Tay-Sachs Disease 2012

From Bioinformatikpedia

By Alice Meier and Jonas Reeb

Summary

Tay-Sachs disease (TSD) is an autosomal recessive, neurodegenerative disorder. It is a form of GM2 gangliosidosis which, in the classic infantile form, is usually fatal by the age of 2 or 3 years. Failure to degrade the ganglioside GM2 leads to accumulation of this product in the central nervous system's neurons, eventually causing the affected cells' premature death <ref name=Myerowitz2002> Myerowitz,R. et al. (2002) Molecular pathophysiology in Tay-Sachs and Sandhoff diseases as revealed by gene expression profiling. Human molecular genetics, 11, 1343-50.</ref>. The disease is named after the first descriptors Warren Tay <ref name=Tay1881> Tay,W. (1881) Symmetrical changes in the region of the yellow spot in each eye of an infant. Transactions of the Ophthalmological Society, 1, 55-57.</ref> and Bernard Sachs <ref name=Sachs1887> Sachs,B. (1887) On arrested cerebral development with special reference to its cortical pathology. Journal of Nervous Mental Disease, 14, 541-554.</ref>.

Phenotype

Infantile TSD

TSD can be further classified into the three forms: infantile (acute), juvenile (subacute) and adult TSD. The most common and classical form of TSD is the infantile variant. A phenotypic feature common to all variants is the "cherry red" macula. Since Hex A deficiency leads to GM2 accumulation in nerve cells, this also applies to the retinal ganglion cells. In the vertebrate eye, these are positioned between the light source and the rod and cone cells that actually register the light. However, since the foveal pit in the macula is the point of highest acuity, it is usually depleted of ganglion cells to improve the achieved resolution <ref name=Bolon2011> Bolon,B. and Butt,M. (2011) Fundamental Neuropathology for Pathologists and Toxicologists: Principles and Techniques John Wiley and Sons.</ref> <ref name=Suvarna2008>Suvarna,J. and Hajela,S. (2008) Cherry-red spot. Journal of Postgraduate Medicine, 54, 54-57. </ref> . This allows a view onto the outer retinal layers, where the red color simply stems from the blood flow. For the rest of the retina, the accumulated GM2 in ganglion nerve cells leads to a decreased transparency and altered color. Therefore the red spot seen in the macula is in fact the only portion of the retina that has the normal color. This phenotypic trait however is not exclusive to TSD. Other storage diseases like Gaucher's disease or Adult Niemann Pick disease also cause a red macula <ref name=Suvarna2008/>.

Other common phenotypes are blindness, closely related with the above mentioned effects that cause the red spot, as well as a disturbance of gait, general detoriations of motor functions and seizures <ref name=Jeyakumar2002>Jeyakumar,M. et al. (2002) Glycosphingolipid lysosomal storage diseases: therapy and pathogenesis. Neuropathology and applied neurobiology, 28, 343-57. </ref>. A startled response to sound has been reported as an early detection method as well <ref name=Schneck1964> Schneck,L. et al. (1964) The startle response and serum enzyme profile in early detection of Tay-Sachs’ disease. The Journal of Pediatrics, 65, 749-756.</ref>, while at a later stage, deafness is another reported symptom <ref name=Gason2003> Gason,A. et al. (2003) Evaluation of a Tay-Sachs disease screening program. Clinical genetics, 63, 386-92.</ref>.

Other forms of TSD

Juvenile and adult TSD are rare. Effects like a deterioration of motor functions and general weakness are present, albeit less strong compared to the infantile form. In the adult variant other prominent features like blindness and seizures are not exhibited anymore <ref name=Jeyakumar2002/>. While patients with juvenile TSD, showing symptoms as early as one year of age, usually die at an age of around 15 years <ref name=Maegawa2006> Maegawa,G. et al. (2006) The natural history of juvenile or subacute GM2 gangliosidosis: 21 new cases and literature review of 134 previously reported. Pediatrics, 118, 1-26.</ref>, the adult variant of TSD is often non-fatal <ref name=Desnick2001> Desnick,R.J. and Kaback,M.M. (2001) Tay-sachs disease Academic Press.</ref>.

Nonetheless there is no cure for any TSD variant <ref name=Desnick2001/>. Although the adult variant might not lead to death, current treatment can only slow the disease's progress <ref name=Maegawa2007> Maegawa,G. et al. (2007) Pyrimethamine as a potential pharmacological chaperone for late-onset forms of GM2 gangliosidosis. Journal of Biological, 282, 9150-9161. </ref>. For more information on the ongoing research in this field, see the according section below.

Cross-references

Prevalence

In the general population TSD is rare with 1 case in 201000 live births and a carrier frequency of 1 in 300 people <ref name=Maegawa2006/>. However, in Ashkenazi Jews (1 in 30) and eastern Quebec French Canadians (1 in 14) the carrier frequency is much higher. Carrier screenings have been set up successfully over 30 years ago to reduce births of infants with TSD, or other common diseases, in the Jewish community <ref name=Charrow2004> Charrow,J. (2004) Ashkenazi Jewish genetic disorders. Familial cancer, 3, 201-6.</ref> <ref name=Schneider2009>Schneider,A. et al. (2009) Population-based Tay-Sachs screening among Ashkenazi Jewish young adults in the 21st century: Hexosaminidase A enzyme assay is essential for accurate testing. American journal of medical genetics. Part A, 149A, 2444-7. </ref> as well as other community with increased risk <ref name="21yearstsdscreening">D’Souza, G., McCann, C. L., Hedrick, J., Fairley, C., Nagel, H. L., Kushner, J. D., & Kessel, R. (2000). Tay-Sachs disease carrier screening: a 21-year experience. Genetic testing, 4(3), 257-63. doi:10.1089/10906570050501470</ref>.

Genetic basis

TSD is caused by mutations in the HEXA gene on human chromosome 15. HEXA codes for the alpha subunit of the dimers beta-Hexosaminidase A and S. The beta subunit is coded for by the gene HEXB on chromosome 5. HEXA is a recessive gene, therefore TSD only occurs in patients that carry a defective copy of HEXA on both autosomal chromosomes. In addition some of the alleles show compound heterozygosity <ref name=Charrow2004/>.

Cross references

Biochemical basis

GM2

GM2 is a ganglioside and therefore composed of a glycosphingolipid with at least one sialic acid attached to the sugar chain. A more specific name for GM2 is β-D-GalNAc-(1→4)-[α-Neu5Ac-(2→3)]-β-D-Gal-(1→4)-β-D-Glc-(1↔1)-N-octadecanoylsphingosine. From this one can derive, that the sialic acid in this case is α-Neu5Ac. Figure 1 shows GM2 with annotated subunits.

Figure 1: Shown is the sceletal formula of the ganglioside GM2. Names of sugars are highlighted in green, the single sialic acid in orange and the components of the sphingolipid in blue. Red numbers denote the numbering of C atoms. The purple lightning symbol highlights the glycosidic bond broken by Hex A. The figure was adapted from Wikipedia

Hexosaminidase

beta-Hexosaminidase A (Hex A) is a heterodimer consisting of an alpha and a beta subunit. Its structure is shown in Figure 2. Hex A is an essential enzyme for the degradation of GM2 and found in lysosomes. In presence of the cofactor GM2-activator protein (GM2AP) the alpha subunit of Hex A catalyzes the removal of β-D-GalNAc from GM2, resulting in GM3 that is then further processed until sphingosine remains. The position of Hex A in the broader picture of glycosphingolipid degradation is depicted in the Kegg pathway hsa00604, shown in Figure 3.

Figure 2: Shown is Hex A with the alpha subunit in red and beta subunit in blue. Figure has been rendered in pymol based on PDB entry 2gjx
Figure 3: Shown is the KEGG pathway for glycosphingolipid biosynthesis. Hex A and its alpha subunit's substrate GM2 are highlighted in red. The figure has been adapter from KEGG

Catalytic activity

The details of the catalytic process have been proposed by Lemieux et al. <ref name=Lemieux2006>Lemieux,M. et al. (2006) Crystallographic Structure of Human beta-Hexosaminidase A: Interpretation of Tay-Sachs Mutations and Loss of GM2 Ganglioside Hydrolysis. Journal of molecular biology, 359, 913-29. </ref> and are outlined in Figure 4. As can be seen, no residues of GM2AP are directly involved in the process. The task of GM2AP is solely the delivery of GM2 to Hex A. The residues of Hexosaminidase that stabilize the complex and carry out the nucleophilic attack might be interesting targets for a later analysis.

Figure 4: Shown is the proposed catalytic process in Hex A. The figure has been adapted from <ref name=Lemieux2006/>. Note that GM2 is only processed by the alpha subunit's active site of Hex A.

From the same publication two high resolution structures are available in the PDB entries 2GK1 and 2GJX. Figure 5 is based on one of these structures and gives an idea of the conditions in the alpha subunit active site. R178 is a mutation site (dbsnp) and the importance of D322 is highlighted in Figure 4. Since the bound substrate is NGT, this shows that the Hex A inhibitor forms at least some of the hyrdogen bonds that are also likely to form with the native substrate GM2. In fact the authors that solved the structure performed a docking with GM2 that also suggests every hydrogen bond shown in Figure 5 <ref name=Lemieux2006/>.

Figure 5: Shown is the active site on the alpha subunit of Hex A as modeled in PDB entry 2gk1. The inhibitor NGT is highlighted in red. The alpha subunits' surface is shown as mesh, while residues that have polar contacts to NGT (according to Pymol), are explicitly shown as sticks and labelled.

Isozymes

While Hex A is the only relevant structure for TSD, homodimeric isozymes consisting of two beta subunits (Hexosaminidase B) and two alpha subunits (Hexosaminidase S) also exist <ref name=Desnick2001/>.

Mutants

Disease causing Hex A mutants exhibit differing effects: Mutations might interfer with posttranslational modifications or directly affect catalytic activity. Premature termination by frameshifts have also been observed. The results are precursor molecules trapped in the endoplasmatic reticulum, failure of alpha subunits to associate with the beta subunit or a completely unfunctional catalytic site <ref name=Desnick2001/>. Interestingly it has been shown that although both alpha and beta subunit are known to be affected by proteolytic cleavage apart from the signal peptide trimming, these cleavages are not necessary for full catalytic activity <ref name=Desnick2001/>. For a list of point mutations please refer to the according section below.

Nomenclature

Since there is contradicting nomenclature used in the literature, in the following, HEXA and HEXB always refer to the genes and their respective sequences. Hexosaminidase A and B denote the respective isozymes, i.e. the alpha/beta and beta/beta heterodimers. This might be abbreviated to Hex A and Hex B. If no further description is given, the text is referring to Hex A. Lastly, the alpha and beta subunit are always explicitly referred to as such.

Cross-references

Distinction to other sphingolipidoses

Sphingolipidoses describes a class of diseases that are related to the degradation of sphingolipids. Since GM2 contains sphingosine (c.f. Figure 1), TSD is one of them. In the following, some related sphingolipidoses will be mentioned.

Hexosaminidase related

While TSD was the first reported <ref name=Tay1881/> <ref name=Sachs1887/>, it is strongly related with two other gangliosidoses: Sandhoff disease and the AB variant are also both autosomal recessive diseases, affect the degradation of GM2, lead to comparable phenotypes and usually have a fatal outcome <ref name=Jeyakumar2002/>. In addition, a B1 variant has been reported that also shows the phenotype of TSD, however could initially not be detected by the usual enzyme assays since only the catalytic site in the alpha subunit is defective <ref name=Desnick2001/> <ref name=Gordon1987>Gordon,B.A. et al. (1987) Tay-Sachs Disease : B 1 Variant. Pediatric Neurology, 4, 54-57. </ref>. Table 1 gives an overview of the four types of GM2 gangliosidosis.


Table 1: The three major types of GM2 gangliosidosis. The column Alt. Name denotes the name based on the scheme introduced by Sandhoff et al. <ref name=Sandhoff1971> Sandhoff,K. et al. (1971) Enzyme alterations and lipid storage in three variants of Tay-Sachs disease. Journal of neurochemistry, 18, 2469-89. </ref>. It describes the types of hexosaminidase isozymes that remain functional. Further names used in the literature are noted as well. The Gene column shows the defective gene and the chromosome it is found on.

Name Alt. Names Gene OMIM
TSD Variant B, Type I GM2-gangliosidosis 15:HEXA 272800
TSD B1 variant Variant B1 15:HEXA 272800
Sandhoff disease Variant 0, Type II GM2-gangliosidosis 5:HEXB 268800
AB variant Variant AB 5:GM2A 272750


Other

There are more related monogenic lipid storage disorders caused by defects of enzymes involved in the glycosphingolipid catabolism. Of these, Gaucher Disease and Fabry Disease are topics of other groups in the practical. Figure 6 gives an overview of the glycosphingolipid catabolism and shows how these diseases relate to each other in the pathway.

Figure 6: Shown are some of the important steps in the glycosphingolipid catabolism and which diseases are caused by malfunction of enzymes. Diseases covered in this year's practical are highlighted in red. The figure has been adapter from <ref name=Jeyakumar2002/>.

Mutations

While there are many mutations known, few stand out for the abundance or historical importance during the investigation of TSD. As an example the frameshift mutation 1278insTATC accounts for 80% of all mutant alleles in the subgroup of Ashkenazi Jews <ref name=Charrow2004/>. Together with another well known mutation, 1421+1G>C, causing a splice site change, these two mutations account for more than 95% of all mutants in this population group. On other hand G269S has long been known in association with the rare late onset type of TSD <ref name=Maegawa2006/> <ref name=Charrow2004/>. A list of mutations collected during a later task can be found here.

Reference sequence

Cross-references

Diagnosis and Prevention

A first indication of TSD is the appearance of the typical "cherry red" macula in the retina of the eye. It marks the onset of TSD and can be spotted by any standard physician <ref name=Navon1971> Navon,R. and Padeh,B. (1971) Prenatal diagnosis of Tay-Sachs genotypes. British medical journal, 4, 17-20.</ref>. Besides this phenotypic diagnosis, molecular screening is used for a precise identification of TSD affected individuals. Screening techniques are also applied in prenatal diagnosis or carrier testing. A prenatal diagnosis detects whether a fetus has two defect copies of the HEXA gene. Carrier testings are conducted for mate selection in high risk populations. Here, the potential parents are checked for whether they are heterozygous carriers of the mutated allele <ref name=TriggsRaine1992> Triggs-Raine,B.L. et al. (1992) A pseudodeficiency allele common in non-Jewish Tay-Sachs carriers: implications for carrier screening. American journal of human genetics, 51, 793-801.</ref>. Two screening methods are common: Enzyme assys and DNA analysis.

Enzyme Assay

Enzyme assay techniques test for a lower concentration level of hexosamindase A. The tests are conducted with blood serum and thus applicable on a large scale <ref name=TriggsRaine1992/>.

DNA Analysis

DNA Analysis employs PCR based techniques to identificate mutations in the HEXA gene. Small tissue samples are obtained and purified. The sample of DNA is amplified and then tested with genetic markers to identify actual mutations <ref name=Schneider2009/>.

Research

Research has as not yet found a cure for TSD. There are however several therapies which are studied in ongoing research.

Gene therapy

The Goal of gene therapy is the transport of the healthy gene into the diseased cells. This is facilitated with a viral vector, e.g. the AAV vector. Late research is done with animal testing. A model organism for TSD research are jacob sheep, who express the same biochemical properties as the human body <ref name=Torres2010> Torres,P. a et al. (2010) Tay-Sachs disease in Jacob sheep. Molecular genetics and metabolism, 101, 357-63.</ref>. The current state of animal research is the identification of the optimal method of vector delivery into disease cells. The investigated possibilities are injection into the cerebrospinal fluid or injection directly into the brain <ref name=TSDConsort>Tay-Sachs Gene Therapy Consortium, A 3-year roadmap to a gene therapy clinical trial for Tay-Sachs Disease.</ref>

Enzyme replacement therapy

An enzyme replacement therapy is not possible intravenously because of the blood-brain barrier. Alternatively the enzyme can be injected directly into the cerebrospinal fluid. Matsuoka et al. <ref name=Matsuoka2011> Matsuoka,K. et al. (2011) Therapeutic potential of intracerebroventricular replacement of modified human β-hexosaminidase B for GM2 gangliosidosis. Molecular Therapy, 19, 1017-24.</ref> designed a genetically engineered a Hexosaminidase B chimera, containing some of the alpha subunit's residues. This enzyme was injected into the cerebrospinal fluid of mice and partially restored GM2 ganglioside degradation in the brain <ref name=Matsuoka2011/>.

Substrate reduction therapy

This approach is aimed at decreasing the synthesis-rate of GM2 gangliosidosis. The inhibition should be strong enough to levels where the residual activity of the mutant catabolic enzyme is sufficient to prevent pathological substrate accumulation. Therefore, substrate reduction therapy is only useful for patients where the enzyme is present and not trapped in the endoplasmatic reticulum or otherwise completely unfunctional. An example compound is N-butyldeoxynojirimycin (NB-DNJ), an imino sugar that inhibits the ceramide-specific glucosyltransferase which catalyses the first step of ganglioside synthesis. This agent has been reported to slow accumulation of stored glycolipid in an in vitro model of Gaucher’s disease and in knockout mouse models of Tay-Sachs and Sandhoff disease <ref name=Lachmann2001>Lachmann,R.H. and Platt,F.M. (2001) Substrate reduction therapy of glycosphingolipid storage disorders. Expert Opinion on Investigational Drugs, 10, 455-466. </ref>.

References

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Tasks



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