Nod2 in Normal and Abnormal Intestinal Immune Function

Abstract

s a m h 3 s N I e h n m h f r a o i he search for genetic associations in Crohn’s disease has led to several fruitful discoveries that hold the otential to identify pathogenetic mechanisms and tarets for future therapy. The first gene to be identified for complex genetic disorder using genome wide scanning as the Nod2 gene. Simultaneous with the laborious ork of fine mapping chromosome 16 performed by ugot et al, the Nunez laboratory had identified the od2 gene in a search for genes containing caspase ctivation and recruitment domains (CARD). Both roups reported the now well-known association between olymorphisms in the Nod2 gene and susceptibility to rohn’s disease. These results were confirmed by many nvestigators in other ethnic groups with the notable xception of Asian populations. A subsequent wave of publications described the pheotype of Crohn’s disease patients carrying mutations in od2. Patients with Nod2 mutations are more likely to ave ileal disease with fibrostenotic complications. In dult populations, Nod2 mutations are not associated ith more frequent need for surgery or with rapid proression to complicated small bowel disease. By contrast, ugathasan et al have demonstrated that in a populaion-based, prospectively ascertained cohort of children ith Crohn’s disease that carriage of Nod2 mutations, in articular the 3020insC mutation resulted in an inreased need for surgery and earlier surgery. As a single genetic or stand-alone diagnostic test, utations in Nod2 are not likely to provide information hat will permit clinical decisions. More recently, the iminovitch group have found an interaction between utations in Nod2 and mutations in the Solute Carrier amily 22A4/22A5 (SLC22A/22A5) gene cluster. Mutaions in these organic cation transporter genes decrease xpression or function of carnitine transport in intestinal pithelial cells especially in response to heat shock or ellular stress. Other phenotypic associations such as the elationship of SLC22A/22A5 and perforating or fibrotenotic disease or need for surgery were not examined. hese data suggest interaction between Nod2 and the rganic cation transporter genes. One model to reconcile he relationship between SLC22A/22A5 and Nod2 is a ecrease in epithelial integrity caused by the former and n inability to mount an inflammatory response to inracellular bacteria caused by the latter (discussed later). e expect that a panel of genetic tests and biological ssays that address Nod2 function may soon provide rognostic and diagnostic information. The discovery of Nod2 immediately sparked tremenous enthusiasm given its homology to other immunoogically active molecules such as the pyrins (familial editerranean fever) and its role as a pattern recognition eceptor for pathogens. The true biologic role of Nod2 in ormal human physiology and in Crohn’s disease pathoenesis has proved elusive, however. Nod2 is normally xpressed in peripheral blood monocytes and Paneth ells. Cytokine stimulation of intestinal epithelial cells in ulture can also lead to expression of Nod2, and some ave described expression of Nod2 in the colon of paients with active Crohn’s disease. The ligand for Nod2 is muramyl dipeptide (MDP), a tructure contained within peptidoglycan and generated fter its lysosomal digestion. Using peripheral blood ononuclear cells (PBMC) from individuals who are omozygous for the truncation mutation of Nod2, 020insC (L1007fsinsC), the Nunez laboratory demontrated that homozygous individuals had decreased FB activation (by EMSA) and decreased expression of L-1 in response to MDP but not LPS. Importantly, ven unaffected individuals, ie, without Crohn’s disease, ad this biochemical defect suggesting that this alone is ot sufficient to cause Crohn’s disease. The more comon genotype of Crohn’s disease patients, however, is eterozygosity for Nod2 mutations. In this assay, PBMC rom heterozygotes behaved similarly to wild-type with espect to NFB activation and IL-1 expression. Using transient expression assay, Chamaillard et al have demnstrated that Nod2 variants can be categorized accordng to their basal ability to activate NFB and their