The genetic basis of inflammatory bowel disease.

Twin studies and large-scale population studies have confirmed an increased sibling risk for both Crohn's disease (CD) and ulcerative colitis (UC). Unlike single gene disorders, CD and UC are thought to result from a complex interplay of multiple genes and environmental factors. The confirmation of CARD15/NOD2 as a CD susceptibility gene in the late 1990s caused much excitement in the field of complex diseases in general and since then, the rapid rate of progress in molecular genetics, with the advent of large-scale affordable genotyping techniques, has resulted in large collaborations and the identification of over 30 inflammatory bowel disease (IBD)-associated genes. In particular, the importance of the innate immune system has been reaffirmed with the identification of IRGM and ATG16L1 genes in the autophagy pathway as CD susceptibility genes. Disturbance in the adaptive immune system, in particular the IL-23/Th17 axis, has also shown to be of importance for IBD overall. In this era of genome-wide association studies it may be possible to, at last, identify the multiple genes involved in IBD and thus improve our understanding of the genotype-phenotype correlation and improve treatment.