210. AAV Vector-Mediated Enzyme Replacement Therapy Is Efficacious in Reversing Storage Pathology and Preventing Purkinje Cell Death in a Mouse Model of Niemann-Pick A Disease

Abstract

Top of pageAbstract Type A Niemann-Pick disease (NPA) is a fatal neurometabolic childhood disorder caused by a genetic deficiency of acid sphingomyelinase (ASM). The lack of functional ASM results in sphingomyelin and cholesterol accumulation within the lysosomal compartment of cells throughout the brain, leading to neurodegeneration. In this study, we investigated the efficacy of using AAV gene therapy as a form of ASM enzyme replacement to correct lysosomal pathology and to prevent Purkinje cell death in a mouse ASM knockout (ASMKO) model of NPA. An AAV serotype-2 vector encoding human ASM under the control of CMV promoter (AAV2-CMV-ASM) was injected into the adult hippocampus of one hemisphere. This resulted in human ASM mRNA and protein expression in all major cell layers of the ipsilateral hippocampus for up to 15 weeks post-injection. Transduced cells were also present in the entorhinal cortex, medial septum, supramammillary nucleus, and contralateral hippocampus in a pattern consistent with retrograde transport of AAV2. There was a substantial reduction of distended lysosomal pathology and an almost complete reversal of cholesterol accumulation in all brain regions expressing human ASM. Following injection of AAV2-CMV-ASM into other brain regions (e.g. striatal and cerebellar circuits), we observed a similar pattern of local and distant (retrograde) transduction and reversal of pathology. Within the cerebellum of ASMKO mice, there is an almost complete loss of Purkinje cells between 7 and 20 weeks of age. AAV2-CMV-ASM was injected into the ASMKO cerebellum between 5 and 7 weeks, and treated mice were sacrificed 15 weeks later (20–22 weeks old). These injections resulted in neuroprotection as evidenced by the preservation of large numbers of human ASM mRNA- and protein-positive Purkinje cells within treated lobules. We also found that intraventricular injection of juvenile (3 week old) ASMKO mice resulted in Purkinje cell transduction, human ASM expression and survival for up to 18 weeks post-injection. These results demonstrate that viral vector delivery via the cerebrospinal fluid is an effective way to target Purkinje cells for genetic manipulation. We are currently investigating different promoters and AAV serotypes for vector optimization. To this end, we have generated an AAV serotype-1 vector encoding human ASM under the control of chicken beta-actin promoter (AAV1-CBA-ASM). Administration into the cerebrum resulted in long-term human ASM expression and widespread clearance of cholesterol deposits for up to 26 weeks. Preliminary results have also shown that injection of AAV1-CBA-ASM into the cerebellum of young ASMKO mice preserved rotorod performance, a test sensitive to proprioceptive deficits caused, in part, by Purkinje cell degeneration. In conclusion, our data show that AAV-mediated human ASM enzyme replacement therapy leads to long-lasting, widespread reversal of lysosomal storage pathology, neuroprotection and functional sparing in the ASMKO mouse model of NPA.