Crohn's disease and genetic hitchhiking at IBD5

Chad D. Huff, David J. Witherspoon, Yuhua Zhang, Chandler Gatenbee, Lee A. Denson, Subra Kugathasan, Hakon Hakonarson, April Whiting, Chadwick T. Davis, Wilfred Wu, Jinchuan Xing, W. Scott Watkins, Michael J. Bamshad, Jonathan P. Bradfield, Kazima Bulayeva, Tatum S. Simonson, Lynn B. Jorde, Stephen L. Guthery

Research output: Contribution to journalArticlepeer-review

32 Scopus citations

Abstract

Inflammatory bowel disease 5 (IBD5) is a 250 kb haplotype on chromosome 5 that is associated with an increased risk of Crohn's disease in Europeans. The OCTN1 gene is centrally located on IBD5 and encodes a transporter of the antioxidant ergothioneine (ET). The 503F variant of OCTN1 is strongly associated with IBD5 and is a gain-of-function mutation that increases absorption of ET. Although 503F has been implicated as the variant potentially responsible for Crohn's disease susceptibility at IBD5, there is little evidence beyond statistical association to support its role in disease causation.We hypothesize that 503F is a recent adaptation in Europeans that swept to relatively high frequency and that disease association at IBD5 results not from503F itself, but fromone or more nearby hitchhiking variants, in the genes IRF1 or IL5. To test for evidence of recent positive selection on the 503F allele, we employed the iHS statistic, which was significant in the European CEU HapMap population (P 5 0.0007) and European Human Genome Diversity Panel populations (P ≤ 0.01). To evaluate the hypothesis of disease-variant hitchhiking, we performed haplotype association tests on high-density microarray data in a sample of 1,868 Crohn's disease cases and 5,550 controls. We found that 503F haplotypes with recombination breakpoints between OCTN1 and IRF1 or IL5 were not associated with disease (odds ratio [OR]: 1.05, P5 0.21). In contrast, we observed strong disease association for 503F haplotypes with no recombination between these three genes (OR: 1.24, P52.6=10-8), as expected if the sweeping haplotype harbored one or more disease-causing mutations in IRF1 or IL5. To further evaluate these disease-gene candidates, we obtained expression data from lower gastrointestinal biopsies of healthy individuals and Crohn's disease patients.Weobserved a 72% increase in gene expression of IRF1 among Crohn's disease patients (P50.0006) and no significant difference in expression of OCTN1. Collectively, these data indicate that the 503F variant has increased in frequency due to recent positive selection and that disease-causing variants in linkage disequilibrium with 503F have hitchhiked to relatively high frequency, thus forming the IBD5 risk haplotype. Finally, our association results and expression data support IRF1 as a strong candidate for Crohn's disease causation.

Original languageEnglish (US)
Pages (from-to)101-111
Number of pages11
JournalMolecular biology and evolution
Volume29
Issue number1
DOIs
StatePublished - Jan 2012
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Ecology, Evolution, Behavior and Systematics
  • Molecular Biology
  • Genetics

Keywords

  • Crohn's disease
  • Genetic hitchhiking
  • IBD5
  • IRF1
  • Positive selection

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