EZH2 Is Required for Breast and Pancreatic Cancer Stem Cell Maintenance and Can Be Used as a Functional Cancer Stem Cell Reporter

Abstract

Although cancer is largely seen as a disease stemming from genetic mutations, evidence has implicated epigenetic regulation of gene expression as a driving force for tumorigenesis. Epigenetic regulation by histone modification, specifically through polycomb group (PcG) proteins such as EZH2 and BMI-1, is a major driver in stem cell biology and is found to be correlated with poor prognosis in many tumor types. This suggests a role for PcG proteins in cancer stem cells (CSCs). We hypothesized that epigenetic modification by EZH2, specifically, helps maintain the CSC phenotype and that in turn this epigenetic modifier can be used as a reporter for CSC activity in an in vitro high-throughput screening assay. CSCs isolated from pancreatic and breast cancer lines had elevated EZH2 levels over non-CSCs. Moreover, EZH2 knockdown by RNA interference significantly reduced the frequency of CSCs in all models tested, confirming the role of EZH2 in maintenance of the CSC population. Interestingly, genes affected by EZH2 loss, and therefore CSC loss, were inversely correlated with genes identified by CSC enrichment, further supporting the function of EZH2 CSC regulation. We translated these results into a novel assay whereby elevated EZH2 staining was used as a reporter for CSCs. Data confirmed that this assay could effectively measure changes, both inhibition and enrichment, in the CSC population, providing a novel approach to look at CSC activity. This assay provides a unique, rapid way to facilitate CSC screening across several tumor types to aid in further CSC-related research.