"In order to take full advantage of reprogramming, it is essential to develop methods to induce pluripotency in the absence of permanent changes in the genome," added Fred H. Gage, PhD, a professor in the Laboratory for Genetics at the Salk Institute and the Vi and John Adler Chair for Research on Age-Related Neurodegenerative Diseases.

By creating iPSCs from human neural stem cells without the use of viruses, the scientists learned something new. While the genetic transcriptional profile of the new iPSCs was closer to that of embryonic stem cells than to human neural stem cells, the iPSCs still carried a transcriptional "signature" of the original neural cell.

"While most of the original genetic memory was erased when the cells were reprogrammed, some were retained," said Muotri. He added that, in the past, it wasn't known if this was caused by the use of viral vectors. "By using a footprint-free methodology, we have shown a safe way to generate human iPSCs for clinical purposes and basic research. We've also raised an interesting question about what, if any, effect the 'memory retention' of these cells might have."

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