A newly developed version of the gene editing technology CRISPR — a non-viral delivery system called CRISPR-Gold — can effectively edit an autism-associated gene in a mouse model of fragile X, reducing the exaggerated repetitive behaviors common with the disorder.
The study, “Nanoparticle delivery of CRISPR into the brain rescues a mouse model of fragile X syndrome from exaggerated repetitive behaviours,” was published in the journal Nature Biomedical Engineering.
CRISPR-Cas9 is a technology that was initially discovered as part of the bacterial immune system. Essentially, Cas9 is an endonuclease (a type of protein) that acts like a pair of molecular scissors and is capable of cutting strands of DNA. Cas9 and other proteins recognize and cut specific stretches of DNA known as CRISPRs.
Since its discovery, researchers have harnessed this technology to direct Cas9 to specific regions in the human genome using a molecule called a guide-RNA. This technology can be used to selectively cut out mutated genes that are associated with disease development.
However, despite the immense potential of CRISPR-Cas9, researchers are still trying to determine the best way to deliver this technology into different tissues of the body using mouse models. In particular, it is hard to efficiently deliver the technology into the brain without causing toxicity.
The best way to deliver CRISPR-Cas9 into the brain is through a viral delivery method, where an inactivated virus is used as a carrier. But there are several limitations to this technique, namely the immune reactions the body can mount against the virus.
To combat that, researchers are working to develop non-viral methods for delivering RNA-guided CRISPR-Cas9 into the brains of adult mice.
If perfected, one of the diseases that CRISPR-Cas9 could be used to treat is fragile X syndrome, for which there is a lack of treatment options.
“CRISPR-based editing of the brain, generated by a local intracranial [into the brain] injection, has great potential for treating FXS [fragile X syndrome] because it will lead to localized gene editing in the brain,” the researchers wrote.
There are several genes that researchers think are attractive targets for CRISPR-based fragile X therapy. One is a gene called mGluR5, whose exaggerated signaling has been shown to be associated not only with fragile X, but also with other autism spectrum disorders (ASDs). Fragile X is the most common single-gene form of autism.
Researchers speculated that editing out the mGluR5 gene (also known as Grm5) could reverse the exaggerated behaviors in people with fragile X and other ASDs.
The team first evaluated the use of a recently developed non-viral delivery system, called CRIPSR-Gold, to deliver two different types of endonucleases (Cas9 and Cpf1) into the brains of two different types of adult mice. This alternative method uses gold nanoparticles to carry the CRISPR-Cas9 complex into the brain by covering a gold nanoparticle with the treated proteins.
CRISPR–Gold was able to effectively deliver CRISPR–Cas9 and Cpf1 into local regions of the brain after the injection.
Next, researchers specifically targeted the mGluR5 gene. They were able to efficiently edit the gene, reducing its activation.
“CRISPR–Gold was able to inhibit 40–50% of the autism causal gene Grm5 in the striatum after an intracranial injection,” the researchers wrote. The striatum coordinates multiple aspects of cognition, including motor and action planning.
Mice treated with mGluR5–CRISPR had no evidence of an increased immune response, the problem encountered when using a viral delivery system.
Importantly, CRISPR-edited mice also had a significant reduction in exaggerated repetitive behaviors. “Taken together, these results demonstrate that gene editing via the nonviral delivery of CRISPR into a local brain region can rescue specific behavioural phenotypes in an autism mouse model,” the researchers wrote.
“Our results demonstrate that CRISPR–Gold has the potential to significantly accelerate the development of new brain-targeted therapeutics,” they added.