Last winter, two female Komodo dragons at separate zoos in England gave their keepers big surprises. With no contact from any male, each of the giant lizards laid a clutch of viable eggs, some of which hatched healthy young (SN: 12/23/06, p. 403). The events made the news because they were the first known examples of the species reproducing by the asexual process of parthenogenesis, or virgin birth.
Without fertilization by sperm, the animals’ eggs had begun dividing. In these surprising cases, the process continued and cute little komodo dragons emerged. Viable young can also result from parthenogenesis in various species of reptiles, plants, insects, fish, and birds.
Mammals normally can’t reproduce by parthenogenesis, but that very fact is making the process interesting in a different way: It suggests a possible solution to the moral issues surrounding embryonic stem cell research. Even if an unfertilized human egg is tricked into beginning to grow, most scientists say that it will lack the capacity to produce a viable pregnancy. Yet the entity could contain stem cells with the ability to develop into nerve cells, heart cells, or any other kind of cell in the body.
Stem cells hold tremendous potential for medicine and basic biological research. But acquiring these primordial cells usually involves extracting them from an embryo that has developed into a blastocyst, a hollow sphere of about 100 cells. The extraction generally destroys the blastocyst, so opponents of embryonic stem cell research say that it also destroys a potential human life.
But what if an embryo never had the potential to develop into a human being? Because parthenogenesis can’t lead to a live birth in people, some researchers argue that an early embryo created by parthenogenesis is merely a ball of cells. Removing embryonic stem cells from such a ball doesn’t pose an ethical dilemma, they say. “It’s a morally unproblematic way” of getting stem cells, says Michael West, chief scientific officer for Advanced Cell Technology of Alameda, Calif.
Recent years have seen swift progress in parthenogenesis. In 1994, Nicholas D. Allen and his colleagues at the Babraham Institute in Cambridge, England, succeeded in creating parthenogentic stem cells from unfertilized mouse eggs. In 2002, Jose Cibelli of Advanced Cell Technology and his colleagues created them from egg cells taken from macaque monkeys.
In the June Cloning Stem Cells, a team of Russian scientists working for Walkersville, Md.–based Lifeline Cell Technology announced the first deliberate creation of parthenogenetic stem cells from human eggs. The same milestone had been achieved accidentally by now-discredited Korean researcher Woo Suk Hwang in 2004, but Hwang claimed that he had created stem cells through cloning rather than parthenogenesis (SN: 8/4/07, p. 69).
But creating human parthenogenetic stem cells is just the first step toward the possibility of using them in medical therapies. The abnormal genetic makeup of these cells—with duplicated DNA from the woman whose egg is used—poses significant scientific hurdles.
Even if scientists can overcome these obstacles, could both women and men benefit from therapies using parthenogenetic cells? Doctors could use an egg from a woman with, say, Parkinson’s disease to produce stem cells that the woman’s body wouldn’t reject, since the cells would share her genetic makeup. But that’s not possible for a man, since parthenogenesis can’t start with sperm.
It’s a strange relic of evolution that human egg cells can undergo parthenogenesis at all.
Animals capable of reproducing by parthenogenesis store an extra set of chromosomes in a pouch called a polar body, in case a sperm doesn’t show up and deliver its DNA. Although people can’t naturally reproduce this way, a woman’s egg also retains an extra set of chromosomes in a polar body until fertilization.
Scientists can jolt the egg into reclaiming the DNA in the polar body. The cell may then begin to develop without being fertilized.
Once the egg has developed into a blastocyst, scientists can extract stem cells to use for research or for stem cell therapies. These cells, however, are far from normal because of abnormalities in a process called imprinting, which normally turns off certain genes in mammals. Chromosomes inherited from the mother will have one pattern of imprinted genes, and the paternal chromosomes will have another. Many imprinted genes are involved in fetal development, so mammal embryos need both patterns in order to develop normally.
Abnormal imprinting is what prevents a mammalian egg activated by parthenogenesis from developing much beyond the blastocyst stage. In such an egg, both members of each pair of chromosomes would have maternal imprinting, resulting in an abnormal pattern of turned-off genes.
When used medically, however, stem cells created via parthenogenesis wouldn’t need to go through the complex baby-building process. Scientists could steer the cells directly into becoming adult heart-muscle cells to treat a heart attack victim or into nerve cells for a person with Parkinson’s disease. The cells wouldn’t need to use their fetal-development genes, explains George Q. Daley of Children’s Hospital Boston.
Although parthenogenetic stem cells aren’t exactly normal, “maybe many cell types would be normal enough to be clinically useful,” West says. Indeed, some evidence points in this direction. The year after Cibelli’s group created parthenogenetic stem cells from macaque eggs, the team steered these cells into becoming working nerve, heart, and fat cells. “The cells seemed very normal,” reports West, who was on the team. “You couldn’t tell any difference.”
More recently, researchers at the University of Pennsylvania in Philadelphia replaced mice’s bone marrow with blood-forming cells derived from parthenogenetic stem cells. Despite the abnormal imprinting, the cells established new bone marrow that functioned normally for at least 4 months, Kenneth J. McLaughlin and his colleagues reported in the Feb. 15 Genes and Development.
A mother’s touch
While experiments have shown that abnormal imprinting might not be a deal breaker, more research is needed to address concerns raised by parthenogenetic cells’ other genetic abnormality: the fact that both members of each pair of chromosomes are inherited from one parent.
A cell whose paired chromosomes are identical is called homozygous. “A lot of bad things can happen if the cell line is homozygous,” says Jeanne F. Loring, a stem cell researcher at the Burnham Institute for Medical Research in La Jolla, Calif. For example, having different paternal and maternal versions of many cancer-related genes prevents either set of genes from having too much influence on whether a cell becomes cancerous. If a gene-copying error results in a cell with two identical copies of the gene, it can upset this balance and lead the cell toward becoming a malignant growth.
In the real world, however, imperfections in egg creation prevent the chromosome pairs from being perfectly identical. In fact, the parthenogenetic stem cells made accidentally by Hwang are only about 40 percent homozygous, Daley and his colleagues report in the September Cell Stem Cell. For the other 60 percent of the genome, corresponding pairs of chromosomes differ from each other as much as they would if they had been inherited from two parents.
The reason for this odd fact lies in how an egg is formed. A cell that will become an egg begins with pairs of each chromosome, one from the woman’s mother and one from her father. First, that cell duplicates these chromosomes, resulting in two maternal and two paternal copies. Then, the chromosomes line up across the center of the cell in preparation for the cell to divide. There, the ends of tube-shaped bundles of maternal DNA can mingle, intertwine, and even swap pieces with the paternal copies.
When the cell divides in half, the somewhat altered chromosomes of each pair part ways. One of these chromosomes—perhaps the maternal one—ends up in the cell that splits again to become the egg and polar body. Because this chromosome had been duplicated at the outset, the egg and polar body each end up with a copy of the maternal chromosome. But because of the random swapping with the paternal DNA, these chromosomes won’t be identical. As a result, in parthenogenetic stem cells, the two chromosomes in any given pair will differ from each other wherever those swaps occurred, thus reducing the cells’ homozygosity.
Scientists might be able to control the lingering cancer risk from homozygosity by simply checking whether any newly created stem cells have matching copies of known cancer-related genes. Doctors could discard any cells that pose too much of a risk. “In all such techniques you need quality control,” says West.
For certain genes, homozygosity might actually be a good thing, because it could extend the benefits of parthenogenetic stem cells to men and postmenopausal women who no longer ovulate. The main reason for using a person’s own cells to make stem cells for medical therapies is to prevent the person’s immune system from rejecting the implanted tissues. As with organ transplants, however, a close match is often good enough. Finding such a match would be easier with parthenogenetic stem cells than with normal cells. That’s because normal cells have two versions of a set of genes called the major histocompatibility complex, which determines whether the cells would be accepted or rejected, whereas the stem cells would often have two identical sets. This greatly reduces the number of possible combinations of these genes, making it easier to find an acceptable match.
“You could find a credible match, maybe not a perfect match, for a lot of people,” Daley says.
Some people and institutions staunchly opposed to making embryonic stem cells by other means remain open to the possibility of making them by parthenogenesis. The Roman Catholic Church, for example, has taken a firm stance against stem cell research that destroys embryos made by cloning and by in vitro fertilization. However, it has adopted a wait-and-see approach to parthenogenetic stem cells, says Tadeusz Pacholczyk, a neurologist and priest at the National Catholic Bioethics Center in Philadelphia.
The Catholic Church probably won’t issue an official decision until scientific research establishes whether such blastocysts are impotent balls of cells or viable human embryos that happen to be defective, Pacholczyk says. “Until we have really clear and convincing evidence whether parthenogenesis makes a true human embryo, the church is not going to step into these waters.”
Kevin FitzGerald, a Jesuit priest and geneticist at Georgetown University in Washington, D.C., agrees that the moral issue revolves around gaining a better understanding of what is produced when scientists cause a human egg to undergo parthenogenesis. “Since you don’t naturally get parthenogenetic offspring in mammals, if you get some sort of [spontaneous] parthenogenetic growth, then yeah, that’s not an embryo,” FitzGerald says. “But if you induce it yourself, what are you creating?” The laboratory techniques that scientists use to trigger parthenogenesis affect egg cells in ways that are still poorly understood. While natural parthenogenesis doesn’t produce viable embryos in mammals, more research is needed to show whether artificially activated eggs might sometimes be viable.
When President George W. Bush vetoed a bill in June that would have allowed federal funding for research on newly created embryonic stem cells, he specifically mentioned parthenogenesis as a method that should not receive federal money. However, the president’s Council on Bioethics has given a conditional nod to parthenogenesis as a possible ethical work-around. In a 2005 report, the council wrote that the blastocyst-like ball of cells created by parthenogenesis “is assumed by most commentators to lack entirely the potential for development as a human being, and is therefore, arguably, not really an embryo.”
The council stopped short of recommending the technique, saying that more evidence is needed to show that such a ball of cells can’t sometimes develop like an embryo.
So if further research makes it clear that artificially induced parthenogenesis can’t produce a viable human embryo, the technique might find acceptance among political and religious leaders who have thus far rejected other kinds of embryonic stem cell research. In its report, the president’s council concluded, “Those who are convinced that parthenogenetic embryos have no chance of development beyond the blastocyst stage are likely to have few ethical objections.”