June 22, 2026 —
A research team led by the University of California, Riverside has developed an AAV9-based gene therapy that restored production of the missing brain protein FMRP, corrected abnormalities in brain circuitry, and improved behavior in a mouse model of Fragile X syndrome. The study was published in Molecular Therapy Nucleic Acids.
Fragile X syndrome is the most common single-gene cause of autism spectrum disorder. It is typically caused by expansion of CGG repeats in the 5′ untranslated region of the FMR1 gene, leading to methylation and gene silencing. As a result, patients have markedly reduced or absent Fragile X messenger ribonucleoprotein, or FMRP, an RNA-binding protein that regulates messenger RNAs involved in synapse formation, maturation, and function.
Without FMRP, neural circuits can become overactive and less efficient, contributing to abnormal synaptic activity, cortical hyperexcitability, sensory hypersensitivity, seizures, anxiety, intellectual disability, repetitive behaviors, developmental delay, and social communication difficulties. Current treatments primarily manage symptoms rather than addressing the underlying loss of FMRP.
The experimental therapy was designed to replace the missing protein rather than repair the original mutation. Researchers used an AAV9 vector to deliver a normal human version of FMR1 isoform 7, one of the most abundant FMRP isoforms in the brain. The therapy was administered to newborn mice lacking FMRP through intracerebroventricular injection at either low or high doses.
High-dose treatment produced the strongest effects. Electroencephalography showed normalization of baseline gamma power, improved responses to sound, reduced background neural activity, and better habituation to repeated auditory stimuli. The therapy also restored abnormal brain-wave coupling patterns associated with Fragile X-related dysfunction.
Behavioral improvements persisted into adulthood. Mice treated with the higher dose showed normalized exploratory behavior, improved social preference, and better performance in probabilistic reversal learning, a measure of cognitive flexibility that evaluates the ability to adapt when previously rewarded behaviors no longer produce rewards.
The researchers emphasized that timing and distribution were critical. Early treatment and broad delivery across the brain appeared necessary for therapeutic benefit. The study found a clear relationship between the proportion of neurons expressing the therapeutic gene and the degree of functional recovery, suggesting that restoring FMRP in a sufficient number of cortical cells is important for correcting behavioral deficits.
While the findings remain preclinical, the study supports the potential of AAV-mediated gene replacement for Fragile X syndrome and other neurodevelopmental disorders caused by loss of a single critical protein. Future work will focus on developing delivery approaches that can achieve broad and safe distribution across the human brain.
