The GM2 gangliosidoses an inherited lysosomal storage disorders that result from a deficiency of the enzyme beta-hexosaminidase. This enzyme catalyzes the biodegradation of fatty acid derivatives known as gangliosides. The diseases are better known by their individual names Tay-Sachs and Sandoff disease.
Tay-Sachs disease, AB variant, and Sandhoff disease might easily have been defined together as a single disease, because the three disorders are associated with failure of the same metabolic pathway and have the same outcome. Classification and naming for many genetic disorders reflects history, because most diseases were first observed and classified based on biochemistry and pathophysiology before genetic diagnosis was available. However, the three GM2 gangliosidoses were discovered and named separately. Each represents a distinct molecular point of failure in a subunit that is required for activation of the enzyme.
Tay–Sachs disease (also known as GM2 gangliosidosis or hexosaminidase A deficiency) is a rare autosomal recessive genetic disorder. In its most common variant (known as infantile Tay–Sachs disease), it causes a progressive deterioration of nerve cells and of mental and physical abilities that begins around six months of age and usually results in death by the age of four. The disease occurs when harmful quantities of cell membrane components known asgangliosides accumulate in the brain's nerve cells, eventually leading to the premature death of the cells. A ganglioside is a form of sphingolipid, which makes Tay–Sachs disease a member of the sphingolipidoses. There is no known cure or treatment.
The disease is named after the British ophthalmologist Waren Tay, who in 1881 first described a symptomatic red spot on the retina of the eye; and after the American neurologist Bernard Sachs of Mount Sinai Hospital, New York, who described in 1887 the cellular changes of Tay–Sachs disease and noted an increased disease prevalence in AshkenaziJewish people.
Research in the late 20th century demonstrated that Tay–Sachs disease is caused by a genetic mutation in the HEXAgene on (human) chromosome 15. A large number of HEXA mutations have been discovered, and new ones are still being reported. These mutations reach significant frequencies in specific populations. French Canadians of southeastern Quebec have a carrier frequency similar to that seen in Ashkenazi Jews, but carry a different mutation.Cajuns of southern Louisiana carry the same mutation that is seen most commonly in Ashkenazi Jews. HEXAmutations are rare and are most seen in genetically isolated populations. Tay–Sachs can occur from the inheritance of either two similar, or two unrelated, causative mutations in the HEXA gene.
As an autosomal recessive disorder, two Tay–Sachs alleles are required for an individual to exhibit symptoms of the disease. Carriers of a single Tay–Sachs allele do not exhibit symptoms of the disease but appear to be protected to some extent against tuberculosis. This accounts for the persistence of the allele in certain populations in that it confers a selective advantage—in other words, being a heterozygote is advantageous.
Tay–Sachs disease is typically first noticed in infants around 6 months old displaying an abnormally em response to sudden noises or other stimulus, known as the "startle response". There may also be listlessness or muscle stiffness (hypertonia). The disease is classified into several forms, which are differentiated based on the onset age of neurological symptoms.
Two parents carrying a mutated gene and passing it on to their offspring cause the disease. Even with both parents carrying the disease in their genome, there is only a 25% chance that they will have a child containing the genetic coding for the disease.
There is currently no cure or treatment for Tay–Sachs disease. Even with the best care, children with infantile Tay–Sachs disease die by the age of 4. Although experimental work is underway, no current medical treatment of the root cause yet exists. Patients receive supportive care to ease the symptoms or extend life. Infants are given feeding tubes when they can no longer swallow. Improvements in life-extending care have somewhat lengthened the survival of children with Tay–Sachs disease, but no current therapy is able to reverse or delay the disease's progress. In late-onset Tay-Sachs, medication (e.g., lithium for depression) can sometimes control psychiatric symptoms and seizures, although some medications (e.g., tricyclic antidepressants, phenothiazines, haloperidol, and risperidone) are associated with significant adverse effects. In 2011, researchers discovered that Pyrimethamine can increase ß- hexosaminidase activity, thus slowing down the progression of Late-Onset Tay–Sachs disease.
Sandhoff disease symptoms are clinically indeterminable from Tay-Sachs Disease. The classic infantile form of the disease has the most severe symptoms and is incredibly hard to diagnose at this early age. The first signs of symptoms begin before 6 months of age and the parents’ notice when the child begins digressing in their development. If the children had the ability to sit up by themselves or crawl they will lose this ability. This is caused by a slow deterioration of the muscles in the child’s body from the buildup of GM2 gangliosides. Since the body is unable to create the enzymes it needs within the central nervous system it is unable to attach to these gangliosides to break them apart and make them non- toxic. With this buildup there are several symptoms that begin to appear such as muscle/motor weakness, sharp reaction to loud noises, blindness, deafness, inability to react to stimulants, respiratory problems and infections, mental retardation, seizures, cherry red spots in the retina, enlarged liver and spleen (hepatosplenomegaly), pneumonia, or bronchopneumonia.
The other two forms of Sandhoff disease have similar symptoms but to a lesser extent. Adult and juvenile forms of Sandhoff disease are more rare then the infantile form. In these cases victims suffer cognitive impairment (retardation) and a loss of muscle coordination that impairs and eventually destroys their ability to walk; the characteristic red spots in the retina also develop. The adult form of the disease, however, is sometimes milder, and may only lead to muscle weakness that impairs walking or the ability to get out of bed.
AB variant are identical with those of infantile Tay-Sachs disease, except that enzyme assay testing shows normal levels of hexosaminidase A. Infantile Sandhoff disease has similar symptoms and prognosis, except that there is deficiency of both hexosaminidase A and hexosaminidase B. Infants with this disorder typically appear normal until the age of 3 to 6 months, when development slows and muscles used for movement weaken. Affected infants lose motor skills such as turning over, sitting, and crawling. As the disease progresses, infants develop seizures, vision and hearing loss,mental retardation, and paralysis.
An ophthalmological abnormality called a cherry-red spot, which can be identified with an eye examination, is characteristic of this disorder. This cherry-red spot is the same finding that Warren Tay first reported in 1881, when he identified a case of Tay-Sachs disease, and it has the same etiology.
The prognosis for AB variant is the same as for infantile Tay-Sachs disease. Children with AB variant die in infancy or early childhood.