To achieve this type of gene shutdown, known as RNA interference,many researchers have tried — with some success — to deliver RNAwith particles made from polymers or lipids. However, thosematerials can pose safety risks and are difficult to target, saysDaniel Anderson, an associate professor of health sciences andtechnology and chemical engineering, and a member of the David H.Koch Institute for Integrative Cancer Research at MIT. The new particles, developed by researchers at MIT, AlnylamPharmaceuticals and Harvard Medical School, appear to overcomethose challenges, Anderson says. Because the particles are made ofDNA and RNA, they are biodegradable and pose no threat to the body.They can also be tagged with molecules of folate (vitamin B9) totarget the abundance of folate receptors found on some tumors,including those associated with ovarian cancer — one of thedeadliest, hardest-to-treat cancers. Anderson is senior author of a paper on the particles appearing inthe June 3 issue of Nature Nanotechnology.
Lead author of the paper is former MIT postdoc Hyukjin Lee, nowan assistant professor at Ewha Womans University in Seoul, SouthKorea. Genetic disruption RNA interference (RNAi), a natural phenomenon that cells use tocontrol their gene expression, has intrigued researchers since itsdiscovery in 1998. Genetic information is normally carried from DNAin the nucleus to ribosomes, cellular structures where proteins aremade. Short interfering RNA (siRNA) disrupts this process bybinding to the messenger RNA molecules that carry DNA’sinstructions, destroying them before they reach the ribosome. siRNA-delivering nanoparticles made of lipids, which Anderson’s laband Alnylam are also developing, have shown some success in turningoff cancer genes in animal studies, and clinical trials are nowunderway in patients with liver cancer.
Nanoparticles tend toaccumulate in the liver, spleen and lungs, so liver cancer is anatural target — but it has been difficult to target suchparticles to tumors in other organs. “When you think of metastatic cancer, you don’t want to juststop in the liver,” Anderson says. “You also want to getto more diverse sites.” Another obstacle to fulfilling the promise of RNAi has been findingways to deliver the short strands of RNA without harming healthytissues in the body. To avoid those possible side effects, Andersonand his colleagues decided to try delivering RNA in a simplepackage made of DNA. Using nucleic acid origami — which allowsresearchers to construct 3-D shapes from short segments of DNA –they fused six strands of DNA to create a tetrahedron (a six-edged,four-faced pyramid). PBL Tube
A single RNA strand was then affixed to eachedge of the tetrahedron. “What’s particularly exciting about nucleic acid origami isthe fact that you can make molecularly identical particles anddefine the location of every single atom,” Anderson says. To target the particles to tumor cells, the researchers attachedthree folate molecules to each tetrahedron. Short protein fragmentscould also be used to target the particles to a variety of tumors. Cosmetic Packaging Tube
Using nucleic acid origami, the researchers have much more controlover the composition of the particles, making it easier to createidentical particles that all seek the right target. This is notusually the case with lipid nanoparticles, says Vinod Labhasetwar,a professor of biomedical engineering at the Lerner ResearchInstitute at the Cleveland Clinic. “With lipid particles,you’re not sure what fraction of the particles are really gettingto the target tissue,” says Labhasetwar, who was not involvedin this study. Circulate and accumulate In studies of mice implanted with human tumors, the researchersfound that once injected, the nucleic acid nanoparticles circulatedin the bloodstream with a half-life of 24 minutes — long enough toreach their targets. The DNA tetrahedron appears to protect the RNAfrom rapid absorption by the kidneys and excretion, which usuallyhappens with RNA administered on its own, Anderson says. Plastic Cosmetic Tubes
“If you take a short interfering RNA and inject it into thebloodstream, it is typically gone in six minutes. If you make abigger nanoparticle using origami methods, it increases its abilityto avoid excretion through the kidneys, thereby increasing its timecirculating in the blood” he says. The researchers also showed that the nucleic acid nanoparticlesaccumulated at the tumor sites. The RNA delivered by the particleswas designed to target a gene for luciferase, which had been addedto the tumor cells to make them glow.
They found that in treatedmice, luciferase activity dropped by more than half. The team is now designing nanoparticles to target genes thatpromote tumor growth, and is also working on shutting off genesinvolved in other genetic diseases. The research was funded by the National Institutes of Health, theCenter for Cancer Nanotechnology Excellence, AlnylamPharmaceuticals and the National Research Foundation of Korea.