Mus musculus Genome sequencing. Mus musculus

Signal Transducers and Activators of Transcription (STATs) are principal transcription factors downstream of cytokine receptors. Although STAT5A is expressed in most tissues it remains to be understood why its premier, non-redundant functions are restricted to prolactin-induced mammary gland development. We report that the ubiquitously expressed Stat5a/b locus is subject to lineage-specific transcriptional control in mammary epithelium. Genome-wide surveys of epigenetic status and transcription factor occupancy uncovered a putative mammary-specific enhancer within the intergenic sequences separating the two Stat5 genes. This region exhibited several hallmarks of genomic enhancers, H3K27 acetylation and binding by MED1. Mammary-specific STAT5 binding was obtained at two canonical STAT5 binding motifs. CRISPR/CAS9-mediated genome editing was used to delete these STAT5 binding sites in mice and determine their biological function. Mutant animals exhibited an 80% reduction of Stat5 levels in mammary epithelium and a concomitant reduction of STAT5-dependent gene expression. Transcriptome analysis identified a class of mammary-restricted genes that was particularly dependent on high STAT5 levels as a result of the intergenic enhancer. Taken together, the mammary-specific enhancer enables a positive feedback circuit that underlies the remarkable abundance of STAT5 and, in turn, controls the efficacy of STAT5-dependent mammary physiology.