Integrating Single Cell RNA-Sequencing Analyses and Functional Assays to Decipher Cell Differentiation Hierarchies in the Mammary Epithelium

The identification and molecular characterization of cellular hierarchies in complex tissues is key to understanding both normal cellular homoeostasis and tumorigenesis. The mammary epithelium is a heterogeneous tissue consisting of two main cellular compartments, an outer basal layer containing myoepithelial cells and an inner luminal layer consisting of estrogen receptor negative (ER^-) ductal cells and secretory alveolar cells (in the fully functional differentiated tissue) and hormone responsive estrogen receptor positive (ER⁺) cells. Recent publications in Nature Communications used single cell RNA-sequencing (scRNA-seq) analysis to decipher epithelial cell differentiation hierarchies in human (Nguyen et al., 2018) and murine (Pal et al., 2017) mammary glands and report the identification of new cell types based on the expression of the luminal progenitor cell marker KIT (c-Kit). However, there are several inaccuracies and unfortunate omissions in the citation of previous research in each of these studies. As a result, the overall conclusions on the significance of these reports, in particular the claimed identification of new cell types is not accurate. Here we discuss these studies in the context of our previous research (Regan et al., 2012), in which we identified c-Kit as a luminal progenitor cell marker and functionally characterized cellular subpopulations analogous to those reported in the recent scRNA-seq studies.