Tay-Sachs Disease 2012

By Alice Meier and Jonas Reeb

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, juvenile 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:

• OMIM
• Wikipedia

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">

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

</figure>

Hexosaminidase

Beta-Hexosaminidase A (Hex A) is an essential enzyme for the degradation of GM2. 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">

Shown is the proposed catalytic process in Hex A. The figure has been adapted from [Lemieux2006].

</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">

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.

</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.

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">

The three major types of GM2 gangliosidosis. The column Alt. Name denotes the name based on the scheme introduced by Sandhoff et al. [Sandhoff1971]. 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

</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">

List of known HEXA mutations. It was adapted from the curated UniProtKB:Swissprot article[REF]. [MORe DBS + refs] were checked as well but did not contain additional information... add the terminating ones! and then really make a short googling

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!

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">

File:Ts.jpg

Standard Agent's Life-Cycle

</figure>