Analysis of single‐nucleotide polymorphisms in Japanese rheumatoid arthritis patients shows additional susceptibility markers besides the classic shared epitope susceptibility sequences

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Abstract
Objective To examine the entire HLA region for loci (other than the DRB1 locus) associated with rheumatoid arthritis (RA) susceptibility, by typing HLA–DRB1 alleles and multiple single‐nucleotide polymorphisms (SNPs) in the Japanese population. Methods The HLA–DRB1 alleles and 88 SNPs distributed over the HLA gene complex were genotyped, for 828 patients with RA and 1,032 control subjects. The data were evaluated for linkage disequilibrium, and case–control associations were analyzed in 2 ways, in the presence or absence of the disease‐susceptibility DRB1 allele, to detect loci independent of the DRB1 allele. Results HLA–DRB1 alleles *0405, *0401, *0901, *0101, *1401, *1602, *0403, and *1405 were significantly associated with RA in the Japanese population. The smallest P value (P = 1.4 × 10−27) was observed in association with an intronic SNP of the NOTCH4 gene, which was due to strong linkage disequilibrium with the HLA–DRB1 allele. A strong association that was independent of HLA–DRB1 shared epitope alleles was observed in 2 SNPs: one in the intron of the MICA gene, the other in the intron of the HLA–DQB2 gene. Their association with RA, independent of HLA–DRB1 shared epitope alleles, was suggestive (P = 0.0024 [corrected P (Pcorr) = 0.068, and P = 0.00037 [Pcorr = 0.012], respectively). Conclusion These findings suggest that 1 or more other loci besides the HLA–DRB1 or other DRB1 (non–shared epitope, non‐*0901) alleles are involved in RA susceptibility/protection.