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A proposed NIH policy would require extra review for certain studies that create chimeras, or animals with both human and animal cells. Show here are a mouse, a rat-mouse chimera, a mouse-rat chimera, and a rat.

Nakauchi et al./The University of Tokyo

NIH moves to lift moratorium on animal-human chimera research

The National Institutes of Health (NIH) today announced that the agency soon expects to lift a moratorium on funding for controversial experiments that add human stem cells to animal embryos, creating an organism that is part animal, part human. Instead, these so-called chimera studies will undergo an extra layer of ethical review but may ultimately be allowed to proceed.

Although scientists who support such research welcomed the move, some were left trying to parse exactly what the draft policy will mean. It is “a step in the right direction,” says Sean Wu, a stem cell researcher at Stanford University in Palo Alto, California, who co-authored a letter to Science last year opposing the moratorium. But "we still don’t know what the outcome will be case by case,” he adds. However, some see the proposal as opening up research in some areas that had been potentially off-limits.

At issue are experiments in which scientists introduce human pluripotent stem cells—cells that can potentially turn into any kind of tissue—into very early embryos of mice and other animals and then let the animals develop. Such experiments can be used to study human development, generate disease models, and potentially grow human organs for transplantation. But the idea of such human-animal chimeras has drawn public concern, and some scientists and ethicists worry that the experiments could produce, say, a supersmart mouse.

Last September, NIH abruptly announced it was suspending funding for studies that introduce human stem cells into animal embryos while the agency considered the ethical issues. Although NIH wasn’t yet funding such research, the pause put on hold future federal support for studies by researchers who want to create pig-human or sheep-human chimeras to generate organs for transplantation. NIH then held a workshop last November to gather input, where the general consensus was that these studies are scientifically valuable.

According to two notices released today, NIH is proposing to replace the moratorium with a new agency review process for certain chimera experiments. One type involves adding human stem cells to nonhuman vertebrate embryos through the gastrulation stage, when an embryo develops three distinct layers of cells that then give rise to different tissues and organs. The other category is studies that introduce human cells into the brains of postgastrulation mammals (except rodent studies, which won’t need extra review).

These proposed studies will go to an internal NIH steering committee of scientists, ethicists, and animal welfare experts that will consider factors such as the type of human cells, where they may wind up in the animal, and how the cells might change the animal’s behavior or appearance. The committee’s conclusions will then help NIH’s institutes decide whether to fund projects that have passed scientific peer review.

NIH also wants to tighten its existing stem cell guidelines to prohibit studies that add human stem cells to primate embryos up to and including the blastocyst stage. (Current guidelines only prohibit adding human pluripotent cells to primate blastocysts.) And the agency wants to extend a current ban on breeding chimeric animals that might carry human eggs or sperm to include chimeras created with any kind of human cell, not just pluripotent stem cells.

“I am confident that these proposed changes will enable the NIH research community to move this promising area of science forward in a responsible manner,” writes NIH's associate director for science policy Carrie Wolinetz in a blog post today. In a call with reporters, she emphasized that the proposal “is not ... a prohibition” on chimera research. “It is merely an extra look.”

NIH is collecting comments on the proposed changes until 4 September, then hopes to issue a final policy and lift the moratorium by late January, Wolinetz said.

Although some scientists are holding their applause, one sees the proposed policy as an indication that NIH is relaxing its chimera policy. Neuroscientist Steve Goldman of the University of Rochester in New York, who injects human stem cells into the brains of mice, notes that even experiments that put human cells into the brains of monkeys or other primates appear to be potentially permissible. Such studies, which could be useful for studying mental illnesses, “had been a very murky zone” until now, Goldman says. The proposed changes suggest “a much more permissive environment.”