Behavioural effects of imprinted genes

Next Steps in Prader-Willi Syndrome Research: On the Relationship between Genotype and Phenotype

This article reviews what we know of the phenotype and genotype of Prader-Willi syndrome and hypothesizes two possible paths from phenotype to genotype. It then suggests research that may strengthen the case for one or other of these hypotheses.

Epigenetic Mechanisms of ART-Related Imprinting Disorders: Lessons From iPSC and Mouse Models

The rising frequency of ART-conceived births is accompanied by the need for an improved understanding of the implications of ART on gametes and embryos. Increasing evidence from mouse models and human epidemiological data suggests that ART procedures may play a role in the pathophysiology of certain imprinting disorders (IDs), including Beckwith-Wiedemann syndrome, Silver-Russell syndrome, Prader-Willi syndrome, and Angelman syndrome. The underlying molecular basis of this association, however, requires further elucidation. In this review, we discuss the epigenetic and imprinting alterations of in vivo mouse models and human iPSC models of ART. Mouse models have demonstrated aberrant regulation of imprinted genes involved with ART-related IDs. In the past decade, iPSC technology has provided a platform for patient-specific cellular models of culture-associated perturbed imprinting. However, despite ongoing efforts, a deeper understanding of the susceptibility of iPSCs to epigenetic perturbation is required if they are to be reliably used for modelling ART-associated IDs. Comparing the patterns of susceptibility of imprinted genes in mouse models and IPSCs in culture improves the current understanding of the underlying mechanisms of ART-linked IDs with implications for our understanding of the influence of environmental factors such as culture and hormone treatments on epigenetically important regions of the genome such as imprints.

Detailed analysis of paternal knockout Grb10 mice suggests effects on stability of social behavior, rather than social dominance

Imprinted genes are highly expressed in monoaminergic regions of the midbrain and their functions in this area are thought to have an impact on mammalian social behaviors. One such imprinted gene is Grb10, of which the paternal allele is generally recognized as mediating social dominance behavior. However, there has been no detailed study of social dominance in Grb10^+/p mice. Moreover, the original study examined tube‐test behavior in isolated mice 10 months of age. Isolation testing favors more territorial and aggressive behaviors, and does not address social dominance strategies employed in group housing contexts. Furthermore, isolation stress impacts midbrain function and dominance related behavior, often through alterations in monoaminergic signaling. Thus, we undertook a systematic study of Grb10^+/p social rank and dominance behavior within the cage group, using a number of convergent behavioral tests. We examined both male and female mice to account for sex differences and tested cohorts aged 2, 6 and 10 months to examine any developments related to age. We found group‐housed Grb10^+/p mice do not show evidence of enhanced social dominance, but cages containing Grb10^+/p and wild‐type mice lacked the normal correlation between three different measures of social rank. Moreover, a separate study indicated isolation stress induced inconsistent changes in tube test behavior. Taken together, these data suggest future research on Grb10^+/p mice should focus on the stability of social behaviors, rather than dominance per se.

Genomic Imprinting and Physiological Processes in Mammals

Complex multicellular organisms, such as mammals, express two complete sets of chromosomes per nucleus, combining the genetic material of both parents. However, epigenetic studies have demonstrated violations to this rule that are necessary for mammalian physiology; the most notable parental allele expression phenomenon is genomic imprinting. With the identification of endogenous imprinted genes, genomic imprinting became well-established as an epigenetic mechanism in which the expression pattern of a parental allele influences phenotypic expression. The expanding study of genomic imprinting is revealing a significant impact on brain functions and associated diseases. Here, we review key milestones in the field of imprinting and discuss mechanisms and systems in which imprinted genes exert a significant role.

Territorial Behavior and Social Stability in the Mouse Require Correct Expression of Imprinted Cdkn1c

Genomic imprinting, the epigenetic process by which transcription occurs from a single parental allele, is believed to influence social behaviors in mammals. An important social behavior is group living, which is enriched in Eutherian mammals relative to monotremes and marsupials. Group living facilitates resource acquisition, defense of territory and co-care of young, but requires a stable social group with complex inter-individual relationships. Co-occurring with increased group living in Eutherians is an increase in the number of imprinted loci, including that spanning the maternally expressed Cdkn1c. Using a 'loss-of-imprinting' model of Cdkn1c (Cdkn1cBACx1), we demonstrated that twofold over expression of Cdkn1c results in abnormal social behaviors. Although, our previous work indicated that male Cdkn1cBACx1 mice were more dominant as measured by tube test encounters with unfamiliar wild-type (WT) males. Building upon this work, using more ecologically relevant assessments of social dominance, indicated that within their normal social group, Cdkn1cBACx1 mice did not occupy higher ranking positions. Nevertheless, we find that presence of Cdkn1cBACx1 animals within a group leads to instability of the normal social hierarchy, as indicated by greater variability in social rank within the group over time and an increase in territorial behavior in WT cage-mates. Consequently, these abnormal behaviors led to an increased incidence of fighting and wounding within the group. Taken together these data indicate that normal expression of Cdkn1c is required for maintaining stability of the social group and suggests that the acquisition of monoallelic expression of Cdkn1c may have enhanced social behavior in Eutherian mammals to facilitate group living.

Dopaminergic and behavioural changes in a loss-of-imprinting model of Cdkn1c

The imprinted gene Cdkn1c is expressed exclusively from the maternally inherited allele as a consequences of epigenetic regulation. Cdkn1c exemplifies many of the functional characteristics of imprinted genes, playing a role in foetal growth and placental development. However, Cdkn1c also plays an important role in the brain, being key to the appropriate proliferation and differentiation of midbrain dopaminergic neurons. Using a transgenic model (Cdkn1c^BACx1) with a twofold elevation in Cdkn1c expression that mimics loss‐of‐imprinting, we show that increased expression of Cdkn1c in the brain gives rise to neurobiological and behavioural changes indicative of a functionally altered dopaminergic system. Cdkn1c^BACX1 mice displayed altered expression of dopamine system‐related genes, increased tyrosine hydroxylase (Th) staining and increased tissue content of dopamine in the striatum. In addition, Cdkn1c^BACx1 animals were hypersensitive to amphetamine as showed by c‐fos expression in the nucleus accumbens. Cdkn1c^BACX1 mice had significant changes in behaviours that are dependent on the mesolimbic dopaminergic system. Specifically, increased motivation for palatable food stuffs, as indexed on a progressive ratio task. In addition, Cdkn1c^BACX1 mice displayed enhanced social dominance. These data show, for the first time, the consequence of elevated Cdkn1c expression on dopamine‐related behaviours highlighting the importance of correct dosage of this imprinted gene in the brain. This work has significant relevance for deepening our understanding of the epigenetic factors that can shape neurobiology and behaviour.

Impulsive choices in mice lacking imprinted Nesp55

Genomic imprinting is the process whereby germline epigenetic events lead to parent-of-origin specific monallelic expression of a number of key mammalian genes. The imprinted gene Nesp is expressed from the maternal allele only and encodes for Nesp55 protein. In the brain, Nesp55 is found predominately in discrete areas of the hypothalamus and midbrain. Previously, we have shown that loss of Nesp55 gives rise to alterations in novelty-related behaviour. Here, we extend these findings and demonstrate, using the Nesp^m/+ mouse model, that loss of Nesp55 leads to impulsive choices as measured by a delayed-reinforcement task, whereby Nesp^m/+ mice were less willing to wait for a delayed, larger reward, preferring instead to choose an immediate, smaller reward. These effects were highly specific as performance in another component of impulsive behaviour, the ability to stop a response once started as assayed in the stop-signal reaction time task, was equivalent to controls. We also showed changes in the serotonin system, a key neurotransmitter pathway mediating impulsive behaviour. First, we demonstrated that Nesp55 is co-localized with serotonin and then went on to show that in midbrain regions there were reductions in mRNA expression of the serotonin-specific genes Tph2 and Slc6a4, but not the dopamine-specific gene Th in Nesp^m/+ mice; suggesting an altered serotonergic system could contribute, in part, to the changes in impulsive behaviour. These data provide a novel mode of action for genomic imprinting in the brain and may have implications for pathological conditions characterized by maladaptive response control.

The evolving landscape of imprinted genes in humans and mice: Conflict among alleles, genes, tissues, and kin

Three recent genome-wide studies in mice and humans have produced the most definitive map to date of genomic imprinting (gene expression that depends on parental origin) by incorporating multiple tissue types and developmental stages. Here, we explore the results of these studies in light of the kinship theory of genomic imprinting, which predicts that imprinting evolves due to differential genetic relatedness between maternal and paternal relatives. The studies produce a list of imprinted genes with around 120-180 in mice and ~100 in humans. The studies agree on broad patterns across mice and humans including the complex patterns of imprinted expression at loci like Igf2 and Grb10. We discuss how the kinship theory provides a powerful framework for hypotheses that can explain these patterns. Finally, since imprinting is rare in the genome despite predictions from the kinship theory that it might be common, we discuss evolutionary factors that could favor biallelic expression.

Neural and behavioral epigenetics; what it is, and what is hype

The ability to examine epigenetic mechanisms in the brain has become readily available over the last 20 years. This has led to an explosion of research and interest in neural and behavioral epigenetics. Of particular interest to researchers, and indeed the lay public, is the possibility that epigenetic processes, such as changes in DNA-methylation and histone modification, may provide a biochemical record of environmental effects. This has led to some fascinating insights into how molecular changes in the brain can control behavior. However, some of this research has also attracted controversy and, as is dealt with here, some overblown claims. This latter problem is partly linked to the shifting sands of what is defined as 'epigenetics'. In this review, I provide an overview of what exactly epigenetics is, and what is hype, with the aim of opening up a debate as to how this exciting field moves forward.