Comparison of the genomic structure and variation in the two human sodium-dependent vitamin C transporters, SLC23A1 and SLC23A2

Vitamin C (L-ascorbic acid) is an essential co-factor for eight mammalian enzymes and quenches reactive oxygen species. Sodium-dependent vitamin C transport is mediated by two transporters, SVCT 1 and SVCT 2, encoded by SLC23A1 and SLC23A2. We characterized the genomic structures of SLC23A1 and SLC23A2, determined the extent of genetic variation and linkage disequilibrium across each gene, analyzed nucleotide diversity to estimate the effect of selective pressure, and compared sequence variation across species. In SLC23A1, the majority of single nucleotide polymorphisms (SNPs) are population-specific in either African Americans or Caucasians, including three of four non-synonymous SNPs. In contrast, most SNPs in SLC23A2 are shared between African Americans and Caucasians, and there are no non-synonymous SNPs in SLC23A2. Our analysis, combined with previous in vitro and in vivo studies, suggests that non-synonymous variation appears to be tolerated in SLC23A1 but not SLC23A2, and that this may be a consequence of different selective pressures following past gene duplication of the sodium-dependent vitamin C transporters. Genetic association studies of these two genes will need to account for the differences in haplotype structure and the population-specific variants. Our data represent a fundamental step toward the application of genetics to refining nutrient recommendations, specifically for vitamin C, and may serve as a paradigm for other vitamins.