Abstract A7: Expression of the developmental gene and transcription factor, ELF5, in human and murine breast cancers

We previously reported that TGFα transgenic mice which received a 60-day course of tamoxifen chemoprevention had a reduced incidence of estrogen receptor positive mammary tumors by ∼50%. Mammary gene expression analysis several weeks post-tamoxifen but months prior to tumor development revealed reduced mRNA levels of the developmental gene, Elf5, in mice that remained tumor free throughout life compared to those that developed tumors. Elf5 belongs to the Elf group of transcription factors, which have ascribed roles in epithelial cell organization during organogenesis and make up a subfamily of the Ets family of transcription factors, some of which have been implicated in cancer. ELF5 is a downstream effector molecule of prolactin and instrumental to lobular development during pregnancy. Other laboratories have reported that Elf5 mRNA expression is highest in mammary epithelial cell isolates enriched with luminal progenitor cells, a subpopulation that has been shown to be increased in some murine models of mammary cancer as well as in breast tissue isolated from women with BRCA1 mutations. These data implicate luminal progenitor cells as tumor forming cells and Elf5 as a participant in cancer development. In this study, our goals were to determine (1) if there were alterations in the proportions of mammary epithelial subpopulations in glands from TGFα mice using flow cytometry and (2) the presence and pattern of ELF5 protein expression in clinical breast biopsies and in mammary glands isolated from TGFα and c-myc mammary cancer prone mice using immunohistochemistry. We found that glands from TGFα mice had an increased fraction of luminal progenitor cells compared with wild type mice which coincided with increased Elf5 mRNA expression. We then identified ELF5 protein in the nuclei of luminal cells in ducts and lobules and in higher proportions within hyperplasias from glands of both TGFα and c-myc transgenic mice. Tumor epithelia from low-grade and highgrade tumors from TGFα and c-myc, respectively, displayed negligible ELF5 protein, while surrounding reactive stroma contained spindle-like cells and endothelial cells displayed intense ELF5 staining. In an initial screen of 25 human breast biopsies ELF5 protein localization was similar to that found in mice, with nuclear ELF5 expressed in a subset of luminal cells of normal ducts and lobules and in hyperplasias but not in in situ carcinomas. ELF5 protein was frequently found in the epithelial cells of high-grade tumors but infrequently observed in low-grade tumors. In a second screen of strictly high-grade cancers (≥Grade 3) ELF5 staining was found in the epithelial compartment in ∼75% of the cases, but expression was predominantly cytoplasmic. Similar to findings in mouse models, immunolocalization of ELF5 in human breast samples was not restricted to the epithelial compartment but included spindle-shaped cells and endothelial cells within the reactive stroma of all tumors, regardless of tumor epithelial ELF5 staining status. These novel data indicate that ELF5 may contribute to breast carcinogenesis in both humans and mice. The data also demonstrate that ELF5 expression is not restricted to epithelia and points to a role of this transcription factor in the tumor reactive stromal microenvironment. Citation Information: Cancer Prev Res 2010;3(12 Suppl):A7.