Reporter gene systems for the identification and characterization of cancer stem cells

Abstract

Cancer stem cells (CSCs) are tumor cells that share functional characteristics with normal and embryonic stem cells. CSCs have increased tumor-initiating capacity and metastatic potential and lower sensitivity to chemo- and radiotherapy, with important roles in tumor progression and the response to therapy. Thus, a current goal of cancer research is to eliminate CSCs, necessitating an adequate phenotypic and functional characterization of CSCs. Strategies have been developed to identify, enrich, and track CSCs, many of which distinguish CSCs by evaluating the expression of surface markers, the initiation of specific signaling pathways, and the activation of master transcription factors that control stemness in normal cells. We review and discuss the use of reporter gene systems for identifying CSCs. Reporters that are under the control of aldehyde dehydrogenase 1A1, CD133, Notch, Nanog homeobox, Sex-determining region Y-box 2, and POU class 5 homeobox can be used to identify CSCs in many tumor types, track cells in real time, and screen for drugs. Thus, reporter gene systems, in combination with in vitro and in vivo functional assays, can assess changes in the CSCs pool. We present relevant examples of these systems in the evaluation of experimental CSCs-targeting therapeutics, demonstrating their value in CSCs research.

Keywords: Cancer, Gene reporter systems, Cancer stem cells, Pluripotency transcription factors, Anticancer drugs, Preclinical analysis, Cancer stem cells marker

Core Tip: Controversial cancer stem cells (CSCs) research has caused confusion in this discipline. CSCs should be analyzed based on their function with regard to their ability to generate serially transplantable tumors. However, such evaluations are expensive and time-consuming and are fraught with ethical issues. Gene reporter assays can be used as a surrogate measure of the presence of CSCs in a sample. When combined with immunophenotyping and functional assays, reporter systems improve the quality of the evidence. However, there is no standard system; thus, the selection of an appropriate system must carefully consider its utility in previous works for the tumor type that is to be analyzed.