Characterization of a Novel NPC1I1061T Knock-in Mouse Model of Niemann-Pick Type C1 Disease

Niemann-Pick Type C (NPC) disease is a fatal pediatric cholesterol storage disease that is caused by mutations in either the npc1 or npc2 genes. Loss of function of either gene results in toxic storage of free cholesterol in the lysosomes due to a cholesterol trafficking defect. In humans the free cholesterol accumulation leads to Purkinje cell loss in the cerebellum and is accompanied by neurodegenerative symptoms that include ataxia, vertical supranuclear gaze palsy, and decreased motor function. Most NPC1 patients succumb to the disease within the first two decades of life. The most prevalent mutation, NPC1I1061T, resides within the cysteine-rich luminal domain of the NPC1 protein and represents 18-23% of all disease alleles. Previous studies from our laboratory have shown that the NPC1I1061T protein exhibits a markedly reduced half-life as compared to wild-type NPC1 due to protein misfolding and consequent targeting for endoplasmic reticulum (ER)-associated degradation (ERAD). More recently, ryanodine receptor antagonists have been shown to increase NPC1I1061T protein stability and reduce cholesterol storage in human skin fibroblasts homozygous for the NPC1I1061T mutation. Histone deacetylase (HDAC) inhibitors have also been shown to augment NPC1I1061T protein and cholesterol egress from the lysosome, possibly via an increase in HSP90 expression. Together, these findings raise the possibility that small molecule-based proteostatic therapies might stabilize mutant NPC1 proteins and have the potential to provide clinical benefit. To address the need for an animal model in which to test proteostatic therapies for NPC1 disease, we generated a NPC1I1061T knock-in mouse model. In comparison to the BALB/c NPC1nih (Npc1-/-) mouse, this mouse models a less severe form of NPC1 disease and displays hallmarks of NPC1 disease, including weight loss, decreased motor coordination, Purkinje cell death, and lysosomal lipid accumulation in brain and liver. We show that the murine NPC1I1061T protein has a markedly reduced half-life in vivo, which is consistent with protein misfolding and rapid ER-associated degradation and can be stabilized by HDAC inhibition. This novel mouse model faithfully recapitulates disease caused by the human NPC1I1061T mutation, and provides the field with a powerful tool for pre-clinical evaluation of proteostatic therapies for NPC1 disease