The Progranulin C-Terminal Domain and AAV-Progranulin Gene Therapy for Frontotemporal Dementia

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Project Summary This application is for F30 support of Shreya Kashyap during her MD/PhD training. The scientific focus of this proposal is to determine the role of the progranulin C-terminal domain (CTD) on AAV-derived progranulin levels in vivo and the effectiveness of AAV-Progranulin gene therapy. Heterozygous loss of function mutations in the progranulin gene (GRN) are one cause of frontotemporal dementia (FTD), a devastating neurodegenerative disease characterized by social behavior and language deficits. Work from the lab of Dr. Erik Roberson, sponsor of the PI, has established the therapeutic potential of AAV-Progranulin gene therapy in progranulin-insufficient mouse models of FTD. The vector used for these experiments has a tag fused to the CTD that precludes progranulin's interaction with its canonical trafficking receptor, sortilin. These experiments show that the lysosomal localization and efficacy of AAV-Progranulin is independent of sortilin. However, the functional consequences of an exposed progranulin CTD, which allows for progranulin-sortilin binding, are still unknown. Notably, sortilin mediates the uptake and degradation of progranulin and sortilin ablation or inhibition boosts both brain and CSF progranulin levels in wild type and progranulin-insufficient mouse models. My preliminary experiments indicate that blocking the progranulin C-terminal domain boosts parenchymal levels of AAV-derived progranulin, and improves efficacy of AAV-Progranulin in rescuing lysosomal pathology and neuroinflammation. The clinical translation of AAV-Progranulin vectors with tags fused to the progranulin CTD is unlikely. Thereby, it is important to determine CTD sequence modifications that improve dosage and levels of AAV-Progranulin. Immune mediated clearance of AAV particles and AAV-transduced cells reduce the effectiveness of AAV- based gene therapies. Progranulin-deficient mice mount an immune response to AAV-murine progranulin, and it is unclear if FTD-GRN patients (who have one functioning progranulin allele) will mount an immune response to AAV-human progranulin. However, as evidenced by recent clinical testing of AAV-glial derived neurotrophic growth factor (AAV-GDNF) and AAV-amino acid decarboxylase (AAV-AADC) in patients with Parkinson's disease, patients can mount immune responses even to AAV-derived proteins with non-foreign epitopes. Thus, it is important to identify sequence modifications that reduce immune cell uptake of secreted AAV-derived progranulin. Because sortilin is expressed by various antigen presenting cells in the brain, blocking the progranulin-sortilin interaction has the potential to decrease the immunogenicity of AAV-Progranulin vectors. The proposed training plan for Shreya Kashyap is sponsored by her project mentor, Dr. Erik Roberson. The overall goal of the training plan is to provide the PI with a solid foundation for a successful career as a physician scientist. A project based both in translational approaches, while focused on a disease-oriented pathogenesis, is the ideal training environment for any aspiring physician scientist.