Contribution of rare genetic variants to drug response in absence epilepsy

Model-Informed Precision Dosing Guidance of Ethosuximide Developed from a Randomized Controlled Clinical Trial of Childhood Absence Epilepsy

Ethosuximide was identified as the optimal option for new-onset childhood absence epilepsy (CAE) in a randomized, two-phase dose escalation comparative effectiveness trial of ethosuximide, lamotrigine, and valproic acid. However, 47% of ethosuximide initial monotherapy participants experienced short-term treatment failure. This study aimed to characterize the initial monotherapy ethosuximide exposure-response relationship and to propose model-informed precision dosing guidance. Dose titration occurred over a 16-20-week period until patients experienced seizure freedom or intolerable side effects. Subjects with initial monotherapy failure were randomized to one of the other two medications and dose escalation was repeated. A population pharmacokinetic model was created using plasma concentration data (n = 1,320), collected at 4-week intervals from 211 unique participants during both the initial and second monotherapy phases. A logistic regression analysis was performed on the initial monotherapy cohort (n = 103) with complete exposure-response data. Eighty-four participants achieved seizure freedom with a wide range of ethosuximide area under the curves (AUC) ranging from 420 to 2,420 μg·h/mL. AUC exposure estimates for achieving a 50% and 75% probability of seizure freedom were 1,027 and 1,489 μg·h/mL, respectively, whereas the corresponding cumulative frequency of intolerable adverse events was 11% and 16%. Monte Carlo Simulation indicated a daily dose of 40 and 55 mg/kg to achieve 50% and 75% probability of seizure freedom in the overall population, respectively. We identified the need for adjusted mg/kg dosing in different body weight cohorts. This ethosuximide proposed model-informed precision dosing guidance to achieve seizure freedom carries promise to optimize initial monotherapy success for patients with CAE.

Effects of the T-type calcium channel CaV3.2 R1584P mutation on absence seizure susceptibility in GAERS and NEC congenic rats models

RATIONALE

Low-voltage-activated or T-type Ca²⁺ channels play a key role in the generation of seizures in absence epilepsy. We have described a homozygous, gain of function substitution mutation (R1584P) in the Ca_V3.2 T-type Ca²⁺ channel gene (Cacna1h) in the Genetic Absence Epilepsy Rats from Strasbourg (GAERS). The non-epileptic control (NEC) rats, derived from the same original Wistar strains as GAERS but selectively in-breed not to express seizures, are null for the R1584P mutation. To study the effects of this mutation in rats who otherwise have a GAERS or NEC genetic background, we bred congenic GAERS-Cacna1hNEC (GAERS null for R1584P mutation) and congenic NEC-Cacna1hGAERS (NEC homozygous for R1584P mutation) and evaluated the seizure and behavioral phenotype of these strains in comparison to the original GAERS and NEC strains.

METHODS

To evaluate seizure expression in the congenic strains, EEG electrodes were implanted in NEC, GAERS, GAERS^-Cacna1hNEC without the R1584P mutation, and NEC^{-Cacna1hGAERS} with the R1584P mutation rats. In the first study, continuous EEG recordings were acquired from week 4 (when seizures begin to develop in GAERS) to week 14 of age (when GAERS display hundreds of seizures per day). In the second study, the seizure and behavioral phenotype of GAERS and NEC^{-Cacna1hGAERS} strains were evaluated during young age (6 weeks of age) and adulthood (16 weeks of age) of GAERS, NEC, GAERS^-Cacna1hNEC and NEC^{-Cacna1hGAERS}. The Open field test (OFT) and sucrose preference test (SPT) were performed to evaluate anxiety-like and depressive-like behavior, respectively. This was followed by EEG recordings at 18 weeks of age to quantify the seizures, and spike-wave discharge (SWD) cycle frequency. At the end of the study, the whole thalamus was collected for T-type calcium channel mRNA expression analysis.

RESULTS

GAERS had a significantly shorter latency to first seizures and an increased number of seizures per day compared to GAERS^-Cacna1hNEC. On the other hand, the presence of the R1584P mutation in the NEC^{-Cacna1hGAERS} was not enough to generate spontaneous seizures in their seizure-resistant background. 6 and 16-week-old GAERS and GAERS^-Cacna1hNEC rats showed anxiety-like behavior in the OFT, in contrast to NEC and NEC^{-Cacna1hGAERS}. Results from the SPT showed that the GAERS developed depressive-like in the SPT compared to GAERS^-Cacna1hNEC, NEC, and NEC^{-Cacna1hGAERS}. Analysis of the EEG at 18 weeks of age showed that the GAERS had an increased number of seizures per day, increased total seizure duration and a higher cycle frequency of SWD relative to GAERS^-Cacna1hNEC. However, the average seizure duration was not significantly different between strains. Quantitative real-time PCR showed that the T-type Ca²⁺ channel isoform Ca_V3.2 channel expression was significantly increased in GAERS compared to NEC, GAERS^-Cacna1hNEC and NEC^{-Cacna1hGAERS}. The presence of the R1584P mutation increased the total ratio of Ca_V3.2 + 25/-25 splice variants in GAERS and NEC^{-Cacna1hGAERS} compared to NEC and GAERS^-Cacna1hNEC.

DISCUSSION

The data from this study demonstrate that the R1584P mutation in isolation on a seizure-resistant NEC genetic background was insufficient to generate absence seizures, and that a GAERS genetic background can cause seizures even without the mutation. However, the study provides evidence that the R1584P mutation acts as a modulator of seizures development and expression, and depressive-like behavior in the SPT, but not the anxiety phenotype of the GAERS model of absence epilepsy.

Site-directed mutagenesis improves the practical application of L-glutamic acid decarboxylase in Escherichia coli

γ-Aminobutyric acid (GABA) is a kind of non-proteinogenic amino acid which is highly soluble in water and widely used in the food and pharmaceutical industries. Enzymatic conversion is an efficient method to produce GABA, whereby glutamic acid decarboxylase (GAD) is the key enzyme that catalyzes the process. The activity of wild-type GAD is usually limited by temperature, pH or biotin concentration, and hence directional modification is applied to improve its catalytic properties and practical application. GABA was produced using whole cell transformation of the recombinant strains Escherichia coli BL21(DE3)-Gad B, E. coli BL21(DE3)-Gad B-T62S and E. coli BL21(DE3)-Gad B-Q309A. The corresponding GABA concentrations in the fermentation broth were 219.09, 238.42, and 276.66 g/L, and the transformation rates were 78.02%, 85.04%, and 98.58%, respectively. The results showed that Gad B-T62S and Gad B-Q309A are two effective mutation sites. These findings may contribute to ideas for constructing potent recombinant strains for GABA production. Practical Application : Enzymatic properties of the GAD from Escherichia coli and GAD site-specific mutants were examined by analyzing their conserved sequences, substrate contacts, contact between GAD amino acid residues and mutation energy (ΔΔG) of the GAD mutants. The enzyme activity and stability of Gad B-T62S and Gad B-Q309A mutants were improved compared to Gad B. The kinetic parameters Km and Vmax of Gad B, Gad B-T62S, and Gad B-Q309A mutants were 11.3 ± 2.1 mM and 32.1 ± 2.4 U/mg, 7.3 ± 2.5 mM and 76.1 ± 3.1 U/mg, and 7.2 ± 3.8 mM and 87.3 ± 1.1 U/mg, respectively. GABA was produced using whole cell transformation of the recombinant strains E. coli BL21(DE3)-Gad B, E. coli BL21(DE3)-Gad B-T62S, and E. coli BL21(DE3)-Gad B-Q309A. The corresponding GABA concentrations in the fermentation broth were 219.09, 238.42, and 276.66 g/L, and the transformation rates were 78.02%, 85.04%, and 98.58%, respectively.

Gain-of-function and loss-of-function GABRB3 variants lead to distinct clinical phenotypes in patients with developmental and epileptic encephalopathies

Many patients with developmental and epileptic encephalopathies present with variants in genes coding for GABA_A receptors. These variants are presumed to cause loss-of-function receptors leading to reduced neuronal GABAergic activity. Yet, patients with GABA_A receptor variants have diverse clinical phenotypes and many are refractory to treatment despite the availability of drugs that enhance GABAergic activity. Here we show that 44 pathogenic GABRB3 missense variants segregate into gain-of-function and loss-of-function groups and respective patients display distinct clinical phenotypes. The gain-of-function cohort (n = 27 patients) presented with a younger age of seizure onset, higher risk of severe intellectual disability, focal seizures at onset, hypotonia, and lower likelihood of seizure freedom in response to treatment. Febrile seizures at onset are exclusive to the loss-of-function cohort (n = 47 patients). Overall, patients with GABRB3 variants that increase GABAergic activity have more severe developmental and epileptic encephalopathies. This paradoxical finding challenges our current understanding of the GABAergic system in epilepsy and how patients should be treated.

Novel treatments in epilepsy guided by genetic diagnosis

In recent years, precision medicine has emerged as a new paradigm for improved and more individualized patient care. Its key objective is to provide the right treatment, to the right patient at the right time, by basing medical decisions on individual characteristics, including specific genetic biomarkers. In order to realize this objective researchers and physicians must first identify the underlying genetic cause; over the last 10 years, advances in genetics have made this possible for several monogenic epilepsies. Through next generation techniques, a precise genetic aetiology is attainable in 30-50% of genetic epilepsies beginning in the paediatric age. While committed in such search for novel genes carrying disease-causing variants, progress in the study of experimental models of epilepsy has also provided a better understanding of the mechanisms underlying the condition. Such advances are already being translated into improving care, management and treatment of some patients. Identification of a precise genetic aetiology can already direct physicians to prescribe treatments correcting specific metabolic defects, avoid antiseizure medicines that might aggravate functional consequences of the disease-causing variant or select the drugs that counteract the underlying, genetically determined, functional disturbance. Personalized, tailored treatments should not just focus on how to stop seizures but possibly prevent their onset and cure the disorder, often consisting of seizures and its comorbidities including cognitive, motor and behaviour deficiencies. This review discusses the therapeutic implications following a specific genetic diagnosis and the correlation between genetic findings, pathophysiological mechanisms and tailored seizure treatment, emphasizing the impact on current clinical practice.

Rare genetic variability in human drug target genes modulates drug response and can guide precision medicine

Interindividual variability in drug response constitutes a major concern in pharmacotherapy. While polymorphisms in genes involved in drug disposition have been extensively studied, drug target variability remains underappreciated. By mapping the genomic variability of all human drug target genes onto high-resolution crystal structures of drug target complexes, we identified 1094 variants localized within 6 Å of drug-binding pockets and directly affecting their geometry, topology, or physicochemical properties. We experimentally show that binding site variants affect pharmacodynamics with marked drug- and variant-specific differences. In addition, we demonstrate that a common BCHE variant confers resistance to tacrine and rivastigmine, which can be overcome by the use of derivatives based on squaric acid scaffolds or tryptophan conjugation. These findings underscore the importance of genetic drug target variability and demonstrate that integration of genomic data and structural information can inform personalized drug selection and genetically guided drug development to overcome resistance.