In a separate analysis, they also found that three regions lacked the chemical modifications, or imprinting, that paternal genes impose on a fertilized embryo to prevent those genes from being activated.
Jeffrey Janus, president and director of research for ISC, agrees that "Dr. Hwang's cells have characteristics found in parthenogenetic cells" but remains cautious, saying "it needs more study."
The Irony of It All
Stem cell experts say that Hwang and his team probably had no clue what they had achieved, because if they had they would have claimed credit for it.
"I think this is every bit as exciting as the SCNT they were claiming," says stem cell researcher Kent Vrana of Pennsylvania State University, who pioneered parthenogenesis in monkeys. "Parthenotes by their very nature are nonviable embryos, so you're not destroying embryos, which has some ethical advantages."
Vrana says the Korean team used a procedure common in attempts to induce parthenogenesis and SCNT alike, in which they injected egg cells with calcium and a protein synthesis inhibitor to mimic what happens when sperm fertilizes an egg.
To achieve SCNT, they first had to extract each egg's DNA and then inject the donor cell nucleus. Daley says the Korean scientists must have inadvertently left the DNA intact in one of the 242 egg cells they injected. "They claimed they verified the removal of the DNA,'' Daley says, "but obviously they didn't."
The injection of the donor nucleus could have failed if the injecting needle pulled it back out when withdrawn from the egg or if the egg somehow rejected the introduced nucleus, Vrana says.
Hwang's group purported to rule out parthenogenesis as an explanation in part by showing that two genes normally activated by paternal DNA were inactive in the cells. But Daley says such experiments are easy to misinterpret and are less conclusive than sequencing SNPs.
"I think they were just so blinded by what they hoped to accomplish, they missed it," Vrana says.
As a result, in late June, more than a year after Science retracted the 2004 paper, researchers at ISC were able to claim the discovery of human parthenogenetic cell lines as their own in the journal Cloning and Stem Cells. The group reported growing multiple parthenogenetic embryonic stem cell lines by incubating eggs in a warm, low-oxygen culture medium.
Before today's announcement, the work was already "awfully convincing," Vrana says, and surprisingly successful: out of some 50 donated eggs, the company grew six cell lines. Parthenogenesis in monkeys typically works only once every 90 eggs, he says.
Banking on Parthenotes
The therapeutic potential of parthenogenetic cells remains to be seen. The lack of imprinting from the paternal DNA may cause the cells to behave abnormally as they develop. Furthermore, they must have matching immune proteins to be transplanted back into a donor.
In principle, tissue banks of parthenogenetic cell lines could include enough different immune protein combinations to treat up to half of the U.S. population—men as well as women—Lanza says. But he adds that if human parthenotes routinely contain as many genetic mismatches as the Korean cells, the number of eggs needed to create such a bank could be prohibitively large.
Daley says his group hopes to acquire donated eggs from women with inherited diseases and use parthenogenesis to create cell lines to study those disorders. In the future, researchers will have to determine whether similar cells are safe and effective when transplanted.
"We're a long, long way," Daley says, "from realizing therapeutic uses of these cells."