Understanding inflammatory bowel disease via immunogenetics

The major inflammatory bowel diseases, Crohn's disease and ulcerative colitis, are both debilitating disorders of the gastrointestinal tract, characterized by a dysregulated immune response to unknown environmental triggers. Both disorders have an important and overlapping genetic component, and much progress has been made in the last 20 years at elucidating some of the specific factors contributing to disease pathogenesis. Here we review our growing understanding of the immunogenetics of inflammatory bowel disease, from the twin studies that first implicated a role for the genome in disease susceptibility to the latest genome-wide association studies that have identified hundreds of associated loci. We consider the insight this offers into the biological mechanisms of the inflammatory bowel diseases, such as autophagy, barrier defence and T-cell differentiation signalling. We reflect on these findings in the context of other immune-related disorders, both common and rare. These observations include links both obvious, such as to pediatric colitis, and more surprising, such as to leprosy. As a changing picture of the underlying genetic architecture emerges, we turn to future directions for the study of complex human diseases such as these, including the use of next generation sequencing technologies for the identification of rarer risk alleles, and potential approaches for narrowing down associated loci to casual variants. We consider the implications of this work for translation into clinical practice, for example via early therapeutic hypotheses arising from our improved understanding of the biology of inflammatory bowel disease. Finally, we present potential opportunities to better understand environmental risk factors, such as the human microbiota in the context of immunogenetics.