Myelin Deficits and Intravenous Gene Therapy in Feline Sandhoff Disease

Abstract

Sandhoff Disease (SD) is a neurodegenerative lysosomal storage disease (LSD) that results in the death of children before 4 years of age. Because there are no FDA-approved therapies available, current treatment strategies are limited to palliation. SD is a form of GM2 gangliosidosis caused by the absence of ß-hexosaminidase (Hex) and subsequent accumulation of GM2 ganglioside in neuronal lysosomes. In previous studies, intracranial administration of adeno-associated viral (AAV) vector in feline models quadrupled lifespan and increased quality of life. To maximize patient benefit, and because gene therapy manufacturing is expensive and labor-intensive, it is crucial to make sure that each dose is as effective as possible. Chapter 1 of this dissertation investigated the possibility of using a bicistronic AAV vector, reducing the risk of intracranial surgery, and achieving greater systemic vector distribution through the intravenous (IV) treatment of SD cats at one month of age. In-life assessments revealed clear clinical benefit of AAV treatment, with the most dramatic improvement seen in the reduction of tremors, the most debilitating feature of feline SD. Other in vivo metrics were measured in cerebrospinal fluid and utilizing magnetic resonance imaging; these revealed partial normalization of AAV-treated cats, with the high dose being more effective than the low dose. Post-mortem assessments revealed dose-dependent reduction of GM2 ganglioside storage and increases in Hex activity that were more substantial in the caudal regions of the brain and in the spinal cord. Immunohistochemistry (IHC) staining revealed partial normalization for 4 separate cell populations (astrocytes, microglia, neurons, oligodendrocytes). The brainstem was the region with the mildest pathology after AAV treatment, along with the spinal cord when included. These results support the dose-dependent efficacy of AAV delivered IV in a feline model of SD. Chapter 2 addressed current gaps in the literature on the origin of myelin deficits in SD, which is increasingly recognized as a barrier to effective gene therapy in multiple LSDs. Myelin pathology was characterized in SD cats by measuring mRNA and protein (IHC) markers of myelinogenesis at 3-4 different stages of feline development. Deficits were noted as early as 1 month of age, which is considered pre-symptomatic. As previously reported in humans and mice, the feline brainstem was one of the first areas to become myelinated in normal cats. These results accentuate the importance of intervening with AAV treatment as early as possible, though the diagnosis of pre-symptomatic children is difficult and rare.