Y. Zeissig1, B. Petersen2, E. Bosse3, S. Milutinovic4, S. Billmann2, M. Kohl1, P. Rosenstiel2, J. Reed4, S. Schreiber3, A. Franke2, S. Zeissig3, 1University Medical Center Schleswig-Holstein Kiel, Department of Pediatrics, Kiel, Germany, 2Institute of Clinical Molecular Biology, Kiel, Germany, 3University Medical Center Schleswig-Holstein Kiel, Department of Internal Medicine I, Kiel, Germany, 4Sanford-Burnham Medical Research Institute, United States
The genetic contribution to inflammatory bowel disease (IBD) has only been partially unraveled and it has been suggested that rare variants that escape detection by genome-wide association studies contribute to the genetic risk in IBD.
Exome-sequencing and functional studies were performed in cases of familial and sporadic early-onset IBD to reveal rare high penetrance variants and novel candidate genes involved in disease etiology.
In a three-year-old patient with a one-year history of severe colonic, stricturing, and fistulising CD refractory to treatment with corticosteroids, azathioprine, and infliximab, exome-sequencing revealed a novel hemizygous nonsense mutation (E99X) in XIAP, the gene encoding for the X-linked inhibitor of apoptosis protein. XIAP was not detectable by flow cytometry or western blot in peripheral blood mononuclear cells (PBMCs) in the CD patient. Relative and absolute frequencies of PBMC subsets, T cell proliferation, apoptosis, and cytokine production upon stimulation and re-stimulation, and immunoglobulin levels were unaltered in the patient. However, while XIAP-deficient PBMCs and monocytes exhibited unaltered responses to toll-like receptor 2, 3, 4, 7, and 9 stimulation and unimpaired NLRP3-dependent IL-1beta secretion, NOD2-dependent IL-8 and IL-1beta release were not detectable across a range of muramyl dipeptide (MDP) concentrations. Furthermore, wildtype but not E99X XIAP associated with NOD2 in a RIP2-dependent manner as revealed by co-immunoprecipitation. Lentiviral XIAP reconstitution in primary monocyte-derived dendritic cells restored MDP-induced NOD2 signaling thus confirming a critical role of XIAP deficiency in the observed NOD2 defects.
Our studies reveal a novel XIAP mutation associated with early-onset IBD and suggest that XIAP deficiency may contribute to disease pathogenesis via impaired NOD2 signaling. In addition, XIAP deficiency is associated with X-linked lymphoproliferative syndrome, a primary immunodeficiency characterized by severe susceptibility to Epstein-Barr virus infection, and thus supports the notion that a subset of IBD cases may result from immunodeficiency, potentially in a NOD2-dependent manner.