For ten years, it’s been known that RNA interference could potentially kill cancer by shutting off malfunctioning genes – but there’s been no efficient way to deliver the RNA.
Usually, the short interfering RNA (siRNA) used is quickly broken down inside the body by enzymes that defend against infection by RNA viruses.
“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,” says Paula Hammond, an engineering professor at MIT.
Now, though, Hammond and her team have come up with a way of packing RNA into microspheres so dense that they can stay intact until they reach their destinations.
And such particles could offer a new way to treat not only cancer, but also any other chronic disease caused by a ‘misbehaving’ gene, says Hammond: “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,” she says.
RNA interference is a natural process that allows cells to fine-tune their genetic expression. Genetic information is normally carried from DNA in the nucleus to ribosomes, where proteins are made. SiRNA binds to the messenger RNA that carries this genetic information, destroying the instructions before they reach the ribosome.
While scientists have found several ways to package up siRNA and artificially replicate this process to target specific genes, it’s difficult to load the packages with enough siRNA, as the short strands don’t pack tightly.
To overcome this, Hammond’s team decided to package the RNA as one long strand that would fold into a tiny, compact sphere. They used an RNA synthesis method known as rolling circle transcription to produce extremely long strands of RNA made up of a repeating sequence of 21 nucleotides. Those segments are separated by a shorter stretch that is recognized by the enzyme Dicer, which chops RNA wherever it encounters that sequence.
As the RNA strand is synthesized, it folds into sheets that then self-assemble into a very dense, sponge-like sphere. Up to half a million copies of the same RNA sequence can be packed into a sphere just two microns across. The spheres are then wrapped in a layer of positively charged polymer, which induces them to pack even more tightly, and helps them to enter cells.
After the spheres enter a cell, the Dicer enzyme chops the RNA at specific locations, releasing the 21-nucleotide siRNA sequences.
The researchers tested their spheres by programming them to deliver RNA sequences that shut off a gene that causes tumor cells to glow in mice. They found that they could achieve the same level of gene knockdown as conventional nanoparticle delivery – but with one-thousandth as many particles.
The researchers now plan to design microspheres coated with polymers that specifically target tumor cells or other diseased cells. They’re also working on spheres that carry DNA, for potential use in gene therapy.
