During the study, which was published in the June issue of Nature Biomedical Engineering, scientists used CRISPR gold nanoparticle carriers to inject a gene-editing enzyme called Cas9 into the striatum — the part of the brain that facilitates voluntary movement — of mice with fragile X syndrome, the most common single-gene cause of autism. Enzymes are proteins that act as catalysts to bring about biochemical reactions.
CRISPR-Cas9, or "Clustered Regularly Interspaced Palindromic Repeats," is a technique that involves identifying the location of mutations in genes and then using Cas9 to cut and remove unwanted portions of DNA along a gene sequence. CRISPR-Gold delivers purified Cas9 protein and guides RNA directly into cells.
"The enzyme we used, Cas9, is like a pair of scissors," said Dr. Hye Young Lee, assistant professor in the Department of Cellular and Integrative Physiology at UT Health San Antonio, who was also the lead author of the study.
"We were able to cut the genetic blueprint, DNA, at a location that causes the exaggerated repetitive behaviors," he explained, EurekAlert reported Tuesday.
The Cas9 enzyme was used to target Metabotropic glutamate receptor 5 (mGluR5), a major excitatory neurotransmitter in the central nervous system that increases communication between neurons in the brain. When researchers performed the technique on the mice, the rodents' repetitive digging behavior slowed down by 30 percent while their leaping behavior decreased by 70. Both of these behaviors are indicative of autistic behavior in mice.
"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," Lee added, the Telegraph reported Monday.
"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," he continued.
In addition, the use of a non-viral approach to deliver Cas9 into the brain is advantageous over other ways to deliver Cas9 into the body, such as through viruses, which has been demonstrated in previous studies.
"Previous attempts by other groups used viral carriers to convey Cas9, which has potential problems because the virus can't be programmed to stop working," said Dr. Bumwhee Lee, postdoctoral fellow in Dr. Hye Young Lee's laboratory, EurekAlert reported Tuesday. "This can result in serious immune reactions and cell toxicity."
"If you inject CRISPR DNA using a virus, you can't control how much Cas9 protein and guide RNA are expressed, so injecting it via a virus has a potential problem," Lee noted. "I think the CRISPR-Gold method is very cool because we can control the amount we wish to inject and that probably minimizes the side effects of using CRISPR, for example, off-target effects."
"The really compelling thing about this paper is that Hye Young was able to show that if you injected CRISPR-Gold into the brain, you could knock out disease-causing genes and actually see fairly significant behavioral changes," CRISPR-Gold inventor Niren Murthy, a UC Berkeley professor of bioengineering recently told Berkeley News. "This is the first time anyone had ever shown that with non-viral delivery."
Autism spectrum disorder (ASD) is a neurodevelopmental disorder that affects communication abilities and social interaction. People with autism usually exhibit repetitive behaviors and obsessive interests and communicate in usual and idiosyncratic ways. Autistic people often have trouble navigating the world and many require some sort of care even as adults. The condition usually shows up in childhood and affects more than 200,000 people a year in the US.
ASD is a recent diagnosis created in 2013 as an umbrella diagnosis for a number of disorders, including Asperger Syndrome, Childhood Degenerative Disorder, and Pervasive Developmental Disorder, Not Otherwise Specified (PDD-NOS), which psychiatrists realized were related disorder on a spectrum with autism and not separate conditions.