Lgr5‐expressing stem cells are not the cells of origin of pyloric neuroendocrine carcinomas in mice

In intestinal and pyloric epithelia, leucine‐rich repeat‐containing G protein‐coupled receptor 5 (Lgr5)‐expressing cells represent long‐lived adult stem cells that give rise to all epithelial cell types, including endocrine cells. Ablation of the Apc gene in Lgr5‐expressing cells leads to intestinal and pyloric adenomas. To assess whether all epithelial tumours of the gastrointestinal tract are derived from LGR5‐positive stem cells, we crossed Lgr5–EGFP–IRES–creER^T2 mice, which express EGFP and Cre recombinase driven by the Lgr5 promoter, with CEA424–SV40–TAg mice, which develop pyloric neuroendocrine carcinomas of epithelial origin. In 19 day‐old mice, single SV40 T antigen (TAg)‐positive cells were identified preferentially at the the bases of pyloric glands, close to the stem cell compartment. However, contrary to previous publications describing subpopulations of LGR5‐positive cells in gastrointestinal neoplasia, we could not detect Lgr5–EGFP‐positive tumour cells in malignant lesions. The lack of expression of the Wnt target gene Lgr5 is probably not caused by suppression of Wnt signalling by TAg, since β‐catenin‐mediated Wnt signalling, as measured by the TOPflash assay, was not inhibited. To determine the cellular origin of CEA424–SV40–TAg tumours, we performed tracing experiments using Lgr5–EGFP–IRES–creERT2:CEA424–SV40–TAg:ROSA26–tdRFP mice. Following tamoxifen induction, it was possible to efficiently trace the progeny of Lgr5‐expressing cells in gastrointestinal tissue via red fluorescent protein (RFP) expression. No RFP‐positive tumour cells were detected, even when RFP gene activation occurred in 7 day‐old mice well before the appearance of TAg‐positive tumour cells. Hence, we conclude that Lgr5‐expressing stem cells probably do not constitute the cells of origin in CEA424–SV40–TAg mice. Consequently, not all epithelial tumours in the pyloric region are initiated by transformation of LGR5‐positive stem cells. Thus, additional long‐lived LGR5‐negative stem cells or progenitor cells with a low turnover rate might exist in the pyloric region, which could give rise to tumours. Copyright © 2015 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.