[ARTICLE] 2020 Epilepsy Benchmarks Area III: Improved Treatment Options for Controlling Seizures and Epilepsy-Related Conditions Without Side Effects

Abstract

The goals of Epilepsy Benchmark Area III involve identifying areas that are ripe for progress in terms of controlling seizures and patient symptoms in light of the most recent advances in both basic and clinical research. These goals were developed with an emphasis on potential new therapeutic strategies that will reduce seizure burden and improve quality of life for patients with epilepsy. In particular, we continue to support the proposition that a better understanding of how seizures are initiated, propagated, and terminated in different forms of epilepsy is central to enabling new approaches to treatment, including pharmacological as well as surgical and device-oriented approaches. The stubbornly high rate of treatment-resistant epilepsy—one-third of patients—emphasizes the urgent need for new therapeutic strategies, including pharmacological, procedural, device linked, and genetic. The development of new approaches can be advanced by better animal models of seizure initiation that represent salient features of human epilepsy, as well as humanized models such as induced pluripotent stem cells and organoids. The rapid advances in genetic understanding of a subset of epilepsies provide a path to new and direct patient-relevant cellular and animal models, which could catalyze conceptualization of new treatments that may be broadly applicable across multiple forms of epilepsies beyond those arising from variation in a single gene. Remarkable advances in machine learning algorithms and miniaturization of devices and increases in computational power together provide an enhanced opportunity to detect and mitigate seizures in real time via devices that interrupt electrical activity directly or administer effective pharmaceuticals. Each of these potential areas for advance will be discussed in turn.

Introductory Vignette by Amanda Jaksha: Clinical Trials—A Parent’s Perspective

CDKL5 deficiency disorder (CDD) is a rare developmental and epileptic encephalopathy that typically presents with refractory epilepsy, often epileptic spasms without hypsarrhythmia, in the first days or months of life. In 2012, at the age of 6.5 years, my daughter was diagnosed with CDD. By then, she had endured thousands of seizures and failed most available AEDs. She narrowly escaped liver failure from drug rash with eosinophilia and systemic symptoms syndrome upon the introduction of a second-generation AED adjunct therapy. Also seasoned in failed treatments for comorbidities of dysmotility, behavior, and sleep, we become cynical about introducing any compounds.

We recently convened serious discussions about seizure control due to a decrease in her quality of life, with puberty onset increasing daily seizure activity. My daughter was a candidate for 2 clinical trials, one a blinded, placebo-controlled study and the other an open-label investigation. It was a simple choice as there was no time for a placebo. Upon completion of the observation period, she received her first dose around 6 weeks later. There was an immediate increase in seizures, and a few days later, a gradual reduction from baseline activity emerged. Anxiety and vocal stimming behaviors decreased substantially, and her gross motor skills became more fluid and sustained. With these improvements, she enjoys more functional access to community and more independence with the ability to ambulate longer distances. She also appreciates expressing more of her voice as she uses her eyes to talk via an eye-gaze communication (AAC) device. She can tell me to go away or that she feels diabolical with higher efficiency and less frustration. While her epilepsy remains refractory, to our surprise and delight, her quality of life has increased beyond anything imagined with this assumed improvement in other neuronal functions.

—Amanda Jaksha, International Foundation for CDKL5 Research

Introduction to Area III

The goals of Epilepsy Benchmark Area III involve identifying areas that are ripe for progress in terms of controlling seizures and patient symptoms in light of the most recent advances in both basic and clinical research. These goals were developed with an emphasis on potential new therapeutic strategies that will reduce seizure burden and improve quality of life for patients with epilepsy. In particular, we continue to support the proposition¹ that a better understanding of how seizures are initiated, propagated, and terminated in different forms of epilepsy is central to enabling new approaches to treatment, including pharmacological as well as surgical and device-oriented approaches. The stubbornly high rate of treatment-resistant epilepsy—one-third of patients²—emphasizes the urgent need for new therapeutic strategies, including pharmacological, procedural, device linked, and genetic. The development of new approaches can be advanced by better animal models of seizure initiation that represent salient features of human epilepsy,³ as well as humanized models such as induced pluripotent stem cells (iPSCs) and organoids.⁴ The rapid advances in genetic understanding of a subset of epilepsies^5,6 provide a path to new and direct patient-relevant cellular and animal models, which could catalyze conceptualization of new treatments that may be broadly applicable across multiple forms of epilepsies beyond those arising from variation in a single gene. Remarkable advances in machine learning algorithms and miniaturization of devices and increases in computational power together provide an enhanced opportunity to detect and mitigate seizures in real time^7,8 via devices that interrupt electrical activity directly or administer effective pharmaceuticals. Each of these potential areas for advance will be discussed in turn.

Seizure Mechanisms

There remains a pressing need to understand the initiation, propagation, and termination of seizures at the network level in different forms of epilepsy in order to devise better treatment strategies. Understanding how neuronal synchrony within a microcircuit reaches a critical threshold, subsequently allowing it to entrain larger populations of neurons, could suggest novel mechanisms that can be engaged to terminate a seizure. Although there are volumes of work on this topic over the decades,^9–11 new advances in stratification of epilepsies through pharmacogenomics¹² and genetic analysis¹³ could provide new understanding of mechanisms in models relevant for human disease. Advances in computational models have reached the point where both interictal and ictal activities can be reliably generated from the same network. The predictions of these models can now be practically verified.^14,15 Additional insights may also follow from a determination of the relative contribution of shared cellular and network mechanisms to different models. Similarly, advances in modeling the process of epileptogenesis suggest interesting new mechanisms, yet highlight the complexity of the problem.¹⁶ These mechanisms could lead to the testing of more effective therapies.

Status epilepticus remains a clinical challenge, with a subset of patients proving refractory to multiple treatments¹⁷ despite the development and approval of new antiseizure medications (ASMs). The persistent seizures associated with this condition focus attention on how little we understand about the processes of seizure initiation, maintenance, and termination. Thus, insight into mechanisms that maintain hypersynchronous firing for prolonged durations in the face of adaptive changes, exhaustion of energy stores, and mounting inflammatory cascades may allow improved treatments that can stop ongoing seizures and status epilepticus. Although a variety of processes are considered relevant to status epilepticus,^18–20 we still lack a clear assessment of the relative contributions of each one. New mechanism-based targets would improve our ability to effectively terminate status epilepticus.

An impressive amount of electrophysiological analysis of mechanisms that can lead to hypersynchronous firing has been performed either in vivo in adult animals or ex vivo in brain slices from rodents that range in age from adolescence to young adulthood. There is a growing opportunity to complement animal tissue work with acute and organotypic human brain slices obtained following surgical resection^21,22 as well as in vivo recordings from depth electrode–implanted patients.²³ However, there is a stark lack of information in some areas, for example, related to features of the neonatal brain that contribute to hypersynchronous activity, apart from changes in chloride (Cl⁻) gradients that render GABAergic transmission excitatory.^24,25 Early-life seizures are an important therapeutic target because many epileptic encephalopathies become apparent early in life. In particular, understanding the mechanisms underlying hypersynchronous firing in neonatal brain could lead to the development of therapies that are more effective for neonatal seizures as opposed to simply modifying the dosing of drugs that showed a positive signal in clinical trials in adults with epilepsy. Strategies could involve use of repurposed drugs, specific combinations of therapies, or the development of new therapies, noting, however, the substantial hurdles for bringing to market drugs for a pediatric population. Although the first uncontrolled trial of the repurposed drug bumetanide did not show efficacy,²⁶ this finding was controversial,^27,28 and the results of a subsequent blinded controlled trial of bumetanide is reported to be more promising (clinicaltrials.govNCT00830531). To this end, new genetic models of ultrarare variants in genes capable of producing seizures and hyperexcitability may provide new models of mechanisms underlying development of an epileptic focus in neonatal animals. Indeed, multiple animal models of genetic epilepsies show seizure activity at an early age, providing an opportunity to study epilepsy in the developing brain.

The role of inflammation has been increasingly recognized in a wide range of neurological diseases, including epilepsy and status epilepticus.^29–31 Neuroinflammation can impact network excitability in several ways, including activating microglia, reshaping synaptic input, and altering ion channel function. Thus, there is the potential to explore anti-inflammatory therapies for use in conjunction with conventional ASMs in the chronic therapy of epilepsies that are thought to be inflammatory in nature, such as Rasmussen encephalitis.³² In addition, the utility of some treatments for seizure categories not conventionally believed to be related to inflammatory mechanisms should be explored. This has the potential to perhaps reduce the refractory rate, or increase seizure control, for some groups of patients.

There is an emerging appreciation of autoimmune encephalitis³³ that involves antibodies against epitopes in proteins that control neuronal excitability, such as the NMDA receptor,³⁴ GAD65,³⁵ and GABA_B receptor subunits.^36,37 Patients with antibody-mediated encephalitis often exhibit nonconvulsive seizures, in addition to memory loss, psychiatric symptoms, and other features. For some epitopes, preclinical data validate the immunoglobulin G fraction as causative for seizures. Treatment with immunotherapy can be effective, but additional therapeutic strategies are needed.^36,38 The full extent of this clinical condition is just now becoming appreciated, and it remains almost certainly underdiagnosed at this point. Thus, future work should focus on earlier recognition of these presentations and early and robust diagnosis in order to achieve potentially effective treatment before the development of irreversible sequelae of neuroinflammation.

Genetic Advances

An important consequence of the many genetic advances that are transforming clinical neurology³⁹ is their ability to suggest new animal models to investigate the underlying disease mechanisms, including compensatory mechanisms that can contribute to a seizure focus.⁴⁰ Such models are relevant to genetic human epilepsies and serve as an important complement to the acquired models of focal epilepsy (eg, pharmacologically induced seizure models) that have become the mainstay for development of in vivo models of chronic recurring seizures. Animal models of single-gene defects offer an opportunity to evaluate windows for therapeutic intervention in patients who have these specific variants, with the possibility that some therapies will be more broadly applicable to multiple epilepsies. In addition, such models offer a new opportunity to study common mechanisms that underlie maladaptive plasticity and can lead to generation of a seizure focus. Novel gene expression programs may be triggered by genetic deficiencies that engage similar mechanisms, and understanding these might allow better understanding of antiepileptic drug utility.⁴⁰ In this respect, the intersection of gene expression data sets may inform key pathways that establish seizure foci regardless of the initial genetic defect driving seizures. In addition, genetic animal models can facilitate the evaluation and validation of strategies such as antisense oligonucleotides, gene replacement, and gene augmentation. The success of new genetic treatments of spinal muscular atrophy with intravenously delivered gene replacement via adeno-associated viral vector in very young infants⁴¹ has created hope for many patients that these therapies can correct other neurological conditions, stimulating work on this problem in academia and, importantly, in industry. Thus, there are actionable opportunities for genetic therapies for epilepsy on the horizon.

In addition to the value of new models suggested by genetic analysis, there are several opportunities to exploit advances in diagnostic and therapeutic genetic approaches. There are now multiple examples of strategies one could use to develop gene therapy employing viral vectors to treat focal and generalized epilepsies in animal models in which a missense variant or truncating mutation has modified the function of a target gene or reduced the gene dose.⁴² Other innovative uses of gene therapy include introduction of potassium channels that could reduce excitability, as well as engineering cells to release neuroactive molecules that can counteract excessive excitability.^43,44 As more animal models are developed for different genes, there will be opportunities to test fundamental approaches that supplement underexpressed alleles or proteins with reduced function, as well as editing gene approaches to correct identified defects. These strategies will require demonstration of utility in animals with measurable defects, and the results will speak to the important question in epilepsy around whether symptoms are driven by the genetic defect, are a feature of maladaptive compensation, or reflect some combination of both. That is, there is a need for proof-of-concept data for oligonucleotide and antisense therapies for application in the treatment of genetically defined monogenic epilepsies, as well as data on effectiveness of the timing of treatment in the context of the development of a seizure focus. Advances are needed in genetic therapy using virus delivery vectors that are already approved for other payloads and access both brain and spinal cord following intrathecal administration. The rare genetic epilepsies might provide a test case for intervention, which can be evaluated in iPSC-based models in vitro, organoids derived from iPSC cells, and animal models now.

One opportunity that the accessibility of multiple new genetic models of human variants associated with epilepsy offers is evaluation of repurposed drugs. This requires a comprehensive functional evaluation of the effects of rare variants in vitro, which for ion channels is accessible. Functional evaluation of drug sensitivity of variant proteins will inform potential use of therapeutics, as will knowledge of the nature of the net functional effects as either gain of function or loss of function, or indeterminant.^45–49 Genetic models—from cellular models to zebrafish and mouse models—harboring variants can then be screened for actions of Food and Drug Administration (FDA)-approved medications for efficacy in reducing electroencephalogram abnormalities and seizures^46,50,51 as a step toward using pharmacological treatments. If the models capture patient-relevant features of epileptogenesis, early treatment within a vulnerable window might have long-lasting consequences.

In addition to these pharmacological approaches, bioinformatics coupled with large-scale data sets have driven the development of computational resources^52,53 that can suggest candidate drugs in the FDA library from patterns of changes in gene expression. Moreover, evaluation of multiple drugs in multiple models might identify candidate drugs as add-on therapies that could be used more broadly than for just for rare genetic conditions. Indeed, a large number of epilepsy models have been or are being made from various genes identified in patients with rare epilepsies (eg, sodium channels, potassium channels, postsynaptic ligand-gated ion channels, synaptic proteins), which will provide patient-relevant models in which to assess new pharmacological strategies. These same models can be used to understand developmental compensation, transformation of the foci with time, and pharmacological sensitivity. It seems likely that some compensatory mechanisms will be shared across these different models and may inform treatment of refractory epilepsy. In addition to rodent models, use of companion models and organisms (fly, zebrafish, mice, iPSC-derived neuronal cultures, and organoids) could provide faster and more efficient drug screening⁴³ as well as evaluation of compensatory mechanisms.

The advances in genetic analysis could also expand our understanding of acquired epilepsies and yield insight into whether persons with genetic predispositions may be at greater risk and merit more aggressive treatment and management. This will require concerted effort to capture genetic information from patients with acute events that lead to seizures or increase seizure risk. With a sufficient sample size, some common polygenic factors might emerge, suggesting genes or genetic patterns that imply risk.⁶ In some cases, one might consider treating the predisposition if it can be identified as the first step to gain adequate seizure control before considering, for example, epilepsy surgery. This same form of analyses could be applied to traumatic brain injury, stroke, hypoxia, and other insults that enhance the likelihood of future seizures.

Refractory Epilepsy

About one-third of patients with epilepsy are in part or fully refractory to treatment, creating an enormous medical, social, and economic burden. Thus, an essential aspect of any future prioritization is the need to develop new or improve existing antiseizure therapies for patients with refractory epilepsy. Efforts should include analysis of sequencing data for patients who fail to show adequate improvement following surgical intervention to determine whether there are shared risk factors, as well as those who successfully respond. Approaches that deserve consideration in this regard include conventional drug development, selection of surgical patients, and genetic analysis of both responsive and refractory patients. Toward this end, several new drugs have entered clinical use following FDA approval, including cannabidiol for Dravet and Lennox-Gastaut syndromes,^54,55 nasally administered midazolam for seizure clusters,⁵⁶ stiripentol for Dravet syndrome,⁵⁷ and everolimus for seizures in patients with tuberous sclerosis complex.⁵⁸ In addition, new treatment approaches for specific epilepsies are under investigation with novel or disease-specific targets, including AMPA receptors containing the TARPγ8 subunit, expressed predominantly in the temporal lobe and of potential relevance to mesial temporal lobe epilepsy⁵⁹; KCNQ (Kv7) potassium channels implicated in KCNQ2 developmental and epileptic encephalopathy⁶⁰; and serotonin systems, representing a target of fenfluramine, which have been reported to cause seizure reduction in patients with Dravet syndrome.⁶¹ New routes of drug administration are also being explored.⁶² It will be important to carefully evaluate the utility of these new medications in refractory epilepsy beyond the initial indications for which they are tested or approved.

In addition to new medications, more effort is needed to understand the mechanisms of pharmacoresistance in order to overcome refractoriness to ASMs. To this end, new animal models together with humanized models in vitro based on genetic data may provide an opportunity to explore mechanisms of resistance for those specific models with clear seizure phenotypes for which the patient is known to be refractory to treatment with conventional anticonvulsants. Work in this area would benefit from integration of information about new targets into existing efforts to develop new medications that are effective against refractory seizures. In addition to traditional targets such as ion channels, neurotransmitter receptors, and neurotransmitter transporters, important targets include mTOR and related pathways, the extracellular matrix, oxidative stress, anti-inflammatory pathways, neurosteroid systems, microRNAs, and epigenetic targets include histone deacetylase.^{30,31,63–67} Cell replacement strategies to introduce engineered cells that can support or release neuroactive substances and oligonucleotide approaches to regulate specific genes for therapeutic gain are also opportunities to identify new ways to treat refractory epilepsy. Moreover, clarification of the mechanisms underlying the ketogenic diet might identify metabolic and lipid targets that are relevant, the role of the gut microbiota,⁶⁸ and allow a “ketogenic diet in a pill” treatment strategy for refractory epilepsy.

Real-Time Management of Seizures

Efforts have been made for decades to predict when seizures will occur and provide an immediate intervention to either prevent or terminate the seizure.⁶⁹ These efforts rely on a range of recording devices, computational algorithms to identify at-risk periods, and active response in the form of electrical/optical stimulation or administration of a drug. Although there has been steady progress with these strategies over the decades, many approaches are maturing to the point where they seem poised to provide a workable and effective therapy for a larger number of patients.⁷⁰ Indeed, the introduction in 2013 of an FDA-approved closed loop device that detects seizures and aborts them by deep brain stimulation has spawned many efforts to refine stimulation parameters for better seizure control.⁷¹ New seizure prediction algorithms⁸ as well as new devices may allow intravenous injection or even direct infusion of antiseizure agents into the brain at the onset of or immediately before a seizure is predicted.⁷² This approach has the capacity to harness the utility of proven pharmacological treatments without the side effects of chronic exposure to drug in blood and brain. Taken a step further, introduction of active drug locally into the epileptic focus could provide more selective treatment of certain epileptic conditions, including refractory epilepsies, localization-related epilepsies, and status epilepticus. Increasing power of computational algorithms⁷ should allow enhanced ability to predict seizures from multiple streams of data, including electrical recordings and peripheral readouts. In addition, miniaturization of devices can improve the ability to deliver electrical, light, or pharmacological stimuli to specific regions both inside and outside of the central nervous system. The emergence of new animal models of genetic epilepsies provides another opportunity to test detection and seizure interruption strategies in homogeneous models that share some basis with human epilepsy and thus might provide robust data that can be translated to patients harboring these variants. A range of models might stimulate improvements in the low signal to noise ratio in seizure prediction and in the abortion of seizures, such as evaluation of new biomarkers that change prior to seizure initiation⁷³ and consideration of circadian rhythms.⁷⁴ Ultimately, these systems need to be suitable for self-management in the home and other nonmedical settings in order to improve adherence and efficacy.

Taken to its logical albeit futuristic conclusion, one might envision a paradigm shift from ASMs in the form of multiple doses of a drug per day and steady-state blood levels (with attendant side effects) to delivery systems that provide anticonvulsants to the brain at the site they are needed and only when they are needed, improving the quality of life of the patient. Further, each patient’s treatment would be customized based on genetic and molecular profiles. This form of precision medicine would eliminate the need for chronic and systemic nonspecific and side effect-laden pharmacotherapy, improving efficacy and possibly reducing the development of pharmacoresistance.

Source: https://doi.org/10.1177/1535759719895279

[ARTICLE] Epilepsy Benchmarks Area I: Understanding the Causes of the Epilepsies and Epilepsy-Related Neurologic, Psychiatric, and Somatic Conditions – Full Text

Abstract

The 2014 NINDS Benchmarks for Epilepsy Research included area I: Understand the causes of the epilepsies and epilepsy-related neurologic, psychiatric, and somatic conditions. In preparation for the 2020 Curing Epilepsies Conference, where the Benchmarks will be revised, this review will cover scientific progress toward that Benchmark, with emphasize on studies since 2016.

Introductory Vignette by Lizbeth Carmichael. Epilepsy, Depression, and SUDEP—A Parent’s Perspective

My son John developed epilepsy in his late teens, and despite medications, his seizures remained severe and uncontrolled. John was a talented and creative musician and a caring and thoughtful brother and son. He had many friends, and he desperately wanted an independent life. As John’s epilepsy progressed, he also experienced declining mental health. John, who was normally a very peaceful individual, had periods of severe irritability and rage. He also became very anxious at times, and this was a sign of an impending seizure. John heard voices and developed paranoia, hallucinations, and depression. Our family was told to see specialists, but we found that the communication and coordination of care between epileptologists and mental health professionals was impossible, even when he was hospitalized and referrals were made. Ultimately, his mental health issues were not understood or addressed and contributed significantly to his decline. John died of sudden unexpected death in epilepsy (SUDEP) in 2012. Our family’s wish is that those around John had been more attuned to the mental health comorbidities that he was experiencing, and that his medical issues were jointly managed as the outcome for him might have been different.

Significant comorbidities often accompany epilepsy and can be more debilitating than the seizures themselves. A better understanding of the underlying mechanisms of epilepsy-associated comorbidities and appropriate clinical care is critical for increased quality of life for those impacted by epilepsy and their families.

Lizbeth Carmichael. Forever John’s Mom. Citizens United for Research in Epilepsy (CURE).

Introduction

In this review, we provide an update on preclinical and clinical advances into our understanding of the many etiologies of the epilepsies, as well as progress in assigning etiology to epilepsy-related neuropsychiatric and somatic comorbidities. Since the most recent summary in this area,¹ expansion in our knowledge of epilepsy genetics and autoimmune epilepsies has continued to result in fewer individuals being labeled with epilepsy of unknown etiology. With the advent of next-generation sequencing technologies, the number of “epilepsy genes” continues to expand. Assigning a causative role to such genes requires verification in not only larger cohorts with statistical rigor but also a number of criteria that take into account normal variation, determination of how a genetic change leads to altered molecular function, and the demonstration of an epilepsy or epilepsy-related phenotype in genetically manipulated model organisms.² Similar considerations apply for autoimmune epilepsies, for which the relative epileptogenic effects of T-cell infiltration and circulating antibodies continue to be clarified.

Preclinical models of genetic, autoimmune, and brain injury-related epilepsies have been essential to advance our knowledge into upstream and downstream cellular and neurophysiological perturbations that may promote hypersynchrony and the transition to the ictal state. It is only with this type of knowledge that we will be able to better inform treatment of epilepsy related to these types of epilepsy. Incomplete penetrance and variable phenotypes in both humans and animal models strongly implicate genetic modifiers of susceptibility, which need to be identified and validated so as to appreciate mechanisms by which epilepsy may be therapeutically modulated.

In parallel with efforts to address the causes of epilepsy and epileptogenesis, there has been an expansion in efforts designed to unravel the genesis of epilepsy’s various psychiatric comorbidities. Generally, these are etiologically related to broad network dysfunction that may be secondary to the underlying epileptogenic lesion (genetic, structural, or unknown) and actively modulated by the burden of ongoing seizures (if present) and antiseizure medications. Animal models of monogenic epilepsies provide the most tractable route to assigning etiology to epilepsy-associated comorbidities, albeit with some limitations in the ability to assess psychiatric comorbidities in various models. Incorporating optogenetic and chemogenetic strategies in these models affords the ability to definitively test whether specific network abnormalities affect seizure risk or impact limbic function or cognitive function or both.

We conclude our review with a set of general recommendations for future research into the causes of epilepsy spectrum disorders that will guide our understanding into epilepsy prevention (area II), treatment options (area III), and the adverse consequences of seizures themselves (area IV).

Key Advances in Area I

Epilepsy Genetics

Advances in our understanding of the genetics of the epilepsies have continued to accrue since the last Benchmarks update and have been reviewed in several excellent publications.^3–6 Many new variants associated with epilepsy are identified as “de novo dominant,” meaning that they are present in the heterozygous state in sporadically affected individuals. At a cellular level, these genes encode proteins that display a broad range of functions that extend well beyond ion channels, including cell adhesion (eg, PCDH19), DNA binding and chromatin remodeling (eg, CHD2), and neurotransmitter release (eg, STXBP1).⁷ The importance of genetic etiologies in focal epilepsy in particular has become even more clear, with the involvement of DEPDC5 and associated GATOR1-complex mTOR repressors in epileptogenic cortical malformations being notable examples.⁸

De novo postzygotic (somatic) mutation has been increasingly recognized to play a role in focal epilepsy, largely involving the mTOR pathway in the pathogenesis of lesional epilepsies such as focal cortical dysplasia and hemimegalencephaly, with a majority of cases explained by this mechanism.^9–11 Extending the discovery of somatic mutation to a new pathway and, interestingly, to both focal cortical dysplasia (type I) and nonlesional focal epilepsy was a report on mosaic variants in the gene SLC35A2, which encodes an UDP-galactose transporter previously associated in nonmosaic form with developmental and epileptic encephalopathy.¹² The discovery of these 2 distinct pathways may point to very different targeted therapies after further study, which is promising but also demands attention to precision in classifying individuals with focal epilepsy and establishing a molecular diagnosis before pursuing experimental therapy.

Although most newly discovered pathogenic variants each seem to be causative in only a small number of individuals, taken together their combined impact is substantial. From the perspective of practicing epileptologists, we now benefit from a relatively high rate of identifiable genetic causes in neonatal and early childhood epilepsies, particularly in those individuals with comorbid intellectual disability, so that more routine usage of next-generation sequencing methods in this population may be warranted.^13,14 Much more research is needed, however, to separate out the effects of seizures, genetic changes, and treatments on the intellectual impairments that are found in the epileptic encephalopathies.¹⁵

Animal models have permitted important insights into the specific mechanisms by which genetic aberrations may promote hyperexcitability. In additional to conventional “knockout” mice, mutants with conditional gene deletions (permitted via Cre-LoxP technology) have helped dissect the individual contributions of specific neuronal populations to seizure generation. For example, mice with a conditional deletion of Lgi1 in parvalbumin-positive interneurons alone are devoid of spontaneous seizures, while conditional deletions of Lgi1 in forebrain glutamatergic neurons result in frequent early-life seizures and premature death,¹⁶ just as in Lgi1 knockout mice.¹⁷ These results not only provide guidance to future gene replacement strategies but also show that while Lgi1 is an extracellularly secreted protein that is expressed in both GABAergic and glutamatergic neurons, restoring Lgi1 expression in glutamatergic neurons may be more likely to ameliorate seizures. The lack of spontaneous seizures in mice with heterozygous deletions of Lgi1 (recapitulating the haploinsufficiency of LGI1 mutation-related lateral temporal lobe epilepsy [TLE]) illustrates an important point with regard to gene dosage in animal models. Similar findings exist with other epilepsy genes, including KCNQ2,¹⁸ CDKL5,¹⁹ and DEPDC5. ²⁰ Heterozygous DEPDC5 variants are found in cases of familial focal epilepsy as well as focal cortical dysplasia–associated epilepsy.^20,21 Mice or rats with homozygous germ line deletions of Depdc5 had embryonic lethality,^22–24 which is itself etiologically nonspecific and may even reflect placental pathology.²⁵ In contrast, rats with heterozygous deletions of Depdc5 do not display spontaneous seizures.²⁴ Mice with a conditional brain-specific homozygous deletion of Depdc5 display extremely rare seizures, together with macrocephaly, impaired survival, and biochemical evidence of mTOR1 complex activation.²² Thus, it appears that for certain genetic variants strongly associated with epilepsy in humans, mice with corresponding gene deletions or transgenic “knock-ins” of variants seen in individuals with the specific epilepsy syndrome may not display spontaneous seizures or even reflex audiogenic seizures, a common expression of epilepsy in mice. This phenomenon may reflect the influences of variations in genetic background or fundamental differences in mechanisms of genetic epileptogenesis between mice and humans.

Confirming the epilepsy-inducing or epilepsy-modifying effects of specific variants may be greatly aided through the use of other vertebrate models, such as zebrafish (Danio rerio). Classically employed as a model to study embryology and development, zebrafish has now been adopted to study a variety of neurological disorders, including epilepsy. This species is amenable to exon deletion via homologous recombination, and specific single-nucleotide variants can be introduced via CRISPR-Cas9 technology.^26–28 As with mice, stereotyped spontaneous or induced seizures can be identified by video tracking and/or electroencephalography. The small size and rapid development of zebrafish also permit high-throughput drug screening²⁹ that may be individualized to identify a treatment for a specific variant.³⁰

Despite the impressive array of genetic advances, the translation of these findings into gene-related or pathway-based clinical treatments has had mixed results.³¹ There are genetic diagnoses for which specific antiepileptic therapies are either indicated or relatively contraindicated (eg, GLUT1 deficiency, pyridoxine dependency, SCN1A-related epilepsy), and mTOR inhibitors are now known to be at least partially effective for tuberous sclerosis complex–associated epilepsy.³² By contrast, the use of quinidine for KCNT1-related epilepsy, initially thought to be promising following the report of a single case,³³ has not been shown to reduce seizure frequency in subsequent studies.³⁴ Overall, these and other findings suggest that simply modulating a causative pathway featuring a rational drug target can lead to variable responses. More work is clearly necessary to bring genetic discoveries from the bench successfully to therapeutic application at the bedside.

Interneuronopathy-Related Epilepsies

Interneuronopathies can be broadly defined as those conditions in which epilepsy or neuropsychiatric comorbidities arise as a consequence of either developmental or functional changes in interneurons. Alterations in interneuron migration or numbers have been identified in multiple epilepsy mouse models, including mice with deletions of Cntnap2,³⁵ Wwox,³⁶ and Syngap1,³⁷ as well as in certain models of acquired epilepsy,^38,39 and after traumatic brain injury.^40,41 Epilepsy that occurs in Dravet syndrome associated with pathogenic variants in SCN1A may also be classified in this category based on evidence that interneurons in Scn1a heterozygous mice display a selective decrease in excitability, and selective deletions of Scn1a in interneurons are sufficient to recapitulate the spectrum of Dravet-related phenotypes.^42–44 The term “interneuronopathy” was first used in the setting of a very severe genetic epilepsy syndrome (X-linked lissencephaly with ambiguous genitalia, XLAG) caused by pathogenic variants in ARX, with significant reductions in interneuron density in hippocampal and cortical regions observed in this condition.^45–47

A more detailed understanding of interneuron development and migration patterns will be critical for developing novel treatments for these specific genetic epilepsy syndromes and will guide our explorations into the therapeutic potential of either transplantation^48,49 and/or optogenetic/chemogenetic manipulations of interneurons.

Tumor-Related Epilepsies

The incidence of epilepsy in individuals with brain tumors ranges from 70% to 80% in glioneuronal tumors (including gangliogliomas and dysembryoplastic neuroepithelial tumors) to 20% to 35% in individuals with brain metastases.⁵⁰ Epileptogenesis associated with gliomas, the most common malignant primary brain tumor, has been a focus of intense research, with 2 nonmutually exclusive mechanisms explored extensively.

For some neurodevelopmental tumors such as ganglioglioma, a genetic profile has become apparent in the form of a BRAF V600E variant, suggesting the possibility of treatment with BRAF inhibitors.⁵¹ Furthermore, in some tumors, malignant glial cells release excessive amounts of glutamate through the cystine/glutamate transporter (SLC7A11), a gene whose expression is upregulated in at least half of all glial tumors.⁵² SLC7A11-mediated glutamate release results in hyperexcitability that spreads to adjacent tissues,⁵³ and in preclinical studies, a currently available SLC7A11 inhibitor (sulfasalazine, utilized in the treatment of Crohn disease) resulted in improved seizure frequency and prolonged survival.⁵⁴ Mutations in isocitrate dehydrogenase (IDH1) are a strong predictor of epilepsy in patients with low-grade glial tumors.⁵⁵ Mutant IDH1 converts isocitrate to 2-hydroxyglutarate (instead of α-ketoglutarate), which is structurally similar to glutamate and sufficient to lengthen burst duration in cultured rat cortical neurons in an NMDA-receptor-dependent fashion.⁵⁵

A second potential mechanism involves the dysregulation of chloride homeostasis in peritumoral cortical neurons through the aberrant downregulation of KCC2 (potassium chloride cotransporter) and upregulation of NKCC1 (sodium potassium chloride cotransporter) within these cells.⁵⁶ Under these conditions, γ-aminobutyric acid (GABA) binding to ionotropic receptors results in depolarization, and inhibitors of NKCC1 (which reverse altered chloride gradients) in preclinical glioma models improve seizure susceptibility.⁵⁷ It remains to be seen whether similar mechanisms of epileptogenesis may be involved in epilepsies related to meningiomas or metastatic lesions, for which preclinical models are less well developed. Clearly, cortically based or invading tumors seem to possess the greatest risk of epilepsy.⁵⁰

Autoimmune Epilepsies

As of 2019, antibodies to at least 11 different antigens have been associated with epilepsy occurring in the context of encephalitis. Antibodies against extracellular antigens raise neuronal excitability and impose synaptic dysfunction either by disrupting specific protein interactions (eg, LGI1, NMDAR), enhancing receptor internalization (AMPAR), or by functioning as an antagonist (GABA-BR).⁵⁸ In contrast, antibodies against intracellular antigens are thought to produce epilepsy as a consequence of direct cytotoxic T-cell infiltration (eg, amphiphysin, GAD-65). The clinical presentation of autoimmune encephalitides is highly variable (signs and symptoms of limbic or motor dysfunction may or may not be present), and seizures may be the presenting symptom, a late symptom, or absent entirely.⁵⁹

Establishing a direct causative link between individual antibodies and their specific mechanisms of epileptogenesis has been possible through experiments in which patient-derived antibodies are infused into mouse or rat models. For example, hippocampal specimens from mice that received intracerebroventricularly infused LGI1 antibodies over 14 days displayed reduced synaptic expression of the voltage-gated potassium channel KV1.1 (KCNA1) together with increases in presynaptic-release probability and postsynaptic current amplitudes, as well as diminished long-term potentiation and impairments in learning and memory.⁶⁰ These mice did not develop spontaneous seizures, suggesting that at least in mice, either longer durations of anti-LGI1 antibody exposure or higher antibody titers may be necessary for seizure generation. In contrast, similar infusions of anti-NMDAR antibodies in mice produced spontaneous seizures without impairments in memory or motor function.⁶¹

Recent genome-wide association studies have revealed that particular human leukocyte antigen (HLA) haplotypes may increase the risk of specific antibody-mediated encephalitides,^59,62,63 just as with other autoimmune conditions such as type I diabetes mellitus or ankylosing spondylitis; these HLA associations provide pathophysiological insights into the genesis of these antibodies. Fortunately, only a minority of patients who display acute symptomatic seizures during active encephalitis go on to develop epilepsy.⁵⁸ Early immunomodulatory therapy appears to be critical to avoid future drug resistance, while other factors, such as medical complications or hypoxia, may also contribute to long-term seizure risk.^58,59

Epilepsy-Related Conditions

Adults have a median of 2 chronic medical conditions, but this number rises to 6 in individuals older than 65 years.⁶⁴ Thus, “comorbidities” are a central aspect of all chronic medical conditions, and epilepsy is no exception. In epilepsy, comorbidities can be broadly divided into those which affect mental health (including sleep), general physical health (including trauma), and reproductive health.^65,66 Together, these comorbidities contribute tremendously to overall disability, impairments in quality of life, and premature mortality.^67,68 Outside of chance or artifactual comorbidities that may reflect various forms of bias,⁶⁴ 4 main mechanisms of comorbidity have been proposed⁶⁹: (1) independent comorbidity (etiologically unrelated to epilepsy), (2) consequent comorbidity (a direct consequence of epilepsy), (3) iatrogenic comorbidity (treatment related), and (4) shared risk factor (in which epilepsy and its comorbidity independently arise from a single etiology). Importantly, shared risk factors may epidemiologically resemble a bidirectional association (in which each condition causes the other).

Psychiatric comorbidities in epilepsy have received the greatest emphasis. Epilepsy is associated with significantly higher rates of mood and anxiety disorders,^70,71 psychosis,⁷² fatigue,⁷³ and autism spectrum disorder.⁷⁴ These entities are each independently associated with varying degrees of intellectual disability. Cross-sectional and/or prospective human data provide a framework for mechanistic hypotheses into their etiology; ultimately, these hypotheses require verification in animal models. Unfortunately, this schema is inherently limited since many psychiatric endophenotypes are either absent entirely (eg, suicidality) or difficult to measure (eg, depressed mood, psychosis) in animal models.

Depression, or major depressive disorder, has and will continue to be a major focus of comorbidity research. Individuals with epilepsy are twice as likely to develop depression over their lifetime,⁷⁰ and either entity can occur first.⁷⁵ The severity of depression is associated with the risk of epilepsy.⁷⁶ Depression and suicidality tend to be more prominent in individuals with TLE compared with those who have genetic generalized epilepsies,^77,78 and within TLE, depression severity correlates with pharmacoresistance but does not correlate with the side⁵⁷ or the extent of hippocampal atrophy,⁷⁹ if present. Improvements in depression that follow temporal lobectomy are strongly associated with improvements in seizure control.⁸⁰ To date, there has been no high-quality evidence to suggest that antidepressants (in conjunction with standard anticonvulsant therapy) are sufficient to either impact epilepsy risk or reduce seizure frequency.⁸¹ On the other hand, behavioral interventions such as cognitive behavioral therapy or mindfulness training have been shown to improve both seizure control and quality of life.⁸² Overall, this body of evidence argues strongly for the presence of shared noniatrogenic neurobiological risk factors that simultaneously raise the risk of depression and epilepsy.

What are these risk factors? Genetic or epigenetic factors may play only a modulatory role since major depression and epilepsy display little to no evidence of genetic overlap (unlike autism and epilepsy).⁷⁸ The roots of epilepsy–depression comorbidity may be related to changes in network functional connectivity. In major depression, such functional rearrangements are broad, bilateral, and vary by depression subtype.^83,84 At least within TLE circuits,⁸⁵ hyperexcitability within the anterior hippocampus (corresponding to the ventral hippocampus in rodents) may be one such anatomical substrate for comorbidity. In mice, ventral hippocampal injections of kainic acid produce epileptic seizures together with memory impairments and anhedonic behavior; these behavioral comorbidities were not observed in mice that received dorsal kainic acid injections.⁸⁶ Hypersynchrony in the anterior/ventral hippocampus region may contribute to depressive symptoms by compromising functional connectivity to ipsilateral frontal regions.⁸⁷

Testing these hypotheses in preclinical models is now possible with optogenetics, in which an anatomically or molecularly defined neuronal population is genetically or virally transduced to express an excitatory or inhibitory ion channel that is activated by light of a specific wavelength. Bilateral optogenetic activations of ventral hippocampal afferent pathways in nonepileptic mice are sufficient to produce depression and anxiety-like symptoms.^88,89 Similarly, the optogenetic inhibition of mossy cells within the dentate gyrus (simulating mossy fiber loss) is sufficient to produce impairments in object memory in mice.⁹⁰ Aside from these focal network derangements, aberrations in a variety of other neuromolecular axes have been proposed as substrates that may raise seizure risk and compromise mood, including disturbances in neurotransmitter signaling (glutamate, GABA, serotonin), dysfunctional hypothalamo–pituitary–adrenal axis signaling, and a host of cellular and secreted mediators of neuroinflammation.^57,91

Looking Forward: Opportunities and Challenges

Given the progress over the past several years and the remaining gaps in knowledge in the field, we have identified some ambitious but feasible future priorities in epilepsy research that we believe should guide our scientific efforts in this area over the next decade. First, it is notable that a large portion of this update has been devoted to genetic advances, given the substantial work in this area. We also recognize that many patients worldwide have epilepsy primarily caused by infection, head injury, birth trauma, hypoxic–ischemic insult, or any of a number of other perturbations of nervous system function. We support an increased focus on investigating the underlying causes and mechanisms of all forms of epilepsy, including these acquired forms of epilepsy, in order to improve our ability to prevent and treat these conditions successfully.

We also support further work on the cognitive and behavioral deficits that accompany epilepsy through experimental animal models, including further use of chemogenetic and optogenetic strategies to study specific cellular populations in the pathogenesis of epilepsy and related conditions. An important question with direct clinical relevance centers on the transition to the ictal state: Since seizures occur only in discrete episodes in most instances, we need a better understanding of what allows them to arise at any particular time and what limits transition to an ictal state at other times.⁹²

We support continued attention on interneuron pathology, central neuronal signaling pathways, and autoimmune factors as underlying mechanistic factors in both genetic and acquired epilepsy syndromes. Further, invoking another less well-studied cell type in the nervous system, we support evaluation of the role of glia in epileptogenesis and seizure propagation. The pathogenesis of infection-related epilepsy, including virus-induced epilepsy and parasite-induced epilepsy, the latter of which is a leading cause of epilepsy worldwide but lacks a relevant animal model, needs further exploration. In general, the links between the brain and immune system and the relationship between inflammation and neural excitability should be critical targets of investigation. Despite the large volume of new advances in epilepsy genetics, we believe there needs to be further characterization of genes associated with the most prevalent early-life syndromes and further research on the use of “rational” therapy design to modulate known pathogenic pathways.

Although some work has been devoted to understanding the causality behind some of the most common epilepsy-related comorbidities, much more is required. We would support further research aimed at disentangling the effects of seizures, genetic changes, and antiseizure medication in contributing to the intellectual impairments that are present in patients with epileptic encephalopathies. In addition, we believe that further timely study of epilepsy etiologies in elderly individuals, who represent a second peak of epilepsy incidence after early childhood, could be highly impactful. Recent findings related to hippocampal hyperexcitability in individuals with Alzheimer disease⁹³ and the discovery of associations between lifestyle risk factors and late-onset epilepsy⁹⁴ provide tantalizing suggestions of important etiological connections in older adults who had multiple medical conditions.

“Doctor, what is causing my seizures?” At the current time, in a significant majority of individuals, including those without a definite brain lesion, an encephalitic prodrome, evidence for a familial epilepsy syndrome, or a comorbid neurodevelopmental syndrome, the answer to this question remains unknown. Fortunately, 65% of individuals will experience seizure freedom with 1 or more currently available antiseizure medications.⁹⁵ To improve the lives of all individuals affected by epilepsy, however, we must address the fundamental causes of epilepsy and its associated conditions. As demonstrated in the introductory vignette, we also have a responsibility to translate our scientific advances toward the treatment of epilepsy and fcognitive and psychiatric comorbidities in a coordinated fashion.

Source: https://doi.org/10.1177/1535759719895280