Dravet syndrome is a rare form of genetic epilepsy that affects about 1:16,000 to 1:21,000 children. The disease strikes otherwise healthy infants typically in the first year of life. The catastrophic syndrome is characterized by dozens of daily, prolonged seizures that trigger an array of profound physiological and social complications. These issues include cognitive dysfunction and delays, severely impaired movement, problems associated with language, speech, growth, nutrition, and sleep, and disruptions in the autonomic nervous system.  Most of the children affected are unresponsive to antiepileptic drugs (AEDs) and cannot be surgically treated. Because the condition is so rare, Dravet syndrome has not qualified for large-scale clinical trials.  Notwithstanding the lack of significant medical study, a team of researchers at the University of California, San Francisco led by Scott C. Baraban may have discovered a potential treatment.  While blindly screening anti-seizure drugs against mutant zebrafish that mimic Dravet syndrome in humans, the efficacy of an antihistamine known as Clemizole inspired further testing of similar drugs.  The finding prompted a small but successful clinical trial of a similar drug that offered promising results and a glimmer of hope for families affected by the disorder.

Nearly 85% of patients diagnosed with Dravet syndrome have a mutation in SCN1A, a gene that encodes for the NaV1.1 protein which comprises the alpha subunit of the sodium channels expressed in neurons and muscles.  Sodium channels play a vital role in generating and transmitting electrical signals across various plasma membranes. In the brain these channels control the flow of sodium ions, which in turn cause the release of neurotransmitters. Communication between neurons is dependent on these neurotransmitters that are released from one neuron and taken up by another.  When mutated, the channels allow too many positively charged ions into the neuron resulting in excessive firing throughout the brain, which ultimately cause seizures.

Scientists came across a strain of zebrafish with a mutation in a SCN1A homologue, scn1Lab, that mimics seizure activity and convulsive behavioral movements found in humans.  Upon this discovery, researchers monitored the seizure activity in scn1Lab mutant zebrafish larvae.  More than 2,300 drug compounds that are already approved by the Food and Drug Administration were screened for repression of seizures, but in order to avoid bias about previously recognized AEDs, their identities remained unknown until after the results were obtained. Drugs that were able to reduce swim velocity above a certain threshold and were nontoxic were then analysed using the electrophysiological assay. This led to the discovery of Clemizole, a histamine receptor (H1) antagonist and a potent inhibitor of both behavioral seizures and seizures detected by an electroencephalogram (EEG), which detects electrical activity in the brain. This was surprising, as seizures typically are suppressed by stimulating histamine H1 receptors, so antihistamines such as Clemizole are not used as AEDs.

To further test Clemizole’s effect on seizure behavior in the zebrafish, scn1 larvae were treated with the compound at various concentrations. The seizures, which were indicated by high swimming velocity, were monitored using automated locomotion tracking software. A reduction in mean swimming velocity of greater than 40% indicated sufficient suppression of convulsions. They found that Clemizole could suppress seizures in the fish, which prompted an investigation into its mechanism of action.  

Because Clemizole is an antihistamine, researchers hypothesized that the compound exerts anti-epileptic seizure activity by binding to serotonin receptors and interfering with the serotonin-signaling pathway. The study proposes that modulators of serotonin signaling  (5-HT agonists) represent the future of treating catastrophic childhood epilepsy by elevating extracellular serotonin levels to inhibit seizures.

To test the proposition, they screened 49 known antihistamines for suppression of seizure activity in the mutant zebrafish. However, none of the antihistamine compounds were successful in suppressing seizures.  The researchers then discovered that while Clemizole has a high antagonist binding affinity for the H1 receptor, it also has a high binding affinity as an agonist for two 5-HT receptors HTR2A and HTR2B, ion channel modulators, and G-protein-coupled receptors, all of which have been associated with the potential to cause seizures, and therefore have been targets for methods of seizure regulation.

Researchers obtained commercially available compound libraries that spanned the above-mentioned targets; in another blind screening, these compounds were tested on scn1Lab mutant zebrafish to determine if they exhibited seizure suppressing activity by reducing swim velocity by at least 40%. Mutant zebrafish were then treated with compounds capable of inhibiting locomotion in the phenotypic-based screening to observe their effect on seizures with an EEG. The brain activity in the zebrafish was measured with a microelectrode that was inserted into the brain of immobilized zebrafish. They concluded that lorcaserin and trazodone, two FDA approved 5-HT agonists, were successful in reducing seizure frequency.  Additionally, the two 5-HT receptor subunits, HTR2A and HTR2B, were found to be evolutionarily conserved between zebrafish and humans, suggesting that trazodone and lorcaserin mimic Clemizole’s mechanism of action in suppressing seizures in zebrafish.  Further screening of trazodone and lorcaserin in a locomotion analysis showed that trazodone and lorcaserin were 80% effective and 50% effective, respectively, in reducing velocity in a concentration-dependent manner.

The results of zebrafish testing revealed that seizures associated with SCN1A mutations can be suppressed with modulation of 5-HT signaling, as increased 5-HT levels in the brain can inhibit seizures. Clemizole is no longer manufactured or available to treat patients, which is why the researchers turned to other available compounds. Trazodone can act as a 5-HT antagonist depending on the concentration, which means it can block serotonin signaling rather than stimulating it and therefore cause increased seizure activity. Because of this, lorcaserin (Belviq) was evaluated as a potential treatment for patients with Dravet syndrome.

Using an FDA compound with a known safety profile, such as Belviq, expedited clinical use.  Five children with Dravet syndrome who demonstrated resistance to at least five AEDs and had a mutation in SNC1A were treated with Belviq under a compassionate use program and monitored for reduced seizure activity.  All five patients in the trial experienced a reduction in their total number of seizures.  One patient was seizure-free for three weeks, another for 2 weeks, and a third patient had 1.2 seizure-free days each week.  Tonic-clonic seizures, which are characterized by a stiffening of muscles and loss of consciousness followed by rapid and rhythmic jerking in the arms, legs, and head, were significantly reduced in 3 of the 5 patients. The most common side effect was a decreased appetite, as Belviq is drug typically used for weight management.  

Dravet syndrome is a devastating form of epilepsy with extremely limited treatment options. Often, those with the condition have a markedly reduced quality of life, require long term care, and have a high risk of sudden unexplained death with epilepsy (SUDEP) and require long-term care.  The results of this study are promising in that Belviq reduced seizure frequency and severity.  Further investigation into the efficacy of treating patients by compounds capable of modulating of serotonergic signalling hopefully will lead to an effective clinical treatment for the patients and families affected by Dravet syndrome.


REFERENCES

  1. Aliesha Griffin, Kyla R. Hamling, Kelly Knupp, Soon Gweon Hong, Luke P. Lee, Scott C. Baraban; Clemizole and modulators of serotonin signalling suppress seizures in Dravet syndrome. Brain 2017; 140 (3): 669-683. doi: 10.1093/brain/aww342
  2. Bagdy, G., Kecskemeti, V., Riba, P. and Jakus, R. (2007), Serotonin and epilepsy. Journal of Neurochemistry, 100: 857–873. doi:10.1111/j.1471-4159.2006.04277.x
  3. Kurtzman, Laura. (2013 September 03). Potential Drug Discovered for Severe Form of Epilepsy. Retrieved March 05, 2017, from https://www.ucsf.edu/news/2013/08/108521/potential-epilepsy-drug-discovered-using-zebrafish
  4. Sirven, Joseph I. (2014 April). How do Ion Channels Cause Epilepsy?. Retrieved March 22, 2017, from http://www.epilepsy.com/article/2014/4/how-do-ion-channels-cause-epilepsy

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