Resequencing of positional candidates identifies low frequency IL23R coding variants protecting against inflammatory bowel disease

Genome-wide association studies (GWAS) have identified dozens of risk loci for many complex disorders, including Crohn's disease(1,2). However, common disease-associated SNPs explain at most similar to 20% of the genetic variance for Crohn's disease. Several factors may account for this unexplained heritability(3-5), including rare risk variants not adequately tagged thus far in GWAS(6-8). That rare susceptibility variants indeed contribute to variation in multifactorial phenotypes has been demonstrated for colorectal cancer(9), plasma high-density lipoprotein cholesterol levels(10), blood pressure(11), type 1 diabetes(12), hypertriglyceridemia(13) and, in the case of Crohn's disease, for NOD2 (refs. 14,15). Here we describe the use of high-throughput resequencing of DNA pools to search for rare coding variants influencing susceptibility to Crohn's disease in 63 GWAS-identified positional candidate genes. We identify low frequency coding variants conferring protection against inflammatory bowel disease in IL23R, but we conclude that rare coding variants in positional candidates do not make a large contribution to inherited predisposition to Crohn's disease.