GM2 gangliosidosis deficiency
Hexosaminidas A deficiency
Tay–Sachs disease ( 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 as gangliosides 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 Ashkenazi Jewish people.
Tay–Sachs disease is typically first noticed in infants around 6 months old displaying an abnormally strong response to sudden noises or other stimulus, known as the "startle response", because they are startled. 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.
- Infantile Tay–Sachs disease. Infants with Tay–Sachs disease appear to develop normally for the first six months after birth. Then, as neurons become distended with gangliosides, a relentless deterioration of mental and physical abilities begins. The child may become blind, deaf, unable to swallow, atrophied, and paralytic. Death usually occurs before the age of four.
- Juvenile Tay–Sachs disease. Juvenile Tay–Sachs disease is rarer than other forms of Tay–Sachs, and usually is initially seen in children between two and ten years old. People with Tay–Sachs disease develop cognitive and motor skill deterioration, dysarthria, dysphagia, ataxia, and spasticity. Death usually occurs between the age of five to fifteen years.
- Adult/Late-Onset Tay–Sachs disease. A rare form of this disease, known as Adult-Onset or Late-Onset Tay–Sachs disease, usually has its first symptoms during the 30s or 40s. In contrast to the other forms, late-onset Tay–Sachs disease is usually not fatal as the effects can stop progressing. It is frequently misdiagnosed. It is characterized by unsteadiness of gait and progressive neurological deterioration. Symptoms of late-onset Tay–Sachs – which typically begin to be seen in adolescence or early adulthood – include speech and swallowing difficulties, unsteadiness of gait, spasticity, cognitive decline, and psychiatric illness, particularly a schizophrenia-like psychosis. People with late-onset Tay–Sachs may become full-time wheelchair users in adulthood.
Until the 1970s and 1980s, when the disease's molecular genetics became known, the juvenile and adult forms of the disease were not always recognized as variants of Tay–Sachs disease. Post-infantile Tay–Sachs was often misdiagnosed as another neurological disorder, such as Friedreich's ataxia.
Tay–Sachs disease is an autosomal recessive genetic disorder, meaning that when both parents are carriers, there is a 25% risk of giving birth to an affected child with each pregnancy. The affected child would have received a mutated copy of the gene from each parent.
Tay–Sachs results from mutations in the HEXA gene on human chromosome 15, which encodes the alpha-subunit of beta-N-acetylhexosaminidase A, a lysosomal enzyme. By 2000, more than 100 different mutations had been identified in the human HEXA gene.These mutations have included single base insertions and deletions, splice phase mutations, missense mutations, and other more complex patterns. Each of these mutations alters the gene's protein product (i.e., the enzyme), sometimes severely inhibiting its function. In recent years, population studies and pedigree analysis have shown how such mutations arise and spread within small founder populations. Initial research focused on several such founder populations:
- Ashkenazi Jews. A four base pair insertion in exon 11 (1278insTATC) results in an altered reading frame for theHEXA gene. This mutation is the most prevalent mutation in the Ashkenazi Jewish population, and leads to the infantile form of Tay–Sachs disease.
- Cajun. The same 1278insTATC mutation found among Ashkenazi Jews occurs in the Cajun population of southern Louisiana. Researchers have traced the ancestry of carriers from Louisiana families back to a single founder couple – not known to be Jewish – who lived in France in the 18th century.
- French Canadians. Two mutations, unrelated to the Ashkenazi/Cajun mutation, are absent in France but common among French Canadians living in eastern Quebec and Acadians from the Province of New Brunswick. Pedigree analysis suggests the mutations were uncommon before the late 17th century.
In the 1960s and early 1970s, when the biochemical basis of Tay–Sachs disease was first becoming known, no mutations had been sequenced directly for genetic diseases. Researchers of that era did not yet know how common polymorphisms would prove to be. The "Jewish Fur Trader Hypothesis," with its implication that a single mutation must have spread from one population into another, reflected the knowledge at the time. Subsequent research, however, has proven that a large variety of different HEXA mutations can cause the disease. Because Tay–Sachs was one of the first genetic disorders for which widespread genetic screening was possible, it is one of the first genetic disorders in which the prevalence of compound heterozygosity has been demonstrated.
Compound heterozygosity ultimately explains the disease's variability, including the late-onset forms. The disease can potentially result from the inheritance of two unrelated mutations in the HEXA gene, one from each parent. Classic infantile Tay–Sachs disease results when a child has inherited mutations from both parents that completely stop the biodegradation of gangliosides. Late onset forms occur due to the diverse mutation base – people with Tay–Sachs disease may technically be heterozygotes, with two differing HEXA mutations that both inactivate, alter, or inhibit enzyme activity. When a patient has at least one HEXA copy that still enables some level of hexosaminidase A activity, a later onset disease form occurs. When disease occurs because of two unrelated mutations, the patient is said to be a compound heterozygote.
Heterozygous carriers (individuals who inherit one mutant allele) show abnormal enzyme activity, but manifest no disease symptoms. This phenomenon is called dominance; the biochemical reason for wild-type alleles' dominance over nonfunctional mutant alleles in inborn errors of metabolism comes from how enzymes function. Enzymes are protein catalysts for chemical reactions; as catalysts, they speed up reactions without being used up in the process, so only small enzyme quantities are required to carry out a reaction. Someone homozygous for a nonfunctional mutation in the enzyme-encoding gene has little or no enzyme activity, so will manifest the abnormal phenotype. A heterozygote (heterozygous individual) has at least half of the normal enzyme activity level, due to expression by the wild-type allele. This level is normally enough to enable normal function and thus prevent phenotypic expression.
Three main approaches have been used to prevent or reduce the incidence of Tay–Sachs:
- Prenatal diagnosis. If both parents are identified as carriers, prenatal genetic testing can determine whether the fetus has inherited a defective gene copy from both parents. Chorionic villus sampling (CVS), the most common form of prenatal diagnosis, can be performed between 10 and 14 weeks of gestation. Amniocentesis is usually performed at 15–18 weeks. These procedures have risks of miscarriage of 1% or less.
- Preimplantation genetic diagnosis. By retrieving the mother's eggs for in vitro fertilization, it is possible to test the embryo for the disorder prior to implantation. Healthy embryos are then selected and transferred into the mother's womb, while unhealthy embryos are discarded. In addition to Tay–Sachs disease, preimplantation genetic diagnosis has been used to prevent cystic fibrosis and sickle cell anemia among other genetic disorders.
- Mate selection. In Orthodox Jewish circles, the organization Dor Yeshorim carries out an anonymous screening program so that couples with Tay–Sachs or another genetic disorder can avoid conception.
In patients with a clinical suspicion for Tay–Sachs disease, with any age of onset, the initial testing involves an enzyme assay to measure the activity of hexosaminidase in serum, fibroblasts or leukocytes. Total hexosaminidase enzyme activity is decreased in individuals with Tay-Sachs as is the percentage of hexosaminidase A. After confirmation of decreased enzyme activity in an individual, confirmation by molecular analysis can be pursued. All patients with infantile onset Tay–Sachs disease have a "cherry red" macula in the retina, easily observable by a physician using an ophthalmoscope. This red spot is a retinal area that appears red because of gangliosides in the surrounding retinal ganglion cells. The choroidal circulation is showing through "red" in this foveal region where all retinal ganglion cells are pushed aside to increase visual acuity. Thus, this cherry-red spot is the only normal part of the retina; it shows up in contrast to the rest of the retina. Microscopic analysis of the retinal neurons shows they are distended from excess ganglioside storage. Unlike other lysosomal storage diseases (e.g., Gaucher disease, Niemann–Pick disease, and Sandhoff disease), hepatosplenomegaly (liver and spleen enlargement) is not seen in Tay–Sachs.
Children with this disease have symptoms that get worse over time. They usually die by age 4 or 5.
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.