Characterizing Intratumoral Heterogeneity: A Role for Cell Plasticity

Abstract

Purpose: While most human tumors are thought to be derived from a single cell, tumor cell populations demonstrate considerable heterogeneity in phenotype and behavior. During characterization of single-cell clonal populations of a primary clear cell ovarian carcinoma, we identified a particularly tumorigenic clone (Clone 31) with epithelial and mesenchymal traits suggesting a possible multi-potent progenitor. We thus wanted to determine whether tumor initiating activity and other cell properties were restricted to progenitor, epithelial-like or mesenchymal-like cells. Methods: We generated single-cell subclones of Clone 31 using FACS. These subclones were subsequently characterized phenotypically, functionally and genetically. In vitro studies included monolayer proliferation, soft agar colony formation and E-cadherin and vimentin expression. In vivo tumor initiating ability was tested using an immunocompromised mouse model. Lastly, each subclone underwent copy number analysis, deep exome sequencing of a panel of genes and RNA sequencing to elucidate mechanism for functional diversity. Results: While Clone 31 displayed a strong ability to initiate tumor growth in vivo, this property was not universally retained among the subclones. While our study was designed to identify processes and properties that correlated with tumorigenicity, we were unable to distinguish any that clearly correlated with this activity. However it is noteworthy that only subclones with mixed epithelial and mesenchymal morphology formed solid tumors in vivo, which Clone 31 is incapable of doing. Further, subclone expression data is suggestive of epithelial mesenchymal plasticity among tumorigenic subclones and thus warrants further investigation. Conclusions: The present study identifies a multi-potent progenitor clone through characterization of its progeny and suggests that cell plasticity may be responsible for the observed heterogeneity in cell behavior.