Regulating normal brain activity is a delicate balance. It involves the convergence of signalling molecules, transporters that carry molecules into or out of cells, and several specialised cell types. Together, these components facilitate the exchange of substances and messages within different areas of the brain, creating essential communication pathways. Disruption of these highly organised networks is associated with several developmental brain disorders, including genetic epilepsy, autism, and intellectual disability as well as neurodevelopmental delay.

One of the most prominently expressed signalling molecules within the brain is called Gamma-aminobutyric acid – or GABA. GABA exerts its functions via a specific transporter protein called GABA transporter 1 – or GAT-1. It is accepted that this relationship is crucial for balance of excitation and inhibition in the brain as GABA is the most important inhibitory neurotransmitter, although the precise mechanisms involved have yet to be established. GABA also affects neural development, and any alterations in GABA or the genes involved in GABA signalling in brain cells may impair brain development, leading to development brain disorders such as epilepsy, autism and intellectual disability. The mechanisms underpinning atypical early brain development in conditions with a genetic component are not fully understood.  

Dr Katty Kang and colleagues at Vanderbilt University Medical Centre in Tennessee have endeavoured to overcome this knowledge gap and investigate how genetic changes affect the flow of proteins into or out of cells, the function of affected proteins and the possibility that unstable protein formations contribute to irregularities in molecular messaging, leading to the symptoms observed in genetic epilepsy and other neurological disorders.

The team applied sophisticated laboratory techniques and machine learning tools to test their theory using various cell and animal models, including models that use brain cells, such as neurons or astrocytes, derived from mice carrying the same genetic variations identified in human patients. The team also used neurons and astrocytes created from patient-derived stem cells. They set out to determine how the genetic variations associated with brain dysfunction alter transporter proteins and signalling molecules, and what the subsequent impact on brain function might be, and how symptoms might be alleviated.

They discovered that many of the genetically modulated brain disorders share common mechanisms, and that the associated effects were observed in cell types such as neurons and astrocytes from all brain regions. Furthermore, their research revealed new knowledge regarding the association between specific genetic modifications and defective brain development. Interestingly, all tested disorders showed a complete or partial loss of GABA uptake within brain cells such as neurons and astrocytes.

Lower detectable protein levels at the cell surface indicated that non-functional transporter proteins with an incorrect configuration cannot reach the cell membrane, meaning there are fewer functional transporters to transfer substances into and out of the cell. This can result in increased levels of GABA in the brain, leading to a molecular imbalance of neurotransmitters, neuronal excitation, and seizures.

Sometimes, genetic variation or environmental factors can result in misshapen proteins, and this is a known cause of neurodevelopmental disorders. When proteins are folded incorrectly, their function can be impaired, and they are more likely to be retained in specialised cell components such as the endoplasmic reticulum for eventual destruction. The researchers confirmed that this internal quality control process was likely consistent across all cell types, regardless of the condition in question. However, retaining the misfolded proteins in the cell can negatively impact non-mutant proteins, resulting in cellular stress. Whether this impacts disease severity is still unclear. However, modulating this process and increasing the transport of proteins across cell membranes could be a novel pharmaceutical target, not just for genetic epilepsy, but for other brain disorders associated with genetic mutations too.

Next, the team assessed the feasibility of using existing medications, already approved for the treatment of other conditions, to mitigate the symptoms of a range of genetically modulated brain disorders. This could help shorten the time to create new treatments, as the average time to develop a completely new brain drug is 12-16 years. They used cell and mouse models to evaluate a compound used in the management of inherited metabolic disorders to see whether mutated protein transporters could be functionally restored.

They established that both GABA uptake and the amount of functional protein present in the cell were increased in both neurons and astrocytes after administering the drug, suggesting that the function of transporter proteins could indeed be restored from the very earliest developmental stages. Furthermore, the severity and duration of seizures in mice were reduced, since the compound targeted the root cause, modifying the disease rather than simply masking the symptoms. Interestingly, an absence in the improvement of intelligence levels in some mice bearing mutant GAT-1 despite seizure control suggests that the protein transporter deficiencies are directly linked with intellectual disability as opposed to this being a consequence of seizure-induced brain alterations.

The mechanisms through which misfolded proteins are processed could be a promising therapeutic target. Indeed, the fact that genetic epilepsy and neurodegenerative diseases share mechanistic similarities in their origin suggests that medicines currently used to treat dementia-type diseases may be repurposed and used for the treatment of genetic epilepsy. Increasing the levels of normal transporter proteins whilst simultaneously reducing the levels of the misfolded versions could also be a potentially attractive therapeutic method. In addition, improving the predictive abilities of computational technologies may help in reducing labour-intensive experiments when investigating the effect of specific genetic modifications on protein stability and function. 

As is usual during the initial stages of groundbreaking research, questions remain unanswered. However, for the first time, Dr. Katty Kang’s team presented a potentially viable treatment intervention for a series of neurodevelopmental disorders with a common genetic anomaly. Such treatments may not only reduce physical symptoms but could also improve other brain functions such as cognition. This is critical as it will help improve disease outcomes and patient quality of life.

The team has collaborated with multiple groups of clinicians worldwide and uncovered that SLC6A1 variants are not only associated with genetic epilepsy. The variants in SLC6A1 can also give rise to a wide spectrum of brain disorders.

It is possible that other serious brain disorders such as schizophrenia may exhibit similar genetic mechanisms, which certainly warrants further investigation. However, confirmation of treatment safety is paramount, and an exploration of combination therapies to optimise outcomes is required.

Further insights are critical to the development of anti-seizure medications, and the unwavering dedication of Dr Kang and colleagues has laid strong foundations for further investigations into the complex web of disordered brain development, adding central pieces to this as-yet unsolved puzzle.