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Crohn disease (CD) is a chronic inflammatory bowel disease (IBD) that may affect any part of the gastrointestinal (GI) tract from the mouth to the anus. CD is generally characterized by transmural skip lesions as well as periods of exacerbations and quiescence.
- ~20–25% of patients with CD are diagnosed in childhood or adolescence.
- Family history of CD is present in 30% of patients <30 years old.
- In adulthood, male = female; in childhood, male > female (1.6:1)
- Higher prevalence in the developed world; increasingly more common in developing countries
- Although the prevalence is highest in Ashkenazi Jews, CD may occur in individuals of all racial and ethnic backgrounds.
- 1st-degree relatives have a 5–25% higher risk of developing CD.
- Children of one parent with CD have a 7–16% risk of developing either CD or ulcerative colitis (UC); if both parents affected, risk of child developing the disease is 33%.
- Sibling relative risk for developing CD is 13–26%.
- CD concordance in monozygotic twins is 30% and in dizygotic twins, 4%.
- CD is complex genetic disease:
- >300 gene loci have been associated with CD.
- >30 known amino acid polymorphisms
- Most associated susceptibility genes are involved in host defense.
- NOD2/CARD15: first gene associated with increased CD susceptibility
- A NOD2 risk allele confers a 2- to 3-fold relative risk of developing CD; risk increases 17-fold if two alleles present.
- Several causative gene defects have been identified in very early-onset IBD (children diagnosed <6 years).
- 8–15% of the healthy population possess at least one of these mutations, and 1% of healthy individuals are homozygous or compound heterozygous; suggests phenotypic expression of disease is subject to polygenic factors, variable penetrance, and other environmental mediators
- Interaction and combination of environmental and genetic factors, intestinal microbiota, and likely one or more undetermined triggers (such as bacterial metabolic products) lead to a dysregulated immune response and chronic intestinal inflammation.
- CD pathogenesis is now attributed to dysfunction of both innate and adaptive immunity.
- Innate immunity: Defects have been identified in epithelial barrier, microbial sensing, and autophagy in CD, for example, patients with the CARD15/NOD2 mutation have dysregulated response to bacterial products.
- Adaptive immunity: abnormal activation of T-helper-1 and Th17 lymphocytes leads to overproduction of inflammatory cytokines such as IL-2, IFNγ, IL-6, TNF-α, and IL-17 which cause invasive intestinal inflammation in CD.
- IL-23 is a significant cytokine in CD. Polymorphisms in the IL-23R gene have been associated with aberrant responses of both the innate and adaptive immune systems.