mêÊÊiê^ÊÊÊÊSSSÊKUÊÊt don't penetrate cell membranes easily and are quite toxic. The researchers then showed that aptamers grown to bind to a less toxic, more cell-permeable dye mole cule could also serve as switches to regu late gene expression in cells. Part of the significance of the ParkeDavis work, too, is that it shows that mol ecules with structures quite different from—and much simpler than—the ami noglycoside antibiotics can bind to RNA at specific sites to interfere with the RNA's NA, far from being a floppy, string- eventually be used in gene therapy, Green biological activity. Some of the Parke-Davis like molecule, sometimes folds into suggests. "We want to be able to intro- compounds have "an established track a proteinlike structure that offers duce genes into cells and turn them on record for delivery and bioavailability that specific binding sites for small molecules. and off at will," he says. "Using a small, we can take advantage of" for further drug Recent research in two laboratories helps cell-permeable molecule as the on-and-off development, Mei says. In earlier work, several laboratoriesconfirm this view of structured RNA and switch would be an ideal way to do that." suggests ways that chemists can put these By providing a way to temporarily turn off including Green's and the Parke-Davis small-molecule interactions to good use. the activity of a particular gene, these group—screened relatively small libraries At Parke-Davis Pharmaceutical Re- switches may also prove useful in func- of aminoglycoside antibiotics to find mole search, Ann Arbor, Mich., Houng-Yau Mei, tional genomics, the task of sorting out cules that bind to specific RNA targets. Anthony W. Czarnik, and their coworkers just what particular genes do. By helping The Wong group has developed aminogly screened a large library of small organic to identify genes that are important in dis- coside mimetic libraries to target certain molecule drug candidates to identify three eases, these switches may also come to be sequences of RNA. At Parke-Davis, the researchers inves quite different heterocyclic molecules that important tools for drug discovery, Green tigated an RNA called TAR that's critical bind to a target RNA at three different lo- suggests. cations {Biochemistry, 37, 14204 (1998)]. The Massachusetts work offers "a new for the activity of HIV-1, the human im This approach is "remarkably close to ex- strategy for controlling gene expression in munodeficiency virus that causes AIDS. isting paradigms for the discovery of drugs vivo using small molecules," comments After first identifying molecules from that inhibit protein function," says Chi-Huey Wong, chemistry professor at their library that bound to the RNA and Czarnik, who is now chief scientific offi- Scripps Research Institute, La Jolla, Calif. interfered with its activity, they put these cer at IUurnina in San Diego. Thus, he sug- "It provides a new method for studying molecules through a second assay to show that they would selectively bind to gests, it could "broaden the range of tar- the expression of a specific gene." gets considered 'reasonable' in the drug Green and Werstuck initially generated their RNA target even within cells, discovery community." aptamers that would bind specifically to where the target RNA was greatly out Meanwhile, Michael R. Green, an inves- aminoglycoside molecules, a class of anti- numbered by other types of RNA. They also tigator at the Howard Hughes Medical In- biotics that are known to function by showed that one of the molecules actually stitute and professor of biochemistry and binding to RNA. These are not ideal trig- inhibits the virus in a model system. molecular biology, and former postdoctor- gering molecules, however, because they Some biotechnology firms are also de al fellow Geoffrey Werstuck veloping rapid methods to of the University of Massa- — ^ ~ — — ——— screen large libraries of small chusetts Medical Center, molecules to find those that Parke-Davis drug candidates bind Worcester, have generated bind to and interfere with to RNA at different sites nonnatural RNA sequences RNA. For example, Scriptgen (aptamers) that bind to prePharmaceuticals, Waltham, GG selected small molecules. Mass., has developed a pro NHo NH U G These aptamers do not interprietary screening method Ν C A fere with the normal functhat identifies small mole C-G < •N^NH. tioning of a messenger RNA cules that bind to a specific G-C in protein synthesis when RNA target by measuring A-U they are inserted near its 5'the increase in the stability G-C end. Adding the small moleof the RNA brought about U cule to cells containing these by the binding, according to NHo engineered mRNAs, howevMichael G. Palfreyman, se / H C 'u HO er, inhibits the function of nior vice president for R&D. A-U ^ \ HO NHo the mRNA. The whole sysIn partnership with other G-C HO tem forms a switch that alcompanies, Scriptgen is us A U HO lows the researchers to coning this technology to devel HOH C C-G HO trol whether a particular op drugs targeted toward OH C-G gene is on or off within a cell hepatitis Β and C viruses G-C [Science, 282, 296 (1998)]. and HIV, he says. CH NH G-C Such a switch might Rebecca Rawls
SMALL MOLECULES TARGET RNA
Binding to specific regions could lead to drugs or to on-and-off switches for genes
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OCTOBER 12, 1998 C&EN 15
