May 24, 2004

The Stem Cell Challenge

What hurdles stand between the promise of human stem cell therapies and real
treatments in the clinic?

By Robert Lanza and Nadia Rosenthal

Stem cells raise the prospect of regenerating failing body parts and curing
diseases that have so far defied drug-based treatment. Patients are buoyed
by reports of the cells' near-miraculous properties, but many of the most
publicized scientific studies have subsequently been refuted, and other data
have been distorted in debates over the propriety of deriving some of these
cells from human embryos.

Provocative and conflicting claims have left the public (and most
scientists) confused as to whether stem cell treatments are even medically
feasible. If legal and funding restrictions in the U.S. and other countries
were lifted immediately, could doctors start treating patients with stem
cells the next day? Probably not. Many technical obstacles must be overcome
and unanswered questions resolved before stem cells can safely fulfill their

For instance, just identifying a true stem cell can be tricky. For
scientists to be able to share results and gauge the success of techniques
for controlling stem cell behavior, we must first know that the cells we are
studying actually possess the ability to serve as the source, or "stem," of
a variety of cell types while themselves remaining in a generic state of
potential. But for all the intensive scrutiny of stem cells, they cannot be
distinguished by appearance. They are defined by their behavior.

Most versatile are embryonic stem (ES) cells, first isolated in mice more
than 20 years ago. ES cells come from the portion of a very early-stage
embryo that would normally go on to form three distinctive germ layers
within a later embryo and ultimately all the different tissues of the body.
ES cells retain this potential ability to produce any cell type in the body,
making them pluripotent.

Most of the existing human ES cell lines in the world were derived from
unused embryos created for couples seeking in vitro fertilization (IVF).
Researchers working with these cells have found that they usually recover
after freezing and thawing and can differentiate into assorted cell types in
a culture dish. But it is becoming clear that not all human ES cell lines
are the same.

Seeking Stemness
Some lines will differentiate into only certain cell types; others grow
sluggishly in culture. To ensure that these cells are pluripotent before
using them in research, two possible tests, already common in nonhuman ES
cell studies, have been proposed by a group of American and Canadian
biologists hoping to set standards for experimentation with human ES cells.
One would involve injecting the ES cells into an animal's body tissue. If
they form a teratoma--a distinctive tumor containing cell types from all
three embryonic layers--their pluripotency is proved. Another way to test
putative ES cells is to mark them, then inject them into a developing animal
embryo. When the animal is born, if the marked cells turn up in all its
tissues, the cell line is deemed pluripotent. But testing human embryonic
stem cells in this manner would create a chimeric animal with human DNA
throughout its body, a prospect many find ethically troubling. What is more,
passing the latter test does not always guarantee that the cells will
differentiate in the lab.

The need to find more reliable markers that distinguish truly pluripotent ES
cells is driving widespread attempts to catalogue the genes that are turned
on or off at various times in cultured ES cells. Having such a gene
expression profile would not only provide a way of identifying pluripotent
ES cells, it would also offer tremendous insight into the properties that
confer their "stemness." Unfortunately, to date, gene expression profiles of
ES cells have yielded conflicting results, and the search for a clear ES
cell signature continues.

Of course, the goal of stem cell research is to replace or regenerate
failing body parts, such as pancreatic insulin-producing cells in diabetics
or dopamine-producing neurons in people with Parkinson's disease. But
techniques for coaxing ES cells to differentiate into desired cell types are
far from perfected.

Left to their own devices in a culture dish, ES cells will spontaneously
differentiate into a hodgepodge of tissue types. With timed administration
of chemicals, we can often direct them to become one cell type or another.
But they seem to prefer to become certain tissues--readily proliferating
into patches of beating heart cells, for example--whereas other tissues are
far more difficult to derive.

Putting Cells to Work
Because we still do not understand the signals that normally instruct these
cells to choose a particular pathway during embryonic development, many
researchers are studying the natural embryonic "niche" to understand
possible environmental cues. Other scientists are trying to profile
embryonic cells' gene expression patterns as they differentiate in order to
find genes that could be turned on or off to direct the cells toward a
particular tissue type.

But deriving what appear to be cells of the desired kind is just half the
battle. ES cells will easily produce dishes full of neurons, for instance,
but these are only useful if they can be placed in a living brain, make
connections and "talk" with surrounding neurons. In 2001 stem cell
researchers thought they had a major breakthrough when Ronald McKay of the
National Institutes of Health reported having generated insulin-producing
cells--a coveted goal in stem cell research--from mouse ES cells. Last year,
though, Douglas A. Melton of Harvard University reproduced McKay's
experiment and found that the cells had absorbed insulin from their culture
medium rather than producing it themselves. Discovering markers to identify
truly functional cells is another urgent task for the stem cell research

It would be ideal if we could simply inject ES cells into the part of the
body we wish to regenerate and let them take their cues from the surrounding
environment. ES cells' pluripotency, however, makes this far too dangerous
an approach for human therapy. The cells might form a teratoma or could
differentiate into an undesirable tissue type, or both. In animal
experiments, teratomas containing fully formed teeth have been reported.

Rather than risk creating a tumor or a tooth in a patient's brain or heart
with direct ES cell injections or struggling to produce specific functional
tissues, many ES cell researchers are now striving for a middle ground. By
coaxing ES cells into a more stable, yet still flexible, progenitor-cell
stage before administering them, we can avoid uncontrolled differentiation
while still taking advantage of environmental cues to generate the desired
cell types.

Even though these progenitor cells can take to their environment and
initiate the generation of new tissue, they would still be subject to attack
by the patient's own body. ES cells and their derivatives carry the same
likelihood of immune rejection as a transplanted organ because, like all
cells, they carry the surface proteins, or antigens, by which the immune
system recognizes invaders. Hundreds of combinations of different types of
antigens are possible, meaning that hundreds of thousands of ES cell lines
might be needed to establish a bank of cells with immune matches for most
potential patients. Creating that many lines could require millions of
discarded embryos from IVF clinics.

Some researchers have speculated that such an extensive bank might not be
necessary, that patients can be desensitized to ES cell derivatives or that
the antigenic properties of the cells themselves can be reduced. But those
feats have yet to be conclusively demonstrated. At present, the only sure
way to circumvent the problem of immune rejection would be to create an ES
cell line using a patient's own genetic material through nuclear transfer or
cloning. This technique has inspired considerable controversy and has its
own practical hurdles to overcome, but it has also produced encouraging
results in animal experiments for regenerating failing tissues.

Turning Back the Clock
Cloning can be viewed as a way to restore embryonic potential to a patient's
old cells. The human body is made of more than 200 kinds of cells, and in
mammals, once a cell is committed to a particular type, there is normally no
turning back. It is said to be "terminally differentiated." An exception to
this rule is when the nucleus containing an unfertilized egg's genetic
material is extracted and the nucleus of a somatic (body) cell is placed
into the egg instead. The egg is tricked into behaving as though it has been
fertilized and begins dividing like a normal embryo. The ES cells derived
from this embryo will contain the donor somatic cell's DNA. But the somatic
cell will have been reprogrammed--reset to a state of stemness, capable of
generating any tissue type.

One of us (Lanza) recently showed that partially differentiated stem cells
from a cloned mouse embryo could be injected into the donor mouse's heart,
where they homed in on the site of injury from a heart attack, replacing 38
percent of the scar with healthy heart tissue within a month. And this year,
for the first time, somatic cell nuclear transfer (SCNT) yielded a human ES
cell line. A few in the scientific community had started to wonder whether
the nuclear-transfer technique would work with primate physiology to produce
therapeutic stem cells. But Woo Suk Hwang of Seoul National University and
his colleagues proved that it could be done. The Korean team announced this
past February that they had created a human embryo through SCNT, grew it
into a blastocyst and derived a pluripotent ES cell line. Their
accomplishment represents a major milestone. It also demonstrates how many
unknowns we still face.

Because Hwang's group had 242 donated eggs to work with, they were able to
experiment with techniques, timing and conditions at every step. Even so,
from hundreds of eggs the effort yielded only a single ES cell line, and the
researchers have said that they are not certain which of their methods was
responsible for that success. Much remains to be learned about the
mysterious mechanism of reprogramming within the egg and all that could go
wrong while creating and culturing a nuclear-transfer embryo.

Scientists are still not sure whether reprogramming itself or other aspects
of handling these embryos might introduce gene mutations that could
predispose the resulting ES cells to senescence or cancer, and more research
is needed to detect these potential problems. Inherited gene mutations, such
as those that cause hemophilia or muscular dystrophy, would have to be
corrected as well before using a patient's own cells to create ES cells. But
techniques for gene-specific modifications routinely performed in mouse ES
cells have been successfully applied to human ES cells, providing a means of
safely correcting mutations before administering cells to patients.

The overall health of ES cells derived from clone embryos has also been
questioned because efforts to produce live animals through cloning have met
with an unusually high rate of deformities and mortality. When a cloned ES
cell line's potential is tested by injecting the cells into a developing
animal blastocyst, though, the resulting animals seem to be perfectly
normal. This outcome suggests that although reproductive cloning is clearly
too unpredictable to consider for humans, ES cells derived by nuclear
transfer, at least for therapeutic purposes, are equivalent to regular ES

Similar safety questions must also be resolved for a different technique
that produces ES cells without nuclear-transfer or IVF embryos. In a process
called parthenogenesis (from Greek for "virgin birth"), an unfertilized egg
can be chemically tricked into beginning cell division as though it has been
fertilized. These pseudo-embryos, or parthenotes, are considerably easier to
grow than nuclear-transfer embryos. In animal studies, parthenotes have
yielded ES cells able to differentiate into multiple tissue types in culture
and to pass the teratoma test, forming cells from all three embryonic germ

Unlike normal body cells, which contain a set of chromosomes from each
parent, parthenotes contain a doubled set of the egg donor's chromosomes.
This duplication gives a parthenote a full complement of genes but prevents
it from being viable if it were implanted in a woman's womb. Having a single
"parent" also means that parthenote cells carry half the normal potential
combinations of antigens, making them much easier to match to patients. A
bank of fewer than 1,000 parthenogenic ES cell lines could probably provide
immunological matches for most of the U.S. population.

How long it will take for any ES cell therapies to be tested in humans will
be determined as much by politics as by the remaining scientific questions.
Well-understood and easy-to-control cell types derived from ES cells, such
as dopamine-producing neurons or the eyes' retinal pigment epithelium cells,
could be ready for human trials in less than two years. In the meantime, the
extraordinary regenerative potential of embryonic stem cells has intensified
the search for similar cells that may be involved in normal healing in the
adult body.

Hidden Potential?
Skin begins repairing itself immediately after being injured. The human
liver can regenerate up to 50 percent of its mass within weeks, just as a
salamander regrows a severed tail. Our red blood cells are replaced at a
rate of 350 million per minute. We know that prolific stem cells must be at
work in such rapidly regenerating tissues. But their very vigor raises
questions about why other organs, such as the brain and heart, seem
incapable of significant self-repair, especially when purported stem cells
have also recently been discovered in those tissues.

The best-known stem cells in the adult body are the hematopoietic stem cells
found in bone marrow, which are the source of more than half a dozen kinds
of blood cells. Their ability to generate a variety of cell types, at least
within a specific tissue family, is why hematopoietic stem cells have been
described as multipotent.

There is great hope that similar multipotent stem cells found in other body
tissues might be drafted into repairing damage without the need to involve
embryos--or better still, that an adult stem cell with more versatility,
approaching the pluripotency of embryonic cells, might be discovered.

But scientists are just beginning to investigate whether natural
regeneration is somehow blocked in tissues that do not repair themselves
easily and, if so, whether unblocking their regenerative capacity will be
possible. The very source, as well as the potential of various adult stem
cells, is still disputed among researchers. We cannot say for sure whether
tissue-specific adult stem cells originate within those tissues or are
descendants of circulating hematopoietic stem cells. Nor do we know how far
these cells can be pushed to differentiate into functional tissues outside
their specific type or whether such transdifferentiation produced in the
laboratory could be reproduced in a living organism.

The idea that certain adult stem cells might have greater potential first
came from observations following human bone marrow transplants, when donor
cells were subsequently found in a wide range of recipients' tissues. These
accounts implied that under the right conditions, stem cells from the bone
marrow could contribute to virtually any part of the body. (Similar claims
have been made for the so-called fetal stem cells found in umbilical cord
blood, which resemble hematopoietic stem cells.)

Attempts to directly test this theory in living organisms, however, have not
found consistent evidence of such plasticity. In March separate reports from
Leora Balsam and her colleagues at Stanford University and from a group led
by Charles E. Murry of the University of Washington both described using
powerful tracking methods to see if hematopoietic stem cells would
incorporate into injured heart muscle, a nonhematopoietic tissue. Neither
group detected contribution of new tissue by the stem cells.

What has increasingly been found is extensive fusion of bone marrow stem
cells to cells in the heart, liver and brain, offering an alternative
explanation for the presumed transdifferentiation. In future studies of
adult stem cell potential, it will be crucial to rule out the possibility
that stem cells are merely fusing to local cells rather than generating new

Still, tissue-specific cells have already produced encouraging results. In
the German TOPCARE-AMI study of patients with severe heart damage following
myocardial infarction, the patients' own heart progenitor cells were infused
directly into the infarcted artery. Four months later the size of the
damaged tissue swath had decreased by nearly 36 percent, and the patients'
heart function had increased by 10 percent.

The small number of stem cells that can be isolated from any adult tissue
remains the biggest technical hurdle to applying this type of research more
widely in the clinic. In mouse bone marrow, stem cells are as rare as one in
10,000, and the ratio may be even greater in humans. In most tissues, there
is no predictable location for stem cells, and we possess only limited tools
for identifying them using surface markers or gene expression signatures.

Once isolated, adult stem cells are also notoriously slow and
labor-intensive to grow. As is true of embryonic cells, so little is
understood about the factors that may control the adult stem cells' fate
that we do not yet know whether extensive time spent in culture could harm
their ability to restore tissues in patients.

Rather than hunting for a patient's stem cells to remove, cultivate and then
replace them, we may be able to summon the body's hidden stores. Increasing
evidence suggests that stem cells, like metastatic tumor cells, respond to
common chemical signals leading them to sites of injury. One of us
(Rosenthal) recently showed in mice that stem cells will travel great
distances to reach an injury when summoned with the help of a protein called

Marshaling the body's own ability to trigger tissue regeneration by stem
cells will require a better grasp of the roles played by such chemical
signals. Rosenthal and her collaborator Antonio Musarò have demonstrated
that IGF-1 helps to beckon stem cells, but we suspect that this molecule may
also take part in causing some of the injured cells to revert to a
multipotent state and begin differentiating into the required tissue types.
This phenomenon, known as epimorphic regeneration, underlies the ability of
newts and zebra fish to regrow entire limbs and organs.

Regenerative medicine's ideal would be to find a means to cause such
controlled dedifferentiation of adult tissue--in essence turning a
terminally differentiated cell back into a stem cell. Many

researchers are looking for the magic molecules that can produce this
transformation, and some very preliminary successes have recently been
reported. But therapeutic regeneration through dedifferentiation is a long
way off and will most likely come from a much better understanding of stem
cells themselves--both adult and embryonic.

Which Way Forward?
As often happens in science, stem cell research has raised as many new
questions as it has answered, but the field is advancing. Early tests of
human adult stem cells in treating cardiovascular disease are encouraging
and will certainly lead to more extensive trials in the near future. Given
much promising experimental evidence in animals, therapeutic trials of human
ES cell derivatives in neurodegenerative disease are probably imminent.

As the appropriate source of cells for both research and eventual
therapeutic applications continues to be hotly debated, restrictions on this
research are slowing progress. But we believe that generating replacement
cells and regenerating organs are feasible and realistic goals. The
remaining hurdles are difficult but not insurmountable.

ROBERT LANZA and NADIA ROSENTHAL are leading stem cell researchers. Lanza is
medical director of Advanced Cell Technology, Inc., as well as adjunct
professor at the Institute of Regenerative Medicine at Wake Forest
University School of Medicine. Lanza's current research centers on embryonic
stem cells; he has also done groundbreaking work in cloning and tissue
engineering. Rosenthal is head of the European Molecular Biology Laboratory
in Rome. She directs the EMBL Mouse Biology program, concentrating her
research on stem cell-mediated regeneration of neuromuscular and cardiac
tissues, embryonic heart development, and developing mouse models of human
diseases. Before joining EMBL, Rosenthal directed a laboratory at the
Harvard Medical School's Cardiovascular Research Center and was a consultant
on molecular medicine for the New England Journal of Medicine.

On 2/15/07, Michael Balter <[log in to unmask]> wrote:
> Mitchel, you are welcome to challenge all my assertions. But please do it,
> rather than tell me that you can do it. Especially on the stem cell issue.
> Michael
> On 2/15/07, Mitchel Cohen <[log in to unmask]> wrote:
> >
> > Problem is, Michael Balter was simply wrong in his denunciation of
> > Jonathan regarding adult stem cells. But he asserted his wrongness with such
> > glee, such joy that one has to question and indeed challenge the other
> > assertions he makes as well.
> >
> > Scientists need to apply science to their own beliefs and motivations.
> > We all need to do that.
> >
> >
> > Mitchel
> >
> >
> >
> > -----Original Message-----
> > >From: Michael H Goldhaber < [log in to unmask]>
> > >Sent: Feb 15, 2007 1:30 PM
> > >To: [log in to unmask]
> > >Subject: Re: Mitchel's Marxism & the Environment talk now on-line
> > >
> > >Michael,
> > >
> > >Thanks, for this. I agree pretty much completely. Science for the
> > >People was originally called Scientists for Social and Political
> > >Action. We were a group of physicists aghast at the ways our science
> > >was tied into war-making. We questioned the idea  that "research
> > >means progress, and progress is good," and this must still be
> > >questioned. (By the way, for a thoughtful, partial anti-reductionism,
> > >see Michael Pollan's recent NY  Times piece on the difference between
> > >food and nutrients.)
> > >
> > >For me, the questioning helped lead me out of physics altogether. One
> > >way I tended was towards the ideas of Marx. But he has now been dead
> > >for almost 125 years, and he wasn't always right even in his
> > >lifetime. One thing that disturbed me about much Marxism (and still
> > >does)  was the way in which it became like a religion, complete with
> > >sacred texts —open to varying interpretations, certainly — but not to
> > >be considered as only a step on the way to better and more currently
> > >relevant understanding.
> > >
> > >That viewpoint has much to do with what I view as scientism, namely
> > >using isolated tidbits of evidence, some indeed laboratory-based, as
> > >if they prove some opinion that goes far beyond what they can
> > >possibly even indicate. Thus, abortion opponents like to use
> > >photomicrographs of embryos to "prove" that they are human beings.
> > >Bush and company used photos of strange trucks to "prove" that Saddam
> > >was engaged in biological warfare. Jonathan uses highly questionable
> > >research from  out-of-the-way "peer-reviewed" journals to 'prove"
> > >whatever he likes.  (One thing I learned as a young physicist was
> > >there is always some peer-reviewed journal somewhere that will
> > >publish anything that remotely resembles a scientific result.)
> > >
> > >There are many serious issues about the contemporary role of science
> > >and the direction of society. It would be nice to have a forum in
> > >which they could be seriously discussed, if enough people care.
> > >
> > >
> > >Best,
> > >Michael
> > >
> > >On Feb 15, 2007, at 3:29 AM, Michael Balter wrote:
> > >
> > >> This post from Jonathan Campbell is an example, to me anyway, of
> > >> the absolute contempt that some so-called leftists have not just
> > >> for science--which could, just perhaps, be justified by the abuses
> > >> of science--but for any standards of evidence whatsoever when it
> > >> comes to argumentation and debate. In other words, first comes the
> > >> politics, and then the "facts" to back them up. Is this any
> > >> different from the way the Bush administration justified the war in
> > >> Iraq? Not in my view.
> > >>
> > >> Let's just take one of Jonathan's statements:
> > >>
> > >> "Why is it that there is no interest in ADULT stem cells, which
> > >> really can be
> > >> obtained easily (from the patient's intestines) and used easily and
> > >> relatively cheaply to re-grow organs?"
> > >>
> > >> Every single clause in this sentence is factually incorrect. If it
> > >> is so easy to grow organs from adult stem cells, perhaps Jonathan
> > >> would like to tell us where this is actually being done.
> > >>
> > >> As for the pharmaceutical industry, it has a lot to answer for. But
> > >> do we base our policies on taking the exact opposite position from
> > >> everything it does? How about the attempts to find an AIDS vaccine.
> > >> Obviously pernicious, and must mean that a pure leftist should be
> > >> for continuing the epidemic, right?
> > >>
> > >> In sum, this is the kind of infantile, ignorant, knee-jerk, no-
> > >> nothing leftism that has landed the American left in the toilet for
> > >> the past 30 years where it will stay until those who want to
> > >> impress us with how hard they work and how tirelessly they engage
> > >> in the struggle actually get their brains in gear and start acting
> > >> and thinking in the real world.
> > >>
> > >> Sorry for the rant, but we had a dream once and too many leftists
> > >> have turned it into a fantasy with this kind of crap.
> > >>
> > >> Michael
> > >>
> > >>
> > >>
> > >>
> > >> On 2/15/07, Jonathan Campbell <[log in to unmask]> wrote:
> > >> The person with chuzpah around here is the Marxist Expert from the
> > >> Columbian
> > >> White Tower.
> > >>
> > >> Mitchel at least is at least DOING something good for the world
> > >> (actually
> > >> many things) and I think his critique of Marxist philosophy with
> > >> regard to
> > >> capitalist progress is right on the money.
> > >>
> > >> Louis, here is something to chew on philosophically (I understand
> > >> you have
> > >> almost a PhD in this): why are so many extremely wealthy
> > >> capitalists and
> > >> capitalist foundations connected to the pharmaceutical industry
> > >> interested
> > >> in a technology (embryonic stem cells) that is supposed to really
> > >> solve
> > >> diseases, when the primary business model of the pharmaceutical
> > >> industry is
> > >> long term illness maintenance, having nothing to do with cures or
> > >> effective
> > >> curative treatment? Why are prominent leftists lining up as a
> > >> cheering squad
> > >> along with the capitalists for this new technology?
> > >>
> > >> Why is it that there is no interest in ADULT stem cells, which
> > >> really can be
> > >> obtained easily (from the patient's intestines) and used easily and
> > >> relatively cheaply to re-grow organs?
> > >>
> > >> Why is the FDA and the pharmaceutical industry so intent on
> > regulating
> > >> natural supplements (via Codex) when "modern medicine" is the
> > >> leading cause
> > >> of death in the US (>650,000 per year) and the leading cause of
> > injury
> > >> (millions per year), while the number of people who have died as a
> > >> result of
> > >> natural supplements is less than a hundred per year, almost
> > >> exclusively the
> > >> result of not following the label and/or sheer stupidity.
> > >>
> > >> Jonathan
> > >>
> > >> ----- Original Message -----
> > >> From: "Louis Proyect" <[log in to unmask]>
> > >> To: < [log in to unmask]>
> > >> Sent: Wednesday, February 14, 2007 1:49 PM
> > >> Subject: Re: Mitchel's Marxism & the Environment talk now on-line
> > >>
> > >>
> > >> > >Stan Goff has kindly posted the talk I gave on January 26, 2007,
> > at
> > >> > >Bertell Ollman's Marxism Seminar, at
> > >> >>
> > >> >>
> > >> >>
> > >> >>You can read the entire talk there -- including the parts that I
> > >> had to
> > >> >>excise due to time constraints (concerning stem cell research and
> > >> the
> > >> >>left) and also post your own comments.
> > >> >>
> > >> >>Thanx.
> > >> >>
> > >> >>Mitchel
> > >> >
> > >> > Amazing. Not a single reference to John Bellamy Foster, Paul
> > >> Burkett,
> > >> > James O'Connor or Mike Davis.
> > >> >
> > >> > What chutzpah.
> > >> >
> > >> > --
> > >> >
> > >> >
> > >> >
> > >>
> > >>
> > >>
> > >> --
> > >>
> > >>
> > >> ******************************************
> > >> Michael Balter
> > >> Contributing Correspondent, Science
> > >> [log in to unmask]
> > >> ******************************************
> > >
> >
> --
> ******************************************
> Michael Balter
> Contributing Correspondent, Science
> [log in to unmask]
> ******************************************