Tay-Sachs Disease 2012

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 gangliosides leads to accumulation of these products in the central nervous system's neurons eventually causing the affected cells' premature death.

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:

• OMIM
• Wikipedia
• HGMD

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 gene 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 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 <xr id="fig:tsd_hsa_00604">

<figure id="fig:tsd_hsa_00604">

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

</figure>

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.

Cross-references

• KEGG gangliosidoses
• KEGG HEXA
• MetaCyc

Distinction to other sphingolipidoses

Hexosaminidase related

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

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. <xr id=fig:tsd_sphingolipid_catabolism/> gives an overview of the glycosphingolipid catabolism and shows how these diseases relate to each other in the pathway.

<figure id="fig:tsd_sphingolipid_catabolism">

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 [Jeyakumar2002].

</figure>

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 population group. On other hand G269S has long been known in association with the rare late onset type of TSD [Maegawa2006, Charrow2004].

<xr id=tbl:tsd_hexa_mutations/> shows a list of some known mutations in the human HEXA gene. Further sources which might contain additional mutations can be found at the end of this section under cross references.

<figtable id="tbl:tsd_hexa_mutations">

List of known HEXA mutations. It is adapted from the curated UniProtKB/Swissprot entry P06865. Further mutations will be added over the course of the practical.

Mutation	Effect	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
N295S	TSD
M301R	TSD (infantile)
304del	TSD (infantile)		Moroccan Jewish.
D314V	TSD
320del	TSD (late infantile)
I335F	TSD
347_352del	TSD
V391M	TSD		mild; associated with spinal muscular atrophy
N399D	TSD	rs1800430
W420C	TSD (infantile)		inactive protein
I436V	TSD	rs1800431
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>

Reference sequence

The reference sequence of the human HEXA gene as given by the Swissprot entry P06865.

>sp|P06865|HEXA_HUMAN Beta-hexosaminidase subunit alpha OS=Homo sapiens GN=HEXA PE=1 SV=2
MTSSRLWFSLLLAAAFAGRATALWPWPQNFQTSDQRYVLYPNNFQFQYDVSSAAQPGCSV
LDEAFQRYRDLLFGSGSWPRPYLTGKRHTLEKNVLVVSVVTPGCNQLPTLESVENYTLTI
NDDQCLLLSETVWGALRGLETFSQLVWKSAEGTFFINKTEIEDFPRFPHRGLLLDTSRHY
LPLSSILDTLDVMAYNKLNVFHWHLVDDPSFPYESFTFPELMRKGSYNPVTHIYTAQDVK
EVIEYARLRGIRVLAEFDTPGHTLSWGPGIPGLLTPCYSGSEPSGTFGPVNPSLNNTYEF
MSTFFLEVSSVFPDFYLHLGGDEVDFTCWKSNPEIQDFMRKKGFGEDFKQLESFYIQTLL
DIVSSYGKGYVVWQEVFDNKVKIQPDTIIQVWREDIPVNYMKELELVTKAGFRALLSAPW
YLNRISYGPDWKDFYIVEPLAFEGTPEQKALVIGGEACMWGEYVDNTNLVPRLWPRAGAV
AERLWSNKLTSDLTFAYERLSHFRCELLRRGVQAQPLNVGFCEQEFEQT

Cross-references

• NCBI Gene