Lysosomal storage diseases (LSDs) consist of >40 distinct disorders, with a cumulative prevalence of 1 in 7,700 live births, similar in frequency to cystic fibrosis and hemophilia. Each LSD has an underlying defect in lysosomal function that leads to widespread storage of undegraded substrates in all tissues. Many lysosomal storage diseases have some level of neurologic involvement, with several previously thought peripheral tissue only subtypes now categorized as neuropathic. Though effective treatments have been developed for peripheral manifestations of some lysosomal diseases, those with neurological components have been virtually untreatable. Tay-Sachs (TSD) and Sandhoff disease (SD) are each caused by a subunit deficiency in the enzyme Hexosaminidase (Hex), and are almost clinically indistinguishable. Intracranial injection of adeno associated viral (AAV) vectors has led to > 4-fold increases in life span and vastly improved quality of life in mice and cats with Sandhoff disease. The newly discovered Tay-Sachs sheep is the only relevant animal model of Tay-Sachs disease (TSD) and may inform on challenges associated with TSD not experienced in the Sandhoff cat and mouse. Additionally, testing therapy in an authentic model of TSD with brain and body size on the same order of magnitude as children, will provide invaluable data on vector safety, biodistribution and efficacy. Sheep with Tay-Sachs experience heterogeneous clinical signs which are representative of the highly variable age of onset and clinical signs experienced by human juvenile patients.
The first aim of this proposal is to identify, characterize and validate biomarkers of sheep Tay-Sachs and understand their relationship to disease progression. This project will make use of ultra-high field (7 Tesla) magnetic resonance imaging, combined functional MRI?electroencephalography, MR spectroscopy, diffusion tensor imaging as well as metabolomics and electrodiagnostic testing; and these techniques will be used to evaluate therapeutic success in Aim 2. Preliminary studies have shown that simultaneous delivery of both Hex subunits is ideal for normal ratios of Hex isozymes, but enzymatic levels were below normal. Therefore, we plan to test a novel bicistronic vector that expresses both Hex subunits in a single construct with superior enzymatic expression using a novel AAV capsid that transduces the brain with greater efficiency than the industry standard AAV9 (Aim 2). To bypass the invasiveness of parenchymal brain injections, we will administer this new AAV by intravenous injection and compare side by side with the ?the gold standard? routes of Thalamic+CSF delivery. Conclusions from this project will ultimately inform future human clinical trials for GM2 gangliosidosis as well as other AAV trials for LSDs.
First reported in 1881, Tay-Sachs and Sandhoff disease are lysosomal storage disorders usually fatal by 5 years of age after a long period of neurodegeneration leading to a semi-vegetative state. Though no treatment exists currently, AAV gene therapy in mouse and larger animal models has proven effective, justifying the pursuit of human clinical trials. The current application builds upon previous work to develop a second- generation gene therapy approach that bypasses invasive brain injections, while effectively spreading therapy across the largest brain size tested thus far, which is ~ the size of a human child.