HEMATOLOGY

Screen Lands Drug to Boost Blood Stem Cells

Zebrafish Finding May Restock Stem Cell Pool in Bone Marrow Transplantation

In Leonard Zon’s zebrafish facility at Children’s Hospital Boston, row upon row of small tanks hold these small, transparent fish at each stage of life: the eggs that filter down from mating adults, larval embryos, babies the size of fingernails, and finally the adult form, which in this lab often includes a transgene or two.

Graham Ramsay

Beginning with a screen in zebrafish, a team led by (left to right) Leonard Zon, Wolfram Goessling, and Trista North found that prostaglandins have an unexpected role in the development of blood stem cells.

Zon’s lab has been using zebrafish to study genes involved in blood development for several years. While the previous work has linked genes to human diseases, a new study from the lab, published in the June 21 Nature, yielded a discovery that has the potential to help people restock their immune system after a bone marrow transplant. The research used zebrafish embryos, slivers just a couple of millimeters long, as a screening tool for compounds that boost blood cell development.

“This is the first time that one could see a potential therapy coming from our work,” said Zon, the Grousbeck professor of pediatrics. “It’s very exciting for us, because you never know if your basic science is going to lead to a potential treatment.”

The Catch
Zebrafish embryos, which are free-swimming and take up drugs through their skin, offer a way to screen efficiently thousands of chemicals to look for a potential effect on blood stem cell development. First author and postdoctoral fellow Trista North exposed wells of the embryos just a day or two old to a library of more than 2,500 drugs and monitored them during the small window of time when blood stem cells form. After months of these studies, she uncovered about 80 molecules that either increased or decreased blood stem cell production; of those, 10 were compounds that affected prostaglandins.

These fatty acids, which have been so well studied for their roles in blood flow, pain, inflammation, and other processes, were not known to have important functions in stem cells. Wolfram Goessling, a co-author on the paper who is an HMS instructor in medicine at Brigham and Women’s Hospital and Massachusetts General Hospital, said that based on previous research, no one had predicted that the prostaglandin pathway would have a major role in stem cell production. “It was really the screen that led the way,” he said.

The team found that prostaglandins caused blood stem cells to flourish during development, while drugs that work against prostaglandins—including common nonsteroidal anti-inflammatory drugs (NSAIDS) like aspirin and ibuprofen—dampened cell production. One drug, a derivative of prostaglandin E2, was particularly effective at boosting stem cells, and the drug also aided stem cell recovery in adult fish after radiation.

With these promising leads in zebrafish, the team was able to partner with two labs on the same floor at CHB to see if the findings translated into mammals. With George Daly’s lab, they showed that the drug, dmPGE2, increases production of mouse embryonic stem cells in culture. As a final proof, they worked with Stuart Orkin’s lab to test the function of dmPGE2 in mice. Mice were irradiated to deplete their hematopoietic systems and then given bone marrow transplants. Some of the marrow was treated beforehand with dmPGE2; the mice that received the treated marrow had an average threefold increase in their stem cell number, and these cells persisted for months after the procedure.

Clinical Trial to Come
A drug that boosts blood stem cells could be useful for several purposes, but the most acute clinical need is in patients receiving umbilical cord blood as a treatment for leukemia or to replenish blood cells after chemotherapy. Currently, bone marrow transplants are available only for those people with a matched donor—those who cannot find a bone marrow match might be eligible to receive cord blood instead. One umbilical cord is usually enough to replenish the blood cells of small children, but older people often require two cords. “Twice the number of stem cells is enough to make a huge impact on patients,” Zon said. “The problem is, you’re putting two independent immune systems into a single person. It would be better to give a single cord with a larger number of stem cells.”

With that goal in mind, the team hopes to conduct a clinical trial of this new potential use of dmPGE2, which originally was developed to treat gastritis and shown to be safe in humans. North said that it is possible to use the drug without having to give it to patients directly. “If you pretreat the blood with this, you might bias stem cells to expand and proliferate,” she said. The researchers are now developing a trial that will test this ex vivo treatment of stem cells prior to transplants. The trial, which will be run by Children’s in conjunction with the Dana–Farber/Harvard Cancer Center, will take advantage of cases in which patients receive two separate cords. One cord will be treated with the drug. Its cells will be traced after treatment to see if they engraft at higher numbers than cells of the untreated cord and if the patients then recover their blood cell counts faster.

Zon said that the finding may spur other researchers to look more closely at prostaglandins and how they might regulate other kinds of stem cells. A drug that helps stem cells proliferate could be a boon in the lab as well as the clinic. As for the implications of this research for prostaglandin inhibitors and their effects on stem cells, Zon said it probably will not change treatment—doctors already avoid prescribing NSAIDs after bone marrow transplants because they promote bleeding—but it offers another reason why these common drugs should be avoided in transplant patients.

—Courtney Humphries

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