The RUNX1 Enhancer Element eR1: A Versatile Marker for Adult Stem Cells

The identification of adult stem cells is challenging because of the heterogeneity and plasticity of stem cells in different organs. Within the same tissue, stem cells may be highly proliferative, or maintained in a quiescent state and only to be activated after tissue damage. Although various stem cell markers have been successfully identified, there is no universal stem cell marker, which is exclusively expressed in all stem cells. Here, we discuss the roles of master developmental regulator RUNX1 in stem cells and the development of a 270 base pair fragment of the Runx1 enhancer (eR1) for use as stem cell marker. Using eR1 to identify stem cells offers a distinct advantage over gene promoters, which might not be expressed exclusively in stem cells. Moreover, RUNX1 has been strongly implicated in various cancer types, such as leukemia, breast, esophageal, prostate, oral, skin, and ovarian cancers—it has been suggested that RUNX1 dysfunction promotes stem cell dysfunction and proliferation. As tissue stem cells are potential candidates for cancer cells-of-origin and cancer stem cells, we will also discuss the use of eR1 to target oncogenic gene manipulations in stem cells and to track subsequent neoplastic changes.