Exploring stem cell dynamics, clonal expansion and pseudopolyps in inflammatory bowel disease.

Abstract

Inflammatory bowel disease (IBD) confers a high risk of development of colitis-associated colorectal cancer in patients with extensive colitis. Crypt fission is a mechanism of clonal expansion in the intestinal epithelium. Although fission is rare in the normal colon, many crypts in IBD patients are in the process of fission. Protumourigenic mutations can spread through the entire inflamed colon relatively quickly indicating that stem cell dynamics are altered in IBD. Some patients with IBD develop pseudopolyps as a result of mucosal ulceration and epithelial regeneration. The aim of this PhD was to investigate the effect of inflammation on niche succession, the crypt cycle and the expansion of clones in the IBD intestine. Pseudopolyps were examined as potential sites for clonal expansion by determining the frequency of mutated pseudopolyps and proliferative potential, and examining their microRNA (miRNA) profile relative to inactive, active and dysplastic mucosa, and adenoma and cancerous tissue. This thesis will show that crypt fission cycles in inflammatory bowel diseased colon are protracted and that each stage of crypt fission appears to be slow. Overall, clonally related adjacent IBD crypts seem to share a more recent common ancestor than non-related IBD crypts, supporting increased crypt fission rates in IBD. The proliferative drive induced by continuous inflammation and mucosal repair in ulcerative colitis (UC) appears to promote the expansion of CCO-deficient patches. Furthermore, niche succession appears to be faster in active IBD. Pseudopolyps are a source of regeneration within the epithelium and, as shown here, have a faster proliferative drive than background mucosa in IBD patients. Pseudopolyps are not genetically inert and are a potential source of protumourigenic mutations in UC. Hence, pseudopolyps are a potential reservoir within the inflamed epithelium where mutations are harboured and where there is no competition from neighbouring epithelium, as it has been denuded following previous inflammation. MiRNA expression in pseudopolyps differs from that of UC-dysplasia and mucosa. In particular, the MiR-29 family was downregulated in pseudopolyps, a miRNA family that has been implicated in intestinal fibrosis formation in stricturing Crohn’s disease. Pseudopolyps have been traditionally thought of as benign, genetically inert and incidental findings characteristic of chronic inflammation. My research runs counter to this view indicating an exciting paradigm shift in the way we consider pseudopolyps, which may eventually alter the endoscopic management of these lesions in the future.