A University of California, Riverside-led research team has developed a gene therapy that restored production of a missing brain protein, corrected abnormalities in brain circuitry, and improved behavior in a mouse model of Fragile X syndrome (FXS). The study, published in the journal Molecular Therapy Nucleic Acids, tested an adeno-associated virus (AAV)-based therapy carrying a normal human version of the FMR1 gene to produce the Fragile X messenger ribonucleoprotein (FMRP) and found that early treatment normalized several measures of brain activity while improving social behavior, exploratory behavior, and cognitive flexibility.
“In a typical brain, FMRP acts like a brake or a volume control,” said senior author Iryna Ethell, PhD, a professor of biomedical sciences at the UC Riverside School of Medicine. “Without it, neural circuits become overactive and less efficient, which contributes to many of the developmental and behavioral challenges associated with FXS.”
FXS is the most common single-gene cause of autism spectrum disorder. According to the researchers, the disorder typically manifests from expansion of CGG repeats in the 5′ untranslated region of FMR1. The mutation causes methylation and silencing of the gene, leading to a major reduction or complete loss of FMRP, an RNA-binding protein that regulates numerous messenger RNAs involved in synapse formation, maturation, and function. Loss of the protein can lead to abnormal synaptic activity and increased cortical hyperexcitability.
FXS can produce sensory hypersensitivity, seizures, anxiety, intellectual disability, developmental delays, repetitive behaviors, and social communication difficulty. Current treatments for this syndrome don’t seek to cure it, rather they are aimed at managing the associated symptoms of anxiety, hyperactivity, irritability, aggression, depression, and seizures.
The therapy developed by the research team was designed to replace missing FMRP rather than repair the original mutation. To do this, the researchers used an AAV9 viral vector to deliver human FMR1 isoform 7, one of the most abundant forms of the protein found in the brain. The therapy was tested in newborn mice lacking FMRP via intracerebroventricular injections at either a low or high doses.
The work built on earlier research that explored the potential of AAV-mediated restoration of FMRP in rodent models. These prior studies used a range of viral serotypes, promoters, delivery routes, and FMRP isoforms and showed they could partially or completely correct specific biochemical, physiological, and behavioral abnormalities. The researchers noted that studies involving mouse and rat FMRP homologs had shown that restoring the protein could improve a range of Fragile X-related deficits.
The current study showed that high-dose treatment produced the strongest positive effects in the mouse models. Electroencephalography showed normalization of baseline gamma power, improvements in responses to sound, reduced background neural activity, and improved habituation to repeated auditory stimuli. The therapy also restored abnormal patterns of brain-wave coupling that have been associated with Fragile X-related dysfunction.
Behavioral testing showed that these improvements persisted into adulthood. Mice receiving the higher dose displayed normalized exploratory behavior, improved social preference, and better performance in probabilistic reversal learning, a measure of cognitive flexibility that requires adapting when previously rewarded behaviors stop producing rewards.
“Fragile X mice tend to persist with an old solution even after the rules change,” Ethell said. “After treatment, they became much better at adapting, performing similarly to mice with normal FMR1 function.”
The researchers noted that their work showed the importance of delivering at therapy for FXS early in its development. They said that widespread distribution of the potential new gene therapy throughout the brain was necessary to achieve a therapeutic benefit. There was a clear relationship between the proportion of neurons expressing the therapeutic gene and the degree of functional recovery, which indicated that restoring FMRP in a sufficient number of cortical cells is critical for correcting any behavioral deficits.
While a promising step, the investigators said that the work was a preclinical study and that future research will now focus on developing delivery methods that can of have broad distribution across the human brain. The team also believes their approach could have broader applications.
“Beyond FXS, the findings may provide a roadmap for treating other genetic neurodevelopmental disorders caused by the loss of a single critical protein,” Ethell said. “Our study shows it may be possible to restore function across complex brain networks by replacing a missing gene. That gives us reason to be optimistic about the future of genetic medicine.”
The post Gene Therapy Restores Brain Function and Behavior in Fragile X Syndrome appeared first on Inside Precision Medicine.