Loss of cyclin D1 in concert with deregulated estrogen receptor alpha expression induces DNA damage response activation and interrupts mammary gland morphogenesis.

We have previously shown that increased and deregulated estrogen receptor alpha expression in the mammary gland leads to the development of proliferative disease and cancer. To address the importance of cyclin D1 in ERalpha-mediated mammary tumorigenesis, we crossed ERalpha-overexpressing mice with cyclin D1 knockout mice. Mammary gland morphogenesis was completely interrupted in the ERalpha-overexpressing cyclin D1-deficient triple transgenic mice. In addition to a highly significant reduction in mammary epithelial cell proliferation, cyclin E was upregulated resulting in DNA damage checkpoint activation and apoptosis. This imbalance between proliferative and apoptotic rates in conjunction with remarkable structural defects and cellular disorganization in the terminal end buds interrupted ductal morphogenesis. Interestingly, the structure of the mammary fat pad was fundamentally altered by the consequences of overexpressing ERalpha in the epithelial cells in the absence of cyclin D1 illustrating how alterations in the epithelial compartment can impact surrounding stromal composition. Transplantation of embryonic ERalpha-overexpressing and cyclin D1-deficient mammary epithelium into the cleared fat pad of wild-type mice did not rescue the aberrant mammary gland phenotype indicating that it was intrinsic to the mammary epithelial cells. In conclusion, although cyclin D1 is not essential for proliferation of normal mammary epithelial cells, ERalpha-overexpressing cells are absolutely dependent on cyclin D1 for proliferation. This differential requirement for cyclin D1 in normal vs abnormal mammary epithelial cells supports the application of cyclin D1 inhibitors as therapeutic interventions in ERalpha-overexpressing breast cancers.

Validation of transgenic models of breast cancer: ductal carcinoma in situ (DCIS) and Brca1-mutation-related breast cancer.

Available mouse models of ductal carcinoma in situ (DCIS) and BRCA1-mutation-related breast cancer are reviewed. The best validated mouse models of human DCIS are the conditional estrogen receptor α in mammary tissue (CERM) model initiated by deregulated estrogen receptor α and the serial explant mouse model initiated by p53 deficiency. At present the most useful and best validated mouse model of BRCA1-mutation-related breast cancer uses the cre-lox system to make a conditional Brca1 deletion targeted to mammary epithelial cells. The major shortcoming of the non-conditional Brca1 models is the high incidence of non-mammary tumor development. The use of mammary gland transplants or explants from these mice into nude hosts is one approach that could be used to circumvent this deficiency. Development and validation of a Brca1-mutation-related mouse model of basal cell breast cancer is an important next step.