The Down-Regulated in Adenoma gene (DRA) was originally identified by subtractive hybridization between a colon adenocarcinoma and adjacent, normal mucosa. It was found to be transcriptionally extinguished in the overwhelming majority of colon adenomas and adenocarcinomas. DRA protein is expressed in the terminally differentiated absorptive mucosal epithelium and, to a lesser degree, in the goblet cells, but not in the stem cell population deeper within the crypt. We (and others) have previously demonstrated that DRA is an anion transporter capable of the inward transport of chloride and sulfate ions. Such a function is consistent with its protein localization. However, we have also discovered another function of DRA that is of more direct interest to the subject of colon tumorigenesis: expression of DRA causes growth suppression. This function is independent of the anion exchange function of DRA. Our working hypothesis is that the Dra knockout mouse, in combination with the p53 knockout and/or the Min mouse will demonstrate colon tumor progression at the level of morphology, histology and/or molecular assessment of cell cycle criteria. The possibility that DRA may be a specific growth suppressor of colonic epithelial cells is a highly innovative concept and of great potential importance given the frequent down-regulation of this gene in human colon tumors. The possibility that a mouse strain could be developed that would better mimic the stages of development of the human disease will add a valuable approach to the study of colon cancer. Our long-term goal is to elucidate the pathway between loss of Dra expression at the cell surface and loss of growth control. This proposal will focus on the growth suppressing function of DRA by simultaneously employing several approaches: 1) we will use a Dra knockout mouse model to explore the in vivo effects of loss of Dra expression on the pathology of tumor development in the mouse intestine and on the cell cycle of the intestinal epithelium, 2) we will use a cell culture model in which DRA expression is expressed ectopically in an Adenoviral construct or controlled with a tetracycline-repressible promoter to examine cell cycle alterations, and 3) we will characterize the molecular domain(s) of DRA that are required for growth suppression.
