Genotype-phenotype relationship in inflammatory bowel disease

Crohn's disease (CD) and ulcerative colitis (UC) are chronic diseases of unknown etiology. Much effort has been made in the last years to clarify the pathogenesis of inflammatory bowel disease (IBD). Data are not conclusive at the moment, but the most important known risk factor for developing IBD is a positive familial history. Genetic analyses have shown a linkage between loci on several chromosomes and IBD (IBD1 gene on chromosome 16 for CD and on chromosome 12 for UC). The association of genotype to specific phenotypes of disease could be hypothesized by the concordance of clinical characteristics in familial IBD, by the association of specific HLA haplotypes to clinically different groups of patients, and by different responses to treatment related to different polymorphisms of other chromosome 6 genes. The clinical heterogeneity of IBD has led to classifications of patients with Crohn's disease based upon clinical features (e.g. Rome and Vienna classifications) that allow the identification of subgroups of patients with similar clinical behavior. The possible drawbacks of these classifications are the lack of validation of intra-interobserver concordance, the absence of prospective evaluations, and stratification into too many subgroups. Furthermore, in our experience, clinical presentation (surgical or medical) seems to have a good correlation with prognosis, is easy identifiable, and can be applied at the time of diagnosis. In UC, extension of disease and clinical behavior correlate with prognosis. For these reasons, studies correlating genotype to phenotype should be performed to improve our knowledge of the diseases and possibly to stratify patients into different subgroups for more personalized treatments, in clinical trials and for research purposes.