Clinical applications of advances in the genetics of IBD.

Our rapidly expanding understanding of the genetics of inflammatory bowel disease (IBD) has led to important clinical applications. It is becoming apparent that genes help determine the clinical phenotype, intestinal and extraintestinal complications, response to treatment, and drug toxicities in these disorders. For example, NOD2/CARD15 mutations are associated with ileal Crohn's disease, possibly with a fibrosing/obstructing phenotype, but do not influence responses to infliximab treatment. Similarly, certain human leukocyte antigen (HLA) haplotypes are associated with aggressive, extensive ulcerative colitis and strongly influence extraintestinal manifestations of IBD, including uveitis and various forms of arthritis. Expression of the glucocorticoid receptor b determines the clinical response to corticosteroids, whereas genetically regulated levels of enzymes metabolizing 6-mercaptopurine/azathioprine may determine clinical responses and toxicities to these important immunosuppressive agents. Once we have a more sophisticated understanding of the mechanisms of genetic defects in IBD, it may be feasible to restore physiologic function to prevent the onset of disease in susceptible individuals. However, because we do not have the ability to prevent disease at the present time, it is premature to screen offspring and first-degree relatives of IBD patients for the NOD2/CARD15 genotype. One can anticipate that it will become feasible to prospectively determine a patient's genotype and to individualize a drug regimen, leading to highly effective, safe treatments for IBD patients on a rational, rather than empiric, basis.