RIP2 polymorphisms in inflammatory bowel diseases

This is the authors' abstract. We don't add key points for this paper.

To the Editor: NOD2 (also known as CARD15 and NLRC2) is a Crohn's disease (CD) susceptibility gene.1 It codes for an intracellular sensor of peptidoglycan related molecules including muramyl dipeptide (MDP).2,3 After its activation by MDP, Nod2 binds directly to Rip2 (also known as Rick/Ripk2/Cardiak) through its caspase recruitment domains (CARD).4,5 The recruitment of Rip2 leads to the activation of the NF-κB and MAP kinase pathways resulting in the transcriptional activation of several innate immune genes including proinflammatory cytokines.6 As a result, Rip2 is seen as a key component of the Nod2 activating pathway. Considering that not all CD patients exhibit NOD2 mutations it can be postulated that genes playing a role in the Nod2 activating pathway could also be mutated in some CD cases. RIP2 is thus an obvious candidate gene for the susceptibility to CD. However, no study focusing on RIP2 polymorphisms in CD was available in the literature until today. We thus decided to specifically look for an association between Rip2 and inflammatory bowel disease (IBD). Our European consortium of gastroenterologists recruited 313 IBD families from Denmark, France, Ireland, Spain, and Sweden.1 The 313 IBD families with a total of 1288 subjects were divided in 170 CD families composed of healthy and CD members (266 CD patients), 53 ulcerative colitis (UC) families composed of healthy and UC members (91 UC patients) and 90 families containing both CD and UC cases (mixed families) and/or indeterminate colitis (151 IBD patients). The recruited families were previously genotyped for the three main NOD2 mutations: R702W, G908R, and L1007fs.1 The CD group was composed of 90 families where at least one NOD2 mutation was segregating and 80 families with no NOD2 mutations. The exons and intron–exon boundaries of RIP2 were resequenced in 12 unrelated CD patients but we failed to identify new mutations. Thus, the only frequent nonconservative genetic variation retained for genotyping was located in the sixth exon (rs2230801). The haplotype structure of the gene was determined according to the HapMap CEU database using and we chose seven single nucleotide polymorphisms (SNPs) (rs390993, rs13262484, rs39764, rs40377, rs419181, rs10097279, and rs40247) able to tag the five haplotypes with a frequency higher than 0.05. The transmission disequilibrium test (TDT) was performed for each individual SNP and for the discernable RIP2 haplotypes using GENEHUNTER. No significant association was observed with IBD, CD, or UC and any single SNP including the nonconservative variant rs2230801. When splitting the CD families in subgroups with or without NOD2 segregating mutations, we also failed to detect any distortion of transmission. Furthermore, the TDT was nonsignificant for all the tested haplotypes with any of the IBD phenotypes. We only observed a distortion of transmission for one haplotype in the subgroup of CD families without NOD2 mutation (T:14 U:5 P = 0.039). However, this association did not remain significant after Bonferroni correction. These results indicate that Rip2 does not seem to play a crucial role in the genetic predisposition to IBD.