Background: Crohn’s disease (CD) and ulcerative colitis (UC) are two major forms of inflammatory bowel disease (IBD). Meta-analyses of genome-wide association studies (GWAS) have identified 163 susceptibility loci for IBD among European populations; however, there is limited information for IBD susceptibility in a Japanese population. Methods: We performed a GWAS using imputed genotypes of 743 IBD patients (372 with CD and 371 with UC) and 3321 controls. Using 100 tag single-nucleotide polymorphisms (SNPs) (P < 5 × 10−5), a replication study was conducted with an independent set of 1310 IBD patients (949 with CD and 361 with UC) and 4163 controls. In addition, 163 SNPs identified by a European IBD GWAS were genotyped, and genetic backgrounds were compared between the Japanese and European populations. Results: In the IBD GWAS, two East Asia-specific IBD susceptibility loci were identified in the Japanese population: ATG16L2–FCHSD2 and SLC25A15–ELF1–WBP4. Among 163 reported SNPs in European IBD patients, significant associations were confirmed in 18 (8 CD-specific, 4 UC-specific, and 6 IBD-shared). In Japanese CD patients, genes in the Th17–IL23 pathway showed stronger genetic effects, whereas the association of genes in the autophagy pathway was limited. The association of genes in the epithelial barrier and the Th17–IL23R pathways were similar in the Japanese and European UC populations. Conclusions: We confirmed two IBD susceptibility loci as common for CD and UC, and East Asian-specific. The genetic architecture in UC appeared to be similar between Europeans and East Asians, but may have some differences in CD.
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