The Other Stem Cells
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Mammalian development normally proceeds in a steadfast forward march. A sperm fertilizes an egg, the egg divides once, those cells divide and the process continues until a ball of cells forms. About four days after fertilization, that ball has developed into a blastocyst, and many of the cells within it are embryonic stem cells. (Researchers can extract those cells, typically from unused embryos created during in vitro fertilization, but that process destroys the blastocyst.) The DNA within ES cells is the same as that found in adult heart, skin and eye cells, but it exists in a very open form. ES cells are pluripotent—they can become any cell in the body—because all of their genes have the potential to be turned on.
Gregory Heisler for Proto
That’s very different from adult cells. In the process of becoming a liver cell, for example, many of a cell’s genes are turned off or disabled. The DNA of those genes is folded tightly, and chemicals called methyl groups attach themselves to the genes, preventing the transcription of genetic code that enables the proteins associated with the genes to form. With whole sections of DNA turned off, a liver cell can make only the proteins needed for the liver to function.
These gene shutdowns were once thought to be permanent. But it turns out that a process called somatic cell nuclear transfer (SCNT) can sometimes reactivate genes. During the 1950s and 1960s, researchers working with frogs developed techniques to extract the nucleus of a differentiated adult cell and stick it into an enucleated, unfertilized egg. Something about the environment of the egg again turned on all of the genes in what had been a differentiated nucleus, reprogramming the adult DNA to its embryonic state, and the newly pluripotent cell was able to grow into a tadpole. In 1996 researchers in Scotland used SCNT to transplant the nucleus of an adult sheep cell into an enucleated sheep egg, producing Dolly, the first cloned mammal. Other animal clones followed.
Cloning was very difficult, however. Few cell transplants worked, and fewer still yielded healthy adult animals. Researchers assumed that whatever happened in the egg to return an adult cell to its pluripotent state must be exceedingly complex.
Then came Yamanaka’s study, published in the journal Cell in August 2006. “To everyone’s jaw-dropping surprise, you needed only four genes,” says stem cell biologist David Scadden, director of the Center for Regenerative Medicine at the MGH and co-director of the Harvard Stem Cell Institute. To find those genes, Yamanaka had compared all the genes (the genome) of an embryonic stem cell with those of an adult cell to find genes that were turned on in the ES cell but turned off in the adult cell. Those, along with genes already known to be important in embryonic stem cells, totaled 24, and, using a virus to carry them, he inserted the genes into skin cells taken from the tail of a mouse. Some of the new cells, as they divided and grew, started to change. They shrank and clumped into balls of cells, reversing their developmental course and eventually becoming surprisingly similar to embryonic stem cells.
Using 24 factors made the process very involved, so Yamanaka set out to learn whether all of the genes were needed. He repeated the experiment with combinations of fewer and fewer genes until he determined that he could create iPS cells with just four genes that code for four transcription factors (a type of protein that turns genes on or off in a cell): Oct3/4, Sox2, c-Myc and Klf4. “They clearly act as some master regulators and kick-start a process that does a lot of things to the cell that eventually gives rise to pluripotent cells,” says Hochedlinger. “But how it works, we don’t know.”
In late 2007, James Thomson at the University of Wisconsin, Madison; In-Hyun Park and George Daley at Harvard University; and Yamanaka each reported creating human iPS cells, using either the same four genes that had worked in mice or, in the case of Thomson, two of those genes, Oct3/4 and Sox2, plus two different ones, Nanog and Lin28.
Throughout these experiments, researchers noticed that only one in about 10,000 skin cells receiving the four genes ever becomes an iPS cell—an inefficiency that could undercut the potential therapeutic value of the cells. An even bigger obstacle is that two of the four factors, c-Myc and Klf4, are oncogenes—they cause cancer. And when viruses containing the four genes insert themselves into the genomes of skin cells, they can activate innate cancer genes and cause tumors.
