Members of the Senate are expected to introduce legislation in the coming days to fund alternative methods of obtaining embryonic-type stem cells that do not require the creation or destruction of human embryos. The Senate is expected to vote on the bill in the very near future, along with a bill to establish a national cord-blood banking system and a bill to permit federal funding for research using stem cells derived from IVF embryos that are destroyed in the process.
Interest in alternative methods has been building among scientists and politicians. According to a July 8, 2005 article in Science entitled "Embryo-Free Techniques Gain Momentum" by Gretchen Vogel, "Ethical concerns about research involving embryos have been driving the search for other ways to derive stem cells, and results may soon be on the horizon." Given President Bush's promise to veto legislation that would allow federal funding for research involving stem cell lines produced after August 9, 2001, such alternative methods are the only way that scientists are likely to obtain new pluripotent stem cell lines for federally funded research during the next three-and-a-half years.
As bioethicist Leon Kass, the chairman of the President's Council on Bioethics, put it, these alternative methods might provide "a technological solution to [the] moral dilemma" of embryo-destroying stem cell research. "Senators will be given a chance ... to enact legislation to increase funding for alternative sources," he wrote. "They should not miss this timely and most promising opportunity for scientific and ethical statesmanship" ("A Way Forward on Stem Cells," Washington Post, July 12, 2005).
A bill to fund non-destructive methods of obtaining embryonic-type stem cells (H.R. 3144) was introduced into the House of Representatives by Congressman Roscoe Bartlett of Maryland on June 30, 2005. The bill would mandate spending $15 million on research in animals to "develop and test techniques for deriving cells from embryos without doing harm to those embryos," "to develop and test techniques for producing human pluripotent stem cells without creating or making use of embryos," and "to isolate, develop and test pluripotent stem cells from postnatal tissues, umbilical cord blood, and placenta."
Four possible methods have been proposed for obtaining "pluripotent" stem cells without destroying or endangering human embryos: (1) Reprogramming the nuclei of adult cells back to pluripotency by fusing them with existing embryonic stem cells or with factors extracted from human oocytes; (2) extracting cells from embryo-like but non-embryonic cellular systems (engineered to lack the essential elements of embryogenesis but still capable of some cell division and growth and still capable of producing pluripotent stem cells); (3) extracting cells from IVF embryos that are already dead; and (4) non-harmful biopsy of living embryos.
The President's Council on Bioethics discussed these methods in a White Paper published in May 2005 entitled Alternative Sources of Human Pluripotent Stem Cells.
1. Reprogramming adult body cells into a pluripotent state
The first of these methods involves reprogramming adult cells so as to "dedifferentiate" them back into pluripotent stem cells. (See Proposal IV in the Council's report.) All cells in a person's body have the same complement of genes, but they become skin, nerve, heart muscle or other kinds of specialized cell depending on which genes in the cell are turned on or off. Scientists are now learning how to change the genetic expression inside specialized adult cells in order to make them pluripotent again, and thus capable of producing all the different cell types of the body.
One potential way of reprogramming somatic cells is to fuse them with existing embryonic stem cells, so that the chemical factors present in the embryonic stem cells restore the somatic cells to pluripotency. Another approach to reprogramming adult cells into a pluripotent state is to use factors extracted from oocytes as the reprogramming tool. Scientists believe that the chemical factors in oocytes may contain the key to reprogramming, and that one can genetically alter somatic cells so that the product is, from the beginning, a pluripotent stem cell, not a totipotent zygote or embryo.
This method would allow scientists to produce pluripotent stem cell lines with the genetic make-up of the original somatic cell. It would enable them to build models of various diseases and produce genetically matched cells for possible immune-free transplantation into patients. It thus gives us all the hypothetical benefits of research or "therapeutic" cloning without creating and destroying cloned embryos.
On June 24, 2005 at the third annual meeting of the International Society for Stem Cell Research in San Francisco, Kevin Eggan of the Harvard Stem Cell Institute announced that his lab has successfully fused a human embryonic stem cell to an adult skin cell. "Eggan showed that the embryonic stem cell 'reprogrammed' the skin cell's nucleus, causing the skin cell to start behaving like a youthful, embryonic stem cell," according to a June 27, 2005 article by Erika Check at news@nature.com. As described in the article, Eggan reports that "the fused cells stopped expressing the genes that characterized the adult cell ... and instead expressed the same genes as the embryonic stem cells. They could also transform themselves into the major cell types of the body, such as neurons, hair follicles and muscle cells."
According to Science writer Gretchen Vogel,
Scientists have known for several years that ES cells can fuse with somatic cells to produce stem cell-like hybrids (Science, 15 March 2002, p. 1989), and previous studies had shown that several key tissue-specific genes turned off in the fused nucleus while key embryonic genes turned on. But Eggan and Chad Cowan of Doug Melton's group at Harvard went a step further, using gene expression arrays for detailed analyses of the hybrid cell lines. The cells, they found, had an almost identical expression profile to that of normal ES cells and one very different from that of fibroblast cells. "There is no longer transcription of fibroblast genes," Eggan says, "and there aren't any deficits of ES cell genes." Apparently, he says, "the ES cell nucleus can win the battle" between the two sets of chromosomes. ["Embryo-Free Techniques Gain Momentum," Science, July 8, 2005]
The hybrid cells produced by Eggan cannot yet be used for transplantation because they contain the DNA of both the somatic cell and the stem cell, twice the amount found in normal cells. Some scientists at the meeting in San Francisco, such as Rudolf Jaenisch, a biologist at the Massachusetts Institute of Technology, cautioned that removing the extra DNA might not be easy to do.
Washington Post science writer Rick Weiss reports that similar experiments involving the fusion of mouse skin and stem cells are being undertaken by Alan Trounson of Monash University in Australia. According to a June 6, 2005 article, Trounson has reported that he has developed a simple way to remove the stem cell's DNA after the skin cell's genes have been reprogrammed. In addition, Yuri Verlinsky of the Reproductive Genetics Institute in Chicago reported at a meeting last month that he has succeeded in making new human embryonic stem cells by first removing a stem cell's DNA before fusing it with it with a human skin cell. However, he says he has not yet finished the tests that would show whether or not these new cells can function the same way that embryonic stem cells do. ("Stem Cell Advances May Make Moral Issue Moot," Washington Post, June 6, 2005)
2. Engineering non-embryonic biological artifacts that can produce pluripotent stem cells
The second approach that has been widely discussed is engineering "biological artifacts" that have some of the developmental capacities of natural embryogenesis (but without the organismal character of human embryos) and that contain cells from which pluripotent stem cell lines can be derived. (See Proposal III in the Council's report.) The fact that embryonic stem cells can be produced within a cellular system that is not an organism is evident in naturally occurring ovarian tumors called teratomas. Teratomas can produce all the primary embryonic cell types and even more advanced tissue and body parts, but no physician would consider a teratoma a living being. Embryonic stem cells likewise lack the capacity to form an organism (they produce teratomas when implanted into the human body), so they too are not embryos.
One proposal that adopts this approach is called "altered nuclear transfer" or ANT, and has been put forward by William Hurlbut, a physician, bioethicist, and member of the President's Council on Bioethcs. As with somatic cell nuclear transfer, or cloning, in altered nuclear transfer, scientists would take the nucleus of an adult or somatic cell and place it in an egg cell or ooctye whose own nucleus has been removed. But first, unlike cloning, they would alter either the somatic cell nucleus or the egg before transfer to prevent the integrated unity that is essential for embryogenesis, creating a "nonembryonic entity."
Hurlbut's proposal is a broad concept with a range of possible approaches, but one specific variation under discussion has raised concerns among certain bioethicists that it would create a disabled embryo rather than a "nonembryonic entity." The Council has recommended that this proposal be tested in animal cells first to demonstrate that the developing entity that is produced is truly not a human embryo and that the cells derived from it are normal human pluripotent cells.
One recently proposed variation of ANT, however, has found wider acceptance among conservative bioethicists and religious leaders. Called oocyte assisted reprogramming (OAR), the method was recently proposed by Markus Grompe, director of the Oregon Stem Cell Center and a board member of the International Society for Stem Cell Research. It combines basic cloning technology with "epigenetic reprogramming." The method was described in an op-ed in the June 20, 2005 Wall Street Journal by Grompe and Robert George, a member of the President's Council on Bioethics.
In their op-ed, Grompe and George write: "In cloning, the nucleus of a somatic cell (such as a skin cell) is transferred to an egg cell whose nucleus has been removed. An electrical stimulus is administered in a way that, if all goes as planned, triggers the development of a new and distinct organism, an embryo, that is virtually identical in its genetic constitution to the organism from which the somatic cell was taken. In OAR, however, the somatic cell nucleus or the egg cytoplasm or both would first be altered before the nucleus is transferred. The modifications would change the expression of certain 'master genes' -- transcription factors that control expression of many other genes by switching them on or off.
"These genetic alterations would permit the egg to reprogram the somatic cell nucleus directly to a pluripotent, but not a totipotent (i.e., embryonic) state. The altered expression of the powerful control gene would ensure that the characteristics of the newly produced cell are immediately different from, and incompatible with, those of an embryo. For optimal reprogramming, master genes known to control the pluripotency of embryonic stem cells would be used, for example the transcription factor known as 'nanog.' Thus, we would reasonably expect to obtain precisely the type of stem cells desired by advocates of embryonic stem-cell research, without ever creating or killing embryos."
Some preliminary studies related to OAR are already underway. Because OAR would use only existing biotechnologies, Grompe believes that it can be tested in animals within a year, and tested in humans within another year. (A more detailed description of OAR with a list of signatories that support its trial in animals that includes bioethicists, religious leaders and scientists can be found online here.)
One moral objection that could be raised to OAR is that it requires the use of human eggs. Some object to the use of human eggs for research because they are currently obtained by superovulating women, a procedure that is often painful and sometimes dangerous. The proponents of OAR, including Hurlbut, oppose the idea of intentional superovulation to obtain eggs for experimentation because it is medically risky and the long term effects on women are not well known. However, there may eventually be abundant sources of eggs from alternative sources. Recent studies suggest that nuclear transfer can be successfully accomplished with immature eggs. There are many immature eggs left over from IVF procedures, and eggs might also be obtained from laboratory stimulation of ovarian tissues obtained during surgical removal of the ovaries for therapeutic reasons. Slices of ovarian tissue from mice have been cultured in vitro and yield a large number of eggs. This is proving more difficult with human ovarian tissue, but there are signs of progress. In the meantime, eggs might also be donated by women whose ovaries are surgically removed or they might be taken from cadavers.
Like the first method, ANT and OAR would allow scientists to create stem cells with a full range of genetic types, and it would allow scientists to control the genetic make-up of each stem cell line with precision.
3. Extracting cells from IVF embryos that are already dead
This method seeks to derive pluripotent stem cells "from early IVF embryos (roughly 4-8 cells) that have spontaneously died (as evidenced by the irreversible cessation of cell division) but some of whose blastomeres appear normal and healthy." (See Proposal I in the Council's report.) Before this approach can be carried out, scientists will have to develop non-invasive, non-harmful tests for deciding embryo death, just as we decide death for developed human beings. To ensure that this procedure is ethical, Drs. Donald Landry and Howard Zucker of Columbia University College of Physicians and Surgeons have proposed that "only those once-frozen embryos that are thawed and that die spontaneously during efforts to produce a child will be eligible for post-mortem extraction."
The drawbacks of this proposal are, first, that it would not allow scientists to control the genomes of the stem cell lines produced, as they could with the previous two proposals. Secondly, it would require significant safeguards and scrupulosity to ensure that only those embryos that die on their own are used, rather than using those spares that "are going to die anyway" but are not really dead. While the proposal has not yet been tested, even in animals, the Council's report concluded that studies could be undertaken immediately in an ethical manner not only in animals but in humans to determine whether "reliable objective criteria for determining death of IVF embryos can be developed."
4. Non-harmful biopsy of living embryos
According to this proposal, pluripotent stem cells could be derived from blastomeres obtained by biopsy of an early human embryo. (See Proposal II in the Council's report.) A "blastomere" is a cell contained within an early embryo up to two days after conception; an embryo at that stage is made up of at most eight blastomeres. Single blastomeres are already being removed from in-vitro embryos for genetic testing before implantation. However, the Council found this proposal to be ethically unacceptable in humans because of the risks imposed "by embryo biopsy and blastomere removal on a born child the embryo might become, solely for research of no benefit to him or her." The Council saw no ethical reason why the approach could not be tried in animals, but did not see "how results from animal experimentation could alter this assessment of ethical propriety in humans."
