Project Summary/Abstract By the year 2050, it is projected that over 72 million people will be 65 years or older in the United States, representing 20% of the population. This rapid, worldwide, demographic shift in age has many societal, economic, and public health implications. One of the most common chronic illnesses and pathological conditions experienced with increasing age is cancer. Head and neck cancer (HNC), in particular, is the seventh most common cancer worldwide and accounts for more than 500,000 deaths annually. Incidence of HNC significantly increases in individuals over the age of 40 and is more common in men. Further, individuals with a prior hematologic cancer diagnosis who have undergone hematopoietic stem cell (HSCT) treatments have an elevated risk for the development of HNC. Primary treatments for HNC are surgery and/or radiation therapy, but these treatments have devastating effects on swallowing function and speech. Multiple causal pathways likely exist for the development of HNC in aged individuals and following HSCT. One likely mechanism promoting both aging and cancer is the acquisition and accumulation of epigenetic modifications and genetic alterations within resident tissue stem cells in the head and neck. Clonal expansion of these mutated stem cells may accelerate the incidence of HNC in the aging population and in those who have undergone HSCT. The purpose of this proposal is to identify underlying cellular and systemic mechanisms contributing to the development of cancer in head and neck tissues (oral cavity, tongue, pharynx, esophagus, neck) with aging and following HSCT, and determine whether somatic mutations in relevant tissue stem cells drive clonal expansion, HNC initiation, and tumor development. Our hypotheses are that frequency of head and neck tissue stem cells will be significantly reduced with increasing age and following HSCT treatment and that epigenetic and genetic modifications of head and neck tissue stem cells will be 1) significantly increased in frequency with age and following HSCT, 2) differentially represented in male and female mice, and 3) capable of driving clonal outgrowth/HNC emergence when introduced into relevant tissue stem cell populations. To address these hypotheses, this proposal has 3 specific aims: 1) determine cell-intrinsic mechanisms of clonal evolution in head and neck somatic tissue stem cells in aged male and female mice, 2) examine effects of environment on epigenetic and genetic profiles of somatic tissue stem cells, and 3) examine whether in vivo introduction of somatic mutations in head and neck tissues are capable of driving clonal outgrowth of tissue stem cells and HNC initiation. Altogether, this study will provide significant new knowledge regarding mechanisms contributing to HNC with age and following HSCT, and enable development of targeted and effective therapies.