Sitting by the window of a posh coastal hotel in Half Moon Bay, Calif., wearing a baby-blue sweater and khakis, Ian Wilmut doesn’t project the image of a scientist who pulled off one of the most dramatic experiments in modern biology. When he and his collaborators unveiled Dolly the cloned sheep in 1997, they ignited the embryonic stem cell research field, struck awe in the public and set off a panic about the imminent cloning of humans. “Dolly was a big surprise to everyone,” recalls stem cell biologist Thomas Zwaka of the Center for Cell and Gene Therapy at the Baylor College of Medicine. Cloned frogs had refused to grow past the tadpole stage, and a seeming success in mice had proved to be a fake. According to scientific consensus back then, cloning adult mammals by the method Wilmut used was biologically impossible.
As Dolly matured, the cloning technology that created her—called somatic cell nuclear transfer (SCNT)—grew into a rich research enterprise. Scientists hoped to eventually be able to take a patient’s cell, place its nucleus into an unfertilized human egg and then harvest embryonic stem cells to treat intractable conditions such as Parkinson’s disease. But the first human clinical trial continues to seem remote, with embryonic cloning constrained by a federal funding ban, deeply controversial ethical issues and technical challenges. In mid-May safety concerns led the U.S. Food and Drug Administration to put on hold a bid by Geron Corporation in Menlo Park, Calif., to conduct trials on patients who have acute spinal cord injury.
Now the 64-year-old Wilmut is one of several high-profile scientists who remain loyal to SCNT in concept but are leading a wholesale charge out of the field and into an alternative technology. That other approach, first demonstrated in 2006 by Shinya Yamanaka of Kyoto University, restores adult cells back to an embryoniclike state called pluripotency, in which they regain the ability to develop into any kind of cell. Any well-appointed lab can apply the comparatively straightforward technique. “It’s really easy—a high school lab can do it,” says Mahendra Rao, who heads up the stem cell and regenerative medicine business at Invitrogen, a life sciences corporation based in Carlsbad, Calif. Yamanaka’s approach also enables scientists to leap over nuclear transfer’s egg supply problems and sidestep qualms about destroying human embryos.
Such practicalities, rather than a lack of inherent scientific value, seem to be driving the SCNT exodus. Wilmut describes his own switch in approach as a by-product of time-consuming responsibilities at the helm of the Scottish Center for Regenerative Medicine in Edinburgh, a post he assumed last year after nearly three decades at the nearby Roslin Institute. With 20 principal investigators demanding his attention, Wilmut’s research on amyotrophic lateral sclerosis (ALS) had slowed to a crawl. “We thought it would be more likely that things could be made to happen quickly,” he says.
Somatic cell nuclear transfer demands enormous skill and expensive equipment. It is easy to damage the unfertilized egg and hard to get the donated nucleus to operate in concert with its new host. Last fall Oregon Health & Science University researchers announced the first-ever success in primates—but the team went through 304 eggs from 14 rhesus macaque females to generate just two cell lines. And one of those had an abnormal Y chromosome. In humans the ability to collect fresh oocytes also remains a huge roadblock, especially because scientists cannot legally pay donors.
Yamanaka’s ability to convert adult mouse cells into embryoniclike stem cells—called induced pluripotent stem cells (iPS cells)—has pumped fresh excitement into regenerative medicine. In this process, scientists use viruses to deliver three to four genes into an adult cell and to reprogram it back to its unspecialized state, enabling it to grow into any type of cell in the body. In a span of months, Yamanaka’s team and three others reported success using human cells from adult skin and joint tissue and newborn foreskin.[break]
Now it’s hard to find a lab concentrating solely on embryonic cloning. Jamie Thomson, the first to pluck viable cells from a human embryo and grow them in culture, for instance, recently took charge of an institute focusing primarily on iPS cells. Although the technique is inefficient so far—less than 1 percent of cells become pluripotent—scientists see the iPS approach as a speedier path to cells suitable for disease research and, ultimately, the clinic.
With iPS, Wilmut enthuses, his team can study cell lines instead of wrestling to get them. “All you have to do is take some skin cells from somebody who apparently has inherited the disease, scatter some ‘magic dust’ on them and wait for three weeks,” he says. “And you’ve got pluripotent cells.” Wilmut and his collaborators, including George Daley of Children’s Hospital Boston and Chris Shaw of King’s College London, hope to use iPS cells to pinpoint mutations involved in ALS.
The method still does not promise quick cures. In ALS, for instance, researchers must speed up disease development and co-culture the various cells involved in the condition. Scientists would like to avoid retroviral vectors, risky because they deliver the genes randomly into the chromosome. Moreover, the new genes might vary in activity level, turn on in surprising ways or negatively influence other genes. Some teams succeeded in making iPS cells without the tumor-producing gene that Yamanaka used, but they also found that, as a result, they ended up with many fewer iPS cells.
Scientists do not fully understand how iPS reprogramming works—the inserted genes might represent a core regulatory circuit, or they might activate other genes. It is also not clear whether the results subtly differ from embryonic stem cells. No one yet has grown the two and made a side-by-side comparison, and survival after transplantation remains an unknown for both.
The iPS cells may force biologists to throw out accepted ideas about what it means to be a differentiated cell, says Zwaka, whose lab is studying characteristics of embryonic stem cells. Perhaps, he suggests, it is not necessary to take an embryonic cell through every step of development to create a particular cell type. There may be a set of “master regulators” that would enable, say, a skin cell to become an adult neuron without passing through the embryonic state.
Despite leaping on the iPS bandwagon, Wilmut and other cloning pioneers insist that embryonic stem cell research should continue. SCNT has offered important lessons about basic biology and will continue to enable studies of cell programming and reprogramming outside the genome. Only embryonic cells can answer questions about fertility and very early human development. Scientists will also likely rely on SCNT to produce mammalian models of diseases such as cystic fibrosis and for agricultural applications such as producing human proteins in animal milk.
“It’s simply too early to start putting one avenue over another,” says Daley, who is also president of the International Society for Stem Cell Research. His lab is using both SCNT and iPS to understand pluripotency. Daley fears that public sentiment may turn against embryonic work and dash hopes that a new U.S. administration will open up opportunities to clone new cell lines for research.
Indeed, as scientists turn their attention elsewhere, opponents of embryonic cell research have seized on the moment to attack. “There is no valid reason for any human cloning” or embryo destruction, wrote Tony Perkins of the Family Research Council. It is hard to escape the sense that SCNT research is on the wane. The ethical barriers and short egg supply remain daunting. If iPS pans out, Wilmut predicts, nuclear transfer to produce cell lines may one day become a history lesson.
Note: This story was originally printed with the title, "No More Cloning Around".