Selected annotated bibliography on stem cell research and cloning.

Rosenthal N. Prometheus's vulture and the stem-cell promise. N Engl J Med 2003;349:267-274.

More questions than answers exist surrounding the powers and potential of stem cells. A great deal of uncertainty abounds about both embryonic and adult stem cells, much of which has been furthered by a flurry of recent publications and a loose set of definitions. Rigorous definitions in stem cell research are necessary; a true stem cell must satisfy the following criteria:

1. It must be clonogenic, capable of unlimited self-renewal by symmetric division.

2. It must be able to divide asymmetrically, one daughter resembling its mother, the other daughter giving rise to multiple types of differentiated cells representing all three primitive embryonic germ layers (the ectoderm, mesoderm, and endoderm).

3. It must originate from an embryonic or adult stem-cell source.

Furthermore, while there is much hope for the properties of stem cells, a great deal of uncertainty still surrounds the properties of human stem cells, and a great number of obstacles remain unresolved. These obstacles include:

1. Although mammalian embryonic stem cells derive from the inner cell mass of the preimplantation embryo, it is not known whether these same cells would proceed to exhibit the same pluripotent properties that they exhibit in vitro.

2. There is no certainty that human stem cells possess the same potential as the stem cells of mice, from which most of the experimental data is derived.

3. While human embryonic stem cells appear capable of unlimited proliferation in vitro, it is still a matter of debate whether prolonged culturing affects their pluripotency.

There are also several matters of speculation which appear to be emerging from the recent literature. For instance, the remarkable plasticity and pluripotency of bone marrow cells raises the possibility of stem cells circulating throughout the body and becoming activated wherever needed. Also, it appears that injury may be a prerequisite for such circulating stem cells participating in differentiated tissue structure and function. While these circulating stem cells appear to be relatively few, it also appears that such reparatory cells induce endogenous regenerative programs, thus requiring relatively small numbers. Lastly, there is some evidence to believe that stem cells may be found in peripheral postmitotic tissues; the skin, a naturally rejuvenating tissue, would be a likely source of stem cells.

Solter D. From teratocarcinomas to embryonic stem cells and beyond: a history of embryonic stem cell research. Nat Rev Genet 2006;7:319-327.

In a salute to basic research and scientific curiosity, this thorough yet concise review of the literature discusses the scientific developments surrounding embryonic stem cell research and cloning and the distinct phases of research that have been carried out over the last 50 years. Included in the review is a discussion of teratocarcinomas, embryonic carcinoma cells, and embryonic stem (ES) cells. The various types and sources of human ES cells (hES) are also discussed, as are possible alternatives to ES cells. A number of key terms are defined in a small glossary.

Prentice DA. Current science of regenerative medicine with stem cells. J Investig Med 2006;54:33-37.

In a review of the literature of embryonic and adult stem cells, the author concludes that adult stem cells may hold more immediate clinical promise than embryonic stem (ES) cells and, due to the many scientific obstacles, that the latter may be better suited for basic scientific studies.

Problems with ES cells include the following:

1. While demonstrating partial behavioral improvement, some experiments have also shown tumorigenesis of the injected ES cells.

2. While a few animal studies indicate some ability of ES cells for cardiac repair, in vitro studies indicate potential problems with arrhythmia induced by ES-derived cardiac cells.

3. To date, it has been difficult to obtain a pure culture of ES-derived functional differentiated cells and to get physiologic integration into damaged tissues.

4. Immune rejection (due largely to the major histocompatibility complex genes) has been an issue; thus far, animal trials have relied upon either immunosuppression or injection into immunoprivileged sites (eg, the brain).

5. The number of matching human ES cells required for treatment remains far from certain, with estimates ranging from 250 to 10,000 potential lines needed.

While results from tests using embryonic stem cells have been modest, those using adult stem cells have demonstrated that adult stem cells possess a remarkable degree of pluripotent flexibility. Several examples now exist of adult stem cells with pluripotent flexibility, including cells from bone marrow, peripheral blood, the inner ear, umbilical cord, nasal mucosa, amniotic fluid, and the placental amniotic membrane. Furthermore, many of these cells can multiply in culture medium for extensive periods of time while retaining their ability to differentiate and still providing sufficient numbers of cells for clinical treatment.

Animal trials using adult stem cells have been remarkably successful and diverse. Adult stem cells have shown themselves to be effective for spinal cord injury, Parkinson disease, ameliorating retinal degeneration (thus giving hope for treatment for diabetic retinopathy and age-related macular degeneration), as an adjunct for cancer chemotherapy (to replace the patient's hematopoietic system or for anemia), for various autoimmune conditions (including scleromyxedema, multiple sclerosis and Crohn disease), metabolic disorders (such as Krabbe disease), bone repair protocols and repair of cardiac damage. Significantly, in some cases, adult stem cells appear to be capable of transdifferentiation. Most exciting perhaps, is the indication in some experiments that the injected adult stem cells do not form new beta cells, but rather, stimulate endogenous precursors within the pancreas to accomplish regeneration.

Ethical aspects of cloning techniques. Advisers to the President of the European Commission on the Ethical Implications of Biotechnology. J Med Ethics 1997;23:349-352.

In this opinion, the group condemns reproductive cloning on moral grounds; three specific conclusions concerning human cloning are given:

1. "Any attempt to produce a genetically identical human individual by nuclear substitution from a human adult or child cell ('reproductive cloning') should be prohibited"; this conclusion is reached in light of the considerations of instrumentalization and eugenics which render cloning ethically unacceptable, as well as potential safety considerations.

2. Any attempt to make genetically identical embryos for clinical use in assisted reproduction should be prohibited for the same ethical objections made against cloning.

3. Multiple cloning is a fortiori unacceptable.

Furthermore, the group believes that research involving nuclear substitution should aim at either throwing further light on the cause of human disease or contributing to the alleviation of suffering; in no case should the manipulated embryo not be replaced in a uterus. Similar restrictions are given concerning the cloning of animals.

A distinction is drawn between reproductive and therapeutic cloning, and, while the former is roundly condemned, the door is not soundly shut on the latter, as long as the given appropriate ethical guidelines are met. Indeed, the group recognizes the important therapeutic implications that such research presents, both to medicine and agriculture. At the same time, emphasis is placed on the possible risks (eg, a fall in the level of genetic diversity to an unacceptable degree) and the high biologic cost (Dolly the sheep, for instance, was the one success out of 277 attempts).

Cameron C, Williamson R. In the world of Dolly, when does a human embryo acquire respect? J Med Ethics 2005;31:215-220.

It is argued that the time of implantation is the most useful point at which embryos should acquire the respect accorded to human beings because from that point a new life is viable and develops. While cloned human beings--shown to be at least theoretically possible in light of the successful cloning of Dolly the sheep--would have the same ethical status as any other person, the cloned embryo (before implantation) does not.

The author points out a number of factors that have changed following the cloning of Dolly the sheep:

* Fertilization is no longer required to create an embryo;

* A sperm is no longer required to create an embryo;

* An egg may not be required to create an embryo;

* One or more embryos may be created that have the same DNA as another living individual.

As the authors show, there is a great deal of divergence about the point at which a human life can be defined as beginning; the Catholic, Muslim, Buddhist, Jewish, scientific and philosophical views of where to find this point are presented herein, no two of which agree. Although an embryo is technically defined as what is created by fertilization, as fertilization is no longer necessary, the authors disregard claims that the life of an embryo--or, by extension, human life--must begin at such a point.

From the moment of implantation, however, the embryo thereafter develops into a recognizable fetus. Incidentally, scientists presently are only able to allow an embryo to develop ex utero for approximately 14 days, after which point the embryo will cease to develop and will die--unless it is successfully implanted in the womb (it is possible, however, that such a limitation may be alleviated in the future).

Furthermore, at about 14 days, the appearance of the primitive streak in the embryo corresponds to the beginning of neural functions. It is also the point at which there is no longer any chance of twinning. For these reasons, many scientists agree that the embryo ought to be accorded "an increment of respect."

Hansen JE. Embryonic stem cell production through therapeutic cloning has fewer ethical problems than stem cell harvest from surplus IVF embryos. J Med Ethics 2002;28:86-88.

Therapeutic cloning is not unethical and should be permitted. The author considers there to be two overriding concerns preventing research on stem cells for therapeutic purposes, neither of which should be considered serious enough to hinder research in this field; these two concerns are as follows:

1. The harvesting of stem cells is immoral because it necessitates the destruction of the pre-embryo at the blastocyst stage, which is regarded by many as a human being with an inalienable right to life.

2. As the technique of therapeutic cloning is the same as that used in the initial stage of reproductive cloning, it is feared that therapeutic cloning will inevitably pave the way ("slippery slope") for reproductive cloning.

The first concern is addressed by looking at the onset of identity and personhood, neither of which, the author claims, are inherent to the blastocyst. Personhood is considered to be essential in the defining of the individual. Several criteria for the moment at which personhood is said to develop are considered. The earliest of these is around 14 days. As argued above (Cf. Cameron C, Williamson R.), implantation is said to be the earliest possible point in time at which personhood can be said to begin to develop: "after implantation, an unquestionably human being may evolve." Should future technical capabilities permit for the development of the blastocyst in culture, the development of the primitive streak after 14 days is considered a safe minimum requirement before a human being can be said to emerge. As twinning can no longer occur after this point, individuality is not considered to be a property of the pre-embryo. As none of these considerations support the blastocyst, the author feels justified in repudiating the first of the two objections. In addition, neither is the identity of the blastocyst necessarily preserved nor its development independent of external conditions.

The second concern--that of the slippery slope--is dismissed because while therapeutic and reproductive cloning may share several preliminary steps, the two are not the same, and requires a separate decision to implant the transnuclear egg cell before it reaches the blastocyst stage where stem cell harvest is possible. If reproductive cloning is morally impermissible, it is reproductive cloning--and not therapeutic cloning--that should be permitted.

McMahan J. Cloning, killing, and identity. J Med Ethics 1999;25:77-86.

It is morally permissible to kill a clone in the early stages of its development as there is, as yet, no "one" who would be harmed by the killing.

Generally, a sharp moral distinction is made between killing a person and preventing a person from existing. The author argues that during the early stages of development, the pre-embryo cannot be considered to be a person, and may thus be killed and used, for instance, as a source of tissues for transplantation. Several reasons are given for this conclusion:

1. Arguments made against the killing of a clone at any point after nuclear transfer that are based on the idea that the clone is or has a soul are precariously based. The author systematically refutes claims that the pre-embryo has a soul (which is capable of consciousness), ending by saying that it is more reasonable to conclude that the soul does not begin to exist before evidence of the existence of an independent, conscious life in the womb.

2. Twinning is no longer possible after the development of a "primitive streak," which coincides with implantation (although it need not necessarily do so in the case of cloning). As it does not seem possible to establish identity and to subsequently twin, it is not unreasonable to believe that a human organism does not exist before about 14 days after fertilization (or, in the case of the clone, after nuclear transfer).

3. A person is not the same thing as an organism: in the author's opinion, a person does not begin to exist until the brain first acquires the capacity to support consciousness. This is not to say that the organism does not have intrinsic worth, but it is not comparable in value to a human person.

4. The fetus is psychologically removed from itself in the future: there is no "egotistic concern" that it should survive; instead any such interest matters only for and in the future of someone else. While, in considering the fetus's life as a whole, it is clear that death would be much worse for it than continuing to exist, the fetus has no such interest in its future.

For all of these reasons, the author concludes that it is largely innocuous to kill a clone in the early stages of its existence. Killing the clone at this stage of existence would be akin not to killing a person but to preventing a person from being--the act may be likened to contraception rather than infanticide. Killing becomes increasingly objectionable as the fetus develops, as there is not one moment when the killing of a fetus ceases to be innocuous and suddenly becomes murderous instead. As such, the author does not suggest what cut-off point should be given, although admitting that this point could be set anywhere from the moment of conception to around the time of birth (provided that there is a sufficiently significant end, such as saving a person's life).

Lo B, Zettler P, Cedars MI, et al. A new era in the ethics of human embryonic stem cell research. Stem Cells 2005;23:1454-1459.

The present ethical debate surrounding human embryonic stem cell (hESC) research has focused on the moral status of the embryo. As such, practical concerns surrounding clinical trials of hESC transplantation have yet to be addressed. These concerns center on the twin needs of protecting participants in phase I trials of hESC transplantation while also respecting the confidentiality of donors of materials used for derivation of hESC lines. There are also significant challenges surrounding a more developed understanding of informed consent on the part of hESC transplantation recipients.

To prevent against the transmission of infectious agents or serious genetic conditions, the authors argue for a broader perspective on protecting recipients of transplanted hESC materials. There are at least four concerns that need to be addressed before the implementation of clinical trials of hESC transplantation:

1. There is likely to be a significant time period between donation and clinical trials involving transplantation. During this lag time, diseases which have long latency periods and which might not have been identified at the time of donation may become apparent in the donor (eg, Creutzfeldt-Jakob Disease).

2. Recipients need to be protected against the inadvertent transmission of serious genetic conditions: for instance, donors may develop cancer or show a strong family history of cancer.

3. Immunosuppressive drugs, which may be needed after cell transplantation, will increase the risk of communicable diseases and cancer in recipients.

4. Many patients may receive transplantation from a single hESC line over time. As such, it will be necessary to continuously update information; such monitoring will necessitate recontacting donors on a periodic basis while protecting their confidentiality. To avoid invading donor privacy, permission to recontact donors must be attained at the initial time of donation.

Regarding informed consent, three points for reducing "therapeutic misconception" in recipients of hESC transplantation in phase I trials are emphasized:

1. Researchers need to communicate the distinction between the long-term hope for effective treatments and the uncertainty inherent in any phase I trial.

2. Investigators in hESC clinical trials must discuss a broader range of information with potential participants than in other clinical trials: while relevant information generally concerns the nature of the intervention being studied and the medical risks and prospective benefits, in hESC transplantation, nonmedical issues may serve an important role.

3. Finally, researchers should verify that potential participants understand the study; simply disclosing the relevant information may not be enough.

Burley J, Harris J. Human cloning and child welfare. J Med Ethics 1999;25:108-113

Human cloning should not be prohibited due to concerns for the resultant child; indeed, a ban on such grounds would be "palpably morally unacceptable." The authors repudiate three hypotheses which purportedly regard the welfare of cloned children as reasons for a ban on cloning:

1. Clones will be harmed by the fearful or prejudicial attitudes people may have about or toward them (H1).

2. Clones will be harmed by the demands and expectations of parents or genotype donors (H2).

3. Clones will be harmed by their own awareness of their origins (H3).

The first hypothesis is repudiated by comparing a case of a 14-year-old girl who has a child: while the child may have been given a relatively poor start to life--by being cloned or conceived at 14--that life is still worth living and better than the alternative: that is, never to have existed at all. In choosing to have either child, there is no immoral act: rather, the decision to impose a ban on cloning is based upon the "moral wrong" of other members of society.

The second hypothesis is dealt with by a discussion of the restrictive procreative policies on autonomy which would logically follow from a ban on cloning. For instance, if cloning was banned because the public might harbor misunderstanding about genetic existentialism, it would be equally valid to prohibit the freedom to procreate whenever it was known that the resultant child would be deformed. As with the first hypothesis, no moral wrong appears to be committed by the clone's progenitor(s).

The final hypothesis is disregarded as there are many possible sources of analogous trauma which a child created by "natural" means might suffer: for instance, learning that one was adopted, that one was the product of in vitro fertilization (which need not always involve the genetic material of both parents), or that one's parents were criminals, drug addicts, etc. Although psychological distress might ensue, such trauma would not be sufficient reason for imposing "draconian restrictions on procreative freedom which the objector would hardly endorse."

McCarthy D. Persons and their copies. J Med Ethics 1999;25:98-104.

There is no basis for objecting to cloning on the grounds that clones would have much worse lives than nonclones.

Some apparent moral deficiencies of cloning--that it is unnatural, that its purpose is to enhance the human race (eugenics), that it is egotistical--are dismissed early in the paper. The author poses a series of questions, of which the answers lead him to the aforementioned conclusion; these four questions--asked with reference to the clone and its original--are as follows:

1. Is having an original conception of the good an important part of a good life?

2. Does the mere fact of genetic identity make it likely that a clone will lead an unoriginal life?

3. Is there much difference, with respect to revealed limitations, between us and clones?

4. Does any such difference in revealed information undermine a clone's ability to lead a normal life?

Presuming that the rights of clones are equal to our own, the rather obvious answers to all four questions lead the author to conclude that there is no evidence that the lives of clones would be significantly worse that the lives of nonclones. Should the lives of clones be shown to be significantly worse of than those of nonclones, the opposite conclusion must be drawn.

Savulescu J. Should doctors intentionally do less than the best? J Med Ethics 1999;25:121-126.

Doctors have an obligation to maximize well being, and while there are instances when submaximization is permissible, such instances must have a good reason to do so. In the case of helping parents have a child, genetic relatedness is not sufficient enough reason for doctors to bring a severely sick or disabled child into existence when they could bring into existence a healthy child who is expected to have a long and rewarding life. Whatever the value of genetic relatedness, it is not an unconditional value, and producing an offspring which is genetically related is not itself a good reason to clone.

We shall not cease from exploration, and the end of all our exploring
will be to arrive where we started and know the place for the first
time.
--T.S. Eliot

Conrad C. Daly, MTh