Angelman syndrome

What is Angelman syndrome?

Angelman syndrome is a neurodevelopmental disorder characterized by severe developmental delay and speech impairment, gait ataxia and/or tremulousness of limbs, microcephaly, seizures, significant sleep difficulties, a happy demeanor, and easily provoked laughter.1 

Angelman syndrome is caused by loss of function of the maternally-inherited copy of ubiquitin-protein ligase E3A (UBE3A) gene, which sits in the chromosome 15q11.2–q13 region.2,3 The timely diagnosis of Angelman syndrome is difficult given the lack of specific clinical caveats or dysmorphic features in infancy and the several genetic mechanisms that impair UBE3A expression.3 

What causes Angelman syndrome?

UBE3A spans approximately 120 kb of genomic DNA and contains 16 exons.1 The coding region of the gene spans approximately 60 kb, from which 3 main mRNA transcripts are derived. These transcripts contain 10 exons, approximately 5 kb in size, and encode 3 protein isoforms (I, II, and III):4–8  

  • Isoform I: Corresponds to the open reading frame (ORF) for E6AP 
  • Isoform II: Amino acid equivalent of isoform 1 with an additional 20 amino acids at the amino terminus 
  • Isoform III: Amino acid equivalent of isoform 1 with an additional 23 amino acids at the amino terminus 

The 3 protein isoforms include the Homologous to E6AP Carboxy Terminus (HECT) domain and are thus capable of functioning as an E3 ligase.1 

E6AP catalyzes the transfer and covalent linkage of activated ubiquitin (a 76 amino acid protein) to target proteins.1 This domain is encoded by exons 916 and forms a bilobed structure with a broad catalytic cleft at the point where the two lobes meet.1 A cysteine residue sits at the catalytic site where it accepts ubiquitin from the E2 ubiquitin-conjugating enzyme.1 Mutations within the cleft interfere with the formation of the ubiquitin-thioester bond; a necessary step before ubiquitin is transferred to substrates.1 In Angelman syndrome, mutations are due to missense or single amino acid insertions or deletions within the HECT domain.9 

The inactivation or deletion of the maternal copy of the UBE3A gene which causes Angelman syndrome occurs by several different genetic mechanisms.10 Importantly, both Angelman syndrome and Prader-Willi syndrome are caused by reciprocal deletion of the human chromosome interval in this region.11 Prader-Willi syndrome, however, is caused by loss of the paternal expression of several contiguous genes, and manifests with a distinct phenotype.11 

Interstitial deletions in the 15q11.2– q13 region 

Interstitial deletions occur in ~7075% of patients, deletions are approximately 4 Mb in size.2 Most of these occur de novo and are of maternal origin.10 Phenotypic differences between patient groups are attributed to genetic mechanisms.10 Of these, patients with chromosome 15 deletions are usually the most severely affected.10 They have a higher incidence of seizures, microcephaly, hypopigmentation, absent speech, and a greater delay in motor milestones.10 According to the Rare Diseases Clinical Research Network (RDCRN) Natural History Study data, non-deletion participants were more advanced than the deletion participants in all aspects of development except expressive language skills.12 

Paternal uniparental disomy (UPD) of chromosome 15  

A second class of Angelman syndrome patients fail to inherit a maternal copy of UBE3A; both copies of chromosome 15 are paternally inherited.10 The occurrence of uniparental disomy is sporadic and accounts for only 23% of cases of Angelman syndrome.10  

Imprinting defects (ID) 

The 15q11.2–q13 chromosome region is subject to genomic imprinting, an epigenetic process that renders genes to be monoallelically expressed in a parent‐of‐origin‐specific manner.10 An ID affects approximately 3–5% of individuals with Angelman syndrome;10,13 these can be caused by genetic or epigenetic alterations affecting methylation and gene expression throughout the imprinted domain in 15q11.2–q13.2 The imprinting center is located in the small nuclear ribonucleoprotein-associated protein N (SNRPN) region.2 Imprinted expression of UBE3A is regulated indirectly by small nucleolar RNA host gene 14 (SNHG14, formerly known as UBE3A-ATS).2 SNHG14 is a non-coding antisense transcript that extends into the UBE3A gene and interferes with its transcription.2  

In individuals with Angelman syndrome, chromosome 15 is inherited in a biparental manner, but the maternal copy lacks methylation of the SNRPN promoter.2 SNHG14 is subsequently expressed, leading to suppression of the maternal UBE3A transcripts.2 By contrast, the paternal allele is unmethylated. In neurons, the paternal long noncoding RNA extends to and overlaps UBE3A as an antisense transcript (UBE3A-ATS) with concomitant silencing of the paternal UBE3A allele.1  

UBE3A gene mutations 

Mutations can be identified in 14–20% of sporadic patients with normal methylation and in ~75% of familial patients.10,14,15 As outlined, the catalytic cleft between the 2 lobes of the HECT domain is the site of many of the reported mutations.9 

In ~10–15% of cases, the molecular exam is normal with no deletions, UPD, IDs, or UBE3A mutations.14 Patients are likely to be heterogeneous and may include those with other disorders.10 

The signs and symptoms of Angelman syndrome

Consensus criteria for the clinical diagnosis of Angelman syndrome have been established.16 To diagnose Angelman syndrome according to the consensus for diagnostic criterion, the clinical features of the individuals are divided by their percentage of frequencies in the syndrome: consistent features (100%), frequent features (>80%), and associated features (20–80%).16 

A wide index of clinical suspicion is necessary for children demonstrating the behavioral features seen in Angelman syndrome as they are the most consistent clinical feature.10  

Angelman syndrome in adults 

Clinical features of Angelman syndrome evolve with age progression.10 Age of puberty onset and secondary sex characteristics are normal.10 The facial characteristics of Angelman syndrome in adults are pronounced and include pointed chin, mandibular prognathism, macrostomia (wide mouth), and a prominent lower lip.10,17 

The estimated prevalence of Angelman syndrome

There is a range in prevalence estimates of Angelman syndrome due to the lack of recent prevalence studies.18 The prevalence of Angelman syndrome has no apparent racial predilection and males and females appear equally affected.19,20 It is estimated to be between 1 in 12,000–20,000 people in the general population.2022  

The most recent studies have calculated incident rates relative to the larger population, following exploration of the number of cases of Angelman syndrome among populations of individuals with severe and epilepsy/intellectual disability.23 While valuable for providing estimates, these studies sample from larger populations and so do not reflect the true incidence of Angelman syndrome; consequently, the disorder is often undiagnosed or misdiagnosed.23 

Determining a diagnosis of Angelman syndrome

The differential diagnosis of Angelman syndrome is often difficult, as infants present with general psychomotor delays and/or seizures.1 Consequently during infancy, differential diagnosis is non-specific and encompasses a range of Angelman syndrome mimics.1 These include Rett’s syndrome, nonspecific cerebral palsy, Lennox-Gastaut syndrome, static encephalopathy with mental retardation, infantile autism, and α-thalassemia X-linked mental retardation syndrome.29 

Testing strategy 

Molecular genetic testing approaches to establish a diagnosis of Angelman syndrome can be based on either the clinical findings or the laboratory findings that suggest the condition.19 DNA methylation analysis identifies ~80% of individuals with Angelman syndrome and is typically the first test ordered.19 

DNA methylation analysis:1 

Individuals with Angelman syndrome caused by a 5–7 Mb deletion of 15q11.2–q13, UPD, or an ID have an unmethylated paternal imprint.3 Methylation can be detected by methylation-specific polymerase chain reaction (MS-PCR), southern blot, or methylation-specific multiplex ligation-dependent probe amplification (MS-MLPA) testing.1  

  • Abnormal DNA methylation analysis: Fluorescent in situ hybridization (FISH) or array comparative genomic hybridization (CGH) analysis.3 If a deletion is found, chromosome rearrangement (ie, translocation or inversion) of one chromosome 15 involving 15q11.2–q13 should be excluded; however, <1% of individuals with Angelman syndrome have this cytogenetically visible chromosome rearrangement.3 Methylation analysis and UPD studies do not detect chromosomal rearrangements.3 
  • Normal DNA methylation analysis: UBE3A sequence analysis is the next appropriate diagnostic test.3 
  • Normal FISH or array CGH analysis: This warrants DNA polymorphism analysis of chromosome 15 which can distinguish between UPD and an ID.3 In the absence of UPD, further DNA analysis may reveal the presence of an ID.3  

Most commercially available DNA methylation analysis tests are unable to distinguish between Angelman syndrome resulting from a deletion, UPD, or an ID.19 As such, subsequent testing is necessary to identify the underlying molecular mechanism, regardless of the outcome of DNA methylation analysis.19 

What is the prognosis of Angelman syndrome?

The clinical features of Angelman syndrome are severe and lifelong, which results in a burden on both the individual and family.22 Lifespan does not appear to be affected; although data is not available.19  

The discrepancies and reported epidemiology and the absence of population-based studies result in a lack of awareness of Angelman syndrome.23 This subsequently results in lengthy diagnostic journeys. Despite the paucity of data on Health-Related Quality of Life (HR-QoL), the impact of Angelman syndrome on both the individual and the families and caregivers is thought to be significant, particularly as individuals rarely obtain the skills necessary to manage daily living tasks independently.23 

Impact on the individual 

Many studies have reported on Angelman syndrome since its description, which has resulted in a clear picture of the health issues of children.34 Less is known is about the course of the syndrome in adulthood.34 

  • In a study of 34 individuals with a mean age of 21.6 years, high hospitalization burden (median of 5.5 hospitalizations per person) occurred in individuals with Angelman syndrome; the most common reasons for hospitalization were seizures, gastrointestinal disorders, and dental work.28 Similarly, another study reported reasons for hospitalization that included dental care, seizures, orthopedic problems, and acute respiratory disorders34 
  • Data obtained from the Angelman Syndrome Natural History Study (ASNHS), a large-scale longitudinal study of individuals with Angelman syndrome in the US, suggests that children with the condition tend to experience surgeries, hospitalizations, and longer hospital stays within the first year of life compared to older children.35 Seizures were found to be one of the most common symptoms associated with Angelman syndrome.35 The study reported the use of antiepileptic drugs (AEDs) regardless of age for the management of seizures, and increasing use of behavioral/psychiatric (psychotropic) medications with age for the management of behavioral issues35 
  • A targeted literature review suggests patients face significant challenges with motor skills including tremors, ataxic gait, and inability to develop functional skills they may otherwise be able to develop cognitively.23 In addition, individuals with Angelman syndrome have significantly impaired communication skills23 
  • A description of the evolution of the clinical phenotypes of 95 individuals (mean age 31.6 years, median 29.0 years, range 18–83 years) with Angelman syndrome revealed that adults experience visual problems, scoliosis, constipation, reflux, and behavioral and sleeping problems. Epilepsy was also reported in 57% of adults and a decline in mobility was noted in 84% of the adults 40 years and older36 

Impact on caregivers 

Although there have been very few systematic explorations of the burden on parents, it has been reported that caregivers report high levels of fatigue and adverse effects on their social life.23 This is notable in cases where the individual with Angelman syndrome experiences sleep disturbances.23 An important impact on caregivers is stress as a result of children’s behavior challenges; parents of children with Angelman syndrome report more stress than parents of children with Prader-Willi syndrome.37 Parenting stress is also associated with negative outcomes for the parent, such as depression.23

AEDs, antiepileptic drugs; CGH, comparative genomic hybridization; CMA, chromosomal microarray; EEG, electroencephalogram; FISH, fluorescent in situ hybridization; HECT, homologous to E6AP COOH-terminus; HR-QoL, Health-Related Quality of Life; ID, imprinting defect; MLPA, multiplex ligation-dependent probe amplification; mRNA, messenger ribonucleic acid; MS-MLPA, methylation-specific multiplex ligation-dependent probe amplification; MS-PCR, methylation-specific polymerase chain reaction; ORF, open reading frame; PCR, polymerase chain reaction; PWS-IC, Prader-Willi syndrome imprinting center; q, longarm; SHR, steroid hormone receptor; SHRE, steroid hormone response element; SNRPN, small nuclear ribonucleoprotein-associated protein N; SR, steroid hormone receptorUb, ubiquitinUBE3A, ubiquitin-protein ligase E3A; UBE3A-ATS, antisense UBE3A; UPD, uniparental disomy. 

References

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  8. Williams CA (2016) Chapter 162 – UBE3A and the Angelman syndrome. In RP Erickson, Wynshaw-Boris AJ, in Epstein’s Inborn Errors of Development: The Molecular Basis of Clinical Disorders of Morphogenesis (pp.1077-1083). New York, NY: Oxford University Press. 
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