Learning when to fold
CAMBRIDGE, Mass.óRNA interference has long since been an attractive method for potential therapeutics, offering the chance to use cells' natural method to control gene expression. Previous methods have consisted of attempts to deliver RNA via particles made of polymers or lipids, but a team of researchers at the Massachusetts Institute of Technology (MIT), Alnylam Pharmaceuticals and Harvard Medical School has crafted new particles with better results, based on a technique known as "nucleic acid origami."
These new particles seem to overcome the safety risks and targeting difficulties associated with existing particles, according to Daniel Anderson, associate professor of health sciences and technology and chemical engineering and a member of the David H. Koch Institute for Integrative Cancer Research at MIT.
The nanoparticles are constructed from DNA and RNA and allow researchers to turn off genes expressed in cancer cells. They are biodegradable and have no potential to harm the body, unlike currently used particles, and can be tagged with folate (vitamin B9) molecules in order to target the numerous folate receptors found on some tumors.
"Control of nanoparticle shape and size has been shown to be important for function. Origami methods have been developed to make all kinds of interesting shapes," says Anderson, a senior author of a recent paper on the team's work. "We reasoned that using origami we could make a siRNA delivery particles that were all the same, biocompatible and biodegradable (because they are made of nucleic acid), small but large enough to avoid renal filtration (so they could circulate longer but still penetrate tissues) and labeled with ligands, to facilitate their specific delivery. We were surprised in the case of folate that no obvious endosomal escape methods needed to be included."
RNA interference is the method by which short interfering RNA (siRNA) interrupts the transference of genetic information from DNA to ribosomes by binding to the messenger RNA molecules carrying DNA instructions and destroying them before they reach the ribosome.
Anderson and his colleagues built their nanoparticles by using nucleic acid origami, which enables the construction of 3D shapes using short segments of DNA. The team fused six strands of DNA to create a tetrahedron, with three folate molecules on each particle and a single strand of RNA attached to each edge of the shape.
"What's particularly exciting about nucleic acid origami is the fact that you can make molecularly identical particles and define the location of every single atom," said Anderson in a press release.
The lipid, lipid-like or polymeric materials that are currently used are effective, but tend to result in larger particles, Anderson notes, and might be associated with toxicity. In addition, such particles are heterogeneous, he adds, "a collection of particles within a certain size range and composition." The origami particles the team has constructed, however, are monodisperse and molecularly identical, which Anderson says allows them to "control the shape and ligand presentation within the particle in a way we can't do with self-assembling particles."
In mouse studies, the nucleic acid nanoparticles were shown to circulate in the bloodstream with a half-life of 24 minutes, which gave them enough time to reach their targets. According to Anderson, the DNA tetrahedron appeared to protect the RNA from being rapidly absorbed and excreted by the kidneys. The nanoparticles also effectively accumulated at the tumor sites. The RNA attached to the tetrahedrons was designed to target a gene for luciferase, which had been added to the tumors to make them glow, and in treated mice, luciferase activity was more than cut in half.
The next step for the research is to spread out into targeting other genes and other genetic diseases.
"We are looking at the utility of these particles in other tumors, in particular ovarian, and with other ligands, and also optimizing their size and shape," says Anderson.
The research was funded by the National Institutes of Health, the Center for Cancer Nanotechnology Excellence, Alnylam Pharmaceuticals and the National Research Foundation of Korea, and appeared in the June 3 issue of Nature Nanotechnology. The paper's lead author is Hyukjin Lee, a former MIT postdoctoral and current assistant professor at Ewha Womans University in Seoul, South Korea.