Biochemical, Biomarker, and Behavioral Characterization of the Grn^R493X Mouse Model of Frontotemporal Dementia

Heterozygous loss-of-function mutations in the progranulin gene (GRN) are a major cause of frontotemporal dementia due to progranulin haploinsufficiency; complete deficiency of progranulin causes neuronal ceroid lipofuscinosis. Several progranulin-deficient mouse models have been generated, including both knockout mice and knockin mice harboring a common patient mutation (R493X). However, the Grn^R493X mouse model has not been characterized completely. Additionally, while homozygous Grn^R493X and Grn knockout mice have been extensively studied, data from heterozygous mice is still limited. Here, we performed more in-depth characterization of heterozygous and homozygous Grn^R493X knockin mice, which includes biochemical assessments, behavioral studies, and analysis of fluid biomarkers. In the brains of homozygous Grn^R493X mice, we found increased phosphorylated TDP-43 along with increased expression of lysosomal genes, markers of microgliosis and astrogliosis, pro-inflammatory cytokines, and complement factors. Heterozygous Grn^R493X mice did not have increased TDP-43 phosphorylation but did exhibit limited increases in lysosomal and inflammatory gene expression. Behavioral studies found social and emotional deficits in Grn^R493X mice that mirror those observed in Grn knockout mouse models, as well as impairment in memory and executive function. Overall, the Grn^R493X knockin mouse model closely phenocopies Grn knockout models. Lastly, in contrast to homozygous knockin mice, heterozygous Grn^R493X mice do not have elevated levels of fluid biomarkers previously identified in humans, including neurofilament light chain (NfL) and glial fibrillary acidic protein (GFAP) in both plasma and CSF. These results may help to inform pre-clinical studies that use this Grn knockin mouse model and other Grn knockout models.