Objective: To investigate whether the interleukin-1 (IL-1) ligand gene cluster at 2q13 encodes for genetic susceptibility to primary osteoarthritis (OA).
Methods: Seven single-nucleotide polymorphisms (SNPs) and a variable-number tandem repeat (VNTR) polymorphism from within the IL-1 ligand genes IL1A, IL1B, and IL1RN were genotyped in a cohort of 557 OA cases and 557 age-matched controls.
Results: None of the variants demonstrated association in the unstratified data set. However, when cases were stratified according to sex and site of disease (hip or knee), 4 SNPs showed marginal evidence for association (P < 0.1) in knee cases (n = 136) and male knee cases (n = 58). For 2 of these SNPs, evidence for association was enhanced when probands from 60 knee-only affected sibling pair families were genotyped and combined with the original knee cases (P < or = 0.05). Further analysis revealed that the associated alleles at 2 of these SNPs were markers for the same haplotype, the frequency of which was significantly elevated when knee cases and knee probands were combined (P = 0.01, odds ratio [OR] 1.4) and when male knee cases and male knee probands were combined (P = 0.009, OR 1.7). Furthermore, linkage analysis of 2q revealed suggestive evidence for linkage to the IL-1 gene clusters in affected sibling pairs concordant for knee OA but no evidence for linkage in affected sibling pairs concordant for hip OA.
Conclusion: The IL-1 ligand cluster encodes for susceptibility to knee OA but not to hip OA, highlighting the genetic heterogeneity of this common, complex disease.