Molecular characterization of the NPC1L1 variants identified from cholesterol low absorbers

Niemann-Pick C1-like 1 (NPC1L1) is an essential protein for dietary cholesterol absorption. Nonsynonymous (NS) variants of NPC1L1 in humans have been suggested to associate with cholesterol absorption variations. However, information concerning the characteristics and mechanism of these variants in cholesterol uptake is limited. In this study, we analyzed the cholesterol uptake ability of the 19 reported NS variants of NPC1L1 identified from cholesterol low absorbers. Among these variants, L110F, R306C, A395V, G402S, T413M, R693C, R1214H, and R1268H could partially mediate cellular cholesterol uptake and were categorized as partially dysfunctional variants. The other 11 variants including T61M, N132S, D398G, R417W, G434R, T499M, S620C, I647N, G672R, S881L, and R1108W could barely facilitate cholesterol uptake, and were classified into the severely dysfunctional group. The partially dysfunctional variants showed mild defects in one or multiple aspects of cholesterol-regulated recycling, subcellular localization, glycosylation, and protein stability. The severely dysfunctional ones displayed remarkable defects in all these aspects and were rapidly degraded through the ER-associated degradation (ERAD) pathway. In vivo analyses using adenovirus-mediated expression in mouse liver confirmed that the S881L variant failed to localize to liver canalicular membrane, and the mice showed defects in biliary cholesterol re-absorption, while the G402S variant appeared to be similar to wild-type NPC1L1 in mouse liver. This study suggests that the dysfunction of the 19 variants on cholesterol absorption is due to the impairment of recycling, subcellular localization, glycosylation, or stability of NPC1L1.