Project Summary In skin epithelium, a population of progenitor cells distinctly capable of proliferation, self-renewal, and terminal differentiation into post-mitotic progeny, is responsible to sustain tissue homeostasis throughout life. In particular, the dynamic choice between renewal and differentiation has emerged as a critical regulator of the long-term fate of epidermal progenitors with cancer-driver oncogenic mutations, which are either rapidly eliminated through increased differentiation or tolerated as growth suppressed clones due to a stringent renewal/differentiation equilibrium. Our long-term objective is to establish how epidermal progenitor cells balance growth in the presence of oncogenic mutations to maintain tissue homeostasis. To do so, we will employ: (i) a mouse model that can initiate expression of oncogenic Hras in a single epidermal progenitor, (ii) intravital imaging to document growth from a single oncogenic cell to a stably integrated clone, (iii) a novel progenitor renewal assay to quantify dynamic cell fate choices accompanying clone expansion, and (iv) our recently developed methods to modify gene function in epidermal and stromal cells surrounding the oncogenic clone, to explore specific cellular and molecular mechanisms we hypothesize function are the interface of oncogenic cells and their microenvironment, and ensure skin homeostasis. This application aims to test the hypotheses that: 1.) Balanced progenitor renewal, critical to epidermal tolerance of oncogenic mutations, is coordinated across the tissue through short- and long-range non-cell autonomous interactions; 2.) Signaling between oncogenic clones and the surrounding normal epidermis, mediated by traditional axon guidance molecules, is required for balanced progenitor renewal and skin homeostasis; and 3.) Skin site-specific interaction between epidermal and stromal compartments can override oncogenic tolerance and lead to loss of tissue homeostasis. The results of our research are expected to immediately integrate our growing understanding of cell autonomous mechanisms of oncogenic tolerance into the broader, tissue-wide context critical to skin homeostasis. We expect these findings to inform future development of comprehensive strategies, focused on both the progenitor cell and its microenvironment, to manipulate its renewal potential and treat conditions marked by unrestrained epidermal growth.