DiagnosisClinical DiagnosisConsensus criteria for the clinical diagnosis of Angelman syndrome (AS) have been developed in conjunction with the Scientific Advisory Committee of the US Angelman Syndrome Foundation [Williams et al 2006; click for full text]. Several recent reviews are available [Van Buggenhout & Fryns 2009, Chamberlain & Lalande 2010, Williams et al 2010a]. Newborns typically have a normal phenotype. Developmental delays are first noted at around age six months. However, the unique clinical features of AS do not become manifest until after age one year, and it can take several years before the correct clinical diagnosis is obvious.Findings typically present in affected individualsNormal prenatal and birth history, normal head circumference at birth, no major birth defectsNormal metabolic, hematologic, and chemical laboratory profilesStructurally normal brain by MRI or CT, although mild cortical atrophy or dysmyelination may be observedDelayed attainment of developmental milestones without loss of skillsEvidence of developmental delay by age six to 12 months, eventually classified as severeSpeech impairment, with minimal to no use of words; receptive language skills and nonverbal communication skills higher than expressive language skillsMovement or balance disorder, usually ataxia of gait and/or tremulous movement of the limbsBehavioral uniqueness, including any combination of frequent laughter/smiling; apparent happy demeanor; excitability, often with hand-flapping movements; hypermotoric behavior; short attention spanFindings in more than 80% of affected individualsDelayed or disproportionately slow growth in head circumference, usually resulting in absolute or relative microcephaly by age two yearsSeizures, usually starting before age three yearsAbnormal EEG, with a characteristic pattern of large-amplitude slow-spike wavesFindings in fewer than 80% of affected individualsFlat occiputOccipital grooveProtruding tongueTongue thrusting; suck/swallowing disordersFeeding problems and/or muscle hypotonia during infancyPrognathiaWide mouth, wide-spaced teethFrequent droolingExcessive chewing/mouthing behaviorsStrabismusHypopigmented skin, light hair and eye color (compared to family); seen only in those with a deletionHyperactive lower-extremity deep-tendon reflexesUplifted, flexed arm position especially during ambulationWide-based gait with out-going (i.e., pronated or valgus-positioned) anklesIncreased sensitivity to heatAbnormal sleep-wake cycles and diminished need for sleepAttraction to/fascination with water; fascination with crinkly items such as certain papers and plasticsAbnormal food-related behaviorsObesity (in the older child; more common in those who do not have a deletion)ScoliosisConstipationSee Figure 1 for clinical photographs of facial findings.FigureFigure 1. Individuals depicted have a genetically confirmed diagnosis of Angelman syndrome. Happy expression and an unstable gait accompanied by uplifted arms are commonly observed. At times, the facial appearance can suggest the diagnosis, but usually (more...)TestingCytogenetic testingFewer than 1% of individuals with AS have a cytogenetically visible chromosome rearrangement (i.e., translocation or inversion) of one number 15 chromosome involving 15q11.2-q13.Typically, the 5- to 7-Mb common deletion is not detectable by routine cytogenetic analysis.Molecular Genetic TestingGene. The cardinal features of AS are caused by deficient expression or function of the maternally inherited UBE3A allele.Clinical testingDNA methylation analysis. Unaffected individuals have a methylated and an unmethylated SNRPN allele in both the Southern blot analysis [Glenn et al 1996] and methylation-specific PCR assay [Kubota et al 1997, Zeschnigk et al 1997]. Individuals with AS caused by a 5- to 7-Mb deletion of 15q11.2-q13, uniparental disomy (UPD), or an imprinting defect (ID) have only an unmethylated (i.e., "paternal") contribution (i.e., an abnormal parent-specific DNA methylation imprint).
Note: (1) Most commercially available DNA methylation analysis tests cannot distinguish between AS resulting from a deletion, from UPD, and from an ID. Further testing is required to identify the underlying molecular mechanism as outlined in Testing Strategy. (2) Methylation-specific multiplex ligation-dependent probe amplification (MLPA) can test for deletion along with the methylation assay amplification [Nygren et al 2005, Procter et al 2006, Ramsden et al 2010].Deletion/duplication analysis / fluorescent in situ hybridization (FISH) / chromosomal microarray (CMA). In 68% of individuals, 5- to 7-Mb deletions are detected by FISH, CMA, or any of a variety of deletion testing methods (see Table 1).
Note: FISH analysis with the D15S10 and/or the SNRPN probe can identify the common deletion, but typically the deletion is not detected by routine cytogenetic analysis.Uniparental disomy (UPD) study. In approximately 7% of individuals, UPD is detected using DNA polymorphism testing, which requires a DNA sample from the affected individual and both parents. SNP-based chromosomal microarray may diagnose whole-chromosome and segmental uniparental isodisomies but cannot detect all instances of uniparental disomy.Imprinting defect (ID). IDs account for approximately 3% of affected individuals. IDs have abnormal DNA methylation and 10%-20% of the IDs are caused by microdeletions (6-200 kb) that include the AS imprinting center (IC). These microdeletions are detected by any of the various methods used for deletion analysis (see Table 1).
The other 80%-90% of IDs are thought to be epigenetic mutations occurring during maternal oogenesis or in early embryogenesis [Buiting 2010].Characterization of the ID as either an IC deletion or epigenetic defect is available primarily through research laboratories.Sequence analysis. Approximately 11% of individuals with AS have an identifiable UBE3A mutation [Malzac et al 1998, Fang et al 1999, Lossie et al 2001].
Note: A few individuals with AS have multiexonic or whole-gene deletions of UBE3A. These deletions are detected by any of various methods used for deletion analysis (see Table 1). Some chromosomal microarray platforms may be able to detect some of these deletions [Lawson-Yuen et al 2006, Sato et al 2007].Table 1. Summary of Molecular Genetic Testing Used in AS after DNA Methylation AnalysisView in own windowParent-Specific DNA Methylation ImprintLocus, Gene, or ChromosomeTest MethodsMutations DetectedMutation Detection Frequency by Test Method ^{1Test AvailabilityAbnormalAS/PWS regionDeletion/ duplication analysis, (including FISH or CMA)5- to 7-Mb deletion of 15q11.2-q13~68% ClinicalChromosome 15UPD studyUPD~7%AS imprinting center Deletion analysis 2, 36- to 200-kb deletions~3%NormalUBE3ASequence analysisSequence variants 4~11%Deletion/ duplication analysis 2, 5Partial- or whole-gene deletionsRare1. Eleven percent of individuals with the presumptive clinical diagnosis of AS have normal results for all testing methods described in this table.2. Testing that identifies deletions/duplications not readily detectable by sequence analysis of the coding and flanking intronic regions of genomic DNA; included in the variety of methods that may be used are: quantitative PCR, long-range PCR, multiplex ligation-dependent probe amplification (MLPA), and chromosomal microarray (CMA) that includes this gene/chromosome segment. Extent of deletion detected may vary by method and by laboratory. 3. Deletion analysis of the AS imprinting center (IC) detects small deletions, which account for 10%-20% of imprinting defects (IDs).4. Examples of mutations detected by sequence analysis may include small intragenic deletions/insertions and missense, nonsense, and splice site mutations.5. Although CMA usually detects large 15q11.2-13 deletions, in rare instances CMA has detected UBE3A multiexonic or whole-gene deletions [Lawson-Yuen et al 2006, Sato et al 2007].Interpretation of test results. For issues to consider in interpretation of sequence analysis results, click here.Possible explanations for the failure to detect AS-causing genetic abnormalities in the 11% or more of individuals with clinically diagnosed AS:Incorrect clinical diagnosisUndetected mutations in the regulatory region(s) of UBE3AOther unidentified mechanisms or gene(s) involved in UBE3A functionTesting StrategyTo confirm/establish the diagnosis in a probandDNA methylation analysis identifies approximately 80% of individuals with AS and is typically the first test ordered.If DNA methylation analysis is normal, UBE3A sequence analysis is the next appropriate diagnostic test.To establish the molecular basis of AS for genetic counseling purposesIf DNA methylation analysis is abnormal, deletion/duplication analysis is performed.If a deletion is found, a chromosome rearrangement (rarely observed) should be excluded. Note: Methylation analysis and UPD studies do not detect chromosomal rearrangements.If deletion/duplication analysis is normal, analysis of DNA polymorphisms on chromosome 15 can be used to detect UPD.If UPD of chromosome 15 is not present, further DNA studies (usually in a research laboratory) can determine if an IC deletion is present.Prenatal diagnosis and preimplantation genetic diagnosis (PGD) for at-risk pregnancies require prior identification of the disease-causing mechanism in the family:Prenatal diagnosis using amniocytes can detect all known mechanisms that cause AS.PGD can detect disease-causing UBE3A mutations or IC deletions that have been previously identified in a family.Note: The relative hypomethylation of the early embryo and the placental tissue makes chorionic villus sampling (CVS) for prenatal diagnosis or PGD problematic for DNA methylation testing.Genetically Related (Allelic) DisordersPrader-Willi syndrome (PWS) is caused by loss of the paternally contributed 15q11.2-q13 region. Although PWS and AS are clinically distinct in older children, some clinical overlap exists (e.g., feeding difficulties, hypotonia, developmental delay) [Cassidy et al 2000] in children younger than age two years.Interstitial duplications of 15q11.2-q13 on the maternally derived chromosome cause a disorder clinically distinct from either AS or PWS. Individuals with dup15q11.2-1q13 do not have facial dysmorphism but have mild to moderately severe learning deficits and may have behaviors in the autism spectrum [Boyar et al 2001].}

Natural HistoryPrenatal history, fetal development, birth weight, and head circumference at birth are usually normal. Young infants with Angelman syndrome (AS) may have difficulties with breast feeding or bottle feeding (as a result of sucking difficulties) and muscular hypotonia. Gastroesophageal reflux may occur. Some infants have a happy affect with excessive chortling or paroxysms of laughter. Fifty percent of children develop microcephaly by age 12 months. Strabismus may also occur. Tremulous movements may be noted prior to age 12 months, often with increased deep-tendon reflexes.AS may first be suspected in a toddler because of delayed gross motor milestones, muscular hypotonia, and/or speech delay [Williams et al 2006].Seizures typically occur between ages one and three years and can be associated with generalized, somewhat specific EEG changes: runs of high-amplitude delta activity with intermittent spike and slow-wave discharges (at times observed as a notched delta pattern); runs of rhythmic theta activity over a wide area; and runs of rhythmic sharp theta activity of 5-6/s over the posterior third of the head, forming complexes with small spikes. These are usually facilitated by or seen only with eye closure [Boyd et al 1997, Rubin et al 1997, Korff et al 2005].Seizure types can be quite varied and include both major motor and minor motor types (e.g., petit mal, atonic) [Galvan-Manso et al 2005, Pelc et al 2008a, Thibert et al 2009, Fiumara et al 2010]. Infantile spasms are rare. Non-convulsive status epilepticus may occur [Pelc et al 2008a]. Brain MRI may show mild atrophy and mild dysmyelination, but no structural lesions [Harting et al 2009, Castro-Gago et al 2010].The average child with AS walks between ages 2.5 and six years [Lossie et al 2001] and at that time may have a jerky, robot-like, stiff gait, with uplifted, flexed, and pronated forearms, hypermotoric activity, excessive laughter, protruding tongue, drooling, absent speech, and social-seeking behavior. Ten percent of children are nonambulatory.Sleep problems are well known in individuals with AS; frequent awakening at night is common [Bruni et al 2004, Didden et al 2004]. Dyssomnias (difficulties in initiating or maintaining sleep), irregular sleep-wake cycles, disruptive night behaviors such as periods of laughter, and sleep-related seizures have been reported [Pelc et al 2008b]. Essentially all young children with AS have some component of hyperactivity; males and females appear equally affected. Infants and toddlers may have seemingly ceaseless activity, constantly keeping their hands or toys in their mouth, and/or moving from object to object. Some behaviors may suggest an autism spectrum problem but social engagement is typically good and stereotypic behaviors such as lining up of toys or fascination with spinning objects or flashing lights rarely occur [Walz 2007].Language impairment is severe. Appropriate use of even one or two words in a consistent manner is rare. Receptive language skills are always more advanced than expressive language skills [Gentile et al 2010]. Most older children and adults with AS are able to communicate by pointing and using gestures and by using communication boards. Effective fluent use of sign language does not occur [Clayton-Smith 1993].Pubertal onset and development are generally normal in AS. Fertility appears to be normal; procreation appears possible for both males and females. Lossie & Driscoll [1999] reported transmission of an AS deletion to a fetus by the affected mother.Young adults appear to have good physical health with the exception of possible seizures. Constipation is common. Many are treated for gastroesophageal reflux symptoms. Scoliosis becomes more common with advancing age.Independent living is not possible for adults with AS; many live at home or in home-like placements. Life span data are not available, but life span appears to be nearly normal.

Genotype-Phenotype CorrelationsAll genetic mechanisms that give rise to AS lead to a somewhat uniform clinical picture of severe-to-profound intellectual disability, movement disorder, characteristic behaviors, and severe limitations in speech and language. However, some clinical differences correlate with genotype [Smith et al 1997, Fridman et al 2000, Lossie et al 2001, Varela et al 2004]. These correlations are broadly summarized below:The 5- to 7-Mb deletion class results in the most severe phenotype with microcephaly, seizures, motor difficulties (e.g., ataxia, muscular hypotonia, feeding difficulties), and language impairment. They also have lower body mass index compared to individuals with UPD or imprinting defects [Tan et al 2011]. There is some suggestion that individuals with larger deletions (e.g., BP1-BP3 [class I] break points) may have more language impairment or autistic traits than those with BP2-BP3 (class II) break points [Sahoo et al 2006] (see Figure 2).Individuals with UPD have better physical growth (e.g., less likelihood of microcephaly), fewer movement abnormalities, less ataxia, and a lower prevalence (but not absence) of seizures than do those with other underlying molecular mechanisms [Lossie et al 2001, Saitoh et al 2005].Individuals with IDs or UPD have higher developmental and language ability than those with other underlying molecular mechanisms. Individuals who are mosaic for the nondeletion ID (approximately 20% of the ID group) have the most advanced speech abilities [Nazlican et al 2004]; they may speak up to 50-60 words and use simple sentences.Individuals with chromosome deletions encompassing OCA2 frequently have hypopigmented irides, skin, and hair. OCA2 encodes a protein important in tyrosine metabolism that is associated with the development of pigment in the skin, hair, and irides (see Oculocutaneous Albinism Type 2).FigureFigure 2. Schematic drawing of chromosome region 15q11.2-q13 indicating the breakpoint regions BP1-BP6. Low copy repeat elements (LCR's) are located within these breakpoint regions (see text for details). Approximately 90% of chromosome deletions resulting (more...)

Differential DiagnosisInfants with AS commonly present with nonspecific psychomotor delay and/or seizures; therefore, the differential diagnosis is often broad and nonspecific, encompassing such entities as cerebral palsy, static encephalopathy, or mitochondrial encephalomyopathy. The tremulousness and jerky limb movements seen in most infants with AS may help distinguish AS from these conditions. The following disorders that mimic AS need to be considered in the differential diagnosis:Mowat-Wilson syndrome can present with happy affect, seizures, prominent mandible, upturned prominent ear lobes, diminished speech, microcephaly, constipation, and, on occasion Hirschsprung disease [Zweier et al 2005]. Congenital heart defects or agenesis of the corpus callosum can also occur. Mowat-Wilson syndrome results from heterozygous mutations in, or deletion of, ZEB2.The characteristic features of the Pitt-Hopkins syndrome (PTHS) are intellectual disability, wide mouth and distinctive facial features, and intermittent hyperventilation followed by apnea [Zweier et al 2007]. Features that may overlap with AS include microcephaly, seizures, ataxic gait, and happy personality [Takano et al 2010]. Diurnal hyperventilation, a salient feature in some, occurs after age three years [Peippo et al 2006]. Inheritance of PTHS is usually autosomal dominant and caused by heterozygous TCF4 mutations or deletions; however, on rare occasion inheritance is autosomal recessive with underlying mutations in NRXN1 or CTNAP2 [Zweier et al 2009].Christianson syndrome, an X-linked disorder caused by mutations in SLC9A6, can mimic AS. The clinical features include apparently happy disposition, severe cognitive delays, ataxia, microcephaly, and a seizure disorder [Christianson et al 1999, Gilfillan et al 2008, Schroer et al 2010]. Affected individuals may have a thin body appearance and may lose ambulation after age ten years. Some may have cerebellar and brain stem atrophy [Gilfillan et al 2008]. Although seizures are present in both conditions, the EEG pattern appears to differ: AS typically shows a generalized high amplitude, slow spike/wave (1.5-3 Hz) pattern while those with an SLC9A6 mutation lack the AS EEG pattern and have a more rapid (10-14 Hz) background frequency [Gilfillan et al 2008]. Female infants with seizures, acquired microcephaly, and severe speech impairment can resemble girls with Rett syndrome, caused by mutation of MECP2. Girls with Rett syndrome usually do not have a distinctive happy demeanor and girls with AS do not have a neuroregressive course or lack purposeful use of their hands. Older girls with undiagnosed Rett syndrome may have features that resemble AS [Watson et al 2001]. Sometimes infants with AS who present with feeding difficulties and muscle hypotonia are misdiagnosed as having Prader-Willi syndrome because the 15q11.2-q13 deletion, detected by chromosomal microarray or FISH, was not proven by DNA methylation analysis to be of maternal origin.Microdeletions of 2q23.1 involving MBD5 may result in severe speech delay, seizures, behavioral disorders, and microcephaly. Some individuals present with an AS-like phenotype [Williams et al 2010b, van Bon et al 2010]. Other chromosome disorders can mimic some of the features of AS, especially 22q13.3 deletion (Phelan-McDermid syndrome) [Precht et al 1998], characterized by nondysmorphic facial features, absent or minimal speech, and moderate to severe developmental delay, sometimes with behavioral features in the autism disorders spectrum. Additional microdeletion disorders, especially newer ones detected by chromosomal microarray may be associated with some features of AS [Brunetti-Pierri et al 2008, Sharkey et al 2009].MECP2 duplication (typically encompassing an approximately 500-kb region at Xq28) in males is characterized by severe developmental impairment, absent speech, seizures, and ataxic gait with spastic paraparesis. Although adult males are typically nonambulatory and are prone to infectious illnesses, children may have relatively nonspecific findings that include features of intellectual disability with autism, absent speech, and unstable gait [Van Esch et al 2005, Friez et al 2006, Lugtenberg et al 2009]. Adenylosuccinate lyase deficiency results in accumulation of succinylpurines leading to psychomotor retardation, autistic features, hypotonia, and seizures [Spiegel et al 2006]. Motor apraxia, severe speech deficits, excessive laughter, a very happy disposition, hyperactivity, a short attention span, mouthing of objects, tantrums, and stereotyped movements have been reported in female sibs [Gitiaux et al 2009]. Diagnostic testing involves detection of succinylaminoimidazole carboxamide riboside (SAICA riboside) and succinyladenosine (S-Ado) in cerebrospinal fluid, urine, and (to a lesser extent) in plasma. The rare metabolic disorder of severe methylene-tetrahydrofolate-reductase (MTHFR) deficiency associated with low methionine and elevated homocysteine blood levels was reported in a boy with happy demeanor, ataxic gait, absent speech, and flattened occiput [Arn et al 1998]. On rare occasions, congenital disorders of glycosylation (CDG) can mimic the features of AS especially if the affected child has unstable gait, speech impairment, and seizures.Note to clinicians: For a patient-specific ‘simultaneous consult’ related to this disorder, go to , an interactive diagnostic decision support software tool that provides differential diagnoses based on patient findings (registration or institutional access required).

ManagementEvaluations Following Initial DiagnosisTo establish the extent of disease in an individual diagnosed with Angelman syndrome (AS), the following evaluations focused on neurologic assessment and good preventive practice are recommended:Baseline brain MRI and EEG
Note: Typically, management of seizures (or assessment of risk for seizures) is not significantly helped by repetitive EEG or MRI testing.Musculoskeletal examination for scoliosis and gait impairment (e.g., extent of foot pronation or ankle subluxation; tight Achilles tendons) and the extent of muscular hypotonia. Orthopedic referral as needed.Ophthalmology examination for strabismus, evidence of ocular albinism (in deletion-positive AS), and visual acuityDevelopmental evaluation focused on: (1) nonverbal language ability and related educational and teaching strategies; and (2) physical therapy to enable optimal ambulationEvaluation for gastroesophageal reflux in infants and young children. Dietary evaluation to assure optimal nutritional status.Treatment of ManifestationsFeeding problems in newborns may require special nipples and other strategies to manage weak or uncoordinated sucking.Gastroesophageal reflux can be associated with poor weight gain and emesis; the customary medical treatment (i.e., upright positioning, motility drugs) is usually effective; sometimes fundoplication as required.Many antiepileptic drugs (AEDs) have been used to treat seizures in individuals with AS; no one drug has proven superior. Medications used for minor motor seizures (e.g., valproic acid, clonazepam, topiramate, lamotrigine, ethosuximide) are more commonly prescribed than medications for major motor seizures (e.g., diphenylhydantoin, phenobarbital) [Nolt et al 2003]. Carbamezapine, although not contraindicated, is infrequently used compared to other common anticonvulsants. Single medication use is preferred, but seizure breakthrough is common. A few individuals with AS have infrequent seizures and are not on AEDs. Some with uncontrollable seizures have benefited from a ketogenic diet.Hypermotoric behaviors are typically resistant to behavioral therapies; accommodation by the family and provision of a safe environment are important.Most children with AS do not receive drug therapy for hyperactivity, although some may benefit from the use of stimulant medications such as methylphenidate (Ritalin^®).Behavioral modification is effective in treating undesirable behaviors that are socially disruptive or self-injurious.A full range of educational training and enrichment programs should be available. Unstable or nonambulatory children may benefit from physical therapy. Occupational therapy may help improve fine motor and oral-motor control. Special adaptive chairs or positioners may be required, especially for extremely ataxic children. Speech therapy is essential and should focus on nonverbal methods of communication. Augmentative communication aids such as picture cards or communication boards should be used at the earliest appropriate time. Attempts to teach signing should begin as soon as the child is sufficiently attentive. Individualization and flexibility in the school are important educational strategies.Special physical provisions in the classroom, along with teacher aides or assistants, may be needed for effective class integration. Children with AS with excessive hypermotoric behaviors need an accommodating classroom space. Many families construct safe but confining bedrooms to accommodate disruptive nighttime wakefulness. Use of sedatives such as chloral hydrate or diphenylhydramines (Benadryl^®) may be helpful. Administration of 0.3 mg melatonin one hour before sleep may be helpful in some, but should not be given in the middle of the night if the child awakens.Strabismus may require surgical correction.Constipation often requires regular use of laxatives such as high fiber or lubricating agents.Orthopedic problems, particularly subluxed or pronated ankles or tight Achilles tendons, can be corrected by orthotic bracing or surgery.Thoraco-lumbar jackets may be needed for scoliosis, and individuals with severe curvature may benefit from surgical rod stabilization.Prevention of Secondary ComplicationsChildren with AS are at risk for medication overtreatment because their movement abnormalities can be mistaken for seizures and because EEG abnormalities can persist even when seizures are controlled.Use of sedating agents such as phenothiazines or other neuroleptic drugs is not advised because they cause negative side effects.Older adults tend to become less mobile and less active; attention to activity schedules may be helpful in reducing the extent of scoliosis and obesity.SurveillanceThe following are appropriate:Annual clinical examination for scoliosisFor older children, evaluation for the development of obesity associated with excessive appetiteAgents/Circumstances to AvoidCarbamezapine, although not contraindicated, is infrequently used compared to other common anticonvulsants. Vigabatrin and tigabine (anticonvulsants that increase brain GABA levels) are contraindicated in individuals with Angelman syndrome. For unknown reasons however, carbamazapine, vigabatrine and tigabine can cause development of other seizure types or non-convulsive status epilepticus. This paradoxic seizure development is not limited to individuals with Angelman syndrome [Pelc et al 2008a]Evaluation of Relatives at RiskSee Genetic Counseling for issues related to testing of at-risk relatives for genetic counseling purposes.Therapies Under InvestigationClinical trials involving oral administration of folate, vitamin B₁₂, creatine, and betaine have been undertaken in an attempt to augment DNA methylation pathways and possibly increase expression of the paternal UBE3A allele in the central nervous system; however, the initial trial did not demonstrate significant clinical benefit [Peters et al 2010]. Click here for more information.Search ClinicalTrials.gov for access to information on clinical studies for a wide range of diseases and conditions.OtherExcessive tongue protrusion causes drooling; available surgical or medication treatments (e.g., surgical reimplantation of the salivary ducts or use of local scopolamine patches) are generally not effective.

Molecular GeneticsInformation in the Molecular Genetics and OMIM tables may differ from that elsewhere in the GeneReview: tables may contain more recent information. —ED.Table A. Angelman Syndrome: Genes and DatabasesView in own windowGene SymbolChromosomal LocusProtein NameLocus SpecificHGMDUBE3A15q11​.2Ubiquitin-protein ligase E3AUBE3A homepage - Mendelian genesUBE3AData are compiled from the following standard references: gene symbol from HGNC; chromosomal locus, locus name, critical region, complementation group from OMIM; protein name from UniProt. For a description of databases (Locus Specific, HGMD) to which links are provided, click here.Table B. OMIM Entries for Angelman Syndrome (View All in OMIM) View in own window 105830ANGELMAN SYNDROME; AS 601623UBIQUITIN-PROTEIN LIGASE E3A; UBE3AMolecular Genetic PathogenesisGenomic imprinting is a phenomenon in mammals in which particular genes, depending on the sex of the parent of origin, are not equally expressed. The cardinal features of AS result from deficient expression or function of the maternally inherited UBE3A allele [Jiang et al 1999, Lossie et al 2001, Nicholls & Knepper 2001]. Ubiquitin-protein ligase E3A is involved in the ubiquitination pathway, which targets selected proteins for degradation.UBE3A displays predominant maternal expression in human fetal brain and adult frontal cortex [Rougeulle et al 1997, Vu & Hoffman 1997, Herzing et al 2001]. In mouse, maternal allele-specific expression is detected in specific brain subregions including hippocampus, Purkinje cells of the cerebellum, mitral cells of the olfactory bulb, and visual cortex [Albrecht et al 1997, Jiang et al 1998, Yashiro et al 2009]. It is possible that there is widespread, if not global, UBE3A allele-specific expression in mouse and in human brain neurons. Primary cell cultures from fetal mouse brain have demonstrated that UBE3A imprinting is limited to neurons, but glial cells show biallelic expression [Yamasaki et al 2003]. Studies with RNA-FISH suggest that preferential maternal expression of UBE3A occurs in lymphoblasts and fibroblasts, but the differential expression between the parental alleles is not as striking as it is in brain [Herzing et al 2002].UBE3A has a large 5' CpG island, but in contrast to genes in the "PWS critical region," DNA methylation does not differ between the maternal and paternal alleles [Lossie et al 2001].Because no differentially methylated region is present in UBE3A, it has been proposed that the imprinted expression of UBE3A may be regulated indirectly through a paternally expressed antisense transcript [Rougeulle et al 1998]. Runte et al [2001] have shown that a long SNURF-SNRPN sense/UBE3A antisense RNA transcript exists in the AS/PWS region, starting from the SNURF-SNRPN IC and extending more than 460 kb to at least the 5' end of UBE3A. It has been proposed that this UBE3A antisense transcript blocks paternal UBE3A expression. Normal allelic variants. UBE3A spans approximately 120 kb of genomic DNA and contains 16 exons. The 5' untranslated region (UTR) extends several kilobases upstream from the initiation site and spans an additional six to nine exons [Kishino et al 1997, Vu & Hoffman 1997, Yamamoto et al 1997, Kishino & Wagstaff 1998], whereas the 3' UTR extends an additional 2.0 kb [Kishino & Wagstaff 1998]. To date, alternative splicing of the 5' UTR accounts for the production of nine adult and two fetal transcripts [Kishino et al 1997, Vu & Hoffman 1997, Yamamoto et al 1997, Kishino & Wagstaff 1998], which are translated into three different protein isoforms. The functions of the different protein isoforms are unknown.Isoform I (NM_130838.1, NP_570853.1) corresponds to the open reading frame for E6-AP (see Normal gene product). Isoform II (NM_000462.3, NP_000453.2) has an additional 20 amino acids.Isoform III (NM_130839.2, NP_570854.1) has an additional 23 amino acids at the amino terminus. Pathologic allelic variantsDeletions of 15q11.2-q13 (65%-75%). Three chromosomal break points (proximal BP1, BP2, and a distal BP3) are involved in most AS-causing deletion events involving 15q11.1-q13. These deletions span approximately 5-7 Mb [Knoll et al 1990, Amos-Landgraf et al 1999, Christian et al 1999] (see Figure 2). Fewer than 10% of individuals with AS may have deletions extending from the BP1/BP2 region to regions more distal, at BP4 or BP5 locations (see Figure 2) [Sahoo et al 2007].
The BP1, BP2, and BP3 regions are characterized by low-copy repeat regions (LCRs) that contain repeats mainly derived from the ancestral HEct domain and RCc1 domain protein 2 (HERC2) genes [Pujana et al 2002]. The BP sites distal to BP3 contain other LCRs (e.g., without HERC2 duplications) that share chromosome 15-derived repeated DNA elements.
Microdeletions that flank the typical deletion region and include areas between BP1 and BP2 [Doornbos et al 2009], BP3 and BP4 [Rosenfeld et al 2011], and the more distal microdeletion syndrome involving region 15q13.3 [Masurel-Paulet et al 2010] have been described. Individuals with these deletions do not exhibit features of the AS.Genomic abnormalities of 15q11.1-q13. It is possible that in otherwise normal individuals preexisting genomic abnormalities may predispose to deletion of 15q11.1-q13 in the germline resulting in offspring with AS.A proportion of mothers who have a child with an AS deletion have been found to have inversions in the 15q11.2-q13 region (the region deleted in the offspring with AS) [Gimelli et al 2003].A kindred in which two individuals had deletions (one deletion causing PWS and the other causing AS) has been previously reported to be associated with an inherited inverted intrachromosomal insertion of 15q11.2-q13 [Collinson et al 2004]. Paternal uniparental disomy of chromosome 15 (3%-7%). In contrast to PWS, the paternal UPD observed in AS is most likely to be postzygotic in origin [Robinson et al 2000]. Paternal UPD of meiotic origin does occur but this mechanism is less common than the maternal UPD associated with PWS.Imprinting defects (3%). This subset of individuals with AS have a defect in the IC that disrupts the resetting of the normal imprint during gametogenesis. Even though these individuals have biparental inheritance of chromosome 15, the maternal 15q11.2-q13 region has a paternal epigenotype and is, therefore, transcriptionally incompetent for the maternal-only expressed gene(s) in this region [Glenn et al 1993, Buiting et al 2001, Buiting et al 2003].
Mapping of these deletions (as well as mapping of the IC deletions that are associated with PWS) has delineated two small regions of deletion overlap (SRO) that define two critical elements in the IC region, the AS-SRO and the PWS-SRO [Buiting et al 1995] (see Figure 2). The PWS-SRO is 4.3 kb in size and overlaps with the SNURF-SNRPN exon1/promoter region [Ohta et al 1999]. IC deletions found in individuals with AS affect the more centromeric SNURF-SNRPN promoter/exon 1 region. The smallest region of overlap in patients with AS and an IC deletion (AS-SRO) is 880 bp in size and maps 35 kb proximal to SNURF-SNRPN exon 1 [Buiting et al 1999, Horsthemke & Buiting 2008]. Most individuals with AS caused by IC defects do not have a deletion of the AS IC region, but rather have epigenetic defects that disrupt IC function.UBE3A (5%-11%). Sequence analysis of individuals with AS reveals that the vast majority of UBE3A mutations result in (or predict) protein truncation [Kishino et al 1997, Matsuura et al 1997, Kishino & Wagstaff 1998, Malzac et al 1998, Lossie et al 2001].
More than 60 mutations have been reported and 60%-70% of these involve small deletions and duplications leading to frameshift mutations [Camprubi et al 2009, Stenson et al 2009, Abaied et al 2010]. Another approximately 25% involve missense and nonsense mutations with the remainder representing splicing defects, gross deletions, and complex rearrangements [Stenson et al 2009].
All mutations noted thus far are predicted to disrupt the HECT ligase domain. Exons 9 and 16, which code for part of the HECT domain, account for a high percentage of all mutations but these coding regions are disproportionately large so the high percentage probably does not represent true hot spots for mutation. It is possible that individuals with milder-effect mutations (e.g., certain missense and in-frame deletions or duplications) may show some, but not all, of the clinical features associated with AS. A few individuals with AS have been found to have complete or partial deletions of UBE3A or to have intragenic deletions. Some types of deletion testing methods may be able to detect some of these deletions (see Table 1) [Lawson-Yuen et al 2006, Sato et al 2007]. For more information, see Table A.Normal gene product. UBE3A produces the 865-amino acid protein E6-associated protein (E6AP), which acts as a cellular ubiquitin ligase enzyme. It is termed “E6-associated” because it was first discovered as the protein able to associate with p53 in the presence of the E6 oncoprotein of the human papilloma virus, type 16. The function of the E6AP enzyme is to create a covalent linkage (e.g., the “ligase” function) between the small (~76-amino acid) ubiquitin molecule and its target protein. After initial ubiquitin attachment, E6AP can then add ubiquitins onto the first ubiquitin to create a polyubiquitylated substrate. Proteins modified in this way can be targeted for degradation through the 26S proteasome complex. The E6AP is the prototype of what is termed the E3 component of the ubiquitin cycle; E1 and E2 proteins respectively activate and transfer the ubiquitin molecule to E3. Then E3 is able to bind to a target protein and transfer and ligate ubiquitin to the target. This ligation reaction occurs mainly in a catalytic region of the E3 enzyme called the homologous to E6AP C terminus) domain [Verdecia et al 2003].Abnormal gene product. Disruption of UBE3A could affect crucial neuronal processes of protein degradation and replacement that would otherwise be balanced or maintained by a functional ubiquitin-proteosome system. The ubiquitin-proteasome pathway is essential for cellular functioning including signal transduction, cell cycle progression, DNA repair, and transcriptional regulation [Ciechanover 1998, Hershko & Ciechanover 1998]. Several E6AP protein targets have been discovered [Kuhne & Banks 1998, Kumar et al 1999, Oda et al 1999, Khan et al 2006, Li et al 2006, Reiter et al 2006, Louria-Hayon et al 2009, Shimoji et al 2009], and two recently identified target proteins, ARC (activity-regulated cytoskeleton-associated protein) and ephexin-5, appear to be crucial components of synaptic plasticity and dendrite growth regulation [Greer et al 2010, Margolis et al 2010, Scheiffele & Beg 2010]. The ARC protein is involved in regulating membrane stabilization of excitatory postsynaptic receptors. The guanine exchange protein, ephexin-5, is known to regulate activity of EphB receptor signaling that is a crucial component of dendritic growth [Margolis et al 2010]. Eph receptors are known to be enriched at synapses and are important in regulating dendritic spine density. The EphB receptors interact with ephrin ligands and regulate dendritic development through small GTPases of the Rho family (Rho, Rac, and Cdc42) by activation of guanine nucleotide exchange factors (GEFs) [Murai & Pasquale 2003]. It is unknown how abnormalities in E6AP-target protein interaction lead to AS but the recently identified targets strongly indicate that the UBE3A protein is crucial to development of normal synapses and neural plasticity.