A Point Mutation R122C in RUNX3 Promotes the Expansion of Isthmus Stem Cells and Inhibits Their Differentiation in the Stomach

Background & Aims: RUNX transcription factors play pivotal roles in embryonic development and neoplasia. We previously identified the single missense mutation R122C in RUNX3 from human gastric cancer. However, how RUNX3^R122C mutation disrupts stem cell homeostasis and promotes gastric carcinogenesis remained unclear. Methods: To understand the oncogenic nature of this mutation in vivo, we generated the RUNX3^R122C knock-in mice. Stomach tissues were harvested, followed by histologic and immunofluorescence staining, organoid culture, flow cytometry to isolate gastric corpus isthmus and nonisthmus epithelial cells, and RNA extraction for transcriptomic analysis. Results: The corpus tissue of RUNX3^R122C/R122C homozygous mice showed a precancerous phenotype such as spasmolytic polypeptide-expressing metaplasia. We observed mucous neck cell hyperplasia; massive reduction of pit, parietal, and chief cell populations; as well as a dramatic increase in the number of rapidly proliferating isthmus stem/progenitor cells in the corpus of RUNX3^R122C/R122C mice. Transcriptomic analyses of the isolated epithelial cells showed that the cell-cycle–related MYC target gene signature was enriched in the corpus epithelial cells of RUNX3^R122C/R122C mice compared with the wild-type corpus. Mechanistically, RUNX3^R122C mutant protein disrupted the regulation of the restriction point where cells decide to enter either a proliferative or quiescent state, thereby driving stem cell expansion and limiting the ability of cells to terminally differentiate. Conclusions: RUNX3^R122C missense mutation is associated with the continuous cycling of isthmus stem/progenitor cells, maturation arrest, and development of a precancerous state. This work highlights the importance of RUNX3 in the prevention of metaplasia and gastric cancer.