UCLA Health researchers have identified a potential drug target for treating fragile X syndrome (FXS), the most common genetic cause of intellectual disability and autism that affects roughly one in 2,000 boys.
Fragile X syndrome is caused by a mutation in a single gene, FMR1, that results in the loss of a protein critical for normal brain development and function. Headed by Carlos Portera-Cailliau, MD, PhD, professor of neurology at UCLA and member of the UCLA Brain Research Institute, the researchers, the team’s work in genetically engineered mice lacking the Fmr1 gene identified the synaptic protein EPAC2 as a potential therapeutic target for fragile X syndrome. Their study showed that blocking EPAC2 in the fragile X mouse model restored abnormal patterns of brain activity and improved several FXS-associated behavioral symptoms.
Pertera-Cailliau is senior and corresponding author of the researchers published paper in Neuron, titled “Translatome profiling reveals opposing alterations in inhibitory and excitatory neurons of fragile X mice and identifies EPAC2 as a therapeutic target.”
Fragile X syndrome is a prototypical neurodevelopmental disorder (NDD) characterized by intellectual disability, social anxiety, atypical sensory processing characterized heightened sensitivity to sensory input such as sound and touch, and a higher risk of seizures. Many also meet the criteria for an autism spectrum disorder diagnosis. “Symptoms of fragile X syndrome (FXS), the leading monogenic cause of intellectual disability and autism, are thought to arise from an excitation/inhibition (E/I) imbalance,” the authors stated.
FXS is caused by mutations in the FMR1 gene, resulting in near complete loss of the fragile X messenger ribonucleoprotein (FMRP), an RNA-binding protein in neurons that plays different roles in cell compartments including the nucleus, axons and dendrites, including regulating mRNA translation at synapses, they explained. As it is caused by a change in a single gene, fragile X syndrome has long been considered a promising candidate for targeted therapies yet clinical trials to date have not produced an effective treatment. “Since the discovery of the genetic basis of FXS in 1991, several clinical trials have been undertaken—without success—and no specific treatments for FXS are currently available,” the investigators continued. “Thus, there is an urgent need to rethink therapeutic strategies for FXS.”
For their newly reported study the researchers used genetically engineered knockout (KO) mice that lack Fmr1 to simulate fragile X syndrome. Using genetic sequencing, they found that levels of the gene EPAC2 were increased in the brain of fragile X mice. This was of potential interest as a target for therapy because the gene’s protein, EPAC2, is localized to synapses and is known to be important for learning and memory.
The researchers then demonstrated that blocking EPAC2 in the fragile X mouse model, either genetically, or using an EPAC2 inhibitor compound, restored cortical circuit function and improved multiple behavioral symptoms associated with fragile X syndrome, including heightened sensitivity to touch, difficulties with social interaction and their susceptibility for seizures. “Perhaps the most exciting result is that treatment with an EPAC2 antagonist can rescue several behavioral phenotypes in Fmr1 KO mice,” the authors stated.
“EPAC2 emerged as an attractive target because it was consistently altered across multiple types of brain cells in our analysis,” said the study’s first author Anand Suresh, PhD, a post-doctoral fellow in the laboratory of Portera-Cailliau. “When we blocked it, either genetically or with a drug compound, we saw meaningful improvements in both brain circuit function and behavior.”
EPAC2 is expressed almost exclusively in the brain, which means drugs targeting it are less likely to cause unwanted effects elsewhere in the body. Suresh said this is an important consideration as researchers continue preclinical studies. “This bodes well for future preclinical trials and safety studies in humans, as compounds that target EPAC2 should not have off-target effects,” the authors stated in their report.
For their study the UCLA investigators used an RNA sequencing technique to examine gene activity separately in two major classes of brain cells: those that excite and those that inhibit neural activity. Fragile X syndrome is thought to arise from an imbalance between these two systems. The analysis revealed striking differences in how the genetic mutation underlying Fragile X syndrome affects each cell type but also identified a small set of genes, including the one that encodes EPAC2, that were dysregulated in both.
The researchers also found that EPAC2 levels appear to rise gradually as the brain matures, suggesting it may be a particularly relevant target for older children and adults with Fragile X syndrome, rather than only in early development. They concluded, “Our results should encourage the development of novel EPAC2 inhibitors for the treatment of FXS. More generally, our study exemplifies how transcriptomic approaches in animal models of neuropsychiatric conditions can be used to prioritize potential novel therapeutic targets.”
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