A Mouse Model of Persistence of Ductal and Alveolar Hyperplasia and Breast Cancer

Background and Innovation Increased breast density on mammograms is associated with an increased breast cancer risk and decreased sensitivity of detection by mammographic exams. The persistence of terminal duct lobular units (TDLUs) is associated with and may account for the increased mammographic density (MD), which affects 40-50% of women in their breast cancer screening age. TDLUs typically begin to involute after cessation of lactation. Persistence of TDLUs was associated with an increased risk for subtypes of breast cancer such as triple negative breast cancer in women of diverse ancestry groups. Lobuloalveolar units (LAs) in mice showed high degree of functional similarity to TDLUs in humans despite some difference in histology. TDLUs are the origin of most breast cancers for humans and mice. A delay in the involution of LAs after lactation was described in a number of genetically engineered mouse models (GEMMs) but they still eventually regressed with time and therefore might not truly reflect the persistence of TDLUs in postmenopausal women. Mouse models of persistent LAs (TDLUs) are not available hampering our ability to examine the mechanism and outcome of TDLU persistence. Hereby we describe a mouse model (iGRB7) of Inducible GRB7 expression in the mammary epithelial cells which was turned on by cre recombinase driven by a beta lactoglobulin gene promoter in lactating mice. 17 out of 19 parous and aging transgenic mice had persistent alveolar and ductal hyperplasia, many with secretory features reminiscent of the persistent TDLUs in humans or breast cancer; 5 out of 11 parous and aging control mice had some but more modest hyperplasia features (p=0.01). Three of the transgenic animals developed palpable mammary gland tumors: high grade invasive ductal carcinoma with various pre-invasive changes. There was a significant increase in the areas occupied by epithelial cells in the mammary glands for iGRB7 mice as well as numbers of alveoli per lobule. These mice had atrophic endometrium suggesting a lack of systemic estrogenic stimulation in these aging females. Mouse models with mammary specific expression of GRB7 therefore represent an innovative and valuable system to investigate the mechanism of TDLU persistence in postmenopausal women and the associated breast cancer risk. Significance and Impact to Veterans Healthcare Breast cancer is a leading cancer problem for Women Veterans. The significance is to examine the mechanisms of the persistence of TDLUs and how it contributes to breast cancer development in women with high breast density, a condition that affects 40-50% of Women Veterans. The latency in the development of invasive and non-invasive breast lesions provides a valuable and convenient platform to investigate the modulation of breast cancer risk by both environmental, such a military exposure, and genetic factors. Study of GRB7 signaling and associated mammary carcinogenesis in our animal model may reveal novel cancer prevention strategies that benefit many at risk Veteran Women and beyond with high mammographic density/persistence of TDLUs. Moreover, ~20% of human breast cancers carry HER-2 gene amplification and over-expression. GRB7 is co-amplified and over-expressed as HER-2. GRB7 has been shown to promote HER-2 positive breast cancer cell growth. The roles of GRB7 in HER-2 driven breast cancer animal models have not been examined. Our work is therefore significant and impactful. Path to translation In this study, we will establish 3-D culture of mammary epithelial cells derived from our animal models. Proteomic and phospho-proteomic analysis of these cultured mammary epithelial cells will reveal signaling proteins modulated by GRB7 in mammary glands. Via genetic and pharmacologic intervention, we will identify therapies, such as pharmacologic or antisense inhibitors, that can be further tested in animal models and then eventually human subjects. From knowing what causes high mammographic density, and understanding why there is associated elevated breast cancer risk, we will develop strategies to counter it.