mTOR-therapy and targeted treatment opportunities in mTOR-related epilepsies associated with cortical malformations.

Dysregulation of the mTOR pathway is now well documented in several neurodevelopmental disorders associated with epilepsy. Mutations of mTOR pathway genes are involved in tuberous sclerosis complex (TSC) as well as in a range of cortical malformations from hemimegalencephaly (HME) to type II focal cortical dysplasia (FCD II), leading to the concept of "mTORopathies" (mTOR pathway-related malformations). This suggests that mTOR inhibitors (notably rapamycin (sirolimus), and everolimus) could be used as antiseizure medication. In this review, we provide an overview of pharmacological treatments targeting the mTOR pathway for epilepsy based on lectures from the ILAE French Chapter meeting in October 2022 in Grenoble. There is strong preclinical evidence for the antiseizure effects of mTOR inhibitors in TSC and cortical malformation mouse models. There are also open studies on the antiseizure effects of mTOR inhibitors, as well as one phase III study showing the antiseizure effect of everolimus in TSC patients. Finally, we discuss to which extent mTOR inhibitors might have properties beyond the antiseizure effect on associated neuropsychiatric comorbidities. We also discuss a new way of treatment on the mTOR pathways.

Review: Mechanistic target of rapamycin (mTOR) pathway, focal cortical dysplasia and epilepsy.

Over the last decade, there has been increasing evidence that hyperactivation of the mechanistic target of rapamycin (mTOR) pathway is a hallmark of malformations of cortical development such as focal cortical dysplasia (FCD) or hemimegalencephaly. The mTOR pathway governs protein and lipid synthesis, cell growth and proliferation as well as metabolism and autophagy. The molecular genetic aetiology of mTOR hyperactivation has only been recently clarified. This article will review the current and still evolving genetic advances in the elucidation of the molecular basis of FCD. Activating somatic mutations in the MTOR gene are to date the most frequent mutations found in FCD brain specimens.

mTOR signaling pathway genes in focal epilepsies.

Focal epilepsies, where seizures initiate in spatially limited networks, are the most frequent epilepsy type, accounting for two-thirds of patients. Focal epilepsies have long been thought to be acquired disorders; several focal epilepsy syndromes are now proven to be (genetically heterogeneous) monogenic disorders. While earlier genetic studies have demonstrated a strong contribution of ion channel and neurotransmitter receptor genes, or synaptic secreted protein genes, later work has revealed a new class of genes encoding components of the mechanistic target of rapamycin (mTOR) signal transduction pathway. The mTOR pathway controls a myriad of biological processes among which cell growth and protein synthesis in response to several extracellular and intracellular. Recently, germline mutations have been found in genes encoding the components of the GATOR1 complex (DEPDC5, NPRL2, NPRL3), a repressor of mTORC1. These mutations are increasingly recognized as causing a wide and yet evolving spectrum of focal epilepsy syndromes, with and without cortical structural abnormalities (usually focal cortical dysplasia). Brain somatic mutations in the gene encoding mTOR (MTOR) have recently been linked to focal cortical dysplasia and other associated brain pathologies including hemimegalencephaly. This chapter reviews the genetics and neurobiology of DEPDC5, NPRL2, and NPRL3, and summarizes the clinical and molecular spectrum of GATOR1-related epilepsies.

Hyperactive behavior in a family with autosomal dominant lateral temporal lobe epilepsy caused by a mutation in the LGI1/epitempin gene.

Autosomal dominant lateral temporal lobe epilepsy (ADLTE) is characterized by focal seizures with auditory features or aphasia. Mutations in the leucine-rich glioma-inactivated 1 (LGI1) gene have been reported in up to 50% of families with ADLTE. Attention-deficit/hyperactivity disorder (ADHD) symptoms have not yet been reported in these families. Clinical data were collected from a family with five affected members. Leucine-rich glioma-inactivated 1 exons and boundaries were sequenced by standard methods. Attention-deficit/hyperactivity disorder symptoms were scored based on the Diagnostic and Statistical Manual of Mental Disorders (DSM-IV) criteria. Affected members had seizures with auditory features and psychic auras, and some experienced nightmares. A heterozygous c.431+1G>A substitution in LGI1 was detected in all members. Significantly more hyperactivity symptoms were found in family members carrying the LGI1 mutation. This study expands the phenotypic spectrum associated with ADLTE due to LGI1 mutation and underlines the need for more systematic evaluation of ADHD and related symptoms.

GLUT1 mutations are a rare cause of familial idiopathic generalized epilepsy.

OBJECTIVE: The idiopathic generalized epilepsies (IGE) are the most common genetically determined epilepsies. However, the underlying genes are largely unknown. We screened the SLC2A1 gene, encoding the glucose transporter type 1 (GLUT1), for mutations in a group of 95 European patients with familial IGE. METHODS: The affected individuals were examined clinically by EEG and brain imaging. The coding regions of SLC2A1 were sequenced in the index cases of all families. Wild-type and mutant transporters were expressed and functionally characterized in Xenopus laevis oocytes. RESULTS: We detected a novel nonsynonymous SLC2A1 mutation (c.694C>T, p.R232C) in one IGE family. Nine family members were affected mainly by absence epilepsies with a variable age at onset, from early childhood to adulthood. Childhood absence epilepsy in one individual evolved into juvenile myoclonic epilepsy. Eight affected and 4 unaffected individuals carried the mutation, revealing a reduced penetrance of 67%. The detected mutation was not found in 846 normal control subjects. Functional analysis revealed a reduced maximum uptake velocity for glucose, whereas the affinity to glucose and the protein expression were not different in wild-type and mutant transporters. CONCLUSION: Our study shows that GLUT1 defects are a rare cause of classic IGE. SLC2A1 screening should be considered in IGE featuring absence epilepsies with onset from early childhood to adult life, because this diagnosis may have important implications for treatment and genetic counseling.

Spectrum of SCN1A gene mutations associated with Dravet syndrome: analysis of 333 patients.

INTRODUCTION: Mutations in the voltage-gated sodium channel SCN1A gene are the main genetic cause of Dravet syndrome (previously called severe myoclonic epilepsy of infancy or SMEI). OBJECTIVE: To characterise in more detail the mutation spectrum associated with Dravet syndrome. METHODS: A large series of 333 patients was screened using both direct sequencing and multiplex ligation-dependent probe amplification (MLPA). Non-coding regions of the gene that are usually not investigated were also screened. RESULTS: SCN1A point mutations were identified in 228 patients, 161 of which had not been previously reported. Missense mutations, either (1) altering a highly conserved amino acid of the protein, (2) transforming this conserved residue into a chemically dissimilar amino acid and/or (3) belonging to ion-transport sequences, were the most common mutation type. MLPA analysis of the 105 patients without point mutation detected a heterozygous microrearrangement of SCN1A in 14 additional patients; 8 were private, partial deletions and six corresponded to whole gene deletions, 0.15-2.9 Mb in size, deleting nearby genes. Finally, mutations in exon 5N and in untranslated regions of the SCN1A gene that were conserved during evolution were excluded in the remaining negative patients. CONCLUSION: These findings widely expand the SCN1A mutation spectrum identified and highlight the importance of screening the coding regions with both direct sequencing and a quantitative method. This mutation spectrum, including whole gene deletions, argues in favour of haploinsufficiency as the main mechanism responsible for Dravet syndrome.

New locus for febrile seizures with absence epilepsy on 3p and a possible modifier gene on 18p.

OBJECTIVE: To report a clinical and genetic study of a large family with febrile seizures (FS) and childhood absence epilepsy (CAE). METHODS: This family was identified through a French campaign for familial epilepsies. It spans four generations and consists of 51 members, 13 of whom were affected. The medical history of all members was obtained by personal information and by consulting the medical files of affected members. All family members gave written consent to participate in the study. RESULTS: All affected members presented FS, with CAE in five and temporal lobe epilepsy (TLE) in one. FS stopped before age 6 years in all but one patient. FS were simple, except in one patient who had a long-lasting complex FS at 8 months of age. He later presented pharmacoresistant TLE and left hippocampal sclerosis was visible on brain MRI. Patients presenting CAE had recorded absences and characteristic EEGs with 3 Hz spike waves. After exclusion of reported loci for FS and generalized epilepsy with FS plus, a genome-wide search allowed us to map a new locus for FS on 3p. We could not exclude another genomic segment on chromosome 18p and all patients presenting epilepsy (CAE and TLE) shared a common haplotype at this locus in addition to the haplotype on 3p. CONCLUSION: These findings emphasize the genetic heterogeneity of febrile seizures. Furthermore, epilepsy in association with febrile seizures might result in this family from an interaction between at least two genes: the gene on 3p and a possible modifier gene on 18p.

[Recent insights into the implication of ion channels in familial forms of epilepsies associated or non associated to febrile convulsions]. / Données récentes sur l'implications des canaux ioniques dans les formes familiales d'épilepsies généralisées idiopathiques associées ou non à des convulsions fébriles.

Major advances have recently been made in the understanding of the genetic bases of monogenic inherited epilepsies. For several idiopathic epilepsies, mutations in genes encoding subunits of ion channels or ligand receptors have been demonstrated. This is the case for some generalized idiopathic epilepsies and generalized epilepsies associated with febrile seizures. In this Article, we review the recent clinical and genetic data of these forms of epilepsy.

Evidence for digenic inheritance in a family with both febrile convulsions and temporal lobe epilepsy implicating chromosomes 18qter and 1q25-q31.

We report a clinical and genetic study of a French family among whom febrile convulsions (FC) are associated with subsequent temporal lobe epilepsy (TLE) in the same individual, without magnetic resonance imaging-identifiable hippocampal abnormalities. Linkage analyses excluded the loci FEB1 and FEB2, previously implicated in FC; the GEFS+1 locus responsible for generalized epilepsy with febrile seizures plus; and the locus implicated in lateral temporal lobe epilepsy. After scanning the entire genome, significant lod scores (>3) for markers on 18qter and suggestive lod scores (>2) for markers on 1q25-q31 were obtained. An analysis of the haplotypes at these two loci supported the hypothesis that two genes segregated with the phenotype. All patients shared common haplotypes for both 1q25-q31 and 18qter chromosomes. All but one unaffected at-risk individuals carried only one, or none, of the disease haplotypes. Under the assumption of digenic inheritance, haplotype reconstruction defined a 26 cM interval on chromosome 1 and a 10 cM interval on chromosome 18. This family suggests that the association between FC and TLE may be observed in the absence of hippocampal structural abnormalities and that they may have, in some cases, a common genetic basis.

First genetic evidence of GABA(A) receptor dysfunction in epilepsy: a mutation in the gamma2-subunit gene.

Major advances in the identification of genes implicated in idiopathic epilepsy have been made. Generalized epilepsy with febrile seizures plus (GEFS+), benign familial neonatal convulsions and nocturnal frontal lobe epilepsy, three autosomal dominant idiopathic epilepsies, result from mutations affecting voltage-gated sodium and potassium channels, and nicotinic acetylcholine receptors, respectively. Disruption of GABAergic neurotransmission mediated by gamma-aminobutyric acid (GABA) has been implicated in epilepsy for many decades. We now report a K289M mutation in the GABA(A) receptor gamma2-subunit gene (GABRG2) that segregates in a family with a phenotype closely related to GEFS+ (ref. 8), an autosomal dominant disorder associating febrile seizures and generalized epilepsy previously linked to mutations in sodium channel genes. The K289M mutation affects a highly conserved residue located in the extracellular loop between transmembrane segments M2 and M3. Analysis of the mutated and wild-type alleles in Xenopus laevis oocytes confirmed the predicted effect of the mutation, a decrease in the amplitude of GABA-activated currents. We thus provide the first genetic evidence that a GABA(A) receptor is directly involved in human idiopathic epilepsy.

Genetics of inherited human epilepsies.

Major advances have recently been made in our understanding of the genetic basis of monogenic inherited epilepsies. Progress has been particularly spectacular with respect to idiopathic epilepsies, with the discovery that mutations in ion channel subunits are implicated. However, important advances have also been made in many inherited symptomatic epilepsies, for which direct molecular diagnosis is now possible, simplifying previously complex investigations, it is expected that identification of the genes implicated in familial forms of epilepsies will lead to a better understanding of the underlying pathophysiological mechanisms of these disorders and to the development of experimental models and new therapeutic strategies, in this article, we review the clinical and genetic data concerning most of the inherited human epilepsies.

Dominant partial epilepsies. A clinical, electrophysiological and genetic study of 19 European families.

Nineteen families with autosomal dominant partial epilepsy were analysed clinically and electrophysiologically in detail. Seventy-one patients were studied as well as 33 non-epileptic at-risk family members. We subdivided the families into those with autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE) (n = 8), familial temporal lobe epilepsy (n = 7) and autosomal dominant partial epilepsy with variable foci (n = 4). However, the application of this nosology to certain families was difficult in cases of non-specific or conflicting clinical and electrophysiological evidence. This was underscored by the observation by depth electrode recordings in one patient that a so-called ADNFLE may originate in an extrafrontal area. The evolution of familial partial epilepsies, which exhibit great intrafamilial variability, is not always benign. The level of pharmacoresistance may reach 30%, close to that seen in classical cryptogenic partial epilepsies. The familial character of a partial epilepsy may be unrecognized in small families as some affected members may have only EEG abnormalities and are clinically asymptomatic, which reflects incomplete clinical penetrance. In view of the recent discoveries of mutations in the alpha4 nicotinic acetylcholine receptor subunit in a few families with ADNFLE, this genetic study focused on genes encoding nicotinic receptor subunits and a candidate region on chromosome 10q. No mutation was detected in the alpha4 and 012 nicotinic acetylcholine receptor subunits. Positive but not significant lod scores were obtained in four families with markers from the candidate region on chromosome 10q.

A second locus for familial generalized epilepsy with febrile seizures plus maps to chromosome 2q21-q33.

We report a clinical and genetic study of a family with a phenotype resembling generalized epilepsy with febrile seizures plus (GEFS+), described by Berkovic and colleagues. Patients express a very variable phenotype combining febrile seizures, generalized seizures often precipitated by fever at age >6 years, and partial seizures, with a variable degree of severity. Linkage analysis has excluded both the beta 1 subunit gene (SCN1B) of a voltage-gated sodium (Na+) channel responsible for GEFS+ and the two loci, FEB1 and FEB2, previously implicated in febrile seizures. A genomewide search, under the assumption of incomplete penetrance at 85% and a phenocopy rate of 5%, permitted identification of a new locus on chromosome 2q21-q33. The maximum pairwise LOD score was 3.00 at recombination fraction 0 for marker D2S2330. Haplotype reconstruction defined a large (22-cM) candidate interval flanked by markers D2S156 and D2S2314. Four genes coding for different isoforms of the alpha-subunit voltage-gated sodium channels (SCN1A, SCN2A1, SCN2A2, and SCN3A) located in this region are strong candidates for the disease gene.