Polymorphism of the interleukin‐1 receptor antagonist gene: A factor in susceptibility to rheumatoid arthritis in a Spanish population

Objective To assess whether an interleukin‐1 receptor antagonist gene (IL1RN) polymorphism is associated with disease susceptibility and/or severity in a Spanish population of patients with rheumatoid arthritis (RA). Methods An 86‐bp variable‐number tandem repeat polymorphism within IL1RN intron 2 was analyzed by polymerase chain reaction in genomic DNA obtained from 247 unrelated patients with RA (group A) and 287 healthy control subjects. The polymorphism analysis was repeated in a second group of 194 patients with RA (group B). Clinical information from patients in group A was used to compare activity and severity data in patients stratified according to the different alleles or genotypes. Odds ratios (ORs) with 95% confidence intervals (95% CIs) were used to determine the strength of the association of the different alleles or genotypes with RA activity or severity. Results In the control group, the allelic frequencies were 76% for IL1RN*1 (4 repeats), 21% for IL1RN*2 (2 repeats), 3% for IL1RN*3 (5 repeats), and 0.3% for IL1RN*4 (3 repeats). In group A patients with RA, both the frequency (OR 1.47, 95% CI 1.1–1.96, P = 0.007) and carriage rate (OR 1.6, 95% CI 1.1–2.2, P = 0.01) of allele IL1RN*2 were significantly increased. The increased frequency of IL1RN*2 was confirmed in group B patients with RA (OR 1.44, 95% CI 1.1–1.97, P = 0.01). In patients with RA, homozygosity for IL1RN*2 was associated with an increased number of affected articular areas during the first year of followup but not with other parameters of disease activity or severity. Conclusion Our results suggest that IL1RN has a role in determining susceptibility to RA in the Spanish population.