Scientists from the New York Stem Cell Foundation Laboratory reported on Wednesday the first success in using the cloning technique that gave rise to Dolly the sheep to generate stem cells using adult human cells.
The advance brings the field of stem cell research closer to its ultimate goal: being able to treat conditions such as diabetes and spinal cord injury by replacing malfunctioning cells with healthy ones, made specially for each individual patient. The discovery also moves science closer to human cloning.
“Though not unexpected, it is nonetheless a landmark,” says Dr. George Daley, a leading stem-cell researcher and professor at Children’s Hospital Boston and Harvard Medical School, of the discovery.
Researchers used a variation of a technique known as somatic cell nuclear transfer (SCNT), which has been studied extensively in animal cells. SCNT involves replacing the genetic material of an egg cell with the DNA from a mature cell (a skin cell, for example). The egg is then stimulated to divide, and if it develops fully, produces a genetically identical clone of the animal from which the mature cell was taken.
Dolly was the first animal to be cloned this way, in 1996. She was followed by cats, dogs, mice and a slew of other animals. However, cloning isn’t what the researchers were after in the new study. SCNT can also be used to generate valuable embryonic stem cells, the mother cells that can develop into any of the more than 200 cell types in the body. Any stem cells made from a donated adult skin cell would share the same DNA as the donor, and, therefore, if these healthy cells were to be used to replace the donor’s damaged cells, they would not risk immune rejection.
In theory, that’s how it should work. But scientists have been trying for more than a decade to coax human eggs and skin cells to merge in the right way to generate stem cells — without success. The genetically altered egg has always stopped progressing after a few rounds of cell division, leading frustrated researchers to wonder whether the human egg was even capable of creating stem cells in this way.
The field has also weathered controversy along the way: South Korean scientist Woo Suk Hwang claimed he had created human stem cells using SCNT in 2004, but it turned out his work was flawed.
Now, the New York-based researchers report in Nature that they have come very close to making SCNT work with human cells. They did not perfect the technique, however; rather than fully replacing the egg’s genetic material with that of another cell, they had to combine DNA from both cells to make the process work. The researchers created two lines of stem cells: one using skin cells from a healthy adult, and another using skin cells from a patient with Type 1 diabetes.
Since the resulting stem cells contained excess chromosomes, they are rendered therapeutically useless, but the discovery is critical — it shows that the human egg does indeed have the ability to reprogram a mature skin cell back to an embryonic state.
Led by Dieter Egli, a senior research fellow at the New York Stem Cell Foundation (NYSCF), researchers began with 270 human egg cells, to which they added the DNA from skin cells of each of the donors. The researchers were able to coax 13 egg cells to develop into blastocysts, and from those, the two stem cell lines were extracted. The stem cells contained the complete set of chromosomes from the skin cell as well as the half-set from the egg (egg and sperm cells each have half the full complement of chromosomes, so that they can combine during fertilization to create a set of 23 pairs).
The key was in leaving the egg’s nucleus intact, Egli says, which suggests that there are certain factors in the egg’s nuclear DNA that are required for the creation of stem cells.
“There was a big question mark in the field whether this was possible,” Egli told reporters regarding the use of SCNT to clone human cells. “Now we need to find ways that allow us to remove the egg genome while still allowing development, and without interfering with those functions.”
For instance, it may be possible that using other types of mature cells, besides skin cells, may silence the egg’s genome. If so, any resulting stem cells would have the normal number of chromosomes. Or, it’s possible that scientists can isolate and harness the exact genes from the egg that are critical to reverting an adult cell back to an embryonic state, avoiding the need to combine cells’ genetic material.
Figuring that out will be important if the nuclear transfer technique has any chance of playing a role in stem cell therapy for disease, particularly because the prospect of collecting enough egg cells to make a viable amount of stem cells is dim. Indeed, many efforts to study SCNT in human cells have been scrapped simply due to a shortage of egg cells.
In the current study, the eggs came from donors in New York, which in 2009 became the first state to compensate women for donating eggs for research purposes. Although fertility clinics may pay women to donate eggs to help infertile couples have children, the morally knotty implications of paying for human tissues for research has stymied scientists’ efforts to collect women’s eggs and made studies like Egli’s challenging.
After much debate, the Empire State Stem Cell Board, which oversees New York’s $600 million stem cell research program, approved compensation for women who donate eggs for stem cell research in the same way donors are paid for providing ooctyes to fertility clinics. The eggs used in Egli’s study came from women who were recruited to donate to Columbia University Medical Center’s infertility program. The women were asked to choose between designating their eggs for use in the research study or for reproductive purposes; they were paid $8,000 regardless of their decision. “We did not have any difficulty recruiting donors for this project,” said Dr. Mark Sauer, a co-author of the paper and program director of assisted reproduction at the Center for Women’s Reproductive Care at Columbia.
Sauer notes also that compensation is critical to obtaining good quality eggs. Other programs that did not pay donors, including one started by the Harvard Stem Cell Institute in 2006, were forced to shut down when women failed to volunteer. “It’s quite clear that if donors are properly compensated, at a rate commensurate with those for donors [who give] for reproductive purposes, we would serve the need for research purposes,” says Sauer.
But even if scientists had enough eggs to perfect the nuclear transfer process, there’s no telling at this point whether the technique will prove to be the best way to regenerate tissues to treat disease. So far, the only two human stem cell trials that have been approved by the Food and Drug Administration involve embryonic stem cells obtained from excess IVF embryos. Although these cells are not immunologically matched to patients who are receiving them, as cells generated with nuclear transfer would be, they may still help treat the spinal cord injury and eye diseases for which they are being tested.
Another stem-cell technique that involves neither eggs nor embryos, but requires only skin cells that can be reverted back to an embryonic state, also holds promise as a source for patient-specific stem cells. But studies of these so-called induced pluripotent stem (iPS) cells have shown that while they are stem cells, they are genetically distinct from embryonic stem cells, and those differences may bar them from safe use in patients.
The new findings, however, may help to improve the iPS process and, in turn, the prospects of stem-cell therapy, Daley says. “Understanding how eggs reprogram may allow us to make iPS cells even better,” he says.
Daley, who co-directs a project at the Harvard Stem Cell Institute to establish a bank of iPS cell lines from patients with various diseases, believes that iPS cells may be the more efficient way to generate patient-specific stem cells. But he notes that advances in any area of stem-cell research have wide-ranging benefits for the field as a whole. “This study is not about taking us a step closer to therapy,” he says of the Nature paper. “Instead, the immediate value is this: we can now compare the fidelity of reprogramming by nuclear transfer against the [iPS] method. We need to compare and contrast the two different strategies to determine which gets us closest to the gold standard of embryonic stem cells.”
In other words, as research progresses, scientists can continue to compare the therapeutic viability of stem cells created in various ways — a pursuit that ultimately brings the potential for treatment within closer reach. “The long-term goal of all of this research is to derive cells for therapeutic purposes,” Egli says.