Fragile X syndrome (FXS) — the most commonly inherited cause of both intellectual disability and autism spectrum disorder — results from a mutation in the FMR1 gene (Fragile X messenger ribonucleoprotein 1). Though it’s just a single gene and should theoretically be a great candidate for gene therapy, preclinical and translational work targeting the FMR1 gene has so far not led to a successful treatment of the disorder.
One issue that Carlos Portera-Cailliau’s lab at the University of California, Los Angeles (UCLA), identified as holding back drug development in this space is that preclinical studies in animal models of FXS have focused solely on ameliorating the behavioral phenotypes but by and large ignored the restoration of circuit function in the brain. “We felt that if a therapeutic intervention in mice could restore circuit function and multiple behaviors, then it would bode well for translation to humans eventually,” Portera-Cailliau told DDN.
To find a new drug target, Portera-Cailliau’s team took a new approach. “We asked what happens to neurons in the brain when the protein FMRP is missing as a result of the mutation in the FMR1 gene. How do neurons dial up or down the expression of other genes and proteins as a result?” he said.
This led them to discover one protein in particular — EPAC2 (exchange protein directly activated by cAMP 2) — could be worthy of targeting with novel therapeutic interventions. The new work was published recently in Neuron.
Finding a new FXS drug target
To identify EPAC2, the researchers started with an unbiased molecular screen to evaluate which genes are dysregulated in a model of FXS in mice. Crucially, they noticed that EPAC2 levels were unusually elevated in cerebral cortex neurons of the mice.
By using an EPAC2 antagonist, the small molecule inhibitor EPAC2-specific inhibitor-05 (ESI-05), as well as genetic breeding of FMR1 knockout mice with mice that have reduced copies of the gene that produces EPAC2, the team showed that decreasing EPAC2 levels in the brain successfully restored the imbalance of excitation and inhibition of the neurons — which is a leading hypothesis behind the behavioral symptoms of FXS. Doing so also improved the behavioral symptoms in the mice, including their susceptibility to seizures, oversensitivity to sensory stimuli, and social avoidance.
The researchers also found that different neurons in the brain did not handle the loss of FMRP protein in the same ways. “Surprisingly, we found that FMRP does very different things in excitatory and inhibitory neurons,” said Portera-Cailliau. “We can only assume that loss of FMRP probably also triggers unique changes in gene expression across different brain regions, which could explain the different symptoms.”
Treating FXS with EPAC2 antagonists
Currently, there are no FDA-approved EPAC2 antagonists to treat any disease, and none have been tested for safety in Phase 1 clinical trials. That could certainly delay hopes for getting this new drug target into human testing for FXS any time soon. “Our hope is that others will also explore EPAC2 as a potential target for therapy and when the preclinical evidence is strong enough this will attract interest from industry to pursue this by improving upon ESI-05 and synthesizing analogs that are more potent,” said Portera-Cailliau.
Our hope is that others will also explore EPAC2 as a potential target for therapy and when the preclinical evidence is strong enough this will attract interest from industry to pursue this by improving upon ESI-05 and synthesizing analogs that are more potent.
—Carlos Portera-Cailliau, UCLA
Portera-Cailliau also noted that more work is needed to replicate their findings and confirm that EPAC2 is a better target than others being pursued for FXS, like SPG601 developed by Spinogenix, which activates BK (Big Potassium) channels, that recently advanced to Phase 2b trials.
Still, Portera-Cailliau is excited about the potential to move into the clinic with an EPAC2 antagonist. "In my lab, we spent the first 12 years or so studying Fragile X mice, looking for differences in synapses and circuits, during early brain development and in adulthood. … We did a lot of homework to understand the disease at the circuit level,” he said. “As a physician scientist, I feel like my laboratory is getting closer and closer to having come full circle, from bench to bedside. It’s very rewarding.”
