The prospect of molecular therapy for Angelman syndrome and other monogenic neurologic disorders

Bailus and Segal BMC Neuroscience 2014, 15:76 http://www.biomedcentral.com/1471-2202/15/76 REVIEW Open Access The prospect of molecular therapy for Angelman syndrome and other monogenic neurologic disorders Barbara J Bailus and David J Segal* Abstract Background: Angelman syndrome is a monogenic neurologic disorder that affects 1 in 15,000 children, and is characterized by ataxia, intellectual disability, speech impairment, sleep disorders, and seizures. The disorder is caused by loss of central nervous system expression of UBE3A, a gene encoding a ubiquitin ligase. Current treatments focus on the management of symptoms, as there have not been therapies to treat the underlying molecular cause of the disease. However, this outlook is evolving with advances in molecular therapies, including artificial transcription factors a class of engineered DNA-binding proteins that have the potential to target a specific site in the genome. Results: Here we review the recent progress and prospect of targeted gene expression therapies. Three main issues that must be addressed to advance toward human clinical trials are specificity, toxicity, and delivery. Conclusions: Artificial transcription factors have the potential to address these concerns on a level that meets and in some cases exceeds current small molecule therapies. We examine the possibilities of such approaches in the context of Angelman syndrome, as a template for other single-gene, neurologic disorders. Keywords: Artificial transcription factor, Engineered zinc finger, TALE, CRISPR, Gene regulation, Gene therapy, Blood–brain barrier, Angelman syndrome, Autism spectrum disorders Review Angelman syndrome is a neurodevelopmental disorder that affects 1 in 15,000 children [1]. The disease is characterized as an autism spectrum disorder with individuals exhibiting severe mental and physical impairments, including a lack of speech and ataxia. In a normal individual, the region encoding the gene UBE3A is epigenetically imprinted throughout neuronal brain cells, with the maternal allele being preferentially expressed and the paternal allele silenced [2]. In Angelman syndrome, expression of the active maternal allele is lost [3]. Loss of the maternal allele, while the paternal allele remains silenced, results in a lack of UBE3A expression. Approximately 70% of all cases involve a large, 5–7-Mb, denovo maternal deletion of the chromosome 15q11-q13 region, which includes the critical UBE3A gene [2]. The remaining known causes of Angelman syndrome involve mutations within UBE3A (~11%), uniparental disomy (~7%), and imprinting defects (~3%) [4]. About 10% of cases present phenotypically as Angelman syndrome but with currently unknown genetic or epigenetic causes. A gradient of severity affecting both motor function and cognitive ability is exhibited among Angelman syndrome individuals, correlating roughly with the size of the deletion. Individuals with point mutations tend to have less severe symptoms. There are no curative treatments for Angelman syndrome. Current treatments focus on behavioral and physical therapies to minimize symptoms. Drug therapies are used to control seizures and sleep disruption. However, the lack of potential therapies is rapidly changing as advances in molecular therapy that focus on altering a specific genes expression approach human clinical trials [5,6]. * Correspondence: Genome Center, MIND Institute, and Department of Biochemistry and Molecular Medicine, University of California, Davis, CA 95616, USA © 2014 Bailus and Segal; licensee BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Bailus and Segal BMC Neuroscience 2014, 15:76 http://www.biomedcentral.com/1471-2202/15/76 Importance of Ube3a and progress made toward restoring UBE3A expression For gene therapy to be a viable treatment option it is essential that the genetic target is known, and that there is evidence that a postnatal intervention would be beneficial. In 1997 mutations in UBE3A or that severely reduced expression of the maternal copy of UBE3A were found to be the causative for Angleman syndrome [3,7,8]. UBE3A is one of the many E3 ubiquitin ligases, which are known to add chains of ubiquitin to specific proteins and thus target them for proteasome degradation. A simple model for Angelman syndrome is that lack of UBE3A increases the concentration or persistence of its target proteins. However, more complex models would also need to consider that some E3 ligases facilitate monoubiquitination, which is associated with signaling rather than degradation, as well as evidence that UBE3A can act as a transcriptional co-regulator [9]. In 2011, a potential role for Ube3a in mouse neuronal synapse firing and longterm potentiation (LTP) was suggested by the discovery of its interactions with Arc and Ephexin 5. Arc was shown to be over-expressed in the absence of Ube3a, causing a depletion of AMPA receptors at the synapse and thus defects in synaptic plasticity, the chemical basis of learning and memory [10]. Also, the degradation of Ephexin5 was found to be mediated by Ube3a, which promoted aberrant excitatory synapse development [11]. More recently, loss of UBe3a was found to affect the cytoskeletal protein actin, providing an explanation for the known defects in dendritic spine density, LTP and learning [12]. However, there are likely many other targets and potential functions of UBE3A. Designing interventions to only one target or activity might produce only a partial benefit, and comprehensive therapy of all downstream effectors might be impractical. A more attractive therapeutic approach would be amelioration of the upstream causative event; that is, restoration of UBE3A expression. The developmental delay of Angelman syndrome generally becomes noticeable after 6 to 12 months of age. Since the brain has been without UBE3A expression throughout development, an important consideration is whether latestage (postnatal) intervention might have clinical value. The first study supporting that it could was based on the observation that αCaM kinase II was inhibited by phosphorylation in a mouse model of Angelman syndrome [13]. Substitutions that prevented the inhibition rescued many of the phenotypes of Angelman mice, suggesting that the major lesion was signaling and not abnormal development. Since αCaM kinase II is predominantly expressed postnatally [14,15], the study also suggested that other postnatal interventions might have efficacy. Since the underlying genetic defects are known, gene therapy could, in principle, (...truncated)