Microarray expression profiling study of Runx1-null and wild type luminal mammary epithelial cells

RUNX1 encodes a RUNX family transcription factor (TF) and was recently identified as a novel mutated gene in human luminal breast cancers. We found that Runx1 is expressed in all subpopulations of murine mammary epithelial cells (MECs) except the secretory alveolar luminal cells. Conditional knockout of Runx1 in MECs by MMTV-Cre led to a decrease in luminal MECs, largely due to a profound reduction in the estrogen receptor (ER)-positive mature luminal subpopulation, a phenotype that could be rescued by loss of either Trp53 or Rb1. Mechanistically RUNX1 represses Elf5, a master regulatory TF gene for alveolar cells, and activates Foxa1, a key mature luminal TF gene involved in the ER program. Collectively, our data identified a key regulator of the ER+ luminal lineage whose disruption may contribute to development of ER+ luminal breast cancer when under the background of either TP53 or RB1 loss. Thoracic and inguinal mammary glands from 3 MMTV-Cre;Runx1L/L;R26Y and 3 MMTV-Cre;Runx1+/+;R26Y adult virgin females were dissected out, minced and digested to single cell suspension. Runx1L is the floxed conditional knockout allele of Runx1. R26Y is a conditional YFP reporter that would be turned on upon Cre-mediated recombination. FACSaria machine was used to sort out the YFP-marked luminal epithelial cell population of each of these 6 mice. Total RNA was isolated with Qiagen RNeasy kit and subsequently amplified by Nugen V2 and applied to Affymetrix mouse genome 430 2.0 arrays.