Harvard Researchers Make Breakthrough in Cell Reprogramming

After three years of endless experiments on mouse cells, scientists at the Harvard Stem Cell Institute have discovered the three transcription factors out of a possible 1,100 that may provide the key to unlocking the secret to growing replacement tissues—a longtime goal of regenerative medicine.

HSCI co-director Douglas A. Melton is the first to report successful "direct reprogramming," a technique that—as its name suggests—directly transforms one type of a fully formed adult cell into another.

Using this technique, Melton and his colleagues were able to turn ordinary mouse exocrine cells of the pancreas into beta cells, vital insulin-producing cells that die off in Type I diabetes patients.

The findings, published last Wednesday on the Web site of the British journal Nature, have implications in finding a new treatment or cure for diabetes while opening doors to a host of new possibilities.

The technique could be applicable, for example, to other disorders that affect the cardiovascular or nervous system such as amyotrophic lateral sclerosis (ALS, also known as Lou Gehrig's disease), as scientists could potentially grow heart or nerve cells.

But direct reprogramming has yet to be introduced in human cells.

George Q. Daley, a member of the executive committee of HSCI not involved in the research, said that several HSCI laboratories—including his own team, whose work was published earlier this month—have already pioneered the direct reprogramming of adult cells back to an embryonic state.

But Daley said that his colleague's findings, which were funded by HSCI and the Maryland-based Howard Hughes Medical Institute are "really the first conclusive evidence" for reprogramming one tissue type into another.

Unlike the technique used to create induced pluripotent stem (iPS) cells by genetically manipulating a patient's cells and reprogramming them into a pluripotent state before morphing into a different type of body tissue, direct reprogramming simply flips an adult cell directly into the desired form, skipping the middle steps.

In a press conference on Tuesday, Melton, who is also an HHMI investigator, likened the identity-change process to a journalist going directly to law school instead of starting over as a kindergartner in order to become a lawyer.

Reprogramming blood or skin cells back to a pluripotent state before coaxing them into the final form takes weeks of repeated cell division, while with direct reprogramming, the cell identity transformation occurs very quickly—20 percent of cells are fully converted within three days.

Despite the relative efficiency of this approach, both Melton and Daley stressed that direct reprogramming does not eliminate the need to continue work with iPS cells or human embryonic stem cells (hES).

"We really wouldn't be where we are today without working with hES cells," said Melton, who is also the co-chair of the new Department of Stem Cell and Regenerative Biology, Harvard's first cross-school department. "They really provide a unique window into human development and human disease, and we really need those to progress our understanding."

Melton added that direct reprogramming is simply another way of approaching the same problem scientists face in regenerative medicine—how to make cells to replace lost or deficient ones.

Additionally, Daley said that though there may be practical advantages of tissue-to-tissue reprogramming, inducing cells back to a pluripotent state provides valuable insight to currently unanswered questions.

Lead author of the paper Qiao "Joe" Zhou, a post-doctoral fellow working in Melton's laboratory, said that finding the correct combination of the three transcription factors to reprogram the cell that makes gut enzymes in mice into a more useful pancreatic cell was the result of three years of repetitive lab work.

Of over 1,100 transcription factors in the mouse genome, only 200 or so are expressed in the cells involved in forming the pancreas, 28 of which are expressed in the part of the gland organ where beta cells are found.

Zhou said that systematic experiments narrowed down the transcription factors down to nine, which were then tested directly in the mice.

As with all iPS work, direct reprogramming uses viruses to integrate the transcription factors into the target cells. Melton said that the retrovirus used in the creation of iPS cells may result in cancer-causing genes, but that the virus used in direct reprogramming does not cause such mutations and has been used for decades in human trials.

Although Melton believes this virus is safe, he conceded that the U.S. Food and Drug Administration's concerns that arise "whenever you use the term 'virus' and talk about injecting a virus into a person" are "legitimate."

He added that his team will now be screening for chemicals that would safely replace the viruses.

Melton, who describes his diabetes research as an "obsession," will be collaborating with James Markmann, the director of transplantation at Harvard-affiliated Mass. General Hospital, to explore the possibility of applying the findings in a clinical context to make new beta cells for patients.

"I wake up every day thinking about how to make beta cells," Melton said.

—Staff writer June Q. Wu can be reached at junewu@fas.harvard.edu.