Genotype-based phenotyping heralds a new taxonomy for inflammatory bowel disease.

Inflammatory bowel disease (IBD) has traditionally been categorized as either ulcerative colitis or Crohn disease on the basis of clinical, radiologic, and histologic criteria. Within these diseases, however, significant heterogeneity is observed, suggesting the existence of phenotypic subtypes, based on features such as location and behavior of disease. Evidence for a possible genetic basis of these subgroups first emerged in the 1990s from epidemiologic studies in multiply affected families. Recent advances in our understanding of the genetics of IBD, in particular the identification of NOD2/CARD15, have provided the opportunity to explore the genetic basis for this heterogeneity. This article reviews recent studies investigating the contribution of genetics to IBD phenotype. Although many of the genes remain unidentified, the emerging data suggests that IBD comprises a heterogeneous family of oligogenic inflammatory disorders in which the specific clinical manifestations of disease in any individual are determined by the interaction of genetic and environmental factors. These data have validated the approach of classifying patients into accurately defined clinical subgroups, and they raise the possibility that a genetic basis for the observed disease heterogeneity may account for the discrepant findings from earlier genetic studies. A future molecular classification will provide the framework to understanding the different biologic mechanisms that underlie the clinical subgroups of IBD and, by patient stratification, permit the unraveling of the complex interaction between the genetic and environmental causes of disease.