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Background Colorectal cancer is the 2nd leading cause of cancer death in Canada. In rapidly dividing tissues such as the intestine or colon, only long-lived, multipotent, self-renewing tissue stem cells have longevity to accumulate mutations and serve as the cellular origin of cancer. In the small intestine, genetic fate mapping studies have demonstrated that there are at least two principal stem cell pools: actively cycling, crypt base cells expressing Lgr5, and quiescent cells situated above the crypt base. Clevers and colleagues have previously shown that Lgr5-expressing cells can give rise to cancer upon mutation. Interestingly, when Lgr5+ stem cells are selectively “killed”, intestinal integrity remains intact and other stem cells restore homeostasis. To determine whether another stem cell population can give rise to cancer in the colon, we examined whether the atypical homeobox protein Hopx, marks stem cells in the colon and whether these cells can give rise to colon cancer. Aims In the present study, we aim to determine whether Hopx-expressing cells are colonic stem cells that contribute to gut healing and can give rise to colonic tumours following the loss of APC. Methods To determine whether Hopx expressing cells show stemness, we crossed Hopx-CreERT mice to R26-TdTomato reporter mice. We then conducted genetic lineage tracing studies in the colon during homeostasis and following dextran sodium sulphate (DSS)-induced colitis. To test the function of Hopx expressing cells, Hopx-CreERT mice were also crossed to R26DTR mice and treated with diphtheria toxin (DT) following tamoxifen. These mice were then exposed to either normal drinking water or DSS to determine the role of Hopx+ cells in colonic regeneration. To test whether Hopx expressing cells can serve as a cellular origin for colon cancer, Hopx-CreERT mice were crossed to Apcf/f (floxed) mice. Results Consistent with the labeling of a stem cell, following tamoxifen, Hopx+ cells expressing tdTomato expanded to lineage trace full colonic crypts within 7 days, and labelling was persistent for greater than 6 months. Interestingly, ablation of Hopx+ cells with DT did not alter weight, histological damage or survival during normal homeostasis, however, Hopx+ cell ablation in mice treated with DSS resulted in increased histological damage. Surprisingly, loss of APC in Hopx-expressing cells did not induce colonic adenomas even after 8 months following tamoxifen administration. Conclusions These findings prove that Hopx expressing cells identify a novel colonic stem cell pool that is redundant in homeostasis, but in the context of injury, are essential for epithelial regeneration. Interestingly, Hopx+ cells do not have the capacity to give rise to colorectal adenomas upon loss of the APC gene. Funding Agencies CIHR

A29 HOPX LABELS A COLONIC STEM CELL THAT CONTRIBUTES TO COLONIC REGENERATION BUT NOT COLONIC TUMORS