Lay Summary

Proposal No.  IBD-0126R2
Principal Investigator:  Török, Helga-Paula, M.D. (replacement PI); Folwaczny, Christian, M.D. (original PI)
Applicant Organization:  Klinikum der Universität München (Germany)
Project Title:  NOD2/CARD15 mutations and apoptosis in Crohn's disease
Period of Award:  July 1, 2005 – March 31, 2007

Recently, variations in a novel gene (NOD2/CARD15), which encodes for an intracellular receptor recognizing specific bacterial products (muramyl dipeptide, MDP), were associated with Crohn’s disease (CD).  The NOD2 protein belongs to a family of receptors of the innate immune system, which represents the first line of defense in mammalians.  Alterations of the NOD2 gene in monocytes from patients with CD lead to reduced synthesis of peptides (cytokines) promoting inflammation.  This observation hardly fits in the current concepts about a disease characterized by an excessive production of cytokines resulting in a chronic relapsing inflammation.  Thus, albeit an unequivocal genetic association between CD and the NOD2 gene exists, the functional link between the genetic background of these patients and the development of the disease remains to be determined.

The programmed cell death (apoptosis) of various cell types within the immune system is of paramount importance for the understanding and treatment of immunological-mediated disorders, such as CD.  Besides elimination of pathogen-infected cells by programmed cell death, apoptosis is important for initiation and critical for the resolution of inflammation and maintenance of host homeostasis.  Mounting evidence is compatible with a disturbed apoptosis of T-lymphocytes in CD.  The structure of NOD2 displaying two CARDs (caspase activation and recruitment domains), and the enhanced cell death in genetically engineered NOD2-overexpressing cells implicates this protein in apoptosis suggesting a functional link between NOD2 mutations and CD.

To provide some insights into this link, induction of apoptosis in different cell types, which are part of the immune system, will be assessed in whole blood preparations and compared between patients who display altered and normal NOD2.  It is well known that apoptosis is closely regulated by intracellular proteases (caspases) structured in a hierarchic manner determining different pathways.  Thus, the next step will be to elucidate which caspases and pathways are involved in NOD2 dependent apoptosis.  Of particular interest are T-lymphocytes for which a resistance towards apoptosis has been described as a major pathologic characteristic in Crohn’s disease.  In order to prove whether the situation in the peripheral blood is transferable to the intestinal mucosa, the final experimental step will be the assessment of apoptosis in immunocompetent cells of the intestinal wall using biopsies obtained from Crohn's disease patients during routine endoscopic examination.

We hypothesize that impaired apoptosis of immunocompetent cells caused by mutations in NOD2 could lead to the inappropriate initiation and resolution of inflammatory processes in response to bacterial stimulant resulting in perturbations of host homeostasis and the state of chronic inflammation which is the hallmark of Crohn's disease.  This could explain the apparent contradiction between decreased in vitro release of pro-inflammatory cytokines from monocytes of CD patients and mutated NOD2, and consistently described increased concentrations of pro-inflammatory cytokines in these patients.