Polymorphic variants in the human bile salt export pump (BSEP; ABCB11): functional characterization and interindividual variability.

Abstract

Our aims were to identify and functionally characterize coding region nonsynonymous single nucleotide polymorphisms in the hepatic efflux transporter, bile salt export pump (BSEP; ABCB11), and to assess interindividual variability in BSEP expression. We identified 24 single nucleotide polymorphisms, including nine nonsynonymous variants, in ABCB11 from genomic DNA of approximately 250 ethnically diverse healthy individuals using denaturing high-performance liquid chromatography analysis and DNA sequencing. Wild type and variant BSEP were generated and functionally characterized for taurocholate transport activity in vitro in HeLa cells using a recombinant vaccinia-based method. BSEP expression was assessed by real-time mRNA analysis, western blot analysis, and immunofluorescence confocal microscopy. For the most part, polymorphisms were rare and ethnic-dependent. In vitro functional studies revealed several rare variants, including 616A>G, 1674G>C, 1772A>G, and 3556G>A, to be associated with significantly impaired taurocholate transport activity while the 890A>G variant trended towards impaired function but was not statistically significant. The 3556G>A variant was associated with reduced cell surface to total protein expression compared with wild-type BSEP. Expression of BSEP by mRNA and protein analysis was determined from a bank of human liver samples. Wide interindividual variability was noted in both mRNA (19-fold) and protein (31-fold) expression levels. The common variant 1331T>C was associated with significantly reduced hepatic BSEP mRNA levels. Accordingly, our study indicates there are functionally relevant polymorphisms in ABCB11 which may be of potential relevance in the predisposition to acquired liver disorders such as drug-induced cholestasis.