Background/Purpose: Growing numbers of studies show increased expression in Osteoarthritis (OA) of inflammatory cytokines, such as IL-1β and TNFα, in joint tissues and peripheral blood mononuclear (PBM) cells. The IL1 receptor antagonist (IL1RN) gene cluster region has been associated with susceptibility to knee OA, thereby further implicating inflammation in OA pathogenesis. In these studies, we examined the association of IL-1RN haplotype with the radiographic severity of symptomatic knee OA (SKOA).

Methods: Genomic DNA from SKOA patients from three cohorts (NYU I, NYU-II and OAI – clinical characteristics are described in Table 1) in this prospective analysis were used to genotype three single nucleotide polymorphisms (SNPs) [rs419598, rs315952 & rs9005) based on our prior publications. Genotyping was accomplished by PCR using validated SNP primers and probes (Applied Biosystems) along with detection using allelic discrimination computation. Genotypes were scored blinded to all patients’ data. Statistical analyses for associations of genotype with OA radiographic severity used Chi square and Fisher’s exact test, with linear regression, adjusting for age, gender and BMI. Non-fasting Heparin plasma IL-1Ra levels were determined using R&D system ELISA kit.

Results: Carriage of two copies of a haplotype consisting of IL1RN rs419598, IL1RN rs315952 and IL1RN rs9005 (TTG-2), had a population frequency of 18%, and was associated with a significantly increased risk of more severe radiographic SKOA (KL1-2 vs. KL3/4; OR > 3.07; 95% CI 1.67 -5.68; p=0.000), compared to SKOA patients with zero copies of TTG (TTG-0) in all three cohorts (Table 1). Furthermore, narrower baseline radiographic medial joint space width (mJSW) was similarly associated with TTG-2 haplotype (Table 2). TTG-2 patients exhibited decreased JSW compared to TTG-0 patients of comparable age (50-80 years), consistent with earlier onset of disease. Biologically, TTG-2 patients in two of the cohorts had reduced plasma concentrations of the “protective” IL1RN gene product, IL-1Ra, (297 to 338 pg/ml; p=0.14) relative to TTG-0 OA patients.

Conclusion: IL-1RN haplotype TTG (rs9005, rs419598 and rs315952) predicted high risk for greater severity of radiographic knee OA. Carriers of two copies of the TTG SNP also exhibited lower plasma IL-1Ra concentrations. These data are consistent with the hypothesis that a relative deficiency of IL-Ra typified by TTG-2 patients results in enhanced IL-1β action in joint tissues and more severe OA. These genetic markers may also be useful as tools for enriching OA clinical trials for disease progressors.   Table 1: Baseline Clinical Characteristics and Association of IL-1RN Haplotype (TTG) With Radiographic Severity In Symptomatic Knee OA Patients  

Table 2: Association of IL-1RN Haplotype (TTG) With Baseline Radiographic Medial Joint Space Width (mJSW) In Symptomatic Knee OA Patients