CD44posCD49fhiCD133/2hi Defines Xenograft-Initiating Cells in Estrogen Receptor–Negative Breast Cancer

Defining the populations of tumor-initating cells that are present in tumors is a first step in developing therapeutics to target these cells. We show here that both CD44(pos)CD24(neg) and CD44(pos)CD24(pos) cell populations in estrogen receptor (ER) alpha-negative breast tumors are tumorigenic in murine xenograft models. We also describe a third population of xenograft-initiating cells (XIC) enriched in CD44(pos)CD49f(hi)CD133/2(hi) cells that display heightened tumorigenicity, self-renewal in vivo, and the capacity to give rise to functional and molecular heterogeneity. Consistent with their capacity for self-renewal, these cells express elevated levels of Sox2, Bmi-1, and/or Nanog and their CpG islands are hypermethylated relative to nontumorigenic cells. These differences in methylome regulation may be responsible for the dramatic functional differences between the two populations. The identification of CD44(pos)CD49f(hi)CD133/2(hi) XIC in ER-negative tumors may lead to expanded understanding of these tumors and ultimately the development of therapeutics designed to specifically target the cells.