From Plants to Worms to Humans: Discovery and Mechanism of RNAi

Much of our understanding of RNAi is based on seminal observations made in model organisms such as the mustard weed Arabidopsis and the nematode worm C. elegans.

A failed genetic experiment leads to surprising biological insight

The first experimental observation of RNAi began with attempts by Jorgensen and colleagues to create more attractive petunia flowers by genetically engineering additional flower pigmentation genes into the plant genome. To their surprise, instead of producing plants with more colorful flowers, many had lost most or all pigmentation, and as a result, became white. Intriguingly, when they looked at the expression of genes involved in the natural pigmentation biosynthesis pathway, they found that the mRNA level of the gene that corresponded to the newly introduced designer pigment gene had been strongly reduced. Thus, the concept of co-suppression was born whereby sequence-related genes could negatively regulate each other.

Long double-stranded RNAs induce specific gene silencing in a nematode worm

The first major breakthrough in understanding the mechanism of this type of gene silencing came from studies conducted by Andrew Fire and Craig Mello. By introducing various forms of long RNA molecules into the nematode worm C. elegans, Fire and Mello observed that it was double-stranded RNAs (dsRNAs) that were the actual inducers of the RNAi gene silencing phenomenon.

dsRNAs are similar in structure to the double-stranded DNA helix that makes up our genome, only that DNA is replaced by its chemical cousin, RNA. While Fire and Mello’s work represented a major advance in understanding the mechanism of RNAi, it also provided for a simple and reproducible method by which long dsRNAs could be used to induce specific gene silencing in lower organisms commonly used in genetic research such as C. elegans and the fruitfly, D. melanogaster. Because of this work and the impact it would have on biomedical research, Andrew Fire and Craig Mello were awarded the 2006 Nobel Prize in Physiology or Medicine, even though they did not show or expect the RNAi mechanism to work in mammalian systems when their early work was performed.