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AN OVERVIEW OF STEM CELL RESEARCH
SUZANNE KADEREIT* & PAMELA J. HINES**
The fascination with stem cells draws us into considering current
complexities in biology, biomedical research and clinical applications. In
each sector, experts are struggling to understand better what stem cells are,
and how to best unleash their enormous potential for knowledge, society
and human health.1
I. STEM CELL SEMANTICS
A stem cell is defined as a cell that self-renews but also can give rise
to several differentiated cell types, such as muscle cells, heart cells or brain
cells.2
During normal cell division, the original cell gives rise to two
identical daughter cells, and is thus supplanted by two new and different
cells.3 For stem cells, cellular division is regulated in a different way.
Sophisticated molecular mechanisms are in place to maintain one of the
two daughter cells in the stem cell state, whereas the other daughter cell
divides and produces progeny of more differentiated and specialized cells.
Without this asymmetric division and active maintenance of the stem cell
phenotype in one cell, the stem cell lineage would be lost rapidly. How this
asymmetric division is exactly regulated is still largely unknown. Only
elucidation of this sophisticated mechanism will allow for the expansion of
stem cells in culture for clinical applications, or expansion directly in the
body.
In order to demonstrate that a cell in question is really a stem cell, it
*
**
1.
2.
3.
Suzanne Kadereit, Ph.D., is the Science Editor of the International Society for Stem
Cell Research at Children’s Hospital/Harvard Medical School.
Pamela J. Hines, Ph.D., is a Senior Editor of Science Magazine, American
Association for Advancement of Science.
Disclaimer: All opinions and statements in this article are those of the authors alone
and do not reflect statements of their respective organizations.
See generally Stewart Sell, Stem Cells: What are They? Where Do They Come From?
Why are They Here? When Do They Go Wrong? Where are They Going?, in STEM
CELLS HANDBOOK 1 (Stewart Sell ed., Humana Press 2004).
Id.
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must be proven that one single isolated cell can give rise to all the
specialized cells it is supposed to produce,4 for example, all of the different
cell types of the blood lineage for the blood stem cells. At the same time,
that single cell must also produce cells that perpetuate the stem cell
phenotype. Only then can that cell be named a stem cell. This requirement
of proof at the single cell level has emerged in the last few years, as it
became apparent that stem cell populations are heterogeneous despite
surface markers held in common: By purifying stem cells for those surface
markers, one nonetheless obtains stem cells of different potentials.
This rigorous corroboration has been difficult to achieve in many
cases, as some tissue-specific stem cells are very rare and often of illdefined phenotype, rendering their isolation difficult.
Categories of Stem Cells
Stem cells have been grouped into several categories: Multipotent,
pluripotent and totipotent stem cells.
1.
Multipotent Stem Cells
A multipotent stem cell is a stem cell that can give rise to several
different specialized cells,5 such as adult stem cells residing in the different
tissues. Thus, a blood stem cell can produce red blood cells, white blood
cells and platelets. To demonstrate that a stem cell is multipotent, the cell
has to be isolated out of its surrounding tissue, and tested at the single cell
level for its capacities both to self-renew and to produce different types of
specialized cells.
2.
Pluripotent Stem Cells
A pluripotent stem cell is a stem cell that can give rise to all of the
cells in the body. The embryonic stem cell is considered a pluripotent stem
cell. One type of experiment that can be used to prove this capacity
depends on the formation of teratomas when the cell in question is injected
into immune deficient mice. Teratomas are benign tumor growths that
contain cells of all three germ layers (endoderm, mesoderm and ectoderm).6
This approach has been taken to verify the potential of several existing
human embryonic stem cell lines. The ultimate proof, however, is to inject
the cells in question into a blastocyst (a very early embryo). Pluripotent
cells will then contribute to all of the tissue in the organism that develops
4.
5.
6.
Id.
Id. at 2-4.
James A. Thomson et al., Embryonic Stem Cell Lines Derived from Human
Blastocysts, 282 SCI. MAG. 1145 (Nov. 6, 1998).
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from this blastocyst. While this is practiced routinely in the mouse, with
mouse embryonic stem cells, for ethical reasons, this is not done with
human embryonic stem cells. With human embryonic stem cells, proof of
pluripotency remains limited to teratoma formation and the observation of
as many specialized cell types as possible after differentiation in cell
culture.
3.
Totipotent Stem Cells
A totipotent cell is one that can give rise to an entire organism. 7 The
fertilized egg cell, or zygote, is totipotent. It is possible that human
embryonic stem cells may have this capability, but again, for ethical
reasons, such proof does not exist for human embryonic stem cells.
A key difference between the pluripotent cell, which can form all the
cells in the body, and a totipotent cell, which can form the embryo from
which all the cells in the body are derived, lies in the extraembryonic
membranes. The zygote not only forms the embryo itself, but also must
form the placenta and various other extraembryonic tissues. Nonetheless,
the pluripotency of the embryonic stem cell is impressive in its scope.
II. THE ADULT STEM CELL
The adult stem cell, or tissue stem cell, is the resident stem cell in any
given tissue that maintains and repairs the tissue by producing the cell
types that make up that given tissue.
Stem cells have been found in bone marrow, skeletal and cardiac
muscle, dental pulp, skin, colon, liver, prostate, mammary gland, testicles,
mouse ovaries, several areas of the eye and ear and fat. Most of these stem
cells have not been well-characterized yet. Adult stem cells that have been
well-characterized and isolated to high purity from their tissue, and thus
have been demonstrated at the single cell level to be capable of giving rise
to progeny of different cell types, include the blood stem cell and the
mesenchymal stem cell.
Both blood and mesenchymal stem cells can be isolated from the
bone marrow and umbilical cord blood, two tissues that are relatively
available for research. Research on other tissue stem cells is hampered by
both the lack of human tissues for research and the scarcity of stem cells in
the small tissue samples that can be obtained.
Blood stem cells give rise to all the cells of the blood lineage and
maintain the blood over the lifetime of the body.8 Deficiencies in the blood
7.
8.
Sell, supra note 2, at 4.
See generally Stuart H. Orkin & Leonard I. Zon, Hematopoiesis and Stem Cells:
Plasticity Versus Developmental Heterogeneity, 3 NATURE IMMUNOLOGY 323 (Apr.
2002).
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stem cells result in fatal diseases if not treated with blood products. When
there are no blood stem cells, the organism dies in early development, prior
to birth. While the blood stem cell can be isolated at high purity from bone
marrow and cord blood, and one single stem cell injected into a bloodless
mouse can provide blood for a lifetime, this cell does not self-maintain in
culture, rendering experimentation difficult. However, culture expansion of
blood stem cells would be highly desirable for clinical purposes. Finally, in
the last two years, results have emerged that suggest that this may be
feasible. Blood stem cells could thus be used on a more routine basis for
immune reconstitution of leukemia, HIV, autoimmune disease, and
possibly cancer.
The mesenchymal stem cell can give rise to bone, cartilage, tendon,
fat, marrow stroma, muscle, and skin.9 For many years it has been possible
to isolate mesenchymal stem cells and support their proliferation in culture
without losing their stem cell potential. Accordingly, after numerous
successful experiments, in which animal models of human disease were
treated with mesenchymal stem cells, use of this stem cell is now moving
into the clinic. The mesenchymal stem cell seems of particular value for
repairing bone and cartilage, and other applications, such as cardiac repair,
are being actively investigated.
It is important to point out that scientists have known about both the
blood stem cells and the mesenchymal stem cells for over one hundred
years.10 However, only in the last twenty years has the blood stem cell been
used to reconstitute tissue for leukemia patients, while the mesenchymal
stem cell is only now approaching early clinical trials.
Another interesting stem cell that has been recently isolated from
human, mouse and rat bone marrow is the Multipotent Adult Progenitor
Cell (MAPC).11 The MAPC is a very rare cell and requires time to grow
out of bone marrow that has been put into culture. It is not yet known how
to isolate this cell from the bone marrow directly, without culture, but once
it has grown out in culture, it grows for many population doublings without
differentiating.12 Even after prolonged culture, it can still respond to
changes in signals so as to give rise to cells of endodermal, ectodermal and
9.
10.
11.
12.
See generally James E. Dennis & Arnold I. Caplan, Bone Marrow Mesenchymal Stem
Cells, in STEM CELLS HANDBOOK 107 (Stewart Sell ed., Humana Press 2004).
Id. at 108; M. William Lensch & George Q. Daley, Origins of Mammalian
Hematopoiesis: In Vivo Paradigms and In Vitro Models, 60 CURRENT TOPICAL DEV.
IN BIOLOGY 127 (2004).
See generally Morayma Reyes et al., Origin of Endothelial Progenitors in Human
Postnatal Bone Marrow, 109 J. CLINICAL INVESTIGATION 337 (Feb. 2002) (reporting
on a study of multipotent adult progenitor cells), available at http://www.jci.org/cgi/
reprint/109/3/337.pdf.
See id. at 337.
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mesodermal lineage, both in culture and when injected into animals. It has
thus been shown to reconstitute the blood system of mice, and to contribute
robustly to liver, lung and gut tissue. When injected into blastocysts of
mice, this stem cell gives rise to most, if not all, of the tissues in the mouse
developing from the injected blastocyst.13 This cell is thus truly
multipotent, if not pluripotent. Whether the MAPC can be used for human
clinical applications remains to be investigated, as the culture requirements
are cumbersome, and applications in medical settings will need far more
cells than what is used on mice in the experimental setting.
Umbilical cord blood also contains, in addition to blood and
mesenchymal stem cells, a very rare stem cell termed the Unrestricted
Somatic Stem Cell (USSC).14 It is not clear at this point if the USSC is the
cord blood equivalent of the MAPC from bone marrow. The USSC can
also give rise to cells of endodermal, ectodermal and mesodermal lineages
in culture, and has been shown to differentiate into neural cells, bone and
cartilage, blood, heart and liver cells when injected into animals.15
III. STEM CELL PLASTICITY
Recently, a lot of attention has been given to a phenomenon
previously deemed highly unlikely, if not impossible: Plasticity of cell
fate.16
For many years, it has been assumed that once a cell went down the
specialization path, it could not turn back and become a less specialized
cell, or even a different specialized cell. Thus a blood cell remains a blood
cell and a heart cell a heart cell. It was even hypothesized that the cells lose
the genes not required for their specialized function on their way toward
their final specialization. This dogma was also applied to the stem cells
residing in the different tissues, in charge of regenerating the cells of that
particular tissue. Thus, it was assumed that a blood stem cell could only
produce blood cells and a brain stem cell only brain tissue.
The creation of Dolly changed this way of thinking.17 Dolly showed
that the DNA from a terminally specialized cell could recapitulate the
expression of embryonic genes, as well as all the ensuing genes required
13.
14.
15.
16.
17.
See id. at 344.
See Gesine Kögler et al., A New Human Somatic Stem Cell from Placental Cord
Blood with Intrinsic Pluripotent Differentiation Potential, 200 J. EXPERIMENTAL MED.
123, 124 (2004), available at http://www.jem.org/cgi/reprint/200/2/123.
Id.
William B. Slayton & Gerald J. Spangrude, Adult Stem Cell Plasticity, in ADULT
STEM CELLS 1, 4-5 (Kursad Turksen ed., Humana Press 2004).
See I. Wilmut et al., Viable Offspring Derived from Fetal and Adult Mammalian
Cells, 385 NATURE 810, 812 (Feb. 27, 1997).
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for proper embryonic, fetal and adult body and tissue development. First,
Dolly was dismissed as an aberration. It was assumed that a rare very
immature stem cell could have made its way into the preparation from
which Dolly was created. Similar arguments had been used in the 1960s to
try to explain away the cloning of frogs. But after Dolly, the idea gathered
steam. Other researchers attempted to prove the validity of the cloning
approach. A particularly convincing experiment used lymphocytes, which
undergo gene rearrangement as part of their specialization, to show that
only the DNA from these terminally specialized cells contributed to the live
clones that were created. In these clones, every single cell in the body had
exactly the same gene rearrangement as the original DNA donor cells.18
It is now known that there are powerful molecular mechanisms in
place, called epigenetic regulation, to stabilize the cellular phenotype. As
the cell specializes more and more, entire gene programs are switched off
and others switched on. Genes that are no longer required are switched off,
often buried in the depth of the DNA organization, and maintained in this
state throughout future cellular divisions by stable molecular modifications
of the DNA molecule. A heart cell, for example, will not express the same
genes as a liver cell, or even a stem cell.
Overall, several layers of regulation are in place. Analogous to an
electrical system, for example, certain proteins might be attached to, or
detached from, a gene and might function like a light switch turning a lamp
on or off. Another layer of regulation would be the transformer at the end
of the neighborhood that delivers power to the house––if it is off, no
amount of switch-flipping will have any effect. And yet, larger, and more
firmly embedded layers of regulation exist. If the Eastern power grid goes
down, then even fixing the neighborhood transformer will be irrelevant.
Similarly, genes can be placed under flexible immediate regulation, or can
essentially be packed away in cold storage, never to be used again.
This type of regulation is crucial for proper control of cell growth and
cellular homeostasis in any given organ, as the reactivation of genes
involved in early development and strong proliferation could lead to
cancer. Of particular importance is the silencing of certain “embryonic
genes” that are required for proper embryonic development, but which,
when expressed in specialized tissue cells, will lead to uncontrolled growth
of the cells.
In the case of stem cells, stringent regulation of gene expression is
even more crucial, as the stem cell has to respond to environmental cues
18.
Konrad Hochedlinger & Rudolf Jaenisch, Monoclonal Mice Generated by Nuclear
Transfer from Mature B and T Donor Cells, 415 NATURE 1035 (Feb. 28, 2002),
available at http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v415/
n6875/full/nature718_fs.html (last visited Feb. 11, 2005).
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and produce progeny on demand, producing the required amount of
specialized cells through intermediate stages of rapidly dividing cells,
while maintaining its stem cell phenotype. It thus must express early
developmental genes, if not embryonic genes, while also silencing them
progressively in the progeny. If a stem cell is taken out of its normal
environment, this tight regulation may possibly become perturbed and the
stem cell may lose its stem cell phenotype and go down the differentiation
pathway.
It may also happen that the stem cell changes its fate, and thus a blood
stem cell may produce muscle cells. If this phenomenon is observed,
however, it is imperative to ensure that one single blood stem cell gave rise
to the muscle cells, and that the source was not a contaminating muscle
stem cell. Moreover, it has recently become apparent that stem cells like to
fuse with other cells, thus masquerading as cells of a different type even
though no new cell has been created. This happens frequently in tissues
such as liver and muscle fibers, tissues which contain cells that fuse on a
regular basis to accomplish their normal function.
Furthermore, some recent reports that claimed stem cell plasticity
were hampered by inadequate experimental approaches.19 This became
apparent when experiments were repeated by other groups with more
advanced and appropriate techniques. It was important to verify claims of
stem cell plasticity, as clinical applications could be vast, provided the stem
cells in question could be grown up to meaningful quantities.
While the debate about stem cell plasticity is on-going, and while it
appears that in certain instances plasticity may take place, the frequency of
this plasticity is extremely low, making rapid advancement towards clinical
applications uncertain.
Moreover, compelling evidence has accumulated in the last two years
that in certain forms of cancer, cells with a stem cell phenotype, fuel cancer
growth. “The hypothesis of the cancer ‘stem cells’ is not entirely new …
[and] stem[s] from the observation that not all the cells within a tumor can
maintain tumor growth….”20 Recently, by applying techniques used in the
stem cell field to identify self-renewing populations at the single cell level,
it became possible to isolate to high purity the cells from the tumors and to
demonstrate their stem cell character. When injected into mice, only the
cancer stem cells regenerated the tumors, whereas injecting the entire rest
of the tumor mass did not lead to tumor growth. Also, it has emerged that
19.
20.
See, e.g., Constance Holden & Gretchen Vogel, Plasticity: Time for a Reappraisal?,
296 SCI. MAG. 2126 (June 21, 2002).
SUZANNE KADEREIT, INT’L SOC’Y FOR STEM CELL RESEARCH, CANCER “STEM CELLS”
OR STEM CELL “CANCER,” at http://www.isscr.org/scientists/TOM/Sept04.htm (Sept.
2004); Philip A. Beachy et al., Tissue Repair and Stem Cell Renewal in
Carcinogenesis, 432 NATURE 324 (Nov. 18, 2004).
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several long-known oncogenic pathways are crucial to the maintenance of
self-renewal in normal stem cells.21
Cancer stem cells have now been demonstrated in certain types of
leukemia, in breast cancer, in pediatric and adult brain tumors, in gastric
tumors, and it is likely that other cancer types will follow suit.22
At this point it is not known whether the resident tissue stem cells
have gone “rogue,” or if a more differentiated cell has reacquired a stem
cell phenotype. It is crucial, however, to take these new findings into
consideration when designing therapies with adult stem cells, particularly
when such therapies entail expansion in cell culture of the adult stem cell
populations, or when the stem cells are derived from older adults, as such
cells may have acquired mutations over the years or through extra rounds
of proliferation.
IV. ADULT STEM CELLS IN THE CLINIC
Some diseases or degenerative afflictions are being treated with adult
stem cells. The vast majority of adult stem cell trials currently underway
are using blood stem cells from bone marrow, mobilized peripheral blood,
or cord blood, to treat blood disorders or diseases such as leukemia,
different types of anemia, systemic lupus, and certain other autoimmune
diseases or immune deficiencies.23 In most of these applications, years of
clinical trials have demonstrated efficient cures, and protocols are now
being refined.
Recently, a handful of clinical trials have begun around the world to
evaluate the use of the patient’s own bone marrow to repair heart tissue and
improve blood flow. At this stage it is not clear whether real improvements
take place, and to what extent this could be used on a broader patient basis.
Moreover, it is not clear whether stem cells participate themselves in the
tissue regeneration or provide support for other cells. It is also not clear
which stem cells in the bone marrow promote the observed improvement of
blood flow in the heart tissue. Recent literature suggests that the relevant
cells could be the mesenchymal stem cell. However, it may be that the key
21.
22.
23.
Muhammad Al-Hajj et al., Therapeutic Implications of Cancer Stem Cells, 14
CURRENT OPINIONS GENETIC DEV., Feb. 2004, at 43.
Muhammad Al-Hajj & Michael F. Clarke, Self-Renewal and Solid Tumor Stem Cells,
23 ONCOGENE 7274, 7274 (Sept. 20, 2004).
See generally Mary J. Laughlin et al., Outcomes After Transplantation of Cord Blood
or Bone Marrow from Unrelated Donors in Adults with Leukemia, 351 NEW ENG. J.
MED. 2265 (2004); Helen A. Papadaki, Autoreactive T-lymphocytes are Implicated in
the Pathogenesis of Bone Marrow Failure in Patients with Systemic Lupus
Erythematosus, 44 LEUKEMIA & LYMPHOMA 1301 (2003); Christiane Vermylen,
Hematopoietic Stem Cell Transplantation in Sickle Cell Disease, 17 BLOOD REV. 163
(2003).
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is another early stem cell, the common precursor of blood and endothelial
lineages.
As mentioned above, mesenchymal stem cells are currently being
evaluated in preliminary clinical trials for bone and cartilage repair. Other
applications such as cardiac muscle regeneration may soon follow.
Other adult stem cells explored in the clinic include stem cells in the
eye. For example, transplantation of corneal epithelial stem cells has been
shown to restore useful vision in some patients with severe ocular-surface
disorders.
Various tissue or cellular grafts are in current use and include, for
example, skin grafts for burn victims, pancreatic insulin-producing cells for
type I diabetes, fetal dopamine neurons for Parkinson’s disease, and
epithelium for bladder reconstruction. It is likely that in cases where longterm engraftment is achieved, stem cells contained in these grafts have
contributed to the tissue regeneration. It is, however, unclear at this point
exactly which adult stem cells have contributed, or how they have been
stimulated to grow and regenerate.
Some additional anecdotal cases of cures or partial cures have been
reported in the lay media, without supporting publication in the peerreviewed scientific literature, and should be interpreted with caution at
best.
Wide-spread use of such therapies and application of similar therapies
to other diseases will, however, be rendered difficult by the same problems
plaguing the solid organ field, namely the scarcity of human organs and
tissue. There are approximately six thousand organ donors each year in the
United States, and an estimated one hundred million patients who could
potentially benefit from stem cell-derived therapies.24
Another problem is to find a matched donor to reduce immune
rejection of the foreign tissue. Despite the availability of immune
suppressive drugs, immune rejection is still a major problem. Immune
rejection could be avoided if one could use the patient’s own tissues. This
is, however, often not possible. For example, some victims of severe burns
do not have enough skin left to generate replacement tissue. Or, for patients
with advanced degenerative diseases, most of the relevant tissue has
already been destroyed. In addition, current observations suggest that adult
stem cells age, and that the regenerative capacity of these cells decreases
with increasing age. Thus, older patients may not be able to provide the
cells necessary for their treatments.
On a last note, as mentioned above, it remains unknown for most
24.
COMMITTEE ON THE BIOLOGICAL AND BIOMEDICAL APPLICATIONS OF STEM CELL
RESEARCH, STEM CELLS AND THE FUTURE OF REGENERATIVE MEDICINE 8 (Nat’l Acad.
Press 2002), available at http://books.nap.edu/books/0309076307/html/R1.html.
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adult stem cell types how to go from a limited amount of source stem cells
to the larger numbers needed to treat disease or to restructure tissues. It
may take years of additional research to elucidate the mechanisms
controlling stem cell self-renewal and differentiation. Moreover, use of
culture dish approaches for expansion of adult stem cell populations will
have to be monitored very closely to avoid the negative consequences of
unwanted proliferation.
V. THE EMBRYONIC STEM CELL
Embryonic stem cells are derived from the inner cell mass (ICM) of
five-day-old human embryos called blastocysts.25 At this stage of
development, no beginning of organ formation has yet taken place, and the
blastocyst resembles a hollow ball of cells that is not visible to the naked
eye.26 The blastocysts used for derivation of embryonic stem cells have
been created during fertility treatments through in-vitro fertilization (IVF).
Blastocysts that are not implanted for pregnancy are frozen and kept for
possible future pregnancies. The parents are charged storage fees for the
storage of those blastocysts. Once payment stops, these blastocysts may be
discarded if not donated for research or to other parents for their fertility
treatment.
Blastocysts used for derivation of embryonic stem cells have not yet
been implanted. Nor are implanted embryos a good source of embryonic
stem cells. Once the embryo implants and develops any further, the ICM is
rapidly lost, as all the cells start specialization and initiation of organ
formation.
Embryonic stem cells from mice were isolated in 1981 by two groups
independently.27 Since then, research using mouse embryonic stem cells
has had an enormous impact on human biomedical advances. The
availability of a large number of mouse cell lines has allowed rapid
progress in the field and the ability to establish numerous mouse models of
human diseases, as well as to pinpoint genes involved in human diseases
and to develop therapeutic approaches. Soon scientists realized that such
cells could be used for regeneration of damaged tissues. Examples of
diseases treated successfully in mice with mouse embryonic stem cells
include: myocardial infarction, severe immune deficiency, diabetes,
Parkinson’s disease, spinal injury, and demyelination, such as occurs in
multiple sclerosis.
25.
26.
27.
Thomson et al., supra note 6.
RICHARD MOLLARD, INT’L SOC’Y FOR STEM CELL RESEARCH, Embryonic Stem Cells,
at http://www.isscr.org/public/ES_cells.pdf (last updated Feb. 2, 2005).
M.J. Evans & M.H. Kaufman, Establishment in Culture of Pluripotential Cells from
Mouse Embryos, 292 NATURE 154 (July 9, 1981).
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For a long time it was thought that it was not possible to derive
human embryonic stem cells. However, drawing on experience with mouse
and monkey embryonic cell derivation, derivation of human embryonic
stem cells was achieved in 1998.28 Since then, many additional cell lines
have been derived around the world, including cell lines from blastocysts
with genetic defects, which will be crucial to the study of certain genetic
diseases and potential drug/therapy development.
Embryonic stem cells are so attractive for research because they grow
well in culture and retain the property of pluripotency during extended
culture growth. Thus, after prolonged culture periods, embryonic stem cells
can still produce a wide array of the cells of the body in culture. Embryonic
stem cells provide, therefore, an unlimited supply of stem cells and
specialized cells for meaningful experiments. Moreover, once the specifics
have been worked out, these cells can give rise to clinically relevant cell
numbers and could thus be used with greater ease in therapies than adult
stem cells.
Apart from the potential for clinical applications, human embryonic
stem cells provide a culture model for numerous biomedical research
applications. They provide a model to study early human development and
some of the disorders that lead to birth defects and childhood cancers.
Many of these conditions begin in utero and thus are difficult to study in
humans. These cells can also be used to perform functional genomics in
humans. Genes can be deleted and the results studied in the developing
specialized cells. Such studies are currently performed in mouse and fish
embryos, in which a gene might be deleted or altered to observe its effect
on heart development, for example. To do the same experiments in humans
is ethically unfeasible. Although this type of research in animals has
yielded a wealth of knowledge, the insights are not always directly
transferable to human growth, metabolism, and physiology.
It will also be interesting to investigate which genes are turned on and
off as the stem cell gives rise to a more specialized cell. This is difficult
with adult stem cells due to scarcity of available cells. Insights gained in
gene expression programs will inform improvements in as-yet inefficient
tissue culture protocols.
The unlimited supply of genetically normal human cells that can be
derived from human embryonic stem cell lines would be of great value for
use in the development of new drugs and pharmaceuticals. The metabolism
of rodent cells is different from that of human cells, and the differences can
affect drug metabolism. Drugs successfully tested in rodent models
sometimes turn out to be toxic in humans. With human embryonic stem
cells being a ready source of specialized cells to test organ-specific drugs,
28.
See generally Thomson et al., supra note 6.
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the costs of drug development may be considerably reduced.
Then there is the potential to use human embryonic stem cells to
generate specialized cells for therapeutic purposes. In principle, any type of
cells in the body can be derived from human embryonic stem cells and
could be used to regenerate failing or damaged organs. One day, perhaps,
entire organs could be grown from embryonic stem cells. But, as this
involves a three-dimensional structure and several different types of
specialized cells, it will be a difficult, but not impossible, goal to achieve.
The field of bioengineering has produced amazing results in recent years,
suggesting that the use of scaffolds may allow entire organs to be grown in
culture.
Cell types that have been derived in culture from human embryonic
stem cells include blood cells, endothelial cells, heart muscle cells, liver
cells, bone cells, cells of the nervous system, and insulin-producing cells.
In most instances, it remains to be seen whether the cultured specialized
cells have the ability to integrate functionally into living tissue and respond
to cues from the tissue. Achievement of these goals with both mouse and
human embryonic stem cells transplanted into mice indicates that it is
feasible.
VI. EMBRYONIC STEM CELLS IN THE CLINIC
Before human embryonic stem cells can be taken into the clinic,
several hurdles will have to be overcome. The first challenge will be to
produce the required specialized cell type in sufficient numbers. While
curing mice requires only a few cells, curing humans will require a
significant scaling-up of current approaches. Also, the therapeutic cells
derived from the embryonic stem cells have to be pure, with no
contaminating undifferentiated embryonic stem cells in the final cellular
preparation, as undifferentiated embryonic stem cells could grow into
teratomas once transplanted. Ongoing mouse studies are addressing this,
and teratoma formation is only seen rarely.
Another challenge will be the delivery of the cells to the required
location within an affected organ. Certain organs, such as the brain and the
heart, may prove difficult in terms of reaching the desired physical target,
although for different reasons: The brain, because of mechanical damage
possibly caused by the needle; and the heart, because its density precludes
easy diffusion of the injected cells. And once delivered, the cells have to
connect functionally with the resident cells in the tissue. They have to be
able to follow the cues from the environment and respond appropriately.
For example, transplanted pancreatic islet cells must produce insulin upon
demand and in the right amount.
But possibly the biggest hurdle will be to circumvent the patient’s
immune system, which may recognize new cells as foreign and kill them.
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As this is a problem that also plagues the existing solid organ transplant
field, there have been gains over the years. The discovery of immune
suppressive drugs was the first step in that direction. Life-long immune
suppression is not a solution, however, as patients have to take drugs on a
daily basis and risk either opportunistic infections or graft rejection,
depending on the dosage of the drugs. One distinct advantage of cell
therapy over solid organ transplantation is that the cells derived from
human embryonic stem cells can be manipulated in culture prior to
injection. The immune system can thus be tricked into not rejecting the
foreign cells. Promising new strategies are currently emerging in the
immunology field which could also be applied. Another approach could be
to establish large banks, with as many as possible cell lines in the hope to
match as many patients as possible. But this approach would only be
incrementally better than the current bone marrow registries, which provide
matched grafts for only twenty to thirty percent (much less for ethnic
minorities) of patients in need of a bone marrow transplant.
VII. NUCLEAR TRANSFER
Stem cells derived from nuclear transfer or therapeutic cloning
present another option to avoid immune rejection. For this, the DNA from
the patient is transferred into an egg which has had its DNA removed. The
egg is then coaxed into dividing as if it had been fertilized. At the
blastocyst stage of development the embryonic stem cells are derived and
can then be driven into the cell type required for the patient. The embryonic
stem cells can also be frozen for later use and derivation of more
therapeutic cells. As the therapeutic cells express the patient’s genes and
proteins, the immune system will not likely reject the cells.
Another important use for nuclear transfer is the possibility to
generate disease-specific embryonic stem cell lines from patients with
certain diseases, to study disease development and to develop drugs. For
example, it is still not known how Parkinson’s or Alzheimer’s disease
develop. It is important to fill this knowledge gap before developing
therapeutic applications from stem cells, as it is possible that the
transplanted cells may follow the fate of the patient’s destroyed cells.
While nuclear transfer was finally achieved recently in humans, it has
proved highly inefficient.29 The use of 242 oocytes yielded only one single
human embryonic stem cell line. Experience from nuclear transfer in
animals suggests that deep biological problems underlie this inefficiency
and may not be overcome easily. A restriction on further progress is thus
the limited availability of human eggs. Moreover, the time currently
29.
Woo Suk Hwang et al., Evidence of a Pluripotent Human Embryonic Stem Cell Line
Derived From a Cloned Blastocyst, 303 SCI. MAG. 1669 (Mar. 12, 2004).
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required for the nuclear transfer procedure is long enough that this
approach is not likely to be useful to patients acutely ill, however, the
process is still worth considering for patients with conditions that develop
more slowly.
Proof of principle for therapeutic cloning has been achieved in mice,
suggesting its feasibility in humans. A group of researchers has
demonstrated that they could cure severe immune deficiency in mice by
combining nuclear transfer and gene therapy.30
VIII. FEDERAL FUNDING FOR RESEARCH
History has shown that research in this country advances at a faster
pace with federal funding. Once proof of principle has been achieved,
increasing resources results in rapid advancement. This is valid for all
research fields. Considerable research progress occurs also in the private
sector, such as in the research and development departments of
pharmaceutical and biotech companies. Companies have different priorities
than public-sector government-supported research, however, which affects
the directions the companies choose to research and whether the results will
be publicly available. Moreover, when companies choose not to publish
their results, there is no peer-review of the findings.
In the stem cell research field in particular, the restriction of federal
funding imposed by President Bush31 has several consequences. First, only
twenty-two cell lines are currently eligible for federal funding, and for
technical reasons it is unlikely that the number will increase dramatically in
the near future. This number is inadequate to provide sufficient material for
experimentation for an entire country and several generations of stem cell
researchers. While a cell line is supposed to grow indefinitely, practically
speaking, that is not the case. Over long periods, the cells change and
accumulate mutations. Chromosomal alterations have already been
observed in a few of the approved cell lines. Second, all the approved cell
lines are “first generation” cell lines, and have thus been derived on mouse
feeder layers. This entails a risk of contamination by murine viruses which
could jump the species barrier, as has been observed recently in certain
instances, such as SARS and the Avian Flu, as well as the recently
demonstrated contamination by murine sugar molecules, which may trigger
an immune response upon transplantation into humans.32 Third, and most
30.
31.
32.
William M. Rideout, III et al., Correction of a Genetic Defect by Nuclear
Transplantation and Combined Cell and Gene Therapy, CELL METABOLISM, Apr. 5,
2002, at 17.
NAT’L INSTS. OF HEALTH, NIH’s Role in Federal Policy, at http://stemcells.nih.gov/
policy/NIHFedPolicy.asp (last modified June 10, 2004).
See generally Maria J. Martin et al., Human Embryonic Stem Cells Express an
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importantly, clinical application of embryonic stem cells will require large
banks of cell lines in order to provide an immunological match for as many
patients as possible. This is certainly not feasible with only twenty-two cell
lines.
There is no doubt in the research field about the great potential of
stem cells in general. For the most rapid progress towards human therapies,
however, both types of stem cells, adult and embryonic, have to be
investigated in more detail and in conjunction with one another. The
strongest research in adult stem cells will be in concert with research in
embryonic stem cells, and vice versa. Both types of stem cells have to be
seen as one research field, consisting of two complementary entities.
Moreover, the clinical applications of stem cell therapies are easy to
imagine in great variety, a factor that both supports the enthusiasm for the
field and can lead to over-enthusiastic hype. Whether a particular disease
will be treated with adult stem cells, embryonic stem cells, or simply by
applying knowledge gained from a better understanding of endogenous
stem cells, all of these approaches combined will yield the greatest
likelihood of success.
Immunogenic Nonhuman Sialic Acid, 11 NATURE MED. 228 (Feb. 2005).
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