Genetic heterogeneity within inflammatory bowel disease
Results of recent clinical, epidemiological and molecular genetic studies all support the model of Crohn's disease (CD) and ulcerative colitis (UC) as related polygenic disorders, sharing some but not all susceptibility loci. Evidence is continuing to emerge that the variability of clinical presentation of inflammatory bowel disease (IBD) reflects a large degree of genetic heterogeneity. Thus, the clinical presentation of disease may reflect not only the effect of different susceptibility loci, but also of allelic variation in susceptibility genes. Genome-wide scanning has allowed dissection of the relationship between CD and UC. The linkage between susceptibility to CD and a 40 centiMorgan region on chromosome 16 has been replicated in both European and North American populations. In addition, data from Oxford have demonstrated linkage between overall susceptibility to IBD and regions on chromosomes 12 (lod score 5.69), 7 (lod score 3.19), and 3 (lod score 2.69); regions on chromosomes 2 and 6 (human leucocyte antigen [HLA]) were linked only in UC. Candidate gene studies involving detailed analysis of relationship between disease behaviour and genotype point to further genetic heterogeneity. The data are most compelling in UC, where HLA Class II genes may predict not only susceptibility to disease, but also severity, extent, need for surgery and the presence of extraintestinal manifestations. Cytokine gene polymorphisms may also determine disease behaviour. Many studies are now in progress throughout the world and may provide insight into the pathogenesis of the inflammatory bowel disorders.