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The newspaper of The Johns Hopkins University February 12, 2007 | Vol. 36 No. 21
'Master Switches' Found for Adult Blood-Forming Stem Cells

By Vanessa Wasta
Johns Hopkins Medicine

Johns Hopkins Kimmel Cancer Center scientists have found a set of "master switches" that keep adult blood-forming stem cells in their primitive state. Unlocking the switches' code may one day enable scientists to grow new blood cells for transplant into patients with cancer and other bone marrow disorders.

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 that 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," said cancer researcher Curt Civin, who led the study. The journal account appeared online the week of Feb. 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," said Civin, the Herman and Walter Samuelson Professor of Cancer Research in the School of Medicine.

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

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 nonreproducing virus to insert genetic instructions for each of the 33 microRNAs into adult stem cells. The cells will 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," Civin said.

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

Johns Hopkins 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 Corp. and Baxter HealthCare Corp. The terms of this arrangement are being managed by the university in accordance with its conflict-of-interest policies.


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