PharmGKB update: I. Genetic variants of the organic cation transporter 2 (OCT2, SLC22A2).
Explore the source record for details and available documents.
Genetic Variation↗
Biomedical subjects
Thomas E Ferrin
Publications and source records attributed to Thomas E Ferrin.
Natural variation in human membrane transporter genes reveals evolutionary and functional constraints.
Membrane transporters maintain cellular and organismal homeostasis by importing nutrients and exporting toxic compounds. Transporters also play a crucial role in drug response, serving as drug targets and setting drug levels. As part of a pharmacogenetics project, we screened exons and flanking intronic regions for variation in a set of 24 membrane transporter genes (96 kb; 57% coding) in 247 DNA samples from ethnically diverse populations. We identified 680 single nucleotide polymorphisms (SNPs), of which 175 were synonymous and 155 caused amino acid changes, and 29 small insertions and deletions. Amino acid diversity (pi(NS)) in transmembrane domains (TMDs) was significantly lower than in loop domains, suggesting that TMDs have special functional constraints. This difference was especially striking in the ATP-binding cassette superfamily and did not parallel evolutionary conservation: there was little variation in the TMDs, even in evolutionarily unconserved residues. We used allele frequency distribution to evaluate different scoring systems (Grantham, blosum62, SIFT, and evolutionarily conservedevolutionarily unconserved) for their ability to predict which SNPs affect function. Our underlying assumption was that alleles that are functionally deleterious will be selected against and thus under represented at high frequencies and over represented at low frequencies. We found that evolutionary conservation of orthologous sequences, as assessed by evolutionarily conservedevolutionarily unconserved and SIFT, was the best predictor of allele frequency distribution and hence of function. European Americans had an excess of high frequency alleles in comparison to African Americans, consistent with a historic bottleneck. In addition, African Americans exhibited a much higher frequency of population specific medium-frequency alleles than did European Americans.
DNA↗
Evolutionary conservation predicts function of variants of the human organic cation transporter, OCT1.
The organic cation transporter, OCT1, is a major hepatic transporter that mediates the uptake of many organic cations from the blood into the liver where the compounds may be metabolized or secreted into the bile. Because OCT1 interacts with a variety of structurally diverse organic cations, including clinically used drugs as well as toxic substances (e.g., N-methylpyridinium, MPP(+)), it is an important determinant of systemic exposure to many xenobiotics. To understand the genetic basis of extensive interindividual differences in xenobiotic disposition, we functionally characterized 15 protein-altering variants of the human liver organic cation transporter, OCT1, in Xenopus oocytes. All variants that reduced or eliminated function (OCT1-R61C, OCT1-P341L, OCT1-G220V, OCT1-G401S, and OCT1-G465R) altered evolutionarily conserved amino acid residues. In general, variants with decreased function had amino acid substitutions that resulted in more radical chemical changes (higher Grantham values) and were less evolutionarily favorable (lower blosum62 values) than variants that maintained function. A variant with increased function (OCT1-S14F) changed an amino acid residue such that the human protein matched the consensus of the OCT1 mammalian orthologs. Our results indicate that changes at evolutionarily conserved positions of OCT1 are strong predictors of decreased function and suggest that a combination of evolutionary conservation and chemical change might be a stronger predictor of function.
Alleles↗
MINRMS: an efficient algorithm for determining protein structure similarity using root-mean-squared-distance.
MOTIVATION: Existing algorithms for automated protein structure alignment generate contradictory results and are difficult to interpret. An algorithm which can provide a context for interpreting the alignment and uses a simple method to characterize protein structure similarity is needed. RESULTS: We describe a heuristic for limiting the search space for structure alignment comparisons between two proteins, and an algorithm for finding minimal root-mean-squared-distance (RMSD) alignments as a function of the number of matching residue pairs within this limited search space. Our alignment algorithm uses coordinates of alpha-carbon atoms to represent each amino acid residue and requires a total computation time of O(m(3) n(2)), where m and n denote the lengths of the protein sequences. This makes our method fast enough for comparisons of moderate-size proteins (fewer than approximately 800 residues) on current workstation-class computers and therefore addresses the need for a systematic analysis of multiple plausible shape similarities between two proteins using a widely accepted comparison metric.
Algorithms↗
BayGenomics: a resource of insertional mutations in mouse embryonic stem cells.
The BayGenomics gene-trap resource (http://baygenomics.ucsf.edu) provides researchers with access to thousands of mouse embryonic stem (ES) cell lines harboring characterized insertional mutations in both known and novel genes. Each cell line contains an insertional mutation in a specific gene. The identity of the gene that has been interrupted can be determined from a DNA sequence tag. Approximately 75% of our cell lines contain insertional mutations in known mouse genes or genes that share strong sequence similarities with genes that have been identified in other organisms. These cell lines readily transmit the mutation to the germline of mice and many mutant lines of mice have already been generated from this resource. BayGenomics provides facile access to our entire database, including sequence tags for each mutant ES cell line, through the World Wide Web. Investigators can browse our resource, search for specific entries, download any portion of our database and BLAST sequences of interest against our entire set of cell line sequence tags. They can then obtain the mutant ES cell line for the purpose of generating knockout mice.
Animals↗
The Structure Superposition Database.
The need for new tools for investigating biological systems on a large scale is becoming acute, particularly with respect to computationally intensive analyses such as comparisons of many three-dimensional protein structures. Structure superposition is a valuable approach for understanding evolutionary relationships and for the prediction of function. But while available tools are adequate for generating and viewing superpositions of single pairs of protein structures, these tools are generally too cumbersome and time-consuming for examining multiple superpositions. To address this need, we have created the Structure Superposition Database (SSD) for accessing, viewing and understanding large sets of structure superposition data. The initial implementation of the SSD contains the results of pairwise, all-by-all superpositions of a representative set of 115 (beta/alpha)8 barrel structures (TIM barrels). Future plans call for extending the database to include representative structure superpositions for many additional folds. The SSD can be browsed with a user interface module developed as an extension to Chimera, an extensible molecular modeling program. Features of the user interface module facilitate viewing multiple superpositions together. The SSD interface module can be downloaded from http://ssd.rbvi.ucsf.edu.
Animals↗
Functional characterization in yeast of genetic variants in the human equilibrative nucleoside transporter, ENT1.
The human equilibrative nucleoside transporter, ENT1, appears to play a critical role in the disposition of nucleosides and nucleoside analogs used clinically as anti-viral and anti-cancer drugs. Recently, we identified variants of ENT1 in an ethnically diverse DNA sample set from 247 individuals, focusing primarily on the coding region. The goal of the present study was to analyse the haplotype structure and functionally characterize the variants of ENT1. We observed that a single haplotype, ENT1*1, accounted for 91.3% of the 494 chromosomes. Functional analysis in Saccharomyces cerevisiae revealed no differences in the kinetics of uptake of nucleosides and nucleoside analogs by the two non-synonymous variant transporters, ENT1-I216T and ENT1-E391K, and the reference ENT1. These results, together with the observation that there are few haplotypes of ENT1, indicate that coding region variants of ENT1 do not contribute to inter-individual differences in response to nucleoside analog drugs.
Amino Acid Sequence↗
Sequence diversity and haplotype structure in the human ABCB1 (MDR1, multidrug resistance transporter) gene.
OBJECTIVES: There is increasing evidence that polymorphism of the ABCB1 (MDR1) gene contributes to interindividual variability in bioavailability and tissue distribution of P-glycoprotein substrates. The aim of the present study was to (1) identify and describe novel variants in the ABCB1 gene, (2) understand the extent of variation in ABCB1 at the population level, (3) analyze how variation in ABCB1 is structured in haplotypes, and (4) functionally characterize the effect of the most common amino acid change in P-glycoprotein. METHODS AND RESULTS: Forty-eight variant sites, including 30 novel variants and 13 coding for amino acid changes, were identified in a collection of 247 ethnically diverse DNA samples. These variants comprised 64 statistically inferred haplotypes, 33 of which accounted for 92% of chromosomes analyzed. The two most common haplotypes, ABCB1*1 and ABCB1*13, differed at six sites (three intronic, two synonymous, and one non-synonymous) and were present in 36% of all chromosomes. Significant population substructure was detected at both the nucleotide and haplotype level. Linkage disequilibrium was significant across the entire ABCB1 gene, especially between the variant sites found in ABCB1*13, and recombination was inferred. The Ala893Ser change found in the common ABCB1*13 haplotype did not affect P-glycoprotein function. CONCLUSION: This study represents a comprehensive analysis of ABCB1 nucleotide diversity and haplotype structure in different populations and illustrates the importance of haplotype considerations in characterizing the functional consequences of ABCB1 polymorphisms.
Base Sequence↗
SNP analysis and presentation in the Pharmacogenetics of Membrane Transporters Project.
The multidisciplinary UCSF Pharmacogenetics of Membrane Transporters project seeks to systematically identify sequence variants in transporters and to determine the functional significance of these variants through evaluation of relevant cellular and clinical phenotypes. The project is structured around four interacting cores: genomics, cellular phenotyping, clinical phenotyping, and bioinformatics. The bioinformatics core is responsible for collecting, storing, and analyzing the information obtained by the other cores and for presenting the results, in particular, for the genomic data. Most of this process is automated using locally developed software written in Python, an open source language well suited for rapid, modular development that meets requirements that are themselves constantly evolving. Here we present the details of transforming ABI trace file data into useful information for project investigators and a description of the types of data analysis and display that we have developed.
Amino Acid Sequence↗
Polymorphisms in a human kidney xenobiotic transporter, OCT2, exhibit altered function.
The completion of the Human Genome Project and the development of high-throughput polymorphism identification methods have allowed researchers to carry out full genetic analyses of many clinically relevant genes. However, few studies have combined genetic analysis with in vitro phenotyping to better understand the relationship between genetic variation and protein function. Many transporters in the kidney are thought to play key roles in defense against a variety of foreign substances. The goal of this study was to understand the relationship between variation in a gene encoding a major renal xenobiotic transporter, OCT2, and transporter function. We report a comprehensive genetic analysis and functional characterization of variants of OCT2. Twenty-eight variable sites in the OCT2 gene were identified in a collection of 247 ethnically diverse DNA samples. Eight caused non-synonymous amino acid changes, of which four were present at >/= 1% in an ethnic population. All four of these altered transporter function assayed in Xenopus laevis oocytes. Analysis of nucleotide diversity (pi) revealed a higher prevalence of synonymous (pi = 22.4 x 10-4) versus non-synonymous (pi = 2.1 x 10-4) changes in OCT2 than in other genes. In addition, the non-synonymous sites had a significant tendency to exhibit more skewed allele frequencies (more negative Tajima's D-values) compared to synonymous sites. The population-genetic analysis, together with the functional characterization, suggests that selection has acted against amino acid changes in OCT2. This selection may be due to a necessary role of OCT2 in the renal elimination of endogenous amines or xenobiotics, including environmental toxins, neurotoxic amines and therapeutic drugs.
Alleles↗