siRNA Structure

Short double-stranded RNAs (dsRNAs) mediate RNAi in human cells

Shortly after Fire and Mello’s discovery, dsRNAs were found to induce similar gene silencing in a variety of other organisms. However, it was still not believed that RNAi could also work in humans. This is because long dsRNAs, typically larger than 30 base pairs in length, induce an innate cellular response, including production of interferons (often called the interferon response).

The first demonstrations that RNAi also functions in humans came from work by two separate groups in Germany.

At the University of Bayreuth, Roland Kreutzer and Stefan Limmer reasoned that short fragments of dsRNA might mediate the RNAi response triggered by the long dsRNAs as observed by Fire and Mello. Kreutzer and Limmer showed these short fragments worked in mammalian cells, and while they did not publish their findings, they did file key patents around the discovery, and left their academic positions to start a company called Ribopharma AG. In June, 2000, Ribopharma AG, which later became Alnylam Europe AG and then Roche Kulmbach AG, thus became the first company formed to focus on the development and commercialization of RNAi therapeutics.

Around the same time, Thomas Tuschl, and his colleagues at the Max Planck Institute would show in a seminal publication that synthetic versions of short double-stranded RNA molecules result in potent RNAi gene silencing in mammalian cells without inducing the interferon response. Moreover, Tuschl found that structurally defined 21 to 23 base-pair small RNAs, known as small interfering RNAs (siRNAs), with 2 nucleotide unpaired overhangs at the 3' ends were the most efficient mediators of RNAi. He also made and tests a wide range of siRNA design features, e.g. "blunt ends," length parameters, and chemical modifications. This work was published in the journal Nature (2001) 411:494-8. and became the scientific content for a key patent in the RNAi field called "Tuschl II".

Targets from the work of Tuschl, and his discovery that siRNAs can be used to silence virtually any human gene this technology has immediately changed the practice of biomedical research.