UC San Diego Researchers Uncover Structural Basis of Congenital Myasthenic Syndromes, Paving Way for Precision Therapies
Key Insights
Cryo-EM reveals 12 high-resolution structures of disease-causing acetylcholine receptor (search) variants, showing for the first time how CMS mutations disrupt nerve-muscle signaling at near-atomic resolution.
Fast-channel CMS mutations prevent receptor opening; researchers discovered a novel drug-binding pocket where positive allosteric modulators can partially restore function in a mutation-specific manner.
The antidepressant reboxetine, already approved in several countries, selectively suppresses abnormal receptor activity in slow-channel CMS, offering a potential repurposing opportunity.
In a landmark study published in Nature, researchers at the University of California San Diego have revealed the structural mechanisms underlying congenital myasthenic syndromes (search) (CMS)—a family of rare inherited neuromuscular disorders that weaken communication between nerves and muscles. Using cryo-electron microscopy (cryo-EM), the team determined 12 high-resolution structures of disease-causing human acetylcholine receptor (search) variants, providing the first near-atomic view of how genetic mutations disrupt the molecular machinery essential for muscle contraction.
The findings offer a roadmap for developing precision medicines tailored to the specific genetic mutation carried by each patient, and identify a potential new therapeutic use for reboxetine, an antidepressant already approved in several countries.
Two Distinct Disease Mechanisms Emerge at the Receptor Level
CMS affects children from birth or early childhood and can cause severe muscle weakness, difficulty walking, impaired breathing, and in the most severe cases, paralysis or death. Although clinicians have long recognized that different mutations disrupt muscle signaling in different ways, the precise molecular consequences have remained elusive.
"We've known for decades which mutations cause these diseases, but not exactly how they damage the receptor or why certain drugs help some patients but not others," said senior author Ryan Hibbs, professor and chair of the Department of Neurobiology at UC San Diego's School of Biological Sciences. "By visualizing these receptors at near-atomic resolution, we can now explain how the mutations disrupt their function and begin designing therapies that target the underlying molecular defect."
The team, which combined cryo-EM with electrophysiology and chemical biology approaches, found that the two major forms of CMS arise through fundamentally different mechanisms. In "fast-channel" CMS, mutations prevent the receptor from opening efficiently when the neurotransmitter acetylcholine binds. In "slow-channel" CMS, mutations cause receptors to remain open too long, progressively damaging the neuromuscular junction.
A New Drug-Binding Pocket for Fast-Channel CMS
For fast-channel CMS, the researchers discovered a previously unknown drug-binding pocket that can partially restore receptor function using positive allosteric modulators—compounds that enhance receptor activity without directly activating it. Critically, different modulators worked better for different patient mutations.
"Our results show that there probably won't be a single drug that works for every patient," Hibbs said. "Instead, different mutations respond differently, opening the door to precision medicine approaches for these disorders."
Reboxetine Emerges as a Candidate for Slow-Channel CMS
The study also investigated slow-channel CMS, revealing exactly how two current treatments—quinidine and fluoxetine—block defective receptors at the structural level. Building on this understanding, the team examined reboxetine, an antidepressant already approved in several countries, and found that it selectively suppresses the abnormal receptor activity responsible for slow-channel disease. Because reboxetine has already undergone extensive safety testing for depression, repurposing it for CMS could potentially accelerate the path toward new treatments.
A Framework for Future Precision Medicine
Beyond identifying specific therapeutic opportunities, the work establishes general principles explaining how dozens of disease-causing mutations affect receptor function.
"Rather than studying one mutation at a time, we've uncovered the common mechanisms that explain two entire classes of congenital myasthenic syndromes (search)," said first author Huanhuan Li, a postdoctoral researcher in Hibbs' laboratory. "That gives us a framework for understanding newly discovered patient mutations and for designing better therapies in the future."
The study highlights the growing role of structural biology in precision medicine. Much of the structural work was performed in UC San Diego's Goeddel Family Technology Sandbox, an advanced facility providing researchers access to next-generation imaging technologies including cryo-EM. Collaborators at Mayo Clinic contributed expertise in receptor physiology, while researchers at UC San Francisco (search) synthesized the experimental compounds.
The research was supported by the National Institutes of Health (grants NS031744, NS120496, and NS130831), the Myasthenia Gravis Foundation of America, and the American Heart Association (25POST1378255).