The Stem Cell

Today the British government tentatively approved the creation of “cybrids,” or human-animal hybrids, for stem cell research. Cybrids are made by putting a human nucleus containing DNA into an empty animal egg (a technique called nuclear transfer). Culturing the egg for a few days will hopefully yield human embryonic stem cells. The idea springs from the dearth of human eggs available for making embryos for stem cell research.

The ruling gives a green light to the U.K. Human Fertilization and Embryology Authority (HEFA) to grant licenses to researchers who want to fuse cow or rabbit eggs with human nuclei and see if they can derive an embryonic stem cell line. HEFA has three months to consider whether they will award the permissions to a handful of laboratories.

One of those laboratories belongs to Steven Minger, a U.K. stem cell biologist. He said in a Times UK report “It is gratifying to see that the HFEA has listened to the broader scientific and bioethical community as well as to the Science and Technology Committee and the Cross-Parliament Scrutiny Committee and agreed to consider licence applications for interspecies somatic cell nuclear transfer.”

(TheStemCellBlog will visit the Minger lab next week, with a report to follow).

Some worry that these embryos might make “cowumans” or “humabbits.” Because of the species differences between humans and animals is vast, and because there no animal nuclear DNA in the picture, this scenario is highly unlikely.

Nonetheless, there will be a close watch on the procedures. “We understand people’s initial resistance to the idea of hybrid embryos, but there would be very strict controls on the way they are used,” Tony Calland, chairman of the BMA Medical Ethics Committee, said in a news report. “The law already prevents such embryos being implanted in a woman, and they must be destroyed within 14 days.”

Others see this as the first step towards the manufacture of “chimeras,” organisms comprising genetically distinct populations of cells that hail from two or more zygotes, the earliest stage of the embryo. In a 2006 state of the union address, George W. Bush said, “Tonight I ask you to pass legislation to prohibit the most egregious abuses of medical research: human cloning in all its forms [including] creating human-animal hybrids.” Bush supported legislation that would imprison scientists for making cellular chimeras.

Bush’s provocative mention of “human-animal hybrids” was more than a rhetorical flourish, joining other loaded terms in the stem cell debate such as ‘embryo’ and ‘cloning.’ Like those words, biology’s ‘chimera’ comes from years of common usage. The chimera of Greek fable was a fire-breathing monster with a lion’s head, a goat’s body and a serpent’s tail, dispatched by the Pegasus-riding Bellerophon. By contrast, cellular chimeras are much tamer. (Genetic mosaics, which are sometimes confused with chimeras, express different genotypes arising from a single zygote. Differential activation of the X chromosome, for example, produces the curious color of calico cats).

Should we run for the hills? Chimeras have not so much arrived; they were already here. The world is home to a thriving menagerie of them. The joined fetal circulatory systems of a cow pregnant with fraternal twins will cause stem cells to seed the blood system of one twin with the other. In humans, placental blood exchange between the mother and fetus results in microchimerism, where progenitor blood cells from the child persist in the mother decades after the child is born. Some estimate that up to 50% of mothers are naturally chimeric, and later-born children can carry cells of elder siblings that slip across the placental membrane during fetal development. In rare cases, a chimera can form after two embryos fuse. For example last year, a US woman was denied welfare benefits after officials determined that her children’s DNA did not match her own. Further tests revealed the mother was an amalgam of two genomes—her blood didn’t match her children’s, but cells from her internal organs did.