Researchers who figured out how to make valued embryonic stem cells out of ordinary skin cells said on Friday they had found a way to cut one cancer-causing ingredient out of the mix.
But it came at a price — the method may be safer, but it is also less efficient.
Shinya Yamanaka of Kyoto University in Japan said the findings, published in the journal Nature Biotechnology, demonstrate that the stem cell breakthrough may have been exciting, but is nowhere near ready to be used in humans.
Earlier this month, teams led by Yamanaka and James Thomson of the University of Wisconsin in Madison each reported separately that they had used four genes to transform ordinary skin cells called fibroblasts into induced pluripotent stem cells — iPS cells for short.
Their reports showed a way to get perfectly matched cells from patients that have at least some of the powers of embryonic stem cells, but without having to use cloning technology or embryos.
The hope is to find a way for new medical treatments that can make use of the body's own regenerative powers.
Yamanaka's team, working with a team at the Gladstone Institute of Cardiovascular Disease in San Francisco, used different genes than Thomson's team did.
One of the four genes in Yamanaka's restorative cocktail is called c-Myc1. They grew live mice from their new cells, but later found that the mice were prone to develop tumors.
So they left out c-Myc1. It worked, although not nearly as well. The new method was about half as efficient, they reported.
"Mice derived from Myc-negative iPS cells did not develop tumors during the study period," they wrote. "Future study is required to determine whether these mice develop tumors later in life," they added.
"Furthermore, we generated human iPS cells from adult dermal fibroblasts without MYC."
Both teams of researchers say they are still trying to fine-tune the precise genetic cocktail needed to turn back the clock on skin cells and make them act as if they came from a days-old human embryo — one with just eight or so cells, each one of which has the power to give rise to all the tissues and cells found in the human body.
Yamanaka's team said it is possible that the other three genes they used — called Oct3/4, Sox2 and Klf4 — may somehow activate Myc that naturally is found in the DNA of the skin cells.
Politicians have welcomed the reports of reprogramming normal cells and said it shows there is no need to continue work on controversial stem cells taken from human embryos.
But most scientists in the field say it is important to continue to work with all kinds of stem cells, as scientists still do not understand quite how they work — or how to use them in treating people.