The Molecular Bases of Tay – Sachs Disease

The Gene Behind It All

This schematic demonstrates the recessive nature of the disease. In order to pass the disease on to their children, both parents must be carriers or affected. However, since the infantile form of TSD results in death within the first 4 years, both parents must be carriers in order to have an affected child.  The interest in this disease is that there is much higher population of carriers within certain populations compared to the entire population as a whole. This means that chances to individuals carrying a defective copy of the gene are much higher, which leads to higher chances, compared to the population as a whole, of having affected children.
Figure 1. This schematic demonstrates the recessive nature of the disease. In order to pass the disease on to their children, both parents must be carriers or affected. However, since the infantile form of TSD results in death within the first 4 years, both parents must be carriers in order to have an affected child.
The interest in this disease is that there is much higher population of carriers within certain populations compared to the entire population as a whole. This means that chances to individuals carrying a defective copy of the gene are much higher, which leads to higher chances, compared to the population as a whole, of having affected children.

TSD is characterized by a mutation in the HexA gene, located on chromosome 15, encoding the α-subunit of the HexA isozyme, which is responsible for the first step in the degradation pathway of the GM2 ganglioside. There are several mutations that have been determined to be responsible for the various forms of the disease; however, non-sense mutations or frameshift mutations that result in a truncated form of the enzyme typically lead to the severe infantile form (Mahuran, 1991).

Following observations that a defect in Hex activity was associated with TSD, it was found that total Hex enzyme activity, when measured with an artificial substrate, was nearly normal (Okada and O’Brien, 1969). Okada and O’Brien in 1969 determined that lack of HexA activity results in manifestation in TSD by establishing that it was this specific enzyme that was missing in their Jewish patients. From this, they recognized that Hex activity is derived from two different gene products, the HexA and HexB genes, resulting in two different isozymes of the Hex enzyme, and that there were elevated levels of the HexB isozyme in their Jewish patients to compensate for the lack of HexA activity. Since this discovery, there have been over 75 different mutations in this gene that have been demonstrated to be responsible for the manifestation of the various forms of TSD (as reviewed in Gravel et al., 2001).

Common mutations of the HexA gene are highlighted in the following table (as reviewed in Mahuran, 1999):

Mutation

Type of Mutation

Effect on the Enzyme

127ins41, exon 11 Frameshift Insertion Absent mRNA
IVS12 + 1G → C1 Single Point Transversion Abnormal splicing resulting in trace levels of abnormal mRNA
1274_1277dupTATC1,2 Frameshift Duplication Premature termination
805G → A1 Single Point Transition Gly → Ser
7.6-kilobase deletion, exon 13 Frameshift Deletion Absent mRNA

1 Common mutations within the Ashkenazi Jew population

2 Common mutations within the Cajun population

3 Common mutations within the French Canadian population

The major mutation in the Ashkenzai Jewish population that is responsible for the high-frequency of TSD in the population is the 4 base-pair insertion found in exon 11 that results in a premature stop codon (as reviewed in Mahuran, 1991). According to Myerowitz and Costigan, this accounts for approximately 70% of tested carriers. This mutation results in the introduction of a stop codon that produces an unstable α-subunit, most likely due to the inability of the ribosome to protect the 3’ end of the mRNA from degradation (Myerowitz and Costigan, 1988).

A second mutation of interest within the Ashkenazi Jewish population is the Gly269Ser mutation, the mutation responsible for the adult form of TSD which can pair with one of the other high-frequency infantile TSD genes commonly found in the Ashkenazi Jewish population (Lemieux et al., 2006). Affected individuals with this mutation have residual HexA activity that is approximately 4% – 8% of non-affected individuals (Ohno and Suzuki, 1988). Modeling studies of this mutation demonstrate that the mutation disrupts the backbone in which the normal 269Gly is involved, which shifts the placement of the His262, a residue important for the coordination of residues within the active site of the enzyme (Lemieux et al., 2006). However, since there is residual enzyme function, this mutation allows for some GM2 ganglioside degradation in the early years of left, delaying the fatal effects of ganglioside accumulation.

The major mutation in the French Canadian population is the 7.6 kilobase deletion near the 5’ end of the gene, resulting in an α-chain that bears a deletion at the front (Myerowitz and Hogikyan, 1986). This discovery demonstrated that even though differing populations may bear the same clinical manifestations of the disease, the genetic component that results in the same outcome may differ from population to population. Therefore, it is vital that screening techniques are able to detect various types of mutations in a varied population. The previous two studies demonstrate that the same clinical manifestation arises from different mutations and that different mutations may be the underlying mechanism as to the different severities of the disease.

Overall, mutations within the HexA gene that result in a defective α-chain of the HexA isozyme render the isozyme ineffective at degrading the GM2 ganglioside. Therefore, as the ganglioside is being delivered to the lysosome for degradation, it continues to accumulate within the lysosome, leading to the development of membranous cytoplasmic bodies (MCB). These MCBs are commonly found within neurons of affected individuals and accumulate as the lysosome becomes filled with the ganglioside (as reviewed in Gravel et al., 2001). Since neurons within the central nervous system (CNS) have high composition of the GM2 ganglioside, the most dramatic effects occur within the neuronal lysosomes, with the increasing prevalence of these MCBs leading to the neurodegeneration that is characteristic of TSD.

Pathophysiology

Individuals affected by the infantile GM2 gangliosidosis have a characteristic course of neurodegeneration that results in death within early childhood. It has been found that apoptosis, programmed cell death, of neurons has been common in affected patient samples and mouse models of the disease.

Studies of gene expression of patient samples, one Tay-Sachs patient and one Sandhoff patient, demonstrate that a large fraction of genes involved in the pro-inflammatory response is elevated (Myerowitz et al., 2002). Utilizing the SAGE (serial analysis of gene expression) methodology, a qualitative and quantitative genetic profile of the mRNA isolated from the cerebral cortex tissues of affected patients was obtained. Comparison between the affected and unaffected individuals shows that there is an increased expression of genes involved in the inflammation and injury response. Many of these genes operate to activate macrophages/microglia and astrocytes, which are cells typically associated with this type of inflammatory response (Myerowitz et al., 2002). Examples of these genes include: CD68 (a macrophage marker), glial fibrillary acid protein and vimentin (both of which are characteristic of activated astrocytes), and osteopontin (a proinflammatory cytokine) among others (Myerowitz et al., 2002). There was also increased expression of enzymes and proteins related to stress and apoptosis, including death-associated protein 6, which is involved in Fas-mediated pathway of programmed cell death (Myerowitz et al., 2002). A decrease expression of neuron-specific genes, such as β-synuclein and neuronal-specific enolase, was also observed in these samples, which demonstrates neuron loss (Myerowitz et al., 2002). Confirmation of the modified expression of these genes was done through immunohistochemical staining of sections of the cortex (Myerowitz et al., 2002).  The authors also found that there is an elevated expression of growth-associated protein 43, which is a protein involved in the elongating of the neuronal axon and promotes neurite, which are structures projected from the neuronal cell body, formation (Myerowitz et al., 2002). The  Myerowitz et al. (2002) believe that this explains the presence of “meganeurites” – swollen neurons with high densities of neurites – that is typically found in GM2 gangliosidosis. Therefore, authors suggest that this may be indicative of neuronal damage in affected individuals. In conclusion, the authors suggest that the accumulation of these gangliosides within the lysosome result in increases expression of genes involved in inflammation, which triggers the loss of neurons, leading to the neurodegeneration that is characteristic to TSD and related GM2 gangliosidoses (Myerowitz et al., 2002).

This is the proposed physiopathology by Myerowitz et al. (2002) that displays how the accumulation of the ganglioside triggers the expression of imflammatory response elements, which leads to apoptosis of the neuronal cells. (citation)
Figure 2. This is the proposed physiopathology by Myerowitz et al. (2002) that displays how the accumulation of the ganglioside triggers the expression of imflammatory response elements, which leads to apoptosis of the neuronal cells. Source: Myerowtiz, R.; Lawson, D.; Mizukami, H.; Mi, Y.; Tifft, C.J.; Prioa, R.L.; Molecular pathophysiology in Tay-Sachs and Sandhoff diseases as revealed by gene expression profiling. Hum Mol Genet. 11: 1343-1350, 2002. doi: 10.1093/hmg/11.11.1343