Clinical spectrum and genotype-phenotype correlations in PRRT2 Italian patients

Missense mutations in the membrane domain of PRRT2 affect its interaction with Nav1.2 voltage-gated sodium channels

PRRT2 is a neuronal protein that controls neuronal excitability and network stability by modulating voltage-gated Na⁺ channel (Nav). PRRT2 pathogenic variants cause pleiotropic syndromes including epilepsy, paroxysmal kinesigenic dyskinesia and episodic ataxia attributable to loss-of-function pathogenetic mechanism. Based on the evidence that the transmembrane domain of PRRT2 interacts with Nav1.2/1.6, we focused on eight missense mutations located within the domain that show expression and membrane localization similar to the wild-type protein. Molecular dynamics simulations showed that the mutants do not alter the structural stability of the PRRT2 membrane domain and preserve its conformation. Using affinity assays, we found that the A320V and V286M mutants displayed respectively decreased and increased binding to Nav1.2. Accordingly, surface biotinylation showed an increased Nav1.2 surface exposure induced by the A320V mutant. Electrophysiological analysis confirmed the lack of modulation of Nav1.2 biophysical properties by the A320V mutant with a loss-of-function phenotype, while the V286M mutant displayed a gain-of-function with respect to wild-type PRRT2 with a more pronounced left-shift of the inactivation kinetics and delayed recovery from inactivation. The data confirm the key role played by the PRRT2-Nav interaction in the pathogenesis of the PRRT2-linked disorders and suggest an involvement of the A320 and V286 residues in the interaction site. Given the similar clinical phenotype caused by the two mutations, we speculate that circuit instability and paroxysmal manifestations may arise when PRRT2 function is outside the physiological range.

Translating Genetic Discovery into a Mechanistic Understanding of Pediatric Movement Disorders: Lessons from Genetic Dystonias and Related Disorders

The era of next-generation sequencing has increased the pace of gene discovery in the field of pediatric movement disorders. Following the identification of novel disease-causing genes, several studies have aimed to link the molecular and clinical aspects of these disorders. This perspective presents the developing stories of several childhood-onset movement disorders, including paroxysmal kinesigenic dyskinesia, myoclonus-dystonia syndrome, and other monogenic dystonias. These stories illustrate how gene discovery helps focus the research efforts of scientists trying to understand the mechanisms of disease. The genetic diagnosis of these clinical syndromes also helps clarify the associated phenotypic spectra and aids the search for additional disease-causing genes. Collectively, the findings of previous studies have led to increased recognition of the role of the cerebellum in the physiology and pathophysiology of motor control-a common theme in many pediatric movement disorders. To fully exploit the genetic information garnered in the clinical and research arenas, it is crucial that corresponding multi-omics analyses and functional studies also be performed at scale. Hopefully, these integrated efforts will provide us with a more comprehensive understanding of the genetic and neurobiological bases of movement disorders in childhood.

Comprehensive Genetic Analysis of Druze Provides Insights into Carrier Screening

BACKGROUND

Druze individuals, like many genetically homogeneous and isolated populations, harbor recurring pathogenic variants (PV) in autosomal recessive (AR) disorders.

METHODS

Variant calling of whole-genome sequencing (WGS) of 40 Druze from the Human Genome Diversity Project (HGDP) was performed (HGDP-cohort). Additionally, we performed whole exome sequencing (WES) of 118 Druze individuals: 38 trios and 2 couples, representing geographically distinct clans (WES-cohort). Rates of validated PV were compared with rates in worldwide and Middle Eastern populations, from the gnomAD and dbSNP datasets.

RESULTS

Overall, 34 PVs were identified: 30 PVs in genes underlying AR disorders, 3 additional PVs were associated with autosomal dominant (AD) disorders, and 1 PV with X-linked-dominant inherited disorder in the WES cohort.

CONCLUSIONS

The newly identified PVs associated with AR conditions should be considered for incorporation into prenatal-screening options offered to Druze individuals after an extension and validation of the results in a larger study.

The intramembrane COOH-terminal domain of PRRT2 regulates voltage-dependent Na+ channels

Proline-rich transmembrane protein 2 (PRRT2) is the single causative gene for pleiotropic paroxysmal syndromes including epilepsy, kinesigenic dyskinesia, episodic ataxia and migraine. PRRT2 is a neuron-specific type-2 membrane protein with a COOH-terminal intramembrane domain and a long proline-rich NH₂-terminal cytoplasmic region. A large array of experimental data indicates that PRRT2 is a neuron stability gene that negatively controls intrinsic excitability by regulating surface membrane localization and biophysical properties of voltage-dependent Na⁺ channels Nav1.2 and Nav1.6, but not Nav1.1. To further investigate the regulatory role of PRRT2, we studied the structural features of this membrane protein with molecular dynamics simulations, and its structure-function relationships with Nav1.2 channels by biochemical and electrophysiological techniques. We found that the intramembrane COOH-terminal region maintains a stable conformation over time, with the first transmembrane domain forming a helix-loop-helix motif within the bilayer. The unstructured NH₂-terminal cytoplasmic region bound to the Nav1.2 better than the isolated COOH-terminal intramembrane domain, mimicking full-length PRRT2, while the COOH-terminal intramembrane domain was able to modulate Na⁺ current and channel biophysical properties, still maintaining the striking specificity for Nav1.2 vs Nav1.1. channels. The results identify PRRT2 as a dual-domain protein in which the NH₂-terminal cytoplasmic region acts as a binding antenna for Na⁺ channels, while the COOH-terminal membrane domain regulates channel exposure on the membrane and its biophysical properties.

Epilepsy Course and Developmental Trajectories in STXBP1-DEE

Background and Objectives

Clinical manifestations in STXBP1 developmental and epileptic encephalopathy (DEE) vary in severity and outcome, and the genotypic spectrum is diverse. We aim to trace the neurodevelopmental trajectories in individuals with STXBP1-DEE and dissect the relationship between neurodevelopment and epilepsy.

Methods

Retrospective standardized clinical data were collected through international collaboration. A composite neurodevelopmental score system compared the developmental trajectories in STXBP1-DEE.

Results

Forty-eight patients with de novo STXBP1 variants and a history of epilepsy were included (age range at the time of the study: 10 months to 35 years, mean 8.5 years). At the time of inclusion, 65% of individuals (31/48) had active epilepsy, whereas 35% (17/48) were seizure free, and 76% of those (13/17) achieved remission within the first year of life. Twenty-two individuals (46%) showed signs of developmental impairment and/or neurologic abnormalities before epilepsy onset. Age at seizure onset correlated with severity of developmental outcome and the developmental milestones achieved, with a later seizure onset associated with better developmental outcome. In contrast, age at seizure remission and epilepsy duration did not affect neurodevelopmental outcomes. Overall, we did not observe a clear genotype-phenotype correlation, but monozygotic twins with de novo STXBP1 variant showed similar phenotype and parallel disease course.

Discussion

The disease course in STXBP1-DEE presents with 2 main trajectories, with either early seizure remission or drug-resistant epilepsy, and a range of neurodevelopmental outcomes from mild to profound intellectual disability. Age at seizure onset is the only epilepsy-related feature associated with neurodevelopment outcome. These findings can inform future dedicated natural history studies and trial design.

Synaptopathies in Developmental and Epileptic Encephalopathies: A Focus on Pre-synaptic Dysfunction

The proper connection between the pre- and post-synaptic nervous cells depends on any element constituting the synapse: the pre- and post-synaptic membranes, the synaptic cleft, and the surrounding glial cells and extracellular matrix. An alteration of the mechanisms regulating the physiological synergy among these synaptic components is defined as “synaptopathy.” Mutations in the genes encoding for proteins involved in neuronal transmission are associated with several neuropsychiatric disorders, but only some of them are associated with Developmental and Epileptic Encephalopathies (DEEs). These conditions include a heterogeneous group of epilepsy syndromes associated with cognitive disturbances/intellectual disability, autistic features, and movement disorders. This review aims to elucidate the pathogenesis of these conditions, focusing on mechanisms affecting the neuronal pre-synaptic terminal and its role in the onset of DEEs, including potential therapeutic approaches.

PRRT2 variants and effectiveness of various antiepileptic drugs in self-limited familial infantile epilepsy

PURPOSE

Self-limited familial infantile epilepsy (SFIE) is largely associated with variants in proline-rich transmembrane protein 2 (PRRT2). However, the detailed phenotype-genotype correlations are unclear, along with the efficacy of various antiepileptic drugs in the treatment of this epilepsy syndrome. In this study, we analysed the PRRT2 variants associated with SFIE in Chinese patients, and the efficacy of different antiepileptic drugs prescribed during follow-up.

METHODS

We retrospectively included 20 patients diagnosed with SFIE and reviewed their clinical characteristics, genetic variants, and treatment responses.

RESULTS

Eighteen of the 20 (90%) patients harboured the common heterozygous variant of PRRT2 c.649dupC p.(Arg217fs). One patient had two heterozygous variants of PRRT2, c.640G>C p.(Ala214Pro) and c.955G>T p.(Val319Leu), and the other patient harboured a novel c.606delA (p.Pro203Hisfs) variant. Nine patients who had first-line treatment of oxcarbazepine (OXC) became seizure-free. However, initial treatment with levetiracetam (LEV) or sodium valproate (VPA) in eight and three patients, respectively, was not effective even after increasing the dosage, and seizure-free status was only achieved after changing the treatment to OXC. The treatment responses suggested a significant difference (P < 0.001) between OXC and other anti-epileptic drugs.

CONCLUSION

OXC as a sodium channel blocker may have a better effect than LEV and VPA in the treatment of PRRT2-associated SFIE. PRRT2 variants may be used as a biomarker to help select antiepileptic drugs for SFIE.

Do we need to reconsider the classification of vestibular migraine?

Introduction: Vestibular migraine (VM) is a complex disease characterized by recurrent episodes of migraine associated with vertigo attacks that are observed in 1-3% of the general population. Given its high prevalence and the impact on the health system, it is important to characterize these patients, in order to offer an accurate diagnosis and a proper treatment. As the diagnosis of VM is based on clinical features, the study of potential biomarkers has gained more interest in the last years, to improve the precision in the diagnosis of this disease. The aim of this review is to summarize the main tests available for the diagnosis of VM, including the accuracy of biomarkers for the diagnosis of VM.Areas covered: This review summarizes the main information on VM, including all diagnosis records published in the field in the last 10 years, and focusing on candidate biomarkers for the diagnosis of VM patients.Expert opinion: There is a limited knowledge in the pathophysiology of VM. The search of biomarkers for diagnosis of VM is needed to improve the precision in the diagnosis promoting clinical and translational research. The potential reclassification of VM will depend upon the discovery and validation of these biomarkers.

The Spectrum of PRRT2-Associated Disorders: Update on Clinical Features and Pathophysiology

Mutations in the PRRT2 (proline-rich transmembrane protein 2) gene have been identified as the main cause of an expanding spectrum of disorders, including paroxysmal kinesigenic dyskinesia and benign familial infantile epilepsy, which places this gene at the border between epilepsy and movement disorders. The clinical spectrum has largely expanded to include episodic ataxia, hemiplegic migraine, and complex neurodevelopmental disorders in cases with biallelic mutations. Prior to the discovery of PRRT2 as the causative gene for this spectrum of disorders, the sensitivity of paroxysmal kinesigenic dyskinesia to anticonvulsant drugs regulating ion channel function as well as the co-occurrence of epilepsy in some patients or families fostered the hypothesis this could represent a channelopathy. However, recent evidence implicates PRRT2 in synapse functioning, which disproves the "channel hypothesis" (although PRRT2 modulates ion channels at the presynaptic level), and justifies the classification of these conditions as synaptopathies, an emerging rubric of brain disorders. This review aims to provide an update of the clinical and pathophysiologic features of PRRT2-associated disorders.

The Genotype and Phenotype of Proline-Rich Transmembrane Protein 2 Associated Disorders in Chinese Children

Objectives: To study the genetic and clinical characteristics of Chinese children with pathogenic proline-rich transmembrane protein 2 (PRRT2) gene-associated disorders. Methods: Targeted next generation sequencing (NGS) was used to identify pathogenic PRRT2 variations in Chinese children with epilepsy and/or kinesigenic dyskinesia. Patients with confirmed PRRT2-associated disorders were monitored and their clinical data were analyzed. Results: Forty-four patients with pathogenic PRRT2 variants were recruited. Thirty-five of them (79.5%) had heterozygous mutations, including 30 frameshifts, three missenses, one nonsense, and one splice site variant. The c.649dupC was the most common variant (56.8%). Eight patients (18.2%) showed whole gene deletions, and one patient (2.3%) had 16p11.2 microdeletion. Thirty-four cases (97.1%) were inherited and one case (2.9%) was de novo. Forty patients were diagnosed with benign familial infantile epilepsy (BFIE), two patients had paroxysmal kinesigenic dyskinesia (PKD) and two had infantile convulsions and choreoathetosis (ICCA). Patients with whole gene deletions had a later remission than patients with heterozygous mutations (13.9 vs. 7.1 months, P = 0.001). Forty-two patients were treated with antiseizure medications (ASMs). At last follow-up, 35 patients, including one who did not receive therapy, were asymptomatic, and one patient without ASMs died of status epilepticus at 12 months of age. One patient developed autism, and one patient showed mild developmental delay/intellectual disability. Conclusion: Our data suggested that patients with whole gene deletions could have more severe manifestations in PRRT2-associated disorders. Conventional ASMs, especially Oxcarbazepine, showed a good treatment response.