Episodic tremors representing cortical myoclonus are characteristic in Angelman syndrome due to UBE3A mutations

Therapeutic approach to neurological manifestations of Angelman syndrome

INTRODUCTION

Angelman syndrome (AS) is a neurogenetic disorder due to deficient expression of the maternal copy of the UBE3A gene, which encodes ubiquitin ligase E3A protein. Severe developmental delay, seizures and other neurological disorders characterizes AS.

AREAS COVERED

In this review, we focus on a comprehensive therapeutic approach to the most disabling neurological manifestations of AS: epilepsy, sleep disturbances, behavioral and movement disorders. Articles were identified through PubMed and Google Scholar up to October 2021.

EXPERT OPINION

Evidence for the treatment of neurological manifestations in AS mainly derives from poor quality studies (case reports, small case series, expert opinions). Seizures can be polymorphic and includes atypical absences, myoclonic, generalized tonic-clonic, unilateral clonic, or atonic attacks. Sodium valproate, levetiracetam and benzodiazepines are the most commonly used anti-seizure medications. Melatonin or mirtazapine seem to improve sleep quality. Antipsychotics, antidepressants and anxiolytics have been proposed for treatment of behavioral manifestations, but no evidence-based studies are available. Non-pharmacological approach may also be useful. Mild dystonia is common but usually does not significantly impact patients' motor performances. Well-conducted clinical trials aimed to evaluate treatment of neurological complications of AS are warranted. Gene and molecular precision therapies represent a fascinating area of research in the future.

Touchscreen Cognitive Deficits, Hyperexcitability, and Hyperactivity in Males and Females Using Two Models of Cdkl5 Deficiency

Many neurodevelopmental disorders (NDDs) are the result of mutations on the X chromosome. One severe NDD resulting from mutations on the X chromosome is CDKL5 deficiency disorder (CDD). CDD is an epigenetic, X-linked NDD characterized by intellectual disability (ID), pervasive seizures and severe sleep disruption, including recurring hospitalizations. CDD occurs at a 4:1 ratio, with a female bias. CDD is driven by the loss of cyclin-dependent kinase-like 5 (CDKL5), a serine/threonine kinase that is essential for typical brain development, synapse formation and signal transmission. Previous studies focused on male subjects from animal models, likely to avoid the complexity of X mosaicism. For the first time, we report translationally relevant behavioral phenotypes in young adult (8–20 weeks) females and males with robust signal size, including impairments in learning and memory, substantial hyperactivity and increased susceptibility to seizures/reduced seizure thresholds, in both sexes, and in two models of CDD preclinical mice, one with a general loss-of-function mutation and one that is a patient-derived mutation.

Genotype-Phenotype Correlations in Angelman Syndrome

Angelman syndrome (AS) is a rare neurodevelopmental disease that is caused by the loss of function of the maternal copy of ubiquitin-protein ligase E3A (UBE3A) on the chromosome 15q11-13 region. AS is characterized by global developmental delay, severe intellectual disability, lack of speech, happy disposition, ataxia, epilepsy, and distinct behavioral profile. There are four molecular mechanisms of etiology: maternal deletion of chromosome 15q11-q13, paternal uniparental disomy of chromosome 15q11-q13, imprinting defects, and maternally inherited UBE3A mutations. Different genetic types may show different phenotypes in performance, seizure, behavior, sleep, and other aspects. AS caused by maternal deletion of 15q11-13 appears to have worse development, cognitive skills, albinism, ataxia, and more autistic features than those of other genotypes. Children with a UBE3A mutation have less severe phenotypes and a nearly normal development quotient. In this review, we proposed to review genotype-phenotype correlations based on different genotypes. Understanding the pathophysiology of the different genotypes and the genotype-phenotype correlations will offer an opportunity for individualized treatment and genetic counseling. Genotype-phenotype correlations based on larger data should be carried out for identifying new treatment modalities.

Myoclonic tremor status as a presenting symptom of adenylosuccinate lyase deficiency

Adenylosuccinate lyase deficiency is a rare autosomal recessive disorder of purine metabolism. The disorder manifests with developmental delay, postnatal microcephaly, hypotonia, involuntary movements, epileptic seizures, ataxia and autistic features. Paroxysmal non-epileptic motor events are not a typical presentation of the disease. We describe an 8-year-old boy who presented with an infantile onset of prolonged episodes of multifocal sustained myoclonic tremor lasting from minutes to days on a background of global developmental delay and gait ataxia. Ictal EEG during these episodes was normal. Ictal surface EMG of the involved upper limb showed a muscular activation pattern consistent with cortical myoclonus. Brain MRI showed mild cerebral atrophy. Whole exome sequencing revealed a novel homozygous variant in the ADSL gene: c.1027G > A; p. Glu343Lys, inherited from each heterozygous parent. There was a marked elevation of urine succinyladenosine, confirming the diagnosis of adenylosuccinate lyase deficiency. In conclusion, myoclonic tremor status expands the spectrum of movement disorders seen in adenylosuccinate lyase deficiency.

Connectivity measures suggest a sub-cortical generator of myoclonus in Angelman syndrome

OBJECTIVE

The clinical and neurophysiological characteristics of myoclonus in Angelman syndrome (AS) have been evaluated in single case or small cohorts, with contrasting results. We evaluated the features of myoclonus in a wide cohort of AS patients.

METHODS

We performed polygraphic EEG-EMG recording in 24 patients with genetically confirmed AS and myoclonus. Neurophysiological investigations included jerk-locked back-averaging (JLBA), cortico-muscular coherence (CMC) and generalised partial directed coherence (GPDC). CMC and GPDC analyses were compared to those obtained from 10 healthy controls (HC).

RESULTS

Twenty-four patients (aged 3-35 years, median 20) were evaluated. Sequences of quasi-continuous rhythmic jerks mostly occurred at alpha frequency or just below (mean 8.4 ± 1.4 Hz), without EEG correlate. JLBA did not show any clear transient preceding the jerks. CMC showed bilateral over-threshold CMC in alpha band that was prominent on the contralateral hemisphere in the patient group as compared to HC group. GPDC showed a significantly higher alpha outflow from both hemispheres toward activated muscles in the patient group, and a significantly higher beta outflow from contralateral hemisphere in the HC group.

CONCLUSIONS

These neurophysiological findings suggest a subcortical generator of myoclonus in AS.

SIGNIFICANCE

Myoclonus in AS has not a cortical origin as previously hypothesised.

Myoclonus in Angelman syndrome

Angelman syndrome (AS) is a neurogenetic imprinting disorder caused by loss of the maternally inherited Ube3a gene and is characterized by generalized epilepsy, limited expressive speech, sleep dysfunction, and movement disorders. Myoclonic seizures are often the first seizure type to appear, and myoclonic status, associated with developmental regression, may occur in the first few years of life. Additionally, there have been rare reports of prolonged episodes of myoclonus without electrographic correlate in adults with AS. The medical records of 200 individuals seen in the Angelman Syndrome Clinic at the Massachusetts General Hospital and the Lurie Center for Autism were retrospectively reviewed to identify and characterize myoclonic seizures and episodes of nonepileptic myoclonus. Myoclonic seizures were reported in 14% of individuals with age of onset occurring before 8years. These are brief events, unless the individual was experiencing myoclonic status, and electroencephalographs show interictal generalized spike and wave activity. Nonepileptic myoclonus occurred in 40% of individuals over 10years of age, and prevalence appears to increase with age. The episodes of nonepileptic myoclonus arise during puberty or later, with age of onset ranging from 10 to 26years. These events were captured on 5 video electroencephalographs and had no electrographic correlate. They can last from seconds to hours, always occurring in the hands and spreading to the face and all extremities in some individuals. Episodes of nonepileptic myoclonus have a discrete beginning and end, lacks a postictal period, and are not associated with significant alteration of consciousness or developmental regression. These episodes can be difficult to treat and are often refractory to medication; however, levetiracetam, clobazam, and clonazepam appear to be effective for some individuals. Myoclonic seizures are common in AS, typically occurring in young children and associated with epileptiform changes on electroencephalographs. Prolonged episodes are associated with developmental regression. In contrast, nonepileptic myoclonus typically begins in adolescence or early adulthood and has no electroencephalogram (EEG) correlate, alteration in consciousness, or regression but can significantly impact quality of life.

Angelman syndrome in adolescence and adulthood: A retrospective chart review of 53 cases

Angelman syndrome is a neurogenetic disorder with varying clinical presentations and symptoms as the individual ages. The goal of this study was to characterize changes over time in the natural history of this syndrome in a large population. We reviewed the medical records of the 53 patients who were born prior to 2000 and seen at the Angelman Syndrome Clinic at Massachusetts General Hospital to assess neurological, sleep, behavioral, gastrointestinal, orthopedic, and ophthalmologic functioning. The average age of this cohort was 24 years. Active seizures were present in 35%, nonepileptic myoclonus in 42%, and clinically significant tremors in 55%. Anxiety was present in 57%, increasing to 71% in those ages 26-43 years. In terms of sleep, 56% reported 8 hr of sleep or more, although 43% reported frequent nocturnal awakenings. Gastrointestinal issues remain problematic with 81% having constipation and 53% gastroesophageal reflux. The majority lived in a parent's home and remained independently mobile, though scoliosis was reportedly present in 30%, and 20% had reported low bone density/osteoporosis. The results of this study suggest that the prevalence of active seizures may decrease in adulthood but that the prevalence of movement disorders such as tremor and nonepileptic myoclonus may increase. Anxiety increases significantly as individuals age while defiant behaviors appear to decrease. Sleep dysfunction typically improves as compared to childhood but remains a significant issue for many adults. Other areas that require monitoring into adulthood include gastrointestinal dysfunction, and orthopedic/mobility issues, such as reported scoliosis and bone density, and ophthalmologic disorders.