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Angelman Syndrome: an Autism Spectrum Disorder

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Angelman Syndrome: an Autism Spectrum Disorder Journal of Student Research (2017) Volume 6, Issue 2: pp. 56-60 Review Article Angelman Syndrome: An Autism Spectrum Disorder Andrew J. Kennedya and Jeffrey O. Hendersona Neurodevelopmental disorders limit the mental, physical, and social lives of affected individuals and their families. These disorders are often related to genetic abnormalities having a distinct chromosomal location. The abnormalities can cause incorrect proteins to be formed or biochemical pathways to be blocked, predominately affecting brain development, but also having pleiotropic effects. Research into defining and correcting these genetic abnormalities is important to help distinguish between unique neurodevelopmental disorders so that proper clinical interventions are available for affected individuals. In the following review, Angelman syndrome, which results from UBE3A gene function being lost at maternal chromosome 15q11.2-q13, will be discussed. Angelman patients suffer from the defining characteristics of speech impairment, uncontrolled laughing and smiling, motor development issues, muscle tension, and possible ataxia. The genetic mechanisms of the disorder as well as possible therapies will be discussed, with future areas of research into genetic therapies to treat Angelman syndrome also put forth. Research into Angelman syndrome can provide an avenue for a clearer understanding of other neurodevelopmental disorders. Keywords: Angelman syndrome, autism spectrum, UBE3A, maternal chromosome 15, neurodevelopmental disorder Introduction skills to be suppressed (Smith and Laan, 2003). PWS affects Angelman syndrome (AS) is a neuro-genetic disorder the same region of chromosome 15 as AS but requires lack of that is often mischaracterized as either profound autism or expression of genes on the paternally inherited chromosome cerebral palsy. AS was first described in 1965 by Dr. Harry 15q11.2-q13 region. PWS clinically manifests as mild Angelman (reviewed by Martins-Tyler et al., 2014) based on intellectual disability and hyperphagia. However, because his observations of three young patients affected by severe clinical manifestations are age dependent, the primary intellectual disability, microcephaly, developmental delay, methodology to distinguish the two syndromes apart is by speech impairment, lower motor skills, inappropriate laughter, DNA methylation analysis (Angulo et al., 2015). Other and ataxia (loss of full control of body movements) (reviewed autism spectrum disorders, including “pseudo-AS”, depend on by Williams et al., 2010). Later studies determined the deletions mapped to other chromosomes, including 2q23.1. phenotypic characteristics of AS are caused by loss-of- Pseudo-AS behaves similarly to AS; deletions knock out function of the ubiquitin ligase E3A (UBE3A) gene on genes regulating pathways important in normal brain and maternal chromosome 15q11.2-q13 (Bailus and Segal, 2014). muscle development and function (Mullegama et al., 2014). As a result of UBE3A being suppressed, or mutated, the ubiquitin-proteasome pathway is disrupted leading to loss of Molecular Genetics of AS proper functioning of many organ systems, specifically brain The primary cause of AS, ~75% of cases, is a de novo neurons (Peters et al., 2009). 3.08 Mb interstitial deletion encompassing the UBE3A gene Smith and coworkers (2003) reported an incidence rate at maternal 15q13.1-13.3 and affecting 15 other flanking for AS at ~ 1 in 10,000 newborns. The incidence rate has genes regulating language development (Pettigrew et al., changed drastically since the time of Harry Angelman as it 2015; Ranasinghe et al., 2015). Once a couple has a child occurred in 1 in 40,000 live births before better diagnostics, with this de novo deletion causing AS, the chances of having clinical phenotyping, and molecular genotyping increased another child with this deletion falls dramatically to ≤ 1%; accuracy. Genotypic testing is necessary to distinguish AS each birth is an independent event. However, the high from phenocopies such as individuals affected by Mowat- probability (1%) of the same mutation occurring in a sib Wilson syndrome, caused by loss-of-heterozygosity at the encompasses the possibilities of either germ line mosaicism ZEB2 locus, and Christianson syndrome, an X-linked disorder for the deletion or a balanced translocation in the mother with mutations in the SLC9A6I locus (Williams et al., 2010). (Williams et al., 2010). Microdeletions and other genetic However, in 10% of patients with phenotypic AS, there is no abnormalities can be tested prenatally in the fetus and can known genetic mechanism. Because AS and its sister disorder lead to a genotypic diagnosis for AS; as with all genetic Prader-Willi syndrome (PWS) result from abnormal testing technologies, these data allow the parents to either imprinting regulation, approximately 78% of genotypic terminate the pregnancy or prepare mentally and/or testing conducted analyzes DNA methylation patterns at the financially to care for a child having a genetic disorder 15q11.2-q13 chromosomal region (Smith and Laan, 2003; (Wapner et al., 2015). The deletion or point mutation, usually Strachan et al., 2015). These tests have a high success rate of leading to protein truncation, of UBE3A is the main cause of determining if a patient has AS instead of a phenocopy or AS; therefore, detecting these genetic abnormalities is the related neuro-disorder. main focus of research into prenatal testing for this disorder. AS is much like other neuro-degenerative disorders in These genetic testing technologies are also beneficial as an that there is a delay in brain development affecting mental, independent methodology for estimating incidence rates. emotional, and social aspects of affected individuals. AS However, ~10% of patients clinically categorized with AS differs in that it is common for ataxia to occur and/or motor have no genetic inconsistencies in maternal chromosome 15; a. Department of Science and Mathematics, Judson University, Elgin, IL, 60123 Journal of Student Research (2017) Volume 6, Issue 2: pp. 56-60 Review Article as a result, an incorrect diagnosis for another autism spectrum mice is rescued by decreasing expression of the synaptic disorder or similar brain development disorder, as mentioned protein ARC. These data demonstrate the complexity of the above, can occur, leading to incorrect treatment modalities neural circuits defective in AS and the multipronged approach (Smith and Laan, 2003). needed to develop clinical therapies to treat this disorder Deletion or inactivation of UBE3A, encoding E6- (Mandel-Brehm et al., 2015). associated protein (UBE3A/E6AP), defines AS from other Uniparental disomy (UPD) causes ~ 5-10% of AS cases. similar syndromes (Li and Qui, 2014; Mandel-Brehm et al., UPD arises when a zygote develops in which both copies of 2015). UBE3A/E6AP is an E3 ubiquitin ligase that conjugates one particular chromosome are inherited from one parent. ubiquitin groups to specific proteins targeting them to the UPD most often occurs from a trisomic conceptus, a zygote proteasome for degradation (Sell and Margolis, 2015). The that has received two homologous chromosomes from one S5a subunit of the proteasome tethers ubiquinated protein to parent and a single chromosome from the other parent. Loss the proteasome in preparation for proteolysis. S5a is mono- of the single chromosome copy soon after first cleavage ubiquinated by UBE3A/E6AP decreasing the enzymatic results in heterodisomy. Alternatively, a monosomic zygote is activity of the proteasome. In UBE3A/E6AP deficient cells, rescued by duplication of the single chromosome, resulting in not only are substrates of UBE3A/E6AP not ubiquinated, but isodisomy (two identical copies of a chromosome; Strachan et the activity of the proteasome is perturbed (Tomaic and al., 2015). Paternal UPD 15 results in a lack of expression of Banks, 2015). UBE3A in neurons. The majority of UPD AS cases are caused Molecular genetic analysis reveals that in mammals, the by paternal isodisomy 15 (Smith and Laan, 2003). UPD paternal allele of UBE3A is silenced by genomic imprinting in induced AS symptoms are not as severe as those cases neurons. Imprinting is the phenomenon by which specific resulting from deletions or point mutations; thus, a lower genes are expressed based on the parental chromosome percentage of patients experience seizures and seizures that do inherited. In the case of UBE3A, the paternal copy is silenced occur are milder (Angulo et al., 2015; Williams et al., 2010). in the brain by an antisense RNA, UBE3A-ATS, expressed from the paternally inherited chromosome (Meng et al., 2013; Detection Strachan et al., 2015). Because the imprinting of UBE3A is Detection of Angelman has changed dramatically over evolutionarily conserved, the mouse ortholog, Ube3a, can be the years ranging from genetic, anatomical, and physiological genetically manipulated to recapitulate AS in mice. To this tests to determine if a patient suffers from UBE3A end, transgenic maternally deficient Ube3a mice (Ube3am-/p+) suppression. Electroencephalography (EEG) is commonly were created to test the hypothesis that lack of maternal used to detect seizures in AS patients with 85% of affected expression of the Ube3a allele is necessary and sufficient to children developing seizures by age three (Ranasinghe et al., cause AS. These mice exhibit AS pathogenesis confirming the 2015). Seizures persist into adulthood usually showing a hypothesis that maternal Ube3A deficiency is the cause of AS decrease
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