Preliminary Characterization of Dosage Effects of UBE3A on Cognitive and Motor Phenotypes in Mouse Models

Background: The ubiquitin protein ligase E3A gene (UBE3A) is located in the 15q11-q13 region of the genome. Mutations within this region have been associated with various types of neurodevelopmental disorders, which suggests UBE3A is dosage-sensitive (Beaudet, 2011; McNamara & Isles, 2013). Maternally derived deletions, mutations, or epimutations that lead to a loss of expression or function of UBE3A cause Angelman syndrome (AS, Jiang et al., 1999), while elevated UBE3A is hypothesized to cause Dup15q syndrome. The discovery of a patient with a severe developmental delay contributed to the idea of gene-dosage effects. In this case study, a maternally inherited 129 Kb duplication in chromosome region 15q11.2 encompassing only the UBE3A gene was described (Noor et al., 2015). Studies of (D)UBE3a in Drosophila (Ferdousy et al., 2011; Hope et al., 2016; Jensen et al., 2013) and rodent models (Daily et al., 2011; Hethorn et al., 2015; Huang et al., 2013) further corroborates the theory.

Objectives: The present experiments were designed to evaluate cognitive and motor phenotypes in mice with mutated Ube3a (deletion), similar to AS, or increased Ube3a (overexpression), modeling Dup15q syndrome. These domains were chosen due to the phenotypic profile of individuals with AS and Dup15q, which includes deficits in motor abilities, coordination, gait, ataxia and intellectual disabilities. Behaviorally, beam walking, rotarod and open field were utilized to test motoric abnormalities. The touchscreen spatial reversal task was chosen due to its conceptual and technical similarities to the NIH Toolbox battery used for testing cognitive functions in children with minimal demand on motor abilities (Silverman et al., 2013).

Methods: Breeding pairs were acquired from The Jackson Laboratory. Mice were bred in a conventional mouse vivarium at the University of California Davis School of Medicine in Sacramento. Motor assays (beam walking, rotarod and open field) were run according to previously published methods (Silverman et al., 2011; Stanley et al., 2005; Wohr et al., 2013). Spatial reversal was tested in the automated Bussey-Saksida touchscreen system for mice (Campden Instruments Ltd/Lafayette Instruments, Lafayette, IL, USA), using a procedure slightly modified from the methods described previously (Buscher et al., 2016; McTighe et al., 2009).

Results: Ube3a mutant mice displayed deficits in open field, rotarod and beam walking when compared to their WT littermates. Our data replicate and extend prior findings (Bruinsma et al., 2015; Huang et al., 2013). Preliminary data indicate impaired touchscreen training for spatial reversal in Ube3a mutants by requiring more days to reach criteria in each training phase compared to WT littermates. No genotype differences were detected in Ube3aoverexpressing mice on motor tasks, while spatial reversal data is currently being analyzed.

Conclusions: The current results suggest a dosage-sensitive effect of Ube3a mutations on behavior indicating that an optimal dose of Ube3a is required for typical motoric and cognitive flexibility. Importantly, the current findings can and will be utilized as outcome measures for preclinical pharmacological and stem cell-delivered genetic intervention strategies.