Genetic and functional studies of two intestinal vitamin C transporters, SLC23A1 and GLUT14, associated with inflammatory bowel disease

Thumbnail Image
2013-11-27, 2013-08-02
Amir Shaghaghi, Mandana
Journal Title
Journal ISSN
Volume Title
American Journal of Clinical Nutrition
Journal of Human Nutrition & Food Science
It had long been known that individuals with inflammatory bowel disease (IBD), comprising Crohn’s disease (CD) and ulcerative colitis (UC), have locally reduced vitamin C levels in the intestinal mucosa, which equates to an overall loss of the antioxidant capacity and increase risk of oxidative tissue damage. The aim of the present work was to expand on this concept, through investigating the role of genetic variations in intestinal vitamin C transporters in vulnerability to IBD and further characterizing their functions. The studies focused on a known intestinal transporter for ascorbic acid, SLC23A1, and a novel intestinal transporter of dehydroascorbic acid, SLC2A14 (GLUT14). To investigate any association between the SLC23A1 and SLC2A14 with IBD, genomic DNA of 311 Caucasian individuals with IBD, participated in the Manitoba IBD Cohort Study, and 142 healthy controls were genotyped for tagging single nucleotide polymorphism (SNP) of each gene, using TaqMan Assays. New splice variants of SLC23A1 and SLC2A14 were determined by In silico analyses, followed by sub-cloning of the splice variants to verify their subcellular locations. Substrates and functions were determined, using the Xenopus laevis oocyte system for each transporter. The presence of the SLC23A1 variant rs10063949 G allele elevated the risk for CD by 150% (Odds ratios (OR) = 2.54, 95% CI 1.83-3.53). An allele dosage effect was confirmed; compared to rs10063949-AA homozygotes the 10063949-AG heterozygotes have a 150% elevated risk and the 10063949-GG homozygotes have a 370% elevated CD risk (OR=2.54, 95% CI 1.38-4.66; OR=4.72, 95% CI 2.53-8.81, p<0.001; respectively). No relation was observed between genetic variants in SLC23A1 and UC. Through database search, a novel 5’exon was discovered for SLC23A1 locus which is located 1078 nucleotides upstream of the canonical first exon. The two first exons are not mutually exclusive since they splice together to create a novel SLC23A1 protein isoform, we named it isoform 1A, with a N-terminus that is elongated by 36 amino acid. The novel SLC23A1 isoform located at the plasma membrane, but mediates only 7% of the ascorbic acid transport exhibited by the shorter isoform. The presence of the SLC2A14 SNP rs2889504 A allele elevated the risk for UC by 260% and CD by 468% (OR: 3.60, 95% CI: 1.95-6.64; OR: 4.68, 95% CI: 2.78-8.50, respectively). The rs10846086-G allele elevated the risk of UC and CD approximately 3-fold (OR: 2.91, 95% CI: 1.49-5.68; OR: 3.00, 95% CI: 1.55-5.78, respectively). The variant rs12815313-T increased the risk for CD by 112% (OR: 2.12, 95% CI: 1.33-3.36). All the genetic variations in SLC2A14 gene, associated with IBD, were independent from each other, strengthening the evidence that functional SNPs in the SLC2A14 locus contribute to IBD. It was identified that the two major GLUT14 isoforms locate to the plasmalemma membrane and mediate cellular uptake of dehydroascorbic acid. Significant expression in extra-testicular tissues was confirmed for SLC2A14, notably in intestinal segments, explaining the association with IBD. Re-analysis of genomic showed a dramatically expanded locus of SLC2A14, containing twenty exons which covered 103,477 nucleotides from the first Transcriptional Start Site (TSS) to the termination of the longest transcript. All together, the presented evidence indicate that functional SNPs in the SLC2A14 gene and SLC23A1 could contribute to vitamin C imbalance in mucosal cells which contributes to an elevated risks of IBD. Furthermore, novel information about genetic and functional characteristics of SLC23A1 and GLUT14 transporters was identified.
Genetic, Intestinal vitamin C transporters, Inflammatory bowel disease
Amir Shaghaghi, Mandana., Bernstein, Charles N., Serrano Leo´n, Alejandra., El-Gabalawy, Hani., Eck, Peter. (2014). Polymorphisms in the sodium-dependent ascorbate transporter gene SLC23A1 are associated with susceptibility to Crohn disease. Am J Clin Nutr, 99, 378–383.
Amir Shaghaghi Mandana, Tu Hongbin, Yurkova Natalia, Levine Mark, Eck Peter. (2013). Identification and Functional Characterization of an Alternative 5’ Exon of the Sodium Dependent Ascorbic Acid Transporter SLC23A1. J Hum Nutr Food Sci, 1: 1001.