Main genetic factors associated with inflammatory bowel diseases and their consequences on intestinal permeability: involvement in gut inflammation.

Crohn's disease (CD) and ulcerative colitis (UC), the two main subtypes of inflammatory bowel diseases (IBD), are chronic relapsing inflammatory disorders of the gastrointestinal tract. IBD are multifactorial diseases with a complex etiology, involving an intricate interaction between environmental and genetic factors. Since the discovery of NOD2 gene in 2001, genome-wide association studies have reported more than 200 IBD susceptibility loci. The strongest associations highlighted five main pathways as altered in IBD: bacterial sensing (NOD2), autophagy (ATG16L1, IRGM…), endoplasmic reticulum stress (XBP1, ARG2…), Th-17 immune pathway (IL23-receptor), and the vitamin D receptors (VDR). The pathophysiology of IBD results from an abnormal immune response toward an altered gut microbiota. Although the primum movens remains unknown, an increased intestinal permeability is clearly involved in the genesis of this abnormal crosstalk, leading to whole tissue inflammation. Thus, an excessive intestinal permeability, or "leaky gut", has been described to precede the development of CD. Moreover, in IBD, intestinal permeability is described to be a sensitive prognostic indicator of relapse in patients with quiescent IBD. Thus, the aim of this review is to highlight the molecular and cellular mechanisms by which the main pathways associated with IBD could contribute to alter the intestinal permeability to favour and/or exacerbate chronic inflammation, leading to debilitating diseases.