Our project focuses on exploring the use of gene therapy to correct life-threatening brain lesions in a mouse model of tuberous sclerosis. Our mouse model is unique in that we generate cells lacking the TSC1 protein hamartin in a random pattern in the brains of Tsc1 conditional knockout mice by injection of a viral vector encoding Cre recombinase into the brain ventricles at the time of birth. We believe this simulates the situation in human TSC brains, such that loss of hamartin occurs in a subset of cells of different types. Further, these mice show a number of brain lesions similar to those seen in TSC patients, including enlarged neurons, clusters of cells of mixed glial and neuronal type, and enlarged ventricles containing cerebrospinal fluid (CSF). Further, these mice have a shortened lifespan likely due, at least in part, to development of hydrocephalus (abnormal expansion of ventricles compressing the brain) due to restricted CSF flow, which also occurs in patients and can cause rapid death. In companion studies using a mouse model in which hamartin is lost in all neurons, we have shown that a viral vector encoding hamartin can deliver this missing protein to these defective neurons and extend the lifespan of the mice. We are using an adeno-associated virus (AAV) vector for these studies, which has proven to be safe and beneficial in Phase 1 clinical trials for neurologic conditions in children and adults, including blindness and Parkinson's disease, and is being evaluated for gene replacement in a variety of inherited human neurologic conditions, including Tay-Sachs disease and Huntington's disease.
In the case of hydrocephalus in tuberous sclerosis complex (TSC) patients, we believe that early treatment of subependymal nodules by gene replacement mediated by AAV vectors could inhibit their growth and thus avoid high risk, invasive neurosurgical intervention, and greatly reduce the risk of rapid death. Enlarging nodules can be monitored by MRI (magnetic resonance imaging), and treatment would involve either intraventricular or intravascular delivery of an AAV vector encoding the defective protein hamartin. Further, the intravascular delivery would target other tissues affected in tuberous sclerosis. Based on our evaluation of this procedure in our preclinical mouse model, if we observed extended lifespan, we would then apply for National Institutes of Health U01 translational funding, which allows 4-5 years for the generation, biodistribution/toxicity testing, and preclinical evaluation of an AAV vector for delivery of human hamartin appropriate for Phase 1 clinical trials in tuberous sclerosis type 1 patients. We feel the potential benefit would greatly outweigh the risk of such a trial.