With two papers highlighting new sources of pluripotent stem cells, and a US presidential race shifting into high gear, a decade-old science policy challenge is inflamed again.
click to enlarge
Not long ago, the phrase “stem cell” was an obscure piece of laboratory jargon, lacking even a consistent definition. It could refer to a fork in the hematopoietic cell lineage, which gives rise to the diverse types of blood cells, or to one of the well-established lines of cultured cells used for knocking out genes in laboratory mice. At the end of 1998, it could also have come up in a journal club meeting, talking about a paper by James Thomson, PhD, and his colleagues at the University of Wisconsin in Madison, who had derived stable lines of human embryonic stem cells.1
Thomson’s paper changed all that. Now, the term comes up on evening news programs, in presidential campaigns, and around dinner tables nationwide, as shorthand for a complex public policy issue. The crux of the discussion is that, while Thomson’s stem cell lines ignited a promising new area of biomedical research, their derivation—which involved breaking up human blastocysts—also touched a match to the smoldering abortion debate.
If there was any risk that the resulting brouhaha would fade, it disappeared last fall, when a pair of reports described new lines of pluripotent stem cells derived from adult somatic cells. Opponents of human embryonic stem cell research seized on these results, proclaiming that there was no longer any need to destroy embryos. Others, however, were quick to point to the work’s limitations. Unfortunately, the politicized commentary has largely ignored both the enormous potential of stem cell science and the genuine challenges still holding it back.
Your stem cells, on drugs
The papers, published online in late November and early December2, 3, provide a good sample of the field’s current frontiers. In one, from Thomson’s group, the researchers demonstrate that a collection of just four factors is sufficient to reprogram human somatic cells, rewinding their developmental clocks and turning them into pluripotent stem cells. These induced pluripotent stem (iPS) cells can differentiate into endodermal, mesodermal, and ectodermal tissues, and are biochemically indistinguishable from embryo-derived stem cells.
“Human [embryonic stem] cells remain controversial because their derivation involves the destruction of human preimplantation embryos, and iPS cells remove this concern,” the researchers write. Well aware of the effect this would have on the political debate, though, the team also emphasized that the new cells are far from ready for clinical use.
For one thing, the researchers used a lentiviral vector to integrate the genes for the four factors into the somatic cells’ genomes, raising the risk that the cells would produce tumors if they were actually put into patients. However, the work could eventually be a boon to federally-funded stem cell scientists, who are forbidden from deriving new stem cell lines from embryos.
Through a university spokesman, Thomson refused to comment for this article, consistent with his current policy of declining all media interviews. But another University of Wisconsin stem cell researcher, Gabriela Cezar, PhD, explains that Thomson’s new results are tantalizing for drug developers.
“It’s scientifically fascinating, but it’s going to take awhile to develop the cell lines and to do the work to see if the reprogrammed cell lines would differentiate in the dish in the same manner,” says Cezar, an assistant professor of animal sciences who previously worked in compound screening labs at Pfizer.
Getting the cells to differentiate correctly is critical for Cezar’s current work, which focuses on using stem cell lines to develop new toxicological tests for candidate drugs. Because embryo-derived human stem cells can differentiate into any type of human tissue, they provide an ideal platform for in vitro drug screening. “Particularly for developmental toxicity, the cells really emulate and recapitulate each step of organogenesis, and so … when we expose the cells to compounds, we could really recapitulate what would be happening during human development,” says Cezar.
Using mass spectrometry, Cezar and her colleagues establish metabolomic profiles for each compound they test, identifying all of the changes that occur in a particular tissue’s metabolites as a result of the test drug. Running known teratogens and other toxins through the system, the team builds profiles of the metabolic changes associated with particular types of toxicity. “That gives us the ruler to gauge for compounds that we don’t know,” says Cezar. Lead compounds that produce a toxic-looking profile could be dropped early, allowing developers to focus on more promising candidates.
Because the predictive tests come from differentiating human cells, they could avoid many of the pitfalls of other in vitro and animal systems.
Rather than testing compounds on immortalized lines derived from a single patient’s brain tumor years ago, for example, the researchers can test them on a panel of developing human neural tissues that grew last week, and is biologically indistinguishable from human fetal tissue.
Follow that mouse
The political firestorm surrounding human stem cells has often distracted attention from the much older technique of manipulating mouse embryonic stem cells. As the other recent paper on iPS cells demonstrates, however, the mouse system continues to point the way for new therapeutic strategies.
Indeed, Thomson’s most recent work built on earlier studies in mice, which identified the four transcription factors that can, in combination, cause developmental reprogramming. In December, Rudolf Jaenisch, MD, and his colleagues at the Whitehead Institute for Biomedical Research in Cambridge, Mass., reported taking this work a step further, developing the mouse iPS cells into a therapy for a murine model of sickle cell anemia. The work immediately suggests a plausible strategy for approaching human stem cell therapies.
In the new study, Jaenisch’s team collaborated with researchers at the University of Alabama in Birmingham, who had inserted a human sickle cell hemoglobin gene into transgenic mice. The mice develop more immature and misshapen red blood cells than wild-type animals, have trouble breathing, lose weight, and die young. “These mice really mimic very well the human disease,” says Jaenisch.
The researchers took cells from the tails of the afflicted mice, inserted the four transcription factors that can induce reprogramming, and successfully derived iPS cell lines. After genetically modifying the cells to correct the sickle cell defect, the team stimulated them to differentiate into hematopoietic progenitors, and injected them back into the mice. Because the cells had come from the same mice, the animals’ immune systems did not reject them, and the transplanted hematopoietic progenitors yielded functional red blood cells that ameliorated the disease.
Besides the sickle cell treatment, the scientists also performed an extensive series of assays on the iPS cells to see if they were different from embryonic stem cells in any way. “By all of the biological assays which are available to us, they are identical,” Jaenisch concludes. Furthermore, he believes it may be possible to induce reprogramming with even fewer than four gene products, simplifying the process.
While the new work certainly bodes well for future human therapies, the team used several tricks in the mouse system that will never work in the clinic: transferring genes into the stem cells with oncogene-containing retroviral vectors, for example. “Obviously retroviruses are unacceptable, so they have to be replaced; oncogenes obviously can’t be used, so they have to be replaced,” says Jaenisch. In addition, homologous recombination is very inefficient in human stem cells, making them inherently harder to modify than the mouse cells.
Like others in the field, Jaenisch argues that human embryonic stem cells will remain a critical research tool, at least in the near future.
“We only were able to do this in vitro because we knew about [embryonic stem] cells,” says Jaenisch, adding that “in human cells, there’s still a lot to be learned.”
About the Author
Originally trained as a microbiologist, Alan Dove has been writing about science and its interfaces with industry and government for more than a decade.
This article was published in Drug Discovery & Development magazine: Vol. 11, No. 2, February, 2008, pp. 24-27.
Filed Under: Drug Discovery