Status Epilepticus, TLE, and GABA-A Receptor Gene Therapy

Temporal lobe epilepsy (TLE) refers to a condition where recurrent seizures arise in the temporal lobe of the brain. This condition is seen in humans and animals. Often, TLE arises following a neural insult such as head trauma or tumor, but can also be triggered by infection. These febrile seizures are often seen in children under the age of five, and subsequent scans can show atrophy of temporal lobe structures such as the hippocampus. The hippocampus is highly interconnected with other temporal lobe structures, so a seizure that originates from or propagates through the hippocampus is likely to result in widespread seizure activity.

While febrile convulsions of short duration (on the order of a few minutes) are somewhat normal in infants, convulsions lasting more than one hour indicate a high risk for developing TLE in the future. TLE resulting in status epilepticus (SE) is of particular concern, as SE is a life-threatening condition where the brain enters a state of persistent seizure, either from one long episode or a series of recurring episodes. Medication may not be effective at controlling SE, and complications are almost inevitable. If SE is the result of TLE, resection of the entire temporal lobe can be successful at eliminating seizure activity. While the brain is very "plastic" in younger children who can recover from this sort of surgery and go on to live almost completely normal lives, such drastic surgery is not desirable as age increases because the brain's ability to compensate for the surgery is diminished.

With this in mind, I turn your attention to a recent publication.

GABA is the brain's major inhibitory neurotransmitter. Receptors for GABA are comprised of multiple protein subunits, the products of multiple genes. Studies in both humans with TLE and in rodent models of TLE find a reduction in the expression of alpha-1 GABA-A receptor subunits in the hippocampal dentate gyrus and an increase in alpha-4 subunit expression . Large functional changes accompanied the shift in receptor subunit expression in the rodent models, prompting one group to attempt to answer the question as to whether alpha-1 GABA-A receptor subunits play a causal role in TLE. They attempted to do this by introducing a viral vector gene transfer into the hippocampal dentate gyrus in order to increase alpha-1 subunit expression, and to see if this increase could be effective at reducing spontaneous seizure activity in rats treated wtih pilocarpine, which produces SE.

Methods
An adenovirus vector was constructed that placed the alpha-1 receptor gene under control of the alpha-4 promoter. The viral vector or control vector was injected into the dentate gyrus, and two weeks later rats were exposed to pilocarpine to induce SE. Seizures were stopped by diazepam administration. Rats were killed 1, 2, or 4 weeks after SE induction.

Results
Adenovirus vector increased dentate gyrus alpha-1 subunit expression to approximately 200% of control (black bar) two weeks after the initiation of SE.

There were no differences in the latency to SE onset, the amount of pilocarpine necessary to produce SE, or the amount of diazepam required to stop SE (Figure 2)

Fewer adenovirus-treated rats developed post-SE spontaneous seizure activity (39%, vs 100% of control rats).

Discussion
So what does all this mean? Basically, adenovirus therapy to increase levels of alpha-1 GABA-A receptor subunit expression was effective at reducing the incidence of spontaneous seizures after a major SE event, suggesting that the alpha-1 subunit might play a role in the development of epilepsy following SE. Since SE is a risk factor for the development of epilepsy, we can see the potential therapeutic value of targeting the alpha-1 subunit either with more specific pharmacological agents or potentially with gene therapy, or combination therapy.

I was impressed that only a transient two-fold increase in alpha-1 subunit expression was required to produce this effect. Genetic knock-ins can sometimes increase expression 5- to 20-fold, and doing so permanently could have significant negative consequences.

There are a few caveats to this study; delivery of an adenovirus vector to dentate gyrus requires intracranial surgery. In humans this is obviously problematic and novel delivery methods for gene therapy would be desirable. Also, increasing alpha-1 receptor subunit expression after seizure was associated with excessive sedation, weight loss, and in some cases supplemental feeding and watering were required. Additionally, the transient nature of the alpha-1 overexpression might be either beneficial or deletorious: if long-term expression is required to control epilepsy then obviously this therapy needs significantly more refinement before moving to human trials. However, if transient therapy is effective at reducing epilepsy then the patient would benefit from reduction in the aforementioned side effects, especially if the end result is sparing a child from traumatic brain surgery to remove an epileptic loci.

While there are some clear problems with this sort of gene therapy, I think it represents an interesting approach to a significant problem. I'd like to see the treatment extended to include more temporal lobe structures than just the dentate, to determine if a more diffuse, yet widespread treatment were more efficacious. I would also like to see future studies explore the possibility of injecting alpha-1 transfected fibroblasts into the brain; autonomous cells producing known quantities of the receptor might be useful for titrating the minimum effective dose for an individual, thus minimizing side effects while potentially extending the efficacy of treatment for the long haul. Of course, getting those receptor subunits transported from the fibroblasts to other cells is an entirely different issue.... ain't neuroscience fun?

Reference
The Journal of Neuroscience, November 1, 2006, 26(44):11342-11346