History and Metabolic Context of Tay – Sachs Disease

A Walk Down Memory Lane

 

This image displays a typical cherry red spot, which is indicative of atrophy of the nerves in the eyes. This typically precedes the eventual blindness that individuals with infantile TSD develop.
Figure 1. This image displays a typical “cherry red”spot, which is indicative of atrophy of the nerves in the eyes. This typically precedes the eventual blindness that individuals with infantile TSD develop.

Tay – Sachs disease (TSD) is named after the two clinicians who first described it: Warren Tay and Bernard Sachs. In 1881, British ophthalmologist Warren Tay made the reported observations of a “cherry red” spot in the center of a child’s eyes that was eventually found to be atrophic, which the wasting away of this region, and accompanied by progressive central nervous system neurodegeneration (Tay 1881). In 1886, Bernard Sachs noted the familial nature of the disease through the observations of a similar set of symptoms manifesting in several children within the same family and made the initial observations of the high frequency of the disease within Jewish children of European descent (Sachs 1896).

Since its discovery in the late 1800s, there have been over hundreds of publications that expand on the pathology and genetic bases of the disease and possible methods of treatment. The structure and composition of the lipids that are responsible for the manifestation of the disease were elucidated in the late 1930s to early 1940s through the work of various authors and the specific structure of the GM2 ganglioside was identified in 1962 by Svennerholm and characterized in 1963 by Makita and Yamakawa  (as reviewed in Gravel et al. 2005). Okada and O’Brien demonstrated in 1969 in a landmark paper that TSD is characterized by the absence of activity of the β-hexosamindase A (HexA) isozyme due to a mutation in the HexA gene that encodes the α-subunit, which is one of the two subunits for the HexA enzyme. Following further characterization of the HexA isozyme, programs concerning genetic education, genetic screening, and genetic counseling were developed as a means of disease prevention. Following the installment of these programs, there was a markedly reduced incidence, estimated to be a 90% reduction, of TSD in North America (Kaback and Desnick, 2001). With the development of sophisticated methods of DNA-based mutation detection, more than 75 different mutations on the HexA gene alone have been identified to be responsible for the manifestation of all forms of TSD (as reviewed in Gravel et al., 2001).

While this website has a focus on the infantile form of TSD, there are two closely related GM2 gangliosidosises that must be defined. One is called Sandhoff Disease (SD), which is caused by the absence both HexA and HexB isozyme activity due to a mutation in the HexB gene, which encodes for the β-subunit (Sandhoff et al., 1968). This mutation results in the lack of both HexA and HexB activity since the β-subunit is a component of both isozymes. The two diseases have very similar clinical manifestations and therefore are closely tied in the literature. In fact, this disease was delineated from its counterpart through observations of a severe case of TSD that displayed lack of both isozyme activities (Sandhoff et al., 1968). A second rare and closely-related disease is the AB variant of GM2 gangliosidosis, which is due to a deficiency of the GM2 Activator Protein (GM2AP), which is responsible for the delivery of the ganglioside to the enzyme for hydrolysis (Sandhoff et al., 1968). Patients with this disease have similar clinical manifestations to TSD due to a lack of GM2AP rather than a deficiency in either isozyme. Therefore, while this project will focus on TSD, conversations and research concerning SD and this AB variant may play a role in TSD research.

Even with the age of recombinant technology, there is no effective treatment of the GM2 gangliosidoses. The methods currently being developed for treatment of TSD are enzyme replacement therapy, gene therapy, and substrate reduction therapy. Enzyme replacement therapy and gene therapy are focused on the re-introduction of the functional enzyme or gene, respectively. Substrate reduction therapy is focused on inhibiting the biosynthesis pathway of the ganglioside before it can accumulate to harmful levels. More information about treatment can be found on the Treatment of Tay – Sachs Diseaes page. 

Symptoms of Tay – Sachs Disease

An individual with TSD will exhibit normal developmental growth following birth until about 6 months of age. Around this time, parents may notice that development begins to slow down, triggering initial concerns. Other common initial symptoms include: persistence of the abnormal startle response, low muscle tone, and the finding of the cherry-red spot by the eye doctor (Bley et al., 2011). A video displaying this abnormal startle response, known as the Moro reflex, can be found here.

Even if infants were able to obtain some of the developmental milestones, such as learning to crawl and sit-up unsupported, affected children typically lose those abilities within the next year. Other symptoms developed at this time include loss of coordination, progressive inability to swallow, loss of vision, and difficulty breathing (National Tay-Sachs & Allied Diseases). A video documenting these symptoms in an affected 2 year old can be found here.

Typically by the end of the first year in life, seizures are common, but aren’t always present in every affected individual. Within the second year of life, the child has difficulty swallowing, progressive loss of vision, increasing seizure activity, and the child’s awareness of the environment slowly deteriorates until the child is in a completely vegetative state (as reviewed in Gravel et al., 2001). Death is typically caused by aspiration pneumonia, which manifests characteristically due to the child’s inability to prevent foods or liquids from falling into their lungs, which allows for bacteria to colonize the lungs (NHS Choices – Tay-Sachs Disease).

Diagnosis

Following the determination of the underlying cause of TSD, various techniques had been developed in order to detect disease. One of the initial diagnosis techniques depended on the use of synthetic fluorogenic substrates that would quantify the activity of the HexA isozyme in tissues cultured from infants as well as second-trimester amniotic fluid if parents had suspicions that their child could be affected during pregnancy (Kaback et al. 1993).

Diagnosis of the disease after the birth of the child follows initial parental observation that the child is showing signs of developmental delay. The disease is normally diagnosed by a checking the levels of HexA in the blood through an activity assay followed by a DNA-test checking to determine if the child carries any of the mutations associated with TSD (National Tay-Sachs & Allied Diseases).  Another method in which diagnosis of the disease can be done is by checking for the cherry-red spot by an ophthalmologist (Bley et al., 2011).

Recent research suggests the use of Lyso-GM2 ganglioside, a derivative of the GM2 ganglioside, as a possible biomarker of TSD and related gangliosidosis (Kodama et al., 2011). Kodoma et al. demonstrate that there are elevated levels of this lyso-GM2 ganglioside the brain of SD mouse models and human plasma samples through the use of high performance liquid chromatography and MALDI-TOF. When SD mouse models were treated with the Hex enzyme, levels of lyso-GM2 ganglioside decreased, suggesting that levels of lyso-GM2 ganglioside are affected by Hex enzyme activity.

Carrier Screening

When the high frequency of the disease was determined in the Ashkenazi Jews, the use of serum testing was implemented to determine whether individuals were carriers of the disease (Kaback et al., 1993). If both partners considering children were found to both be carriers of TSD, they were provided comprehensive genetic counseling that would allow for them to, if they desired, to only carry to term pregnancies that were shown to have an unaffected child (Kaback et al., 1993). Preimplantation genetic diagnosis (PGD) is a method that utilizes a family-specific fluorescent real-time PCR test to analyze a single cell from the biopsied embryo to determine whether the child will be affected and returning only unaffected embryos to the uterus for full term pregnancy (Altarescu et al., 2012). This method is less invasive than other means of genetic counseling such as chorionic villus sampling or amniocentesis and can be performed earlier than 10-11 weeks into the pregnancy (Altarescu et al., 2012). Following implementation of this carrier-screening and genetic counseling, disease incidence has significantly decreased, especially within the Jewish populations (Kaback et al., 1993).

However, since the DNA screening techniques have a focus on the Jewish population, there is a high false-positive and false-negative rate when screening other populations. Therefore, a new technique called “next-generation DNA sequencing” was developed in 2013 by Hoffman et al. to expand the number of mutations that were being used to determine if an individual was affected or a carrier of the disease. This new screening technique is used to extensively screen for a larger pool of mutations in other populations, such as the French Canadian and the Cajuns, which have been demonstrated to have a high-carrier rate of the disease. Next-generation sequencing is a high-throughput technique that concurrently generates thousands or millions of DNA sequences from short strands of DNA in a short span of time. This is advantageous over traditional means of screening because it allows for the detection of rare missense variants or novel mutations that aren’t common in the population and allows for the inclusion of as many variants as desired.

Defining the Biomolecules

Gangliosides

The manifestation of TSD is due to the accumulation of the GM2 ganglioside within neuronal lysosomes. The gangliosides are composed of the hydrophobic ceramide group, which is embedded within the plasma membrane, and a hydrophilic oligosaccharide, a simple sugar polymer, that contains one or more N-acetylneuraminic acid (common name, sialic acid) (as reviewed in Gravel et al., 2001). They are typical components of the outer leaflet of the plasma membrane and have been demonstrated to be involved in cell-to-cell communication and cell differentiation. Specific function of gangliosides has not been defined, but it has been suggested that they are binding sites for viruses and bacterial toxins, as well as acting as co-receptors, a receptor that binds to a signaling molecule in addition to the primary receptor, for hormones (as reviewed in Gravel et al., 2001). Gangliosides are found primarily in the central nervous system and are highly concentrated in the regions in the nerve endings and dendrites, strengthening the speculation that they are vital to cell-to-cell communication (as reviewed in Gravel et al., 2001).         

Gravel et al. supplies a comprehensive overview of literature concerning the biosynthesis and degradation of the gangliosides. Gangliosides are synthesized in the cytosolic leaflet of the endoplasmic reticulum starting with the ceramide group. The oligosaccharide component is built in a step-wise manner via a series of transferases building on the ceramide at the Golgi apparatus. Following synthesis at this site, the gangliosides are transported to the plasma membrane by vesicle flow. The degradation of the gangliosides occurs in the lysosome via a series of hydrolases. The degradation of the GM2 ganglioside is specifically initiated by the HexA isozyme, which is responsible for the removal of the terminal N-acetylgalactosaminyl residue.

This shows the reaction catalyzed by HexA with the GM2 Activator Protein being an important cofactor.
Figure 2. This shows the reaction catalyzed by HexA with the GM2 Activator Protein being an important cofactor. HexA initiates the first step of degradation of the GM2 ganglioside by removal of the N-acetylgalactosamine of the ganglioside.

 

β-hexosaminidase A (HexA), β-hexosaminidase A (HexB) and GM2 Activator Protein (GM2AP)

The HexA isoyme is responsible for the removal of the terminal residue on the GM2 ganglioside and a mutation of the α-subunit of this enzyme results in TSD.

HexA is one of major isozymes for the β-hexosamindase enzymes, of which there are three: HexA, HexB, and HexS. HexA is composed of two non-identical subunits, an acidic α-subunit and a basic β-subunit; HexB is a homodimer of the β-subunits; and the rare and typically non-functional HexS is a homodimer of the α-subunits (as reviewed in Gravel et al., 2001). The latter two isozymes have no major physiological function. The individual subunits are encoded by two different genes, HexA and HexB, with both genes showing about 60% homology (Myerowtiz et al., 1985).  It has been demonstrated that the more thermostable β-subunit is more effective at hydrolyzing neutral water-soluble substrates whereas the α-subunit is capable of hydrolyzing both neutral and negatively charged substrates, such as the GM2 ganglioside, due to the subunits ability to bind with the GM2 Activator Protein-GM2 ganglioside complex (Kytzia et al., 1983; Hou et al, 1996). The  α-subunit also contains the key residue Arg424 that is responsible for binding with the N-acetylgalactosamine of the GM2 ganglioside (Lemieux et al., 2006). However, dimerization of two subunits is vital for enzyme activity (Lemieux et al., 2006).

Since the GM2 ganglioside is embedded in the cell membrane, it must be transported to the HexA enzyme through the use of the GM2 Activator Protein, which retrieves the ganglioside from the cell membrane and binds to the HexA enzyme for ganglioside degradation. It has been demonstrated that the α-subunit is responsible for binding to the activator protein.
Figure 3. Since the GM2 ganglioside is embedded in the cell membrane, it must be transported to the HexA enzyme through the use of the GM2 Activator Protein, which retrieves the ganglioside from the cell membrane and binds to the HexA enzyme for ganglioside degradation. It has been demonstrated that the α-subunit is responsible for binding to the activator protein.

The α-subunit of the HexA enzyme is vital for GM2 ganglioside degradation because it contains the catalytic site for negative substrate hydrolysis and also a binding site for the GM2 Activator Protein (GM2AP), which functions to present the enzyme with the ganglioside for degradation (Hirabayashi et al. 1983). The GM2AP binds to the ceramide moiety of the ganglioside and transports it from the membrane to the HexA isozyme for catalysis. The α-subunit on the HexA isozyme has been demonstrated to contain a loop structure, consisting of residues Gly280, Ser281, Glu282, and Pro283, that is vital to GM2AP-GM2 ganglioside complex binding, which confers the specificity to the HexA isozyme for the GM2 ganglioside (Lemieux et al., 2006).