The RNA Surprise
Until recently, the rather unglamorous role biologists had attributed to RNA was that of a passive messenger, delivering genetic information from DNA to the protein-making machinery of the cell. In this process, the DNA code of a gene is transcribed into an RNA copy, which the cellular machinery translates into a protein. In RNA interference, short bits of RNA block the process by destroying the message en route. The double-stranded RNA fragments lead cutting enzymes to the RNA that carries the genetic message. The messenger RNA is then chopped up and marked for destruction: the gene’s message is effectively “silenced.”
Biologists have known for years that single-stranded RNA molecules designed to pair with a messenger RNA could shut down protein production, but this artificial process is unreliable even in the lab. Nature, though, does regulate genes using RNA, specifically double-stranded molecules.
The first hints of the phenomenon appeared back in 1990, but at the time, researchers didn’t connect what they had observed with RNA. That year, plant biologist Rich Jorgensen, then at DNA Plant Technology in Oakland, CA, was trying to make purple petunias a deeper shade of purple. He inserted a new, supercharged copy of the gene that controls production of purple pigment. To his surprise, he got white petunias. Jorgensen recognized the importance of this paradoxical effect, but he could not explain why adding more of a gene had turned that gene off.
The next clue came in 1995, when geneticists at Cornell University cloned a gene in the microscopic soil worm C. elegans. To verify their discovery, they used a standard lab method to turn the gene off: they added a single strand of RNA that matched the messenger RNA. This complementary strand bound to the messenger, stopping it from being translated into a protein. Unexpectedly, a noncomplementary single strand of RNA they were using as an experimental control and which should have done nothing, also shut down the gene.
In 1998 biochemist Andrew Fire, then at the Carnegie Institution of Washington, and geneticist Craig Mello, at the University of Massachusetts Medical School, solved the mystery. Injecting complementary single strands of RNA into worms, they got an astonishingly potent silencing effect when the two strands combined. After demonstrating that double-stranded RNA was the real silencing agent, Fire and Mello coined the term “RNA interference,” and a new field was born. In retrospect, Jorgensen’s supercharged purple genes yielded double-stranded RNAs that had the same effect on the native purple genes, essentially shutting them off.
The double-stranded RNA seemed to provide a more stable and reliable means for shutting off specific genes than did the single strands, and labs that were studying organisms including plants, worms, and flies eagerly adopted the new method. RNA interference didn’t work in mammals, though: the immune system destroys cells that contain double-stranded RNA to defend against RNA viruses like those that cause hepatitis A and C. Then came Tuschl and Elbashir’s revelation in Banff that very short RNA segments, which they dubbed “small interfering RNA,” did work in human cells. At that point, says Sharp, “the whole field took off.”