Research Brief - Gene Silencing Tool Leads to New Understanding of Health and Disease

[Guest guest]

Research Brief: Gene Silencing Tool Leads to New Understanding of Health and

Disease

by Alison

April 16, 2004

It is not every day that scientists reveal nature's best secrets, the ones

that promise to deepen understanding of the biology of living things. The

discovery of gene silencing by RNA interference (RNAi) is this kind of

breakthrough. Researchers are using RNAi to reveal the function of genes in

animals, plants, and humans. RNAi also offers a promising new approach to

treating AIDS and a host of other diseases. Underscoring the importance of

RNAi, Science magazine declared advances in this field to be the top

scientific achievement of 2002.

Although scientists only discovered the existence of RNAi within the last

decade, they now know that organisms have been using the process for

millions of years. Researchers believe that RNAi's natural role is to tune

the activity of genes, reducing their expression for purposes of growth

and/or self-defense. When viruses infect cells, for example, they command

their infected host to produce specialized RNAs to enhance viral survival.

Researchers believe that RNAi is an ancient mechanism used to wipe away such

unwanted, extra RNA, and some scientists speculate that it may even play a

role in our immunity.

Unexpected Discovery

RNAi's use as a research tool began with an experiment gone wrong.

Scientists studying the genetics of plant growth noticed a curious result.

The researchers were attempting to deliver an extra " purple " gene to

petunias, but the flowers instead bloomed stark white. The result evaded

genetic logic and fascinated biomedical researchers, who yearned to

understand how adding genetic material could somehow silence an inherited

trait.

The mystery remained until, a few years later, two NIGMS-supported

geneticists identified a similar process in animals. RNAi, they learned,

operates like a molecular " mute button " to quiet individual genes. The two

researchers, Fire, Ph.D., of the Carnegie Institution of Washington

in Baltimore and Craig Mello, Ph.D., of the University of Massachusetts

Medical School in Worcester, had been using a molecular tool called

antisense RNA to dampen gene activity in roundworms and tease apart genetic

factors that contribute to cell growth and tissue formation. In this

technique, researchers cause the normally single-stranded RNA to bind to an

opposite, or " anti-, " strand. This blocks the RNA from delivering the

instructions to make a protein.

To their surprise, Fire and Mello discovered that the activity of their

laboratory preparation did not depend on the antisense RNA itself, but

instead on a contaminant that was produced during the synthesis of the

antisense RNA. The contaminant, it turns out, was a molecule of

double-stranded RNA. Fire and Mello quickly learned that they could mute

specific genes simply by feeding their experimental worms double-stranded

RNA with the same sequence as the gene they wished to target.

RNAi took the research world by storm. NIGMS grantee Hannon, Ph.D.,

of Cold Spring Harbor Laboratory on Long Island, New York, dropped

everything he was doing when he learned that RNAi could be used as a tool in

fruit flies as well as worms. Using this popular insect model system, Hannon

has uncovered a link between RNAi and Fragile X syndrome, which is the most

common inherited form of mental retardation. Also at Cold Spring Harbor

Laboratory, NIGMS grantee Shiv Grewal, Ph.D., led a team that discovered

RNAi's pivotal role in the normal functioning of yeast cells, which share

many features with the cells of humans. Grewal, who is now at the National

Cancer Institute, has learned that the molecules that normally carry out

RNAi help to organize chromosomes so they can be pulled apart during cell

division, one of the most basic steps in the lives of all cells.

Basic scientists investigating gene function began to try RNAi in other

organisms and found that the technique could be applied nearly universally

to manipulate gene activity in many different model systems. However,

researchers had sporadic and unpredictable success getting RNAi to work in

cells from mammals. In 1999, the situation brightened when Hannon's group

and a team of NIGMS-supported scientists from the Whitehead Institute for

Biomedical Research in Cambridge, Massachusetts-including Bartel,

Ph.D., Sharp, Ph.D., Tuschl, Ph.D., and Zamore,

Ph.D.-developed systems for conducting RNAi experiments in a test tube.

Collectively, these approaches revealed RNAi's molecular modus operandi and

led the way toward getting the technique to work in mammalian cells. The

researchers learned that in RNAi, the double-stranded RNA is first chewed up

into smaller RNA pieces by a newly discovered enzyme named Dicer. The

scientists realized that RNAi silences gene activity through the action of

these tiny RNA snippets, which they dubbed short interfering RNAs (siRNAs).

RNAi to the Rescue

Researchers predict that in addition to RNAi's potential for solving many of

the mysteries encoded in our genes, the technique holds promise for new

therapies. NIGMS grantee Yang Shi, Ph.D., of Harvard Medical School in

Boston, Massachusetts, is one of several researchers who have crafted clever

tools for getting living cells to produce specific siRNAs. These methods

have greatly enhanced researchers' ability to explore RNAi's medical promise

in mammalian cells. For example, in recent lab tests with isolated cells,

Sharp and others have succeeded in using RNAi to kill HIV, the virus that

causes AIDS. Biologists are now working hard on the very challenging problem

of developing ways to deliver siRNAs to the body in order to make a

practical means for treating, and perhaps preventing, disease.