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Tay-Sachs disease

Tay-Sachs disease (abbreviated TSD, also known as GM2 gangliosidosis or Hexosaminidase A deficiency) is an autosomal recessive genetic disorder. In its most common variant known as infantile Tay-Sachs disease it presents with a relentless deterioration of mental and physical abilities which commences at 6 months of age and result ultimately in death usually by the age of four.

It is caused by a genetic defect in a single gene with one defective copy of that gene inherited from each parent. The disease occurs when harmful quantities of gangliosides accumulate in the nerve cells of the brain, eventually leading to the premature death of those cells. There is currently no cure or treatment. Tay-Sachs disease is a rare disease. Other autosomal disorders such as cystic fibrosis and sickle cell anemia are far more common.

The disease is named after the British ophthalmologist Warren Tay who first described the red spot on the retina of the eye in 1881, and the American neurologist Bernard Sachs of Mount Sinai Hospital, New York who described the cellular changes of Tay-Sachs and noted an increased prevalence in the Eastern European Jewish (Ashkenazi) population in 1887.

Research in the late 20th century demonstrated that Tay-Sachs disease is caused by a genetic mutation on the HEXA gene on chromosome 15. A large number of HEXA mutations have been discovered, and new ones are still being reported. These mutations reach significant frequencies in several populations. French Canadians of southeastern Quebec have a carrier frequency similar to Ashkenazi Jews, but they carry a different mutation. Many Cajuns of southern Louisiana carry the same mutation that is most common in Ashkenazi Jews. Most HEXA mutations are rare, and do not occur in genetically isolated populations. The disease can potentially occur from the inheritance of two unrelated mutations in the HEXA gene.

Classification and symptoms
Tay-Sachs disease is classified in variant forms, based on the time of onset of neurological symptoms. The variant forms reflect diversity in the mutation base.

Infantile TSD. Infants with Tay-Sachs disease appear to develop normally for the first six months of life. Then, as nerve cells become distended with gangliosides, a relentless deterioration of mental and physical abilities occurs. The child becomes blind, deaf, and unable to swallow. Muscles begin to atrophy and paralysis sets in. Death usually occurs before the age of 4.
Juvenile TSD. Extremely rare, Juvenile Tay-Sachs disease usually presents itself in children between 2 and 10 years of age. They develop cognitive, motor, speech difficulties (dysarthria), swallowing difficulties (dysphagia), unsteadiness of gait (ataxia), and spasticity. Patients with Juvenile TSD usually die between 5–15 years.
Adult/Late Onset TSD. A rare form of the disorder, known as Adult Onset Tay-Sachs disease or Late Onset Tay-Sachs disease (LOTS), occurs in patients in their 20s and early 30s. LOTS is frequently misdiagnosed, and is usually non-fatal. It is characterized by unsteadiness of gait and progressive neurological deterioration. Symptoms of LOTS, which present in adolescence or early adulthood, include speech and swallowing difficulties, unsteadiness of gait, spasticity, cognitive decline, and psychiatric illness, particularly schizophrenic-like psychosis.

Late Onset Tay-Sachs (LOTS)
Until the 1970s and 80s, when the molecular genetics of the disease became known, the juvenile and adult forms of the disease were not always recognized as variants of TSD. Post-infantile Tay-Sachs was often mis-diagnosed as another neurological disorder, such as Friedreich ataxia. Patients with LOTS frequently become full-time wheelchair users in adulthood, but many live full adult lives if psychiatric and physical difficulties are accommodated. Psychiatric symptoms and seizures can be controlled with medications.

Journalist Janet Silver Ghent describes the experience of Vera, whose Russian Jewish family immigrated to the United States when she was a child: “Twenty years ago, when she was 14, Vera Pesotchinsky’s speech became slurred at times, so her parents sent her to a speech therapist. Later, she began to have coordination problems and occasionally fell. She never could peel potatoes.” Vera’s mother insisted that something was wrong, taking her to specialists in neurology and psychiatry. After 12 years and many misdiagnoses, the Pesotchinsky family received a definitive diagnosis of LOTS. Despite her disability, Vera graduated from Wellesley College and received an MBA from Santa Clara University. Ghent reports that Vera lives independently as an adult, working daily in a family business, and that she is adamant about not becoming a “Tay-Sachs poster child.” Vera describes how she lives with her illness: “You can fall apart and be a wreck or do what you can with it. If I didn’t do what I do, I’d get worse.”

Diagnosis
Development of improved testing methods has allowed neurologists to diagnose Tay-Sachs and other neurological diseases with greater precision. But Tay-Sachs disease is sometimes misdiagnosed at first, because clinicians are not aware that it is not exclusively a Jewish disease.

All patients with Tay-Sachs disease have a “cherry-red” spot, easily observable by a physician using an ophthalmoscope, in the back of their eyes (the retina). This red spot is the area of the retina which is accentuated because of gangliosides in the surrounding retinal ganglion cells (which are neurons of the central nervous system). The choroidal circulation is showing through “red” in this region of the fovea where all of the retinal ganglion cells are normally pushed aside to increase visual acuity. Thus, the cherry-red spot is the only normal part of the retina seen. Microscopic analysis of neurons shows that they are distended from excess storage of gangliosides. Without molecular diagnostic methods, only the cherry red spot, characteristic of all GM2 gangliosidosis disorders, provides a definitive diagnostic sign.

Journalist Amanda Pazornik describes the experience of the Arbogast family: “Payton was a beautiful baby girl — but she would not sit up. Four months passed, and similar milestones seemed to slip away. She wouldn’t roll over. She wouldn’t play with her toys. She still wouldn’t sit up. Payton’s symptoms progressively worsened. Loud noises inexplicably startled her. An inability to coordinate muscle movement between her mouth and tongue caused her to choke on food and produce excessive saliva.” Because neither of Peyton’s parents were Jewish, Her doctors did not suspect Tay-Sachs disease until she was 10 months old, when her ophthalmologist noticed the cherry red spots in her eyes. Payton died in 2006 at the age of 3½.

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