A mouse model Niemann-Pick type C disease to test proteostasis therapies

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Niemann-Pick disease type C is an invariably fatal autosomal recessive lipid storage disease. Patients develop a clinically heterogeneous phenotype that includes progressive neurodegeneration and early death. Disease is commonly caused by loss-of-function mutations in the NPC1 gene (95% of cases), encoding a multipass transmembrane glycoprotein required for exporting unesterified cholesterol from late endosomes and lysosomes. The most common disease-causing mutation (~20% of cases) is an isoleucine to threonine substitution at position 1061 (I1061T). I1061T NPC1 misfolds in the endoplasmic reticulum (ER) and is rapidly degraded by the proteasome and ER-autophagy. Importantly, transient over-expression of I1061T in vitro or treatment of I1061T NPC1 patient fibroblasts with ryanodine receptor antagonists drives trafficking of mutant NPC1 to the lysosome where it is still functional. These observations have spurred interest in developing proteostasis modulators to treat disease. These efforts have relied on gene targeted mice in which the I1061T mutation was inserted into the mouse Npc1 gene to test therapeutic strategies. However, our preliminary data indicate that there are marked differences in how the human and mouse I1061T NPC1 proteins are handled by the cellular quality control machinery. These previously unappreciated differences underscore the critical need to develop a new model system that reliably reproduces human I1061T NPC1 proteostasis. The rationale for this project is that developing humanized I1061T NPC1 mice will enable in vivo testing of proteostatic therapeutics for Niemann-Pick C. To attain the overall objective of this application, we will use genetic and biochemical approaches to pursue the following specific aim: Develop and characterize humanized I1061T NPC1 mice. We expect that targeting the mouse Npc1 gene to express human I1061T NPC1 protein will generate a robust model of disease, in which misfolding and trafficking of the mutant NPC1 protein closely mimics the behavior of human I1061T NPC1 as occurs in Niemann-Pick C patients.