Interaction of Three Regiospecific Amino Acid Residues Is Required for OATP1B1 Gain of OATP1B3 Substrate Specificity

The human organic anion-transporting polypeptides OATP1B1 (<i>SLCO1B1</i>) and OATP1B3 (<i>SLCO1B3</i>) are liver-enriched membrane transporters of major importance to hepatic uptake of numerous endogenous compounds, including bile acids, steroid conjugates, hormones, and drugs, including the 3-hydroxy-3-methylglutaryl Co-A reductase inhibitor (statin) family of cholesterol-lowering compounds. Despite their remarkable substrate overlap, there are notable exceptions: in particular, the gastrointestinal peptide hormone cholecystokinin-8 (CCK-8) is a high affinity substrate for OATP1B3 but not OATP1B1. We utilized homologous recombination of linear DNA by <i>E. coli</i> to generate a library of cDNA containing monomer size chimeric OATP1B1–1B3 and OATP1B3–1B1 transporters with randomly distributed chimeric junctions to identify three discrete regions of the transporter involved in conferring CCK-8 transport activity. Site-directed mutagenesis of three key residues in OATP1B1 transmembrane helices 1 and 10, and extracellular loop 6, to the corresponding residues in OATP1B3, resulted in a gain of CCK-8 transport by OATP1B1. The residues appear specific to CCK-8, as the mutations did not affect transport of the shared OATP1B substrate atorvastatin or the OATP1B1-specific substrate estrone sulfate. Regions involved in gain of CCK-8 transport by OATP1B1, when mapped to the crystal structures of bacterial transporters from the major facilitator superfamily, are positioned to suggest these regions could readily interact with drug substrates. Accordingly, our data provide new insight into the molecular determinants of the substrate specificity of these hepatic uptake transporters with relevance to targeted drug design and prediction of drug–drug interactions

Genome-Wide Association Study of Serum Creatinine Levels during Vancomycin Therapy

<div><p>Vancomycin, a commonly used antibiotic, can be nephrotoxic. Known risk factors such as age, creatinine clearance, vancomycin dose / dosing interval, and concurrent nephrotoxic medications fail to accurately predict nephrotoxicity. To identify potential genomic risk factors, we performed a genome-wide association study (GWAS) of serum creatinine levels while on vancomycin in 489 European American individuals and validated findings in three independent cohorts totaling 439 European American individuals. In primary analyses, the chromosome 6q22.31 locus was associated with increased serum creatinine levels while on vancomycin therapy (most significant variant rs2789047, risk allele A, β = -0.06, p = 1.1 x 10^-7). SNPs in this region had consistent directions of effect in the validation cohorts, with a meta-p of 1.1 x 10^-7. Variation in this region on chromosome 6, which includes the genes <i>TBC1D32/C6orf170</i> and <i>GJA1</i> (encoding connexin43), may modulate risk of vancomycin-induced kidney injury.</p></div

Association of genome-wide SNPs to peak creatinine while on vancomycin therapy in the primary cohort.

<p>A) Manhattan plot, where each dot represents a genotyped SNP, arranged along the x-axis by position of the SNP on each chromosome. The y-axis plots-log10(p-value) for the linear regression analysis of peak creatinine, adjusted for sex, age at time of vancomycin therapy, height, weight, vancomycin dose / dosing interval, vancomycin trough, and baseline serum creatinine measurements as described in the methods. B) LocusZoom plot of 6q22.31 locus, including genotyped and imputed SNPs. Each dot represents a SNP, arranged by position on chromosome 6 along the x-axis, and the color indicates degree of linkage disequilibrium with the index SNP, rs2789047. The left y-axis plots-log10(p-value) for each SNP. The blue line indicates estimated recombination rate, quantified on the right y-axis. Known genes in the region are indicted below the x-axis.</p

Cohort demographics for primary and validation cohorts investigated by genome-wide association for the outcome of peak serum creatinine during vancomycin therapy.

<p>*Median (interquartile range)</p><p>^N, %.</p><p>Cohort demographics for primary and validation cohorts investigated by genome-wide association for the outcome of peak serum creatinine during vancomycin therapy.</p

Meta-analysis results for chromosome 6 SNP associations to peak creatinine during vancomycin therapy.

<p>RAF—Risk Allele Frequency; SE—Standard Error.</p><p>*—Imputed SNP in Mayo and VGER cohorts.</p><p>Meta-analysis results for chromosome 6 SNP associations to peak creatinine during vancomycin therapy.</p

Identification of primary cohort.

<p>Electronic medical records data were searched to identify 5,665 individuals exposed to vancomycin. Automated and manual algorithms were used to determine if each satisfied inclusion / exclusion criteria, as described in the methods, resulting in 882 confirmed cases. After exclusion of those without DNA, those who failed quality control (QC), and those of non-European-American ancestry, 745 individuals remained. Of those, 489 had serum creatinine measurements for the primary analysis.</p