Researchers uncovered a novel pathway that causes epilepsy

Researchers have uncovered a novel biological pathway that can lead to seizures when disrupted. The findings also provide a new approach to improve the diagnosis of epilepsy, for which a genetic cause cannot be found in about 50 % of individuals with the condition. 

Researchers at Baylor College of Medicine and the Duncan Neurological Research Institute (Duncan NRI) at Texas Children’s Hospital show that epilepsy can be caused not only by defects in a single gene, as has been traditionally considered, but also by specific combinations of two and potentially more defective genes. The study, published in the Journal of Clinical Investigation, can lead to improvements in genetic diagnosis and treatment for many unsolved or drug-resistant cases.

“Epilepsy is a neurological condition that causes recurrent seizures and affects about 50 million people worldwide – roughly 1 in every 130 people,” said corresponding author Dr. Hugo Bellen, Distinguished Service Professor of the Department of Molecular and Human Genetics at Baylor and chair in neurogenetics at the Duncan NRI. “Although scientists have discovered more than 1,000 genes that can individually cause epilepsy when disrupted, more than half of patients with a suspected genetic cause have no genetic diagnosis.”

Relatively few genes linked to seizures have been studied in detail. Most of them have been associated with increased synaptic activity, the communication between brain cells. “But what about other seizure-associated genes that do not clearly fall into this category?” said first author Dr. Shenzhao Lu, instructor of molecular and human genetics working in the Bellen lab. “To help these individuals with seizures, we need to find out how the genetic disruptions cause the condition. In the current study, we focused on a group of seizure-associated genes that are involved in actin biology.” 

Actin is a protein that is present in all cells and forms filaments, like tiny construction blocks that connect to build or reshape the cell’s structure. These filaments form the foundation for the cytoskeleton, structures inside cells that are essential for many cellular processes such as cell adhesion, cell motility and the transport of materials within cells. Cells control the formation of actin filaments using numerous actin regulatory genes.
In 2022, Lu and colleagues in the Bellen lab found for the first time an association between defective variants of the human gene TIAM1 and a neurological disorder with seizures. The TIAM1 protein is abundant in neurons, where it helps regulate the formation of actin filaments.

In the current study, the researchers worked with the laboratory fruit fly to understand how a defective TIAM1 gene, called sif in fruit flies, could lead to epilepsy. “We found that sif mutant flies have seizures,” Lu said. “These flies make defective actin filaments that are shorter than those in normal flies and accumulate in clusters inside neurons. We observe this in fruit fly and human cells. Interestingly, not all types of neurons are affected. The excitatory neurons that produce the chemical messenger glutamate, called glutaminergic neurons, are the main type of neurons altered by the defective gene.”

The team expected that neurons with defects in actin filaments would show problems in their structure or in their connections that could explain seizures. “We found that neurons in flies lacking the sif gene were not structurally different from normal neurons, but they were overly active,” Lu said. “If their structure and connections seemed normal, what was causing the seizures?”

The literature shows that actin also is associated with mitochondrial division. Mitochondria are structures inside cells that provide the energy needed to carry on cellular functions. “We observed that neurons with defects in actin filaments also had more mitochondria, which were smaller than those in normal cells,” Lu said. “These mitochondria are more active and produce higher levels of reactive oxygen species (ROS).”

ROS are highly reactive forms of oxygen molecules that are naturally produced in our bodies. They are useful and necessary in small amounts, but when produced excessively ROS can damage cells. “Too much ROS can lead to increased glutamatergic transmission and then seizures,” Lu said. “The findings point to a novel actin-mitochondria-glutamate (AMG) pathway involving epilepsy-associated actin regulatory genes.”
Importantly, inhibiting components of this pathway can reduce seizures in the fruit fly.  Preventing mitochondrial fragmentation with the drug Mdivi-1 significantly suppressed seizures in sif mutants and treating sif mutants with an anti-ROS drug called NACA reduced seizures and suppressed increased glutamatergic transmission.

In addition, the team shows that combining two defective genes of the AMG pathway can cause seizures. “People with epilepsy of unknown origin show more defective AMG genes when compared to people without the condition,” Bellen said. “Modeling of these gene combinations observed in patients in the fruit fly confirmed that many of them increase the susceptibility to seizures. Altogether, the findings show a novel biological pathway that can lead to seizures when disrupted and can be used to identify pairs of seizure-associated genes for improved diagnosis. The novel mechanism also suggests potential therapeutic targets.”

Other contributors to this work include Mengqi Ma, Shabab B. Hannan, Mingxi Deng, Hu Chen, Zhijian Yu, Lindsey D. Goodman, Haein Kim, Yun Zhao, Sandeep Kumar Dubey, Wen-Wen Lin, Xueyang Pan, Debdeep Dutta, Vishnu Anand Cuddapah, Jill A. Rosenfeld, Xi Luo, Zhandong Liu and Joshua M. Shulman. The authors are affiliated with one or more of the following institutions: Baylor College of Medicine, Duncan NRI and Baylor Genetics Laboratories.

This work was supported in part by NIH grant U01 AG072439, the Huffington Foundation, the Neurogenetics Chair of the NRI, an NRI Zoghbi Scholar Award, the TNPO2 Foundation and the BrightFocus Foundation. Further support was provided by the Effie Marie Cain Chair in Alzheimer’s Disease Research at Baylor College of Medicine, the Huffington Foundation Chair in Parkinson’s Disease Research at Texas Children’s Hospital, the German Research Foundation, a Walter Benjamin Fellowship and the Cancer Prevention and Research Institute of Texas award CPRIT RP240131.