In the early 2000s, RNAi was a hot topic. The science world was abuzz with all the possibilities that harnessing this natural process could hold. And why not? The idea of posttranscriptionally silencing genes using only a small fragment of double-stranded RNA is huge—big enough to earn the scientists who discovered it a Nobel Prize in 2006.
The process of RNAi starts with short (~70 nucleotieds), double-stranded fragments of RNA called short hairpin RNAs (shRNA). These shRNAs are exported into the cytoplasm and cleaved by the enzyme Dicer into smaller pieces of RNA that are about 21 nucleotides long and are referred to as small interfering RNAs (siRNA). The siRNAs reduce or stop expression of proteins through a sequence of events where the antisense strand of the siRNA is incorporated into and RNA-induced silencing complex (RISC), which then attaches to and degrades its complimentary messenger RNA, thereby reducing or completely stopping expression.
It turned out, however, that harnessing the promise of RNAi was a little trickier than anticipated. Early attempts to develop siRNA therapies failed, and by 2010 excitement had waned, taking more than a few business ventures with it. The hype might have died down, but that doesn’t mean the promise is gone. Unlike in those very early days of RNAi excitement, RNAi has now been well documented in mammals, and we know that it is highly conserved. This means that siRNAs that perform well in one species are fairly certain to perform in other species as well (1).
While siRNAs released directly into the blood stream are quickly degraded by enzymes and are not able to cross cell membranes, today scientists are finding ways to sneak siRNAs safely through the blood and into cells by imbedding them in lipid nanoparticles (LPN). Researchers have discovered that when released into the blood stream, these nanoparticles generally end up in the liver. This is due, in part, to the vascular nature of the liver and in part because the LPNs are rapidly coated in apolipoprotein E (ApoE) once they are released into the blood stream. ApoE binds to receptors on hepatocytes, the primary functional cells of the liver.
Taking advantage of this mechanism, many researchers have focused on therapies related to diseases linked to the liver. Currently, there are several siRNA therapies under development, including ones targeting Hepatitis C and Hepatitis B viruses. Liver-linked diseases aren’t the only ones that RNAi scientists have set their sites on. There are several companies pursuing RNAi-centered approaches to cancer treatments. Imagine developing a treatment that could turn off an oncogene like k-RAS or PLK1. All of these therapies are still in development and/or undergoing clinical trials, but the early results have been encouraging.
Right now, it looks like the promising future that some many saw for RNAi in the early 2000s might just become a reality after all. And while that is exciting, I think there is a lesson to be learned from the rise, fall and resurrection of RNAi. It is the lesson of balance between the practical–questioning, testing, re-evaluating, rethinking and retesting– and the imagining and dreaming. With RNAi, science might have overbalanced a bit in the directions of dreams. But now the practical pieces have caught up, and some of those dreams have a chance at becoming reality.
- Bender, E. (2014) The Second Coming of RNAi. The Scientist, Published online September 1, 2014.