Evidence for genetic heterogeneity in IBD .1. The interleukin-2 receptor antagonist in the predisposition to suffer from ulcerative colitis

Objective: To investigate a polymorphism within intron 2 of the interleukin-1 receptor antagonist (IL-1ra) gene in a Dutch white population of patients with ulcerative colitis and Crohn's disease. Design: Genotype and allele frequencies of the polymorphic region in the IL-1ra gene were determined in 111 unrelated patients with ulcerative colitis, 92 patients with Crohn's disease, and 86 healthy controls. Methods: The polymorphic region of the IL-1ra gene was amplified by the polymerase chain reaction (PCR). The resultant products were analyzed by electrophoresis on 2% agarose gels and stained with ethidium bromide. Amplification of the second exon of HLA-DRB1, HLA-DQA1, and HLA-DQB1 genes was performed by PCR, and Dot-blot analysis with biotin-labelled sequence-specific oligonucleotide probes was used for HLA typing. The standard perinuclear antineutrophil cytoplasmic antibodies (pANCA) indirect immunofluorescence assay was performed according to the protocol described by the First International Workshop on ANCA. Results: The frequency of allele 2 of the IL-1ra gene in ulcerative colitis (27.0%) and Crohn's disease patients (25.5%) did not differ significantly from healthy controls (23.8%). However, the estimate of the relative risk for carriers of allele 2 for ulcerative colitis [odds ratio (OR): 1.35, 95%-confidence interval (CI) from 0.73 to 2.49] was in agreement with a previous British study. The exact test for homogeneity of odds ratios provided no evidence for heterogeneity between both populations (P=0.35) and therefore confirmed the genetic association between ulcerative colitis and the IL-1ra allele 2 in a different population. Our data confirm that, in ulcerative colitis, the presence of this allele is a genetic marker for severity of the disease. A significant association was demonstrated between the carriership of allele 2 of the IL-1ra gene and the trend over the three localizations in ulcerative colitis (P=0.023). The same approach for Crohn's disease when compared with healthy controls did not provide evidence for a similar association. The meta-analysis, based on the British data and our data, yielded: OR=1.06, 95%-Cl from 0.71 to 1.59, and P=0.767. No association between IL-1ra gene polymorphism, and pANCA and the HLA-DR2 allele was found in ulcerative colitis. Conclusion: The observations confirm that the allele 2 of the IL-1ra gene is a marker for genetic susceptibility and severity in ulcerative colitis and contributes to the definition of the immunogenetic heterogeneity of the disease.