| Tay–Sachs disease | |
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| Classification and external resources | |
Cherry red spot as seen in Tay Sachs disease. The center of the fovea appears bright red because it is surrounded by a milky halo. |
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| ICD-10 | E75.0 |
| ICD-9 | 330.1 |
| OMIM | 272800 272750 |
| DiseasesDB | 12916 |
| MedlinePlus | 001417 |
| eMedicine | ped/3016 |
| MeSH | D013661 |
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 causes a relentless deterioration of mental and physical abilities that commences around six months of age and usually results in death by the age of four.[1]
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 cell membrane components known as 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.
The disease is named after 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 Ashkenazi Jewish 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 populations. French Canadians of southeastern Quebec have a carrier frequency similar to Ashkenazi Jews, but they carry a different mutation. 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.
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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.[1][2]
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 Tay–Sachs disease. Post-infantile Tay–Sachs was often mis-diagnosed as another neurological disorder, such as Friedreich ataxia.[5] People with LOTS frequently become full-time wheelchair users in adulthood. Psychiatric symptoms and seizures can be controlled with medication.[6]
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.[1] This also means that mutations can be passed down through generations without manifesting as a genetic disorder.[7] Autosomal genes are chromosomal genes that are not located on one of the sex chromosomes. Every individual carries two copies of each autosomal gene, one copy from each parent. When both parents carry a mutation, the classic 25% Mendelian ratio determines the likelihood of disease.[1] As with all genetic disease, Tay–Sachs disease may arise from a novel mutation, although such mutations are rare.
Autosomal recessive diseases occur when a child has two defective copies of an autosomal gene, when neither copy can be transcribed or expressed as a functional enzyme product.
The disease results from mutations on chromosome 15 in the HEXA gene encoding the alpha-subunit of beta-N-acetylhexosaminidase A, a lysosomal enzyme. By 2000, more than 100 mutations had been identified in the HEXA gene,[8] and new mutations are still being reported. These mutations have included base pair insertions and deletions, splice site mutations, point mutations, and other more complex patterns. Each of these mutations alters the protein product, and thus inhibits the function of the enzyme. In recent years, population studies and pedigree analysis have shown how such mutations arise and spread within small founder populations. Initial research focused on such founder populations:
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 polymorphism 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 of the time. Subsequent research has proven that a large number of HEXA mutations can cause the disease. Because Tay–Sachs disease 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 was demonstrated.[13]
Compound heterozygosity ultimately explains the variability of the disease, including 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 inactivate the biodegradation of gangliosides. Late onset forms of the disease occur because of the diverse mutation base. Patients may technically be heterozygotes, but with two different HEXA mutations that both inactivate, alter, or inhibit enzyme activity. When a patient has at least one copy of the HEXA gene that still enables hexosaminidase A activity, a later onset form of the disease occurs. When disease occurs because of two unrelated mutations, the patient is said to be a compound heterozygote.[14]
Heterozygous carriers, individuals who inherit one mutant allele, show abnormal enzyme activity, but have no symptoms of the disease. Bruce Korf explains why carriers of recessive mutations generally do not manifest the symptoms of genetic disease: "The biochemical basis for the dominance of wild-type alleles over mutant alleles in inborn errors of metabolism can be understood by considering how enzymes function. Enzymes are proteins that catalyze chemical reactions, so only small quantities are required for a reaction to be carried out. In a person homozygous for a mutation in the gene encoding an enzyme, little or no enzyme activity is present, so he or she will manifest the abnormal phenotype. A heterozygous individual expresses at least 50% of the normal level of enzyme activity due to expression of the wild-type allele. This is usually sufficient to prevent phenotypic expression."[15]
Tay–Sachs disease is caused by insufficient activity of an enzyme called hexosaminidase A that catalyzes the biodegradation of fatty acid derivatives known as gangliosides. Hexosaminidase A is a vital hydrolytic enzyme, found in the lysosomes, that breaks down phospholipids. When Hexosaminidase A is no longer functioning properly, the lipids accumulate in the brain and interfere with normal biological processes. Gangliosides are made and biodegraded rapidly in early life as the brain develops. Patients and carriers of Tay–Sachs disease can be identified by a simple blood test that measures hexosaminidase A activity.[1]
Hydrolysis of GM2-ganglioside requires three proteins. Two of them are subunits of hexosaminidase A, and the third is a small glycolipid transport protein, the GM2 activator protein (GM2A), which acts as a substrate specific cofactor for the enzyme. Deficiency in one of these proteins leads to storage of the ganglioside, primarily in the lysosomes of neuronal cells. Tay–Sachs disease (along with GM2-gangliosidosis and Sandhoff disease) occurs because a genetic mutation inherited from both parents deactivates or inhibits this process. Most Tay–Sachs mutations appear not to affect functional elements of the protein. Instead, they cause incorrect folding or assembly of the enzyme, so that intracellular transport is disabled.[16]
The development of improved testing methods has allowed neurologists to diagnose Tay–Sachs and other neurological diseases with greater precision. All patients with Tay–Sachs disease have a "cherry red" macula, easily observable by a physician using an ophthalmoscope, in the retina.[1] 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.[17] Unlike other lysosomal storage diseases (i.e. Gaucher disease, Niemann-Pick disease, Sandhoff disease), hepatosplenomegaly is not a feature of Tay–Sachs disease.[18]
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.[19]
Three main approaches have been used to prevent or reduce the incidence of Tay–Sachs disease:
Historically, Eastern European people of Jewish descent (Ashkenazi Jews) have a high incidence of Tay–Sachs and other lipid storage diseases. Documentation of Tay–Sachs in this Jewish population reaches back to 15th century Europe. In the United States, about 1 in 27 to 1 in 30 Ashkenazi Jews is a recessive carrier. French Canadians and the Cajun community of Louisiana have an occurrence similar to the Ashkenazi Jews. Irish Americans have a 1 in 50 chance of a person being a carrier. In the general population, the incidence of carriers (heterozygotes) is about 1 in 300.[2]
Three general classes of theories have been proposed to explain the high frequency of Tay–Sachs carriers in the Ashkenazi Jewish population:
Tay–Sachs disease was one of the first genetic disorders for which epidemiology was studied using new molecular data. Studies of Tay–Sachs disease mutations using new molecular techniques such as linkage disequilibrium and coalescence analysis has brought an emerging consensus among researchers in support of the founder effects theory.[26][27][28]
With development and acceptance of the germ theory of disease in the 1860s to the 1870s, the possibility that science could explain, prevent or cure illness prompted medical doctors to undertake more precise description and diagnosis of disease. Warren Tay and Bernard Sachs, two physicians described the progression of the disease precisely and provided differential diagnostic criteria to distinguish it from other neurological disorders with similar symptoms.[29][30]
The World War I time was a period of nativism of hostility to immigrants. Jewish immigration to the United States peaked in the period 1880–1924, with the immigrants arriving from Russia and countries in Eastern Europe. Opponents of immigration often questioned whether immigrants from southern and eastern Europe could be assimilated into American society. Reports of Tay–Sachs disease contributed to a perception among nativists that Jews were an inferior race. Reuter writes, "The fact that Jewish immigrants continued to display their nervous tendencies in America where they were free from persecution was seen as proof of their biological inferiority and raised concerns about the degree to which they were being permitted free entry into the US."[31]
In 1969, John S. O'Brien showed that Tay–Sachs disease was caused by a defect in a enzyme. He also proved that Tay–Sachs disease patients could be diagnosed by enzyme assay of hexosaminidase A.[32] Further development of enzyme assay testing demonstrated that levels of both hexosaminidases A and B could be measured in patients and carriers, allowing reliable detection of heterozygotes. During the early 1970s, researchers developed protocols for newborn testing, carrier screening, and pre-natal diagnosis.[33][34] By the end of the 1970s, researchers had identified three variant forms of GM2 gangliosidosis, including Sandhoff disease and AB variant, accounting for false negatives in carrier testing.[35]
Since carrier testing for Tay–Sachs began in 1971, millions of Ashkenazi Jews have been screened as carriers. Jewish communities embraced the cause of genetic screening from the 1970s on. The success with Tay–Sachs disease has led Israel to become the first country which offers free genetic screening and counseling for all couples. Israel has become a leading center for research on genetic disease. Both the Jewish, Arab, and Palestinian populations in Israel contain ethnic and religious minority groups, and Israel's success with Tay–Sachs disease has led to the development of screening programs for other diseases. The success also opened discussions and debates about the proper scope of genetic testing for other disorders.[36]
Because Tay–Sachs disease was one of the first autosomal recessive genetic disorders for which there was an enzyme assay test (prior to polymerase chain reaction testing methods), it was intensely studied as a model for all such diseases, and researchers sought evidence of a selective process. A continuing controversy is whether heterozygotes (carriers) has selective advantage. Neil Risch writes: "The anomalous presence of four different lysosomal storage disorders in the Ashkenazi Jewish population has been the source of long-standing controversy. Many have argued that the low likelihood of four such diseases — particularly when four are involved in the storage of glycosphingolipids — must reflect past selective advantage for heterozygous carriers of these conditions."[26]
This controversy among researchers has reflected three debates among geneticists at large:
Enzyme replacement therapy techniques have been investigated for lysosomal storage disorders, and could potentially be used to treat Tay–Sachs disease. The goal would be to replace the missing enzyme, a process similar to insulin injections for diabetes. However, the HEXA enzyme has proven to be too large to pass through the blood into the brain through the blood-brain barrier. Blood vessels in the brain develop junctions so small that toxic (or large) molecules cannot enter into nerve cells and cause damage. Researchers have also tried instilling the enzyme into cerebrospinal fluid, which bathes the brain. However, neurons are unable to take up the large enzyme efficiently even when it is placed next to the cell, so the treatment is still ineffective.[38]
Options for gene therapy have been explored for Tay–Sachs and other lysosomal storage diseases. If the defective genes could be replaced throughout the brain, Tay–Sachs could theoretically be cured. However, researchers working in this field believe that they are years away from the technology to transport the genes into neurons, which would be as difficult as transporting the enzyme. Use of a viral vector, promoting an infection as a means to introduce new genetic material into cells, has been proposed as a technique for genetic diseases in general. Hematopoetic stem cell therapy (HSCT), another form of gene therapy, uses cells that have not yet differentiated and taken on specialized functions. Yet another approach to gene therapy uses stem cells from umbilical cord blood in an effort to replace the defective gene. Although the stem cell approach has been effective with Krabbé disease.[39]
Recent research has revealed that this disease exists in flocks of Jacob sheep.[40] Furthermore, the biochemical mechanism for this disease in the Jacob sheep (diminished activity of hexosaminidase A resulting in increased concentrations of GM2 ganglioside) is virtually identical to that observed in humans.[41] Sequencing of the cDNA of the HEXA gene of affected Jacobs reveals an identical number of nucleotides and exons as in the human HEXA gene, and 86% identity in nucleotide sequence.[40] A missense mutation (now referred to as the G444R mutation)[42] was found in the HEXA cDNA of the affected sheep caused by a single nucleotide change at the end of exon 11 resulting in skipping of exon 11. This model of Tay–Sachs disease provided by the Jacob sheep is the first to offer promise as a means for trials of gene therapy which may eventually prove to be useful in the treatment of the disease in humans.[40]
Other highly experimental methods being researched involve Substrate reduction therapy, the aim of which is to use alternative enzymes to increase the brain's catabolism of GM2 gangliosides to a point where residual degradative activity is sufficient to prevent substrate accumulation.[43][44] One experiment has demonstrated that, by using the enzyme sialidase, the genetic defect can be effectively bypassed and GM2 gangliosides can be metabolized so that they become almost inconsequential. If a safe pharmacological treatment can be developed, one that causes the increased expression of lysosomal sialidase in neurons, a new form of therapy, essentially curing the disease, could be on the horizon.[45] Metabolic therapies under investigation for Late-Onset Tay–Sachs disease include treatment with the drug OGT 918 (Zavesca).[46]
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