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Media Contact: Sarah Goodwin 23 June 2008    
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Gene Silencer and Quantum Dots Reduce Protein Production to a Whisper
Scientists at Emory University and the University of Washington in Seattle have succeeded in using nanotechnology known as quantum dots to usher gene therapy into the body.

Published online this week in the Journal of the American Chemical Society, the report shows that the technique is 10 to 20 times more effective than existing methods for injecting the gene-silencing tools, known as siRNA, into cells.

More than 15 years ago scientists discovered a way to stop a particular gene in its tracks. The Nobel Prize-winning finding holds tantalizing promise for medical science, but so far it's been difficult to apply the technique, known as RNA interference, in living cells.

"This work helps to overcome the longstanding barrier in the siRNA field: How to achieve high silencing efficiency with low toxicity," says co-author Shuming Nie, professor in the Wallace H. Coulter Department of Biomedical Engineering at Georgia Tech and Emory University.

Xiaohu Gao, a UW assistant professor of bioengineering and co-author of a study says the research teams believe this will make a very important impact to the field of siRNA delivery.

Other co-authors are Maksym Yezhelyev, post-doctoral fellow, hematology and medical oncology at Emory and Ruth O'Regan, associate professor, hematology and medical oncology, Emory Winship Cancer Institute; and Lifeng Qi at the UW.

Short pieces of RNA can disable production of a protein by silencing a stretch of genetic code. Research laboratories regularly use the technique to figure out what a particular gene does. In the body, RNA interference could be used to treat conditions ranging from breast cancer to deteriorating eyesight.

The recent experiments used quantum dots, fluorescent balls of semiconductor material just six nanometers across (lining up 9,000 grains end to end would equal the width of a human hair). Quantum dots' unique nanotechnology properties cause them to emit light of different colors depending on their size. The dots are being developed for cellular imaging, solar cells and light-emitting diodes.

This finding is one of the first applications of quantum dots to drug delivery.

Each quantum dot was surrounded by a proton sponge that carries a positive charge. Without any quantum dots attached, the siRNA's negative charge would prevent it from penetrating the cell's wall. With the quantum-dot chaperone, the electrically neutral siRNA crosses the cellular wall, escapes from the endosome (a fatty bubble that surrounds incoming material) and accumulates in the cellular fluid, where it can do its work disrupting protein manufacture.

Key to the newly published approach is that researchers can adjust the chemical makeup of the quantum dot's proton-sponge coating, allowing the scientists to precisely control how tightly the dots attach to the siRNA.

Researchers compared the quantum dots' effectiveness at silencing genes with three commercial reagents, or reaction-causing substances, used in labs. When siRNA was delivered with quantum dots the production of a test protein dropped to 2 percent. When siRNA was used with the commercial reagents, production only dropped to between 13 percent and 51 percent. On average, the quantum dots were 18 times as efficient as the commonly used chemicals.

Central to the finding is that fluorescent quantum dots allow scientists to watch the siRNA's movements. Previous siRNA trackers gave off light for less than a minute, while quantum dots, developed for imaging, emit light for hours at a time. In the experiments the authors were able to watch the process for many hours to track the gene-silencer's path.

The new approach is also five to 10 times less toxic to the cell compared to existing chemicals, meaning the quantum dot chaperones are less likely to harm cells. The ideal delivery vehicle would have no effect; the only biological change would be siRNA blocking cells' production of an unwanted protein.

The exact reason that the quantum dots were more effective than previous techniques is, however, still a mystery.

The researchers believe the improvement is caused by the endosome escape, and the ability of the quantum dots to separate from the siRNA.

Quantum dots are not yet approved for use in humans. The authors are now transferring their techniques to particles of iron oxide, which have been approved by the U.S. Food and Drug Administration. They are also working to target cancer cells by attaching to specific markers on the cells' surface.

"Looking forward, this work will have important implications in in-vivo siRNA therapeutics, which will require the use of nontoxic iron oxide and biodegrable polymeric carriers rather than quantum dots," says Nie. The research was funded by grants from the National Institutes of Health, the National Science Foundation and the Georgia Cancer Coalition.

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