PROJECT SUMMARY Gastric cancer is currently the fourth leading cause of cancer deaths worldwide and is diagnosed in approximately one million people per year. The most common type of gastric cancer is intestinal-type, which develops through a series of metaplastic and dysplastic lesions of the stomach lining beginning with pyloric metaplasia. The hallmark of pyloric metaplasia is the transdifferentiation of gastric chief cells into spasmolytic polypeptide-expressing metaplastic (SPEM) cells. While gastric carcinogenesis is histologically well defined, the molecular drivers of the cellular and morphological changes that occur in each of these stages have not yet been discovered. Transcription factors are proteins that control cell fate via gene expression during development and in adult progenitor cell differentiation. To determine if transcription factors also control cell fate in metaplastic processes in the stomach, my lab probed for the expression of select factors in carcinogenic gastric tissue and found a strong upregulation of the master transcription factor SOX9 in SPEM cells. To follow up on this finding, I characterized two novel transgenic mouse models of pyloric metaplasia with Sox9 knock- out in chief cells and found that SPEM cell formation and subsequent carcinogenesis was severely disrupted. I hypothesize that SOX9 is necessary for chief cells to transdifferentiate into SPEM cells which are essential for carcinogenesis. To understand the significance of SOX9 in SPEM cells and carcinogenesis, I propose three specific aims. First, I will investigate the functional role of SOX9 by performing an in vitro experiment using organoids from Sox9 knock-out tissues to determine if SOX9 is required for chief cells to transdifferentiate. I will also examine whether Sox9 knock-out chief cells are able to stimulate to transdifferentiate into SPEM cells. Second, I will perform transcriptomic profiling to identify direct downstream genes of SOX9 to elucidate its transcriptional mechanism in SPEM cell identify and function. Finally, I will investigate the success of microenvironment recruitment which is essential to the progression of carcinogenesis without SOX9 upregulation in chief cells. This will elucidate a biological mechanism by which SPEM cells drive carcinogenesis. Together, these data will provide functional and transcriptional mechanisms of SOX9 in SPEM cells during gastric cancer development. This research will move the field forward by uncovering the molecular mechanisms of gastric carcinogenesis and will provide potential therapeutic targets for preventing gastric cancer development.