Effects of nonsense genetic variants on OATP1B1 expression and transport activity in vitro

Organic anion transporting polypeptide 1B1 (OATP1B1, encoded by SLCO1B1) is an influx transporter expressed in the basolateral membrane of hepatocytes, playing a key role in the hepatic uptake and clearance of various drugs and endogenous compounds from the plasma. Single nucleotide variants in the SLCO1B1 gene have been shown to alter plasma drug concentration by affecting transporter activity. Furthermore, several variants leading to premature termination of translation have been observed, together with loss-of-function SLCO1B3 variants, in patients with Rotor syndrome. In this study, we investigated whether selected nonsense variations (c.757C>T, c.1738C>T, c.1877T>A, c.1905-1909del, c.1925-1929del, c.1929-1932del, c.1928T>G, c.1968dup, c.1976C>G) affect the transport activity and membrane expression of OATP1B1. HEK293 cells were transduced with baculovirus constructs carrying either a variant or reference SLCO1B1 sequence and the uptake of 2',7'-dichlorofluorescein or rosuvastatin was measured. Protein abundance was measured using a quantitative targeted absolute proteomics approach. Compared to reference OATP1B1, the c.1968dup and c.1976C>G variants, located near the C-terminus, retained 30-40 % of transport activity. In contrast, the remaining variants, located before or in the last transmembrane helix, resulted in complete loss of OATP1B1 function. Furthermore, all variants reduced OATP1B1 expression in the cell membranes. These results suggest that most nonsense variants result in a loss of OATP1B1 activity, but those located in the C-terminal loop may retain some activity. Moreover, carriers of the studied variants may exhibit increased plasma concentrations of OATP1B1 substrates.