Pluripotent microRNAs replace oncogene

Researchers from the University of California, San Francisco have used three microRNAs (miRNAs) to reprogram mouse fibroblasts into induced pluripotent stem cells.

The miRNAs miR-291-3p, miR-294 and miR-295 – collectively known as the embryonic stem cell-specific cell cycle-regulating (ESCC) miRNAs – were able to do the job of the oncogene cMyc, with improvements.

cMyc is one of four genes used to induce pluripotency in stem cells but it has the disadvantage of being an oncogene and can promote uncontrolled cell growth.

The other three genes commonly used in what is known as the Yamanaka method, named after the pioneer of induced pluripotent stem cells, Japan’s Shinya Yamanaka, are the transcription factors Sox2, Oct4 and Klf4. The latter two are also oncogenes.

In a study published in Nature Biotechnology, the UCLA team, led by Robert Blelloch and Robert Judson, used retroviruses to introduce Oct4, Sox2 and Klf4 into mouse embryonic fibroblasts, and then transfected the cells with mimics of the three miRNAs.

They found that the miRNAs increased the efficiency of reprogramming by the three genes. Efficiency was not higher than cells transfected with the full complement of four, however.

The miR-294 mimic showed the greatest effects. Using just miR-294, the mimic was able to increase the number of colonies to around 75 per cent of that achievable with four-factor reprogramming.

Why this should be so they are unsure, although analysis of their data showed that cMyc binds to the promoter region of the miR-290 cluster, and that the miRNAs may be downstream effectors of cMyc during reprogramming.

The effects of the two reprogramming systems were different, however. While the four-factor reprogramming method does not produce large quantities of colonies with ES cell-like morphologies, the miRNA method produces a predominantly uniform population, the researchers found.

They also found that teratomas that formed when the cells were injected into immunodeficient mice behaved differently. In cMyc, more than a third of the teratomas invaded the underlying body wall, while none of the miRNA iPS cell teratomas did.

Their conclusion is that miRNAs can replace cMyc and perhaps eventually substitute for all DNA elements in the generation of iPS cells.

The team is now working to replace all four of the transcription factors with miRNAs, and to see whether they can by-pass the embryonic stem cell-like stage altogether.