In a pioneering experiment, researchers from Texas and California have reduced the obsessive and repetitive behavior of mice with fragile X syndrome, a form of inherited autism spectrum disorder, using CRISPR-Cas9 gene-editing enzymes that were delivered to brain cells via gold nanoparticles.
The impressive efficacy and apparent safety demonstrated by their results, described in the journal Nature Biomedical Engineering, are poised to open the floodgates for investigations exploring CRISPR's potential for other neurological diseases, chronic pain, and even addiction.
Traditional CRISPR-Cas9 delivery – employing modified viruses to inject genes encoding the molecular machinery into cells’ genomes – has proven ill-suited for applications in the brain because it is difficult to control the quantity of Cas9 proteins and guiding RNA molecules that are produced. Additionally, the foreign proteins that make up Cas9 are then expressed indefinitely in brain tissue, often inducing a harmful immune response and leading to unintended changes in neural functioning (yikes).
To remedy these issues, UC Berkeley researcher Niren Murthy developed a non-viral CRISPR platform, called CRISPR-Gold, wherein preassembled Cas9-RNA complexes are stuck onto gold nanoparticles. The resulting clusters are then covered with a polymer that enables them to enter cell membranes.
After CRISPR-Gold first proved its promise by successfully modifying genes in diseased muscle cells, Murthy teamed up with lead author Hye Young Lee to test it in a neurological condition. Fragile X syndrome (FXS) represented an ideal initial target.
“FXS is the most common inherited form of intellectual disability and a common single-gene form of autism spectrum disorders (ASDs), accounting for ~2.1% of patients,” the authors wrote. “Current drug treatments, such as psychostimulants, antidepressants, and antipsychotics are ineffective because they do not address the underlying [cause] of FXS; they only target individual symptoms.”
In addition to hindered intellectual capacity, humans with FXS display exaggerated repetitive tics and anxiety. Prior to this study, scientists hypothesized that the obsessive behavior patterns characteristic of FXS and other ASDs arise from hyperactivity of a neuron-to-neuron signaling receptor called metabotropic glutamate receptor 5, or mGluR5, but they lacked conclusive evidence. It was also unknown where in the brain this unwanted over-signaling occurred.
Murthy and Lee’s subsequent series of experiments in mice not only demonstrated that CRISPR-Gold has the potential to improve obsessive behavior associated with an ASD, they also established that mGluR5 activity in a region called the striatum is chiefly responsible for the disease state – meaning that future therapies could be spared the challenge of targeting large areas of the brain.
To begin, they created a CRISPR-Gold system that disabled the gene for mGluR5. Then, based on previous findings that the striatum mediates repetitive behavior, the scientists injected the CRISPR-Gold directly into the striatum of mice bred to have FXS. Just two weeks later, the animals’ obsessive symptoms were remarkably reduced – compulsive marble burying dropped by 30 percent and periodic leaping went down by 70 percent. No adverse health effects were observed.
Furthermore, brain tissue analysis showed that the number of mGluR5 proteins expressed in the striatum were 40 to 50 percent lower than pretreatment levels.
"There are no treatments or cures for autism yet, and many of the clinical trials of small-molecule treatments targeting proteins that cause autism have failed," Professor Lee said in a statement. "This is the first case where we were able to edit a causal gene for autism in the brain and show rescue of the behavioral symptoms."
Speaking to New Atlas, Lee was optimistic about the future of the treatment platform, though a great deal more work needs to be done in animals before it is considered in humans.
"CRISPR-Gold can be used to treat a variety of genetic diseases, such as Huntington's disease," he said. "But it's not limited to monogenic diseases; it can also be used against polygenic diseases, once we figure out the entire network of genes involved."