Interaction between RANKL and HLA-DRBI genotypes may contribute to younger age at onset of seropositive rheumatoid arthritis in an inception cohort

Objective. To determine whether the RANKL and HLA-DRB1 "shared epitope" (SE) genotypes contribute to the development of rheumatoid arthritis (RA). Methods. We studied 237 patients with early RA (within 15 months of symptom onset) who were seropositive for rheumatoid factor. HIA-DRB1 genotyping was performed using the polymerase chain reaction (PCR)-based oligonucleotide probe assay. RANKL polymorphisms were analyzed using PCR pyrosequencing for SNP1 and fluorescence-based PCR for the presence or absence of the TAAA insertion. Results. The presence of SE-containing DRB1*04 alleles was associated with an earlier age at RA onset (mean +/- SD 47 +/- 12.7 years versus 53 +/- 12.5 years in SE- patients; P = 0.0004). The 2 novel RANKL polymorphisms were in strong linkage disequilibrium (P < 0.0001) and were associated with earlier ages at disease onset (e.g., for the CC versus CT/TT genotypes, 44 +/- 13.5 years versus 51 +/- 12.7 years; P = 0.0080). The mean age at disease onset in SE+ patients with the RANKL-CC genotype (35 +/- 7.2 years) was a mean of 18 years younger than in SE- patients with RANKL-CT/TT (53 12.5 years; P < 0.0001) and was 17 years younger than in SE- patients with RANKL-CC (52 +/- 13.2 years; P = 0.0005). The proportion of patients with both the SE and RANKL risk alleles was highest (23%) in those who developed RA during their third decade of life (ages 20-30 years), with a declining trend among those who developed RA during their fourth (16%), fifth (5%), and sixth or later (0%) decades. Interestingly, 92% of the patients diagnosed as having RA between ages 20 and 30 years carried at least 1 of the RA-associated DRB1*04 alleles, suggesting a strong influence of the SE in the early onset of RA. The majority of patients who developed RA symptoms in their third to fifth decades (74 of 119 [62%]) carried at least 1 copy of the DRB1*04 alleles; in contrast, fewer than half of the patients who developed RA in their sixth decade or later (50 of 118 [42%]) had DRB1*04 alleles. RANKL genotypes were not associated with erosive disease at baseline or with the yearly progression rate of radiographic joint damage. Conclusion. This study provides the first evidence that novel RANKL polymorphisms were associated with an earlier age at RA onset in SE+, but not SE-, patients and that an interaction between SE-containing HLA-DRB1 and RANKL polymorphisms increased the disease penetrance, resulting in a mean age at RA onset that was 18-20 years younger. Our results also suggested genetic differences between patients with early-onset and those with late-onset RA.