Fierce: a new mouse deletion of Nr2e1; violent behaviour and ocular abnormalities are background-dependent

Abstract

A new spontaneous mouse mutation named fierce (frc) is deleted for the nuclear receptor Nr2e1 gene (also known as Tlx, mouse homolog of Drosophila tailless). The fierce mutation is genetically and phenotypically similar to Nr2e1 targeted mutations previously studied on segregating genetic backgrounds. However, we have characterized the fierce brain, eye, and behavioural phenotypes on three defined genetic backgrounds (C57BL/6J, 129P3/JEms, and B6129F1). The data revealed many novel and background-dependent phenotypic characteristics. Whereas abnormalities in brain development, hypoplasia of cerebrum and olfactory lobes, were consistent on all three backgrounds, our novel finding of enlarged ventricles in 100% and overt hydrocephalus in up to 30% of fierce mice were unique to the C57BL/6J background. Developmental eye abnormalities were also background-dependent with B6129F1-frc mice having less severe thinning of optic layers and less affected electroretinogram responses. Impaired regression of hyaloid vessels was observed in all backgrounds. Furthermore, retinal vessels were deficient in size and number in 129P3/JEms-frc and B6129F1-frc mice but almost entirely absent in C57BL/6J-frc mice. We present the first standardized behavioural tests conducted on Nr2e1 mutant mice and show that C57BL/6J-frc and B6129F1-frc mice have deficits in sensorimotor assays and are hyperaggressive in both sexes and backgrounds. However, C57BL/6J-frc mice were significantly more aggressive than B6129F1-frc mice. Overall, this extensive characterization of the fierce mutation is essential to its application for the study of behavioural, and brain and eye developmental disorders. In addition, the background-dependent differences revealed will enable the identification of important genetic modifiers.

Introduction

The study of heritable behaviour has been impeded by an evolving code of human ethics and a relative lack of animal models for behaviour [3]. Recently, behavioural studies using mouse model systems have emerged based on the powerful mouse genetics of single gene mutations. Using mouse models, numerous genes have been reported to contribute to reproductive, maternal, ingestive, and social behaviours [2], [29], [31]. In addition, genes involved in serotonin metabolism [6], [25], adrenergic alpha2C receptor, neuronal nitric oxide synthase [30], and enkephalins [23] have been implicated in aggressive behaviour in mice. These models have provided interesting insights into portions of behaviour that are influenced genetically.

To interpret the effects of a genetic lesion accurately, genetic background must be considered [6], [13], [33]. This may be particularly relevant for variable phenotypes such as behaviour studied in highly inbred strains recognized as having identifiable characteristics [6], [24]. Firstly, studying behavioural mutants on controlled backgrounds using inbred or F1 animals allows mutants to be compared with otherwise genetically identical littermates. This strategy also allows for reproducibility in time and place. Secondly, the use of multiple backgrounds highlights genetic variability and more closely parallels the differences between human individuals. This is especially true of F1 hybrid animals that provide the genetic variability of a heterozygous state at all loci on a controlled, reproducible, and non-segregating background. In addition, phenotypic differences between strains can lead to the identification of genetic modifiers in mice, and ultimately, in humans. Thus, the applicability of mouse genetics is enhanced by phenotypic assessment on multiple, defined backgrounds.

Two targeted disruption mutations in mice have been described for the gene Nr2e1 (nuclear receptor subfamily 2, group E, member 1), the mouse homologue of the Drosophila nuclear receptor tailless (also known as Tlx). These mutant mice have an extensive developmental defect of the anterior brain, aggressive behaviour [27], and severe impairment of retinal and optic nerve development [40]. However, a detailed characterization of the extent of aggressive behaviours has not been performed and this, along with brain and eye phenotypes, have only been described on mixed and segregating 129 and C57BL/6 genetic backgrounds. Furthermore, sensorimotor evaluation has not been performed on these mice—an evaluation of which is essential to the interpretation of other behaviours [5], [29].

Here we report on a new mouse mutation named ‘fierce’ (frc) that includes a spontaneous deletion of Nr2e1. The frc mutation has been studied on three commonly used genetic backgrounds, C57BL/6J, 129P3/JEms, and B6129F1, to comprehensively examine behaviour, brain, and eye phenotypes [9], [34]. Whereas many of the elements of the fierce phenotype are consistent on all three genetic backgrounds indicating their fundamental importance, we have identified some elements that are background-dependent, thus highlighting the genetic heterogeneity and potential for the identification of modifier genes in aggressive behaviour and brain development. In addition, we conducted a panel of sensorimotor, aggression, and mating tests on C57BL/6J and the hybrid B6129F1 mouse backgrounds. Such detailed behavioural characterization increases the value of this important mouse model permitting further inquiry not only into the consistent phenotypes of brain development and aggression but also into traits exposed on genetically distinct mouse backgrounds.