In 1998, a group of scientists injected double stranded RNA into the worm Caenorhabditis elegans. This introduction of the RNA into the worm was able to completely silence the corresponding gene such that no protein was produced. But, who cares about a worm, right?
Wrong. This set of research experiments introduced an entirely novel mechanism of gene regulation that occurs even in humans. These observations, and the work that followed it, would generate a tool used widely in research labs throughout the world.
Eight years later the Nobel Prize was awarded to 2 of these researchers: Craig Mello and Andy Fire. Their finding, initially discovered in the worm, offers an opportunity to understand proteins and their functions, and a potential therapeutic tool to cure disease. Who knew that a little underdog worm could become a science all star?
Before 1998, there had been a lot of confusing observations that RNA could silence a gene. It made some sense that an antisense transcript could silence an mRNA, potentially by binding to the transcript and preventing its translation. The confusing observation was that both the sense and antisense strands to the mRNA were effective. Furthermore, the silencing often lasted more than one generation, even though most RNAs were degraded.
Let's talk a little bit about the experiment in 1998. The scientists studied the gene unc-22, whose protein product is important for worm mobility. Inactivation of the unc-22 gene results in a worm that twitches, or is completely unable to move. The authors showed that the injection of double stranded RNA was the most effective at silencing the unc-22 gene, resulting in twitching or loss of mobility. The ds-RNA had to be complementary to the unc-22 gene for the silencing to be effective. Similar scenarios where observed when double stranded RNA was injected to silence different genes._CITATION_UUID_ 5B9702AFC85245C3AFE8662267933434_ This process was coined RNAi, or RNA interference.
This landmark study raised more questions than it answered. It was still unclear whether the gene was silenced at the level of transcription or translation. Nonetheless, this study provided some important observations. It provided a mechanism by which genes could be silenced at will. The worm was an ideal organism for these studies because of the ease of injections. Later work showed that worms could also be fed the RNAs or simply soaked in them. Up until recently this technique was thought to be limited to worms. It turns out though that we may be able to eat food and influence the gene expression of genes in our bodies. Interested? Check out this brain snack.
Check out this awesome link about how RNA from rice can influence gene expression. http://blogs.discovermagazine.com/80beats/2011/09/21/what-you-eat-affects-your-genes-rna-from-rice-can-survive-digestion-and-alter-gene-expression/
While much remains unknown about the details of RNAi, we do know have some more insights into the process. It is a natural defense mechanism against viruses. We know that double stranded RNAs are processed into small interfering RNAs by an endonuclease called Dicer. This then interferes with translation.
In addition to the direct silencing of translation, more recently researchers showed that some of the components of the RNAi machine are used to affect gene regulation pre transcription. Here small RNAs can actually alter the expression of a gene by changing the chromatin environment surrounding a gene. By making a gene more heterochromatic, the genes are less open and able to recruit the necessary transcription factors.
We mentioned that RNAi is a powerful tool to study disease states. What do we mean? Many human conditions result from the loss of expression of a gene product. Now imagine that a researcher can design an experiment that lets him effectively "turn off" that gene in an otherwise healthy cell. The researcher can begin to ask questions about why the disease state occurs at the cellular level.
Other times disease states are thought to occur because of the overproduction of a gene product. In this case researchers and drug companies are figuring out ways to selectively introduce small RNAs to humans in order to combat the overproduction of the gene product. The delivery of the small RNAs to the correct cells in the human body is one of the most significant challenges of RNAi therapeutics. This same strategy can be used to target cancer cells. Can you think of how might RNAi be used to induce the death of a cancer cell?