As Korean cloners grab the limelight, a Chicago team is claiming an easier route to embryonic stem cells
WITH great fanfare a South Korean team announced last week that it had used therapeutic cloning to create human embryonic stem cells that were genetically matched to specific people. But their technique could already be obsolete. Another team headed by Yuri Verlinsky of the Reproductive Genetics Institute, based in Chicago, claims to have produced patient-matched embryonic stem cells without resorting to therapeutic cloning. "It would be a much better approach," he says.
Verlinsky says he has already created 10 embryonic stem cell (ESC) lines using his new "stembrid" technique. Unlike therapeutic cloning, it uses existing ESCs instead of human eggs, and so would be much cheaper and easier. What's more, because no embryos are destroyed, it would bypass many ethical issues. "If it is as he said, it would be spectacular," says Jacques Cohen of Tyho-Galileo Research Laboratories, who has been investigating alternative sources of ESCs. But he and other experts say Verlinsky must do a lot more work to prove his claim.
While ESCs show great promise for treating many diseases, such as heart disease, diabetes and Parkinson's, non-matching ESCs would be rejected by patients' immune systems unless they take immunosuppressant drugs. This is why many stem cell researchers are trying to create ESCs that are identical to people's own cells.
Last year Woo Suk Hwang of Seoul National University claimed to have created an ESC line from a cloned human embryo for the first time. But there were doubts about the results. The technique requires an adult cell and an egg, and because in Hwang's experiment both came from the same person, it was difficult to prove the embryo really was cloned (New Scientist, 21 February 2004, p 6). The process was also very inefficient, taking 242 eggs to create just one ESC line.
Now Hwang has created 11 more ESC lines from cloned embryos in an impressive study that answers all the criticisms of his original study. He has also greatly increased the efficiency of the process: the 11 lines came from just 185 fresh eggs donated by 18 unpaid volunteers, meaning an average of only 17 eggs was needed per ESC line (Science, DOI: 10.1126/science.1112286).
The donor "adult" cells came from patients aged 2 to 56, with a variety of conditions ranging from spinal injuries to an inherited immune condition. The work proves that matching ESCs can be derived via therapeutic cloning from donors of any age and sex.
"This is an enormous stride in the long journey to determine whether nuclear-transfer-derived human embryonic stem cells might be eventually suitable for transplantation medicine," says Gerald Schatten of the University of Pittsburgh School of Medicine, the only non-Korean member of the team.
But if Verlinsky's claims stand up, there might be a much easier way to create matching ESCs. His method starts with an existing human ESC line. The nuclei containing the DNA of the ESCs are removed by spinning them in a centrifuge. Verlinsky would not reveal the details of his technique, but other teams glue cells to a surface and spin them until the denser nucleus is forced out.
Then the team take cells from adults and fuse them with the enucleated ESCs. The idea is that the cytoplasm of an enucleated ESC will reprogram the donor nuclei, turning the fused cell into an ESC too. The result, Verlinsky told a conference on pre-implantation genetics in London last week, is new lines of ESCs that are genetically identical to the adult donor. He calls the new cells stembrids.
His claim stunned the audience. How, asked stem cell expert Alan Trounson from Monash University in Australia, could he be sure that the stembrids contain only the genetic material of the adult donors? Verlinsky's response was that he fused male adult cells with a female ESC line, and the resulting cells were male. However, Verlinsky's team has not yet tested the HLA proteins on the cell surface to prove that the stembrids are an immune match for the adult donors. By contrast, Hwang's team has shown that the ESCs derived via therapeutic cloning have the surface markers that make them compatible with the donor's immune system.
The biggest question, though, is whether the stembrids really are true ESCs. Verlinsky says the cells express a number of ESC markers. His team has also shown the cells can differentiate into a number of cell types, including heart muscle cells, neurons and blood stem cells, although the results have not yet been published.
Trounson, whose team has been experimenting with a similar approach, says far more detailed studies will be needed. Besides looking at further cell surface markers, Verlinsky's team must show that, like true ESCs, the stembrids are capable of forming cancers called teratomas, which contain a mixture of different cell types, when injected into immune-compromised mice, Trounson says. "Until they have done these studies, we must remain very sceptical."
Delegates at the meeting were dismayed that Verlinsky has applied for a patent on the stembrid method (US 2004/0259249). But he insists that he does not intend to stop other researchers using the method. "I have not done it so I can charge you, I have done it so no one else can charge me," he assured them.
Verlinsky's method would have huge advantages if it really does work. Obtaining the large numbers of fresh human eggs needed for therapeutic cloning is not possible for legal reasons in many countries and is very expensive in countries where it is legal, such as the US. By contrast, there is a virtually limitless supply of existing ESCs for fusion with adult cells.
Then there is the ethical issue, which is again exercising Congress. With Verlinsky's method, federally funded researchers could try to derive stembrids using already-approved ESC lines.
Verlinsky still has a lot to do to convince others of his claims, however. Marilyn Monk, an embryologist at the Institute of Child Health in London, summed up the general feeling afterwards: "It just seems too easy."
Source: Eurekalert & othersLast reviewed: By John M. Grohol, Psy.D. on 21 Feb 2009
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