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Background & Aims Metaplasia and dysplasia in the corpus are reportedly derived from de-differentiation of chief cells. However, the cellular origin of metaplasia and cancer remained uncertain. Therefore, we investigated whether pepsinogen C (PGC) transcript–expressing cells represent the cellular origin of metaplasia and cancer using a novel Pgc-specific CreERT2 recombinase mouse model. Methods We generated a Pgc-mCherry-IRES-CreERT2 (Pgc-CreERT2) knock-in mouse model. Pgc-CreERT2/+ and Rosa-EYFP mice were crossed to generate Pgc-CreERT2/Rosa-EYFP (Pgc-CreERT2/YFP) mice. Gastric tissues were collected, followed by lineage-tracing experiments and histologic and immunofluorescence staining. We further established Pgc-CreERT2;KrasG12D/+ mice and investigated whether PGC transcript–expressing cells are responsible for the precancerous state in gastric glands. To investigate cancer development from PGC transcript–expressing cells with activated Kras, inactivated Apc, and Trp53 signaling pathways, we crossed Pgc-CreERT2/+ mice with conditional KrasG12D, Apcflox, Trp53flox mice. Results Expectedly, mCherry mainly labeled chief cells in the Pgc-CreERT2 mice. However, mCherry was also detected throughout the neck cell and isthmal stem/progenitor regions, albeit at lower levels. In the Pgc-CreERT2;KrasG12D/+ mice, PGC transcript–expressing cells with KrasG12D/+ mutation presented pseudopyloric metaplasia. The early induction of proliferation at the isthmus may reflect the ability of isthmal progenitors to react rapidly to Pgc-driven KrasG12D/+ oncogenic mutation. Furthermore, Pgc-CreERT2;KrasG12D/+;Apcflox/flox mice presented intramucosal dysplasia/carcinoma and Pgc-CreERT2;KrasG12D/+;Apcflox/flox;Trp53flox/flox mice presented invasive and metastatic gastric carcinoma. Conclusions The Pgc-CreERT2 knock-in mouse is an invaluable tool to study the effects of successive oncogenic activation in the mouse corpus. Time-course observations can be made regarding the responses of isthmal and chief cells to oncogenic insults. We can observe stomach-specific tumorigenesis from the beginning to metastatic development.

Enhancer element (270bp) in the non-coding region between P1 and P2 promoters of Runx1 gene, termed eR1, drives the expression of Runx1 in hematopoietic stem cells (1). This element also targets the expression in stem cells in multiple tissues, including stomach (fundus and corpus) (2), intestine, colon, pancreas, liver, mammary gland and lung. Location of the cells identified by eR1 in each tissue was different from that previously shown by other stem cell markers, such as Lgr5. Nevertheless, in vivo lineage tracing results showed that eR1+ cells are differentiated into all the cell types in the tissue until one year post tamoxifen injection. To demonstrate the stem/progenitor activity in eR1+ cells, we isolated functional units of each organ from eR1-CreERT2; Rosa-tdTomato mice 1 day after tamoxifen injection, dissociated them into single cells and incubated in vitro. Mature organoids were generated from eR1+ cells, suggesting that eR1+ cells have potential to function as stem cells. When we examined the organoids generated from the stomach fundus, all the cells within the organoid showed Tomato color, suggesting they were originated from eR1+ cells. On the contrary, the antral organoids were mixture of Tomato positive and Tomato negative cells. Possible explanation for the mixture of color would be that there are more than one type of stem cells in the antrum and eR1+ and eR1- cells jointly made organoids. Since Lgr5+ stem cells have been described at the base (3) and CCK2R stem cells at +4 position (4) of antrum, whereas eR1+ stem cells are at isthmus of antrum (2), we must examine whether eR1- cells are Lgr5+ cells or CCK2R cells. Interestingly, mixture of eR1+ and eR1- organoids were observed in other GI tract organs. We found that contribution of eR1+ is 100% in proximal stomach and gradually decreased towards rectum (18%). Growth factor requirement of eR1+ stem cells were analyzed by withdrawing each factor. We found that eR1+ stem cells do not require Wnt and jagged1 but eR1- cells require Wnt in antrum. We are studying step-wise carcinogenesis using these organoids. Reference 1. Ng et al. Stem Cells 2010, 28:1869-81. 2. Matsuo J, Yamamura A et al, Gastroenterology 2016, PMID 27670082. 3. Barker et al, Cell Stem Cells 2010, 6:25-36. 4. Hayakawa et al, Gut 2015, 64:544-553. Citation Format: Akihiro Yamamura, Junichi Matsuo, Yi Hui Melissa Lim, Naing Naing Mon, Dede Liana Heng, Michiaki Unno, Khay Guan Yeoh, Motomi Osato, Yoshiaki Ito. Runx1 enhancer element marks stem cells in multiple organs [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr LB-138. doi:10.1158/1538-7445.AM2017-LB-138