Martin Hirst

The adult mammary gland is composed of cells with in vivo regenerative potential and the highly specialized differentiated cells they produce.   These hierarchically organized cell types can be isolated in large quantities at high purity (>95%) providing an ideal model to study in vivo differentiation end points. While significant effort has been dedicated to profiling epigenetic changes associated with the gain or loss of pluripotency, relatively little is known about the epigenetic changes that accompany terminal stages of differentiation where fully functional differentiated cell types with limited lifespans arise. We sought to address this gap by generating epigenomic profiles from highly purified primary human mammary myoepithelial, luminal and progenitor cell types isolated from disease free human subjects.   From these data consensus epigenetic signatures were defined for each cell type and a shared lineage state identified. Unlike the directionality of epigenomic programming that characterizes early differentiation, we found both gains and losses of epigenetic lineage restriction in differentiated cell types compared to a common progenitor. Epigenetic signatures were also generated for de novo derived variant human mammary epithelial cell lines.   We demonstrate that these pre-malignant cell lines are epigenetically distinct from genetically matched normal breast cell types and are characterized by common and cell line specific epigenetic reprogramming events.   Overall, our analysis provides a comprehensive view of highly purified genetically matched normal human mammary cell types and insight into the epigenetic events that define the terminal stages of cellular differentiation.