Although researchers have been investigating cancer treatments based on RNA interference – a method that can switch off malfunctioning genes with short snippets of RNA – for the past 10 years, they still need to find a technique to transport RNA efficiently.

Short interfering RNA (siRNA) – the type used for RNA interference – usually deteriorates rapidly inside the body, by enzymes that protect against RNA virus infections.

Paula Hammond, the David H. Koch Professor in Engineering at MIT, explained:

“It’s been a real struggle to try to design a delivery system that allows us to administer siRNA, especially if you want to target it to a specific part of the body.”

In the February 26 issue of the journal Nature Materials, Hammond and her team reveal they have developed a innovative delivery system that packs RNA into microspheres so compact that the RNA are able to reach their destinations before they deteriorate. Like current delivery techniques, the new system breaks down expression of specific genes effectively, although with a significantly smaller dose of particles.

Hammond, a member of MIT’s David H. Koch Institute for Integrative Cancer Research, explains that such particles show promise for developing a new method to treat cancer and also other chronic disease caused by a “misbehaving gene.”

Hammond said:

“RNA interference holds a huge amount of promise for a number of disorders, one of which is cancer, but also neurological disorders and immune disorders.”

Jong Bum Lee, a former postdoc in Hammond’s lab is lead author of the report. Other authors include, Postdoc Jinkee Hong, Daniel Bonner PhD ’12 and Zhiyong Poon PhD ’11.

RNA interference was discovered in 1998 and is a naturally occurring process. RNA interference allows cells to adjust their genetic expression. Usually genetic information is transferred from DNA in the nucleus to ribosomes, cellular structures where proteins are produced.

siRNA attaches to the carrier RNA that transports this genetic information, demolishing instructions before they are able to reach the ribosome.

Researchers are currently in the process of developing several ways to synthetically mirror this process to target specific genes, including encasing siRNA into nanoparticles made of lipids or inorganic materials, such as gold. Even though several of these methods have demonstrated some success, one challenge is that it is hard to pack large quantities of siRNA onto those carries as the short strands do not compact tightly.

Using an RNA synthesis technique called rolling circle transcription, the researchers were able to overcome this problem by encasing the RNA as one long strand that folds into a tiny, compact sphere. This method allowed the researchers to create very long strands of RNA made up of a repeating sequence of 21 nucleotides. Those segments are divided by a shorter stretch that is identified by the enzyme Dicer, which breaks RNA wherever it encounters that sequence.

Because the RNA strand is synthesized, it is able to fold into a very compact, sponge-like sphere. In a sphere with a diameter of only 2 microns, up to half a million copies of the same RNA sequence can be compact.

The team then wrap the spheres in a layer of positively charged polymer, triggering the spheres to compact more tightly (down to a 200-nanometer diameter) and helps them to penetrate into cells.

Once inside the cell, the Dicer enzyme breaks down the RNA in specific areas, discharging the 21-nucleotide siRNA sequences.

The team programmed the spheres to transport RNA sequences that switch off a gene causes tumor cells to glow in rodents. The researchers discovered they were able to achieve the same amount of gene breakdown as current nanoparticle delivery systems, but with approximately one-thousandth as many particles.

The spheres gather at tumor sites through a phenomenon frequently used to transport nanoparticles: “leaky” blood vessels surrounding tumors have tiny pores, allowing extremely small particles to enter.

The team plan to conduct future studies in order to design microspheres coated with polymers that specifically target tumor cells or other diseased cells. In addition, the researchers are currently developing spheres to transport DNA, for potential use in gene therapy.

Written by Grace Rattue