Gene Therapy Reverses Fragile X Deficits in Mouse Study

For decades, researchers have known that the loss of one protein—fragile X messenger ribonucleoprotein (FMRP)—causes fragile X syndrome (FXS), the most common inherited form of intellectual disability. As a regulator of neuronal protein expression, FMRP is vital for synapse formation, and its loss leads to cognitive symptoms like delayed speech development and learning disabilities as well as behavioral symptoms including sensory sensitivities.¹ While many seemingly promising therapies have worked well in mice, none have proved successful for people with FXS in clinical trials yet.

Now, scientists at Cincinnati Children’s Hospital Medical Center believe they may have a strategy that could translate from mice to humans. In a study published in Gene Therapy, the researchers developed a gene therapy that expresses human FMRP on an adeno-associated viral (AAV) vector that has already been shown to work well in people.² They found that the gene therapy restored multiple clinically relevant symptoms in an FXS mouse model, including sensory hyperexcitability, adaptation to change, and altered brain activity.

“These findings are important because they show that restoring FMRP can improve several fragile X-related traits in a model designed with clinical translation in mind,” said Christina Gross, a study coauthor and researcher at Cincinnati Children’s Hospital Medical Center, in a statement. “By pairing gene replacement with outcomes that can help bridge mouse studies and future human trials, this work gives the field a stronger foundation for developing therapies that address the root biology of fragile X syndrome.”

The researchers delivered an AAV9 vector expressing the human FMR1 gene, which encodes FMRP, and injected it into mice lacking the Fmr1 gene via two different delivery routes: intracerebroventricular (ICV) and intravenous (IV). With ICV injections, the researchers deliver the gene therapy directly to the brain via the cerebrospinal fluid, which means that it doesn’t need to cross the blood-brain barrier; IV injections, on the other hand, are delivered into the bloodstream.

The team found that the gene therapy led to expression of FMRP throughout the brain of adolescent mice. In particular, the ICV injection induced more protein expression in the prefrontal areas of the brain, while IV injection led to expression throughout a wider swath of the brain and slightly higher expression in the midbrain. These results suggested that delivering the gene therapy via both ICV and IV might lead to the broadest coverage of FMRP expression.

People with FXS often have sensory hypersensitivity, which can make common sounds or light feel overwhelming. In FXS mice, this manifests as seizures triggered by sound. Gross and her team found that their gene therapy significantly reduced seizure susceptibility in FXS mice. When the researchers looked at behavioral symptoms, they saw that the therapy also reduced the time FXS mice spent digging, indicating that the mice were having an easier time adjusting to change in their environment.

Researchers have also noticed that both people and mice with FXS have an increased level of electrical activity in their gamma frequency brain waves, called gamma power, when studied via electroencephalogram (EEG).³ The activity of these brain waves correlates with cognitive activities like working memory and attention. To see how their gene therapy affected FXS mice’s gamma power, the team fit the mice with scalp multi-electrode arrays, similar to a human EEG. When delivered in either a high dose via ICV or low doses via both ICV and IV, the gene therapy reduced the gamma power in FXS mice.

For these experiments, the researchers used mice that ranged in age from three- to eight-weeks-old, which would be the equivalent of toddler age to young adulthood in people. For study coauthor Ernest Pedapati, a pediatric psychiatrist and physician-scientist at Cincinnati Children’s Hospital Medical Center, the fact that the gene therapy rescued the seizure phenotype, adversity to change behavior, and heightened gamma brain activity indicates that restoring FMRP expression even later in life could lead to positive outcomes for people with FXS.

“This suggests that certain FXS-related deficits are reversible or can be improved by re-expression of FMRP after large parts of brain development have already occurred,” he said in the statement.