The gene editing system CRISPR-Cas9 has generated excitement for its potential use in eliminating disease-causing irregularities in DNA. But most CRISPR systems rely on viruses to carry Cas9 components into cells, where they then go to work cutting DNA.
Such viral delivery systems carry risks: Some patients already have antibodies against the viruses in their bodies or they could develop them, raising the possibility of immune rejection.
Scientists at MIT have developed an alternative to viral delivery of Cas9: nanoparticles that can carry the CRISPR machinery into cells and edit their genomes. And they’ve proven the promise of their system by targeting the gene PCSK9, which causes a rare inherited form of high cholesterol. In mice, they used their nanoparticle-based CRISPR system to eliminate the PCSK9 gene in more than 80% of liver cells. The research is published in the journal Nature Biotechnology.
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The MIT team, led by chemical engineering professor Daniel Anderson, had previously developed a nonviral CRISPR delivery system that required a high-pressure injection into the liver, but that raised the risk of damaging the organ. They later eliminated the need for the injection using a nanoparticle delivery system, but they still needed to use a virus to deliver one component, an RNA guide called sgRNA.
So they analyzed the bond between sgRNA and Cas9, experimenting with chemical modifications until they hit on a cocktail that could be used in place of the virus without interfering with the interactions between the two molecules. When they injected the nanoparticles into mice, with the goal of cutting PCSK9, they discovered that the PCSK9 protein virtually disappeared, and the cholesterol levels in the animals dropped by 35%.
There are two drugs on the market that target PCSK9 in people with familial hypercholesterolemia, a form of high cholesterol that’s often unresponsive to traditional statins: Repatha, sold by Amgen, and Praluent from Sanofi and Regeneron.
But those must be taken at regular intervals for life. The MIT team believes their approach could be used to develop a one-time cure for the disorder.
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The MIT team is one of many searching for ways to translate CRISPR advances into usable cures. CRISPR-Cas9 has so far proven impractical in nondividing cells such as those in the brain, so scientists at Max Planck Institute for Neuroscience in Florida have been working on an alternative gene-editing system for neurons, for example. Using CRISPR to edit RNA instead of DNA is also being explored as a way to not only expand the potential uses for the technology, but also to make gene edits that are reversible.
The next step for the MIT team will be to identify more liver diseases that might be targeted with their nanoparticle-based CRISPR system and to further develop their technology for human testing. The research team includes MIT professor Robert Langer, who has been instrumental in launching several biotech startups inspired by discoveries made at the school.
"I think having a fully synthetic nanoparticle that can specifically turn genes off could be a powerful tool not just for PCSK9 but for other diseases as well," said Anderson in a press release. "The liver is a really important organ and also is a source of disease for many people. If you can reprogram the DNA of your liver while you're still using it, we think there are many diseases that could be addressed."