Understanding the Early Events in Breast Carcinogenesis by Inactivating p16INK4a in Primary Human Mammary Epithelial Cells
Cancer cells arise from normal cells that have acquired the ability to expand beyond the constraints of a tissue microenvironment. Normal human cells have many mechanisms to prevent carcinogenesis. During the process of carcinogenesis one of the properties a cell must acquire is the ability to evade anti-proliferative signals. Many stimuli cause normal cells to activate a cell cycle arrest. The p16INK4a gene is an important tumor suppressor that activates a cell cycle arrest in response to stimuli like stress, DNA damage, and senescence. Inactivation of p16INK4a occurs in many tumor types and allows the cells to bypass many anti-proliferative signals. Inactivation of p16INK4a is found in about 30% of breast tumors (Rocco and Sidransky, 2001), yet the consequences of that inactivation are not completely understood. We sought to improve our understanding of p16INK4a inactivation in breast carcinogenesis by inactivating p16INK4a in primary human mammary epithelial cells (HMEC). Studying primary human cells allowed us to understand the consequences of p16INK4a inactivation in cells with a wild-type genetic background. Additionally, this system allowed us to examine the role of p16INK4a in early breast carcinogenesis, with a long-term goal of identifying avenues for early detection and preventive intervention of breast cancer. In Chapter II, we describe how inactivation of p16INK4a modulates the levels and functions of another important tumor suppressor gene, p53. The Rb pathway is identified as being necessary for p16INK4a to modulate p53. In Chapter III, we further our understanding of the modulation of p53 by p16INK4a and determine that ATM and the DNA damage response may be involved. Regulation of cell cycle checkpoints following p16 inactivation is also examined. In Chapter IV, we perform global gene expression analysis to identify new genes and pathways modulated by inactivation of p16INK4a in HMEC. We identify chitinase-3-like-1 as a novel p16INK4a regulated gene that is overexpressed in the basal-like subtype of invasive breast tumors. Chitinase-3-like-1 may be useful as a serum biomarker or therapeutic target for basal-like breast tumors.