Crohn's disease-associated NOD2 Mutants

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Nucleotide-binding oligomerization domain 2 (NOD2) is a monocyte-restricted member of a protein family critically involved in the activation of NF-KB in response to certain intracellular microbial infection. Dysfunctional mutations in the NOD2 gene underline the occurrence of inflammatory bower disease (IBD) in a substantial group of patients with Crohn's disease (CD). CD is characterized by an exaggerated T helper I (Thl)-type immune response. There are three most common mutations in the NOD2 gene associated with CD: an insertion of a C at nucleotide position 3020, designated 3020insC, which results in a frame shift mutation and the deletion of the terminal leucine-rich repeats (LRR) of the protein, and two amino acid substitution mutants also in the LRR region: R702W and G908R. Recent human clinical studies revealed defective IL-IO production in the mononuclear cells of CD patients homozygous for 3020insC. IL-lO is a critical immunoregulatory cytokine produced by antigen-presenting cells and activated T and B lymphocytes. The chronically impaired IL-lO production in these CD patients may contribute to persistent inflammation in the intestinal mucosa. Our own data indicate that, in contrast to the prevalent view, 3020insC is not simply a loss-of-function mutant. Instead, it can act as an inhibitor of IL-lO production. Furthermore, we have detected direct interactions between 3020insC with p38 mitogen-activated protein kinase (MAPK) and heterogeneous nuclear ribonucleoprotein Al (hnRNP AI), which we identified as a novel and constitutive transcription factor for IL-lO. 3020insC targets the DNA-binding activity of hnRNP Al by inhibiting its phosphorylation and nuclear translocation. Most importantly, we have confirmed that the phosphorylation of hnRNP Al and DNA-binding activity are indeed impaired in 3020insC-CD patients. In addition, we have identified for the first time a novel activity of the R702W and G908R mutants: they can stimulate and enhance IL-12/IL-23 gene expression. We hypothesize that the acquired property of 3020insC as an IL-lO inhibitor may result in chronically impaired IL-IO levels, whereas the R702W and G908R mutations may make individuals more prone to produce elevated levels of IL-12 and IL-23 in the myeloid compartment. In all, these three mutations cause either impairment of regulatory mechanisms or inappropriate proinflammatory cytokine production, which, combined with other contributory factors, contribute to homeostatic imbalance and persistent inflammation in the intestinal mucosa over time leading to the development and pathogenesis of CD. We propose to investigate the mechanisms whereby R702W and G908R regulate IL-12/IL-23 gene expression.