Safety and efficacy of deoxycytidine/deoxythymidine combination therapy in POLG-related disorders: 6-month interim results of an open-label, single arm, phase 2 trial.

Background: DNA polymerase gamma (POLG)-related disorders are a group of rare neurodegenerative mitochondrial diseases caused by pathogenic variants in POLG, the gene encoding POLG. Patients may experience a range of signs and symptoms, including seizures, vision loss, myopathy, neuropathy, developmental impairment or regression, and liver failure. The diseases follow a progressive, degenerative course, with most affected individuals dying within 3 months-12 years of diagnosis. At present, there are no effective treatments for POLG-related disorders. Methods: In this study we report the interim 6-month data from a long term open-label, single arm phase 2 trial, in which we assessed the safety and efficacy of combination therapy with deoxycytidine and deoxythymidine (dC/dT) in children with POLG-related disorders. dC/dT was given enterally in powder form, dissolved in water. The primary outcome measures included Newcastle Mitochondrial Disease Scale (NMDS) score, serum growth differentiation factor 15 (GDF-15; a biomarker of mitochondrial dysfunction), electroencephalography (EEG), seizure diary, and blood and urine tests to assess end organ and mitochondrial function. Secondary outcome measures included recording of all adverse events to evaluate the safety of the intervention. The trial is registered with ClinicalTrials.gov, NCT04802707 (https://clinicaltrials.gov/ct2/show/NCT04802707). Data were collected from 14 October, 2021 to 13 December, 2023. Findings: We present 6-month interim data from the first ten people with POLG-related disorders enrolled in the trial, six with Alpers-Huttenlocher syndrome, two with ataxia-neuropathy spectrum, and two who do not fit into a classical POLG-related phenotype. During the 6 months of treatment, NMDS score improved from a mean of 27.3 at baseline to 20.7 at 6 months (estimated difference 6.0; 95% CI 2.5-∞). GDF-15 values remained stable or decreased in all patients; the mean decreased from 1031 pg/ml to 729 pg/ml (estimated difference 200; 95% CI 12-∞). 8/10 patients had abnormal baseline EEG; improvement in EEG was seen in 5 of these 8. There were no significant changes in other blood and urine testing. Regarding adverse events, two patients experienced diarrhea that spontaneously resolved. Interpretation: dC/dT is a promising treatment option for people with POLG-related disorders. Further research is needed to assess the long-term safety and efficacy in POLG-related disorders, as well as safety and efficacy in other mitochondrial DNA depletion disorders. Funding: This study was primarily funded by the Liam Foundation, with additional funding from the Savoy Foundation, Grand Défi Pierre Lavoie Foundation, and Fonds de Recherche du Québec - Santé.

Understanding of Consciousness in Absence Seizures: A Literature Review.

Absence seizures are classically associated with behavioral arrest and transient deficits in consciousness, yet substantial variability exists in the severity of the impairment. Despite several decades of research on the topic, the pathophysiology of absence seizures and the mechanisms underlying behavioral impairment remain unclear. Several rationales have been proposed including widespread cortical deactivation, reduced perception of external stimuli, and transient suspension of the default mode network, among others. This review aims to summarize the current knowledge on the neural correlates of impaired consciousness in absence seizures. We review evidence from studies using animal models of absence epilepsy, electroencephalography, functional magnetic resonance imaging, magnetoencephalography, positron emission tomography, and single photon emission computed tomography.

Koolen-de Vries Syndrome: a journey from diagnosis to treatments.

The Koolen-de Vries Syndrome Foundation was founded in 2013 with the mission to educate, increase awareness, promote research and develop treatments for individuals living with Koolen-de Vries Syndrome (KdVS) and their families. With this aim, the foundation has focused on: developing scientific resources through patient cell and animal models, providing seed funding to basic and clinical researchers, establishing a natural history study of KdVS and increasing patient engagement. Projects have been prioritized across these areas of focus with an emphasis on expanding international research on KdVS, supporting translational research, establishing an international natural history study and conducting studies to assess patient priorities. With the incredible growth amongst our research and patient community in the last decade, our goal is to have our first clinical trial for KdVS in 2026.

Koolen de-Vries Syndrome: a journey from diagnosis to treatments The Koolen-de Vries Syndrome Foundation ('KdVSF') was founded in 2013 with the mission to develop treatments for all individuals diagnosed with KdVS. With this aim, we have focused on several research priorities for our community: developing cell and animal models for KdVS for our researchers to utilize for experiments, providing research grants to KdVS basic and clinical researchers, establishing a natural history study of KdVS and increasing patient engagement and diversity. The KdVS research and patient community has expanded tremendously over the last decade, and there is growing excitement over the possible treatments currently being investigated amongst KdVS researchers. With our current focus on translational research and research aimed at identifying treatment strategies in KdVS patients, our goal is to have our first clinical trial for KdVS in late 2026.

Sulthiame use in children with pharmacoresistant epilepsies: A retrospective study.

OBJECTIVE: This retrospective study aimed to assess the efficacy and tolerability of sulthiame as an add-on treatment in children with pharmacoresistant epilepsies. METHODS: All patients with epilepsy who received sulthiame at Montreal Children's Hospital over an 11-year period were included. Medical charts were reviewed, and extracted data included patient age and sex, seizure types, epilepsy syndrome, electroencephalography (EEG) reports, brain imaging reports, antiseizure treatments trialed, starting and final dose of sulthiame, duration of sulthiame treatment, adverse events attributed to sulthiame, and seizure frequency before and after sulthiame treatment. EEG studies were also analyzed and spike-wave index (SWI) in the first 10 min of sleep was calculated. RESULTS: Sixteen patients were included, all of whom had pharmacoresistant epilepsies (mean of 9.9 trials of other antiseizure treatments). Six had genetic diagnoses, four had in utero/perinatal acquired brain injury, one had a suspected focal cortical dysplasia, and five were idiopathic. Ten patients had developmental and epileptic encephalopathy with spike-wave activation in sleep, three had Lennox-Gastaut syndrome, and one each had sleep-related hyperkinetic epilepsy, self-limited epilepsy with centrotemporal spikes, and mixed generalized and multifocal epilepsy. Of the 12 patients with uncontrolled seizures at the time of sulthiame initiation, 4 had improvement in seizure frequency, including 2 who became seizure free. Eight patients had EEG data available that allowed calculation of sleep SWI; from this group, SWI decreased from 81.1% +/- 17.6% to 45.1% +/- 36.5% (p = .007). The most common adverse events reported were somnolence/drowsiness, aggression, and increased seizure frequency. Of the patients with genetic etiologies, significant positive responses were seen in patients with pathogenic variants in NDUFS1 and SATB1. SIGNIFICANCE: These data demonstrate the therapeutic potential of sulthiame, even in patients with highly pharmacoresistant epilepsy. Improvements may be seen in both seizure frequency and sleep SWI.

Sociocultural Factors Influence on Burden and Stress of Caregivers of Children with Epilepsy.

We evaluated stress and burden in epilepsy patient caregivers in a pediatric neurology clinic. Caregivers of 102 children with epilepsy completed the Caregivers' Assessment of Difficulty Index and a questionnaire regarding caregiver sociocultural characteristics. A multiple linear regression statistical analysis found that caregiver burden was significantly increased for those who had a second child with a chronic disease, sole caregivers and for those with children with drug-resistant epilepsy. Caregiver stress was significantly increased for caregivers with a native language other than English or French, caregivers who had a second child with a chronic disease and sole caregivers.

Genotype-phenotype correlation in CLCN4-related developmental and epileptic encephalopathy.

CLCN4-related disorder is a rare X-linked neurodevelopmental condition with a pathogenic mechanism yet to be elucidated. CLCN4 encodes the vesicular 2Cl-/H+ exchanger ClC-4, and CLCN4 pathogenic variants frequently result in altered ClC-4 transport activity. The precise cellular and molecular function of ClC-4 remains unknown; however, together with ClC-3, ClC-4 is thought to have a role in the ion homeostasis of endosomes and intracellular trafficking. We reviewed our research database for patients with CLCN4 variants and epilepsy, and performed thorough phenotyping. We examined the functional properties of the variants in mammalian cells using patch-clamp electrophysiology, protein biochemistry, and confocal fluorescence microscopy. Three male patients with developmental and epileptic encephalopathy were identified, with differing phenotypes. Patients #1 and #2 had normal growth parameters and normal-appearing brains on MRI, while patient #3 had microcephaly, microsomia, complete agenesis of the corpus callosum and cerebellar and brainstem hypoplasia. The p.(Gly342Arg) variant of patient #1 significantly impaired ClC-4's heterodimerization capability with ClC-3 and suppressed anion currents. The p.(Ile549Leu) variant of patient #2 and p.(Asp89Asn) variant of patient #3 both shift the voltage dependency of transport activation by 20 mV to more hyperpolarizing potentials, relative to the wild-type, with p.(Asp89Asn) favouring higher transport activity. We concluded that p.(Gly342Arg) carried by patient #1 and the p.(Ile549Leu) expressed by patient #2 impair ClC-4 transport function, while the p.(Asp89Asn) variant results in a gain-of-transport function; all three variants result in epilepsy and global developmental impairment, but with differences in epilepsy presentation, growth parameters, and presence or absence of brain malformations.

Defining the Genetic Landscape of Congenital Mirror Movements in 80 Affected Individuals.

BACKGROUND: Congenital mirror movements (CMM) is a rare neurodevelopmental disorder characterized by involuntary movements from one side of the body that mirror voluntary movements on the opposite side. To date, five genes have been associated with CMM, namely DCC, RAD51, NTN1, ARHGEF7, and DNAL4. OBJECTIVE: The aim of this study is to characterize the genetic landscape of CMM in a large group of 80 affected individuals. METHODS: We screened 80 individuals with CMM from 43 families for pathogenic variants in CMM genes. In large CMM families, we tested for presence of pathogenic variants in multiple affected and unaffected individuals. In addition, we evaluated the impact of three missense DCC variants on binding between DCC and Netrin-1 in vitro. RESULTS: Causal pathogenic/likely pathogenic variants were found in 35% of probands overall, and 70% with familial CMM. The most common causal gene was DCC, responsible for 28% of CMM probands and 80% of solved cases. RAD51, NTN1, and ARHGEF7 were rare causes of CMM, responsible for 2% each. Penetrance of CMM in DCC pathogenic variant carriers was 68% and higher in males than females (74% vs. 54%). The three tested missense variants (p.Ile164Thr; p.Asn176Ser; and p.Arg1343His) bind Netrin-1 similarly to wild type DCC. CONCLUSIONS: A genetic etiology can be identified in one third of CMM individuals, with DCC being the most common gene involved. Two thirds of CMM individuals were unsolved, highlighting that CMM is genetically heterogeneous and other CMM genes are yet to be discovered. © 2024 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Acute encephalopathy with biphasic seizures and late reduced diffusion with concurrent transverse myelitis.

We describe a patient with acute encephalopathy with biphasic seizures and late reduced diffusion (AESD) with unique features, including concurrent transverse myelitis. A 2-year-old previously healthy girl had clinical findings consistent with AESD, occurring in association with influenza A infection. The posterior brain regions were most severely affected, resulting in cortical blindness. She also developed bilateral limb weakness, and spine MRI revealed transverse myelitis in the cervical region. She was treated acutely with intravenous methylprednisolone. Serum anti-myelin oligodendrocyte glycoprotein and anti-aquaporin-4 antibodies were negative, as was an anti-extractable nuclear antigen panel. Although her clinical presentation was severe, she improved dramatically over the following months, and 6 months following initial presentation, her parents felt she had returned to baseline. This is the first report of AESD occurring in combination with transverse myelitis. The co-occurrence of the two conditions is unlikely to be coincidental, suggesting that there may be a shared or overlapping immunological pathway involved. The patient's recovery was impressive, which could partially relate to the acute treatment with corticosteroids.

Utility of genetic testing in the pre-surgical evaluation of children with drug-resistant epilepsy.

We evaluated the utility of genetic testing in the pre-surgical evaluation of pediatric patients with drug-resistant focal epilepsy. This single-center retrospective study reviewed the charts of all pediatric patients referred for epilepsy surgery evaluation over a 5-year period. We extracted and analyzed results of genetic testing as well as clinical, EEG, and neuroimaging data. Of 125 patients referred for epilepsy surgical evaluation, 86 (69%) had some form of genetic testing. Of these, 18 (21%) had a pathogenic or likely pathogenic variant identified. Genes affected included NPRL3 (3 patients, all related), TSC2 (3 patients), KCNH1, CHRNA4, SPTAN1, DEPDC5, SCN2A, ARX, SCN1A, DLG4, and ST5. One patient had ring chromosome 20, one a 7.17p12 duplication, and one a 15q13 deletion. In six patients, suspected epileptogenic lesions were identified on brain MRI that were thought to be unrelated to the genetic finding. A specific medical therapy choice was allowed due to genetic diagnosis in three patients who did not undergo surgery. Obtaining a molecular diagnosis may dramatically alter management in pediatric patients with drug-resistant focal epilepsy. Genetic testing should be incorporated as part of standard investigations in the pre-surgical work-up of pediatric patients with drug-resistant focal epilepsy.

Epilepsy in Legius syndrome: Coincidence or causation?

Legius syndrome is a rare genetic disorder, caused by heterozygous SPRED1 pathogenic variants, which shares phenotypic features with neurofibromatosis type 1 (NF1). Both conditions typically involve café-au-lait macules, axillary freckling, and macrocephaly; however, patients with NF1 are also at risk for tumors, such as optic nerve gliomas and neurofibromas. Seizure risk is known to be elevated in NF1, but there has been little study of this aspect of Legius syndrome. The reported epilepsy incidence is 3.3%-5%, well above the general population incidence of ~0.5%-1%, but the few reports in the literature have very little data regarding epilepsy phenotype. We identified two unrelated individuals, both with Legius syndrome and epilepsy, and performed thorough phenotyping. One individual's mother also had Legius syndrome and now-resolved childhood epilepsy, as well as reports of more distant relatives who also had multiple café-au-lait macules and seizures. Both probands had experienced childhood-onset focal seizures, with normal brain MRI. In one patient, EEG later showed apparently generalized epileptiform abnormalities. Based on the data from this small case series and literature review, seizure risk is increased in people with Legius syndrome, but the epilepsy prognosis appears to be generally good, with patients having either self-limited or pharmacoresponsive courses.

mTOR Pathway Somatic Pathogenic Variants in Focal Malformations of Cortical Development: Novel Variants, Topographic Mapping, and Clinical Outcomes.

Background and Objectives: Somatic and germline pathogenic variants in genes of the mammalian target of rapamycin (mTOR) signaling pathway are a common mechanism underlying a subset of focal malformations of cortical development (FMCDs) referred to as mTORopathies, which include focal cortical dysplasia (FCD) type II, subtypes of polymicrogyria, and hemimegalencephaly. Our objective is to screen resected FMCD specimens with mTORopathy features on histology for causal somatic variants in mTOR pathway genes, describe novel pathogenic variants, and examine the variant distribution in relation to neuroimaging, histopathologic classification, and clinical outcomes. Methods: We performed ultra-deep sequencing using a custom HaloPlexHS Target Enrichment kit in DNA from 21 resected fresh-frozen histologically confirmed FCD type II, tuberous sclerosis complex, or hemimegalencephaly specimens. We mapped the variant alternative allele frequency (AAF) across the resected brain using targeted ultra-deep sequencing in multiple formalin-fixed paraffin-embedded tissue blocks. We also functionally validated 2 candidate somatic MTOR variants and performed targeted RNA sequencing to validate a splicing defect associated with a novel DEPDC5 variant. Results: We identified causal mTOR pathway gene variants in 66.7% (14/21) of patients, of which 13 were somatic with AAF ranging between 0.6% and 12.0%. Moreover, the AAF did not predict balloon cell presence. Favorable seizure outcomes were associated with genetically clear resection borders. Individuals in whom a causal somatic variant was undetected had excellent postsurgical outcomes. In addition, we demonstrate pathogenicity of the novel c.4373_4375dupATG and candidate c.7499T>A MTOR variants in vitro. We also identified a novel germline aberrant splice site variant in DEPDC5 (c.2802-1G>C). Discussion: The AAF of somatic pathogenic variants correlated with the topographic distribution, histopathology, and postsurgical outcomes. Moreover, cortical regions with absent histologic FCD features had negligible or undetectable pathogenic variant loads. By contrast, specimens with frank histologic abnormalities had detectable pathogenic variant loads, which raises important questions as to whether there is a tolerable variant threshold and whether surgical margins should be clean, as performed in tumor resections. In addition, we describe 2 novel pathogenic variants, expanding the mTORopathy genetic spectrum. Although most pathogenic somatic variants are located at mutation hotspots, screening the full-coding gene sequence remains necessary in a subset of patients.

Electroencephalographic Patterns in Pediatric Patients With Multisystem Inflammatory Syndrome in Children and Coronavirus Disease 2019 Coinfection.

BACKGROUND: Children with recent or acute coronavirus disease 2019 (COVID-19) infections are susceptible to a number of neurological complications, including encephalopathy and seizures. Within the phenomenon of multisystem inflammatory syndrome in children (MIS-C), patients may be encephalopathic or have other nervous system sequelae. The electroencephalographic (EEG) patterns accompanying neurological complications of COVID-19 infection have been reported but primarily in case reports or small case series. METHODS: In this study, we reviewed all reports of EEG patterns seen in pediatric patients with presentations attributed to COVID-19 infection. RESULTS: Fifty patient reports were identified, drawn from 27 articles. We separately analyzed patients whose primary neurological concern was (1) encephalopathy, (2) seizures, or (3) other neurological abnormalities. Patients with acute encephalopathy tend to have EEG showing diffusely slow background, often in the delta range; however, the pattern of slowing is sometimes anterior or posterior predominant and may evolve over the course of illness. CONCLUSIONS: Patients with COVID-19 infection presenting with seizures may have focal or bilateral semiologies, but postictal EEG rarely shows interictal epileptiform discharges and is more likely to also show diffuse slowing. However, subclinical seizures and nonconvulsive status epilepticus have been reported, so prolonged EEG monitoring may still be indicated.

Ataxia and Diplopia: A New SCN8A-Related Phenotype.

Objectives: The objective of this study was to describe the first patient with recurrent ataxia and diplopia in association with a pathogenic variant in SCN8A. Methods: We identified a girl with a heterozygous SCN8A pathogenic variant and performed thorough phenotyping. Results: A 10-year-old girl was previously well with normal intelligence. She had recurrent diplopia, dysmetria, and unsteady gait, which occurred only in the context of febrile illnesses. EEG during her initial acute episode showed multifocal epileptiform discharges, with similar findings seen on a follow-up study 3 months later when she was well. Brain MRI finding was normal. A gene panel identified a de novo SCN8A variant, p.Arg847Gln, classified as likely pathogenic. One year after her initial presentation, the girl is well and developmentally normal and has never had an event concerning for seizure. Discussion: This case presentation demonstrates that SCN8A pathogenic variants should be considered in children with transient ataxia, dysmetria, and diplopia in the context of viral febrile illnesses, even if there is no history of seizures. While there are clinical and molecular data suggesting that SCN8A dysfunction can cause temperature-sensitive phenotypes, further research is necessary to determine how the functional changes caused by our patient's SCN8A variant result in her unique phenotype.

Sleep and respiratory abnormalities in adults with developmental and epileptic encephalopathies using polysomnography and video-EEG monitoring.

This study evaluated sleep and respiratory abnormalities, and their relationship with seizures, in adults with developmental and epileptic encephalopathies (DEEs). We studied consecutive adults with DEEs undergoing inpatient video-EEG monitoring and concurrent polysomnography between December 2011 and July 2022. Thirteen patients with DEEs were included (median age: 31 years, range: 20-50; 69.2% female): Lennox-Gastaut syndrome (n = 6), Lennox-Gastaut syndrome-like phenotype (n = 2), Landau-Kleffner syndrome (n = 1), epilepsy with myoclonic-atonic seizures (n = 1), and unclassified DEEs (n = 3). Sleep architecture was often fragmented by epileptiform discharges and seizures resulting in arousals (median arousal index: 29.0 per h, range: 5.1-65.3). Moderate-to-severe obstructive sleep apnea (OSA) was observed in seven patients (53.8%). Three patients (23.1%) had tonic seizures that frequently occurred with central apnea; one met criteria for mild central sleep apnea. Of the patients with tonic seizures, two had other identifiable seizure manifestations, but in one patient, central apnea was commonly the only discernable seizure manifestation. Polysomnography during video-EEG is an effective diagnostic tool in detecting sleep and seizure-related respiratory abnormalities. Clinically significant OSA may increase the risk of comorbid cardiovascular disease and premature mortality. Treatment of epilepsy may improve sleep quality, and conversely, improved sleep, may decrease seizure burden.

SCN1A as a therapeutic target for Dravet syndrome.

INTRODUCTION: Dravet syndrome is a severe early infancy-onset developmental and epileptic encephalopathy. Patients have drug-resistant seizures, as well as significant co-morbidities, including developmental impairment, crouch gait, sleep disturbance, and early mortality. The underlying cause is mutations in SCN1A, encoding the sodium channel subunit NaV1.1, in >90% of patients. At present, approved Dravet syndrome treatments are symptomatic, primarily aimed at reducing seizure frequency, but having little to no effect on co-morbidities. AREAS COVERED: We discuss the potential to treat Dravet syndrome by targeting NaV1.1 directly. Anti-seizure medications that act as sodium channel inhibitors are generally minimally effective and can actually exacerbate seizures. However, other interventions are currently under investigation, including gene therapies that increase the amount of functional NaV1.1. Some of these interventions have encouraging pre-clinical data from in vitro and animal models. EXPERT OPINION: Increasing functional NaV1.1 via antisense oligonucleotides or virus-borne vectors is the most promising avenue for meaningful improvement in Dravet syndrome treatment, with the potential to not only reduce seizures but also address the multiple co-morbidities associated with this disease. However, human clinical trial data are necessary to determine safety and to clarify if, and to what extent, these interventions modify the natural history of Dravet syndrome.

Cation leak: a common functional defect causing HCN1 developmental and epileptic encephalopathy.

Pathogenic variants in HCN1 are an established cause of developmental and epileptic encephalopathy (DEE). To date, the stratification of patients with HCN1-DEE based on the biophysical consequence on channel function of a given variant has not been possible. Here, we analysed data from eleven patients carrying seven different de novo HCN1 pathogenic variants located in the transmembrane domains of the protein. All patients were diagnosed with severe disease including epilepsy and intellectual disability. The functional properties of the seven HCN1 pathogenic variants were assessed using two-electrode voltage-clamp recordings in Xenopus oocytes. All seven variants showed a significantly larger instantaneous current consistent with cation leak. The impact of each variant on other biophysical properties was variable, including changes in the half activation voltage and activation and deactivation kinetics. These data suggest that cation leak is an important pathogenic mechanism in HCN1-DEE. Furthermore, published mouse model and clinical case reports suggest that seizures are exacerbated by sodium channel blockers in patients with HCN1 variants that cause cation leak. Stratification of patients based on their 'cation leak' biophysical phenotype may therefore provide key information to guide clinical management of individuals with HCN1-DEE.

Awake Craniotomy Language Mapping in Children With Drug-Resistant Epilepsy due to Focal Cortical Dysplasia.

BACKGROUND: Language mapping during awake craniotomy can allow for precise resection of epileptogenic lesions, while reducing the risk of damage to eloquent cortex. There are few reports in the literature of language mapping during awake craniotomy in children with epilepsy. Some centers may avoid awake craniotomy in the pediatric age group due to concerns that children are unable to cooperate with such procedures. METHODS: We reviewed pediatric patients from our center with drug-resistant focal epilepsy who underwent language mapping during awake craniotomy and subsequent resection of the epileptogenic lesion. RESULTS: Two patients were identified, both female, aged 17 years and 11 years at the time of surgery. Both patients had frequent and disabling focal seizures despite trials of multiple antiseizure medications. Both patients had resection of their epileptogenic lesions with the aid of intraoperative language mapping; in both cases pathology was consistent with focal cortical dysplasia. Both patients had transient language difficulties in the immediate postoperative period but no deficits at six-month follow-up. Both patients are now seizure-free. CONCLUSIONS: Awake craniotomy should be considered in pediatric patients with drug-resistant epilepsy in whom the suspected epileptogenic lesion is in close proximity to cortical language areas.

Clinical and functional heterogeneity associated with the disruption of retinoic acid receptor beta.

PURPOSE: Dominant variants in the retinoic acid receptor beta (RARB) gene underlie a syndromic form of microphthalmia, known as MCOPS12, which is associated with other birth anomalies and global developmental delay with spasticity and/or dystonia. Here, we report 25 affected individuals with 17 novel pathogenic or likely pathogenic variants in RARB. This study aims to characterize the functional impact of these variants and describe the clinical spectrum of MCOPS12. METHODS: We used in vitro transcriptional assays and in silico structural analysis to assess the functional relevance of RARB variants in affecting the normal response to retinoids. RESULTS: We found that all RARB variants tested in our assays exhibited either a gain-of-function or a loss-of-function activity. Loss-of-function variants disrupted RARB function through a dominant-negative effect, possibly by disrupting ligand binding and/or coactivators' recruitment. By reviewing clinical data from 52 affected individuals, we found that disruption of RARB is associated with a more variable phenotype than initially suspected, with the absence in some individuals of cardinal features of MCOPS12, such as developmental eye anomaly or motor impairment. CONCLUSION: Our study indicates that pathogenic variants in RARB are functionally heterogeneous and associated with extensive clinical heterogeneity.