Next Steps for Stem Cells
New methods to reprogram adult cells could create novel models of disease.
Searching the brain of an Alzheimer’s patient for clues into the origin of the disease is like trying to find the cause of a plane crash in the wrecked aftermath. However, a recent breakthrough in stem-cell research could generate new cellular models that allow scientists to study disease with unprecedented accuracy, from its earliest inception to a cell’s final biochemical demise.
Last November, two groups of scientists announced that they had independently achieved one of the stem-cell field’s biggest goals: the ability to reprogram adult cells into embryonic-like stem cells without the need for human embryos. (See “Stem Cells without the Embryos.”) The findings garnered extensive media attention, largely because the new method obviated the need for human embryos, a major ethical minefield that has stymied research.
But scientists at stem-cell labs around the world are excited for another reason. The technique creates cells that are genetically matched to an individual, meaning that it’s now possible to create novel cell models that capture all the genetic quirks of complex diseases. “Being able to have human cells with human disease in a dish accessible for testing is a real boon to technology and to science,” says Evan Snyder, director of the Stem Cells and Regeneration Program at the Burnham Institute, in La Jolla, CA.
While animal models exist for many human diseases, they typically only incorporate certain aspects of the disease and can’t capture the complexity of human biology. In addition, some disorders known to have a significant genetic component, such as autism, have proved difficult to model in animals.
To reprogram cells, scientists from Wisconsin and Japan independently engineered skin cells to express four different genes known to be expressed in the developing embryo. For reasons not yet clear to scientists, this treatment turns back the developmental clock. The resulting cells are pluripotent, meaning that they can develop into any type of cell in the body, and they can apparently divide indefinitely in their undifferentiated state. The first two published studies on the new technique reprogrammed cells from a skin-cell line, while a third study, published last month, generated stem cells from the skin biopsy of a healthy volunteer.
No one has yet generated cell lines from a patient, although scientists have been talking about doing so for years. Previously, the only way to make such models for complex genetic diseases was through human therapeutic cloning, also known as nuclear transfer, which is fraught with technical and ethical issues and has not yet been achieved. (See “Stem Cells Reborn” and “The Real Stem Cell Hope.”) “Assuming that these procedures are as easy to do as it seems, it’s definitely more tractable than nuclear transfer,” says Snyder. His own lab is trying to generate such models, as is “probably everyone else you could call on your rolodex,” he says.
To generate a disease-specific cell model, scientists would take some cells from a patient with a particular disease and revert them to an embryonic state. The cells would then be prodded to develop into the tissue type damaged in that disease, such as dopamine neurons in Parkinson’s disease or blood cells in sickle-cell anemia. By comparing the differentiation process in cells derived from healthy and diseased people, scientists could observe how that disease unfolds at a cellular level. They could also use the cells to test drugs that might correct those biochemical abnormalities. “We want to use these cells to ask and answer questions that can’t be asked and answered any other way,” says M. William Lensch, a research scientist at the Harvard Stem Cell Institute and Children’s Hospital Boston.
The relative simplicity of the approach–and the fact that it can be supported by federal funding–means that many more scientists are likely to attempt reprogramming than cloning. (In 2001, President Bush limited federal funding for embryonic stem-cell research to embryonic stem-cell lines already in existence.) According to Story Landis, chair of the Stem Cell Task Force at the National Institutes of Health, in Bethesda, MD, the funding agency has already announced two programs to fund reprogramming research and would welcome applications to derive cell lines from patients.
While no one has yet announced that he or she has derived a disease-specific cell model, George Daley’s lab at Harvard may be in the lead. Last month, he and his team published a paper in Nature showing that they can reprogram cells from a skin biopsy from a healthy person, and they are already trying to repeat the feat with tissue from patients. Ultimately, they are interested in developing models of sickle-cell anemia and Fanconi anemia, a hereditary disease in which the bone marrow doesn’t produce enough new cells to replenish the blood.
For example, patients with Fanconi anemia often suffer from skeletal problems, and their cells show an impaired ability to repair DNA. “We don’t have any idea why kids with DNA repair defect would get a blood disease, and why they sometimes get these bone abnormalities,” says Lensch, who works with Daley. But with stem-cell lines developed from a patient, “we could push the cells to develop into bone and blood, and try to learn about the links between the two.”
Such models could also help resolve long-held debates about specific diseases, such as Alzheimer’s. By differentiating reprogrammed cells from Alzheimer’s patients into neurons and comparing them with neurons derived from healthy embryonic stem cells or with cells with mutations that mimic a rare, hereditary form of the disease, scientists will be able to determine how much of Alzheimer’s is due to the environment versus genes, as well as how similar the sporadic form of the disease is to the hereditary form. (Most drugs on the market for Alzheimer’s were developed using models that mimic the hereditary form of the disease and have shown limited efficacy in patients.) “This is a whole new world of investigation,” says Lawrence Goldstein, a neuroscientist at the University of California, San Diego, whose lab is about to begin collecting skin cells from Alzheimer’s patients.
Despite the excitement, Lensch and others caution against abandoning other embryonic stem-cell research, especially therapeutic cloning. “We’re in the early stages of this research, where we’re excited about the possibilities but still need to show it’s both useful and representative of the disease,” says Snyder. In addition, he says, embryonic stem cells and perhaps cloned stem cells will be needed as controls for future studies.
Scientists also say that it’s too soon to tell how easy it will be to generate stem-cell lines from patients: the genetic variations that lead to the disease could also impact the reprogramming process. “With some genetic disease, I think it will be really difficult,” says Lensch.
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