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Who hasn't wanted to turn back time, to rewrite the fateful moment when we passed up a job, blew an exam or left words unspoken that might have set life on a different path? Impossible for us—but not for our cells.
In a feat ranked the number one breakthrough of 2008 by the journal Science, Children's Hospital Boston scientists have reset the cell's developmental clock. With a dash of genes, they reverted ordinary skin cells to an embryonic state, opening wide the cells' future possibilities. Blood? Brain? Heart? Lung? The reprogrammed cells, called induced pluripotent stem (iPS) cells, can potentially become these and more.
These changeling cells make a future of cell-based medicine more likely than ever. They appear as malleable as embryonic stem cells (ESCs)—the primordial cells that give rise to all others and can make endless copies of themselves. Grown in the lab, genetically corrected if needed and coaxed to become a specific tissue, iPS cells could allow doctors to patch a scarred heart, reawaken damaged nerves or reboot an immune system incapable of fighting infection. Because iPS cells are derived from skin or other body cells, not from embryos, they are far less controversial than ESCs. And because they come from a patient's own cells, they would be rejection-proof. In essence, we'd become our own medicine chests.
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George Daley, MD, PhD, director of the Stem Cell Transplantation Program and associate director of the Stem Cell research program at Children's, warns that it's far too early to know if iPS cells will deliver this future. "There are many aspects of the biology of reprogramming we still need to understand to make the process efficient and safe enough for clinical use," he says. "In our own lab, we're moving iPS and ESC research forward in tandem."
Daley's lab has been a prime mover in catapulting iPS cells from idea to reality. It was one of three labs worldwide to create the first human iPS cells, the first lab to create them from a living donor rather than banked tissues and the first to create a repository of iPS cells from patients with specific diseases. In Hyun Park, PhD, the lab's iPS cell specialist, has taught scores of visiting scientists how to generate the cells, and the Daley lab contributed the vast majority of the lines that established the Harvard Stem Cell Institute's iPS Core, which houses and distributes disease-specific iPS lines.
The Daley team's growing repository includes cells bearing single-gene disorders like muscular dystrophy, as well as genetically complex conditions like Parkinson's disease and type 1 diabetes. Also represented are genetic blood disorders, a primary interest of both Daley and Stem Cell Program Director Leonard Zon, MD. Both see children with severe anemias, leukemias, immune deficiencies and other life-threatening blood disorders in Children's Division of Hematology/Oncology (Daley is the Division's associate chief).
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Cell therapy for blood diseases will likely be among the first to reach the clinic. Blood cells are already being delivered to patients, in the form of bone marrow transplants, and Daley has already shown that cell therapy can correct blood disorders. Working with mice, he successfully combined gene and cell therapy to correct severe combined immune deficiency (SCID). Now he is working to turn human iPS cells into the blood stem cells needed to restore a patient's complete blood system. In a converging line of work, Zon is identifying drugs that stimulate blood stem cells to generate the legions of cells needed for transplant (see sidebar below).
Even before they reach the clinic, iPS cells will produce fresh insights into disease. Immune deficiencies, type 1 diabetes, muscular dystrophy and myriad other disorders are rooted in human development, their beginnings lost in the prehistory of fetal life. But iPS cells can recapture those beginnings.
"We can ask, where does the first muscle cell come from? Where does the first blood cell come from?" says Daley lab senior scientist Willy Lensch, PhD. "How is that different in this disease versus that disease?"
Those are the types of questions Luigi Notarangelo, MD, a world expert on immune deficiencies, hopes to answer. Children with these disorders cannot pet a cat, play in a sandbox or even hug a parent without risking life-threatening infection. "Understanding the biology of these diseases is not simple," says Notarangelo, director of Children's Research and Molecular Diagnosis Program in Primary Immunodeficiencies. "First of all, they are extremely rare. In addition, they are genetically heterogeneous. Each patient may have his or her own mutation even in the same gene."
With iPS cells, Notarangelo can precisely model each patient's genetic defect. He and Daley are now collaborating to create iPS cells representing eight of the 14 variations of SCID, the most severe immune deficiency. Notarangelo will map the differing ways these variants hobble immune response and ultimately use the iPS cells to investigate drugs and other interventions that may reverse disease.
IPS cells show similar promise for probing cancer. Sandra Ryeom, PhD, a cancer researcher with Children's Vascular Biology Program, turned to human iPS cells to explore why people with Down syndrome rarely get cancer. Collaborating with the late Judah Folkman, MD, she theorized that the extra copy of chromosome 21 that causes Down syndrome also gives patients an extra dose of genes that block angiogenesis, or development of blood vessels that feed a tumor. Her mouse studies bore out the theory and also zeroed in on the responsible genes, and a Down syndrome iPS line from Daley provided the confirmation she needed in humans.
When undifferentiated iPS cells are inserted into animals, they give rise to teratomas, tumors containing a crazy quilt of tissues: bone, hair, heart, brain—and blood vessels. If the angiogenesis theory was correct, teratomas from Down iPS cells should have fewer blood vessels than those from healthy volunteers. They did: Blood vessels budded, but never fully formed. Now Ryeom plans to use the Down iPS lines to tease out the precise role the Down genes play in blood vessel formation and their potential as cancer treatments.
Ryeom worked with undifferentiated cells, but most research and all clinical applications will require transforming iPS cells into the specific cell types damaged in disease. The challenge is significant. Like a garden, the human body has a multitude of local environments that are ideal for growing a particular set of cells. To propagate vigorous bunches of blood cells, for example, scientists must recreate the cells' natural growing conditions. Daley has succeeded in differentiating human iPS cells into mature red and white blood cells, and other labs have reported success generating brain cells.
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Healthy blood stem cells, precursors to all blood cell types, are needed in large quantities to treat leukemia and other blood disorders. The usual source is a bone-marrow transplant, but umbilical-cord blood is an attractive alternative: Collection is non-invasive, a perfect donor-recipient match isn't required and cord blood has fewer mature immune cells so poses a lower risk of rejection.
A disadvantage, however, is that each umbilical cord yields a limited number of stem cells, and patients over age 2 generally need two cords to get enough cells to restore immune function. Recently, Leonard Zon, MD, director of the Stem Cell Program at Children's Hospital Boston, found another way to get the quantity of cells needed: an existing drug, prostaglandin E2 (PGE2) stimulates blood stem cell production in both zebrafish and mouse models. If it works well in humans, it might be possible to get enough blood stem cells from a single umbilical cord.
A clinical trial beginning this spring at the Dana-Farber Cancer Institute and Massachusetts General Hospital will test this idea. First, 12 patients with leukemia will receive stem cells from two donor cords—one treated with PGE2, the other left untreated. By "tagging" the cells from the treated cord, the researchers can compare how quickly the cells engraft in the patient's bone marrow and bring up his or her blood-cell count. "We would hope that the blood cells produced by the treated cord would come back more quickly," says Zon, who also sees patients in Children's Division of Hematology/Oncology.
In a later phase, Zon's team plans to treat both cords with PGE2 and transplant the cells into patients with leukemia to determine whether this further accelerates the return of blood cell levels and shortens the period of immunodeficiency post-therapy. Zon estimates that it should be clear in about two years whether the therapy is successful. "My hope is that PGE2 will make transplantation much safer and ensure more long-term success," he says.
—Yvonna Reekie |
Children's urologist Carlos Estrada, MD, is collaborating with the Daley lab on yet another challenge—growing bladder cells to help patients with conditions like spina bifida that compromise bladder function. The traditional solution, which uses a length of intestine to reconstruct the bladder, is fraught with complications, including urinary tract infections, metabolic abnormalities, bladder stones and, most alarmingly, a heightened risk for bladder cancer. "This surgery has been used for 40 years," says Estrada. "But every time I do one I say, there has to be something better."
That "something better" may be bladders made from the cells growing in Estrada's lab. He has successfully converted mouse ESCs and iPS cells into the bladder's smooth muscle cells and—a first in the field—created the specialized epithelial cells that line the bladder. The experiments worked so well that Estrada at first thought his results were false—in just nine days, he had cells expressing the full range of uroplakins—proteins found exclusively in bladder epithelium.
Estrada is itching to repeat these experiments with the human iPS cells sitting in his freezer. "We're the only group doing iPS differentiation into urological tissues," he says. "It's an extraordinary time at an extraordinary place with extraordinary potential to help our patients."
Before iPS-based cells and tissues can be used in patients, much more needs to be understood about reprogramming itself. Lensch likens creating iPS cells to baking a soufflé. Oven temperature, the freshness of the eggs, how long they were whipped—all determine whether a soufflé rises or falls. So, too, many variables influence reprogramming.
For example, Daley and his team are exploring the role of telomeres, which regulate a cell's longevity. They are asking whether iPS lines made from differing source cells (from blood rather than skin, for example) form one cell type more readily than another; preliminary evidence suggests they do.
They are also seeking safer, more efficient reprogramming methods. The original techniques used viruses and genes that could cause cancer, and with current methods, only one in several thousand mature cells actually reprograms. But the science is moving at a breathtaking pace, with encouraging progress. "Every time I hold a group meeting, I learn some startling new fact," says Daley. "It's wonderful because the frontier is unexplored."
Daley is realistic about the challenges ahead but also optimistic, particularly since President Obama's executive order allowing more federal funding for ESC research (see related
article). Daley attended the White House signing ceremony on March 9, a satisfying culmination to years of public advocacy. "We can now use all tools available to study these exciting cells," he says.
That flexibility will speed the day when a plug of skin or dab of bone marrow can be cultured, reprogrammed and used to heal what ails us. |
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