Introduction AK155 (interleukin-26) is a member of the “IL-10 superfamily”. It maps to the IBD2 locus (chromosome 12q15). AK155 is expressed by Th1 and Th17 lymphocytes. The receptor is expressed on epithelial cells, including colon carcinoma and keratinocyte-derived cell lines.1 Activation of the receptor complex results in secretion of IL-10 and IL-18 with surface expression of CD54.1 AK155 mRNA expression is elevated in Crohn's colitis. This expression correlates with IL-8 and IL-22.2
Methods We sequenced AK155 from genomic DNA (gDNA) of patients with IBD to identify novel disease-associated single nucleotide polymorphisms (SNPs). These were then used for allele association studies. gDNA was obtained from peripheral blood mononuclear cells of 30 IBD patients (15 UC and 15 CD). Combinations of primers were used which amplified the open reading frame, introns 1, 2 and 4, 35 base pairs of the 5'UTR immediately proximal to the start codon, 115 bp of intron 3 and 124 bp of the 3'UTR beyond the stop codon. PCR products were sequenced using the ABI 377 automated sequencer.
Results Four novel SNPs were identified. Each SNP was found in a single individual from the 30 patients studied. The first coding SNP (C87T) in exon one was silent. The second coding SNP (C316T) in exon three lead to a substitution in the amino acid sequence of AK155 of an arginine residue to a cysteine. The other two SNPs were identified within introns two and three. The C316T SNP polymorphism was used for an allelic association study in IBD. gDNA samples were obtained from 130 IBD patients and 50 controls. The C316T SNP was identified in one of the 130 patients and no control. This patient was a 38-year-old woman with Crohn's colitis. C316T is in proximity to the intron two SNP but again the intronic SNP was identified in a single individual from the 180 samples.
Each SNP was checked on five separate occasions using different oligonucleotide primer combinations and DNA polymerase enzymes. It was identified with each of these sequencing primers and polymerase combinations.
Conclusion AK155 is both a positional and functional candidate gene for IBD. We were unable to identify a disease-associated polymorphism. There are limitations to the techniques used to perform our association studies. We are limited by the numbers of patients and genetically heterogeneous nature of our IBD population. We believe these SNPs should be studied in large numbers of multiply affected families.
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