AAV-Mediated Progranulin Delivery to a Mouse Model of Progranulin Deficiency Causes T Cell-Mediated Toxicity

Adeno-associated virus-mediated gene replacement is emerging as a safe and effective means of correcting single-gene mutations affecting the CNS. AAV-mediated progranulin gene (GRN) delivery has been proposed as a treatment for GRN-deficient frontotemporal dementia and neuronal ceroid lipofuscinosis, and recent studies using intraparenchymal AAV-Grn delivery to brain have shown moderate success in histopathologic and behavioral rescue in mouse 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, overexpression resulted in dramatic and selective hippocampal toxicity and degeneration affecting neurons and glia. Hippocampal degeneration was preceded by T cell infiltration and perivascular cuffing. GRN delivery with an ependymal-targeting AAV for selective secretion of progranulin into the cerebrospinal fluid 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. Our results highlight the importance of careful consideration of target gene biology and cellular response to overexpression prior to progressing to the clinic.

Toward a gene therapy for neurological and somatic MPSIIIA

Mucopolysaccharidosis Type IIIA (MPSIIIA) represents an unmet medical need. MPSIIIA shares with many other lysosomal storage disorders (LSD) the characteristic of being a severe neurodegenerative disease accompanied by mild somatic involvement. Thus, the main target organ for the development of new treatments is the central nervous system (CNS), but overall clinical efficacy would be greatly enhanced by simultaneous correction of peripheral disease. We have recently developed a novel treatment for MPSIIIA based on the delivery to the cerebrospinal fluid of serotype 9 adeno-associated virus (AAV9)-derived vectors. This gene therapy strategy corrected both CNS and somatic pathology in animal models through widespread transduction of CNS, peripheral nervous system (PNS), and liver. The work set the grounds for the clinical translation of the approach to treat MPSIIIA in humans. Here we discuss some important considerations that further support the applicability of this treatment to MPSIIIA and other LSD with CNS and somatic involvement.