Severe paroxysmal dyskinesias without epilepsy in a RHOBTB2 mutation carrier

Deregulated ion channels contribute to RHOBTB2-associated developmental and epileptic encephalopathy

While de novo missense variants in the BTB domains of atypical RhoGTPase RHOBTB2 cause a severe developmental and epileptic encephalopathy, de novo missense variants in the GTPase domain or bi-allelic truncating variants are associated with more variable neurodevelopmental and seizure phenotypes. Apart from the observation of RHOBTB2 abundance resulting from BTB-domain variants and increased seizure susceptibility in Drosophila overexpressing RhoBTB, our knowledge on RHOBTB2-related pathomechanisms is limited. We now found enrichment for ion channels among the differentially expressed genes from RNA-Seq on fly heads overexpressing RhoBTB. Subsequent genetic interaction experiments confirmed a functional link between RhoBTB and paralytic, the orthologue of human sodium channels, including epilepsy associated SCN1A, in vivo. We then performed patch-clamp recordings on mature neurons differentiated from human induced pluripotent stem cells with either homozygous frameshifts or patient-specific heterozygous missense variants in the GTPase or the BTB domains. This revealed significantly altered neuronal activity and excitability resulting from BTB domain variants but not from GTPase domain variants or upon complete loss of RHOBTB2. Our study indicates a role of deregulated ion channels in the pathogenesis of RHOBTB2-related developmental and epileptic encephalopathy and points to specific pathomechanisms underlying the observed genotype-phenotype correlations regarding variant zygosity, location and nature.

RHOBTB2 Variant p.Arg511Gln Causes Developmental and Epileptic Encephalopathy Type 64 in an Infant: A Case Report and Hotspot Variant Analysis

BACKGROUND

Developmental and epileptic encephalopathies (DEEs) are a heterogeneous group of brain disorders. Variants in the Rho-related BTB domain-containing 2 gene (RHOBTB2) can lead to DEE64, which is characterized by early-onset epilepsy, varying degrees of motor developmental delay and intellectual disability, microcephaly, and movement disorders. More than half of the variants are located at Arg483 and Arg511 within the BTB domain; however, the underlying mechanism of action of these hotspot variants remains unexplored.

METHODS

We performed whole-exome and Sanger sequencing on the patient and his parents. We collected recurrent variant information from the literature on RHOBTB2 variants. We used Discovery Studio software to analyze the folding free energy of variant proteins, and the AlphaFold database to analyze structural alterations in mutant proteins.

RESULTS

The patient presented with early-onset epilepsy, developmental delay, and brain structural abnormalities. Genetic analysis revealed a de novo variant in RHOBTB2, c.1532G>A, p.(Arg511Gln). To date, 60 cases of DEE patients with RHOBTB2 variants have been reported, with approximately 50% of variants located at Arg483 and Arg511. Among them, p.Arg511Gln, p.Arg483His, and p.Arg511Trp have an incidence rate exceeding 10%. The folding free energy of these high-frequency variants proteins is reduced, which may lead to increased structural stability.

CONCLUSION

This study highlights the importance of RHOBTB2 hotspot variants in DEE64 and provides insights into their potential mechanisms of action. We recommend RHOBTB2 gene testing for patients with relevant clinical manifestations to facilitate precise diagnosis and treatment of DEE.

Exploring the Spectrum of RHOBTB2 Variants Associated with Developmental Encephalopathy 64: A Case Series and Literature Review

Background

Rho-related BTB domain-containing protein 2 (RHOBTB2) is a protein that interacts with cullin-3, a crucial E3 ubiquitin ligase for mitotic cell division. RHOBTB2 has been linked to early infantile epileptic encephalopathy, autosomal dominant type 64 (OMIM618004), in 34 reported patients.

Methods

We present a case series of seven patients with RHOBTB2-related disorders (RHOBTB2-RD), including a description of a novel heterozygous variant. We also reviewed previously published cases of RHOBTB2-RD.

Results

The seven patients had ages ranging from 2 years and 8 months to 26 years, and all had experienced seizures before the age of one (onset, 4-12 months, median, 4 months), including various types of seizures. All patients in this cohort also had a movement disorder (onset, 0.3-14 years, median, 1.5 years). Six of seven had a baseline movement disorder, and one of seven only had paroxysmal dystonia. Stereotypies were noted in four of six, choreodystonia in three of six, and ataxia in one case with multiple movement phenotypes at baseline. Paroxysmal movement disorders were observed in six of seven patients for whom carbamazepine or oxcarbazepine treatment was effective in controlling acute or paroxysmal movement disorders. Four patients had acute encephalopathic episodes at ages 4 (one patient) and 6 (three patients), which improved following treatment with methylprednisolone. Magnetic resonance imaging scans revealed transient fluid-attenuated inversion recovery abnormalities during these episodes, as well as myelination delay, thin corpus callosum, and brain atrophy. One patient had a novel RHOBTB2 variant (c.359G>A/p.Gly120Glu).

Conclusion

RHOBTB2-RD is characterized by developmental delay or intellectual disability, early-onset seizures, baseline movement disorders, acute or paroxysmal motor phenomena, acquired microcephaly, and episodes of acute encephalopathy. Early onsets of focal dystonia, acute encephalopathic episodes, episodes of tongue protrusion, or peripheral vasomotor disturbances are important diagnostic clues. Treatment with carbamazepine or oxcarbazepine was found to be effective in controlling acute or paroxysmal movement disorders. Our study highlights the clinical features and treatment response of RHOBTB2-RD.

Genotype-phenotype correlations in RHOBTB2-associated neurodevelopmental disorders

PURPOSE

Missense variants clustering in the BTB domain region of RHOBTB2 cause a developmental and epileptic encephalopathy with early-onset seizures and severe intellectual disability.

METHODS

By international collaboration, we assembled individuals with pathogenic RHOBTB2 variants and a variable spectrum of neurodevelopmental disorders. By western blotting, we investigated the consequences of missense variants in vitro.

RESULTS

In accordance with previous observations, de novo heterozygous missense variants in the BTB domain region led to a severe developmental and epileptic encephalopathy in 16 individuals. Now, we also identified de novo missense variants in the GTPase domain in 6 individuals with apparently more variable neurodevelopmental phenotypes with or without epilepsy. In contrast to variants in the BTB domain region, variants in the GTPase domain do not impair proteasomal degradation of RHOBTB2 in vitro, indicating different functional consequences. Furthermore, we observed biallelic splice-site and truncating variants in 9 families with variable neurodevelopmental phenotypes, indicating that complete loss of RHOBTB2 is pathogenic as well.

CONCLUSION

By identifying genotype-phenotype correlations regarding location and consequences of de novo missense variants in RHOBTB2 and by identifying biallelic truncating variants, we further delineate and expand the molecular and clinical spectrum of RHOBTB2-related phenotypes, including both autosomal dominant and recessive neurodevelopmental disorders.

Genetic Links to Episodic Movement Disorders: Current Insights

Episodic or paroxysmal movement disorders (PxMD) are conditions, which occur episodically, are transient, usually have normal interictal periods, and are characterized by hyperkinetic disorders, including ataxia, chorea, dystonia, and ballism. Broadly, these comprise paroxysmal dyskinesias (paroxysmal kinesigenic and non-kinesigenic dyskinesia [PKD/PNKD], paroxysmal exercise-induced dyskinesias [PED]) and episodic ataxias (EA) types 1-9. Classification of paroxysmal dyskinesias has traditionally been clinical. However, with advancement in genetics and the discovery of the molecular basis of several of these disorders, it is becoming clear that phenotypic pleiotropy exists, that is, the same variant may give rise to a variety of phenotypes, and the classical understanding of these disorders requires a new paradigm. Based on molecular pathogenesis, paroxysmal disorders are now categorized as synaptopathies, transportopathies, channelopathies, second-messenger related disorders, mitochondrial or others. A genetic paradigm also has an advantage of identifying potentially treatable disorders, such as glucose transporter 1 deficiency syndromes, which necessitates a ketogenic diet, and ADCY5-related disorders, which may respond to caffeine. Clues for a primary etiology include age at onset below 18 years, presence of family history and fixed triggers and attack duration. Paroxysmal movement disorder is a network disorder, with both the basal ganglia and the cerebellum implicated in pathogenesis. Abnormalities in the striatal cAMP turnover pathway may also be contributory. Although next-generation sequencing has restructured the approach to paroxysmal movement disorders, the genetic underpinnings of several entities remain undiscovered. As more genes and variants continue to be reported, these will lead to enhanced understanding of pathophysiological mechanisms and precise treatment.

Skin Conditions and Movement Disorders: Hiding in Plain Sight

Skin manifestations are well-recognized non-motor symptoms of Parkinson's disease (PD) and other hypokinetic and hyperkinetic movement disorders. Skin conditions are usually well visible during routine clinical examination and their recognition may play a major role in diagnostic work-up. In this educational review we: (1) briefly outline skin conditions related to Parkinson's disease, including therapy-related skin complications and their management; (2) discuss the role of skin biopsies in early diagnosis of PD and differential diagnosis of parkinsonian syndromes; and focus more on areas which have not been reviewed in the literature before, including (3) skin conditions related to atypical parkinsonism, and (4) skin conditions related to hyperkinetic movement disorders. In case of rare hyperkinetic movement disorders, specific dermatological manifestations, like presence of angiokeratomas, telangiectasias, Mongolian spots, lipomas, ichthyosis, progeroid skin changes and others may point to a very specific group of disorders and help guide further investigations.

RHOBTB2 gene associated epilepsy and paroxysmal movement disorder: two cases report and literature review

Background

RHOBTB2 gene is associated with developmental and epileptic encephalopathy-64(DEE-64), which is characterized by epilepsy, developmental delay, microcephaly, unspecific facial dysmorphism, and paroxysmal movement disorders. Most previous studies showed that the phenotypes of RHOBTB2 gene include developmental and epileptic encephalopathy(DEE) and DEE with paroxysmal movement disorders. Only one study showed that patient with RHOBTB2 variant had paroxysmal movement disorders with no epilepsy.

Case presentations

Two cases with RHOBTB2 variants are presented here: Case one was diagnosed as DEE, he had recurrent afebrile focal status epilepticus and paroxysmal extrapyramidal symptoms in infancy. Interictal electroencephalogram (EEG) showed focal discharges. Brain magnetic resonance imaging (MRI) showed cortical dysplasia. Epilepsy of case one was refractory. Nevertheless, case two only showed paroxysmal movement disorders alone in adolescence. Video EEG showed focal discharges during an interictal dystonic episode and he brain MRI was normal.

Conclusion

The phenotypes of RHOBTB2 gene include DEE, paroxysmal movement disorders, and DEE with paroxysmal movement disorders. RHOBTB2 can be one of the pathogenic genes of paroxysmal movement disorders.

Paroxysmal Movement Disorders

Paroxysmal movement disorders (PxMDs) are a clinical and genetically heterogeneous group of movement disorders characterized by episodic involuntary movements (dystonia, dyskinesia, chorea and/or ataxia). Historically, PxMDs were classified clinically (triggers and characteristics of the movements) and this directed single-gene testing. With the advent of next-generation sequencing (NGS), how we classify and investigate PxMDs has been transformed. Next-generation sequencing has enabled new gene discovery (RHOBTB2, TBC1D24), expansion of phenotypes in known PxMDs genes and a better understanding of disease mechanisms. However, PxMDs exhibit phenotypic pleiotropy and genetic heterogeneity, making it challenging to predict genotype based on the clinical phenotype. For example, paroxysmal kinesigenic dyskinesia is most commonly associated with variants in PRRT2 but also variants identified in PNKD, SCN8A, and SCL2A1. There are no radiological or biochemical biomarkers to differentiate genetic causes. Even with NGS, diagnosis rates are variable, ranging from 11 to 51% depending on the cohort studied and technology employed. Thus, a large proportion of patients remain undiagnosed compared to other neurological disorders such as epilepsy, highlighting the need for further genomic research in PxMDs. Whole-genome sequencing, deep-sequencing, copy number variant analysis, detection of deep-intronic variants, mosaicism and repeat expansions, will improve diagnostic rates. Identifying the underlying genetic cause has a significant impact on patient care, modification of treatment, long-term prognostication and genetic counseling. This paper provides an update on the genetics of PxMDs, description of PxMDs classified according to causative gene rather than clinical phenotype, highlighting key clinical features and providing an algorithm for genetic testing of PxMDs.

Paroxysmal Genetic Movement Disorders and Epilepsy

Paroxysmal movement disorders include paroxysmal kinesigenic dyskinesia, paroxysmal non-kinesigenic dyskinesia, paroxysmal exercise-induced dyskinesia, and episodic ataxias. In recent years, there has been renewed interest and recognition of these disorders and their intersection with epilepsy, at the molecular and pathophysiological levels. In this review, we discuss how these distinct phenotypes were constructed from a historical perspective and discuss how they are currently coalescing into established genetic etiologies with extensive pleiotropy, emphasizing clinical phenotyping important for diagnosis and for interpreting results from genetic testing. We discuss insights on the pathophysiology of select disorders and describe shared mechanisms that overlap treatment principles in some of these disorders. In the near future, it is likely that a growing number of genes will be described associating movement disorders and epilepsy, in parallel with improved understanding of disease mechanisms leading to more effective treatments.

Genetic Dystonias: Update on Classification and New Genetic Discoveries

PURPOSE OF REVIEW

Since the advent of next-generation sequencing, the number of genes associated with dystonia has been growing exponentially. We provide here a comprehensive review of the latest genetic discoveries in the field of dystonia and discuss how the growing knowledge of biology underlying monogenic dystonias may influence and challenge current classification systems.

RECENT FINDINGS

Pathogenic variants in genes without previously confirmed roles in human disease have been identified in subjects affected by isolated or combined dystonia (KMT2B, VPS16, HPCA, KCTD17, DNAJC12, SLC18A2) and complex dystonia (SQSTM1, IRF2BPL, YY1, VPS41). Importantly, the classical distinction between isolated and combined dystonias has become harder to sustain since many genes have been shown to determine multiple dystonic presentations (e.g., ANO3, GNAL, ADCY5, and ATP1A3). In addition, a growing number of genes initially linked to other neurological phenotypes, such as developmental delay, epilepsy, or ataxia, are now recognized to cause prominent dystonia, occasionally in an isolated fashion (e.g., GNAO1, GNB1, SCN8A, RHOBTB2, and COQ8A). Finally, emerging analyses suggest biological convergence of genes linked to different dystonic phenotypes. While our knowledge on the genetic basis of monogenic dystonias has tremendously grown, their clinical boundaries are becoming increasingly blurry. The current phenotype-based classification may not reflect the molecular structure of the disease, urging the need for new systems based on shared biological pathways among dystonia-linked genes.