Issues in the search for candidate genes in mice as potential animal models of human aggression

Mouse model systems to study sex chromosome genes and behavior: relevance to humans

Sex chromosome genes directly influence sex differences in behavior. The discovery of the Sry gene on the Y chromosome (Gubbay et al., 1990; Koopman et al., 1990) substantiated the sex chromosome mechanistic link to sex differences. Moreover, the pronounced connection between X chromosome gene mutations and mental illness produces a strong sex bias in these diseases. Yet, the dominant explanation for sex differences continues to be the gonadal hormones. Here we review progress made on behavioral differences in mouse models that uncouple sex chromosome complement from gonadal sex. We conclude that many social and cognitive behaviors are modified by sex chromosome complement, and discuss the implications for human research. Future directions need to include identification of the genes involved and interactions with these genes and gonadal hormones.

Sex chromosome complement and gonadal sex influence aggressive and parental behaviors in mice

Across human cultures and mammalian species, sex differences can be found in the expression of aggression and parental nurturing behaviors: males are typically more aggressive and less parental than females. These sex differences are primarily attributed to steroid hormone differences during development and/or adulthood, especially the higher levels of androgens experienced by males, which are caused ultimately by the presence of the testis-determining gene Sry on the Y chromosome. The potential for sex differences arising from the different complements of sex-linked genes in male and female cells has received little research attention. To directly test the hypothesis that social behaviors are influenced by differences in sex chromosome complement other than Sry, we used a transgenic mouse model in which gonadal sex and sex chromosome complement are uncoupled. We find that latency to exhibit aggression and one form of parental behavior, pup retrieval, can be influenced by both gonadal sex and sex chromosome complement. For both behaviors, females but not males with XX sex chromosomes differ from XY. We also measured vasopressin immunoreactivity in the lateral septum, which was higher in gonadal males than females, but also differed according to sex chromosome complement. These results imply that a gene(s) on the sex chromosomes (other than Sry) affects sex differences in brain and behavior. Identifying the specific X and/or Y genes involved will increase our understanding of normal and abnormal aggression and parental behavior, including behavioral abnormalities associated with mental illness.

A model system for study of sex chromosome effects on sexually dimorphic neural and behavioral traits

We tested the hypothesis that genes encoded on the sex chromosomes play a direct role in sexual differentiation of brain and behavior. We used mice in which the testis-determining gene (Sry) was moved from the Y chromosome to an autosome (by deletion of Sry from the Y and subsequent insertion of an Sry transgene onto an autosome), so that the determination of testis development occurred independently of the complement of X or Y chromosomes. We compared XX and XY mice with ovaries (females) and XX and XY mice with testes (males). These comparisons allowed us to assess the effect of sex chromosome complement (XX vs XY) independent of gonadal status (testes vs ovaries) on sexually dimorphic neural and behavioral phenotypes. The phenotypes included measures of male copulatory behavior, social exploration behavior, and sexually dimorphic neuroanatomical structures in the septum, hypothalamus, and lumbar spinal cord. Most of the sexually dimorphic phenotypes correlated with the presence of ovaries or testes and therefore reflect the hormonal output of the gonads. We found, however, that both male and female mice with XY sex chromosomes were more masculine than XX mice in the density of vasopressin-immunoreactive fibers in the lateral septum. Moreover, two male groups differing only in the form of their Sry gene showed differences in behavior. The results show that sex chromosome genes contribute directly to the development of a sex difference in the brain.