Insights into the Pathogenesis of Inflammatory Bowel Diseases: Genetics and Microbiota

Abstract

Insights into the Pathogenesis of Inflammatory Bowel Diseases: Genetics and Microbiota Abstract. An inadequate immune response against bacteria of the gastrointestinal tract is the basic mechanism mediating the pathophysiology of inflammatory bowel diseases (IBD). The risk of IBD is partially heritable and approximately 12 % of patients have a family history of IBD. Large genome-wide association studies (GWAS) were able to identify 240 genetic regions associated with IBD. Many of the implicated genes have a function in the immune system, are associated with primary immunodeficiencies or the defense against mycobacteria. Together these 240 genetic regions form an excellent framework for further investigations into the pathogenesis and therapy of IBD. However, GWAS so far were able to unravel only a fraction of the genetic IBD risk. New strategies like genome wide sequencing are currently used to identify additional (rare) genetic variants. In rare cases, IBD is also inherited as a monogenetic disease. Moreover, there likely is significant interaction between genes and environmental factors which can only be unraveled if both, genes and the environment are simultaneously considered. Interestingly, the information provided by genetic risk factors for IBD is unable to predict the clinical course of IBD. New GWAS therefore focus on IBD prognosis and first insights have already been made. The gastrointestinal tract harbors a huge number of microorganisms (microbiota). It remains an enormous challenge for the immune system to contain this bacterial load while enabling the host to benefit from the many essential contributions of the microbiota. In IBD, the microbiota is altered to a dysfunctional (dysbiotic) state showing reduced diversity and a higher amount of potential pathogenic Proteobacteriae, such as Escherichia coli. In IBD, the microbiota is also more dynamic in its composition over time compared to health. Further, IBD dysbiosis is more pronounced in Crohn's disease than in ulcerative colitis. In animal experiments, dysbiosis could be transferred by fecal microbiota transplantation from one mouse to another, triggering inflammation in the recipient. In contrast, a healthy microbiota can downregulate the immune response of the host, for instance by bacterial short chain fatty acids (SCFA) synthesis. In addition, some bacteria with close physical contact to the intestinal wall also have specific immunosuppressive properties. So far, the highly complex network of microbiota, genetics, immune system and environment is only partially understood. The microbiota is a potential therapeutic target which up to now can only be non-specifically influenced by antibiotics, probiotics, prebiotics or fecal microbiota transplantation. A better understanding of the microbiota will likely yield in the discovery of new therapeutic options in the future.