Gene therapy reverses Fragile X deficits in mice

A gene therapy designed to replace a missing brain protein restored normal brain activity and improved behavior in a mouse model of Fragile X syndrome (FXS), according to a University of California, Riverside-led study.

The findings, published in Molecular Therapy Nucleic Acids, suggest that gene therapy may one day address the underlying cause of FXS rather than simply treating its symptoms.

FXS affects approximately 2-3% of individuals diagnosed with autism and is one of the best-defined genetic causes of neurodevelopmental disability. The condition occurs when a mutation in the FMR1 gene prevents the production of Fragile X messenger ribonucleoprotein (FMRP), a protein that regulates communication between brain cells.

"In a typical brain, FMRP acts like a brake or a volume control," said Iryna Ethell, the paper's senior author and a professor of biomedical sciences in the UCR 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."

Ethell and her team used a modified adeno-associated virus, AAV9, to deliver a healthy human version of the FMR1 gene into newborn mice lacking FMRP. The therapy carried human FMR1 isoform 7, one of the most abundant forms of the protein in the brain.

The researchers administered either a low or high dose shortly after birth and later examined brain activity and behavior.

They found the high-dose treatment produced significant improvements. Treated mice showed normalized gamma brain-wave activity, reduced background neural noise, improved responses to sound, normal exploratory behavior, stronger social interactions, and improved cognitive flexibility - the ability to adapt when circumstances change.

One test measured probabilistic reversal learning, which requires animals to change strategies when a previously rewarded behavior no longer produces a reward.

"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 results also highlighted the importance of timing. The therapy was administered during an early developmental period when the brain remains highly adaptable.

"The developing brain has critical windows when neural circuits are still being formed," Ethell said. "Our findings suggest that restoring FMRP during those windows may allow the brain to develop more normally."

The researchers found that broad distribution of the therapy throughout the brain was essential. While some low-dose animals benefited when they produced sufficient levels of the protein, the high dose delivered more consistent therapeutic effects because it reached a larger portion of the brain.

Current treatments for FXS primarily address symptoms such as anxiety, attention difficulties, seizures, and behavioral challenges. Gene therapy, by contrast, aims to restore production of the protein associated with the disorder.

"For many years, treatments have focused on reducing the consequences of losing FMRP," Ethell said. "What makes this approach exciting is that it targets the root cause of the condition itself."

Ethell explained the therapy does not repair the original mutated FMR1 gene. Instead, it delivers a "construct" - a functional copy of the gene to brain cells. The construct was engineered by Neurogene Inc. with regulatory elements designed to keep protein production within a normal range.

Although the results are promising, Ethell emphasized that the research remains at the preclinical stage.

"This was a study in mice, and human brains are much larger and more complex," she said. "The next challenge is developing delivery methods that can safely achieve broad distribution throughout the human brain."

Future studies will evaluate versions of the therapy that can be administered intravenously, a more clinically practical approach than the direct brain injections used in the current study. Intravenous administration is not expected to be limited by the blood-brain barrier - a protective network of cells that restricts the passage of many substances from the bloodstream into the brain.

Ethell also said that early diagnosis could be important if similar therapies reach the clinic.

"Beyond FXS, the findings may provide a roadmap for treating other genetic neurodevelopmental disorders caused by the loss of a single critical protein," she 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."

Ethell was joined in the study by Anna O. Norman, Alexandra Varallo, Aarushi Sahni, Nadia Farooq, Courtney Scaramella, and Khaleel A. Razak at UCR; Dominik Biezonski, Suzanne R. Burstein, Juliana Benito, and Stuart Cobb at Neurogene Inc., New York; and Ralph D. Hector, Jim Selfridge, and Stuart Cobb at the University of Edinburgh, United Kingdom.

The research was supported by Neurogene Inc, National Institute of Neurological Disorders and Stroke, and FRAXA Research Foundation. The development of the gene therapy construct was supported by the Simons Initiative for the Developing Brain.

The title of the paper is "Neonatal expression of human FMRP isoform rescues cortical processing deficits and improves behaviors in a mouse model of Fragile X Syndrome."

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