Exogenous ERα Expression in the Mammary Epithelium Decreases Over Time and Does Not Contribute to p53-Deficient Mammary Tumor Formation in Mice

Approximately 75% of all breast cancers express the nuclear hormone receptor estrogen receptor alpha (ER alpha). However, the majority of mammary tumors from genetically engineered mouse models (GEMMs) are ER alpha-negative. To model ER alpha-positive breast cancer in mice, we exogenously introduced expression of mouse and human ER alpha in an existing GEMM of p53-deficient breast cancer. After initial ER alpha expression during mammary gland development, expression was reduced or lost in adult glands and p53-deficient mammary tumors. Chromatin immunoprecipitation (ChIP)-sequencing analysis of primary mouse mammary epithelial cells (MMECs) derived from these models, in which expression of the ER alpha constructs was induced in vitro, confirmed interaction of ER alpha with the DNA. In human breast and endometrial cancer, and also in healthy breast tissue, DNA binding of ER alpha is facilitated by the pioneer factor FOXA1. Surprisingly, the ER alpha binding sites identified in primary MMECs, but also in mouse mammary gland and uterus, showed an high enrichment of ERE motifs, but were devoid of Forkhead motifs. Furthermore, exogenous introduction of FOXA1 and GATA3 in ER alpha-expressing MMECs was not sufficient to promote ER alpha-responsiveness of these cells. Together, this suggests that species-specific differences in pioneer factor usage between mouse and human are dictated by the DNA sequence, resulting in ER alpha-dependencies in mice that are not FOXA1 driven. These species-specific differences in ER alpha-biology may limit the utility of mice for in vivo modeling of ER alpha-positive breast cancer.