Researchers probing the brain tissue of people with severe forms of epilepsy make a surprising breakthrough: a protein involved in circadian rhythms, called CLOCK, may play a role.
There are drugs available to treat the condition, but side effects can be significant and not all cases of epilepsy respond well.
Although certain genetic mutations have been identified that are responsible for inherited forms of epilepsy, these only account for a minority of cases. In most cases, the exact causes are unknown.
Most individuals with epilepsy have focal epilepsy, meaning that the seizures arise in a specific part of the brain where the tissue is abnormal. To date, the reasons why these small regions of tissue become epileptogenic are not understood.
In severe cases of focal epilepsy that do not respond to treatment, the brain area responsible for the seizures may be surgically removed. And recently, a team examined these excised pieces of tissue, looking for any differences that might shed more light on the condition.
The corresponding author for the study was Dr. Judy Liu, an assistant professor of neurology at Brown University in Providence, RI. The results are published this week in the journal Neuron.
Dr. Liu studied brain tissue from eight patients with either focal cortical dysplasia or tuberous sclerosis complex.
Focal cortical dysplasia is one of the most common causes of epilepsy that does not respond to treatment. Tuberous sclerosis is a rare genetic condition that causes non-cancerous tumors to develop. If these tumors develop in the brain, it can cause epilepsy.
When brain tissue is removed to treat epilepsy, healthy tissue is sometimes removed to get access to the area of interest. For this study, these healthy sections were used as a control.
Dr. Liu and her colleagues analyzed the tissue’s transcriptome, or a survey of the messenger RNA (mRNA) in any given population of cells. This tells scientists which portions of the DNA are being expressed and producing proteins.
The researchers were looking for any differences between healthy and epileptogenic tissues. They had no specific target molecule in mind, but one clear winner emerged: there was a decrease in the expression of mRNA coding for a protein called circadian locomotor output cycles kaput (CLOCK).
“This was an unbiased screen. We were hoping to find targets we can use for therapy, and we were also trying to figure out molecular differences in different types of epilepsy. But the effect on CLOCK was so consistent we didn’t manage to stratify different causes.”
Dr. Judy Liu
CLOCK is a key player in the regulation of our circadian rhythms. Mice with mutated versions of the gene are unable to maintain normal daily rhythms of sleep and wakefulness. In the unhealthy brain tissue from participants with epilepsy, CLOCK was reduced in both excitatory and inhibitory neurons.
Because CLOCK is important for regulating a range of genes, other CLOCK-associated proteins were also absent or reduced in the brain tissue.
To further investigate how these findings might influence the brain, the researchers created mouse models. One model had CLOCK “knocked out” of excitatory neurons, and the other had CLOCK removed from inhibitory neurons.
The mice without CLOCK in their excitatory neurons showed symptoms of epilepsy similar to those in the human patients, including an increased susceptibility to seizures – especially when waking up. This connection with sleep makes sense, given CLOCK’s role in the sleep/wake cycle.
Using the mouse model, they also found that excitatory neurons that lacked CLOCK were not, in fact, more excitable. They received less inhibitory inputs from surrounding cells, in effect unleashing them and potentially giving them a lower threshold for the onset of seizures.
Exactly how CLOCK reduces inhibitory signals is not known. Dr. Liu explains, “That’s a great mystery at this point. It could be because the inhibitory synapses are affected, and there is some evidence that’s the case, but also the activity of a certain subset of inhibitory neurons could be altered.”
The researchers hope that these initial findings will be a basis for further investigation. Now that CLOCK has been identified as a molecule of interest, it will help to focus future investigations into the molecular mechanisms behind focal epilepsy.
Epilepsy is widespread and difficult to treat, with breakthroughs few and far between. Dr. Liu and colleagues plan to continue this work.
“My idea of what epilepsy therapy should look like is we do the same mapping to figure out the seizure focus and then we treat the focus itself,” she says. “We have neurosurgeons who can access the tissue, we have drugs. We need delivery systems that can target the drugs.”
There is still much to learn about CLOCK and its role in epilepsy, but now that scientists know the molecule is involved, research can begin chipping away at the details.