Immunomodulatory Effects of Intracranial Gene Therapy in Feline Sandhoff Disease
Type of Degreedissertation
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Sandhoff disease (SD) is a lysosomal storage disorder characterized by absence of hydrolytic enzyme β-N-acetylhexosaminidase (Hex), which results in storage of GM2 ganglioside in neurons and unremitting neurodegeneration. Neuronal loss initially affects fine motor skills, but rapidly progresses to a vegetative state and death by the age of five in humans. A well-established feline model of SD has allowed disease characterization in a large animal model and provided necessary means to test safety and efficacy of therapeutic interventions before initiating human clinical trials. When treated with intracranial adeno-associated viral (AAV) gene delivery of human Hex, two SD cats lived to 7.0 and 8.2 months of age, compared to an untreated life span of 4.5 ± 0.5 months. Due to a pronounced humoral immune response to both the AAV vector and human Hex, a feline cDNA for Hex was cloned into AAVrh8 vectors. Cats treated with vectors expressing the species-specific transgene produced enzymatic activity >75 fold normal at injection site, a substantially mitigated humoral immune response, and lived to 10.4 ± 3.7 months, or 2.3 times longer than untreated cats. High levels of circulating antibodies did not preclude successful transduction of AAV or sustained Hex expression. Furthermore, sera antibody titers did not correlate with lifespan and were not an indicator of clinical outcome. Decreasing the dose of treatment lowered sera antibody titers; however, clinical symptoms were not as effectively attenuated. Furthermore, directly targeting the ventricular system did not increase sera antibody titers compared to injecting the brain parenchyma. After intracranial injection, AAVrh8 demonstrated a surprisingly strong tissue tropism for the liver, sciatic nerve, and quadriceps, likely explaining the pronounced humoral immune response. Intracranial delivery of AAV vectors expressing feline Hex also attenuated neuroinflammation, characterized in untreated SD cats by marked expansion and activation of microglia, upregulation of major compatibility complex II, and increased expression of macrophage inflammatory protein-1. The therapy also demonstrated a peripheral effect by normalizing blood, CSF, MRI and electro-diagnostic biomarkers of disease progression. These studies support the therapeutic potential of AAV mediated gene therapy for SD and provide highly translatable data in support of initiation of human clinical trials.