Pathobiology of Crohn's Disease Risk Nod2 Mutation

Crohn's disease (CD) is an autoimmune disease in the gastrointestinal tract that affects 400,000 – 600,000 individuals in the US. A number of genes have known association with the disease, including Nod2, which is an intracellular protein that recognizes bacterial cell wall component, muramyl dipeptide, and activates the NF-kB pathway. Three Nod2 mutations, R702W, G908R, and 1007fs, increase the odds of developing CD in a dose dependent manner. Although the genetic link between Nod2 mutations and CD is well established, the mechanism by which these mutations contribute to the disease phenotype is not known. Based on the location of the CD risk mutations, it is expected that the mutations affect ligand binding and/or regulation of Nod2 activation. Elucidating the mechanism of Nod2 activation is important so that the pathobiology of the mutations can be correctly modeled. To understand Nod2 signaling at a molecular level, we modeled the mechanism of Nod2 activation using biochemical data collected on Nod2 as well as structural data on other members of the NB-ARC family, including Apaf-1 and CED-4, in both active and inactive conformations. The resulting three step activation mechanism for Nod2 was confirmed using FRET based assays. The study further led to a hypothesis regarding the molecular basis of R702W pathobiology. We present data from functional studies that suggest loss of ligand sensitivity in the R702W mutant, which can be reversed using a designed peptide inhibitor. The study points to a possible therapeutic strategy to treat a subclass of CD patients carrying the R702W mutation.