Decreased benzodiazepine receptor and increased GABA level in cortical tubers in tuberous sclerosis complex

Treatment-Resistant Epilepsy and Tuberous Sclerosis Complex: Treatment, Maintenance, and Future Directions

Tuberous sclerosis complex (TSC) is a neurogenetic disorder that affects multiple organ systems, including the heart, kidneys, eyes, skin, and central nervous system. The neurologic manifestations have the highest morbidity and mortality, in particular in children. Clinically, patients with TSC often present with new-onset seizures within the first year of life. TSC-associated epilepsy is often difficult to treat and refractory to multiple antiseizure medications. Refractory TSC-associated epilepsy is associated with increased risk of neurodevelopmental comorbidities, including developmental delay, intellectual disability, autism spectrum disorder, and attention hyperactivity disorder. An increasing body of research suggests that early, effective treatment of TSC-associated epilepsy during critical neurodevelopmental periods can potentially improve cognitive outcomes. Therefore, it is important to treat TSC-associated epilepsy aggressively, whether it be with pharmacological therapy, surgical intervention, and/or neuromodulation. This review discusses current and future pharmacological treatments for TSC-associated epilepsy, as well as the importance of early surgical evaluation for refractory epilepsy in children with TSC and consideration of neuromodulatory interventions in young adults.

Bumetanide Effects on Resting-State EEG in Tuberous Sclerosis Complex in Relation to Clinical Outcome: An Open-Label Study

Neuronal excitation-inhibition (E/I) imbalances are considered an important pathophysiological mechanism in neurodevelopmental disorders. Preclinical studies on tuberous sclerosis complex (TSC), suggest that altered chloride homeostasis may impair GABAergic inhibition and thereby E/I-balance regulation. Correction of chloride homeostasis may thus constitute a treatment target to alleviate behavioral symptoms. Recently, we showed that bumetanide-a chloride-regulating agent-improved behavioral symptoms in the open-label study Bumetanide to Ameliorate Tuberous Sclerosis Complex Hyperexcitable Behaviors trial (BATSCH trial; Eudra-CT: 2016-002408-13). Here, we present resting-state EEG as secondary analysis of BATSCH to investigate associations between EEG measures sensitive to network-level changes in E/I balance and clinical response to bumetanide. EEGs of 10 participants with TSC (aged 8-21 years) were available. Spectral power, long-range temporal correlations (LRTC), and functional E/I ratio (fE/I) in the alpha-frequency band were compared before and after 91 days of treatment. Pre-treatment measures were compared against 29 typically developing children (TDC). EEG measures were correlated with the Aberrant Behavioral Checklist-Irritability subscale (ABC-I), the Social Responsiveness Scale-2 (SRS-2), and the Repetitive Behavior Scale-Revised (RBS-R). At baseline, TSC showed lower alpha-band absolute power and fE/I than TDC. Absolute power increased through bumetanide treatment, which showed a moderate, albeit non-significant, correlation with improvement in RBS-R. Interestingly, correlations between baseline EEG measures and clinical outcomes suggest that most responsiveness might be expected in children with network characteristics around the E/I balance point. In sum, E/I imbalances pointing toward an inhibition-dominated network are present in TSC. We established neurophysiological effects of bumetanide although with an inconclusive relationship with clinical improvement. Nonetheless, our results further indicate that baseline network characteristics might influence treatment response. These findings highlight the possible utility of E/I-sensitive EEG measures to accompany new treatment interventions for TSC.

Clinical Trial Registration

EU Clinical Trial Register, EudraCT 2016-002408-13 (www.clinicaltrialsregister.eu/ctr-search/trial/2016-002408-13/NL). Registered 25 July 2016.

Electrophysiological and Behavioral Evidence for Hyper- and Hyposensitivity in Rare Genetic Syndromes Associated with Autism

Our study reviewed abnormalities in spontaneous, as well as event-related, brain activity in syndromes with a known genetic underpinning that are associated with autistic symptomatology. Based on behavioral and neurophysiological evidence, we tentatively subdivided the syndromes on primarily hyper-sensitive (Fragile X, Angelman) and hypo-sensitive (Phelan-McDermid, Rett, Tuberous Sclerosis, Neurofibromatosis 1), pointing to the way of segregation of heterogeneous idiopathic ASD, that includes both hyper-sensitive and hypo-sensitive individuals. This segmentation links abnormalities in different genes, such as FMR1, UBE3A, GABRB3, GABRA5, GABRG3, SHANK3, MECP2, TSC1, TSC2, and NF1, that are causative to the above-mentioned syndromes and associated with synaptic transmission and cell growth, as well as with translational and transcriptional regulation and with sensory sensitivity. Excitation/inhibition imbalance related to GABAergic signaling, and the interplay of tonic and phasic inhibition in different brain regions might underlie this relationship. However, more research is needed. As most genetic syndromes are very rare, future investigations in this field will benefit from multi-site collaboration with a common protocol for electrophysiological and event-related potential (EEG/ERP) research that should include an investigation into all modalities and stages of sensory processing, as well as potential biomarkers of GABAergic signaling (such as 40-Hz ASSR).

Brain concentrations of glutamate and GABA in human epilepsy: A review

An imbalance between excitation and inhibition has been a longstanding proposed mechanism regarding ictogenesis and epileptogenesis. This imbalance is related to increased extracellular glutamate in the brain and/or reduction in GABA concentrations, leading to excitotoxicity, seizures, and cell death. This review aims to summarize the microdialysis and magnetic resonance spectroscopy (MRS) literature investigating glutamate and GABA concentrations in epilepsy patients, present limitations, and suggest future directions to help direct the search for novel epilepsy treatments. The majority of microdialysis studies demonstrated increased glutamate in epileptic regions either compared to control regions or to baseline levels; however, sample sizes were small, with some statistical comparisons missing. For the MRS research, two of six studies reported significant changes in glutamate levels compared to controls, though the results were mixed, with one reporting increased and the other reporting decreased glutamate levels. Eleven of 20 studies reported significant changes in Glx (glutamate + glutamine) or Glx ratios, with most reporting increased levels, except for a few epilepsy syndromes where reduced levels were reported. Few studies investigated GABA concentrations, with one microdialysis and four spectroscopy studies reporting increased GABA levels, and one study reporting decreased GABA in a different brain region. Based on this review, future research should account for medication use; include measurements of GABA, glutamate, and glutamine; use high-tesla strength MRI; and further evaluate the timing of microdialysis. Understanding the importance of brain glutamate and GABA levels in epilepsy may provide direction for future therapies and treatments.

Neuron-Glia Interactions in Tuberous Sclerosis Complex Affect the Synaptic Balance in 2D and Organoid Cultures

Tuberous sclerosis complex (TSC) is a genetic disease affecting the brain. Neurological symptoms like epilepsy and neurodevelopmental issues cause a significant burden on patients. Both neurons and glial cells are affected by TSC mutations. Previous studies have shown changes in the excitation/inhibition balance (E/I balance) in TSC. Astrocytes are known to be important for neuronal development, and astrocytic dysfunction can cause changes in the E/I balance. We hypothesized that astrocytes affect the synaptic balance in TSC. TSC patient-derived stem cells were differentiated into astrocytes, which showed increased proliferation compared to control astrocytes. RNA sequencing revealed changes in gene expression, which were related to epidermal growth factor (EGF) signaling and enriched for genes that coded for secreted or transmembrane proteins. Control neurons were cultured in astrocyte-conditioned medium (ACM) of TSC and control astrocytes. After culture in TSC ACM, neurons showed an altered synaptic balance, with an increase in the percentage of VGAT⁺ synapses. These findings were confirmed in organoids, presenting a spontaneous 3D organization of neurons and glial cells. To conclude, this study shows that TSC astrocytes are affected and secrete factors that alter the synaptic balance. As an altered E/I balance may underlie many of the neurological TSC symptoms, astrocytes may provide new therapeutic targets.

Molecular Imaging of Brain Tumor-Associated Epilepsy

Epilepsy is a common clinical manifestation and a source of significant morbidity in patients with brain tumors. Neuroimaging has a pivotal role in neuro-oncology practice, including tumor detection, differentiation, grading, treatment guidance, and posttreatment monitoring. In this review, we highlight studies demonstrating that imaging can also provide information about brain tumor-associated epileptogenicity and assist delineation of the peritumoral epileptic cortex to optimize postsurgical seizure outcome. Most studies focused on gliomas and glioneuronal tumors where positron emission tomography (PET) and advanced magnetic resonance imaging (MRI) techniques can detect metabolic and biochemical changes associated with altered amino acid transport and metabolism, neuroinflammation, and neurotransmitter abnormalities in and around epileptogenic tumors. PET imaging of amino acid uptake and metabolism as well as activated microglia can detect interictal or peri-ictal cortical increased uptake (as compared to non-epileptic cortex) associated with tumor-associated epilepsy. Metabolic tumor volumes may predict seizure outcome based on objective treatment response during glioma chemotherapy. Advanced MRI, especially glutamate imaging, can detect neurotransmitter changes around epileptogenic brain tumors. Recently, developed PET radiotracers targeting specific glutamate receptor types may also identify therapeutic targets for pharmacologic seizure control. Further studies with advanced multimodal imaging approaches may facilitate development of precision treatment strategies to control brain tumor-associated epilepsy.

Hyperexcitability of the local cortical circuit in mouse models of tuberous sclerosis complex

Tuberous sclerosis complex (TSC) is a neurogenetic disorder associated with epilepsy, intellectual disabilities, and autistic behaviors. These neurological symptoms result from synaptic dysregulations, which shift a balance between excitation and inhibition. To decipher the synaptic substrate of hyperexcitability, we examined pan-neuronal Tsc1 knockout mouse and found a reduction in surface expression of a GABA receptor (GABAR) subunit but not AMPA receptor (AMPAR) subunit. Using electrophysiological recordings, we found a significant reduction in the frequency of GABAR-mediated miniature inhibitory postsynaptic currents (GABAR-mIPSCs) but not AMPAR-mediated miniature excitatory postsynaptic currents (AMPAR-mEPSCs) in layer 2/3 pyramidal neurons. To determine a subpopulation of interneurons that are especially vulnerable to the absence of TSC1 function, we also analyzed two strains of conditional knockout mice targeting two of the prominent interneuron subtypes that express parvalbumin (PV) or somatostatin (SST). Unlike pan-neuronal knockout mice, both interneuron-specific Tsc-1 knockout mice did not develop spontaneous seizures and grew into adults. Further, the properties of AMPAR-mEPSCs and GABAR-mIPSCs were normal in both Pv-Cre and Sst-Cre x Tsc1^fl/fl knockout mice. These results indicate that removal of TSC1 from all neurons in a local cortical circuit results in hyperexcitability while connections between pyramidal neurons and interneurons expressing PV and SST are preserved in the layer 2/3 visual cortex. Our study suggests that another inhibitory cell type or a combination of multiple subtypes may be accountable for hyperexcitability in TSC.

Cellular antiseizure mechanisms of everolimus in pediatric tuberous sclerosis complex, cortical dysplasia, and non-mTOR-mediated etiologies

The present study was designed to examine the potential cellular antiseizure mechanisms of everolimus, a mechanistic target of rapamycin (mTOR) pathway blocker, in pediatric epilepsy cases. Cortical tissue samples obtained from pediatric patients (n = 11, ages 0.67–6.75 years) undergoing surgical resections for the treatment of their pharmacoresistant epilepsy were examined electrophysiologically in ex vivo slices. The cohort included mTOR‐mediated pathologies (tuberous sclerosis complex [TSC] and severe cortical dysplasia [CD]) as well as non–mTOR‐mediated pathologies (tumor and perinatal infarct). Bath application of everolimus (2 μm) had practically no effect on spontaneous inhibitory postsynaptic activity. In contrast, long‐term application of everolimus reduced spontaneous excitatory postsynaptic activity, burst discharges induced by blockade of γ‐aminobutyric acid A (GABA_A) receptors, and epileptiform activity generated by 4‐aminopyridine, a K⁺ channel blocker. The antiseizure effects were more pronounced in TSC and CD cases, whereas in non–mTOR‐mediated pathologies, the effects were subtle at best. These results support further clinical trials of everolimus in mTOR pathway–mediated pathologies and emphasize that the effects require sustained exposure over time.

A one-pot radioiodination of aryl amines via stable diazonium salts: preparation of 125I-imaging agents

An operationally simple, one-pot, two-step tandem procedure that allows the incorporation of radioactive iodine into aryl amines via stable diazonium salts is described. The mild conditions are tolerant of various functional groups and substitution patterns, allowing late-stage, rapid access to a wide range of ¹²⁵I-labelled aryl compounds and SPECT radiotracers.

Neurobiological bases of autism-epilepsy comorbidity: a focus on excitation/inhibition imbalance

Autism spectrum disorders (ASD) and epilepsy are common neurological diseases of childhood, with an estimated incidence of approximately 0.5-1% of the worldwide population. Several genetic, neuroimaging and neuropathological studies clearly showed that both ASD and epilepsy have developmental origins and a substantial degree of heritability. Most importantly, ASD and epilepsy frequently coexist in the same individual, suggesting a common neurodevelopmental basis for these disorders. Genome-wide association studies recently allowed for the identification of a substantial number of genes involved in ASD and epilepsy, some of which are mutated in syndromes presenting both ASD and epilepsy clinical features. At the cellular level, both preclinical and clinical studies indicate that the different genetic causes of ASD and epilepsy may converge to perturb the excitation/inhibition (E/I) balance, due to the dysfunction of excitatory and inhibitory circuits in various brain regions. Metabolic and immune dysfunctions, as well as environmental causes also contribute to ASD pathogenesis. Thus, an E/I imbalance resulting from neurodevelopmental deficits of multiple origins might represent a common pathogenic mechanism for both diseases. Here, we will review the most significant studies supporting these hypotheses. A deeper understanding of the molecular and cellular determinants of autism-epilepsy comorbidity will pave the way to the development of novel therapeutic strategies.

Late stage iodination of biologically active agents using a one-pot process from aryl amines

A tandem process has been developed for the general preparation of aryl iodide compounds from anilines that is also applicable for the late-stage iodination of biologically active agents.

mTOR in Brain Physiology and Pathologies

TOR (target of rapamycin) and its mammalian ortholog mTOR have been discovered in an effort to understand the mechanisms of action of the immunosuppressant drug rapamycin extracted from a bacterium of the Easter Island (Rapa Nui) soil. mTOR is a serine/threonine kinase found in two functionally distinct complexes, mTORC1 and mTORC2, which are differentially regulated by a great number of nutrients such as glucose and amino acids, energy (oxygen and ATP/AMP content), growth factors, hormones, and neurotransmitters. mTOR controls many basic cellular functions such as protein synthesis, energy metabolism, cell size, lipid metabolism, autophagy, mitochondria, and lysosome biogenesis. In addition, mTOR-controlled signaling pathways regulate many integrated physiological functions of the nervous system including neuronal development, synaptic plasticity, memory storage, and cognition. Thus it is not surprising that deregulation of mTOR signaling is associated with many neurological and psychiatric disorders. Preclinical and preliminary clinical studies indicate that inhibition of mTORC1 can be beneficial for some pathological conditions such as epilepsy, cognitive impairment, and brain tumors, whereas stimulation of mTORC1 (direct or indirect) can be beneficial for other pathologies such as depression or axonal growth and regeneration.

mTOR inhibition in epilepsy: rationale and clinical perspectives

Despite a large number of available medical options, many individuals with epilepsy are refractory to existing therapies that mainly target neurotransmitter or ion channel activity. A growing body of preclinical data has uncovered a molecular pathway that appears crucial in many genetic and acquired epilepsy syndromes. The mammalian target of rapamycin (mTOR) pathway regulates a number of cellular processes required in the growth, metabolism, structure, and cell-cell interactions of neurons and glia. Rapamycin and similar compounds inhibit mTOR complex 1 and decrease seizures, delay seizure development, or prevent epileptogenesis in many animal models of mTOR hyperactivation. However, the exact mechanisms by which mTOR inhibition drives decreased seizure activity have not been completely determined. Nonetheless, these preclinical data have led to limited use in humans with epilepsy due to tuberous sclerosis complex and polyhydramnios, megalencephaly, and symptomatic epilepsy with promising results. Currently, larger controlled studies are underway using mTOR inhibitors in individuals with tuberous sclerosis complex and intractable epilepsy.

Early neurodevelopmental screening in tuberous sclerosis complex: a potential window of opportunity

BACKGROUND

Infants born with tuberous sclerosis complex, a genetic condition resulting from a mutation in TSC1 or TSC2, are at increased risk for intellectual disability and/or autism. Features of epilepsy, neuropathology, genetics, as well as timing and type of mechanism-based medications have been proposed as risk factors. Neurodevelopmental outcomes have been reported among these studies; however, few include data about the individuals' early neurodevelopmental profile, a factor that may contribute significantly to these outcomes. Further, there is no clinical standard for the neurodevelopmental assessment of these infants. The paucity of data regarding the natural history of neurodevelopment in infants with tuberous sclerosis complex and the lack of a gold standard for neurodevelopmental evaluation present a significant challenge for clinicians and researchers.

METHOD

During the first year of life, we tracked the onset of infantile spasms, the type and timing of antiepileptic treatments, and the associated response of two age-matched infants with tuberous sclerosis complex. We also employed Capute Scales as a part of a structured neurodevelopmental evaluation to characterize and compare their neurodevelopmental profiles.

RESULTS

Infant 1 developed infantile spasms with confirmed hypsarrhythmia at 4 months of age. Treatment with vigabatrin was initiated within 24 hours with near immediate cessation of seizures and no further seizures to date. Expressive language delay was detected at 12 months and treated with speech and/or language therapy. Infant 2 developed complex partial seizures at 1 month. Treatment included levetiracetam, oxcarbazepine, and the ketogenic diet. Vigabatrin was initiated on detection of hypsarrhythmia after 4 months. Intractable epilepsy persists to date. Global developmental delay was evident by 8 months and treated with physical, occupational, and speech and/or language therapy.

CONCLUSION

Many risk factors have been associated with intellectual disability and/or autism in individuals with tuberous sclerosis complex; however, few data are available regarding practical clinical tools for early identification. In our case series, inclusion of the Capute Scales as a part of routine medical care led to the identification of developmental delays in the first 12 months of life and selection of targeted neurodevelopmental interventions. Development of a risk-based assessment using this approach will be the focus of future studies as it may provide a potential window of opportunity for both research and clinical purposes. In research, it may serve as an objective outcome measure. Clinically, this type of assessment has potential for informing clinical treatment decisions and serving as a prognostic indicator of long-term cognitive and psychiatric outcomes.

Diffusion tensor imaging and related techniques in tuberous sclerosis complex: review and future directions

In this article, the authors aim to introduce the nonradiologist to diffusion tensor imaging (DTI) and its applications to both clinical and research aspects of tuberous sclerosis complex. Tuberous sclerosis complex is a genetic neurocutaneous syndrome with variable and unpredictable neurological comorbidity that includes refractory epilepsy, intellectual disability, behavioral abnormalities and autism spectrum disorder. DTI is a method for modeling water diffusion in tissue and can noninvasively characterize microstructural properties of the brain. In tuberous sclerosis complex, DTI measures reflect well-known pathological changes. Clinically, DTI can assist with detecting the epileptogenic tuber. For research, DTI has a putative role in identifying potential disease biomarkers, as DTI abnormalities of the white matter are associated with neurocognitive morbidity including autism. If indeed DTI changes parallel phenotypical changes related to the investigational treatment of epilepsy, cognition and behavior with mTOR inhibitors, it will facilitate future clinical trials.

Excitatory/inhibitory synaptic imbalance leads to hippocampal hyperexcitability in mouse models of tuberous sclerosis

Neural circuits are regulated by activity-dependent feedback systems that tightly control network excitability and which are thought to be crucial for proper brain development. Defects in the ability to establish and maintain network homeostasis may be central to the pathogenesis of neurodevelopmental disorders. Here, we examine the function of the tuberous sclerosis complex (TSC)-mTOR signaling pathway, a common target of mutations associated with epilepsy and autism spectrum disorder, in regulating activity-dependent processes in the mouse hippocampus. We find that the TSC-mTOR pathway is a central component of a positive feedback loop that promotes network activity by repressing inhibitory synapses onto excitatory neurons. In Tsc1 KO neurons, weakened inhibition caused by deregulated mTOR alters the balance of excitatory and inhibitory synaptic transmission, leading to hippocampal hyperexcitability. These findings identify the TSC-mTOR pathway as a regulator of neural network activity and have implications for the neurological dysfunction in disorders exhibiting deregulated mTOR signaling.

A circuitry and biochemical basis for tuberous sclerosis symptoms: from epilepsy to neurocognitive deficits

Tuberous sclerosis complex (TSC) is an autosomal dominant monogenetic disorder that is characterized by the formation of benign tumors in several organs as well as brain malformations and neuronal defects. TSC is caused by inactivating mutations in one of two genes, TSC1 and TSC2, resulting in increased activity of the mammalian Target of Rapamycin (mTOR). Here, we explore the cytoarchitectural and functional CNS aberrations that may account for the neurological presentations of TSC, notably seizures, hydrocephalus, and cognitive and psychological impairments. In particular, recent mouse models of brain lesions are presented with an emphasis on using electroporation to allow the generation of discrete lesions resulting from loss of heterozygosity during perinatal development. Cortical lesions are thought to contribute to epileptogenesis and worsening of cognitive defects. However, it has recently been suggested that being born with a mutant allele without loss of heterozygosity and associated cortical lesions is sufficient to generate cognitive and neuropsychiatric problems. We will thus discuss the function of mTOR hyperactivity on neuronal circuit formation and the potential consequences of being born heterozygous on neuronal function and the biochemistry of synaptic plasticity, the cellular substrate of learning and memory. Ultimately, a major goal of TSC research is to identify the cellular and molecular mechanisms downstream of mTOR underlying the neurological manifestations observed in TSC patients and identify novel therapeutic targets to prevent the formation of brain lesions and restore neuronal function.