To harness the full power of stem cells, you might need an eraser.
Not an ordinary eraser, of course. More of a drug, really. But if you use it right, it can erase the tiny labels that tell cells where to start reading important chapters in DNA, their inner instruction manual.
When cells can't read that manual, they regain their full stem cell power — the power to become any kind of cell in the body.
In a surprising new finding, University of Michigan Medical School scientists have shown that a drug developed at U-M can achieve this — at least in mice.
It's the first time scientists have been able to get stem cells to revert to their original state by erasing specific labels called epigenetic markers. The drug specifically targets markers on histones, the protein "spools" that DNA coils around to create structures called chromatin.
A "magic" eraser
Writing in the journal Cell Stem Cell, the team reports that more than half of mouse epiblast stem cells treated with the drug reversed course within three days. They regained an embryonic "be anything" state, also called pluripotency.
In addition to generating pluripotent stem cells, the team showed that mice bred using the cells grew up healthy.
"We've demonstrated that we don't have to manipulate the pluripotent genes to get to the ground state, but rather that we can block all other options of where the cell 'wants' to go. Then the only option is going back to the ground, or naïve, pluripotent state," says Yali Dou, Ph.D., senior author of the new paper and an associate professor of pathology and biological chemistry.
We've demonstrated that we don't have to manipulate the pluripotent genes to get to the ground state, but rather that we can block all other options of where the cell 'wants' to go.Yali Dou, Ph.D.
The researchers used a relatively new compound called MM-401, which U-M scientists originally designed for use in treating leukemia. Now, they're working to see if the MM-401 eraser technique works with human stem cells that bear some resemblance to mouse epiblast stem cells. They will share the drug with any other researchers who want to try the technique.
However, it's far too early to see the approach as a way to avoid the use of human embryos for research or potential treatments. Currently, embryos left over from infertility treatments are the only source of human embryonic stem cells.
Other techniques can reprogram "adult" cells in the human body taken from skin, for example — but the cells still carry baggage from their previous state.
Still, the new achievement shows the power of altering epigenetic labels without altering the DNA that's coiled up in tight little packages called chromatin. Similar attempts by other teams to restore pluripotency to mouse cells from the epiblast stem cell state have yielded far lower amounts, or nonviable cells. And, they've required cocktails of multiple drugs, given over the long term, to achieve it.
The U-M team shows that using MM-401 for just a few days, and then stopping its use, is enough. The team that designed MM-401 is led by one of the new study's co-authors, Shaomeng Wang, Ph.D.. He's the Warner-Lambert/Parke-Davis Professor at the Medical School who holds appointments in internal medicine and pharmacology, as well as an appointment in the U-M College of Pharmacy.
Epigenetic maneuvers
Epigenetic labels signal to the cell's DNA-reading machinery where they should start uncoiling a chromosome in order to read it. The drug MM-401 targets the labels that come from the activity of a gene called MLL1.
MLL1 plays a key role in the uncontrolled explosion of white blood cells that's the hallmark of leukemia, which is why U-M researchers originally developed MM-401 to interfere with it. But it also plays a much more mundane role in regular cell development, and the formation of blood cells and the cells that form the spinal cord in later-stage embryos.
It does this by placing tiny tags — called methyl groups — on histones. Without those labels, the cell's DNA-reading machinery doesn't know where to start reading. It's as if the invitation to open the instruction manual has vanished.
Stem cells don't harness the power of MLL1 until they're older. Using MM-401 to block MLL1's normal activity in cells that have started down the path to adulthood takes advantage of the fact that histone marks were missing before the cell needed them. The cells couldn't continue on their journey to becoming different types of cells. But they could still function as healthy pluripotent stem cells.
As part of the work, Sundeep Kalantry, Ph.D., an assistant professor of human genetics at U-M, showed that female stem cells treated with MM-401 had un-silenced one of their X chromosomes. This step is crucial for showing that cells have returned to true pluripotency.
The researchers also looked at how changes in MLL1 expression affected other gene expression in cells, through work by Alexey Nesvizhskii, Ph.D., and his colleagues in Pathology and Computational Medicine and Bioinformatics.
"People have been focused on other epigenetic changes that are more dramatic, but ignored methylation by the MLL family," says Dou, whose team members included first author and postdoctoral fellow Hui Zhang, Ph.D. "Deleting MLL1 entirely causes failure later in differentiation. But inhibiting it with a drug temporarily leaves no trace behind."
Disclosure: The work was funded by the National Institutes of Health (GM082856, OD008646, GM094231, CA117307) and by the Leukemia and Lymphoma Society.
U-M holds a patent on MM-401 and has licensed it to Ascentage Pharma, a China-based company co-founded by Wang in which the university holds equity.