Isolation of cancer stem cells by selection for miR-302 expressing cells

Cancer stem cells receive increasing interest because they are believed to be a major reason for long-term therapy failure. The reason for the therapy resistance of cancer stem cells lies partially in their multi-drug resistance and partially in the ability to rest mitotically inactive in the hypoxic center of tumors. Due to their variable number and their often low proliferation rate, cancer stem cells are difficult to purify in decent quantities and to grow in cell culture systems, where they are easily outcompeted by faster growing more 9differentiated9, i.e. less stem cell-like tumor cells. Here we present a proof of principle study based on the idea to select cancer stem cells by means of the expression of a stem cell-specific gene. We inserted a selectable egfp-neo coding sequence in the last exon of the non-coding murine miR-302 host gene. As a stem cell specific regulatory element, we used 2.1 kb of the genomic region immediately upstream of the miR-302 host gene transcription start. Stable transgenic CJ7 embryonic stem cells were used to induce teratomas. After three weeks, tumors were removed for analysis and primary cultures were established. Stem-like cells were selected from these culture based on G418 selection. When the selection was removed, stem cell morphology and miR-302 expression were rapidly lost, indicating that it were not the original ES cells that have been isolated. In conclusion, we show the possibility to use drug resistance expressed from a regulatory sequence of a stem cell-specific marker, to isolate and propagate cancer stem cells that otherwise might be hidden in the majority of tumor cells.