Mechanisms of genetic and epigenetic (tumor/stem) cell plasticity.

PL03-03 One of the major problems with treating cancer is that it is detected too late. Early lesions and transitions often occur ten to fifteen years prior to our ability to detect the disease by palpation or imaging. By the time many cancers are detected they have long since acquired the ability to generate micrometasases or populations of tumor cells that can develop drug resistance. Our best approach to treating or preventing cancer is early detection and an understanding of the earliest steps in the transition of normal cells to malignant ones. However, there are considerable obstacles to studying the early steps of cancer formation in vivo. For one, the multitude of random genetic aberrations that are generated during the carcinogenic process mask the specific non-random and mechanistically-related aberrations that would provide insights into how the lesion is initiated. Finding the critical alterations in a sea of changes is difficult. Furthermore, the pre-malignant lesions are small, sporadic and arise in a non-synchronized fashion, making detection difficult and analysis almost impossible. Finally, at the present time, these cells have not been successfully cultured in vitro. Our ignorance about the earliest molecular changes prevents studies that would identify individuals at risk or identify targets for prevention strategies. An in vitro model that mimics the transitions that occur as normal epithelial cells become malignant would provide major advances in several aspects of dealing with the disease. Early detection of lesions could be used for risk assessment and diagnosis of the disease at early stages. Additionally, and perhaps, the most significant advance would be the insights a model system may provide to the initiation of lesions and the transition of these lesions to malignancy allowing the translation of that information into preventive agents. It is for these reasons that we began studying human epithelial cells in vitro, focusing on mammary epithelial cells in particular. Recently, studies of human epithelial cells and fibroblasts from healthy individuals have been providing novel insights into how early epigenetic and genetic events affect genomic integrity and fuel carcinogenesis. Key epigenetic changes, such as the hypermethylation of the p16 promoter sequence, create a previously unappreciated pre-clonal phase of tumorigenesis in which a subpopulation of epithelial cells is positioned for progression to malignancy (Nature 409:636, 2001). These key changes generate epigenetic and genetic mosaicism, precede the clonal outgrowth of pre-malignant lesions and occur frequently in healthy, disease-free individuals (Cancer Cell 5:263, 2004; JBC 281: 24790-24802, 2006). Prior work from our laboratory has identified biomarkers that may be useful for risk assessment as well as provide targets for the elimination of these cells. Understanding more about these early events should provide novel molecular candidates for prevention and therapy of cancer. References Romanov, S, Krystyna Kozakiewicz, Charles R. Holst, Martha R. Stampfer, Larisa M. Haupt, and Tlsty TD. Normal Human Mammary Epithelial Cells Spontaneously Emerge from Senescence and Acquire Genomic Instability. Nature, 409:633-637, 2001 Crawford, Y. Gauthier, M., Joubel, A., Kozakiewicz, K. and Tlsty TD. Histologically Normal Human Mammary Epithelia with Silenced p16INK4a Over-express COX-2, Promoting a Premalignant Program. Cancer Cell 5:. 263-273, 2004 Reynolds PA, Sigaroudinia M, Zardo G, Wilson MB, Benton GM, Miller CJ, Hong C, Fridlyand J, Costello JF, Tlsty TD. Tumor Suppressor p16INK4A Regulates Polycomb-Mediated DNA Hypermethylation in Human Mammary Epithelial Cells. J Biol Chem 281: 24790-24802, 2006