Humanized mice carrying a pathogenic GRN deletion as a pre-clinical platform for targeted gene therapies in frontotemporal dementia.

Frontotemporal dementia (FTD) is an early onset dementia characterized by neuropathology and changes to patient behaviour. Haploinsufficiency of the gene progranulin (GRN) is a major cause of FTD, for which there are no effective therapies. Corrective gene therapies that restore GRN expression are of clinical interest, but current in vivo systems have limitations. We developed a novel strain of mice expressing a human GRN transgene bearing a four base pair deletion in exon 5 (GRNc.388_391delCAGT) that causes FTD. Characterization of mice expressing the mutant transgene (GRNmEx5) indicates that GRNmEx5 is expressed at low levels and retains partial function. The GRNmEx5 protein partially rescues progranulin nullizygous-associated neuropathology and transcriptomic dysfunction. Following characterization, we sought to determine if mice expressing GRNmEx5 in the absence of mouse progranulin (Grn-/-; GRNmEx5 mice) could enable pre-clinical gene therapy development. Using CRISPR/Cas9 with lipid nanoparticle delivery, we achieved 8.5% correction of GRNc.388_391delCAGT in target cells in Grn-/-; GRNmEx5 mice, demonstrating both effective in vivo homology-directed repair and the utility of Grn-/-; GRNmEx5 mice for developing novel progranulin-associated FTD therapies. The Grn-/-; GRNmEx5 model provides insight into progranulin biology, increases our understanding of a pathogenic variant that causes FTD, and facilitates the development of GRN gene therapies.