Experiment advances cell reprogramming understanding

The announcement last year by scientists in Japan, at the Harvard Stem Cell Institute (HSCI), and at the Whitehead Institute that they had each — independently — coaxed adult cells into reverting to an embryonic stem cell-like state was arguably the biggest news in developmental biology since the cloning of Dolly the ewe.

This creation of what are being called induced pluripotent (iPS) cells was hailed by opponents of embryonic stem cell research as the end of the need for research using embryonic stem cells, and was seen by those who do stem cell research as potentially providing them with another important tool with which to study normal development and the progression of disease.

But for all their excitement, scientists noted that attempts to reprogram adult cells were only succeeding about .1 percent of the time, which left them with the question, “Could any cell be reprogrammed, or was it only possible to reprogram adult stem cells that were present at a rate of about .1 percent in the cell populations on which the experiments were being conducted?”

Now Konrad Hochedlinger and colleagues Matthias Stadtfeld and Kristen Brennand of HSCI and the Massachusetts General Hospital (MGH) have answered that question, advancing the understanding of iPS cells by demonstrating that fully differentiated, i.e., adult, mouse cells can be reprogrammed to produce iPS cells.

In a paper in the latest issue of Current Biology, Hochedlinger, an assistant professor in Harvard’s new Department of Stem Cell and Regenerative Biology, reports genetically marking pancreatic beta cells — the cells that produce insulin — before successfully reprogramming them into iPS cells.

Harvard Stem Cell Institute Co-Director Doug Melton described Hochedlinger’s latest work as “a well-designed experiment that proves the process of reprogramming works on fully differentiated cells, not a rare undifferentiated cell in the starting population.  It also opens the door to making reprogrammed cells from other adult tissues, not just skin.”

While scientists — including Hochedlinger — had previously reported reprogramming adult cells, the cells they reprogrammed were not genetically marked, so if, for example, they were reprogramming adult skin cells, they had no way of knowing if the percentage of cells that did convert to iPS state had started as fully differentiated cells, or if they were skin stem cells.

“This success shows that a fully specialized cell can still give rise to an iPS cell,” Hochedlinger explained during an interview. “This means that adult stem cells are not rare cells giving rise to iPS cells, which in turn means that the reason reprogramming is so inefficient must be due to some other explanation.”

Which also means that this is a good news/complicated news story. The fact that fully differentiated cells can be reprogrammed is good news, because it means that researchers theoretically can reprogram any cell type in the body. But it’s a complicated news story because it also means that there is an extremely important part of the reprogramming process that researchers still do not understand. Why does the process fail approximately 99.9 percent of the time?

“There is an additional factor, or factors involved in this process, and we don’t know what it is, or they are,” said Hochedlinger. “We think that it’s epigenetic — that there are all sorts of chemical modifications that are imposed on the cell during differentiation, and they need to be erased. These restrictions appear to be very efficient,” he continued, “explaining why we only succeed in reversing them about .1 percent of the time.

“We know that the [four] genes we are using to reprogram the cells do work,” Hochedlinger said, “but there may be one, two, or three other factors out there” involved in reprogramming. “The challenge now is to identify those factors that will make this process more efficient.

“Hopefully,” the scientist concluded, “in the next couple of years we will find ways to make the process to produce iPS cells 10 to 100 times more efficient than it is now.”

The research was primarily funded by a Harvard Stem Cell Institute Seed Grant and Hochedlinger’s NIH Director’s Innovator Award.