Anxiety affects many people, and for some, medication doesn’t help. Researchers have developed a novel, noninvasive method of delivering CRISPR/Cas9 gene-editing technology into the brain – previously a challenge – to knock out a gene that causes anxiety. While the technique has so far only been used on mice, the findings open the door to developing new therapeutics, especially for people resistant to medication.
Mental health is an essential part of our lives. Poor mental health affects our well-being, ability to work, and relationships with family, friends, and the wider community. Anxiety disorders – including generalized anxiety disorder, panic disorder, and social anxiety disorder – are characterized by excessive fear and worry severe enough to cause significant distress or impair functioning.
According to the World Health Organization (WHO), in 2019, 301 million people worldwide were living with an anxiety disorder, including 58 million children and adolescents. They also noted an increase in the prevalence of anxiety by 25% with the onset of the COVID-19 pandemic. Although anxiety is commonly treated with daily medication, this can have side effects and, for some, doesn’t relieve their anxiety symptoms.
Now, researchers have developed a noninvasive method of delivering CRISPR/Cas9 gene editing technology to the brain to knock out a gene associated with anxiety and depression in mice. They say it’s the first successful demonstration of noninvasive CRISPR/Cas9 delivery capable of passing the blood-brain barrier to enable genetic modification.
True to its name, the blood-brain barrier (BBB) is a structural and functional roadblock that stops harmful substances, such as bacteria and viruses, from entering the brain through the blood, while allowing essential nutrients in. While this is highly advantageous for maintaining our health, the BBB is so effective that it can prevent therapeutic agents from accessing the brain.
Precision gene editing using CRISPR/Cas9 has shown great potential to treat various diseases, including muscular dystrophy, HIV, and lung cancer. The CRISPR system uses an enzyme (Cas9) that, directed by a guide RNA (gRNA), snips out a particular section of DNA. It can remove problematic genes, such as those that cause disease. But, as with other therapeutic agents, the BBB provides a challenge to the CRISPR system.
In the current study, the researchers experimented with intranasal delivery of the CRISPR/Cas9 system to see if it would successfully cross the BBB and knock out the serotonin receptor (HTR2A) gene, which modulates the availability of serotonin. This neurotransmitter performs many functions, including regulating mood. Too little serotonin has been implicated in anxiety and depression, which is why people with these conditions are often prescribed selective serotonin reuptake inhibitors (SSRIs), which increase serotonin levels in the brain.
Therapeutic agents delivered intranasally – through the nose – reach the central nervous system via the nerve pathways inside the nasal cavity. It’s not only a practical delivery method, it’s noninvasive.
The researchers administered a viral vector, an inactivated adeno-associated virus (AAV), into the noses of mice to deliver the gRNA to neurons in the brain so that it could bind to the target HTR2A gene, which Cas9 then cut out. AAVs are commonly used as vectors to deliver CRISPR/Cas9 cargo because they’re considered safe and have a low likelihood of causing an immune response. The researchers used the AAV9 subtype, a highly efficient vector for delivering cargo to neurons throughout the central nervous system.
Five weeks after administering the gene-editing package, the mice’s anxiety was tested using a light-dark behavioral test and a marble-burying test. In the light-dark test, mice are given a choice between exploring a brightly lit chamber or a dark chamber. Anxious mice will tend to spend more time in the dark chamber. For the marble-burying test, glass marbles are placed in a grid pattern in sawdust, and the mice are allowed to explore the cage for a fixed period. Anxious mice will bury more marbles.
The researchers found an 8.47-fold decrease in HTR2A expression in treated mice. Regarding the marble-burying test, these mice demonstrated a 14.8% decrease in the number of marbles buried compared with controls. And, for the light-dark test, treated mice spent considerably more time in the lit box (a 35.7% increase) and made more entries into the lit box (a 27.5% increase). These results were comparable to mice treated with the benzodiazepine diazepam, whose time in the lit box was 40%, leading the researchers to conclude that their HTR2A-targeting therapy performed on par with the drug.
“Our results suggest that even with a low percentage of neuronal gene editing, significant anxiolytic [anxiety reducing] effects are observed,” said the researchers. “Delivery within the CNS [central nervous system] was accomplished using a noninvasive intranasal delivery platform that can bypass the BBB, typically a major hurdle for large cargo such as CRISPR/Cas9.”
The researchers say that their proof-of-concept study demonstrates that certain traits can be modified long-term, which has important implications for the development of new medications to treat anxiety and depression, especially for those who are resistant to medications.
The study was published in the journal PNAS Nexus.
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