Identification of novel genetic determinants in the high prevalence early-onset inflammatory bowel disease population in Scotland
Limbergen, Johan Emiel van
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Background & aims: The inflammatory bowel diseases (IBD), Crohn‟s disease (CD) and ulcerative colitis (UC), are common causes of chronic gastrointestinal morbidity, affecting up to 1 in 250 of the general population in Northern Europe. Up to 25% of IBD is diagnosed during childhood or adolescence. The aims for this thesis were to study the epidemiology, natural history and novel genetic determinants of childhood onset IBD in Scotland. Methods: The existing repository of childhood onset and adult onset IBD patients, established at the Western General Hospital in Edinburgh, was used and expanded. Thus, anatomical location and behaviour of disease were assessed in 416 childhood onset (276 CD, 99 UC, 41 IBDU diagnosed before 17th birthday) and 1297 adult patients (596 CD, 701 UC) using the Montreal classification. Additional phenotypic (at diagnosis and at regular follow-up intervals) and epidemiological data were gathered. In this cohort, genotyping of germline variants in putative susceptibility genes (NOD1/CARD4, IL23R, ATG16L1, IRGM, FLG) was performed to enable single variant and haplotype-tagging association studies. Genotypic data of population-matched healthy controls were obtained locally (n=342) and from the Wellcome Trust Case Control Consortium (n=2937). Results: Compared with adults, childhood-onset CD was characterized by a more extensive, “panenteric” phenotype (ileocolonic plus upper GI; p<0.0001 OR23.3; 95% CI (13.4–40.6) with less isolated ileal (p<0.0001 OR 0.06 (0.03–0.1) or colonic disease (p<0.0001, OR 0.3 (0.2–0.5)). In 39%, the anatomic extent increased within 2 years. UC was also more extensive in children at diagnosis vs adults (p<0.0001 OR 5.1 (2.7–9.4)). In population-matched and age, sex and postcode-matched case-control analysis, childhood onset IBD and CD was associated with asthma (p<0.0001 OR 1.7 (1.3-2.1) and (p=0.005 OR 2.5 (1.3-4.8), respectively). Inherited variation of NOD1/CARD4 was not a strong determinant of disease susceptibility in the Scottish population (both in single marker and haplotype-tagging studies, all p>0.05 after Bonferroni correction). We found that the allelic frequency of rs11209026*A located within the IL23R gene, differed significantly between IBD / CD cases and controls (p=0.01 OR 0.51(0.3-0.9) and p=0.04 OR 0.5 (0.3-0.98)). Using a gene-wide haplotype-tagging strategy, we demonstrated that the multiple association signals of the IL23R locus are independent of rs11209026 in childhood onset IBD and CD. In Scottish children, the effect of germline variation of ATG16L1 and IRGM on CD susceptibility was relatively small (OR< 1.4), and appeared less than in adult disease. Genotype–phenotype analysis demonstrated an association of pure ileal disease with the ATG16L1 rs2241880G-allele (p=0.02 OR 1.3 (1.03–1.7)). Using binary logistic regression analysis, we confirmed the effect of rs2241880 genotype (GG) on ileal disease versus colonic disease (p=0.03 OR 2.4 (1.05–5.6)). Null alleles of the epithelial barrier protein FLG have no important effect on IBD susceptibility (p>0.4), but contribute to the high prevalence of atopy, notably co-existent eczema and food allergy (p=0.0003 OR 3.3 (1.7–6.6) and p=0.0001 OR 4.5 (2.0–10.0), respectively). Conclusion: Childhood onset IBD is characterised by extensive intestinal involvement and progression of disease after diagnosis. Genetic association studies in childhood and adult IBD have provided evidence for a large number of new genomic loci. These loci encode genes involved in a number of homeostatic mechanisms: innate pattern recognition receptors, the differentiation of Th17-lymphocytes, autophagy, maintenance of epithelial barrier integrity and the orchestration of the secondary immune response.