Background and ObjectivesDevelopmental and epileptic encephalopathies (DEEs) with early burst-suppression EEG (EIDEE-BS) are among the most severe neonatal epileptic syndromes, typically presenting in the first months of life with refractory seizures and profound neurodevelopmental impairment. Although variants in the KCNQ2, STXBP1, and SCN2A genes are recognized as major causes, the full genetic spectrum remains uncertain. We aimed to delineate the electroclinical characteristics, genetic etiologies, and long-term outcomes in a large MRI-negative EIDEE-BS cohort.MethodsWe retrospectively analyzed 110 patients with BS EEG enrolled from a database of 1,540 individuals with suspected genetic epilepsies (2008-2023). Clinical, EEG, and genetic data were systematically collected. Patients were stratified into 4 groups: KCNQ2, STXBP1, "other pathogenic variants," and "without a genetic diagnosis." EEG traces were reviewed independently, and outcomes were assessed through long-term follow-up.ResultsPathogenic or likely pathogenic variants were identified in 62.7% of patients and involved 23 genes, including 2 copy number variants. KCNQ2 (n = 24) and STXBP1 (n = 16) accounted for one-third of diagnoses, whereas SCN2A (n = 3) and KCNT1 (n = 2) were less frequent. In KCNQ2 cases, seizures and BS onset occurred earlier than in STXBP1 cases: mean 2 days vs 6 weeks for seizures and 3 days vs 2 months for BS, respectively. A typical BS pattern (bursts longer than suppressions) strongly correlated with KCNQ2 and STXBP1 variants. Novel associations were found with DPM1, GRIN2A, KCNT2, PIGO, PURA, WWOX, and candidate genes (KMT2E, SNAP25, and SYT1). Most variants were de novo heterozygous; however, recessive and X-linked inheritance patterns were also observed. Mortality was high (25%), primarily from status epilepticus and complications of severe disability. Most patients (72.5%) had persistent seizures at follow-up (a mean of 6.5 years), as well as profound intellectual disabilities, irrespective of genotype.DiscussionThis large series highlights the strong monogenic basis of EIDEE-BS. KCNQ2, STXBP1, and SCN2A were the most commonly affected genes. Early EEG features, particularly BS timing and morphology, can help anticipate the underlying genotype and guide precision therapy, including the early use of sodium channel blockers in selected cases. These findings support recent ILAE reclassification efforts and underscore the importance of comprehensive genomic testing for improved diagnosis and counseling.

OBJECTIVE:This study was undertaken to characterize the functional impact of novel SLC12A5 variants in two unrelated patients with early onset developmental and epileptic encephalopathy (DEE) and to investigate the mechanisms underlying KCC2 dysfunction. METHODS:Clinical, genetic, and functional analyses were performed in two patients (Cases A and B) with DEE. SLC12A5 encodes two KCC2 splice isoforms (KCC2a and KCC2b). Functional effects of the identified variants on KCC2b ion transport, phosphorylation, mRNA processing, and KCC2-dependent synaptogenesis were assessed using in vitro assays in heterologous expression systems and primary neurons, supported by in silico structural modeling. RESULTS:Both patients developed severe neonatal onset DEE characterized by developmental delay, axial hypotonia, extrapyramidal features, and bilateral migratory seizures within 24 h of birth. Both cases resulted in early mortality (Case A at 9 years; Case B at 6 months). Sequencing revealed distinct biallelic compound heterozygous SLC12A5 variants in both individuals, each inherited from one unaffected parent. Functional analyses demonstrated that in Case A, one variant markedly reduced KCC2-mediated ion transport, whereas the second variant preserved transport activity but exhibited an altered phosphorylation profile at Ser940, located on the intracellular C-terminal region. This variant also disrupted wild-type (WT) KCC2-dependent excitatory synapse formation in immature rat hippocampal neurons. In Case B, one variant disrupted normal mRNA transcript processing consistent with loss of expression, and the second variant exhibited reduced ion transport activity. SIGNIFICANCE:These data demonstrate that SLC12A5-related DEE can result from combined impairment of KCC2-dependent chloride homeostasis and disruption of chloride-independent KCC2 functions critical for early neuronal development. This work expands the mutational and mechanistic spectrum of SLC12A5-DEE and highlights the importance of KCC2 regulatory roles in early brain development, providing new knowledge and tools for basic research and potential avenues for targeted precision therapies.

Small nuclear RNAs (snRNAs) are essential components of the spliceosome. De novo variants in snRNA genes RNU4-2 (ReNU syndrome), RNU5B-1 and RNU2-2 have been linked to dominant neurodevelopmental disorders (NDDs), revealing a large unexpected contribution of noncoding RNA genes to genetic diseases. Here, through international collaborations, we analyze systematically 200 potentially functional snRNA genes in a French cohort of 34,329 people with rare disorders. We report RNU2-2 variants in 141 individuals, including 35 with recurrent dominant pathogenic variants and 91 affected members from 73 families with biallelic variants. Recessive RNU2-2 NDD is at least twice as frequent as the dominant form and often involves a de novo variant in trans with an inherited allele, consistent with the high mutability of snRNA genes. Dominant and recessive RNU2-2 NDDs share overlapping clinical features, with frequent epilepsy. Blood transcriptomics and DNA methylation analyses revealed subtle, variant-specific effects on splicing and episignatures. Our results support a gradient-of-impact model bridging dominant and recessive inheritance, and establish RNU2-2 variants as a principal contributor to NDDs, nearly as prevalent as ReNU syndrome.

Pathogenic KCNQ2 variants are the most common genetic cause of neonatal-onset epilepsies, with phenotypes ranging from self-limited (familial) neonatal epilepsy (SeL(F)NE) to severe developmental and epileptic encephalopathy (KCNQ2-DEE). Sodium channel blockers (SCBs) have shown promise for seizure control in these disorders, but their impact on neurodevelopmental outcomes and possible relationship with timing of initiation remain incompletely understood. We leveraged a large, multicentre international cohort comprising 282 individuals with pathogenic KCNQ2 variants to retrospectively assess the effectiveness of antiseizure medications (ASMs), particularly SCBs, on seizure control and neurodevelopment. Individuals were grouped according to the predicted variant-specific functional effects: loss-of-function (LOF) variants known to be associated with SeL(F)NE or DEE respectively, and gain-of-function (GOF) variants. Epilepsy course, ASM effectiveness, and neurodevelopmental milestones were systematically collected and analysed, including time-to-event and adjusted outcome analyses. SCBs, especially carbamazepine (CBZ) and oxcarbazepine (OXC), emerged as the most effective ASMs in both LOF groups. In LOF KCNQ2-DEE, time-to-event analyses showed that early SCB initiation (≤1 month) was associated with earlier seizure offset. Early SCB initiation was also associated with significantly more favourable neurodevelopmental outcomes, including higher rates of attaining major motor milestones. This association remained significant after adjustment for seizure control by 1 month and total ASM burden. Considerable phenotypic variability persisted, with some individuals experiencing severe impairment despite early seizure control and SCB initiation, suggesting that variant severity and additional genetic or biological modifiers contribute to outcome heterogeneity.Our results support the use of SCBs, particularly CBZ and OXC, as first-line therapy in (LOF) KCNQ2-DEE and SeL(F)NE. Earlier SCB initiation was associated with earlier seizure offset and more favourable developmental outcomes, underscoring the importance of early genetic diagnosis and timely SCB therapy. We however emphasise that early treatment is not universally transformative and further prospective work, including exploration of targeted therapies and standardised neurodevelopmental assessments, is needed to optimise long-term outcomes in this heterogeneous population.

Background: Pathogenic KCNQ2 variants are the most common genetic cause of neonatal-onset epilepsies, with phenotypes ranging from self-limited (familial) neonatal epilepsy (SeL(F)NE) to severe developmental and epileptic encephalopathy (KCNQ2-DEE). Sodium channel blockers (SCBs) have shown promise for seizure control in these disorders, but their impact on neurodevelopmental outcomes and possible relationship with timing of initiation remain incompletely understood. Methods: We leveraged a large, multicentre international cohort comprising 282 individuals with KCNQ2 pathogenic variants to retrospectively assess the effectiveness of antiseizure medications (ASMs), particularly SCBs, on seizure control and neurodevelopment. Individuals were grouped according to the predicted variant-specific functional effects: loss-of-function (LOF) variants known to be associated with SeL(F)NE or DEE respectively, and gain-of-function (GOF) variants. Epilepsy course, ASM effectiveness, and neurodevelopmental milestones were systematically collected and analysed. Results: SCBs, especially carbamazepine (CBZ) and oxcarbazepine (OXC), emerged as the most effective ASMs in both LOF groups. In LOF KCNQ2-DEE, early SCB initiation within the first month of life was associated with significantly more favourable neurodevelopmental trajectories, including higher rates of achievement of major motor milestones. Early seizure freedom itself was a strong predictor of improved neurodevelopment, with the positive effect of SCBs likely mediated by their ability to control seizures. However, considerable phenotypic variability persisted, with some individuals experiencing severe impairment despite early seizure control and SCB initiation. Variant severity and possible genetic modifiers likely contribute to this heterogeneity, underscoring the need for precision therapies beyond nonspecific ASM approaches. Conclusion: Our results strongly support the use of SCBs as first-line therapy in (LOF) KCNQ2-DEE and SeL(F)NE due to their high effectiveness. Moreover, SCBs appear most beneficial when initiated during the neonatal period, with earlier treatment linked to earlier seizure offset and better developmental outcomes. These results highlight the importance of early genetic diagnosis and timely SCB therapy, and support CBZ or OXC as first-line agents. We however emphasise that early treatment is not universally transformative, and further work, including exploration of targeted therapies but also standardised neurodevelopmental assessments, is needed to optimise long-term outcomes in this heterogeneous population. ### Competing Interest Statement S.W. received consultancy fees from UCB, Xenon Pharmaceuticals, Lundbeck, Knopp Biosciences, Encoded Therapeutics, Angelini Pharma, and Roche. C.F. has served on scientific advisory board for longboard pharmaceuticals and biocodex and has received speaker honoraria from Nutricia, UCB, and Jazz Pharmaceuticals. S.A. is Deputy Editor for Epilepsia. He has received personal fees for lectures or advice from: Biocodex, Eisai, Encoded, GRIN therapeutics, Jazz Pharmaceuticals, Longboard, Lundbeck, Neuraxpharm, Nutricia, Mosaica, Proveca, Servier, Stoke, Stream neuroscience, UCB Pharma. He has been investigators for: Eisai, Lundbeck, Proveca, Takeda, UCB Pharma. I.E.S. has served on scientific advisory boards for Biocodex, BioMarin, CAMP4 Therapeutics, Chiesi, Eisai, Encoded Therapeutics, Knopp Biosciences, Longboard Pharmaceuticals, Mosaica Therapeutics, Takeda Pharmaceuticals, UCB; has received speaker honoraria from Akumentis, Biocodex, BioMarin, Chiesi, Eisai, GlaxoSmithKline, Liva Nova, Nutricia, Stoke Therapeutics, Zuellig Pharma; has received funding for travel from Biocodex, BioMarin, Eisai, Encoded Therapeutics, GlaxoSmithKline, Stoke Therapeutics, UCB; has served as an investigator for Anavex Life Sciences, Biohaven Ltd, Bright Minds Biosciences, Cerebral Therapeutics, Cerecin Inc, Cereval Therapeutics, Encoded Therapeutics, EpiMinder Inc, ES-Therapeutics, GW Pharma, Longboard Pharmaceuticals, Marinus, Neuren Pharmaceuticals, Neurocrine BioSciences, Ovid Therapeutics, Praxis Precision Medicines, Shanghai Zhimeng Biopharma, SK Life Science, Supernus Pharmaceuticals, Takeda Pharmaceuticals, UCB, Ultragenyx, Xenon Pharmaceuticals, Zogenix, Zynerba; and has consulted for Atheneum Partners, Biohaven Pharmaceuticals, Care Beyond Diagnosis, Cerecin Inc, Eisai, Epilepsy Consortium, Longboard Pharmaceuticals, Praxis, Stoke Therapeutics, UCB, Zynerba Pharmaceuticals; and is a Non-Executive Director of Bellberry Ltd and a Director of the Australian Academy of Health and Medical Sciences. She may accrue future revenue on pending patent WO61/010176 (filed: 2008): Therapeutic Compound; has a patent for SCN1A testing held by Bionomics Inc and licensed to various diagnostic companies; has a patent molecular diagnostic/theranostic target for benign familial infantile epilepsy (BFIE) [PRRT2] 2011904493 & 2012900190 and PCT/AU2012/001321 (TECH ID:2012-009). The remaining authors report no competing interests. ### Funding Statement C.M. received funding from University of Antwerp-BOF (FFB200262). S.W. received funding from Fonds Wetenschappelijk Onderzoek (FWO 1861424N), GSKE - UCB Award, European Partnership for Personalized Medicine (EPPerMed BEATKCNQ), KCNQ2e.v. L.V. and S.W. received funding from the European Joint Programme on Rare Disease JTC 2020 (TreatKCNQ). L.V. recieved funding from the Agence Nationale de la Recherche (ANR 19-CE17-0018-02). M.M. was supported by Aix Marseille Univ, Agence Nationale de la Recherche (ANR 19‐CE17‐0018‐02). A.T.G.C received funding from the National Health and Medical Research Council (NHMRC) Postgraduate Scholarship, Australia. E.C.C. received funding from the Jack Pribaz Foundation, the KCNQ2 Cure Alliance, the Miles Family Fund, and the parents of Raz Fisher. This work was supported by funding from the Australian National Health and Medical Research Council (GNT1091593, GNT1172897, GNT2006841, GNT2010562, GNT2033247) and Medical Research Future Fund Australia (GNT2007707). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Informed consent for participation and publication was obtained from all parents or legal guardians according to the Declaration of Helsinki. The study was approved by the Human Research Ethics Committees of the University Hospital of Antwerp (Belgium, number: 20/50/683) and the Committee for the Protection of Persons (Comite de Protection des Personnes Sud-Est III, France, IDRCB: 2020-A01363-36). Data are reported in line with the Strengthening Reporting of Observational Studies in Epidemiology (STROBE) statement. I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data supporting the findings of this study are available to those eligible upon request to the corresponding author.

INTRODUCTION:The neurodevelopmental outcome of 'Cystic' malformations of the posterior fossa with marked opening of the fourth ventricle, such as Dandy Walker malformation (DWM) and large Blake's pouch cyst (BPC), is a major issue. This study aimed to refine relevant MRI criteria for distinguishing DWM from BPC and identify prognostic factors. PATIENTS AND METHODS:Inclusion criteria were prenatal retrocerebellar fluid space diameter > 10 mm, marked opening of the fourth ventricle with a tegmento-vermian angle (TVA) > 40°, and postnatal follow-up > 2 years. RESULTS:27 patients were classified as follows: 6 DWM characterized by an overall upward orientation of the tentorium, an open tegmento-tentorial angle (TTA > 78 ) and a high TVA (median 132°); 15 BPC with a normal downward orientation of the proximal part of the tentorium (TTA < 68°) and distal upward displacement (median TVA 74°); 3 PHACE syndromes (Posterior fossa abnormalities, Haemangioma, Arterial cerebrovascular anomalies, Cardiac defects, Eye anomalies) and 3 unclassified. Four prognostic factors were identified, (i) diagnosis: DWM (two deaths, three learning disabilities and one typical development (TD)) versus BPC (five learning disabilities [4/5 with associated malformation or genetic defects] and 10 TD); (ii) associated versus isolated (36% vs. 87% TD); (iii) obstructive ventriculomegaly versus no hydraulic complications (20% vs. 91% TD); and (iv) the foetal TVA value and clinical outcome (correlation coefficient = 0.561, p = 0.006).

Variants in spliceosomal small nuclear RNA (snRNA) genes RNU4-2 (ReNU syndrome), RNU5B-1, and RNU2-2 have recently been linked to dominant neurodevelopmental disorders (NDDs), revealing a major, previously overlooked role for noncoding snRNAs in human disease. Here, we systematically analysed 200 potentially functional snRNA genes in a French cohort comprising 26,911 individuals with rare disorders and through international collaborations. We identify de novo and biallelic variants in RNU2-2 associated with both dominant and recessive NDDs in 126 individuals from 108 unrelated families. Recessive RNU2-2 NDD is at least twice as frequent as the dominant NDD caused by n.4G>A and n.35A>G, and often arises from a de novo variant in trans with an inherited allele, reflecting the high mutability of snRNA genes. Dominant and recessive RNU2-2-NDDs share overlapping clinical features with frequent epilepsy. Blood transcriptomics and DNA methylation analyses revealed subtle, variant-specific effects on splicing and episignatures. Our findings support a gradient-of-impact model and a continuum between dominant and recessive inheritance, establishing RNU2-2 variants as a frequent cause of NDDs, nearly as prevalent as ReNU syndrome.

BACKGROUND:Epileptic disorders are a heterogeneous group of neurological conditions, with many cases linked to monogenic causes, particularly in developmental and epileptic encephalopathies (DEE). Identifying pathogenic variants aids treatment, prognosis, and family planning. In France, genetic testing is coordinated through the EpiGene network. METHODS:We analyzed clinical and genetic data from 2563 epilepsy patients referred to four diagnostic labs (2016-2023). Epilepsy syndromes were classified via pre-test questionnaires, and genotyping used various gene panels, including a 68-gene core panel. Multivariate logistic regression assessed diagnostic rates and genotype-phenotype correlations. RESULTS:Overall, 27.0% of patients had pathogenic/likely pathogenic variants, mainly within the core panel (24%). SCN1A and KCNQ2 were the most frequently mutated genes. Diagnostic yield varied by syndrome, with Dravet Syndrome Spectrum (DSS) and early-infantile DEE (EIDEE) showing the highest rates (41% and 34%, respectively). Genetic heterogeneity differed across syndromes, from DSS (predominantly SCN1A) to Infantile Epileptic Spasms Syndrome (IESS, 12%), involving ≥ 26 genes. Outside DEE, self-limited neonatal epilepsy (SeLNE) had the highest yield (50%). Earlier seizure onset was associated with a higher likelihood of a positive molecular diagnosis, whereas intellectual disability severity and drug resistance were not independently predictive of diagnostic outcome. Genotype-phenotype correlations highlighted that objective clinical data (e.g., age of onset) can outperform syndrome labels (e.g., EIDEE) in predicting diagnosis. CONCLUSION:This large cohort study refines the genetic landscape of epilepsy, informs classification challenges, and enhances genetic testing strategies, ultimately improving patient care and future research directions.

ObjectivesTo present a case series of novel CHD2 variants in patients presenting with genetic epileptic and developmental encephalopathy.BackgroundCHD2 gene encodes an ATP-dependent enzyme, chromodomain helicase DNA-binding protein 2, involved in chromatin remodeling. Pathogenic variants in CHD2 are linked to early-onset conditions such as developmental and epileptic encephalopathy, drug-resistant epilepsies, and neurodevelopmental disorders. Approximately 225 diagnosed patients from 28 countries exhibit various allelic variants in CHD2, including small intragenic deletions/insertions and missense, nonsense, and splice site variants.ResultsWe present the molecular and clinical characteristics of 17 unreported individuals from 17 families with novel pathogenic or likely pathogenic variants in CHD2. All individuals presented with severe global developmental delay, childhood-onset myoclonic epilepsy, and additional neuropsychiatric features, such as behavioral including autism, ADHD, and hyperactivity. Additional findings include abnormal reflexes, hypotonia and hypertonia, motor impairment, gastrointestinal problems, and kyphoscoliosis. Neuroimaging features included hippocampal signal alterations (4/10), with additional volume loss in 2 cases, inferior vermis hypoplasia (7/10), mild cerebellar atrophy (4/10), and cerebral atrophy (1/10).DiscussionOur study broadens the geographic scope of CHD2-related phenotypes, providing valuable insights into the prevalence and clinical characteristics of this genetic disorder in previously underrepresented populations.

Abstract Objective Adenylosuccinate lyase (ADSL) deficiency is a rare inherited metabolic disorder with a wide phenotypic presentation, classically grouped into three types (neonatal, type I, and type II). We aim to better delineate the pathological spectrum, focusing on the electroclinical characteristics and phenotypic differences of patients with ADSL deficiency. Patients and Methods Seven patients, from four different families, underwent serial electroencephalogram (EEG), clinical assessment, and neuroimaging. We also performed a systematic review of the cases published in the literature, summarizing the available clinical, neurophysiological, and genetic data. Results We report seven previously unreported ADSL deficiency patients with long‐term follow‐up (10–34 years). From the literature review, we collected 81 previously reported cases. Of the included patient population, 58 % (51/88) were classified as having ADSL deficiency type I, 28% (25/88) as having type II, and 14% (12/88) as having neonatal. The most frequently reported pathogenic variants are p.R426H homozygous (19 patients), p.Y114H in compound heterozygosity (13 patients), and p.D430N homozygous (6 patients). In the majority (89.2%), disease onset was within the first year of life. Epilepsy is present in 81.8% of the patients, with polymorphic and often intractable seizures. EEG features seem to display common patterns and developmental trajectories: (i) poor general background organization with theta‐delta activity; (ii) hypsarrhythmia with spasms, usually adrenocorticotropic hormone‐responsive; (iii) generalized epileptic discharges with frontal or frontal temporal predominance; and (iv) epileptic discharge activation in sleep with an altered sleep structure. Imaging features present consistent findings of cerebral atrophy with frontal predominance, cerebellar atrophy, and white matter abnormalities among the three types. Significance ADSL deficiency presents variable phenotypic expression, whose severity could be partially attributed to residual activity of the mutant protein. Although a precise phenotype‐genotype correlation was not yet feasible, we delineated a common pattern of clinical, neuroradiological, and neurophysiological features.

INTRODUCTION:Our objective was to evaluate the outcome of fetuses with first- and second-trimester fetal cytomegalovirus infection (CMVi) according to prenatal imaging patterns, especially fetuses presenting with mild imaging features (MF), being currently of uncertain prognosis.MATERIAL AND METHODS:In a retrospective study of 415 suspected CMVi cases, 59 cases were confirmed. Among prenatal imaging features, microcephaly, cortical disorder, and cerebellar hypoplasia as well as severe IUGR and fetal hydrops were considered as severe imaging features (SF). Other imaging features were considered as MF. Postnatal outcome was classified as "normal outcome," "mild sequelae" characterized mainly by sensorineural disorder (SND) and "severe sequelae" characterized by cognitive impairment.RESULTS:Only first-trimester (T1) and second-trimester (T2) CMVi cases were included in our study (n = 49) since all third-trimester cases (n = 10) had normal imaging and outcome. Sixteen fetuses had normal prenatal imaging and normal outcome, except one showing SND. Abnormal ultrasound findings were present in 33 fetuses, including SF noted in 16 fetuses, related exclusively to first-trimester CMVi. Termination of pregnancy was performed in 18 cases. Twelve first-trimester infected fetuses presented SF, whereas 6 fetuses (T1: n = 5, T2: n = 1) presented isolated MF. Four fetal deaths were encountered. Live-born babies with abnormal imaging included 10 fetuses with MF and one with SF. Among the 10 live babies with isolated MF, SND was encountered in 5 cases, whereas 5 children demonstrated normal outcome. Overall, 50% of our babies showing MF suffered from SND. No case of cognitive disorders was reported in babies showing only MF.CONCLUSION:SF were encountered only in first-trimester CMVi and should be distinguished from MF. Among our 10 live babies with prenatal MF following first- or second-trimester infection, 50% showed SND, whereas none presented severe sequelae. In 16 fetuses displaying normal fetal imaging, SND was encountered in one first-trimester case (6%).

De novo mutations in GNAO1, the gene encoding the major neuronal G protein Gαo, cause a spectrum of pediatric encephalopathies with seizures, motor dysfunction, and developmental delay. Of the >80 distinct missense pathogenic variants, many appear to uniformly destabilize the guanine nucleotide handling of the mutant protein, speeding up GTP uptake and deactivating GTP hydrolysis. Zinc supplementation emerges as a promising treatment option for this disease, as Zn2+ ions reactivate the GTP hydrolysis on the mutant Gαo and restore cellular interactions for some of the mutants studied earlier. The molecular etiology of GNAO1 encephalopathies needs further elucidation as a prerequisite for the development of efficient therapeutic approaches. In this work, we combine clinical and medical genetics analysis of a novel GNAO1 mutation with an in-depth molecular dissection of the resultant protein variant. We identify two unrelated patients from Norway and France with a previously unknown mutation in GNAO1, c.509C>G that results in the production of the Pro170Arg mutant Gαo, leading to severe developmental and epileptic encephalopathy. Molecular investigations of Pro170Arg identify this mutant as a unique representative of the pathogenic variants. Its 100-fold-accelerated GTP uptake is not accompanied by a loss in GTP hydrolysis; Zn2+ ions induce a previously unseen effect on the mutant, forcing it to lose the bound GTP. Our work combining clinical and molecular analyses discovers a novel, biochemically distinct pathogenic missense variant of GNAO1 laying the ground for personalized treatment development.

BACKGROUND AND OBJECTIVE:GLUT1 deficiency syndrome (Glut1DS) is a treatable neurometabolic disease that causes a wide range of neurologic symptoms in children and adults. However, its diagnosis relies on an invasive test, that is, a lumbar puncture (LP) to measure glycorrhachia, and sometimes complex molecular analyses of the SLC2A1 gene. This procedure limits the number of patients able to receive the standard of care. We wished to validate the diagnostic performance of METAglut1, a simple blood test that quantifies GLUT1 on the erythrocyte surface.METHODS:We performed a multicenter validation study in France, involving 33 centers. We studied 2 patient cohorts: a prospective cohort consisting of patients with a clinical suspicion of Glut1DS explored through the reference strategy, that is, LP and analyses of the SLC2A1 gene, and a retrospective cohort that included patients previously diagnosed with Glut1DS. All patients were blind-tested with METAglut1.RESULTS:We analyzed 428 patients in the prospective cohort, including 15 patients newly diagnosed with Glut1DS, and 67 patients in the retrospective cohort. METAglut1 was 80% sensitive and >99% specific for the diagnosis of Glut1DS. Concordance analyses showed a substantial agreement between METAglut1 and glycorrhachia. In the prospective cohort, the positive predictive value of METAglut1 was slightly higher than that of glycorrhachia. METAglut1 succeeded to identify patients with Glut1DS with SCL2A1 mosaicism and variants of unknown significance.DISCUSSION:METAglut1 is an easily performed, robust, and noninvasive diagnostic test for the diagnosis of Glut1DS, which allows wide screening of children and adults, including those with atypical forms of this treatable condition.CLASSIFICATION OF EVIDENCE:This study provides Class I evidence that a positive METAglut1 test accurately distinguishes patients with suspected GLUT1 deficiency syndrome from other neurologic syndromes as compared with invasive and genetic testing.

BackgroundPrior studies have revealed remarkable phenotypic heterogeneity in KCNQ2-related disorders, correlated with effects on biophysical features of heterologously expressed channels. Here, we assessed phenotypes and functional properties associated with KCNQ2 missense variants R144W, R144Q, and R144G. We also explored in vitro blockade of channels carrying R144Q mutant subunits by amitriptyline.MethodsPatients were identified using the RIKEE database and through clinical collaborators. Phenotypes were collected by a standardized questionnaire. Functional and pharmacological properties of variant subunits were analyzed by whole-cell patch-clamp recordings.FindingsDetailed clinical information on fifteen patients (14 novel and 1 previously published) was analyzed. All patients had developmental delay with prominent language impairment. R144Q patients were more severely affected than R144W patients. Infantile to childhood onset epilepsy occurred in 40%, while 67% of sleep-EEGs showed sleep-activated epileptiform activity. Ten patients (67%) showed autistic features. Activation gating of homomeric Kv7.2 R144W/Q/G channels was left-shifted, suggesting gain-of-function effects. Amitriptyline blocked channels containing Kv7.2 and Kv7.2 R144Q subunits.InterpretationPatients carrying KCNQ2 R144 gain-of-function variants have developmental delay with prominent language impairment, autistic features, often accompanied by infantile- to childhood-onset epilepsy and EEG sleep-activated epileptiform activity. The absence of neonatal seizures is a robust and important clinical differentiator between KCNQ2 gain-of-function and loss-of-function variants. The Kv7.2/7.3 channel blocker amitriptyline might represent a targeted treatment.FundingSupported by FWO, GSKE, KCNQ2-Cure, Jack Pribaz Foundation, European Joint Programme on Rare Disease 2020, the Italian Ministry for University and Research, the Italian Ministry of Health, the European Commission, the University of Antwerp, NINDS, and Chalk Family Foundation.

PURPOSE:KLHL20 is part of a CUL3-RING E3 ubiquitin ligase involved in protein ubiquitination. KLHL20 functions as the substrate adaptor that recognizes substrates and mediates the transfer of ubiquitin to the substrates. Although KLHL20 regulates neurite outgrowth and synaptic development in animal models, a role in human neurodevelopment has not yet been described. We report on a neurodevelopmental disorder caused by de novo missense variants in KLHL20.METHODS:Patients were ascertained by the investigators through Matchmaker Exchange. Phenotyping of patients with de novo missense variants in KLHL20 was performed.RESULTS:We studied 14 patients with de novo missense variants in KLHL20, delineating a genetic syndrome with patients having mild to severe intellectual disability, febrile seizures or epilepsy, autism spectrum disorder, hyperactivity, and subtle dysmorphic facial features. We observed a recurrent de novo missense variant in 11 patients (NM_014458.4:c.1069G>A p.[Gly357Arg]). The recurrent missense and the 3 other missense variants all clustered in the Kelch-type β-propeller domain of the KLHL20 protein, which shapes the substrate binding surface.CONCLUSION:Our findings implicate KLHL20 in a neurodevelopmental disorder characterized by intellectual disability, febrile seizures or epilepsy, autism spectrum disorder, and hyperactivity.

BACKGROUND:The EPIGENE network was created in 2014 by four multidisciplinary teams composed of geneticists, pediatric neurologists and neurologists specialized in epileptology and neurophysiology. The ambition of the network was to harmonize and improve the diagnostic strategy of Mendelian epileptic disorders using next-generation sequencing, in France. Over the years, five additional centers have joined EPIGENE and the network has been working in close collaboration, since 2018, with the French reference center for rare epilepsies (CRéER). RESULTS:Since 2014, biannual meetings have led to the design of four successive versions of a monogenic epilepsy gene panel (PAGEM), increasing from 68 to 144 genes. A total of 4035 index cases with epileptic disorders have been analyzed with a diagnostic yield of 31% (n = 1265/4035). The top 10 genes, SCN1A, KCNQ2, STXBP1, SCN2A, SCN8A, PRRT2, PCDH19, KCNT1, SYNGAP1, and GRIN2A, account for one-sixth of patients and half of the diagnoses provided by the PAGEM. CONCLUSION:These results suggest that a gene-panel approach is an efficient first-tier test for the genetic diagnosis of Mendelian epileptic disorders. In a near future, French patients with "drug-resistant epilepsies with seizure-onset in the first two-years of life" can benefit from whole-genome sequencing (WGS), as a second line genetic screening with the implementation of the 2025 French Genomic Medicine Plan. The EPIGENE network has also promoted scientific collaborations on genetic epilepsies within CRéER.