South Korean researcher Woo Suk Hwang caused a scandal in 2005 by falsifying data about his attempts to make the first embryonic stem cells from cloned human embryos. However, new research shows that Hwang’s team accidentally made stem cells by another method that some scientists believe could be as important as cloning.
The stem cells produced in Hwang’s lab came from an embryo that grew from an unfertilized egg, not a clone, according to a new genetic analysis of the cells.
Some species can reproduce without fertilization by triggering an egg cell to develop into an embryo on its own, a process called parthenogenesis. A human embryo made this way can’t develop into a fetus, so some scientists and bioethicists believe that parthenogenesis could sidestep the moral issues involved in harvesting stem cells from viable embryos.
“It’s an unfortunate irony, because had [Hwang’s team] realized they had made parthenogenetic cells, that in itself would have been an interesting and important achievement,” says George Q. Daley of the Harvard Stem Cell Institute in Boston, who led the team that performed the new analysis.
Examining parthenogenetic stem cells from mouse embryos, Daley and his colleagues found that paired chromosomes were too similar to each other to have come from a cloned embryo. Certain harmless mutations in DNA are normally different in the two chromosomes that pair up after fertilization, because one chromosome comes from the mother and the other from the father.
In the parthenogenetic mouse stem cells, however, these mutation patterns were identical for long stretches of each pair of chromosomes, showing that both members of each pair had come from an egg. Even parts of chromosomes that weren’t identical, which were mostly at their ends, were consistent with a parthenogenetic scenario.
Daley’s group then examined the mutation patterns in the human stem cells produced by Hwang, and found the same fingerprint of parthenogenesis observed in the mouse cells, the group reports online and in an upcoming Cell Stem Cell.
The team’s method is “exactly the right way to ask whether these cells come from a duplicated genome or a [cloned] genome,” comments Jeanne F. Loring, a stem cell researcher at the Burnham Institute for Medical Research in La Jolla, Calif.
The mistake made by Hwang’s group could be due to a similarity in the laboratory procedures for cloning and for parthenogenesis. In both cases, researchers apply the same chemical shock to trick an egg cell into acting as if it had been fertilized by a sperm. If Hwang’s team had failed to remove an egg’s original chromosomes before shocking it, the scientists would have created a parthenogenetic embryo instead of a cloned one.