Researchers successfully used this nanoparticle, made from several strands of DNA and RNA, to turn off a gene in tumor cells. Image: Hyukjin Lee and Ung Hee Lee |
Using
a technique known as “nucleic acid origami,” chemical engineers have built tiny
particles made out of DNA and RNA that can deliver snippets of RNA directly to
tumors, turning off genes expressed in cancer cells.
To
achieve this type of gene shutdown, known as RNA interference, many researchers
have tried—with some success—to deliver RNA with particles made from polymers
or lipids. However, those materials can pose safety risks and are difficult to
target, says Daniel Anderson, an associate professor of health sciences and
technology and chemical engineering, and a member of the David H. Koch
Institute for Integrative Cancer Research at Massachusetts Institute of
Technology (MIT).
The
new particles, developed by researchers at MIT, Alnylam Pharmaceuticals, and Harvard Medical
School, appear to overcome those
challenges, Anderson
says. Because the particles are made of DNA and RNA, they are biodegradable and
pose no threat to the body. They can also be tagged with molecules of folate
(vitamin B9) to target the abundance of folate receptors found on some tumors,
including those associated with ovarian cancer—one of the deadliest,
hardest-to-treat cancers.
Anderson is senior author of a
paper on the particles appearing in the Nature Nanotechnology. Lead
author of the paper is former MIT postdoctoral student Hyukjin Lee, now an
assistant professor at Ewha Womans University
in Seoul, South Korea.
Genetic disruption
RNA interference (RNAi), a natural phenomenon that cells use to control their
gene expression, has intrigued researchers since its discovery in 1998. Genetic
information is normally carried from DNA in the nucleus to ribosomes, cellular
structures where proteins are made. Short interfering RNA (siRNA) disrupts this
process by binding to the messenger RNA molecules that carry DNA’s
instructions, destroying them before they reach the ribosome.
siRNA-delivering
nanoparticles made of lipids, which Anderson’s
laboratory and Alnylam are also developing, have shown some success in turning
off cancer genes in animal studies, and clinical trials are now underway in
patients with liver cancer. Nanoparticles tend to accumulate in the liver,
spleen, and lungs, so liver cancer is a natural target—but it has been
difficult to target such particles to tumors in other organs.
“When
you think of metastatic cancer, you don’t want to just stop in the liver,” Anderson says. “You also
want to get to more diverse sites.”
Another
obstacle to fulfilling the promise of RNAi has been finding ways to deliver the
short strands of RNA without harming healthy tissues in the body. To avoid
those possible side effects, Anderson and his colleagues decided to try
delivering RNA in a simple package made of DNA. Using nucleic acid origami—which
allows researchers to construct 3D shapes from short segments of DNA—they fused
six strands of DNA to create a tetrahedron. A single RNA strand was then
affixed to each edge of the tetrahedron.
“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,” Anderson says.
To
target the particles to tumor cells, the researchers attached three folate
molecules to each tetrahedron. Short protein fragments could also be used to
target the particles to a variety of tumors.
Using
nucleic acid origami, the researchers have much more control over the
composition of the particles, making it easier to create identical particles
that all seek the right target. This is not usually the case with lipid
nanoparticles, says Vinod Labhasetwar, a professor of biomedical engineering at
the Lerner Research Institute at the Cleveland Clinic. “With lipid particles,
you’re not sure what fraction of the particles are really getting to the target
tissue,” says Labhasetwar, who was not involved in this study.
Circulate and accumulate
In studies of mice implanted with human tumors, the researchers found that once
injected, the nucleic acid nanoparticles circulated in the bloodstream with a
half-life of 24 min—long enough to reach their targets. The DNA tetrahedron
appears to protect the RNA from rapid absorption by the kidneys and excretion,
which usually happens with RNA administered on its own, Anderson says.
“If
you take a short interfering RNA and inject it into the bloodstream, it is
typically gone in six minutes. If you make a bigger nanoparticle using origami
methods, it increases its ability to avoid excretion through the kidneys,
thereby increasing its time circulating in the blood,” he says.
The
researchers also showed that the nucleic acid nanoparticles accumulated at the
tumor sites. The RNA delivered by the particles was designed to target a gene
for luciferase, which had been added to the tumor cells to make them glow. They
found that in treated mice, luciferase activity dropped by more than half.
The
team is now designing nanoparticles to target genes that promote tumor growth,
and is also working on shutting off genes involved in other genetic diseases.
