Gene Silencing in an Origami Package

Potentially, therapeutic RNA interference can knock out essential functions in tumors. The trouble is getting small interfering RNA (siRNA) to cancer cells, and only cancer cells, without causing an immune reaction. Now researchers have reported on using “DNA origami” to create a siRNA-carrying nanoparticle that shows promise in initial mouse studies.

Investigators at Massachusetts Institute of Technology (MIT), Harvard Medical School, and Alnylam Pharmaceuticals (Cambridge, MA), who developed the particles, say the design control afforded by the DNA-folding technique may lead to greater consistency and safety in therapeutic siRNA delivery.

The gene-silencing particle is a pyramid of DNA decorated with 6 siRNA strands, each of which carries a tumor-targeting folate molecule at its tip. This structure is dictated by the sequence of the DNA and RNA strands themselves. When mixed together in the right order, they will self-assemble into a 30-nm spiky pyramid shape.

Daniel Anderson, PhD, an associate professor of chemical engineering at MIT who led the work, says the origami design method enabled them to finely control the size of the nanoparticles and the exact number and location of the folate molecules. This level of control isn't possible with other nanoparticle drug carriers such as liposomes.

After confirming that particles carrying luciferase-gene-silencing siRNA decreased expression of luciferase in HeLa cells in vitro, the researchers went on to test the particles in vivo. They labeled the nanoparticles with a contrast agent and injected them in mice with human xenograft KB tumors.

The researchers then tracked the particles for 24 hours with computed tomography and fluorescence imaging. The origami particles accumulated in the kidney and the tumor, but not in other major organs. The particles also circulated in the blood for about 24 minutes, 4 times longer than naked siRNA. “They take several trips around the body,” says Anderson.

To determine whether there was gene silencing in the mice, they did another set of studies in mice whose tumors expressed luciferase, whose expression was targeted by the siRNA particle. Two days after injection, imaging showed their tumors were 60% less bright. Blood samples showed no significant increase in IFN-α immune response.

Accumulation of siRNA-carrying nanoparticles in tumors has been demonstrated in mouse studies and even in human studies before, notes Peter Beal, PhD, professor of chemistry at the University of California, Davis. What sets this work apart is the ability to precisely control the position and density of the ligands using the origami design method. “This may lead to novel interactions with specific cell-surface receptors that may lead to more precise targeting, which would allow for lower doses and could increase safety,” Beal says.

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