AAV-mediated progranulin delivery to a mouse model of progranulin deficiency causes T cell-mediated hippocampal degeneration

Adeno-associated virus (AAV)-mediated gene replacement is emerging as a safe and effective means of correcting single-gene mutations, and use of AAV vectors for treatment of diseases of the CNS is increasing. AAV-mediated progranulin gene (GRN) delivery has been proposed as a treatment for GRN-deficient frontotemporal dementia (FTD) and neuronal ceroid lipofuscinosis (NCL), and two recent studies using focal intraparenchymal AAV-Grn delivery to brain have shown moderate success in histopathologic and behavioral rescue in mouse FTD models. Here, we used AAV9 to deliver GRN to the lateral ventricle to achieve widespread expression in the Grn null mouse brain. We found that despite a global increase in progranulin throughout many brain regions, overexpression of GRN resulted in dramatic and selective hippocampal toxicity and degeneration affecting both neurons and glia. Histologically, hippocampal degeneration was preceded by T cell infiltration and perivascular cuffing, suggesting an inflammatory component to the ensuing neuronal loss. GRN delivery with an ependymal-targeting AAV for selective secretion of progranulin into the cerebrospinal fluid (CSF) similarly resulted in T cell infiltration as well as ependymal hypertrophy. Interestingly, overexpression of GRN in wild-type animals also provoked T cell infiltration. These results call into question the safety of GRN overexpression in the CNS, with evidence for both a region-selective immune response and cellular proliferative response following GRN gene delivery. Our results highlight the importance of careful consideration of target gene biology and cellular response to overexpression in relevant animal models prior to progressing to the clinic.\n\nSignificance StatementGene therapies using adeno-associated viral (AAV) vectors show great promise for many human diseases, including diseases that affect the central nervous system (CNS). Frontotemporal dementia (FTD) and neuronal ceroid lipofuscinosis (NCL) are neurodegenerative diseases resulting from loss of one or both copies of the gene encoding progranulin (GRN), and gene replacement has been proposed for these currently untreatable disorders. Here, we used two different AAV vectors to induce widespread brain GRN expression in mice lacking the gene, as well as in wild-type mice. Unexpectedly, GRN overexpression resulted in T cell infiltration, followed by marked hippocampal neurodegeneration. Our results call into question the safety of GRN overexpression in the CNS, with wider implications for development of CNS gene therapies.