Master Switches Found For Adult Blood Stem Cells

The scientists located the control switches not at the gene level, but farther down the protein production line in more recently discovered forms of ribonucleic acid, or RNA. MicroRNA molecules, once thought to be cellular junk, are now known to switch off activity of the larger RNA strands which allow assembly of the proteins that let cells grow and function.

"Stem cells are poised to make proteins essential for maturing into blood cells, but microRNAs keep them locked in their place," says cancer researcher Curt Civin, M.D., Ph.D., who led the study. The journal account will appear online the week of February 5 in the early edition of the Proceedings of the National Academy of Sciences.

To halt protein assembly, microRNAs pair up with matching full-length RNA, then fold and twist it, rendering the larger RNA useless. But the RNA pairings are not perfect, and one microRNA can latch on to several hundred RNA strands. "They act like a single circuit breaker to efficiently control hundreds of RNAs," says Civin, the Herman and Walter Samuelson Professor of Cancer Research.

"We're looking for ways to flip these microRNA switches, to control when stem cells grow into new blood cells," says Robert Georgantas, Ph.D., research associate at the Johns Hopkins Kimmel Cancer Center and first and corresponding author of the study.

To identify the key microRNAs, Georgantas sifted through thousands of RNA pieces with a custom-built, computer software program. Its algorithms let the software, fed data from samples of blood and bone marrow from healthy donors, match RNA pairs. The outcome was a core set of 33 microRNAs that match with more than 1,200 of the larger variety RNA already known to be important for stem-cell maturation.

Georgantas and Civin currently are testing whether these pair predictions are valid by using a non-reproducing virus to insert genetic instructions for each of the 33 microRNAs into adult stem cells. They'll then be cultured in Petri dishes. MicroRNA-155 -- the first microRNA tested -- was predicted to stop stem cells from developing into red and white blood cells. As expected, stem cells without microRNA-155 matured: they formed approximately 75 red and 150 white blood cell colonies per dish. Stem cells with microRNA-155 matured into far fewer red and white cell colonies -- about seven and 30 per dish, respectively.

"Using microRNAs to stall an adult blood stem cell in its early stage could help us grow new ones in test tubes, and perhaps give us more insight into stem-cell maturation for other tissue types," says Civin.

Civin and his team have filed for patents on the microRNA technology. The research was funded by the National Institutes of Health, National Cancer Institute, National Foundation for Cancer Research, and Kimmel Foundation for Cancer Research.

Additional authors include Richard Hildreth, Sebastien Morisot, and Jonathan Alder from Johns Hopkins; Chang-gong Liu, George A. Calin, and Carlo Croce from Ohio State University; and Shelly Heimfeld from the Fred Hutchinson Cancer Research Center.

The Johns Hopkins University holds patents on the CD34 monoclonal antibodies and inventions related to stem cells. Civin is entitled to a share of the sales royalty received by the University under licensing agreements between the University, Becton Dickinson Corporation and Baxter HealthCare Corporation. The terms of this arrangement are being managed by the Johns Hopkins University in accordance with its conflict-of-interest policies.

Citation: Georgantas, RW, et al, "CD34+ Hematopoietic Stem-Progenitor Cell MicroRNA Expression and Function. A Circuit Diagram of Differentiation Control." PNAS online, early edition, Feb 5, 2007.