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Scientists Cure Sickle Cell Anemia in Mice: Adult Skin Cells Coaxed to Produce Embryonic-like Properties
Published in UAB Insight, Winter 2008
A team of scientists from UAB and the Whitehead Institute for Biomedical Research have cured mice with humanized sickle cell anemia using a method that involves reprogramming skin fibroblasts to an embryonic-stem-cell-like state.
This process, which does not involve the use of embryos, is the first proof-of-principle in animals of a therapy using induced pluripotent stem (iPS) cells (Science. 2007;318:1920-1923).
The iPS cells have many of the same critical properties that have made embryonic stem cells the focus of intense research.
“The iPS cells express a number of the key genes expressed by embryonic stem cells, can be converted into virtually any cell type — brain, blood, lung, and liver, for example — and can divide to make more cells with the same attributes,” says UAB Department of Biochemistry and Molecular Genetics Chair Tim M. Townes, MD, coauthor of the Science paper.
“Like embryonic stem cells, iPS cells hold tremendous therapeutic promise. Once skin cells are converted to iPS cells, we can correct disease-causing mutations in vitro and transplant corrected cells back into the body where they start generating normal cells,” he says.
This scenario is precisely what the UAB/Whitehead Institute team achieved with the sickle cell mouse models, and Townes believes that adapting the technique for use in humans is only a matter of time.
The technique used to create iPS cells, which hold vast therapeutic potential, avoids the ethical storm surrounding embryonic stem cell work. “The process of generating iPS cells bypasses creation of a blastocyst that could potentially develop into a human being,” Townes says.
The iPS cells have another advantage over embryonic stem cells derived
from fertilized eggs: Because tissue created with iPS cells is
genetically identical to the recipient’s tissue, therapy based on this
technique would avoid or much reduce the need for immunosuppression.
“Without using cloning,” Townes says, “tissue created from embyronic
stem cells would not be a genetic match for recipients.”
Curing Sickle Cell Anemia
As part of UAB’s 20-year search for a sickle cell anemia cure,
scientists in Townes’ lab have developed transgenic knock-in mice with
the full pathology of the human disease. “We replaced mouse globin
genes with human globin genes. The mice make only human hemoglobin, and
more importantly, human sickle hemoglobin. Curing the disease in these
mice means a cure is possible in humans,” he says.
Townes and colleagues isolated skin cells from the tail tips of the
mice and used retroviruses to introduce four genes expressing
transcription factors that convert fibroblasts to iPS cells. The four
transcription factors — Oct3/4, Sox2, c-Myc, and Klf4 — are known to be
important regulators of stem-cell-like properties such as pluripotency.
Next, scientists “used an engineered construct created in the UAB lab
and corrected the sickle mutation in iPS cells with gene-specific
targeting,” Townes says. The cells were then transformed into
hemapoietic stem cells and transplanted back into the diseased mice,
where they began making red blood cells free from the sickle mutation.
“The animals’ sickle cell symptoms resolved completely, and at 4 months
posttransplant they have not rejected transplanted cells, which are
genetically identical to their own cells,” Townes says. He notes that
although long-term monitoring is necessary, the cells appear
self-renewing, and scientists expect them to produce all blood cell
lineages for the animals’ lifetimes.
The paper by the UAB/Whitehead Institute team is the first to describe
the successful use of iPS cells derived from a human disease model to
correct a mutation and treat a disease. “These findings are a major
step toward developing a cure for sickle cell anemia in humans,” Townes
says. He and his team will continue focusing their iPS research on
sickle cell anemia, working toward UAB’s long-held goal of curing the
disease in humans.
Animals to Humans
Regenerative medicine strategies based on iPS cells are advancing rapidly.
Japanese
scientist Shinya Yamanaka first described the revolutionary technique
used by Townes and colleagues in 2006 (Cell. 2006;126:663-676). A
little more than a year later, two labs, Yamanaka’s at Kyoto University
and James A. Thomson’s at the University of Wisconsin — independently
announced the successful reprogramming of human skin cells into iPS
cells (Science. 2007;318:1920-1923) and (Cell. 2007;131:1-12).
To achieve the conversion, Yamanaka used the same four transcription
factors that transformed skin fibroblasts into iPS cells in mice.
Thomson, who more than a decade ago was the first to isolate human
embryonic stem cells, substituted the genes NANOG and LIN28 for Klf4
and c-MYC. Like the tranformed mouse cells, iPS cells generated from
human skin fibroblasts have virtually the same properties as embryonic
stem cells.
Scientists must overcome several major issues before iPS-based therapy
is ready for use in humans, however. A primary challenge is finding a
safer delivery system for the transcription factors, Townes says.
“The retroviral vectors used to deliver the transcription factors into
skin cells create too much unpredictability to use in humans. The
retroviruses integrate randomly into the genome, where they could
potentially disrupt genes and cause cancers,” he says. “A system for
introducing these factors without insertion at random will have to be
developed before we can move iPS therapy from animals to humans.”
Townes’ group and others are investigating alternate delivery systems,
including transiently expressed viruses and viruses that can be deleted.
Experts also are concerned about the reprogramming technique’s
potential to stimulate the growth of tumors. One of the transcription
factors used for conversion — c-Myc — is known to cause cancer. Mice
treated with iPS cells reprogrammed with c-Myc are prone to tumors, as
are their progeny.
Scientists are already tackling this problem. Yamanaka et al reported
successful reprogramming of both human and mouse adult fibroblasts to
iPS cells with a three-factor formula that does not include c-Myc
(Nature Biotechnology. 2007;26:101-106).
The Future
Many public figures, including political and religious leaders who
oppose embryonic stem cell research, are hailing iPS-based approaches
to regenerative medicine as an end to the stem cell controversy and
ultimately, as a potential solution to a number of devastating medical
conditions.
David T. Curiel, MD, PhD, director of UAB’s Gene Therapy Center, says
the stem cell story highlights one of the most fundamental debates in
science — whether to marshal all available resources toward a single
goal or to pursue more open-ended research that may produce answers to
current questions as well as to those that have not yet been asked.
“With stem cells it is clear that we must be in the second category,”
he says. “Stem cells represent a novel biological principle of enormous
potential. Central to this is the recognition that intact adults
possess pluripotential cells that can repopulate organs and offer the
possibility for regeneration and repair. Yet, like every other
revolutionary technology that has preceded it, the fruits are not
likely to be realized quickly. It is possible that the diseases now
thought to be the most approachable will not be easily solved and that
applications that have not yet been considered will represent the first
true success stories.”
The field of gene therapy has followed a parallel path. When it emerged
on the scientific landscape more than 20 years ago, gene therapy
promised “a wide array of medical possibilities,” Curiel says.
“Although gene therapy has yet to deliver a clear genetic cure in a
human interventional context, it has enabled many other novel
technologies, including the transfer of genes that alters cells’ basic
biology to one that is pluripotential and stem-like.”
Curiel and other experts question how soon iPS therapy can be made safe
for direct therapy in humans. The regulatory hurdles involved in
translating such work to humans are likely to be formidable, and iPS
techniques may ultimately be most utilized for basic research — using
the cells to discover human disease mechanisms and test novel drugs,
for example.
Curiel is exploring various avenues of stem cell research. Last January
he traveled to Spain to work with scientists at the Centro Nacional de
Investigación Cardiovascular who are studying adult cardiac stem cells
for myocardial regeneration. In addition, he and colleagues at UAB have
proposed an investigation of mesenchymal stem cells as novel
cell-delivery vehicles for therapy of ovarian cancer.
Many scientists strongly advise against abandoning human embryonic stem
cell research in favor of an isolated focus on cell reprogramming
techniques. Harvard University, for example, plans to continue its
efforts to produce human embryonic stem cells through nuclear transfer.
“The iPS research that is now underway may be ahead of our ability to
understand the impact of the technology, but that is the nature of
science,” Curiel says. “This makes it critical to avoid focusing all
stem cell efforts in a single direction.”
For more information:
Dr. Tim Townes
Dr. David Curiel
http://www.health.uab.edu/42599/
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Stem Cells & Cloning 
