Association of NOD2 leucine-rich repeat variants with susceptibility to Crohn's disease

Crohn's disease and ulcerative colitis, the two main types of chronic inflammatory bowel disease, are multifactorial conditions of unknown aetiology. A susceptibility locus for Crohn's disease has been mapped to chromosome 16. Here we have used a positional-cloning strategy, based on linkage analysis followed by linkage disequilibrium mapping, to identify three independent associations for Crohn's disease: a frameshift variant and two missense variants of NOD2, encoding a member of the Apaf-1/Ced-4 superfamily of apoptosis regulators that is expressed in monocytes. These NOD2 variants alter the structure of either the leucine-rich repeat domain of the protein or the adjacent region. NOD2 activates nuclear factor NF-kB; this activating function is regulated by the carboxy-terminal leucine-rich repeat domain, which has an inhibitory role and also acts as an intracellular receptor for components of microbial pathogens. These observations suggest that the NOD2 gene product confers susceptibility to Crohn's disease by altering the recognition of these components and/or by over-activating NF-kB in monocytes, thus documenting a molecular model for the pathogenic mechanism of Crohn's disease that can now be further investigated.

A frameshift mutation in NOD2 associated with susceptibility to Crohn's disease

Crohn's disease is a chronic inflammatory disorder of the gastrointestinal tract, which is thought to result from the effect of environmental factors in a genetically predisposed host. A gene location in the pericentromeric region of chromosome 16, IBD1, that contributes to susceptibility to Crohn's disease has been established through multiple linkage studies, but the specific gene(s) has not been identified. NOD2, a gene that encodes a protein with homology to plant disease resistance gene products is located in the peak region of linkage on chromosome 16 (ref. 7). Here we show, by using the transmission disequilibium test and case-control analysis, that a frameshift mutation caused by a cytosine insertion, 3020insC, which is expected to encode a truncated NOD2 protein, is associated with Crohn's disease. Wild-type NOD2 activates nuclear factor NF-kappaB, making it responsive to bacterial lipopolysaccharides; however, this induction was deficient in mutant NOD2. These results implicate NOD2 in susceptibility to Crohn's disease, and suggest a link between an innate immune response to bacterial components and development of disease.

Cryopyrin activates the inflammasome in response to toxins and ATP

A crucial part of the innate immune response is the assembly of the inflammasome, a cytosolic complex of proteins that activates caspase-1 to process the proinflammatory cytokines interleukin (IL)-1beta and IL-18. The adaptor protein ASC is essential for inflammasome function, binding directly to caspase-1 (refs 3, 4), but the triggers of this interaction are less clear. ASC also interacts with the adaptor cryopyrin (also known as NALP3 or CIAS1). Activating mutations in cryopyrin are associated with familial cold autoinflammatory syndrome, Muckle-Wells syndrome and neonatal onset multisystem inflammatory disease, diseases that are characterized by excessive production of IL-1beta. Here we show that cryopyrin-deficient macrophages cannot activate caspase-1 in response to Toll-like receptor agonists plus ATP, the latter activating the P2X7 receptor to decrease intracellular K+ levels. The release of IL-1beta in response to nigericin, a potassium ionophore, and maitotoxin, a potent marine toxin, was also found to be dependent on cryopyrin. In contrast to Asc-/- macrophages, cells deficient in the gene encoding cryopyrin (Cias1-/-) activated caspase-1 and secreted normal levels of IL-1beta and IL-18 when infected with Gram-negative Salmonella typhimurium or Francisella tularensis. Macrophages exposed to Gram-positive Staphylococcus aureus or Listeria monocytogenes, however, required both ASC and cryopyrin to activate caspase-1 and secrete IL-1beta. Therefore, cryopyrin is essential for inflammasome activation in response to signalling pathways triggered specifically by ATP, nigericin, maitotoxin, S. aureus or L. monocytogenes.

Nod2 is a general sensor of peptidoglycan through muramyl dipeptide (MDP) detection

Nod2 activates the NF-kappaB pathway following intracellular stimulation by bacterial products. Recently, mutations in Nod2 have been shown to be associated with Crohn's disease, suggesting a role for bacteria-host interactions in the etiology of this disorder. We show here that Nod2 is a general sensor of peptidoglycan through the recognition of muramyl dipeptide (MDP), the minimal bioactive peptidoglycan motif common to all bacteria. Moreover, the 3020insC frameshift mutation, the most frequent Nod2 variant associated with Crohn's disease patients, fully abrogates Nod2-dependent detection of peptidoglycan and MDP. Together, these results impact on the understanding of Crohn's disease development. Additionally, the characterization of Nod2 as the first pathogen-recognition molecule that detects MDP will help to unravel the well known biological activities of this immunomodulatory compound.

The immunological and genetic basis of inflammatory bowel disease

The inflammatory bowel diseases (IBDs), Crohn's disease and ulcerative colitis, are chronic inflammatory disorders of the gastrointestinal tract. Enormous progress has been made recently in understanding the pathogenesis of these diseases. Through the study of patients and mouse models, it has emerged that Crohn's disease is driven by the production of interleukin-12 (IL-12) and interferon-gamma (IFN-gamma), whereas ulcerative colitis is probably driven by the production of IL-13. A second area of progress is in the identification of specific genetic abnormalities that are responsible for disease. The most important finding is the identification of mutations in the gene that encodes NOD2 (nucleotide-binding oligomerization domain 2) protein in a subgroup of patients with Crohn's disease. Here, we discuss these recent findings and the implications for therapy.

Nod2-Dependent Regulation of Innate and Adaptive Immunity in the Intestinal Tract

The gene encoding the Nod2 protein is frequently mutated in Crohn's disease (CD) patients, although the physiological function of Nod2 in the intestine remains elusive. Here we show that protective immunity mediated by Nod2 recognition of bacterial muramyl dipeptide is abolished in Nod2-deficient mice. These animals are susceptible to bacterial infection via the oral route but not through intravenous or peritoneal delivery. Nod2 is required for the expression of a subgroup of intestinal anti-microbial peptides, known as cryptdins. The Nod2 protein is thus a critical regulator of bacterial immunity within the intestine, providing a possible mechanism for Nod2 mutations in CD.

Host recognition of bacterial muramyl dipeptide mediated through NOD2. Implications for Crohn's disease

NOD2, a protein associated with susceptibility to Crohn's disease, confers responsiveness to bacterial preparations of lipopolysaccharide and peptidoglycan, but the precise moiety recognized remains elusive. Biochemical and functional analyses identified muramyl dipeptide (MurNAc-L-Ala-D-isoGln) derived from peptidoglycan as the essential structure in bacteria recognized by NOD2. Replacement of L-Ala for D-Ala or D-isoGln for L-isoGln eliminated the ability of muramyl dipeptide to stimulate NOD2, indicating stereoselective recognition. Muramyl dipeptide was recognized by NOD2 but not by TLR2 or co-expression of TLR2 with TLR1 or TLR6. NOD2 mutants associated with susceptibility to Crohn's disease were deficient in their recognition of muramyl dipeptide. Notably, peripheral blood mononuclear cells from individuals homozygous for the major disease-associated L1007fsinsC NOD2 mutation responded to lipopolysaccharide but not to synthetic muramyl dipeptide. Thus, NOD2 mediates the host response to bacterial muropeptides derived from peptidoglycan, an activity that is important for protection against Crohn's disease. Because muramyl dipeptide is the essential structure of peptidoglycan required for adjuvant activity, these results also have implications for understanding adjuvant function and effective vaccine development.

Nod1 detects a unique muropeptide from gram-negative bacterial peptidoglycan

Although the role of Toll-like receptors in extracellular bacterial sensing has been investigated intensively, intracellular detection of bacteria through Nod molecules remains largely uncharacterized. Here, we show that human Nod1 specifically detects a unique diaminopimelate-containing N-acetylglucosamine-N-acetylmuramic acid (GlcNAc-MurNAc) tripeptide motif found in Gram-negative bacterial peptidoglycan, resulting in activation of the transcription factor NF-kappaB pathway. Moreover, we show that in epithelial cells (which represent the first line of defense against invasive pathogens), Nod1is indispensable for intracellular Gram-negative bacterial sensing.

Nod2, a Nod1/Apaf-1 family member that is restricted to monocytes and activates NF-kappaB

Apaf-1 and Nod1 are members of a protein family, each of which contains a caspase recruitment domain (CARD) linked to a nucleotide-binding domain, which regulate apoptosis and/or NF-kappaB activation. Nod2, a third member of the family, was identified. Nod2 is composed of two N-terminal CARDs, a nucleotide-binding domain, and multiple C-terminal leucine-rich repeats. Although Nod1 and Apaf-1 were broadly expressed in tissues, the expression of Nod2 was highly restricted to monocytes. Nod2 induced nuclear factor kappaB (NF-kappaB) activation, which required IKKgamma and was inhibited by dominant negative mutants of IkappaBalpha, IKKalpha, IKKbeta, and IKKgamma. Nod2 interacted with the serine-threonine kinase RICK via a homophilic CARD-CARD interaction. Furthermore, NF-kappaB activity induced by Nod2 correlated with its ability to interact with RICK and was specifically inhibited by a truncated mutant form of RICK containing its CARD. The identification of Nod2 defines a subfamily of Apaf-1-like proteins that function through RICK to activate a NF-kappaB signaling pathway.

An essential role for NOD1 in host recognition of bacterial peptidoglycan containing diaminopimelic acid

Nucleotide-binding oligomerization domain protein 1 (NOD1) belongs to a family that includes multiple members with NOD and leucine-rich repeats in vertebrates and plants. NOD1 has been suggested to have a role in innate immune responses, but the mechanism involved remains unknown. Here we report that NOD1 mediates the recognition of peptidoglycan derived primarily from Gram-negative bacteria. Biochemical and functional analyses using highly purified and synthetic compounds indicate that the core structure recognized by NOD1 is a dipeptide, gamma-D-glutamyl-meso-diaminopimelic acid (iE-DAP). Murine macrophages deficient in NOD1 did not secrete cytokines in response to synthetic iE-DAP and did not prime the lipopolysaccharide response. Thus, NOD1 mediates selective recognition of bacteria through detection of iE-DAP-containing peptidoglycan.

Reduction in diversity of the colonic mucosa associated bacterial microflora in patients with active inflammatory bowel disease

BACKGROUND AND AIMS

The intestinal bacterial microflora plays an important role in the aetiology of inflammatory bowel disease (IBD). As most of the colonic bacteria cannot be identified by culture techniques, genomic technology can be used for analysis of the composition of the microflora.

PATIENTS AND METHODS

The mucosa associated colonic microflora of 57 patients with active inflammatory bowel disease and 46 controls was investigated using 16S rDNA based single strand conformation polymorphism (SSCP) fingerprint, cloning experiments, and real time polymerase chain reaction (PCR).

RESULTS

Full length sequencing of 1019 clones from 16S rDNA libraries (n = 3) revealed an overall bacterial diversity of 83 non-redundant sequences-among them, only 49 known bacterial species. Molecular epidemiology of the composition of the colonic microflora was investigated by SSCP. Diversity of the microflora in Crohn's disease was reduced to 50% compared with controls (21.7 v 50.4; p<0.0001) and to 30% in ulcerative colitis (17.2 v 50.4; p<0.0001). The reduction in diversity in inflammatory bowel disease was due to loss of normal anaerobic bacteria such as Bacteroides species, Eubacterium species, and Lactobacillus species, as revealed by direct sequencing of variable bands and confirmed by real time PCR. Bacterial diversity in the Crohn's group showed no association with CARD15/NOD2 status.

CONCLUSIONS

Mucosal inflammation in inflammatory bowel disease is associated with loss of normal anaerobic bacteria. This effect is independent of NOD2/CARD15 status of patients.

NODs: intracellular proteins involved in inflammation and apoptosis

NOD (nucleotide-binding oligomerization domain) proteins are members of a family that includes the apoptosis regulator APAF1 (apoptotic protease activating factor 1), mammalian NOD-LRR (leucine-rich repeat) proteins and plant disease-resistance gene products. Several NOD proteins have been implicated in the induction of nuclear factor-kappaB (NF-kappaB) activity and in the activation of caspases. Two members of the NOD family, NOD1 and NOD2, mediate the recognition of specific bacterial components. Notably, genetic variation in the genes encoding the NOD proteins NOD2, cryopyrin and CIITA (MHC class II transactivator) in humans and Naip5 (neuronal apoptosis inhibitory protein 5) in mice is associated with inflammatory disease or increased susceptibility to bacterial infections. Mammalian NOD proteins seem to function as cytosolic sensors for the induction of apoptosis, as well as for innate recognition of microorganisms and regulation of inflammatory responses.

CARD15/NOD2 mutational analysis and genotype-phenotype correlation in 612 patients with inflammatory bowel disease

CARD15/NOD2 encodes a protein involved in bacterial recognition by monocytes. Mutations in CARD15 have recently been found in patients with Crohn disease (CD), a chronic inflammatory condition of the digestive tract. Here, we report the mutational analyses of CARD15 in 453 patients with CD, including 166 sporadic and 287 familial cases, 159 patients with ulcerative colitis (UC), and 103 healthy control subjects. Of 67 sequence variations identified, 9 had an allele frequency >5% in patients with CD. Six of them were considered to be polymorphisms, and three (R702W, G908R, and 1007fs) were confirmed to be independently associated with susceptibility to CD. Also considered as potential disease-causing mutations (DCMs) were 27 rare additional mutations. The three main variants (R702W, G908R, and 1007fs) represented 32%, 18%, and 31%, respectively, of the total CD mutations, whereas the total of the 27 rare mutations represented 19% of DCMs. Altogether, 93% of the mutations were located in the distal third of the gene. No mutations were found to be associated with UC. In contrast, 50% of patients with CD carried at least one DCM, including 17% who had a double mutation. This observation confirmed the gene-dosage effect in CD. The patients with double-dose mutations were characterized by a younger age at onset (16.9 years vs. 19.8 years; P=.01), a more frequent stricturing phenotype (53% vs. 28%; P=.00003; odds ratio 2.92), and a less frequent colonic involvement (43% vs. 62%; P=.003; odds ratio 0.44) than were seen in those patients who had no mutation. The severity of the disease and extraintestinal manifestations were not different for any of the CARD15 genotypes. The proportion of familial and sporadic cases and the proportion of patients with smoking habits were similar in the groups of patients with CD with or without mutation. These findings provide tools for a DNA-based test of susceptibility and for genetic counseling in inflammatory bowel disease.

NOD-LRR PROTEINS: Role in Host-Microbial Interactions and Inflammatory Disease

Nods are cytosolic proteins that contain a nucleotide-binding oligomerization domain (NOD). These proteins include key regulators of apoptosis and pathogen resistance in mammals and plants. A large number of Nods contain leucine-rich repeats (LRRs), hence referred to as NOD-LRR proteins. Genetic variation in several NOD-LRR proteins, including human Nod2, Cryopyrin, and CIITA, as well as mouse Naip5, is associated with inflammatory disease or increased susceptibility to microbial infections. Nod1, Nod2, Cryopyrin, and Ipaf have been implicated in protective immune responses against pathogens. Together with Toll-like receptors, Nod1 and Nod2 appear to play important roles in innate and acquired immunity as sensors of bacterial components. Specifically, Nod1 and Nod2 participate in the signaling events triggered by host recognition of specific motifs in bacterial peptidoglycan and, upon activation, induce the production of proinflammatory mediators. Naip5 is involved in host resistance to Legionella pneumophila through cell autonomous mechanisms, whereas CIITA plays a critical role in antigen presentation and development of antigen-specific T lymphocytes. Thus, NOD-LRR proteins appear to be involved in a diverse array of processes required for host immune reactions against pathogens.

RICK/Rip2/CARDIAK mediates signalling for receptors of the innate and adaptive immune systems

The immune system consists of two evolutionarily different but closely related responses, innate immunity and adaptive immunity. Each of these responses has characteristic receptors-Toll-like receptors (TLRs) for innate immunity and antigen-specific receptors for adaptive immunity. Here we show that the caspase recruitment domain (CARD)-containing serine/threonine kinase Rip2 (also known as RICK, CARDIAK, CCK and Ripk2) transduces signals from receptors of both immune responses. Rip2 was recruited to TLR2 signalling complexes after ligand stimulation. Moreover, cytokine production in Rip2-deficient cells was reduced on stimulation of TLRs with lipopolysaccharide, peptidoglycan and double-stranded RNA, but not with bacterial DNA, indicating that Rip2 is downstream of TLR2/3/4 but not TLR9. Rip2-deficient cells were also hyporesponsive to signalling through interleukin (IL)-1 and IL-18 receptors, and deficient for signalling through Nod proteins-molecules also implicated in the innate immune response. Furthermore, Rip2-deficient T cells showed severely reduced NF-kappaB activation, IL-2 production and proliferation on T-cell-receptor (TCR) engagement, and impaired differentiation to T-helper subtype 1 (TH1) cells, indicating that Rip2 is required for optimal TCR signalling and T-cell differentiation. Rip2 is therefore a signal transducer and integrator of signals for both the innate and adaptive immune systems.

CARD15 mutations in Blau syndrome

We have identified three missense mutations in the nucleotide-binding domain (NBD) of CARD15/NOD2 in four French and German families with Blau syndrome. Our findings indicate that, in addition to Crohn disease, CARD15 is involved in the susceptibility to a second granulomatous disorder.

NOD2 is a negative regulator of Toll-like receptor 2–mediated T helper type 1 responses

The mechanism by which mutations in CARD15, which encodes nucleotide-binding oligomerization domain 2 (NOD2), cause Crohn disease is poorly understood. Because signaling via mutated NOD2 proteins leads to defective activation of the transcription factor NF-kappa B, one proposal is that mutations cause deficient NF-kappa B-dependent T helper type 1 (T(H)1) responses and increased susceptibility to infection. However, this idea is inconsistent with the increased T(H)1 responses characteristic of Crohn disease. Here we used Card15(-/-) mice to show that intact NOD2 signaling inhibited Toll-like receptor 2-driven activation of NF-kappa B, particularly of the NF-kappa B subunit c-Rel. Moreover, NOD2 deficiency or the presence of a Crohn disease-like Card15 mutation increased Toll-like receptor 2-mediated activation of NF-kappa B-c-Rel, and T(H)1 responses were enhanced. Thus, CARD15 mutations may lead to disease by causing excessive T(H)1 responses.

Signalling pathways and molecular interactions of NOD1 and NOD2

The NOD (nucleotide-binding oligomerization domain) proteins NOD1 and NOD2 have important roles in innate immunity as sensors of microbial components derived from bacterial peptidoglycan. The importance of these molecules is underscored by the fact that mutations in the gene that encodes NOD2 occur in a subpopulation of patients with Crohn's disease, and NOD1 has also been shown to participate in host defence against infection with Helicobacter pylori. Here, we focus on the molecular interactions between these NOD proteins and other intracellular molecules to elucidate the mechanisms by which NOD1 and NOD2 contribute to the maintenance of mucosal homeostasis and the induction of mucosal inflammation.

NOD2 (CARD15) mutations in Crohn's disease are associated with diminished mucosal alpha-defensin expression

BACKGROUND

Mutations in NOD2, a putative intracellular receptor for bacterial peptidoglycans, are associated with a subset of Crohn's disease but the molecular mechanism linking this protein with the disease pathogenesis remains unclear. Human alpha defensins (HD-5 and HD-6) are antibiotic effector molecules predominantly expressed in Paneth cells of the ileum. Paneth cells also express NOD2. To address the hypothesis that the function of NOD2 may affect expression of Paneth cell defensins, we compared their expression levels with respect to NOD2 mutations in Crohn's disease.

METHODS

Forty five Crohn's disease patients (24 with NOD2 mutations, 21 with wild-type NOD2) and 12 controls were studied. Real time reverse transcription-polymerase chain reaction was performed with mucosal mRNA for HD-5, HD-6, lysozyme, secretory phospholipase A2 (sPLA2), tumour necrosis factor alpha, interleukin 8, and human hypoxanthine phosphoribosyltransferase (housekeeping gene). Immunohistochemistry with anti-HD-5 and histological Paneth cell staining were performed in 10 patients with NOD2 mutations or wild-type genotypes.

RESULTS

Ileal expression of HD-5 and HD-6, but not sPLA2 or lysozyme, were diminished in affected ileum, and the decrease was significantly more pronounced in patients with NOD2 mutations. In the colon, HD-5, HD-6, and sPLA2 were increased during inflammation in wild-type but not in NOD2 mutated patients. In both the colon and ileum, proinflammatory cytokines and lysozyme were unaffected by NOD2 status. Immunohistochemistry identified Paneth cells as the sole source of HD-5.

CONCLUSION

As alpha defensins are important in the mucosal antibacterial barrier, their diminished expression may explain, in part, the bacterial induced mucosal inflammation and ileal involvement of Crohn's disease, especially in the case of NOD2 mutations.

Nod2 mutation in Crohn's disease potentiates NF-kappaB activity and IL-1beta processing

Variants of NOD2, an intracellular sensor of bacteria-derived muramyl dipeptide (MDP), increase susceptibility to Crohn's disease (CD). These variants are thought to be defective in activation of nuclear factor kappaB (NF-kappaB) and antibacterial defenses, but CD clinical specimens display elevated NF-kappaB activity. To illuminate the pathophysiological function of NOD2, we introduced such a variant to the mouse Nod2 locus. Mutant mice exhibited elevated NF-kappaB activation in response to MDP and more efficient processing and secretion of the cytokine interleukin-1beta (IL-1beta). These effects are linked to increased susceptibility to bacterial-induced intestinal inflammation and identify NOD2 as a positive regulator of NF-kappaB activation and IL-1beta secretion.

Functional variants of OCTN cation transporter genes are associated with Crohn disease

Crohn disease is a chronic, inflammatory disease of the gastrointestinal tract. A locus of approximately 250 kb at 5q31 (IBD5) was previously associated with susceptibility to Crohn disease, as indicated by increased prevalence of a risk haplotype of 11 single-nucleotide polymorphisms among individuals with Crohn disease, but the pathogenic lesion in the region has not yet been identified. We report here that two variants in the organic cation transporter cluster at 5q31 (a missense substitution in SLC22A4 and a G-->C transversion in the SLC22A5 promoter) form a haplotype associated with susceptibility to Crohn disease. These variants alter transcription and transporter functions of the organic cation transporters and interact with variants in another gene associated with Crohn disease, CARD15, to increase risk of Crohn disease. These results suggest that SLC22A4, SLC22A5 and CARD15 act in a common pathogenic pathway to cause Crohn disease.

Peptidoglycan molecular requirements allowing detection by Nod1 and Nod2

Nod1 and Nod2 are mammalian proteins implicated in the intracellular detection of pathogen-associated molecular patterns. Recently, naturally occurring peptidoglycan (PG) fragments were identified as the microbial motifs sensed by Nod1 and Nod2. Whereas Nod2 detects GlcNAc-MurNAc dipeptide (GM-Di), Nod1 senses a unique diaminopimelate-containing GlcNAc-MurNAc tripeptide muropeptide (GM-TriDAP) found mostly in Gram-negative bacterial PGs. Because Nod1 and Nod2 detect similar yet distinct muropeptides, we further analyzed the molecular sensing specificity of Nod1 and Nod2 toward PG fragments. Using a wide array of natural or modified muramyl peptides, we show here that Nod1 and Nod2 have evolved divergent strategies to achieve PG sensing. By defining the PG structural requirements for Nod1 and Nod2 sensing, this study reveals how PG processing and modifications, either by host or bacterial enzymes, may affect innate immune responses.

The contribution of NOD2 gene mutations to the risk and site of disease in inflammatory bowel disease

BACKGROUND & AIMS

Mutations in the NOD2 gene are strongly associated with susceptibility to Crohn's disease (CD). We analyzed a large cohort of European patients with inflammatory bowel disease to determine which mutations confer susceptibility, the degree of risk conferred, their prevalence in familial and sporadic forms of the disease, and whether they are associated with site of disease.

METHODS

Individuals were genotyped for 4 NOD2 mutations: P268S, R702W, G908R, and 3020insC. Allelic transmission distortion to 531 CD- and 337 ulcerative colitis-affected offspring was assessed by the transmission disequilibrium test. Association was also tested in an independent cohort of 995 patients with inflammatory bowel disease and 290 controls. Cases were stratified by disease site and compared across NOD2 genotypes.

RESULTS

R702W, G908R, and 3020insC were strongly associated with CD but not with ulcerative colitis. Linkage disequilibrium was observed between P268S and the other mutations, forming 3 independent disease haplotypes. Genotype relative risks were 3.0 for mutation heterozygotes and 23.4 for homozygotes or compound heterozygotes. The frequency of NOD2 mutations was higher in cases from families affected only with CD and was significantly increased in ileal-specific disease cases compared with colon-specific disease (26.9% vs. 12.7%, P = 0.0004).

CONCLUSIONS

The R702W, G908R, and 3020insC mutations are strong independent risk factors for CD and are associated particularly with ileal disease.

Deficient host-bacteria interactions in inflammatory bowel disease? The toll-like receptor (TLR)-4 Asp299gly polymorphism is associated with Crohn's disease and ulcerative colitis

BACKGROUND AND AIMS

Elicitation of an innate immune response to bacterial products is mediated through pattern recognition receptors (PRRs) such as the toll-like receptors (TLRs) and the NODs. The recently characterised Asp299Gly polymorphism in the lipopolysaccharide (LPS) receptor TLR4 is associated with impaired LPS signalling and increased susceptibility to Gram negative infections. We sought to determine whether this polymorphism was associated with Crohn's disease (CD) and/or ulcerative colitis (UC).

METHODS

Allele frequencies of the TLR4 Asp299Gly polymorphism and the three NOD2/CARD15 polymorphisms (Arg702Trp, Gly908Arg, and Leu1007fsinsC) were assessed in two independent cohorts of CD patients (cohort 1, n = 334; cohort 2, n = 114), in 163 UC patients, and in 140 controls. A transmission disequilibrium test (TDT) was then performed on 318 inflammatory bowel disease (IBD) trios.

RESULTS

The allele frequency of the TLR4 Asp299Gly polymorphism was significantly higher in CD (cohort 1: 11% v 5%, odds ratio (OR) 2.31 (95% confidence interval (CI) 1.28-4.17), p = 0.004; and cohort 2: 12% v 5%, OR 2.45 (95% CI 1.24-4.81), p = 0.007) and UC patients (10% v 5%, OR 2.05 (95% CI 1.07-3.93), p = 0.027) compared with the control population. A TDT on 318 IBD trios demonstrated preferential transmission of the TLR4 Asp299Gly polymorphism from heterozygous parents to affected children (T/U: 68/34, p = 0.01). Carrying polymorphisms in both TLR4 and NOD2 was associated with a genotype relative risk (RR) of 4.7 compared with a RR of 2.6 and 2.5 for TLR4 and NOD2 variants separately.

CONCLUSION

We have reported on a novel association of the TLR4 Asp299Gly polymorphism with both CD and UC. This finding further supports the genetic influence of PRRs in triggering IBD.

CARD15/NOD2 functions as an antibacterial factor in human intestinal epithelial cells

BACKGROUND & AIMS

Mutations in the CARD15/NOD2 gene, a putative intracellular pattern recognition receptor, have been linked to the risk for Crohn's disease. Because intestinal epithelial cells play a role as the barrier to luminal microorganisms, we investigated the expression and function of CARD15/NOD2 in intestinal epithelial cells.

METHODS

Expression of CARD15/NOD2 messenger RNA (mRNA) in intestinal epithelial cell lines and primary intestinal epithelial cells was assessed by reverse-transcription polymerase chain reaction (RT-PCR). Regulation of expression of CARD15/NOD2 by cytokines was determined by Northern blot using the SW480 cell line. Active CARD15/NOD2 protein in SW480 cells was assessed by the combination of immunoprecipitation and immunoblotting using anti-CARD15/NOD2 antisera. To identify the functional role of CARD15/NOD2 in intestinal epithelial cells, gentamicin protection assays of Salmonella typhimurium were performed using Caco2 cells stably transfected with either wild-type CARD15/NOD2 or the 3020insC mutant associated with Crohn's disease.

RESULTS

CARD15/NOD2 mRNA was expressed in both intestinal epithelial cell lines and primary intestinal epithelial cells. CARD15/NOD2 mRNA and protein were up-regulated by tumor necrosis factor alpha (TNFalpha) in SW480 cells. The number of viable internalized S. typhimurium in Caco2 cells stably transfected with CARD15/NOD2 expression plasmid was lower than untransfected Caco2 cells or MOCK transfectant. In contrast, expression of a variant associated with Crohn's disease was unable to constrain bacterial survival.

CONCLUSIONS

CARD15/NOD2 is expressed in intestinal epithelial cells and may serve as a key component of innate mucosal responses to luminal bacteria as an antibacterial factor. Failure in this activity may contribute to the development of Crohn's disease.