Using genetic linkage in an Amish population, we identified a coding mutation (K269E) in Sortilin associated with increased low-density lipoprotein cholesterol (LDL-C) and homeostatic model assessment of insulin resistance (HOMA-IR), and decreased C-peptide/insulin. This is the first time a naturally-occurring coding mutation in Sortilin has been linked to physiological function. Sortilin (SORT1) is a member of the vacuolar protein sorting 10 (VPS10) family of receptors that are involved in post-Golgi protein trafficking. We introduced the Sortilin K269E variant into mice and found that Western diet-fed, male Sortilin K269E mice had elevated LDL, insulin, and HOMA-IR, and decreased C-peptide/insulin, indicating that the variant is causal for these phenotypes in the humans. We show that mice harboring the Sortilin K269E variant had decreased clearance of both LDL and insulin, and our results indicate that mistrafficking of the LDL and insulin receptors in hepatocytes may be the underlying cause, ultimately resulting in elevated levels of LDL and insulin in the plasma. The Sortilin K269E mice had additional metabolic phenotypes, including increased adiposity, hepatic steatosis, gluconeogenesis, and plasma ketones, consistent with a broad role of Sortilin in mammalian metabolism and the increased insulin resistance. The phenotypes described above were observed in Western diet fed male homozygous-bred WT and Sortilin K269E mice. Subsequently, we bred mice heterozygous for the variant to obtain WT, Sortilin K269E heterozygous, and Sortilin K269E homozygous co-housed littermates. Surprisingly, upon measurement of plasma LDL, insulin, and HOMA-IR, and body weight, we found that all Western diet fed male heterozygous-bred mice, including the WT mice, had levels that were more similar to that of the homozygous-bred Sortilin K269E mice than the homozygous-bred WT mice. This unexpected result implicates a much more complex role of Sortilin K269E in metabolism. These data suggest that the phenotypes elicited by the Sortilin K269E mutation may be through an epigenetic parent-of-origin effect, or through a factor that is excreted in the feces of the mice, and due to the mice being coprophagic, is transmitted to the WT mice when co-housed with the Sortilin K269E mice. We are currently investigating these possibilities. Overall, this work has unveiled a novel site in Sortilin important for its role in lipid and insulin metabolism.