SOMATIC EVOLUTION AND PRE-CANCER OF TP53 MUTANT CLONES - PROJECT SUMMARY/ABSTRACT Somatic mutations are continuously acquired in all cells in all tissues and accumulate with age to number hundreds or thousands per cell (depending on the tissue). These mutations are considered to be founding events in cancer, even though it may take decades from first mutation to malignancy development. Remarkably, even though many clone-driving mutations can be cancer-causing, and virtually all tissues harbor some of these clones, not everyone gets cancer. Furthermore, even though all genes have acquired mutations across all tissues, the constellation of mutations that causes positive selection of clones is unique to each tissue. Even TP53, which encodes a broadly important tumor-suppressor, is most often mutated in association with certain cancer types, such as breast, ovarian, pancreatic, and hematologic. Importantly, TP53 mutations are not exclusive to cancers, and are commonly found in histologically normal tissues, including breast, lung, blood, endometrium, skin, esophagus, colon, and bladder. We hypothesize that a key role of pre-cancer mutations is to promote expansion of a nascent clone in competition with other cells in a tissue-specific manner. Here, we will establish a strategy to test this concept in a novel mouse model, in the context of Trp53 mutations. We hypothesize that p53-mutant cells are positively selected for in only some tissues and mostly with age, leading to increased p53-mutant clonal expansion and tumor incidence. We have established a novel mosaic mouse model bearing the Trp53R245W mutation (equivalent of the human TP53R248W hotspot) in just a few cells in a sea of normal cells. TP53R248W is a commonly occurring mutation that leads to poor cancer prognosis in humans and in mice. Preliminary data indicate our ability to generate mosaic mice with unbiased contribution of control cells across tissues and to measure the overall degree of mosaicism in young mice. Therefore, in aim 1, we will examine the relationship between tissue-specific expansion of p53mut clones and oncogenesis. Preliminary data also indicate that expression of Trp53R245W in blood or breast tissue of young adult mice leads to clonal expansion. In aim 2, we will determine the role of aging on p53mut-driven clonal expansion and cancer initiation. Understanding how age exacerbates tissue-specific clonal expansion will aid in the identification of the trigger(s) that tip p53-mutant cells into expansion and those that push it further into a malignant state. Given that TP53 is the most commonly mutated tumor suppressor across cancers, fundamental insights into how it impacts clonal expansion as a function of age has the potential to identify factors that inhibit nascent clonal outgrowth. Despite a great deal of work on p53, the role of p53 mutations in clonal evolution as a pre-requisite to cancer has been little studied. If the concept is established in this developmental/exploratory R21 proposal, the work will offer a new perspective on cancer initiation that may have significant therapeutic implications. Mice develop cancer in ways very similar to humans, and thus they are an ideal setting in which to look at the very first stages of cancer. Mice allow us to examine many tissues at different stages of life with a deep level of analysis, which would not be possible to do with cell-culture strategies or humans.
