Due to current inability of delivering either the enzyme or gene product to the brain without the invasive injection of the product directly into the brain and the selectivity of the blood-brain-barrier, enzyme replacement therapy and gene replacement therapy have many hurdles to overcome before they are feasible means of treatment for human individuals affected by TSD. Since inhibition of enzymes can be accomplished through the use of small inhibitors, substrate reduction therapy does not have the same problem as the previously mentioned therapies because the small molecule may be able to pass through the blood-brain-barrier. Chaperones have the same advantage of being small molecules, but their effect is to assist in the proper 3-dimensional folding of the enzyme in order to allow for proper transport However, the use of these two methods to treat individuals with the infantile form of TSD is not feasible because it relies on residual activity of the enzyme, which is not typically present in those affected with the infantile form.
Proposals for Future Research
A possible combination therapy utilizing both substrate reduction in conjunction with the introduction of either a recombinant enzyme or gene through the use of a viral vector may be an effective treatment. Most therapies involving introduction of either the enzyme or gene show low uptake in the brain, which does not deliver a sufficient amount of enzyme to adequately reduce the amount of the ganglioside to prevent the pathology. However, introduction of the ganglioside biosynthesis inhibitor in combination with low levels of HexA enzyme expression in the brain may be adequate to bring levels of ganglioside within the neurons to an appropriate level in which individuals are effectively “cured”.
A great strategy already in the works is the development of a recombinant β-subunit that can have the catalytic activity of the α-subunit, effectively generating a heterodimer HexB that is able to bind to the GM2AP-GM2 ganglioside. So far there have been two great papers that demonstrate a recombinant HexB that is able to hydrolyze the GM2 ganglioside, but not in a GM2AP driven manner. Therefore, I propose looking at these following residues as potential sites of mutation to generate a β-subunit with GM2AP-binding ability: E106D and N114E. My choice of these two residues in addition to the residues selected by Sinici et al. (2013) is based on a study by Wendeler et al. (2006) that looked at the hydrolysis activity of designed variants of HexA containing a single point mutation and they found that variants containing point mutations of D106K and E114K had decreased catalytic activity due to the weakened interaction of the HexA to the GM2AP (Wendeler et al., 2006). Therefore, these may be other vital residues that contribute to the binding site of the HexA isozyme to the GM2AP
Lastly, making use of mouse strategy to avoid GM2 ganglioside accumulation with the lack of HexA available may be a feasible way to combat the disease. In mouse models of TSD, where the HexA gene is disrupted, mice show accumulation of the GM2 ganglioside in the neuronal lysosomes, but they do not exhibit the clinical manifestations of the disease (Phaneuf et al., 1996). This is most likely to a difference in mice metabolism of the GM2 ganglioside where, in the absence of theg HexA isozyme, the GM2 ganglioside is then converted to GA2, a derivative of the GM2 ganglioside via sialidase, which is responsible for the hydrolysis of the terminal sialic acid linkage in the polysaccharide unit (Phaneuf et al., 1996). It was found that there was a increased percentage of GA2 in TSD mouse models, suggesting this was the method in which they bypassed clinical manifestation of GM2 ganglioside accumulation. It was also found that there was subsequent catabolism by the HexB isozyme (Phaneuf et al., 1996). Therefore, modulation of human sialidase activity would be a potential route for TSD treatment.
Due to the progressive nature of the disease and its growing prevalence in certain populations, there is a great necessity to find a cure. Several foundations have been founded to raise awareness of TSD and other related diseases and help fund research in hopes of one day finding a cure for this and related diseases.