Abstract

Tay-Sachs is a disease caused by a mutation to the gene which codes for Hex A. Without Hex A, a cell cannot degrade G_M2 ganglioside into G_M3 ganglioside. This results in a build up of ganglioside’s in lysosomes of neurons. The result is varying degrees of mental deterioration. New DNA-based screening is currently being developed to replace the enzyme-based screening techniques that have been used since 1969. This will not only speed up the diagnosis, but also allow for earlier detection of Tay-Sachs by using the parents genotypes.

Introduction

Tay-Sachs disease is one of three autosomal recessive, lysosomal storage disorders, collectively known as the G_M2 gangliosidoses. They result from accumulation of G_M2 ganglioside in lysosomes, primarily of neurons. The clinical symptoms of Tay-Sachs vary from infantile lethal neurodegenerative disease to less severe adult onset forms. The latter are often characterized by motor neuron impairments. The recognition of the high incidence of this disease among Ashkenazi Jews and the identification of the deficiency of hexosaminidase A as the basic defect were essential findings leading to the establishment of mass carrier screening programs for this disease [2]. Recently, research has focused on the DNA-based diagnostics that are anticipated to play a role in future carrier screening programs [1].

G_M2 ganglioside hydrolysis

The lysosomal hydrolase, beta-hexosaminidase, occurs predominantly in two forms, hexosaminidase A (Hex A) and hexosaminidase B (Hex B). Hex A is comprised of one alpha and one beta subunit while Hex B is comprised of two beta subunits [3]. While both subunits contain similar active sites, only the alpha subunit can hydrolyze G_M2 ganglioside. Dimer formation is required for either subunit to become active.

The Hexosaminidase A activity that is blocked in Tay-Sachs disease is that of cleaving the terminal N-acetylgalactosamine off of G_M2 ganglioside.

                                                                           Hex A
                                             GalNAc-Gal-Glc-Cer ---------> Gal-Glc-Cer  + GalNAc
                                                              l                                   l
	                                                   NANA	                        NANA
                                                    GM2 Ganglioside       GM3 Ganglioside

Fig. 1 Shows the action of Hex A cleaving the GalNAc off of the G_M2 ganglioside and converting it to the G_M3 ganglioside. This is the blocked reaction in Tay-Sachs disease. (GalNAc=N-acetylgalactosamine; Gal=Galactose; Glc=Glucose; Cer=Ceramide; NANA=Sialic Acid ) [ 6 ]

There is also a third protein required for G_M2 ganglioside hydrolysis called the G_M2-activator. It has been demonstrated that the G_M2-activator extracts ganglioside G_M2 from micelles or liposomes and forms a ganglioside activator complex. This complex is required for the Hex A enzymatic activity [ 4 ] .

The G_M2-gangliosidose

There are three forms of G_M2-gangliosidoses associated with either of the three gene products necessary for G_M2 hydrolysis. Tay-Sachs disease is caused by mutations in the HEXA gene which affect the a subunit unique to Hex A. Patients with Tay--Sachs disease retain the ability to produce normal and often elevated amounts of Hex B [ 5 ] .

Sandhoff disease, results from HEXB gene mutations and affects the beta subunit. Since the beta subunit is common to both Hex A and Hex B, patients with Sandhoff disease have deficiencies of both isoenzymes. The small amount of residual activity associated with this variant is from a labile a homodimer, Hex S. Despite the active a subunits of Hex S, it cannot perform G_M2 hydrolysis because it is incapable of binding to the G_M2 activator protein; therefore, Hex S is not considered a physiological functional form of Hex [ 5 ] .

Mutations in the G_M2 activator protein result in the rare AB--variant form of G_M2-gangliosidosis. Patients with this disease produce normal levels of both Hex A and Hex B [ 5 ] .

Clinical Features

The G_M2 gangliosidoses show extreme variability in clinical expression. Patients are generally classified according to the age at onset of clinical symptoms and the age at death. Within the three groups of G_M2 gangliosidoses, there are four recognized categories: infantile, juvenile, adult, and chronic. Typically, the earlier the age of onset the more severe the disease.

Infantile Tay-Sachs disease is clinically and biochemically the best defined of the G_M2 gangliosidoses. Affected patients usually begin to show evidence of neurological disease by six months of age. More profound neurological symptoms are evident later in the first year of life and thereafter motor and mental deterioration progress rapidly. Death usually occurs by the age of five years. Biochemically, these patients are believed to lack any residual activity towards G_M2 ganglioside [1].

Less severe phenotypes with later onset (juvenile, adult, and chronic) are also associated with Tay-Sachs disease. The juvenile form usually presents at two to six years of age with ataxia and progressive dementia culminating in a vegetative state and death by ten to fifteen years of age. The adult and chronic forms show varying symptoms in the clinic. These symptoms include spinocerebellar and lower motor neuron dysfunction and psychosis in one-third of patients [1].

Histology and Specific Symptoms

In the absence of Hex A, G_M2 ganglioside accumulates in many tissues (e.g., heart, liver, spleen), but it is the involvement of neurons in the central and autonomic nervous systems and retina that dominate as recognizable symptoms. The weight of the brain in individuals surviving one or more years can be increased more than fifty percent, due to the accumulation of ganglioside in the cells. On histologic examination, the neurons are ballooned with cytoplasmic vacuoles, each of which comprises a markedly distended lysosome filled with gangliosides (Fig 2A). Under the electron microscope, several types of cytoplasmic inclusions can be visualized, the most prominent being whorled configurations within lysosomes composed of "onionskin" layers of membranes (Fig 2B)[6].

In time there is progressive destruction of neurons, proliferation of microglia, and accumulation of complex lipids in phagocytes within the brain substance. A similar process occurs in the neurons throughout the basal ganglia, brain stem, spinal cord, and dorsal root ganglia, and the neurons of the autonomic nervous system [6].

The ganglion cells in the retina are similarly swollen with G_M2 ganglioside. A "cherry red spot" will appear in the macula. As the retinal ganglion cells die from the accumulation of lipid, the cherry red spot may disappear in patients with protracted disease [6].

Fig 2A Ganglion cell in Tay-Sachs disease under the light microscope. The arrow points to the distended lysosomes filled with ganglioside. 2B Lysosome in Tay-Sachs disease under the electron microscope. The arrow points to a lysosome with the characteristic whorled configuration inside lysosomes in Tay-Sachs diseased].

Diagnosis: Present and Future

Tay-Sachs disease is the prototype of lysosomal storage disease. While is was first described over a century ago, the defective enzyme was not identified until 1969, making possible the development of enzyme-based diagnostic and carrier screening techniques. This led to the establishment of the successful international Tay-Sachs screening program, primarily for the high risk Ashkenazi Jewish population. In the past five years the development of recombinant DNA technology has allowed researchers to characterize 95-99% of the mutations causing Tay-Sachs disease in this high risk ethnic group. Knowledge of the exact mutations responsible for the disease coupled with the powerful polymerase chain reaction technique has now made DNA-based screening and diagnosis possible. While the enzyme-based test has proven to be reliable and economical, it cannot differentiate variant phenotypes and requires the presence of specialized testing centers. Although the DNA-based test is less economical, it can provide carrier couple’s with their exact genotype and thus, predict the general phenotype of an unborn child. Furthermore, as the catalogue of mutations leading to human disease increases, more economical DNA methodologies will be developed. In the future it would be expected that a lab using a single DNA-based technology could diagnose and screen for a myriad of human diseases including Tay--Sachs[l].

References:

1. Mahuran, D.J., Triggs-Raine, B., Feigenbaum, A., Gravel, R. (1990). The molecular basis of Tay-Sachs disease: mutation identification and diagnosis. Clin. Biochem. 23:409-415.

2. Navon, R., Proia, R. (1991). Tay-Sachs disease in Moroccan Jews deletion of a phenylalanine in the alpha-subunit of beta--hexosaminidase. Am. J. Hum. Genet. 48:412-419.

3. Gray, R.G.F., Green, A., Rabb, L., Broadhead, D.M., Besley, G.T.N. (1990). A case of the B1 variant of G_M2-gangliosidosis. J. Inher. Metab. Dis. 13:280-282.

4. Meier, E., Schwarzmann, G., Furst, W., Sandhoff, K. (1991). The human GM2 activator protein. J. Biological Chem. 266:1879-1887.

5. Mahuran, D.J. (1991). The biochemistry of HEXA and HEXB gene mutations causing G_M2 gangliosidosis. Biochimica et Biophysica Acta. 1096:87-94.

6. Robbins, S., Ranzi, R., Kumar, V., (Eds). (1984). Pathologic Basis of Disease. Philadelphia, PA: Saunders Co. 142-145.