Twenty Years After Dolly the Sheep, We're No Closer to Cloning Humans

On July 5, 1996, after 276 failed attempts, a team of scientists at the Roslin Institute at the University of Edinburgh achieved something remarkable: They managed to clone a sheep. The sheep, named Dolly Parton because the cell from which they'd cloned her had been taken from a sheep's mammary gland, would be the first cloned mammal to survive into adulthood.

Dolly's existence was kept under wraps until February 22, 1997, when she finally got to meet the world. Journalists from around the globe crowded into her stable for her unveiling, anxious to see the feat of genetic engineering that the New York Times described as "anticipated and dreaded more than any other." After seeing her, the media went wild, quickly inciting hysteria about what would come next. If scientists knew how to clone a sheep, how soon would it be before they started cloning humans?

"The primary concern about human cloning was the notion of so-called designer babies," Alta Charo, a professor of law and bioethics at the University of Wisconsin, told me. "They were ridiculous fantasies about essentially xeroxing people that had no relationship to what was actually going on."

Still, the concerns were great enough that a week after Dolly was unveiled, President Bill Clinton issued a ban on the use of federal funds for human cloning research until a national bioethics advisory commission had time to consider the implications. Charo, who served on Clinton's advisory commission, said they "recommended that human cloning not be pursued at that time because it was simply too unsafe to come close to an ethical approach to reproductive medicine."

But that was two decades ago. Since the Clinton moratorium on federal funding for reproductive human cloning, has anything changed?

Today, as in 20 years ago, there are three main types of cloning: gene cloning, reproductive cloning, and therapeutic cloning. Gene cloning reproduces segments of DNA, and therapeutic cloning creates embryonic stem cells with the goal of growing tissues that could replace damaged tissues in humans. Reproductive cloning, the type used to create Dolly, produces genetic copies of an entire animal.

Reproductive cloning is accomplished through a technique called somatic cell nuclear transfer. This involves taking the nucleus from any adult cell and inject it into the cell of a fertilized egg whose nucleus has been removed. "You're essentially hijacking the fertilized egg's cellular machinery to take this adult nucleus," said Charo. In the case of Dolly, that adult cell nucleus was taken from the udder of a six-year-old sheep.

Over the last two decades, a number of different animals have been cloned in the same way—cows, horses, cats, dogs, pigs. Even Dolly has a number of ancestors from the same cell line, many of which are alive and kicking today. (Dolly passed away in 2003 due to a lung disease, unrelated to cloning.)

Despite the success in cloning various mammals through nuclear transfer, cloning is still very difficult to pull off. The process is prone to introducing genetic errors, which results in many cloned offspring dying young.Some companies, like South Korea's Sooam Biotech Research Foundation, have offered the service to pet owners, who can get their pets cloned for about $100,000. In the past decade, the company has cloned about 800 dogs for individuals and government agencies, and as of 2016 was producing about 500 cloned embryos every day.

While cloning animals like sheep and cows is no small task, cloning humans and other primates through nuclear transfer has proved even more difficult. The main reason has to do with spindle proteins that are located very close to the chromosomes in primate eggs. When scientists attempt to remove the nucleus from the primate egg, they also end up removing or destroying the spindle proteins, which makes it impossible for the cell to divide and grow into a human.

That hasn't stopped a number of organizations from falsely claiming to have successfully cloned a human. In 2002, Clonaid—the human cloning branch of the Raelian UFO cult—claimed that the first cloned human baby had been born, although no evidence was ever provided to substantiate these claims. In 2004, a South Korean researcher published two papers in the journal Science, where he had claimed to have cloned human cells, but the research later turned out to be fake.

These days, Charo said no one is really researching reproductive cloning for humans. It's simply too inefficient and error prone to make it worthwhile. Some researchers have suggested cloning technology could be used to rebirth lost children or prevent disease, but Charo said no one has filed an application with the Food and Drug Administration to get started on this kind of research.

There is the potential for therapeutic cloning in a human context, which involves cloning a human embryo from which embryonic stem cells with the same DNA as the original cell can be harvested. Embryonic stem cells are unique because they can develop into almost any type of cell in an organism, which means they have the potential to one day be used to grow healthy tissues in a lab that can replace diseased tissues in a human. So far, the US government says there is "no evidence" that human embryos have been produced for this purpose. Based on the huge ethical questions raised by producing a human embryo to harvest it for stem cells, it is unlikely that this will change anytime soon.

"Both as a research tool and as a reproductive option, cloning is still theoretically available, but is not as nearly as often used," Charo told me. "The deeper ethical issue is really how you determine when something is safe enough for particular use. That is not just a technical issue, it's an ethical question about value judgments. If this is going to be used for human reproduction, what's the level of safety that's required before we permit it? It's a question public health and obligations to future generations who might be affected by your actions today."

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