Inherited RORB pathogenic variants: Overlap of photosensitive genetic generalized and occipital lobe epilepsy

Exploring the Genetic Landscape of Epilepsy With Eyelid Myoclonia: A Comprehensive Review on Clinical Features and Diagnostic Challenges

Jeavons syndrome (JS), also known as epilepsy with eyelid myoclonia (EEM), is an idiopathic epileptic syndrome that primarily affects children. JS constitutes a significant portion of idiopathic generalized epilepsies and overall epileptic conditions and is characterized by frequent eyelid myoclonia. JS is often triggered by factors such as eyelid closure and exposure to light, leading to absence seizures with photoparoxysmal responses. Although previous studies indicate that some genes have demonstrated an association with the syndrome, no definitive causative gene has yet been identified. The current review therefore aims to shed emphasis on the potential value genetic testing holds in the context of EEM, as well as the need to investigate potential early diagnosis and management strategies in future research.

Molecular and Phenotypic Characterization of the RORB-Related Disorder

BACKGROUND AND OBJECTIVES

Heterozygous variants in RAR-related orphan receptor B (RORB) have recently been associated with susceptibility to idiopathic generalized epilepsy. However, few reports have been published so far describing pathogenic variants of this gene in patients with epilepsy and intellectual disability (ID). In this study, we aimed to delineate the epilepsy phenotype associated with RORB pathogenic variants and to provide arguments in favor of the pathogenicity of variants.

METHODS

Through an international collaboration, we analyzed seizure characteristics, EEG data, and genotypes of a cohort of patients with heterozygous variants in RORB. To gain insight into disease mechanisms, we performed ex vivo cortical electroporation in mouse embryos of 5 selected variants, 2 truncating and 3 missense, and evaluated on expression and quantified changes in axonal morphology.

RESULTS

We identified 35 patients (17 male, median age 10 years, range 2.5-23 years) carrying 32 different heterozygous variants in RORB, including 28 single-nucleotide variants or small insertions/deletions (12 missense, 12 frameshift or nonsense, 2 splice-site variants, and 2 in-frame deletions), and 4 microdeletions; de novo in 18 patients and inherited in 10. Seizures were reported in 31/35 (89%) patients, with a median age at onset of 3 years (range 4 months-12 years). Absence seizures occurred in 25 patients with epilepsy (81%). Nineteen patients experienced a single seizure type: absences, myoclonic absences, or absences with eyelid myoclonia and focal seizures. Nine patients had absence seizures combined with other generalized seizure types. One patient had presented with absences associated with photosensitive occipital seizures. Three other patients had generalized tonic-clonic seizures without absences. ID of variable degree was observed in 85% of the patients. Expression studies in cultured neurons showed shorter axons for the 5 tested variants, both truncating and missense variants, supporting an impaired protein function.

DISCUSSION

In most patients, the phenotype of the RORB-related disorder associates absence seizures with mild-to-moderate ID. In silico and in vitro evaluation of the variants in our cohort, including axonal morphogenetic experiments in cultured neurons, supports their pathogenicity, showing a hypomorphic effect.

RORB, an Alzheimer's disease susceptibility gene, is associated with viral encephalitis, an Alzheimer's disease risk factor

BACKGROUND

Viral encephalitis increases later-life risk of Alzheimer's disease (AD) by a factor of 31.

METHODS

To further evaluate this finding, we examined the relationship of West Nile virus (WNV) to Alzheimer's disease in 50 US states. In addition, we performed a genome wide association study (GWAS) of viral encephalitis cases in UK Biobank (UKBB) to see if encephalitis genes might be related to AD.

RESULTS

WNV was significantly associated with deaths from Alzheimer's disease in 50 US states (r = 0.806, p < 0.001). One gene, RORB-AS1, was most significantly related on GWAS to viral encephalitis. RORB-AS1 (RORB Antisense RNA 1) is an RNA gene. Diseases associated with RORB-AS1 include childhood epilepsy and idiopathic generalized epilepsy. The closely related RORB (Related Orphan Receptor B) is a marker of selectively AD vulnerable excitatory neurons in the entorhinal cortex; these neurons are depleted and susceptible to neurofibrillary inclusions during AD progression. RORB variants significantly decreased the risk of AD, independent of the significant effects of epilepsy, age, and years of education. The total effect size of variant RORB on AD prevalence is small, 0.19%, probably the reason RORB has not turned up on genome wide association studies of AD. But the decrease in effect size on AD, no variant versus varian is larger 0.20-0.16%. To produce the 31-fold increase in AD risk associated with viral encephalitis, non-variant RORB may need to interact with encephalitis virus.

LIMITATIONS

A weakness in our correlative analysis is possible confounding by the ecological fallacy (or ecological inference fallacy), a logical fallacy in the interpretation of statistical data where inferences about the nature of individuals are derived from inference for the group to which those individuals belong. In this case, inferences about individuals are being drawn from the characteristics of U.S. states where they reside, rather than from the individuals themselves. A weakness in our GWAS is that UK Biobank had only 18 cases of viral encephalitis and none of these had AD.

CONCLUSION

data presented here confirm the association of viral encephalitis with AD and suggest that WNV infection is a significant AD risk factor. In addition, GWAS suggests that the gene RORB, an AD vulnerability factor, is significantly related to viral encephalitis.

FUTURE PROSPECTS

A human WNV vaccine could reduce Alzheimer's disease morbidity and mortality.

An allelic series of spontaneous Rorb mutant mice exhibit a gait phenotype, changes in retina morphology and behavior, and gene expression signatures associated with the unfolded protein response

The Retinoid-related orphan receptor beta (RORβ) gene encodes a developmental transcription factor and has 2 predominant isoforms created through alternative first exon usage; one specific to the retina and another present more broadly in the central nervous system, particularly regions involved in sensory processing. RORβ belongs to the nuclear receptor family and plays important roles in cell fate specification in the retina and cortical layer formation. In mice, loss of RORβ causes disorganized retina layers, postnatal degeneration, and production of immature cone photoreceptors. Hyperflexion or "high-stepping" of rear limbs caused by reduced presynaptic inhibition by Rorb-expressing inhibitory interneurons of the spinal cord is evident in RORβ-deficient mice. RORβ variants in patients are associated with susceptibility to various neurodevelopmental conditions, primarily generalized epilepsies, but including intellectual disability, bipolar, and autism spectrum disorders. The mechanisms by which RORβ variants confer susceptibility to these neurodevelopmental disorders are unknown but may involve aberrant neural circuit formation and hyperexcitability during development. Here we report an allelic series in 5 strains of spontaneous Rorb mutant mice with a high-stepping gait phenotype. We show retinal abnormalities in a subset of these mutants and demonstrate significant differences in various behavioral phenotypes related to cognition. Gene expression analyses in all 5 mutants reveal a shared over-representation of the unfolded protein response and pathways related to endoplasmic reticulum stress, suggesting a possible mechanism of susceptibility relevant to patients.

New Insights in 9q21.13 Microdeletion Syndrome: Genotype-Phenotype Correlation of 28 Patients

The implementation of array comparative genomic hybridisation (array-CGH) allows us to describe new microdeletion/microduplication syndromes which were previously not identified. 9q21.13 microdeletion syndrome is a genetic condition due to the loss of a critical genomic region of approximately 750kb and includes several genes, such as RORB and TRPM6. Here, we report a case of a 7-year-old boy affected by 9q21.13 microdeletion syndrome. He presents with global developmental delay, intellectual disability, autistic behaviour, seizures and facial dysmorphism. Moreover, he has severe myopia, which was previously reported in only another patient with 9q21.13 deletion, and brain anomalies which were never described before in 9q21.13 microdeletion syndrome. We also collect 17 patients from a literature search and 10 cases from DECIPHER database with a total number of 28 patients (including our case). In order to better investigate the four candidate genes RORB, TRPM6, PCSK5, and PRUNE2 for neurological phenotype, we make, for the first time, a classification in four groups of all the collected 28 patients. This classification is based both on the genomic position of the deletions included in the 9q21.3 locus deleted in our patient and on the different involvement of the four-candidate gene. In this way, we compare the clinical problems, the radiological findings, and the dysmorphic features of each group and of all the 28 patients in our article. Moreover, we perform the genotype-phenotype correlation of the 28 patients to better define the syndromic spectrum of 9q21.13 microdeletion syndrome. Finally, we propose a baseline ophthalmological and neurological monitoring of this syndrome.

Disease-causing mutations in genes encoding transcription factors critical for photoreceptor development

Photoreceptor development of the vertebrate visual system is controlled by a complex transcription regulatory network. OTX2 is expressed in the mitotic retinal progenitor cells (RPCs) and controls photoreceptor genesis. CRX that is activated by OTX2 is expressed in photoreceptor precursors after cell cycle exit. NEUROD1 is also present in photoreceptor precursors that are ready to specify into rod and cone photoreceptor subtypes. NRL is required for the rod fate and regulates downstream rod-specific genes including the orphan nuclear receptor NR2E3 which further activates rod-specific genes and simultaneously represses cone-specific genes. Cone subtype specification is also regulated by the interplay of several transcription factors such as THRB and RXRG. Mutations in these key transcription factors are responsible for ocular defects at birth such as microphthalmia and inherited photoreceptor diseases such as Leber congenital amaurosis (LCA), retinitis pigmentosa (RP) and allied dystrophies. In particular, many mutations are inherited in an autosomal dominant fashion, including the majority of missense mutations in CRX and NRL. In this review, we describe the spectrum of photoreceptor defects that are associated with mutations in the above-mentioned transcription factors, and summarize the current knowledge of molecular mechanisms underlying the pathogenic mutations. At last, we deliberate the outstanding gaps in our understanding of the genotype-phenotype correlations and outline avenues for future research of the treatment strategies.

Nomenclature of Genetic Movement Disorders: Recommendations of the International Parkinson and Movement Disorder Society Task Force - An Update

In 2016, the Movement Disorder Society Task Force for the Nomenclature of Genetic Movement Disorders presented a new system for naming genetically determined movement disorders and provided a criterion-based list of confirmed monogenic movement disorders. Since then, a substantial number of novel disease-causing genes have been described, which warrant classification using this system. In addition, with this update, we further refined the system and propose dissolving the imaging-based categories of Primary Familial Brain Calcification and Neurodegeneration with Brain Iron Accumulation and reclassifying these genetic conditions according to their predominant phenotype. We also introduce the novel category of Mixed Movement Disorders (MxMD), which includes conditions linked to multiple equally prominent movement disorder phenotypes. In this article, we present updated lists of newly confirmed monogenic causes of movement disorders. We found a total of 89 different newly identified genes that warrant a prefix based on our criteria; 6 genes for parkinsonism, 21 for dystonia, 38 for dominant and recessive ataxia, 5 for chorea, 7 for myoclonus, 13 for spastic paraplegia, 3 for paroxysmal movement disorders, and 6 for mixed movement disorder phenotypes; 10 genes were linked to combined phenotypes and have been assigned two new prefixes. The updated lists represent a resource for clinicians and researchers alike and they have also been published on the website of the Task Force for the Nomenclature of Genetic Movement Disorders on the homepage of the International Parkinson and Movement Disorder Society (https://www.movementdisorders.org/MDS/About/Committees--Other-Groups/MDS-Task-Forces/Task-Force-on-Nomenclature-in-Movement-Disorders.htm). © 2022 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson Movement Disorder Society.

Visually sensitive seizures: An updated review by the Epilepsy Foundation

Light flashes, patterns, or color changes can provoke seizures in up to 1 in 4000 persons. Prevalence may be higher because of selection bias. The Epilepsy Foundation reviewed light-induced seizures in 2005. Since then, images on social media, virtual reality, three-dimensional (3D) movies, and the Internet have proliferated. Hundreds of studies have explored the mechanisms and presentations of photosensitive seizures, justifying an updated review. This literature summary derives from a nonsystematic literature review via PubMed using the terms "photosensitive" and "epilepsy." The photoparoxysmal response (PPR) is an electroencephalography (EEG) phenomenon, and photosensitive seizures (PS) are seizures provoked by visual stimulation. Photosensitivity is more common in the young and in specific forms of generalized epilepsy. PS can coexist with spontaneous seizures. PS are hereditable and linked to recently identified genes. Brain imaging usually is normal, but special studies imaging white matter tracts demonstrate abnormal connectivity. Occipital cortex and connected regions are hyperexcitable in subjects with light-provoked seizures. Mechanisms remain unclear. Video games, social media clips, occasional movies, and natural stimuli can provoke PS. Virtual reality and 3D images so far appear benign unless they contain specific provocative content, for example, flashes. Images with flashes brighter than 20 candelas/m² at 3-60 (particularly 15-20) Hz occupying at least 10 to 25% of the visual field are a risk, as are red color flashes or oscillating stripes. Equipment to assay for these characteristics is probably underutilized. Prevention of seizures includes avoiding provocative stimuli, covering one eye, wearing dark glasses, sitting at least two meters from screens, reducing contrast, and taking certain antiseizure drugs. Measurement of PPR suppression in a photosensitivity model can screen putative antiseizure drugs. Some countries regulate media to reduce risk. Visually-induced seizures remain significant public health hazards so they warrant ongoing scientific and regulatory efforts and public education.

Genotype-Phenotype Correlations in Relation to Newly Emerging Monogenic Forms of Autism Spectrum Disorder and Associated Neurodevelopmental Disorders: The Importance of Phenotype Reevaluation after Pangenomic Results

ASD genetic diagnosis has dramatically improved due to NGS technologies, and many new causative genes have been discovered. Consequently, new ASD phenotypes have emerged. An extensive exome sequencing study carried out by the Autism Sequencing Consortium (ASC) was published in February 2020. The study identified 102 genes which are de novo mutated in subjects affected by autism spectrum disorder (ASD) or similar neurodevelopmental disorders (NDDs). The majority of these genes was already known to be implicated in ASD or NDDs, whereas approximately 30 genes were considered “novel” as either they were not previously associated with ASD/NDDs or very little information about them was present in the literature. The aim of this work is to review the current literature since the publication of the ASC paper to see if new data mainly concerning genotype–phenotype correlations of the novel genes have been added to the existing one. We found new important clinical and molecular data for 6 of the 30 novel genes. Though the broad and overlapping neurodevelopmental phenotypes observed in most monogenic forms of NDDs make it difficult for the clinical geneticist to address gene-specific tests, knowledge of these new data can at least help to prioritize and interpret results of pangenomic tests to some extent. Indeed, for some of the new emerging genes analyzed in the present work, specific clinical features emerged that may help the clinical geneticist to make the final diagnosis by associating the genetic test results with the phenotype. The importance of this relatively new approach known as “reverse phenotyping” will be discussed.

Candidate Genes for Eyelid Myoclonia with Absences, Review of the Literature

Eyelid myoclonia with absences (EMA), also known as Jeavons syndrome (JS) is a childhood onset epileptic syndrome with manifestations involving a clinical triad of absence seizures with eyelid myoclonia (EM), photosensitivity (PS), and seizures or electroencephalogram (EEG) paroxysms induced by eye closure. Although a genetic contribution to this syndrome is likely and some genetic alterations have been defined in several cases, the genes responsible for have not been identified. In this review, patients diagnosed with EMA (or EMA-like phenotype) with a genetic diagnosis are summarized. Based on this, four genes could be associated to this syndrome (SYNGAP1, KIA02022/NEXMIF, RORB, and CHD2). Moreover, although there is not enough evidence yet to consider them as candidate for EMA, three more genes present also different alterations in some patients with clinical diagnosis of the disease (SLC2A1, NAA10, and KCNB1). Therefore, a possible relationship of these genes with the disease is discussed in this review.

Eyelid myoclonia with absences, intellectual disability and attention deficit hyperactivity disorder: a clinical phenotype of the RORB gene mutation

Eyelid myoclonia with absences is recently included in the category of childhood epileptic syndromes. It is clinically characterized by brief seizures of eyelid myoclonia, sometimes followed by absences, and it is associated to EEG generalized discharges of polyspikes or polyspike-waves, which are triggered by eyes closure in a well-lit room. This epileptic syndrome probably has a genetic origin, as well as other genetic generalized epilepsies, in particular photosensitive epilepsies. We describe the case of a patient affected by eyelid myoclonia with absences, intellectual disability, and attention deficit hyperactivity disorder (ADHD), with a de novo mutation of the RORB gene (retinoid-related orphan receptor β); this gene is involved in vivo in different neuronal processes among which are migration and differentiation. We suggest that its mutation in our patient can be considered the cause of the aberrant functioning of the cerebral cortex, which is clinically expressed by epilepsy and neurodevelopment disorders.

Generalized, focal, and combined epilepsies in families: New evidence for distinct genetic factors

OBJECTIVE

To determine the roles of shared and distinct genetic influences on generalized and focal epilepsy operating in individuals who manifest features of both types (combined epilepsies), and in families manifesting both generalized and focal epilepsies in separate individuals (mixed families).

METHODS

We analyzed the deeply phenotyped Epi4K cohort of multiplex families (≥3 affected individuals per family) using methods that quantify the aggregation of phenotypes within families and the relatedness of individuals with different phenotypes within family pedigrees.

RESULTS

The cohort included 281 families containing 1021 individuals with generalized (n = 484), focal (304), combined (51), or unclassified (182) epilepsies. The odds of combined epilepsy was higher in relatives of participants with combined epilepsy than in relatives of those with other epilepsy types (odds ratio [OR] 5.2, 95% confidence interval [CI] 1.7-16.1, P = .004). Individuals with combined epilepsy co-occurred in families more often than expected by chance (P = .03). Within mixed families, individuals with each type of epilepsy were more closely related to relatives with the same type than to relatives with other types (P < .001).

SIGNIFICANCE

These findings suggest that distinct genetic influences underlie the recently recognized entity of combined epilepsies, just as generalized epilepsies and focal epilepsies each have distinct genetic influences. Mixed families may in part reflect chance co-occurrence of these distinct genetic influences. These conclusions have important implications for molecular genetic studies aimed at identifying genetic determinants of the epilepsies.