By GINA KOLATA
Published: November 21, 2007
Two teams of scientists are reporting today that they turned human skin cells into what appear to be embryonic stem cells without having to make or destroy an embryo — a feat that could quell the ethical debate troubling the field.
All they had to do, the scientists said, was add four genes. The genes reprogrammed the chromosomes of the skin cells, making the cells into blank slates that should be able to turn into any of the 220 cell types of the human body, be it heart, brain, blood or bone. Until now, the only way to get such human universal cells was to pluck them from a human embryo several days after fertilization, destroying the embryo in the process.
The reprogrammed skin cells may yet prove to have subtle differences from embryonic stem cells that come directly from human embryos, and the new method includes potentially risky steps, like introducing a cancer gene. But stem cell researchers say they are confident that it will not take long to perfect the method and that today’s drawbacks will prove to be temporary.
Researchers and ethicists not involved in the findings say the work should reshape the stem cell field. At some time in the near future, they said, today’s debate over whether it is morally acceptable to create and destroy human embryos to obtain stem cells should be moot.
“Everyone was waiting for this day to come,” said the Rev. Tadeiusz Pacholczyk, director of education at the National Catholic Bioethics Center. “You should have a solution here that will address the moral objections that have been percolating for years,” he added.
The two independent teams, from Japan and Wisconsin, note that their method also creates stem cells that genetically match the donor without having to resort to the controversial step of cloning. If stem cells are used to make replacement cells and tissues for patients, it would be invaluable to have genetically matched cells because they would not be rejected by the immune system. Even more important, scientists say, is that genetically matched cells from patients will enable them to study complex diseases, like Alzheimer’s, in the lab.
Until now, the only way to get embryonic stem cells that genetically matched an individual would be to create embryos that were clones of that person and extract their stem cells. Just last week, scientists in Oregon reported that they did this with monkeys, but the prospect of doing such experiments in humans has been ethically fraught.
But with the new method, human cloning for stem cell research, like the creation of human embryos to extract stem cells, may be unnecessary.
“It really is amazing,” said Dr. Leonard Zon, director of the stem cell program at Harvard Medical School’s Children’s Hospital.
And, said Dr. Douglas Melton, co-director of the Stem Cell Institute at Harvard University, it is “ethically uncomplicated.”
For all the hopes invested in it over the past decade, embryonic stem cell research has not yet produced any cures or major therapeutic discoveries. Stem cells are so malleable that they may pose risk of cancer, and the new method of obtaining stem cells includes steps that raise their own safety concerns.
Still, the new work could allow the field to vault significant problems, including the shortage of human embryonic stem cells and restrictions on federal funding for such research. Even when scientists have other sources of funding, they report that it is expensive and difficult to find women who will provide eggs for such research.
The new discovery is being published online today in Cell, in a paper by Shinya Yamanaka of Kyoto University and the Gladstone Institute for Cardiovascular Disease in San Francisco, and in Science, in a paper by James Thomson and his colleagues at the University of Wisconsin.
While both groups used just four genes to reprogram human skin cells, two of the four genes used by the Japanese scientists were different from two of the four used by the American group. All the genes in question, though, act in a similar way – they are master regulator genes whose role is to turn other genes on or off.
The reprogrammed cells, the scientists report, appear to behave exactly like human embryonic stem cells.
“By any means we test them they are the same as embryonic stem cells,” Dr. Thomson says.
He and Dr. Yamanaka caution, though, that they still must confirm that the reprogrammed human skin cells really are the same as stem cells they get from embryos. And while those studies are underway, Dr. Thomson and others say, it would be premature to abandon research with stem cells taken from human embryos.
Another caveat is that , so far, scientists use a type of virus, a retrovirus, to insert the genes into the cells’ chromosomes. Retroviruses slip genes into chromosomes at random, sometimes causing mutations that can make normal cells turn into cancers.
In addition, one of the genes that the Japanese scientists insert actually is a cancer gene.
The cancer risk means that the resulting stem cells would not be suitable for replacement cells or tissues for patients with diseases, like diabetes, in which their own cells die. They would, though, be ideal for the sort of studies that many researchers say are the real promise of this endeavor — studying the causes and treatments of complex diseases.
For example, researchers want to make embryonic stem cells from a person with a disease like Alzheimer’s and turn the stem cells into nerve cells in a petri dish. Then, scientists hope, they may be able to understand what goes awry in Alzheimer’s patients when their brain cells die and how to prevent or treat the disease.
But even the retrovirus drawback may be temporary, scientists say. Dr. Yamanaka and several other researchers are trying to get the same effect by adding chemicals or using more benign viruses to get the genes into cells. They say they are starting to see success.
It is only a matter of time until retroviruses are not needed, Dr. Melton predicted.
“Anyone who is going to suggest that this is just a side show and that it won’t work is wrong,” Dr. Melton said.
The new discovery was preceded by work in mice. Last year, Dr. Yamanaka published a paper showing that he could add four genes to mouse cells and turn them into mouse embryonic stem cells.
He even completed the ultimate test to show that the resulting stem cells could become any type of mouse cell. He used them to create new mice, whose every cell came from one of those stem cells. Twenty percent of those mice, though, developed cancer, illustrating the risk of using retroviruses and a cancer gene to make cells for replacement parts.
Scientists were electrified by the reprogramming discovery, Dr. Melton said. “Once it worked, I hit my forehead and said, ‘it’s so obvious,’ ”he said. “But it’s not obvious until it’s done.”
Some were skeptical about Dr. Yamanaka’s work and questioned whether such an approach would ever work in humans.
“They said, ‘That’s very good with mice. But let’s see if you can do it with a human,”’ Dr. Zon recalled.
But others set off in what became an international race to repeat the work with human cells.
“Dozens, if not hundreds of labs, have been attempting to do this,” said Dr. George Daley, associate director of the stem cell program at Children’s Hospital.
Few expected Dr. Yamanka would succeed so soon. Nor did they expect that the same four genes would reprogram human cells.
“This shows it’s not an esoteric thing that happened in the mouse,” said Rudolf Jaenisch, a stem cell researcher at M.I.T.
Ever since the birth of Dolly the sheep, scientists knew that adult cells could, in theory, turn into embryonic stem cells. But they had no idea how to do it without cloning, the way Dolly was created.
With cloning, researchers put an adult cell’s chromosomes into an unfertilized egg whose genetic material was removed. The egg, by some mysterious process, then does all the work. It reprograms the adult cell’s chromosomes, bringing them back to the state they were in just after the egg was fertilized. Those reprogrammed genes then direct the development of an embryo. A few days later, a ball of stem cells emerges in the embryo. Since the embryo’s chromosomes came from the adult cell, every cell of the embryo, including its stem cells, are exact genetic matches of the adult.
The abiding question, though, was, How did the egg reprogram the adult cell’s chromosomes? Would it be possible to reprogram an adult cell without using an egg?
About four years ago, Dr. Yamanaka and Dr. Thomson independently hit upon the same idea. They would search for genes that are being used in an embryonic stem cell that are not being used in an adult cell. Then they would see if those genes would reprogram an adult cell.
Dr. Yamanaka worked with mouse cells and Dr. Thomson worked with human cells from foreskins.
The researchers found more than 1,000 candidate genes. So both groups took educated guesses, trying to whittle down the genes to the few dozen they thought might be the crucial ones and then asking whether any combinations of those genes could turn a skin cell into a stem cell.
It was laborious work, with no guarantee of a payoff.
“The number of factors could have been one or ten or 100 or more,” Dr. Yamanaka said in a telephone interview from his lab in Japan.
If many genes were required, the experiments would have failed, Dr. Thomson said, because it would be impossible to test all the gene combinations.
The mouse work went more quickly than Dr. Thomson’s work with human cells. As soon as Dr. Yamanaka saw that the mouse experiments succeeded, he began trying the same brute force method in human skin cells that he ordered from a commercial laboratory. Some were face cells from a 36 year old white woman and others were connective tissue cells from joints of a 69 year old white man.
Dr. Yamanaka said he thought it would take a few years to find the right genes and the right conditions to make the human experiments work. Feeling the hot breath of competitors on his neck, he was in his lab every day for 12 to 14 hours a day, he said.
A few months later, he succeeded.
“We did work very hard,” Dr. Yamanaka said. “But we were very surprised.”