By Vignesh Subramanian ’24
Epilepsy is a chronic central nervous system disorder characterized by uncontrolled electrical activity in the brain that triggers recurrent, spontaneous seizures. The majority of new epilepsy cases involve focal epilepsy, in which seizure activity originates in a particular “focus” or region of abnormal brain tissue and may emanate outwards, typically affecting one hemisphere of the brain. Focal epilepsy is largely unexplained by genetic causes and often requires surgical resection of medication-resistant foci. However, given the inherent risks of excising brain tissue, researchers have long sought to fully understand the molecular basis of errant signaling in order to develop alternative therapies. Researchers led by Dr. Peijun Li of Brown University thus compared healthy and seizure-producing tissue to identify contributing proteinic and neurotransmitter-based factors.
The researchers first collected samples of resected tissue with epileptogenic foci from patients with focal cortical dysplasia (FCD) and tuberous sclerosis complex (TSC), two disorders involving neuronal disorganization that commonly cause focal epilepsy. After being compared to control tissue, the samples were submitted to microarray analysis to screen their transcriptomes – the full collection of RNA molecules transcribed from genes to produce proteins – for causative mutations. Upon noticing a decrease in RNA expression for the production of a protein known as Circadian Locomotor Output Cycles Kaput (CLOCK) in the epileptogenic tissue, researchers created two mouse models – each with the gene encoding CLOCK knocked out, but in different types of signaling neurons – to assess the relationship between absence of CLOCK and susceptibility to seizures.
The finding that CLOCK RNA expression was significantly lower (by 30% in a majority of FCD & TSC cases) in epileptogenic tissue during the initial transcriptome analysis highlighted a correlation between a lacking presence of CLOCK protein, which regulates sleep and circadian cycles, and seizure occurrence. Upon creation of the knockout lines, the researchers found that among excitatory neurons (which release neurotransmitters that promote the firing of action potentials), those with decreased CLOCK levels demonstrated deficient reception of inhibitory postsynaptic currents, resulting in increased overall excitability.
These findings suggest that despite generally being only as disposed to conduct epileptiform discharges (seizure-associated waveforms of brain activity) as other neurons, excitatory neurons lacking CLOCK demonstrated lower thresholds for seizure onset. Furthermore, the seizures observed primarily occurred during sleep – a typical symptom of focal epilepsy. Such veritable proof that the correlation between CLOCK presence and seizure susceptibility is not coincidental may ultimately yield a new molecular target for future molecular therapies, presenting an opportunity for further research.
 P. Li, et al., Loss of CLOCK Results in Dysfunction of Brain Circuits Underlying Focal Epilepsy. Neuron 96, 387-401 (2017). doi: 10.1016/j.neuron.2017.09.044.
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