In fall 1976 the first recorded Ebola outbreak ravaged a small village in the Democratic Republic of the Congo (formerly Zaire). The virus, named for the river valley where it was found, causes a deadly hemorrhagic fever. It spread quickly via contact with blood and contaminated needles killing nearly 90 percent of the 318 villagers it infected. Since then about 2,300 human cases have been reported, according to the U.S. Centers for Disease Control and Prevention, 85 percent of which were fatal.
After identifying a new strain called "Ebola-Reston" during a 1989 outbreak scare in Reston, Va., (and earning a central role in Richard Preston's book, The Hot Zone ) Thomas Geisbert had tried everything to quash the virus, which continues to threaten civilians and medical aid providers in Africa as well as scientists who work in the highest level biocontainment facilities around the world. Vaccines have protected monkeys, and therefore might protect humans from the ensuing fever when given prophylactically (before exposure), but such treatments offer little hope for those already exposed to the virus.
After 15 years of trial and error, Geisbert and colleagues at the National Emerging Infectious Diseases Laboratory Institute at Boston University and the U.S. Army Medical Research Institute of Infectious Diseases at Fort Detrick in Frederick, Md., changed their strategy. Instead of boosting host immunity, they would target the virus itself by silencing its genes. If they could mute the genes responsible for its replication perhaps they could prevent Ebola's rapid and rampant spread though its host, at least buying time for other treatments, such as vaccines, to kick in.
The silent treatment
Gene silencing is a naturally occurring process, also known as RNA interference (RNAi). Some stretches of our DNA are transcribed into RNA sequences, which are then translated into the proteins that keep our cells working. But other, shorter stretches are transcribed into small interfering RNA (siRNA). These tiny molecules aren't translated into proteins. Rather, they exist as short strands of RNA that bind to portions of other RNA strands with complementary sequences, preventing them from being translated into protein. The blocking process allows gene expression to be finely controlled, like the volume on a radio.
Shortly after the process of RNAi broke into the scientific literature at the turn of the millennium, scientists began looking for ways to induce the process artificially to combat diseases. In 2004 Geisbert and colleagues applied this strategy to Ebola, selecting the genes they would target. To stop the virus in its tracks they developed synthetic siRNAs that would bind to and silence a gene essential for replication—polymerase L . In preliminary cell culture studies the researchers successfully silenced the gene and inhibited replication. The next step was to test the approach in Ebola-infected animals (guinea pigs), but this introduced a new problem.
Enzymes in blood and bodily tissues break down "naked" siRNAs. In order for the treatment to make it to the virus intact, it had to be packaged into a molecular vehicle that would be taken up by infected cells without breaking down en route or provoking an immune response. The team's first siRNA packaging attempt (which used synthetic vehicles called "polyethylenimine polyplexes") didn't work, and only 20 percent of the infected guinea pigs survived.