Research reportBehavioral deficits in an Angelman syndrome model: Effects of genetic background and age
Highlights
► Ube3am−/p+ mice had impaired reversal learning in the Morris water maze. ► Deficient acquisition of spatial learning varied across background strain and age. ► Aberrant phenotypes included deficits in rearing, rotarod ability, and marble-burying. ► The C57BL/6J background conferred susceptibility to a range of abnormal behaviors.
Introduction
Angelman syndrome (AS) is a severe genomic imprinting disorder with phenotypes that typically manifest early in childhood. AS is characterized by intellectual disability, speech impairment, motor dysfunction, sleep disturbances, epilepsy, inappropriate laughter, and an unusually happy demeanor [1], [2]. In most cases, AS arises from the deletion or mutation of maternal UBE3A [3], [4], which encodes ubiquitin protein ligase E3A. In neurons, only the maternal copy of UBE3A is active, while the paternally inherited UBE3A allele is silenced [5], [6]. Evidence from mouse lines with targeted disruption of the maternal Ube3a allele supports an important role for UBE3A in neuronal morphology, synaptic function, and the maturation of neocortical circuits in the brain [7], [8], [9], [10]. Maternal Ube3a-deficient mice (Ube3am−/p+ mice) have abnormal phenotypes that resemble many of the clinical symptoms observed in AS, including motor dysfunction, cognitive deficits, and enhanced susceptibility to seizures [11]. Ube3am−/p+ mice also have overt deficiencies in hippocampal long-term potentiation and dendritic spine density, in line with deficits in behavioral tasks, such as contextual fear conditioning and spatial learning in the Morris water maze, which are mediated by the hippocampus [9], [11], [12], [13], [14].
Although no effective treatment currently exists for AS, recent findings utilizing genetic mouse models for the disorder have suggested that abnormal phenotypes can be rescued [12], [14], [15]. For example, van Woerden et al. [14] were able to genetically rescue the deficits in rotarod performance, quadrant selectivity in the water maze, and contextual fear conditioning that characterize Ube3am−/p+ mice. Our research group recently demonstrated that topoisomerase inhibitors can unsilence paternally inherited Ube3a [16], but the possible beneficial effects of these drugs on abnormal behaviors relevant to AS have not yet been established. While the Ube3am−/p+ mice provide a well-validated model for preclinical efficacy testing, there is a critical need to identify the optimal phenotypes to target for reversal in drug discovery studies. Importantly, there are known strain-specific differences in behavioral phenotypes [11]. Moreover, even in the most carefully controlled studies, it can be difficult to find behavioral phenotypes sufficiently penetrant for inter-species, inter-laboratory, and intra-laboratory reproducibility [17]. We therefore sought to identify AS phenotypes of sufficient magnitude and consistency to be suitable for screening potential therapeutics. Toward this goal, the present studies evaluated Ube3am−/p+ mice on two different genetic backgrounds, either 129S7/SvEvBrd-Hprtb-m2 (129) or C57BL/6J (B6), using multi-component phenotyping regimens and testing at different ages. Because clinical studies have linked genotype to differential developmental trajectories in AS [18], separate cohorts of B6 mice were evaluated, beginning from either adolescence or adulthood, to examine both phenotypic trajectories and reproducibility.
Section snippets
Animals
Subjects were heterozygous mice with maternal deficiency of Ube3a (Ube3am−/p+) and wild type (Ube3am+/p+) littermates, on two different background strains: 129S7/SvEvBrd-Hprtb-m2 (129) and C57BL/6J (B6) [11]. The Ube3am+/p− mice on a 129 strain background were developed by the Beaudet laboratory [11] and were obtained from Jackson Laboratory (Bar Harbor, ME). The Ube3am−/p+ mice on a B6 background were originally developed by the Beaudet laboratory [11] and were backcrossed at least 10
Weight
In line with previous reports [14], maternal-deficiency of Ube3a led to significantly increased body weight in almost all of the Ube3am−/p+ groups (Fig. 1). Higher body weights were most pronounced in the female mice on a 129 background [repeated measures ANOVA; genotype main effect, F(1,10) = 7.33, p = 0.0221], and on a B6 background [genotype × age interaction, F(4,44) = 13.25, p < 0.0001]. The increased body weights were also observed in the male mice on a B6 background [B6 males, cohort 1; genotype ×
Discussion
The present studies demonstrate that AS-like behavioral phenotypes in Ube3am−/p+ mice can vary across genetic background, age at testing, and cohort group. In particular, Ube3am−/p+ mice on the B6 background exhibited motor deficits and impaired acquisition of spatial learning, dependent on age and cohort. Marked deficits in spatial learning acquisition were found in B6 mice at 16 weeks of age, but not at 8 weeks of age, suggesting progressive loss of cognitive function across time. In
Acknowledgments
The authors wish to thank Dr. Yong-hui Jiang for his kind generosity in providing Ube3a breeding pairs for our colony. This work was supported by the Simons Foundation, the Angelman Syndrome Foundation, NIMH grant 1R01MH093372 to BDP, and NICHD grant P30HD03110 to Dr. Joe Piven. These sponsors did not have involvement in study design, data collection, analysis, or interpretation, writing the report, or decision to submit the article for publication.
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