Myc-less mice produce stem cells

Last week, Shinya Yamanaka showed how he could generate induced pluripotent stem cells from human fibroblasts using a cocktail of four transcription factors.

This week, he and his team from Kyoto University show the same can be done with three, excluding the tumour-causing gene c-Myc.

In a new paper published today in Nature Biotechnology, Yamanaka describes a modified protocol for the generation of iPS cells that does not require the Myc retrovirus.

Myc was one of a cocktail of four transcription factors used in the initial experiments, along with Oct3/4, Sox2 and Klf4.

In new studies using the three factors but excluding Myc, the researchers say they have obtained significantly fewer non-iPS background cells as well has high quality iPS cells.

Myc is an essential transcription factor but the c-Myc gene is a known proto-oncogene highly expressed in tumours. Embryonic stem cell research has long been dogged by the issue of tumorigenesis.

Yamanaka's new paper, however, shows that none of the chimeric mice used in the study developed tumours once Myc was excluded, and importantly that the results were reproducible using skin cells from an adult human.

In this study, Yamanaka's team used mouse embryonic fibroblasts containing a green fluorescent protein transgene driven by the Nanog gene as a selection marker.

"Nanog-selected iPS cells are indistinguishable from ES cells in morphology, proliferation and gene expression, and give rise to germline-competent chimeras," the researchers write.

In previous experiments the team had found that Myc was essential for the production of iPSs, until the Nanog reporter cells were used.

The difference was one of timing - in the earlier study, drugs were added seven days after transduction. In this study they were added 14 days after. "This suggested that iPS cell generation without Myc is slower than with Myc," they write.

They have concluded that the omission of Myc results in a less efficient but more specific induction of mouse embryonic fibroblasts to iPS cells.