Abstract P6-03-01: Molecular Dissection of Breast Luminal Cell Transcription Factor Networks
Authors: Bargiacchi FG , Earp HS , Perou CM 
Published: August 2014

Abstract
During mammary development, cellular differentiation and lineage commitment are driven by distinct growth cycles under the control of local epithelial and mesenchymal paracrine signaling mechanisms. Within the mammary gland are multipotent stem cells, which give rise to luminal and myoepithelial lineages. The mechanisms that govern differentiation into these distinct cell populations are not completely understood, yet critically important for a complete understanding of normal development and breast tumorigenesis. Recent studies have shown that retroviral transduction of mouse and human fibroblasts with four transcription factors can initiate the conversion of a somatic cell into an embryonic stem cell-like state, with capabilities of differentiating into cell types of all three germ layers. The generation of induced pluripotent stem cells (iPS) illustrates that transformation of a mature cell into a more immature state can erase most epigenetic programming critical for the differentiation of that cell. Our goal is to determine whether mammary specific transcription factors can directly transdifferentiate fibroblasts or human mammary epithelial cells (HMEC) into functionally differentiated ER+/luminal cells.
To address this goal of creating ER+/luminal cells via an iPS type approach, we have developed a model in which an hTERT –immortalized human mammary epithelial cell line are transduced utilizing a multifunctional lentiviral expression vector system, which allows the simultaneous expression and/or silencing of multiple candidate genes. We are investigating the abilities of various combinations of transcription factors (TFs) to convert somatic cells to fully functional differentiated mammary epithelial cells. Nine candidate TFs (including ER, GATA3, FOXA1, XBP1 and CDKN2C) were chosen based upon their differential expression in genomic transcriptional data set of primary human breast tumors, and in fluorescence-activated cell sorting (FACS) of normal breast cell populations. These TFs are highly expressed in ER+/luminal cells, and some are known to be important lineage-specific factors. Ectopic overexpression of these cDNAs in HMECs indicated that these transcription factors have both unique and overlapping contributions towards inducing the expression of genes responsible for luminal differentiation. Examining the expression patterns of individually transduced cells using a classifier of differentiation status (Differentiation Score, Prat et al. 2010) identified several genes, including ESR1 and GATA3, as causing a transition towards the luminal subtype. These individual gene data will guide the selection of combinations, since we anticipate that, as was seen for iPS cells, multiple genes will be required for transdifferentiation into the luminal cell lineage. Since there are presently no cell lines or mouse models of Luminal A/ER+ breast cancers, the artificial creation of such of a cell line would be of great value.
Citation Information: Cancer Res 2012;72(24 Suppl):Abstract nr P6-03-01.