Scientists have come the closest yet to producing valid stem cells from human embryos created by a similar cloning technique to the one that resulted in the birth of Dolly the sheep – the first clone of an adult mammal.
It is a significant step towards using a patient's own skin cells to create embryonic stem cells in the laboratory that can be used in transplant operations to treat diseased tissues and organs, from ailments such as diabetes and heart disease to spinal injuries and Parkinson's disease.
The breakthrough is one of the most significant developments since the discrediting of work published in 2005 by South Korean scientists led by Woo-suk Hwang of Seoul National University, who claimed to have produced the first stem cells from cloned human embryos. It later emerged that the work was flawed and was retracted.
In the latest study, researchers have merged the genetic material of human skin cells with 270 spare IVF eggs to create embryonic stem cells that appear to be capable of developing into specialised cells and tissues. Unlike the Dolly work, the study was not aimed at producing cloned embryos for transplant into the womb but at producing stem cells for clinical use.
However, the embryonic stem cells produced by this technique cannot be used to treat patients because they contain three sets of chromosomes rather than the usual two – the extra set coming from the unfertilised egg cell's nucleus, which was not removed.
The scientists said that retaining the genetic material of the unfertilised egg, rather than ejecting it before merging it with the skin cell, was the only way they could be sure of producing an embryo that would develop to the crucial "blastocyst" stage, when embryonic stem cells can be extracted.
Although the resulting "triploid" embryonic stem cells carried three sets of chromosomes, tests showed that they were capable of developing normally into specialised tissues, just like ordinary stem cells. Next, the scientists hope to find ways of eliminating the extra chromosomes so the stem cells can be used in transplant operations.
"We need to find a way of removing the genome of the oocyte [egg cell] without removing the function that it performs in terms of development. This function is probably not the oocyte genome itself but something associated with it," said Dieter Egli of the New York Stem Cell Foundation, a senior scientist on the study published in the journal Nature.
"Our hope is that we can eventually overcome the remaining hurdles and use patient-specific stem cells to treat and cure people who have diabetes and other diseases. What I know is that this will never happen if we don't make a start. We are definitely closer now," Dr Egli said.
Professor Robin Lovell-Badge of the National Institute for Medical Research in north London said that although the study did not produce useful stem cells, the work may reveal a way to overcome the problems so that stem cells can be made with the normal complement of genetic material.