Imprinted genes and mental dysfunction

Exploring transgenerational inheritance in epigenotypes of DAT heterozygous rats: Circadian anomalies and attentional vulnerability

Dopamine (DA) is mainly involved in locomotor activity, reward processes and maternal behaviors. Rats with KO gene for dopamine transporter (DAT), coding for a truncated DAT protein, are in hyperdopaminergic conditions and thus develop stereotyped behaviors and hyperactivity. Our aim was to test the prior transgenerational modulation of wild and truncated alleles as expressed in heterozygous DAT rats: specifically, we addressed the possible sequelae due to genotype and gender of the ancestors, with regard to behavioral differences in F1, F2, F3 rats. We studied non-classical DAT heterozygotes (HETs) based on two specular lines, with putative grand-maternal vs. grand-paternal imprinting. MAT females (F1; offspring of KO male and WT female) mated with a KO male to generate MIX offspring (F2). Specularly, PAT females (F1; offspring of KO female and WT male) mated with a KO male to generate PIX offspring (F2). Similarly to PAT, we obtained MUX (F2; HET offspring of MAT sire and KO dam); we also observed the F3 (MYX: HET offspring of KO male and MUX female, thus with DAT-KO maternal grandmother like also for PIX). We studied their circadian cycle of locomotor activity and their behavior in the elevated-plus-maze (EPM). Locomotor hyper-activity occurs in F1, the opposite occurs in F2, with MYX rats appearing undistinguishable from WT ones. Open-arm preference emerged in PIX and MIX rats. Only MAT and MYX rats showed a significant vulnerability for ADHD-like inattentive symptoms (duration of rearing in the EPM; Viggiano et al., 2002). A risk-taking profile is evident in the F2 phenotype, while inattentiveness from F1 progeny tends to be transferred to F3. We hypothesize that DAT-related phenotypes result from effective inheritance through pedigree of imprints that are dependent on grandparents, suggesting a protective role for gestation within a hyperdopaminergic uterus. For major features, similar odd (F1, F3) generations appear opposed to even (F2) ones; for minor specific features, the phenotype transfer may affect the progenies with a male but not a female DAT-KO ancestor.

Exploring sex differences: insights into gene expression, neuroanatomy, neurochemistry, cognition, and pathology

Increased knowledge about sex differences is important for development of individualized treatments against many diseases as well as understanding behavioral and pathological differences. This review summarizes sex chromosome effects on gene expression, epigenetics, and hormones in relation to the brain. We explore neuroanatomy, neurochemistry, cognition, and brain pathology aiming to explain the current state of the art. While some domains exhibit strong differences, others reveal subtle differences whose overall significance warrants clarification. We hope that the current review increases awareness and serves as a basis for the planning of future studies that consider both sexes equally regarding similarities and differences.

Intergenerational longitudinal associations between parental reading/musical traits, infants' auditory processing, and later phonological awareness skills

The intergenerational transmission of language/reading skills has been demonstrated by evidence reporting that parental literacy abilities contribute to the prediction of their offspring's language and reading skills. According to the "Intergenerational Multiple Deficit Model," literacy abilities of both parents are viewed as indicators of offspring's liability for literacy difficulties, since parents provide offspring with genetic and environmental endowment. Recently, studies focusing on the heritability of musical traits reached similar conclusions. The "Musical Abilities, Pleiotropy, Language, and Environment (MAPLE)" framework proposed that language/reading and musical traits share a common genetic architecture, and such shared components have an influence on the heritable neural underpinnings of basic-level skills underlying musical and language traits. Here, we investigate the intergenerational transmission of parental musical and language-related (reading) abilities on their offspring's neural response to a basic auditory stimulation (neural intermediate phenotype) and later phonological awareness skills, including in this complex association pattern the mediating effect of home environment. One-hundred and seventy-six families were involved in this study. Through self-report questionnaires we assessed parental reading abilities and musicality, as well as home literacy and musical environment. Offspring were involved in a longitudinal study: auditory processing was measured at 6  months of age by means of a Rapid Auditory Processing electrophysiological paradigm, and phonological awareness was assessed behaviorally at 5  years of age. Results reveal significant correlations between parents' reading skills and musical traits. Intergenerational associations were investigated through mediation analyses using structural equation modeling. For reading traits, the results revealed that paternal reading was indirectly associated with children's phonological awareness skills via their electrophysiological MisMatch Response at 6  months, while maternal reading was directly associated with children's phonological awareness. For musical traits, we found again that paternal musicality, rather than maternal characteristics, was associated with children's phonological phenotypes: in this case, the association was mediated by musical environment. These results provide some insight about the intergenerational pathways linking parental reading and musical traits, neural underpinnings of infants' auditory processing and later phonological awareness skills. Besides shedding light on possible intergenerational transmission mechanisms, this study may open up new perspectives for early intervention based on environmental enrichment.

Evidence for parent-of-origin effects in autism spectrum disorder: a narrative review

Autism spectrum disorder (ASD) is a heterogeneous group of early-onset neurodevelopmental disorders known to be highly heritable with a complex genetic architecture. Abnormal brain developmental trajectories that impact synaptic functioning, excitation-inhibition balance and brain connectivity are now understood to play a central role in ASD. Ongoing efforts to identify the genetic underpinnings still prove challenging, in part due to phenotypic and genetic heterogeneity.This review focuses on parent-of-origin effects (POEs), where the phenotypic effect of an allele depends on its parental origin. POEs include genomic imprinting, transgenerational effects, mitochondrial DNA, sex chromosomes and mutational transmission bias. The motivation for investigating these mechanisms in ASD has been driven by their known impacts on early brain development and brain functioning, in particular for the most well-documented POE, genomic imprinting. Moreover, imprinting is implicated in syndromes such as Angelman and Prader-Willi, which frequently share comorbid symptoms with ASD. In addition to other regions in the genome, this comprehensive review highlights the 15q11-q13 and 7q chromosomal regions as well as the mitochondrial DNA as harbouring the majority of currently identified POEs in ASD.

Quantitative genome-wide association analyses of receptive language in the Danish High Risk and Resilience Study

BACKGROUND

One of the most basic human traits is language. Linguistic ability, and disability, have been shown to have a strong genetic component in family and twin studies, but molecular genetic studies of language phenotypes are scarce, relative to studies of other cognitive traits and neurodevelopmental phenotypes. Moreover, most genetic studies examining such phenotypes do not incorporate parent-of-origin effects, which could account for some of the heritability of the investigated trait. We performed a genome-wide association study of receptive language, examining both child genetic effects and parent-of-origin effects.

RESULTS

Using a family-based cohort with 400 children with receptive language scores, we found a genome-wide significant paternal parent-of-origin effect with a SNP, rs11787922, on chromosome 9q21.31, whereby the T allele reduced the mean receptive language score by ~ 23, constituting a reduction of more than 1.5 times the population SD (P = 1.04 × 10-8). We further confirmed that this association was not driven by broader neurodevelopmental diagnoses in the child or a family history of psychiatric diagnoses by incorporating covariates for the above and repeating the analysis.

CONCLUSIONS

Our study reports a genome-wide significant association for receptive language skills; to our knowledge, this is the first documented genome-wide significant association for this phenotype. Furthermore, our study illustrates the importance of considering parent-of-origin effects in association studies, particularly in the case of cognitive or neurodevelopmental traits, in which parental genetic data are not always incorporated.

Reciprocal F1 Hybrids of Two Inbred Mouse Strains Reveal Parent-of-Origin and Perinatal Diet Effects on Behavior and Expression

Parent-of-origin effects (POE) in mammals typically arise from maternal effects or imprinting. In some instances, such POE have been associated with psychiatric disorders, as well as with changes in a handful of animal behaviors. However, POE on complex traits such as behavior remain largely uncharacterized. Moreover, although both behavior and epigenetic effects are known to be modified by perinatal environmental exposures such as nutrient deficiency, the architecture of such environment-by-POE is mostly unexplored. To study POE and environment-by-POE, we employ a relatively neglected but especially powerful experimental system for POE-detection: reciprocal F1 hybrids (RF1s). We exposed female NOD/ShiLtJ×C57Bl/6J and C57Bl/6J×NOD/ShiLtJ mice, perinatally, to one of four different diets, then after weaning recorded a set of behaviors that model psychiatric disease. Whole-brain microarray expression data revealed an imprinting-enriched set of 15 genes subject to POE. The most-significant expression POE, on the non-imprinted gene Carmil1 (a.k.a. Lrrc16a), was validated using qPCR in the same and in a new set of mice. Several behaviors, especially locomotor behaviors, also showed POE. Bayesian mediation analysis suggested Carmil1 expression suppresses behavioral POE, and that the imprinted gene Airn suppresses POE on Carmil1 expression. A suggestive diet-by-POE was observed on percent center time in the open field test, and a significant diet-by-POE was observed on one imprinted gene, Mir341, and on 16 non-imprinted genes. The relatively small, tractable set of POE and diet-by-POE detected on behavior and expression here motivates further studies examining such effects across RF1s on multiple genetic backgrounds.

Epigenetic factors and autism spectrum disorders

Autism is a complex neurodevelopmental disorder that has significant phenotypic overlap with several diseases, many of which fall within the broader category of autism spectrum disorders (ASDs). The etiology of the disorder is unclear and seems to involve a complex interplay of polygenic as well as environmental factors. We discuss evidence that suggests that epigenetic dysregulation is highly implicated as a contributing cause of ASDs and autism. Specifically, we examine neurodevelopmental disorders that share significant phenotypic overlap with ASDs and feature the dysregulation of epigenetically modified genes including UBE3A, GABA receptor genes, and RELN. We then look at the dysregulated expression of implicated epigenetic modifiers, namely MeCP2, that yield complex and varied downstream pleiotropic effects. Finally, we examine epigenetically mediated parent-of-origin effects through which paternal gene expression dominates that of maternal contributing to contrasting phenotypes implicated in ASDs. Such preliminary evidence suggests that elucidating the complex role of epigenetic regulations involved in ASDs could prove vital in furthering our understanding of the complex etiology of autism and ASDs.

Genomic scan of selective sweeps in thin and fat tail sheep breeds for identifying of candidate regions associated with fat deposition

BACKGROUND

Identification of genomic regions that have been targets of selection for phenotypic traits is one of the most important and challenging areas of research in animal genetics. However, currently there are relatively few genomic regions identified that have been subject to positive selection. In this study, a genome-wide scan using ~50,000 Single Nucleotide Polymorphisms (SNPs) was performed in an attempt to identify genomic regions associated with fat deposition in fat-tail breeds. This trait and its modification are very important in those countries grazing these breeds.

RESULTS

Two independent experiments using either Iranian or Ovine HapMap genotyping data contrasted thin and fat tail breeds. Population differentiation using FST in Iranian thin and fat tail breeds revealed seven genomic regions. Almost all of these regions overlapped with QTLs that had previously been identified as affecting fat and carcass yield traits in beef and dairy cattle. Study of selection sweep signatures using FST in thin and fat tail breeds sampled from the Ovine HapMap project confirmed three of these regions located on Chromosomes 5, 7 and X. We found increased homozygosity in these regions in favour of fat tail breeds on chromosome 5 and X and in favour of thin tail breeds on chromosome 7.

CONCLUSIONS

In this study, we were able to identify three novel regions associated with fat deposition in thin and fat tail sheep breeds. Two of these were associated with an increase of homozygosity in the fat tail breeds which would be consistent with selection for mutations affecting fat tail size several thousand years after domestication.

Serotonin transporter gene polymorphism and psychiatric disorders: is there a link?

Though still in infancy, the field of psychiatric genetics holds great potential to contribute to the development of new diagnostic and therapeutic options to treat these disorders. Among a large number of existing neurotransmitter systems, the serotonin system dysfunction has been implicated in many psychiatric disorders and therapeutic efficacy of many drugs is also thought to be based on modulation of serotonin. Serotonin transporter gene polymorphism is one of the most extensively studied polymorphisms in psychiatric behavioral genetics. In this article, we review the status of evidence for association between the serotonin gene polymorphism and some common mental disorders like affective disorders, post-traumatic stress disorder, obsessive-compulsive disorder, suicide, autism, and other anxiety and personality disorders. Going beyond traditional association studies, gene-environment interaction, currently gaining momentum, is also discussed in the review. While the existing information of psychiatric genetics is inadequate for putting into practice genetic testing in the diagnostic work-up of the psychiatric patient, if consistent in future research attempts, such results can be of great help to improve the clinical care of a vast majority of patients suffering from such disorders.

Epigenetics in Developmental Disorder: ADHD and Endophenotypes

Heterogeneity in attention-deficit/hyperactivity disorder (ADHD), with complex interactive operations of genetic and environmental factors, is expressed in a variety of disorder manifestations: severity, co-morbidities of symptoms, and the effects of genes on phenotypes. Neurodevelopmental influences of genomic imprinting have set the stage for the structural-physiological variations that modulate the cognitive, affective, and pathophysiological domains of ADHD. The relative contributions of genetic and environmental factors provide rapidly proliferating insights into the developmental trajectory of the condition, both structurally and functionally. Parent-of-origin effects seem to support the notion that genetic risks for disease process debut often interact with the social environment, i.e., the parental environment in infants and young children. The notion of endophenotypes, markers of an underlying liability to the disorder, may facilitate detection of genetic risks relative to a complex clinical disorder. Simple genetic association has proven insufficient to explain the spectrum of ADHD. At a primary level of analysis, the consideration of epigenetic regulation of brain signalling mechanisms, dopamine, serotonin, and noradrenaline is examined. Neurotrophic factors that participate in the neurogenesis, survival, and functional maintenance of brain systems, are involved in neuroplasticity alterations underlying brain disorders, and are implicated in the genetic predisposition to ADHD, but not obviously, nor in a simple or straightforward fashion. In the context of intervention, genetic linkage studies of ADHD pharmacological intervention have demonstrated that associations have fitted the "drug response phenotype," rather than the disorder diagnosis. Despite conflicting evidence for the existence, or not, of genetic associations between disorder diagnosis and genes regulating the structure and function of neurotransmitters and brain-derived neurotrophic factor (BDNF), associations between symptoms-profiles endophenotypes and single nucleotide polymorphisms appear reassuring.

The role of imprinted genes in mediating susceptibility to neuropsychiatric disorders

Imprinted genes, which are thought to comprise <1% of the mammalian genome, are defined by their parent-of-origin specific monoallelic expression arising as a consequence of differential epigenetic marking of alleles in the paternal and maternal germlines. Such genes are highly represented in the brain and placental transcriptomes, and have been shown to exert significant influence on fundamental developmental processes in these organs. Converging evidence from work in man and animal models has shown that imprinted genes can influence a variety of brain and behavioral endophenotypes. In this article, we review the current evidence that imprinted gene dysfunction is associated with vulnerability to several common psychiatric disorders. We also discuss how studying imprinted gene (dys)function may provide mechanistic insights into two important areas in modern psychiatry: first, how environmental factors (especially in utero) interact with genetic liability via epigenetic mechanisms to predispose to later mental illness, and second, the molecular underpinnings of sex-specific vulnerability to psychiatric disorders.

Human intelligence and polymorphisms in the DNA methyltransferase genes involved in epigenetic marking

Epigenetic mechanisms have been implicated in syndromes associated with mental impairment but little is known about the role of epigenetics in determining the normal variation in human intelligence. We measured polymorphisms in four DNA methyltransferases (DNMT1, DNMT3A, DNMT3B and DNMT3L) involved in epigenetic marking and related these to childhood and adult general intelligence in a population (n = 1542) consisting of two Scottish cohorts born in 1936 and residing in Lothian (n = 1075) or Aberdeen (n = 467). All subjects had taken the same test of intelligence at age 11yrs. The Lothian cohort took the test again at age 70yrs. The minor T allele of DNMT3L SNP 11330C>T (rs7354779) allele was associated with a higher standardised childhood intelligence score; greatest effect in the dominant analysis but also significant in the additive model (coefficient = 1.40_additive; 95%CI 0.22,2.59; p = 0.020 and 1.99_dominant; 95%CI 0.55,3.43; p = 0.007). The DNMT3L C allele was associated with an increased risk of being below average intelligence (OR 1.25_additive; 95%CI 1.05,1.51; p = 0.011 and OR 1.37_dominant; 95%CI 1.11,1.68; p = 0.003), and being in the lowest 40^th (p_additive = 0.009; p_dominant = 0.002) and lowest 30^th (p_additive = 0.004; p_dominant = 0.002) centiles for intelligence. After Bonferroni correction for the number variants tested the link between DNMT3L 11330C>T and childhood intelligence remained significant by linear regression and centile analysis; only the additive regression model was borderline significant. Adult intelligence was similarly linked to the DNMT3L variant but this analysis was limited by the numbers studied and nature of the test and the association was not significant after Bonferroni correction. We believe that the role of epigenetics in the normal variation in human intelligence merits further study and that this novel finding should be tested in other cohorts.

Global methylation profiling of lymphoblastoid cell lines reveals epigenetic contributions to autism spectrum disorders and a novel autism candidate gene, RORA, whose protein product is reduced in autistic brain

Autism is currently considered a multigene disorder with epigenetic influences. To investigate the contribution of DNA methylation to autism spectrum disorders, we have recently completed large-scale methylation profiling by CpG island microarray analysis of lymphoblastoid cell lines derived from monozygotic twins discordant for diagnosis of autism and their nonautistic siblings. Methylation profiling revealed many candidate genes differentially methylated between discordant MZ twins as well as between both twins and nonautistic siblings. Bioinformatics analysis of the differentially methylated genes demonstrated enrichment for high-level functions including gene transcription, nervous system development, cell death/survival, and other biological processes implicated in autism. The methylation status of 2 of these candidate genes, BCL-2 and retinoic acid-related orphan receptor alpha (RORA), was further confirmed by bisulfite sequencing and methylation-specific PCR, respectively. Immunohistochemical analyses of tissue arrays containing slices of the cerebellum and frontal cortex of autistic and age- and sex-matched control subjects revealed decreased expression of RORA and BCL-2 proteins in the autistic brain. Our data thus confirm the role of epigenetic regulation of gene expression via differential DNA methylation in idiopathic autism, and furthermore link molecular changes in a peripheral cell model with brain pathobiology in autism.

Genomic imprinting in the development and evolution of psychotic spectrum conditions

I review and evaluate genetic and genomic evidence salient to the hypothesis that the development and evolution of psychotic spectrum conditions have been mediated in part by alterations of imprinted genes expressed in the brain. Evidence from the genetics and genomics of schizophrenia, bipolar disorder, major depression, Prader-Willi syndrome, Klinefelter syndrome, and other neurogenetic conditions support the hypothesis that the etiologies of psychotic spectrum conditions commonly involve genetic and epigenetic imbalances in the effects of imprinted genes, with a bias towards increased relative effects from imprinted genes with maternal expression or other genes favouring maternal interests. By contrast, autistic spectrum conditions, including Kanner autism, Asperger syndrome, Rett syndrome, Turner syndrome, Angelman syndrome, and Beckwith-Wiedemann syndrome, commonly engender increased relative effects from paternally expressed imprinted genes, or reduced effects from genes favouring maternal interests. Imprinted-gene effects on the etiologies of autistic and psychotic spectrum conditions parallel the diametric effects of imprinted genes in placental and foetal development, in that psychotic spectrum conditions tend to be associated with undergrowth and relatively-slow brain development, whereas some autistic spectrum conditions involve brain and body overgrowth, especially in foetal development and early childhood. An important role for imprinted genes in the etiologies of psychotic and autistic spectrum conditions is consistent with neurodevelopmental models of these disorders, and with predictions from the conflict theory of genomic imprinting.

Psychosis and autism as diametrical disorders of the social brain

Autistic-spectrum conditions and psychotic-spectrum conditions (mainly schizophrenia, bipolar disorder, and major depression) represent two major suites of disorders of human cognition, affect, and behavior that involve altered development and function of the social brain. We describe evidence that a large set of phenotypic traits exhibit diametrically opposite phenotypes in autistic-spectrum versus psychotic-spectrum conditions, with a focus on schizophrenia. This suite of traits is inter-correlated, in that autism involves a general pattern of constrained overgrowth, whereas schizophrenia involves undergrowth. These disorders also exhibit diametric patterns for traits related to social brain development, including aspects of gaze, agency, social cognition, local versus global processing, language, and behavior. Social cognition is thus underdeveloped in autistic-spectrum conditions and hyper-developed on the psychotic spectrum.;>We propose and evaluate a novel hypothesis that may help to explain these diametric phenotypes: that the development of these two sets of conditions is mediated in part by alterations of genomic imprinting. Evidence regarding the genetic, physiological, neurological, and psychological underpinnings of psychotic-spectrum conditions supports the hypothesis that the etiologies of these conditions involve biases towards increased relative effects from imprinted genes with maternal expression, which engender a general pattern of undergrowth. By contrast, autistic-spectrum conditions appear to involve increased relative bias towards effects of paternally expressed genes, which mediate overgrowth. This hypothesis provides a simple yet comprehensive theory, grounded in evolutionary biology and genetics, for understanding the causes and phenotypes of autistic-spectrum and psychotic-spectrum conditions.

What are imprinted genes doing in the brain?

As evidence for the existence of brain-expressed imprinted genes accumulates, we need to address exactly what they are doing in this tissue, especially in terms of organisational themes and the major challenges posed by reconciling imprinted gene action in brain with current evolutionary theories attempting to explain the origin and maintenance of genomic imprinting. We are at the beginning of this endeavor and much work remains to be done but already it is clear that imprinted genes have the potential to influence diverse behavioral processes via multiple brain mechanisms. There are also grounds to believe that imprinting may contribute to risk of mental and neurological disease. As well as being a source of basic information about imprinted genes in the brain (e.g., via the newly established website, www.bgg.cardiff.ac.uk/imprinted_tables/index. html), we have used this chapter to identify and focus on a number of key questions. How are brain-expressed imprinted genes organised at the molecular and cellular levels? To what extent does imprinted action depend on neurodevelopmental mechanisms? Do imprinted gene effects interact with other epigenetic influences, especially early on in life? Are imprinted effects on adult behaviors adaptive or just epiphenomena? If they are adaptive, what areas of brain function and behavior might be sensitive to imprinted effects? These are big questions and, as shall become apparent, we need much more data, arising from interactions between behavioral neuroscientists, molecular biologists and evolutionary theorists, if we are to begin to answer them.

Genomic imprinting and human psychology: cognition, behavior and pathology

Imprinted genes expressed in the brain are numerous and it has become clear that they play an important role in nervous system development and function. The significant influence of genomic imprinting during development sets the stage for structural and physiological variations affecting psychological function and behaviour, as well as other physiological systems mediating health and well-being. However, our understanding of the role of imprinted genes in behaviour lags far behind our understanding of their roles in perinatal growth and development. Knowledge of genomic imprinting remains limited among behavioral scientists and clinicians and research regarding the influence of imprinted genes on normal cognitive processes and the most common forms of neuropathology has been limited to date. In this chapter, we will explore how knowledge of genomic imprinting can be used to inform our study of normal human cognitive and behavioral processes as well as their disruption. Behavioural analyses of rare imprinted disorders, such as Prader-Willi and Angelman syndromes, provide insight regarding the phenotypic impact of imprinted genes in the brain, and can be used to guide the study of normal behaviour as well as more common but etiologically complex disorders such as ADHD and autism. Furthermore, hypotheses regarding the evolutionary development of imprinted genes can be used to derive predictions about their role in normal behavioural variation, such as that observed in food-related and social interactions.

Aberrant epigenetic regulation could explain the relationship of paternal age to schizophrenia

The causal mechanism underlying the well-established relation between advancing paternal age and schizophrenia is hypothesized to involve mutational errors during spermatogenesis that occur with increasing frequency as males age. Point mutations are well known to increase with advancing paternal age while other errors such as altered copy number in repeat DNA and chromosome breakage have in some cases also been associated with advancing paternal age. Dysregulation of epigenetic processes may also be an important mechanism underlying the association between paternal age and schizophrenia. Evidence suggests that advancing age as well as environmental exposures alter epigenetic regulation. Errors in epigenetic processes, such as parental imprinting can have serious effects on the offspring both pre- and postnatally and into adulthood. This article will discuss parental imprinting on the autosomal and X chromosomes and the alterations in epigenetic regulation that may lead to such errors.

The 39,XO mouse as a model for the neurobiology of Turner syndrome and sex-biased neuropsychiatric disorders

Turner syndrome (TS) is a developmental disorder most frequently arising from the loss of a complete X chromosome (karyotype 45,XO). The disorder is characterised by physiological abnormalities (notably short stature and ovarian dysfunction), emotional anomalies (including heightened anxiety) and by a neuropsychological profile encompassing deficits in visuospatial skills, memory, attention, social cognition and emotion recognition. Moreover, TS subjects are at significantly increased risk of developing attention deficit hyperactivity disorder (ADHD) and autism. At the neuroanatomical level, TS subjects display abnormalities across a number of brain structures, including the amygdala, hippocampus and orbitofrontal cortex. The TS phenotype arises due to reduced dosage of X-linked genes, and may also be modulated by parental origin of the single X chromosome. In this review, we discuss the utility of a mouse model of TS, the 39,XO mouse, in which the parental origin of the single X chromosome can be varied. This model provides the opportunity to investigate the effects of X-linked gene dosage/parent-of-origin effects on neurobiology in the absence of gross physiological abnormalities. Initial findings indicate that several features of the TS behavioural phenotype may be accurately recapitulated in the mouse. Furthermore, as X-linked gene dosage/imprinting can influence sex-specific neurobiology, investigations in the 39,XO mouse are also likely to offer insights into why certain neuropsychiatric disorders (including ADHD and autism) affect the sexes differently.

Genomic imprinting and the social brain

Genomic imprinting refers to the parent-of-origin-specific epigenetic marking of a number of genes. This epigenetic mark leads to a bias in expression between maternally and paternally inherited imprinted genes, that in some cases results in monoallelic expression from one parental allele. Genomic imprinting is often thought to have evolved as a consequence of the intragenomic conflict between the parental alleles that occurs whenever there is an asymmetry of relatedness. The two main examples of asymmetry of relatedness are when there is partiality of parental investment in offspring (as is the case for placental mammals, where there is also the possibility of extended postnatal care by one parent), and in social groups where there is a sex-biased dispersal. From this evolutionary starting point, it is predicted that, at the behavioural level, imprinted genes will influence what can broadly be termed bonding and social behaviour. We examine the animal and human literature for examples of imprinted genes mediating these behaviours, and divide them into two general classes. Firstly, mother-offspring interactions (suckling, attachment and maternal behaviours) that are predicted to occur when partiality in parental investment in early postnatal offspring occurs; and secondly, adult social interactions, when there is an asymmetry of relatedness in social groups. Finally, we return to the evolutionary theory and examine whether there is a pattern of behavioural functions mediated by imprinted genes emerging from the limited data, and also whether any tangible predictions can be made with regards to the direction of action of genes of maternal or paternal origin.

It is not all hormones: Alternative explanations for sexual differentiation of the brain

Males and females of many species differ with regard to neurodevelopment, ongoing brain function and behavior. For many years, it was assumed that these differences primarily arose due to hormonal masculinization of the male brain (and to a lesser extent hormonal feminization of the female brain). Recent elegant experiments in model systems have revealed that, while gonadal hormones undoubtedly play an important role in sexual differentiation of the brain, they are not the only possible mechanism for this phenomenon. In the present review, we discuss the concept that genes residing upon the sex chromosomes (which are asymmetrically inherited between males and females) may influence sexually dimorphic neurobiology directly, and suggest possible mechanisms. Future work will be directed towards understanding the extent and specificity with which sex-linked genes and hormones define brain structure and function, and towards elucidating potential interactions between the two mechanisms. Ultimately, it is hoped that such studies will provide insights into why men and women are differentially vulnerable to certain mental disorders, and will enable the development of effective sex-tailored therapeutics.

Imbalanced genomic imprinting in brain development: an evolutionary basis for the aetiology of autism

We describe a new hypothesis for the development of autism, that it is driven by imbalances in brain development involving enhanced effects of paternally expressed imprinted genes, deficits of effects from maternally expressed genes, or both. This hypothesis is supported by: (1) the strong genomic-imprinting component to the genetic and developmental mechanisms of autism, Angelman syndrome, Rett syndrome and Turner syndrome; (2) the core behavioural features of autism, such as self-focused behaviour, altered social interactions and language, and enhanced spatial and mechanistic cognition and abilities, and (3) the degree to which relevant brain functions and structures are altered in autism and related disorders. The imprinted brain theory of autism has important implications for understanding the genetic, epigenetic, neurological and cognitive bases of autism, as ultimately due to imbalances in the outcomes of intragenomic conflict between effects of maternally vs. paternally expressed genes.

Modes of imprinted gene action in learning disability

BACKGROUND

It is now widely acknowledged that there may be a genetic contribution to learning disability and neuropsychiatric disorders, stemming from evidence provided by family, twin and adoption studies, and from explicit syndromic conditions. Recently it has been recognized that in some cases the presentation of genetic syndromes (or discrete aspects of disorders) is dependent on the sex of the transmitting parent. Such 'parent-of-origin effects' can be explained by a number of genetic mechanisms, a predominant one of which is genomic imprinting. Genomic imprinting refers to the parent of origin-specific epigenetic marking of an allele of a gene, such that for some genes it is mainly the maternally inherited allele only that is expressed, whereas for others expression occurs mainly from the paternal copy.

METHODS

Here we discuss the contribution of imprinted genes to mental dysfunction and learning disability, using clinical examples of association studies and explicit imprinting disorders (with particular emphasis to Angelman and Prader-Willi syndromes), and evidence from animal work.

RESULTS

Clinical and animal studies strongly suggest that imprinted genes contribute to brain functioning, and when the genes or epigenetic processes are disrupted, this can give rise to neuropsychiatric problems. Another system to which imprinted genes provide a large contribute is the placenta and foetal development. Epidemiological studies suggest that this is also a key area in which dysregulation can give rise to learning difficulties.

CONCLUSIONS

Disruption of imprinted genes, or the epigenetic processes controlling them, can contribute to learning disability. These effects can be divided into two types: direct effects, such as those seen in explicit imprinting disorders such as Angelman and Prader-Willi syndromes, and indirect effects as manifest via changes in foetal programming.

Imprinted gene expression in the brain

In normal mammals, autosomal genes are present in duplicate (i.e. two alleles), one inherited from the father, and one from the mother. For the majority of genes both alleles are transcribed (or expressed) equally. However, for a small subset of genes, known as imprinted genes, only one allele is expressed in a parent-of-origin dependent manner (note that the 'imprint' here refers to the epigenetic mechanism through which one allele is silenced, and is completely unrelated to classical 'filial imprinting' manifest at the behavioural level). Thus, for some imprinted genes expression is only (or predominantly) seen from the paternally inherited allele, whilst for the remainder, expression is only observed from the maternally inherited allele. Early work on this class of genes highlighted their importance in gross developmental and growth phenotypes. Recent studies in mouse models and humans have emphasised their contribution to brain function and behaviour. In this article, we review the literature concerning the expression of imprinted genes in the brain. In particular, we attempt to define emerging organisation themes, especially in terms of the direction of imprinting (i.e. maternal or paternal expression). We also emphasise the likely role of imprinted genes in neurodevelopment. We end by pointing out that, so far as discerning the precise functions of imprinted genes in the brain is concerned, there are currently more questions than answers; ranging from the extent to which imprinted genes might contribute to common mental disorders, to wider issues related to how easily the new data on brain may be accommodated within the dominant theory regarding the origins and maintenance of imprinting, which pits the maternal and paternal genomes against each other in an evolutionary battle of the sexes.

Imprinted genes and mother-offspring interactions

A subset of mammalian genes is subject to genomic imprinting. These imprinted genes show parent of origin specific monoallelic or parental allele-biased expression, such that for some genes, it is mainly the maternally inherited allele that is expressed, whereas for others, expression occurs mainly from the paternal copy. Evolutionary theory predicts that these genes will have a role in the mother-offspring interaction in mammals, and indeed many imprinted genes have a role in growth and placental function, and consequently influence prenatal development. In addition to the developing foetus, there is increasing evidence to suggest that imprinted genes influence the pre-weaning mother-offspring relationship, and consequently the development of the offspring into adulthood. In this review, we present an overview of the role imprinted genes play in the mother-offspring relationship using examples from the human and animal literature.

X-linked imprinting: effects on brain and behaviour

Imprinted genes are monoallelically expressed in a parent-of-origin-dependent manner and can affect brain and behavioural phenotypes. The X chromosome is enriched for genes affecting neurodevelopment and is donated asymmetrically to male and female progeny. Hence, X-linked imprinted genes could potentially influence sexually dimorphic neurobiology. Consequently, investigations into such loci may provide new insights into the biological basis of behavioural differences between the sexes and into why men and women show different vulnerabilities to certain mental disorders. In this review, we summarise recent advances in our knowledge of X-linked imprinted genes and the brain substrates that they may act upon. In addition, we suggest strategies for identifying novel X-linked imprinted genes and their downstream effects and discuss evolutionary theories regarding the origin and maintenance of X-linked imprinting.

Genomic imprinting and assisted reproduction

Imprinted genes exhibit a parent-of-origin specific pattern of expression. Such genes have been shown to be targets of molecular defects in particular genetic syndromes such as Beckwith-Wiedemann and Angelman syndromes. Recent reports have raised concern about the possibility that assisted reproduction techniques, such as in vitro fertilization or intracytoplasmic sperm injection, might cause genomic imprinting disorders. The number of reported cases of those disorders is still too small to draw firm conclusions and the safety of these widely used assisted reproduction techniques needs to be further evaluated.

Expression patterns of the novel imprinted genes Nap1l5 and Peg13 and their non-imprinted host genes in the adult mouse brain

Recent work has implicated imprinted gene functioning in neurodevelopment and behaviour and defining the expression patterns of these genes in brain tissue has become a key prerequisite to establishing function. In this work we report on the expression patterns of two novel imprinted loci, Nap1l5 and Peg13, in adult mouse brain using in situ hybridisation methods. Nap1l5 and Peg13 are located, respectively, within the introns of the non-imprinted genes Herc3 and the Tularik1 (T1)/KIAA1882 homologue in two separate microimprinted domains on mouse chromosomes 6 and 15. These 'host' genes are highly expressed in brain and consequently we were interested in assessing their expression patterns in parallel to the imprinted genes. The brain expression of all four genes appeared to be mainly neuronal. The detailed expression profiles of Nap1l5 and Peg13 were generally similar with widespread expression that was relatively high in the septal and hypothalamic regions, the hippocampus and the cerebral cortex. In contrast, there was some degree of dissociation between the imprinted genes and their non-imprinted hosts, in that, whilst there was again widespread expression of Herc3 and the T1/KIAA1882 homologue, these genes were also particularly highly expressed in Purkinje neurons and piriform cortex. We also examined expression of the novel imprinted genes in the adrenal glands. Nap1l5 expression was localised mainly to the adrenal medulla, whilst Peg13 expression was observed more generally throughout the adrenal medulla and the outer cortical layers.

Joint analysis of the DRD5 marker concludes association with attention-deficit/hyperactivity disorder confined to the predominantly inattentive and combined subtypes

Attention-deficit/hyperactivity disorder (ADHD) is a highly heritable, heterogeneous disorder of early onset, consisting of a triad of symptoms: inattention, hyperactivity, and impulsivity. The disorder has a significant genetic component, and theories of etiology include abnormalities in the dopaminergic system, with DRD4, DAT1, SNAP25, and DRD5 being implicated as major susceptibility genes. An initial report of association between ADHD and the common 148-bp allele of a microsatellite marker located 18.5 kb from the DRD5 gene has been followed by several studies showing nonsignificant trends toward association with the same allele. To establish the postulated association of the (CA)(n) repeat with ADHD, we collected genotypic information from 14 independent samples of probands and their parents, analyzed them individually and, in the absence of heterogeneity, analyzed them as a joint sample. The joint analysis showed association with the DRD5 locus (P=.00005; odds ratio 1.24; 95% confidence interval 1.12-1.38). This association appears to be confined to the predominantly inattentive and combined clinical subtypes.

Behavioural, pharmacological, morpho-functional molecular studies reveal a hyperfunctioning mesocortical dopamine system in an animal model of attention deficit and hyperactivity disorder

Clinical and experimental evidence suggest an involvement of dopamine systems, mainly the mesocorticolimbic one (MCL), in Attention-Deficit Hyperactivity Disorder (ADHD). However, it remains to be ascertained whether the systems are hyper- or hypo-functioning, for the implications of the functional state. Indeed, differential functional states of the MCL branches are suggested to be the neural substrate of different ADHD variants. This review covers published and unpublished data from the Naples-High Excitability (NHE) rat, an animal model of ADHD, featuring its main aspects, with no hypertension. Therefore, a multiple approach based on morphological studies of dopamine, norepinephrine, glutamate, acetylcholine and GABA systems, synaptic (Calcium/Calmodulin kinase II) and extrasynaptic (chondroitin sulphates) environments, and molecular biology and pharmacological studies on the dopamine system has been carried out. Morphological findings suggest dopamine neurons in the Ventral Tegmental Area (VTA) to be hypertrophic in NHE rats. The mesostriatal and mesolimbic dopamine branches appear to be normal in basal conditions. However, the striatal interface is probably defective following activation. Conversely, the prefrontal cortex, which represents the second main target of VTA dopamine neurons, has many alterations at the basal level. Therefore, the emerging picture is the association of a hyperinnervating and hyperfunctioning mesocortical branch of the dopamine system. Thus, the evidence gathered so far might improve our understanding of the neural substrates of neuropsychiatric disorders such as ADHD, schizophrenia and drug addiction.

Presence of atopy in first-degree relatives as a predictor of a female proband's depression: results from the Northern Finland 1966 Birth Cohort

BACKGROUND

Recent investigations suggest a common genetic rather than environmental cause to explain the association between IgE-mediated atopic allergies and depression.

OBJECTIVE

Taking into account psychosocial confounding factors, we investigated separately and at the epidemiologic level the effects of maternal, paternal, and sibling atopy on the cumulative incidence of a child's depression.

METHODS

We used an unselected, genetically homogenous, general population birth cohort of 12,058 live-born children in Finland. The 31-year prospective follow-up included questionnaire information on atopic disorders of the cohort members' parents and siblings. The probands' own atopic conditions were evaluated by means of skin prick tests, and information on lifetime depression diagnoses was gleaned from postal questionnaires and national hospital discharge registers. Potential confounders were mother's parity, father's social class, maternal smoking during pregnancy, proband's regular daily smoking, and proband's dwelling place. Total variable information was available from 4068 cohort members.

RESULTS

Among female probands, the presence of atopy in parents was the strongest predictor for lifetime depression (P <.001), and sibling atopy and parental atopy were the strongest predictors for hospital-treated depression (P =.018 and P =.036, respectively). After controlling for confounders, it was noticed that maternal atopy increased a female proband's risk of lifetime depression up to 1.9-fold (odds ratio, 1.9; 95% CI, 1.1-3.0). The corresponding risk increased over 4-fold if parental-maternal atopy was combined with proband's own atopy.

CONCLUSIONS

Our findings suggest that maternal inheritance could be a significant causative factor in the association between atopy and depression of female probands.

Mouse models for neurological disease

The mouse has many advantages over human beings for the study of genetics, including the unique property that genetic manipulation can be routinely carried out in the mouse genome. Most importantly, mice and human beings share the same mammalian genes, have many similar biochemical pathways, and have the same diseases. In the minority of cases where these features do not apply, we can still often gain new insights into mouse and human biology. In addition to existing mouse models, several major programmes have been set up to generate new mouse models of disease. Alongside these efforts are new initiatives for the clinical, behavioural, and physiological testing of mice. Molecular genetics has had a major influence on our understanding of the causes of neurological disorders in human beings, and much of this has come from work in mice.