This commissioned working paper was discussed at
the Council's December 2004
meeting. The views expressed here do not represent the official views
of the Council or of the United States Government.
Altered Nuclear
Transfer as a Morally Acceptable Means for the Procurement of Human
Embryonic Stem Cells
William B. Hurlbut,
M.D.
Program in Human Biology
Stanford University
Introduction
With the sequencing of the human genome and our increasing knowledge
of the molecular mechanisms of basic cell functions, we are entering
an era of rapid advance in the field of developmental biology.
Current scientific interest in embryonic stem cells is a logical
step in the progress of these studies and holds the hope of providing
important research tools as well as possible therapeutic applications.
The ethical controversy surrounding human embryonic stem cell
(ESC) research arises from the fact that to obtain these cells living
human embryos must be disaggregated and destroyed. Many Americans
oppose such embryo destruction, believing that there is an implicit
dignity and inviolability in the individual continuity of a human
life from fertilization to natural death. Many others, however,
believe that the benefits of advances in biomedical science outweigh
these moral concerns.
The present conflict over the moral status of the human embryo reflects
deep differences in our basic convictions and is unlikely to be
resolved through deliberation or debate. Likewise, a purely political
solution will leave our country bitterly divided, eroding the social
support and sense of noble purpose that is essential for the public
funding of biomedical science. These concerns are already encoded
in the Dickey Amendment that prohibits the use of federal funds
for embryo-destructive research and is the legislative foundation
of the President's executive order restricting funding to ESC lines
created before August 9, 2001. While there are currently no federally
legislated constraints on the use of private funds for this research,
there is a consensus opinion in the scientific community that without
NIH support for newly created ESC lines progress in this important
realm of research will be severely constrained.
Notwithstanding this apparently irresolvable impasse, there may be
morally uncontroversial ways to obtain embryonic stem cells. Drawing
on our increasing understanding and control of developmental biology it
may be possible to direct the organic powers of embryological
development to generate ESCs even apart from the living human organism
that is their natural origin.
Altered Nuclear Transfer
There are several possible approaches that might allow the production
of ESCs without the creation and destruction of a human embryo.
The ideal solution, one that many scientists believe will eventually
be possible, would be the direct reprogramming of adult cells to
the functional equivalence of ESCs. In natural embryogenesis ESCs
are produced within a restricted area (the inner cell mass) of a
4-5 day old embryo (known as a blastocyst). Over the first few
days of development, a series of cell signals induces the specific
pattern of gene expression that characterizes ESCs and gives them
their pluripotency, their capacity to subsequently produce all the
cell types of the human body. With an understanding of the exact
molecular nature of these signals it should be possible to bypass
embryogenesis and directly induce this transformation in adult cells.
Unfortunately, it may be many years before our scientific knowledge
and control of these factors will make this approach feasible.
More immediately, there may be ways to obtain ESCs by harnessing
partial organic trajectories apart from the full natural system
of embryonic development. Using the techniques of somatic cell
nuclear transfer (SCNT), but with the intentional alteration of
the nucleus before transfer, we could construct a biological
entity that, by design and from its very beginning, lacks the attributes
and capacities of a human embryo. Studies with mice already provide
evidence that such a project of Altered Nuclear Transfer (ANT) could
generate functional ES cells from a system that is not an organism,
but is biologically (and morally) more akin to the partial organic
potential of a tissue or cell culture.1
This proposal shifts the ethical debate from the question of when
a normal embryo is a human being with moral worth, to the more fundamental
question of what component parts and organized structure
constitute the minimal criteria for considering an entity a human
organism.
The Paradigm of Systems Biology
The moral argument for Altered Nuclear Transfer is grounded in the
emerging science of systems biology. According to this radical
revision of our prevailing reductionistic views, an organism is a
living whole, a dynamic network of interdependent and integrated parts.
There are essential subsystems of growth (cells, tissues and organs),
but a living being is more than the sum of its parts, and the parts
are dependent on the integrated unity of the whole. Fully constituted,
the organism is a self-sustaining and harmonious whole, a unified
being with an inherent principle of organization that orders and
guides its continuity of growth. In the human embryo, this principle
of organismal unity is an engaged and effective potential-in-process,
an activated dynamic of development in the direction of the mature
human form. Incompletely constituted or severed from the whole,
subsystems with partial trajectories of development may temporarily
proceed forward with a certain biological momentum. Ultimately,
however, they fail to rise to the level of the coordinated coherence
of a living organism and become merely disorganized cellular growth.
Failures of Fertilization and Partial Development
The activation of the egg by the penetration of the sperm (or
the equivalent events in nuclear transfer/cloning) triggers the
transition to active organismal existence. But without all of the
essential elements (a full complement of chromosomes, proper chromatin
configuration, the cytoplasmic factors for gene expression, etc.),
there can be no living whole, no organism, and no human embryo.
Recent scientific evidence suggests that such a 'failure of
fertilization' is, in fact, the fate of most early natural initiations
in reproduction. The artificial and intentional construction
of a biological entity lacking any of these essential elements,
yet bearing a partial developmental potential (similar to that in
the aberrant products of fertilization), may make it possible to
procure ES cells without producing a human embryo.
There are natural biological precedents for entities that lack
the qualities and characteristics of an organism, yet are capable
of generating ES cells. Teratomas are germ cell tumors that generate
all three primary embryonic germ layers as well as more advanced
cells and tissues, including partial limb and organ primordia.
Yet these chaotic, disorganized, and nonfunctional masses lack entirely
the structural and dynamic character of organisms. Likewise, failures
of fertilization due to abnormal complements of chromosomes or improper
chromatin configurations (imprinting) may still proceed along partial
trajectories of organic growth without being actual organisms.
Trisomies of chromosome number one, for example, will grow to the
blastocyst stage but will not implant. Even an enucleated oocyte,
when artificially activated, has the developmental momentum to divide
to the eight-cell stage.
These natural examples of partial generative potential (described
by some as pseudo-embryos), together with other observations of
early embryonic processes, have led to a diverse array of suggestions
for ways that ES cells might be produced without the moral ambiguity
of the creation and destruction of full human embryos. These suggestions
include the use of aneuploidies, polyploidies, viable cells from
embryos in arrested development, parthenotes, and chimeras of human
nuclear material and animal oocytes. Each presents its own particular
technical challenges and raises unique and unfamiliar moral considerations.
The scientific prospects for ANT remain largely unexplored, but
as stated by Rudolph Jaenisch in testimony to the President's
Council on Bioethics (July 2003), they are within the reach of our
current technology. There are numerous potential approaches involving
alteration of the genes necessary for early intercellular organization,
formation of extra-embryonic structures, or the primary patterning
of organogenesis. One possibility is the alteration of 'cdx2,'
a gene essential for differentiation of the trophectoderm (the tissues
that ultimately form the placenta). In experiments with mouse models,
when this gene is not expressed there is only a partial and disorganized
developmental process resulting in a visibly abnormal blastocyst.
Nonetheless, there is the formation of an inner cell mass from which
functional ES cells have been harvested. For ANT, this gene might
be temporarily silenced (using RNA interference) by altering the
somatic cell nucleus or the cytoplasm prior to transfer, so that
once the ES cells have been procured the gene could be re-expressed
to allow fully potent ES cells.2
The limited biological entity created by such a procedure would
fail to establish even the most fundamental features of organismal
infrastructure, and would be incapable of implantation. It would
have no inherent principle of unity, no coherent drive in the direction
of the mature human form, and no claim on the moral status due to
a developing human life. Rather, such a partial organic potential
would more rightly be designated a biological 'artifact'
(a human creation for human ends). The fact that some part of such
a constructed entity will carry a certain momentum of development
is morally analogous to the fact that we can grow skin in a tissue
culture and may one day grow whole organs or limbs. Lacking crucial
elements in its fundamental constitution, such an entity would never
rise to the level of a living being.3 When the overarching integration of essential
parts and functions is not present (or, as in the 'brain dead'
organ donor, no longer present), there is no living organism and
therefore there is no being with human moral status.
Ethical Harnessing of Partial Developmental
Potential
The intention in creating such an intrinsically limited "artifact"
would not be one of reproduction (and disaggregation), but simply
the desire to draw on natural organic potential through technological
manipulation of biological materials. This intention is in keeping
with the purposes of scientific research and medical therapy in
which many "unnatural" manipulations are used for human
benefit.
The crucial principle of any approach, however, must be the preemptive
nature of the intervention. This process does not involve
the creation of an embryo that is then altered to transform it into
a non-embryonic entity. Rather, the proposed genetic alteration
is accomplished ab initio, the entity is brought into
existence with a genetic structure insufficient to generate
a human embryo. From the beginning and at every point along its
development it cannot be designated a living being. No human embryo
would be created; hence, none would be violated, mutilated or destroyed
in the process of stem cell harvesting. If such a limited biological
entity were accorded a certain cautionary respect (as with all human
tissues), even though not the full protection of human life, this
project would not compromise any fundamental moral principles.
Moreover, such techniques could be developed using animal models
and confidently extended to work with human cells without engaging
in research that involves the destruction of human embryos.4
Over the course of the previous century we contended with ethical
controversies over blood transfusion, tissue and organ transplantation,
and the transfection of human genes into experimental animals.
In this century we will be confronted by a series of even more challenging
ethical questions related to the dynamic systems of developmental
biology. Just as we have learned that neither genes, nor cells,
nor even whole organs define the locus of human moral standing,
in this era of developmental biology we will come to recognize that
cells and tissues with 'partial generative potential' may
be used for medical benefit without a violation of human dignity.
Conclusion
The moral distinctions essential to discern and define the categories
of organism, embryo and human being will be vital as we go forward
with scientific research involving ES cells, chimeras, and laboratory
studies of fertilization and early embryogenesis. Advances in
developmental biology will depend on clarifying these categories
and defining the moral boundaries in a way that at once defends
human dignity while clearing the path for scientific progress
At this early stage in our technological control of developing
life, we have an opportunity to break the impasse over stem cell
research and provide moral guidance for the biotechnology of the
future. This may require a constructive reformulation of some aspects
of moral philosophy, together with creative exploration of scientific
possibilities, but any postponement of this process will only deepen
the dilemma as we proceed into realms of technological advance unguided
by forethought. We must initiate the cooperative dialogue that
is essential to frame the moral principles that can at once defend
human dignity and promote the fullest prospects for scientific progress
and its medical applications.
1. Personal communication with Janet Rossant, Mount Sinai Hospital,
Toronto, Canada. Also, see "Cdx2 is essential for axial elongation
in mouse development," Kallayanee Chawengsaksophak , et al. PNAS
May 18, 2004, vol 101, no 20, 7641-7645.
2.
Although the trophectoderm is the source of the extraembryonic
membranes, it is properly considered part of the embryo. Indeed,
studies confirm that normal trophectoderm is essential for normal
embryogenesis. Janet Rossant, a world authority on early embryonic
development has done important studies with chimeric mice made
by combining cells from two different early embryos, one tetraploid
and the other the source of the ES cells that are joined to the
tetraploid cells. Referring to studies using tetraploid mice
with gene knockouts in their trophectoderm, she states "the analysis
of mutations in candidate patterning genes in chimeric mice has
played a key role in elucidating the importance of extra-embryonic
tissues as sources of patterning signals in the early mouse embryo."
More specifically, she says: "Embryological and gene expression
studies have shown that the early patterning of the antero-posterior
axis of the mouse embryo at around the time of gastrulation requires
signalling and transcriptional activity in both the extra-embryonic
and embryonic tissues." (Beddington and Robertson 1999). She
goes on to cite several genes such as Hnf4 stating that her studies
indicate that "this outcome indicates that Hnf4 in the extraembryonic
tissues is crucial for normal gastrulation." Citing a series
of other studies on genes essential for early development she
concludes "Therefore, these studies showed that Amn, Nodal,
Akd and Foxa2 function is essential in the extra-embryonic tissues
for normal embryogenesis and that the early patterning events
must involve a complex interplay between embryonic and extra-embryonic
tissues." ( Rossant, Mouse Embryo Chimeras: tools for Studying
Mammalian Development, Development 130,6155-6163).
A deficiency at the first differentiation of
cell type (the formation of the trophectoderm) means the absence
of the most fundamental order. But one could rightly argue that
later deficiencies in axes formation, alignment of basic body
plan and organogenesis also reveal a level of disorder that precludes
organismal existence. At gastrulation the primary pattern of
the developing being is laid down and many aspects of interference
at this level would indicate a fundamental failure in the minimal
composition of organization and integration. However, for the
purposes of the proposed project, the earlier incapacity in the
most primary differentiation of trophectoderm seems the most morally
defensible. It is important to recognize that this deficiency
is almost certainly more fundamental than that which causes teratomas.
3.
As Thomas Aquinas observed eight centuries ago, an animal's
life is not like its image on a fresco which one can scrape off
inch by inch; an animal is a whole, unified being that is either
alive or not alive.
4. With increasing knowledge of genetics and developmental biology
it is clear that many of the basic mechanisms of animal development
have been conserved across millions of years of phylogenetic process.
This means that, with some confidence, we can extend our knowledge
of animal models (such as the mouse) to an understanding of the
principles and patterns of human development. With reasonable caution,
including extensive studies with non-human primates, the techniques
described in this proposal could be developed for application with
human cells.
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