Outcomes of Progranulin Gene Therapy in the Retina are Dependent on Time of Delivery

Neuronal ceroid lipofuscinosis (NCL) is a family of neurodegenerative diseases caused by mutations to genes related to lysosomal function. One variant, CNL11, is caused by mutations to the gene encoding the protein progranulin. Primarily secreted by microglia, progranulin regulates neuronal lysosomal function once endocytosed. Absence of progranulin causes cerebellar atrophy, seizures, ataxia, dementia and vision loss. As progranulin gene therapies targeting the brain are developed, it is also advantageous to focus on the retina, as its characteristics are beneficial for gene therapy development: the retina is easily visible through direct imaging, can be assessed through quantitative methods in vivo, requires smaller amounts of AAV and AAV can be administered via a less invasive surgery. In this study we characterize the retinal degeneration in a progranulin knockout mouse model of CLN11 and study the effects of gene replacement at different time points. All mice heterologously expressing progranulin showed reduction in lipofuscin deposits and microglia infiltration. While mice that receive systemic AAV9.2YF-scCAG-PGRN at post-natal day 3 or 4 show a reduction in retina thinning, mice injected intravitreally at months 1 and 6 with 7m8-scCAG-PGRN show no improvement, and mice injected at 12 months of age show increased retinal thinning in comparison to their controls. Thus, delivery of progranulin proves to be time-sensitive, requiring early administration for optimal therapeutic benefit.