Nanomedicines made from self-assembling DNA structures could last longer inside the bloodstream with a lipid bilayer coating similar to the ones worn by some viruses (ACS Nano 2014, DOI: 10.1021/nn5011914). This protection strategy could make it possible to test new kinds of DNA nanotherapies in animals and bring them to the clinic, the developers say.
DNA is a versatile building block for making nanoparticles with precise shapes that can perform complex tasks. For example, in 2012, researchers used DNA to fashion a drug-carrying box with two locks made from DNA; the box opens to release the drug only if both locks are bound to certain proteins on a target cell (Science, DOI: 10.1126/science.1214081). But researchers struggle to test this and other DNA nanotechnology in animals. Any free-floating DNA in the bloodstream is rapidly destroyed by enzymes. Researchers haven’t yet figured out how to package these DNA machines so that they can survive long enough in the body to do their work.
To solve this problem, William M. Shih, a synthetic biologist at Harvard University, looked to nature for inspiration. Viruses, which are essentially groups of genes that use living cells to replicate, have already developed strategies to endure in the bloodstream. One is to coat themselves with a protective lipid bilayer.
Shih decided to mimic this strategy. He first designed a simple octahedron-shaped wireframe of DNA using software his group had previously developed. Given the dimensions of a structure, this software generates a recipe list of DNA strands that will self-assemble into the desired shape. A DNA synthesis company makes the strands, and the researchers mix them in the lab.
Each strut in the DNA octahedron is made up of six 28-nm-long double helices held together by shorter strands. The Harvard group created attachment points on the interior and exterior surfaces of the octahedron using single-stranded pieces of DNA. On the interior, these handles bind to complementary DNA strands that carry fluorescent dyes so that the scientists could track the particles in animals. The exterior handles bind to complementary strands carrying lipids.
This created a 50-nm-diameter DNA octahedron with “greasy plugs” on it, Shih says. “Then we take this hairball and mix it with a solution of giant liposomes in surfactant.” The lipids from the liposomes stick to the plugs, eventually creating a continuous coating on the DNA frame. The completed structure is about 70 to 80 nm in diameter.
In a proof-of-principle test, the researchers compared the lifetimes of these virus-like, lipid-coated particles with two naked DNA controls. After injecting the DNA structures into the animals, the researchers monitored the particles’ fluorescence to determine their fate. Both of the unprotected DNA structures were immediately broken down and excreted. The lipid-coated DNA octahedra stayed in the body 15 to 20 times longer and had a half-life in the blood of four hours.
Yamuna Krishnan, a biochemist at the Tata Institute of Fundamental Research, in Bangalore, says this method also could protect other kinds of nucleic acids in the body, such as RNA designed to silence genes, or RNAi. “This could be a major conceptual design step toward solving a very difficult issue,” she says.