachs appear normal from birth until about six months of age. After that point, they begin to a experience development complications. Over time, they lose cognitive function, eventually losing the ability to crawl, turn over, or sit up. Other symptoms may encompass inability to swallow and difficulty breathing. By the age of two, infants begin experiencing seizures, loss of mental and muscle function, and overall becoming unconscious of their environment. Most do not live past the age of four or five.



Children afflicted with Tay-Sachs initially begin with a lack of coordination and muscle weakness. Similar to infantile Tay-Sachs, children slowly begin to lose their ability to walk, eat on their own, and communicate; eventually, affected children begin to suffer from seizures as well. In addition, they are also vulnerable to infection, often contracting pneumonia multiple times. Fortunately, the severity of this form is not as severe as infantile Tay-Sachs.

Late-onset Tay-Sachs is perhaps the hardest form to diagnose properly. Adults with these disease of often report of being misdiagnosed with some other illness because they do not possess the cherry red spot that is present on the retina Like the other two forms, adults experience muscle weakness and lack of coordination. The consequences of late-onset Tay-Sachs are not as severe. Many do experience problems breathing and swallowing, but they can still care for themselves. In addition, individual with the disease do not have shortened life spans like the other two forms.

Currently, there are no cures of Tay-Sachs; there are only ways to manage the symptoms.