Fireworks in Brain
Credit: Hiroshi Watanabe/Getty Images

Researchers at University College London (UCL) have developed a new gene therapy approach for epilepsy and potentially other neurological and psychiatric diseases that works by reducing the excitability of overactive brain cells. The new closed-loop gene therapy approach enables the on-demand expression of a gene that tamps down the activity of the overactive neurons, but doesn’t affect those that are acting normally. Tests in mice confirmed that that the gene therapy reduced the activity of neurons responsible for triggering spontaneous seizures.

The researchers suggest that their approach could feasibly be used to damp down the overexcitability of specific brain cells in relevant neurological and psychiatric disorders—including epilepsy, but also potentially schizophrenia and Parkinson’s disease—where it is only a subpopulation of neurons that demonstrates this unwanted overreactivity. The self-regulating gene therapy also only switches on when the neurons are exhibiting pathological overactivity.

“We invented a gene therapy that switches on only in overactive cells, and switches itself off if activity returns to normal,” said Gabriele Lignani, PhD, associate professor and group leader at UCL Queen Square Institute of Neurology. “We harnessed the ability of certain DNA sequences to control gene expression in response to metabolic signals. By re-directing this activity-sensing mechanism to drive the production of molecules that stop brain cells from firing, we showed that epileptic seizures can be suppressed.”

Lignani is co-corresponding author of the team’s published paper in Science, in which they concluded, “Activity-dependent gene therapy is a promising on-demand cell-autonomous treatment for brain circuit disorders.”

Many neurodevelopmental and neuropsychiatric circuit disorders are characterized by intermittent episodes of pathological activity, the authors explained. Drug treatments aim to reduce the frequency and/or severity of such episodes, but success is often limited. “For example, 30% of people with epilepsy are refractory to pharmacological treatment, even with drugs that act in a use-dependent manner on their molecular targets,” the researchers noted.

Genetic therapies represent a promising approach to treating such disorders, because they can modulate neuronal excitability in a region-specific and cell type–specific manner, but current gene therapy approaches tend to indiscriminately target all the neurons in a given brain region rather than just those specific problematic circuits that are responsible for triggering the episode.

“ … a limiting factor is that they do not discriminate between neurons involved in circuit pathologies and ‘healthy’ surrounding or intermingled neurons,” the investigators further noted. “There is a need to develop methods that select and treat only those neurons involved in the generation of crises.” So, in epilepsy, for example, only subpopulations of neurons are involved in seizures. “Targeting these neurons specifically would be an important step toward a rational treatment with minimal side effects.”

The new approach developed by the UCL team is a gene therapy strategy that self-selects pathologically overreactive neurons and downregulates their excitability in a closed-loop feedback system. To create the gene therapy, the team coupled the promoter for the Fos gene, which is known to switch on in response to stimulation, to control the Kcna1 gene which encodes a potassium channel, in an adeno-associated viral vector.  “We used an immediate early gene promoter to drive the expression of Kv1.1 potassium channels specifically in hyperactive neurons …” the investigators commented.

The team tested this gene therapy approach in mice, and in miniature brain-like structures—human cortical assembloids (hCAs)—created using skin-derived human stem cells. They found that during periods of intense neuronal activity, Fos promoted the expression of Kcna1, but only in hyperactive neurons and only for as long as they exhibited abnormal activity. The treatment proved to be highly effective in calming neuronal excitability following an induced seizure, and also in suppressing spontaneous seizures, without having any negative effects on cognition.

The new form of gene therapy was more effective than either previous gene therapies or antiseizure drugs tested in the same model, with around an 80% reduction in spontaneous seizures in epileptic mice. “In a chronic model of epilepsy, we achieved a greater decrease in seizure frequency than previously reported for gene therapy with constitutive promoters or with widely used antiseizure drugs, and without deleterious effects on normal behaviors,” they wrote. “This approach is specific for neurons that participate in pathological network activity but is also time-limited in that transgene expression persists only for as long as neurons are hyperactive.”

The researchers suggested that this strategy for gene therapy could feasibly be used for other disorders in which only some brain cells are overactive, potentially including schizophrenia, Parkinson’s disease, migraine and cluster headache, and obsessive-compulsive disorder. “A cell-autonomous self-regulated tool that normalizes network dynamics has potential clinical applications beyond epilepsy,” they stated. “Several other neuropsychiatric diseases are characterized by circuit hyperactivity.”

Co-corresponding author Dimitri Kullmann PhD, principal investigator at UCL Queen Square Institute of Neurology, added, “Our findings indicate that the activity of brain cells can be normalized, and that this approach can be used to treat important neuropsychiatric diseases that do not always respond to medication. The gene therapy is self-regulated and can therefore be used without deciding a priori which brain cells need to be targeted. Importantly, it could in principle, be extended to many other disorders such as Parkinson’s disease, schizophrenia and pain disorders, where some brain circuits are overactive.”

The authors further pointed out that it may be relatively straightforward to progress their technology towards clinical trials. “Given that the components of activity-dependent gene therapy are normal mammalian genetic elements packaged in a well-tolerated viral vector that is already in the clinic, there is a relatively straightforward path to first-in-human studies. The translational potential is underlined by the preliminary evidence of effectiveness in hCAs.”

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