Received for publication August 25, 2005.
Revised October 20, 2005.
Accepted for publication October 27, 2005.

Genetic Variants of the Human Dipeptide Transporter PEPT1

Abstract

We tested whether genetic polymorphisms affect activity of the dipeptide transporter PEPT1, which mediates bioavailability of peptidomimetic drugs. All 23 exons and adjoining intronic sections of PEPT1 (SLC15A1) were sequenced in 247 individuals of various ethnic origins (Coriell collection). Of 38 single nucleotide polymorphisms (SNPs), 21 occurred in intronic and non-coding regions, and 17 in exonic coding region, of which 9 were nonsynonymous. Eight nonsynonymous variants were cloned into expression vectors and functionally characterized after transient transfection into Cos7 and CHO cells. None of the variants had altered transport activity for various ligands, supporting previous results, except for the new, low frequency PEPT1-F28Y. This variant displayed significantly reduced cephalexin uptake attributable to increased K_m. Altered pH-dependence of substrate transport suggested a role for F28Y in H⁺-driven translocation. Haplotype analysis revealed significant differences among ethnic populations. To search for cis-acting polymorphisms affecting transcription and mRNA processing, we measured allelic PEPT1 mRNA expression in human intestinal biopsy samples, using a frequent marker SNP in exon 17. Of 24 heterozygous samples, significant differences in allelic mRNA levels of 20-30% were observed in 7 tissues. However, the small difference suggests that cis-acting regulatory factors have only limited effects on transporter activity. We also measured the relative formation of a splice variant (PEPT1-RF). PEPT1-RF mRNA levels ranged from 2 to 44% of total PEPT1-related mRNA, with potential consequences for drug absorption. Together with previous results, this study reveals a relatively low level of genetic variability, in polymorphisms affecting both protein function and gene regulation.

Key words: Drug absorption, Haplotype, SNP, Transporter, cis-regulation, protein function

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