Difference between revisions of "Tay-Sachs Disease 2012"
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== Diagnosis and Prevention == |
== Diagnosis and Prevention == |
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Revision as of 12:31, 19 April 2012
By Alice Meier and Jonas Reeb
Contents
Summary
Tay-Sachs disease (TSD) is a form of GM2 gangliosidosis. Failure to degrade gangliosides leads to accumulation of these products in the central nervous system's neurons and usually to death of the patient.
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 macula is the point of highest acuity, it is usually depleted of ganglion cells to improve the achieved resolution [Suvarna2008]. 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 [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 [Jeyakumar2002]. A startled response to sound has been reported as an early detection method as well [Schneck1964].
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 vairant other prominent features like blindness and seizures are not exhibited anymore [Jeyakumar2002]. While patients with juvenile TSD, showing symptoms as early as one year of age, usually die at an age of around 15 years [Maegawa2006], the adult variant of TSD is often non-fatal [ref! book? TODO].
Nonetheless there is no cure for any TSD variant [Desnick2001]. Although the adult variant might not lead to death, current treatment can only slow the disease's progress [Maegawa2007]. For more information on the ongoing research in this field, see the according section below.
Cross-references
See also description of this disease in:
Prevalence
In the general population TSD is rare with 1 case in 201000 live births and a carrier frequency of 1 in 300 people [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 in the Jewish community [Charrow2004,Schneider2009].
Genetic basis
TSD is caused by mutations in HEXA on chromosome 15. HEXA codes for the alpha subunit of the alpha/beta heterodimer beta-Hexosaminidase A. The beta subunit is coded for by HEXB. HEXA is a recessive gene, therefore TSD only occurs in patients that carry a defective copy of HEXA on both autosomal chromosomes.
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 of 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. <xr id=fig:tsd_gm2/> shows GM2 with annotated subunits.
<figure id="fig:tsd_gm2">
</figure>
Hexosaminidase
beta-Hexosaminidase A (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 (cf. KEGG pathway [hsa00604]).
Catalytic activity
The details of the catalytic process have been proposed in [Lemieux2006] and are outlined in <xr id="fig:tsd_hexa_catalysis">. As can be seen, no residues of GM2AP are directly involved in the process. The task of GM2AP is 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 id="fig:tsd_hexa_catalysis">
</figure>
From the same publication two high resolution structures are available in the PDB entries 2GK1 and 2GJX. <xr id="fig:tsd_hexa_active_site"> 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 <xr id="fig:tsd_hexa_catalysis">. 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 also performed a docking with GM2 that also suggests every hydrogen bond shown in figure <xr id="fig:tsd_hexa_active_site"> [Lemieux2006].
<figure id="fig:tsd_hexa_active_site">
</figure>
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 [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 [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 . For a list of single 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 subunits are always explicitly referred to as such.
Cross-references
Distinction to other sphingolipidoses
While TSD was the first reported [Tay1881,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 [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 [Desnick2001, Gordon1987]. <xr id="tbl:tsd_types_overview"/> gives an overview of the four types of GM2 gangliosidosis.
<figtable id="tbl:tsd_types_overview">
| 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 |
</figtable>
- Other shingolipidoses: (that others cover!) also see overview picture src: http://apps.who.int/classifications/icd10/browse/2010/en#/E75.0
- adapted version in folder
- Gaucher
- Fabry
Mutations
While there are many mutations known, few stand out for the abundance or place in the investigation of TSD. As an example the frameshift mutation 1278insTATC accounts for 80% of all mutant alleles in the subgroup of Ashkenazi Jews [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 group. On other hand G269S has long been known in association with the rare late onset type of TSD [Maegawa2006, Charrow2004].
<figtable id="tbl:tsd_hexa_mutations">
| Mutation | Effect | Reference | dbSNP | Comment |
|---|---|---|---|---|
| P25S | TSD (late infantile) | |||
| L39R | TSD (infantile) | |||
| L127F | TSD | |||
| L127R | TSD (infantile) | |||
| R166G | TSD (late infantile) | |||
| R170Q | TSD (infantile) | ;; inactive or unstable protein | ||
| R170W | TSD (infantile) | |||
| R178C | TSD (infantile) | ;; inactive protein | ||
| R178H | TSD (infantile) | ;; inactive protein | ||
| R178L | TSD (infantile) | rs28941770 | ||
| Y180H | TSD | rs28941771 | ||
| V192L | TSD (infantile) | |||
| N196S | TSD | |||
| K197T | TSD | |||
| V200M | TSD | rs1800429 | ||
| H204R | TSD (infantile) | |||
| S210F | TSD (infantile) | |||
| F211S | TSD (infantile) | |||
| S226F | TSD | |||
| R247W | TSD | in HEXA pseudodeficiency | ||
| R249W | TSD | in HEXA pseudodeficiency | ||
| G250D | TSD (juvenile) | |||
| G250S | TSD | |||
| R252H | TSD | |||
| R252L | TSD | |||
| D258H | TSD (infantile) | |||
| G269D | TSD | |||
| G269S | TSD | ; late onset; inhibited subunit dissociation | ||
| S279P | TSD (late infantile) | |||
| S293I | TSD | rs1054374 | in | |
| N295S | TSD | |||
| M301R | TSD (infantile) | |||
| D314V | TSD | |||
| I335F | TSD | |||
| V391M | TSD | ; mild; associated with spinal muscular atrophy | ||
| N399D | TSD | rs1800430 | in | |
| W420C | TSD (infantile) | ;; inactive protein | ||
| I436V | TSD | rs1800431 | in | |
| G454S | TSD (infantile) | |||
| G455R | TSD (late infantile) | |||
| C458Y | TSD (infantile) | |||
| W474C | TSD | ; subacute | ||
| E482K | TSD (infantile) | |||
| L484Q | TSD (infantile) | |||
| W485R | TSD (infantile) | |||
| R499C | TSD (infantile) | |||
| R499H | TSD (juvenile) | |||
| R504C | TSD (infantile) | rs28942071 | ||
| R504H | TSD (juvenile) | ;; inhibited subunit dissociation |
</figtable>
no mapping done, since that is task 5!
Cross-referenzes
Diagnosis and Prevention
TSD can be diagnosed well be the appearence of the "cherry red" macula in the retina of the eye. It marks the onset of TSD and can be spotted by any standard physician [Navon1971]. Beside this phenotypic diagnosis molecular screening is used for a precise identification of TSD individuals. Screening techniques are also applied for prenatal diagnosis or carrier testing. A prenatal diagnosis detects whether a fetus has two defect copies of the HEXA gene. Carriers testing is done for mate selection in high risk populations. Here the potential parents get to know whether they are heterozygous carriers of the mutated allele [Triggs-Raine1992]. Methods for Screening:
Enzyme Essay Enzyme assay techniques test for a lower concentration leven of hexosamindase A. The test are conducted with blood serum and thus applicable on a large scale [Triggs-Raine1992].
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 [Schneider2009].
Research
Research has as yet not revealed a treatment for TSD patients. 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 conducted 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 [Torres2010]. 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 [Disease].
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. designed a genetically engineered Hexosaminidase B with a part of the alpha-subunit based on homology modelling. This enzyme was injected into the cerebrospinal fluid of mice and partially restored GM2 ganglioside degradation in the brain [Matsuoka2011].
Substrate reduction therapy
This approach is aimed at decreasing the synthesis-rate of GM2 gangliosidosis. This inhibition should be strong enough as to levels where the residual activity of the mutant catabolic enzyme is sufficient to prevent pathological substrate accumulation. An example herefore is N-butyldeoxynojirimycin (NB-DNJ), an imino sugar that inhibits the ceramide-specific glucosyltransferase that 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 diseases [Lachmann2001].
Tasks
- Task 2: Sequence Alignments
- Task 3: Proteinsequence-based predictions
- Task 4: Homology modelling
- Task 5: SNPS, databases
- Task 6: Sequence-based mutation analysis
- Task 7: Structure-based mutation analysis
- Task 8: MD simulation
- Task 9: Normal mode analysis
- Task 10: MD simulation analysis
Templates
Ref books etc
Or, better,: <ref>Template:Cite book </ref>
Ref images
Multiple work but might need some manual tweaking, see commented out code. Using only a single images works. See <xr id=fig:singleimg/>. blabla bal <figure id="fig:singleimg">
</figure>
