Turning back time

Dolly the sheep — that cuddly media darling of the late 1990s — actually came 35 years after what was perhaps an even more significant cloning breakthrough: a much less-feted 1960s amphibian, that might have been dubbed Molly the frog had the popular press been on the case back then. The legacy of Dolly and Molly is that we now know that it is possible to turn back developmental time and make a complete organism from a nucleus taken from an adult cell. It is difficult to imagine breakthroughs in developmental research that could have greater social and medical repercussions in future than these.

The groundwork for these crucial breakthroughs was laid more than 50 years ago. In 1952, Robert Briggs and Thomas King, working on a frog, Rana pipiens, became the first to successfully transplant living nuclei in multicellular organisms. They transplanted blastula nuclei into enucleated eggs, which then developed into normal embryos. This was a huge technical breakthrough and provided a spur for those interested in finding out exactly what caused the changes seen during cell differentiation.

However, the successful transplants that Briggs and King performed were of undifferentiated nuclei. Until it was possible to accomplish the same feat with a differentiated nucleus, it would remain an open question as to whether the genome itself somehow changed during development or whether it was the way genes were expressed that was responsible for differentiation.

It was John Gurdon's work on Xenopus laevis in the late 1950s and early 1960s, culminating in his seminal 1962 paper, that resolved this conundrum: genes were not lost or changed during cell differentiation — they were just differentially expressed. In his landmark study, Gurdon transplanted intestinal epithelium-cell nuclei from Xenopus tadpoles into enucleated frog eggs and managed to produce 10 normal tadpoles: Molly and her fellow clones. The logical consequence of Gurdon's success — that the nuclei of differentiated cells retain their totipotency — provided a key conceptual advance in developmental biology. However, Gurdon didn't immediately receive the universal acclaim he deserved.

The problem was that Briggs and King's earlier work on Rana pipiens indicated that nuclear transfer might not be able to reverse embryonic stem-cell differentiation, so many were sceptical about Gurdon's success in Xenopus. However, Gurdon's subsequent production of normal fertile adult animals from eggs with genetically marked nuclei transplanted from differentiated tadpole cells silenced the detractors. It proved once-and-for-all that the genome remained intact during differentiation and that the epigenetic changes to the somatic-cell nucleus were reversible.

It was more than three decades before a breakthrough of a similar magnitude to Gurdon's was to come. The year before their landmark 1997 publication, Ian Wilmut and his team managed to clone a sheep from an embryo-derived cell line that they induced to be quiescent. However, the real breakthrough came when they successfully applied a similar strategy to produce a normal sheep cloned from adult cells.

The scientific and social implications of this success were huge. Although Gurdon's seminal amphibian work had shown that the nuclei from differentiated cells could have their developmental clock turned back, Dolly was the first clone to be produced from an adult cell. Coupled with the fact that this success came in a mammal, the possibility that humans could be cloned from adult cells was an obvious one and saturation media coverage was the result.

However, the scientific legacy of Briggs, King, Gurdon and Wilmut is a much broader one: the knowledge that differentiation of a cell is not irreversible and that nuclei can be reprogrammed has driven expansion of such fields as stem-cell biology and epigenetics. Moreover, should cloning fulfill its enormous therapeutic potential, the debt the world owes these pioneers will be equally large.