Genomic imprinting in the brain

Cognitive benefits of folic acid supplementation during pregnancy track with epigenetic changes at an imprint regulator

BACKGROUND

The human ZFP57 gene is a major regulator of imprinted genes, maintaining DNA methylation marks that distinguish parent-of-origin-specific alleles. DNA methylation of the gene itself has shown sensitivity to environmental stimuli, particularly folate status. However, the role of DNA methylation in ZFP57's own regulation has not been fully investigated.

METHODS

We used samples and data from our previously described randomised controlled trial (RCT) in pregnancy called Folic Acid Supplementation in the Second and Third Trimester (FASSTT), including follow-up of the children at age 11. Biometric and blood biochemistry results were examined for mothers and children. Methylation of ZFP57 was analysed by EPIC arrays, pyrosequencing and clonal analysis, and transcription assessed by PCR-based methods. Functional consequences of altered methylation were examined in cultured cells with mutations or by inhibition of the main DNA methyltransferases. DNA variants were examined using pyrosequencing and Sanger sequencing, with results compared to published studies using bioinformatic approaches. Cognitive outcomes were assessed using the Wechsler Intelligence Scale for Children 4th UK Edition (WISC-IV), with neural activity during language tasks quantified using magnetoencephalography (MEG).

RESULTS

Here we show that methylation at an alternative upstream promoter of ZFP57 is controlled in part by a quantitative trait locus (QTL). By altering DNA methylation levels, we demonstrate that this in turn controls the expression of the ZFP57 isoforms. Methylation at this region is also sensitive to folate levels, as we have previously shown in this cohort. Fully methylated alleles were associated with poorer performance in the Symbol Search and Cancellation subtests of WISC-IV in the children at age 11 years. There were also differences in neural activity during language tasks, as measured by MEG. Analysis of published genome-wide studies indicated other SNPs in linkage disequilibrium with the mQTL were also associated with neurodevelopmental outcomes.

CONCLUSIONS

While numbers in the current RCT were small and require further validation in larger cohorts, the results nevertheless suggest a molecular mechanism by which maternal folic acid supplementation during pregnancy may help to counteract the effects of folate depletion and positively influence cognitive development in the offspring.

Epigenetic control and genomic imprinting dynamics of the Dlk1-Dio3 domain

Genomic imprinting is an epigenetic process whereby genes are monoallelically expressed in a parent-of-origin-specific manner. Imprinted genes are frequently found clustered in the genome, likely illustrating their need for both shared regulatory control and functional inter-dependence. The Dlk1-Dio3 domain is one of the largest imprinted clusters. Genes in this region are involved in development, behavior, and postnatal metabolism: failure to correctly regulate the domain leads to Kagami-Ogata or Temple syndromes in humans. The region contains many of the hallmarks of other imprinted domains, such as long non-coding RNAs and parental origin-specific CTCF binding. Recent studies have shown that the Dlk1-Dio3 domain is exquisitely regulated via a bipartite imprinting control region (ICR) which functions differently on the two parental chromosomes to establish monoallelic expression. Furthermore, the Dlk1 gene displays a selective absence of imprinting in the neurogenic niche, illustrating the need for precise dosage modulation of this domain in different tissues. Here, we discuss the following: how differential epigenetic marks laid down in the gametes cause a cascade of events that leads to imprinting in the region, how this mechanism is selectively switched off in the neurogenic niche, and why studying this imprinted region has added a layer of sophistication to how we think about the hierarchical epigenetic control of genome function.

Associations between parental broader autism phenotype and child autism spectrum disorder phenotype in the Study to Explore Early Development

The autism spectrum disorder phenotype varies by social and communication ability and co-occurring developmental, behavioral, and medical conditions. Etiology is also diverse, with myriad potential genetic origins and environmental risk factors. Examining the influence of parental broader autism phenotype-a set of sub-clinical characteristics of autism spectrum disorder-on child autism spectrum disorder phenotypes may help reduce heterogeneity in potential genetic predisposition for autism spectrum disorder. We assessed the associations between parental broader autism phenotype and child phenotype among children of age 30-68 months enrolled in the Study to Explore Early Development (N = 707). Child autism spectrum disorder phenotype was defined by a replication of latent classes derived from multiple developmental and behavioral measures: Mild Language Delay with Cognitive Rigidity, Mild Language and Motor Delay with Dysregulation (e.g. anxiety/depression), General Developmental Delay, and Significant Developmental Delay with Repetitive Motor Behaviors. Scores on the Social Responsiveness Scale-Adult measured parent broader autism phenotype. Broader autism phenotype in at least one parent was associated with a child having increased odds of being classified as mild language and motor delay with dysregulation compared to significant developmental delay with repetitive motor behaviors (odds ratio: 2.44; 95% confidence interval: 1.16, 5.09). Children of parents with broader autism phenotype were more likely to have a phenotype qualitatively similar to broader autism phenotype presentation; this may have implications for etiologic research.

Epigenetics, chromatin and brain development and function

Research investigating epigenetics and chromatin function in brain and behaviour has mushroomed over the last two decades. And yet epigenetics as a biological concept predates the discovery in the 1950s of DNA as the principle mode of inheritance by over a decade. This review explores the past, present and future research into epigenetics as it relates to understanding brain development and function

Linking genetics to epigenetics: The role of folate and folate-related pathways in neurodevelopmental disorders

There is growing evidence that epigenetic dysregulation plays a role in neurodevelopmental disorders. In humans, folate is one of the main donors of the methyl group required for the synthesis of S-adenosylmethionine, which in turn is needed for DNA and histone methylation as key neurodevelopment processes. Folate deficiency during pregnancy has been correlated with neural tube defects and with a higher incidence of neurocognitive and/or neurobehavioral deficits. A similar outcome may be exerted by gene polymorphisms in folate or folate-related pathways. This has been documented by numerous case/control association studies performed on neurodevelopmental disorders such as autism spectrum disorder and attention deficit hyperactivity disorder. In this regard, the folate cycle represents a "perfect model" of how genetics influences epigenetics. Gene variants in folate and folate-related pathways can be considered risk factors for neurodevelopmental disorders and should therefore be assessed by genetic testing in pregnant women. High-risk women should be considered for folate supplementation during pregnancy. Here, we review all published case/control association studies on gene polymorphisms in folate and folate-related pathways performed on neurodevelopmental disorders, provide an overview of neurodevelopment and DNA methylation changes occurring at this time, and describe the biological basis of neurodevelopmental disorders and recent evidence of their epigenetic dysregulation.

Methods of epigenome editing for probing the function of genomic imprinting

The curious patterns of imprinted gene expression draw interest from several scientific disciplines to the functional consequences of genomic imprinting. Methods of probing the function of imprinting itself have largely been indirect and correlational, relying heavily on conventional transgenics. Recently, the burgeoning field of epigenome editing has provided new tools and suggested strategies for asking causal questions with site specificity. This perspective article aims to outline how these new methods may be applied to questions of functional imprinting and, with this aim in mind, to suggest new dimensions for the expansion of these epigenome-editing tools.

Genetics of Prader-Willi syndrome and Prader-Will-Like syndrome

The Prader-Willi syndrome (PWS) is a human imprinting disorder resulting from genomic alterations that inactivate imprinted, paternally expressed genes in human chromosome region 15q11-q13. This genetic condition appears to be a contiguous gene syndrome caused by the loss of at least 2 of a number of genes expressed exclusively from the paternal allele, including SNRPN, MKRN3, MAGEL2, NDN and several snoRNAs, but it is not yet well known which specific genes in this region are associated with this syndrome. Prader-Will-Like syndrome (PWLS) share features of the PWS phenotype and the gene functions disrupted in PWLS are likely to lie in genetic pathways that are important for the development of PWS phenotype. However, the genetic basis of these rare disorders differs and the absence of a correct diagnosis may worsen the prognosis of these individuals due to the endocrine-metabolic malfunctioning associated with the PWS. Therefore, clinicians face a challenge in determining when to request the specific molecular test used to identify patients with classical PWS because the signs and symptoms of PWS are common to other syndromes such as PWLS. This review aims to provide an overview of current knowledge relating to the genetics of PWS and PWLS, with an emphasis on identification of patients that may benefit from further investigation and genetic screening.

The interplay between DNA methylation, folate and neurocognitive development

DNA methylation provides an attractive possible means for propagating the effects of environmental inputs during fetal life and impacting subsequent adult mental health, which is leading to increasing collaboration between molecular biologists, nutritionists and psychiatrists. An area of interest is the potential role of folate, not just in neural tube closure in early pregnancy, but in later major neurodevelopmental events, with consequences for later sociocognitive maturation. Here, we set the scene for recent discoveries by reviewing the major events of neural development during fetal life, with an emphasis on tissues and structures where dynamic methylation changes are known to occur. Following this, we give an indication of some of the major classes of genes targeted by methylation and important for neurological and behavioral development. Finally, we highlight some cognitive disorders where methylation changes are implicated as playing an important role.

Brain-expressed imprinted genes and adult behaviour: the example of Nesp and Grb10

Imprinted genes are defined by their parent-of-origin-specific monoallelic expression. Although the epigenetic mechanisms regulating imprinted gene expression have been widely studied, their functional importance is still unclear. Imprinted genes are associated with a number of physiologies, including placental function and foetal growth, energy homeostasis, and brain and behaviour. This review focuses on genomic imprinting in the brain and on two imprinted genes in particular, Nesp and paternal Grb10, which, when manipulated in animals, have been shown to influence adult behaviour. These two genes are of particular interest as they are expressed in discrete and overlapping neural regions, recognised as key "imprinting hot spots" in the brain. Furthermore, these two genes do not appear to influence placental function and/or maternal provisioning of offspring. Consequently, by understanding their behavioural function we may begin to shed light on the evolutionary significance of imprinted genes in the adult brain, independent of the recognised role in maternal care. In addition, we discuss the potential future directions of research investigating the function of these two genes and the behavioural role of imprinted genes more generally.

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 imprinting as an adaptative model of developmental plasticity

Developmental plasticity can be defined as the ability of one genotype to produce a range of phenotypes in response to environmental conditions. Such plasticity can be manifest at the level of individual cells, an organ, or a whole organism. Imprinted genes are a group of approximately 100 genes with functionally monoallelic, parental-origin specific expression. As imprinted genes are critical for prenatal growth and metabolic axis development and function, modulation of imprinted gene dosage has been proposed to play a key role in the plastic development of the unborn foetus in response to environmental conditions. Evidence is accumulating that imprinted dosage may also be involved in controlling the plastic potential of individual cells or stem cell populations. Imprinted gene dosage can be modulated through canonical, transcription factor mediated mechanisms, or through the relaxation of imprinting itself, reactivating the normally silent allele.

Why are autism spectrum conditions more prevalent in males?

Autism Spectrum Conditions (ASC) are much more common in males, a bias that may offer clues to the etiology of this condition. Although the cause of this bias remains a mystery, we argue that it occurs because ASC is an extreme manifestation of the male brain. The extreme male brain (EMB) theory, first proposed in 1997, is an extension of the Empathizing-Systemizing (E-S) theory of typical sex differences that proposes that females on average have a stronger drive to empathize while males on average have a stronger drive to systemize. In this first major update since 2005, we describe some of the evidence relating to the EMB theory of ASC and consider how typical sex differences in brain structure may be relevant to ASC. One possible biological mechanism to account for the male bias is the effect of fetal testosterone (fT). We also consider alternative biological theories, the X and Y chromosome theories, and the reduced autosomal penetrance theory. None of these theories has yet been fully confirmed or refuted, though the weight of evidence in favor of the fT theory is growing from converging sources (longitudinal amniocentesis studies from pregnancy to age 10 years old, current hormone studies, and genetic association studies of SNPs in the sex steroid pathways). Ultimately, as these theories are not mutually exclusive and ASC is multi-factorial, they may help explain the male prevalence of ASC.

THE INHERITANCE OF COGNITIVE SKILLS: DOES GENOMIC IMPRINTING PLAY A ROLE?

Genomic imprinting refers to the differential expression of a gene based on parental origin. Animal and clinical studies have suggested that genomic imprinting is influential in brain development, with the maternal genome playing a disproportionate role in the development of the cortex. The present study investigated this phenomenon in a nonclinical human population, using intrafamilial correlations. Broadly consistent with predictions, it was found that abilities mediated by frontal, parietal, and temporal lobes, but not occipital lobes, were more closely correlated between children and mothers versus fathers. The implications of these findings for the prevailing theory of the evolution of genomic imprinting, and for the general study of genetics and behavior, are discussed.

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.

Epigenetics of Complex Diseases: From General Theory to Laboratory Experiments

Despite significant effort, understanding the causes and mechanisms of complex non-Mendelian diseases remains a key challenge. Although numerous molecular genetic linkage and association studies have been conducted in order to explain the heritable predisposition to complex diseases, the resulting data are quite often inconsistent and even controversial. In a similar way, identification of environmental factors causal to a disease is difficult. In this article, a new interpretation of the paradigm of "genes plus environment" is presented in which the emphasis is shifted to epigenetic misregulation as a major etiopathogenic factor. Epigenetic mechanisms are consistent with various non-Mendelian irregularities of complex diseases, such as the existence of clinically indistinguishable sporadic and familial cases, sexual dimorphism, relatively late age of onset and peaks of susceptibility to some diseases, discordance of monozygotic twins and major fluctuations on the course of disease severity. It is also suggested that a substantial portion of phenotypic variance that traditionally has been attributed to environmental effects may result from stochastic epigenetic events in the cell. It is argued that epigenetic strategies, when applied in parallel with the traditional genetic ones, may significantly advance the discovery of etiopathogenic mechanisms of complex diseases. The second part of this chapter is dedicated to a review of laboratory methods for DNA methylation analysis, which may be useful in the study of complex diseases. In this context, epigenetic microarray technologies are emphasized, as it is evident that such technologies will significantly advance epigenetic analyses in complex diseases.

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.

Further evidence of a maternal parent-of-origin effect on chromosome 10 in late-onset Alzheimer's disease

The chromosome 10q region has recently received a great deal of attention in late-onset Alzheimer's disease (LOAD), given the growing evidence of linkage to LOAD, or to A-beta levels, reported by several groups. In a recent paper we reported evidence of linkage in this region in our subset of the NIMH AD genetics initiative pedigrees, approaching genome-wide significance (non-parametric LOD score = 3.27), when only families with maternal disease origin were analyzed [Bassett et al. (2002); Am J Med Genet 114:679-686]. We have now extended this work, using an independent subset of NIMH AD pedigrees from the University of Alabama at Birmingham (UAB), and show further evidence of linkage using parent-of-origin information. As in our Hopkins sample, maternal but not paternal pedigrees show significantly increased linkage in the chromosome 10q region compared to the unstratified sample. Combining data from our previous fine-mapping work on this region and five new markers genotyped in all pedigrees results in a non-parametric LOD score of 3.73 in the same region, a value that reaches genome wide significance for linkage, with an empirical P value = 0.003. These results support our earlier findings and narrow the region of interest. In combination with findings from other groups, these results provide further evidence that this chromosome 10 region harbors a gene implicated in LOAD, and our use of parent-of-origin information has been useful in further narrowing the region of interest.

APeg3, a novel paternally expressed gene 3 antisense RNA transcript specifically expressed in vasopressinergic magnocellular neurons in the rat supraoptic nucleus

Vasopressin (VP) and oxytocin (OT) play critical roles in the regulation of salt and water balance, lactation, and various behaviors and are expressed at very high levels in specific magnocellular neurons (MCNs) in the hypothalamo-neurohypophysial system (HNS). In addition to the cell-specific expression of the VP and OT genes in these cells, there are other transcripts that are preferentially expressed in the VP or OT MCNs. One such gene, paternally expressed gene 3 (Peg3), is an imprinted gene expressed exclusively from the paternal allele that encodes a Kruppel-type zinc finger-containing protein involved in maternal behavior and is abundantly expressed in the VP-MCNs. We report here the robust expression in the VP-MCNs of an RNA, which we designate APeg3 that is transcribed in the antisense direction to the 3' untranslated region of the Peg3 gene. The APeg3 mRNA is about 1 kb in size, and the full-length sequence of APeg3, as determined by 5' and 3' RACE, contains an open reading frame that predicts a protein of 93 amino acids and is predominantly expressed in VP-MCNs. Both Peg3 and APeg3 gene expression in the VP-MCNs increase during systemic hyperosmolality in vivo, demonstrating that both of these genes are osmoregulated.

Increased body fat in mice with a targeted mutation of the paternally expressed imprinted gene Peg3

Peg3 encodes a C2H2 type zinc finger protein that is implicated in a novel physiological pathway regulating core body temperature, feeding behavior, and obesity in mice. Peg3+/- mutant mice develop an excess of abdominal, subcutaneous, and intra-scapular fat, despite a lifetime of lower food intake than wild-type animals. However, they start life with reduced fat reserves and are slower to enter puberty. These mice maintain a lower core body temperature, fail to respond to a cold challenge, and have lower metabolic activity as measured by oxygen consumption. Plasma leptin levels are significantly higher than in wild types, and Peg3+/- mice appear to have developed leptin resistance. Administration of exogenous leptin resulted in a significant reduction in food intake in wild-type mice that was not observed in Peg3+/- mutants. This mutation, which is strongly expressed in hypothalamic tissue during development, has the capacity to regulate multiple events relating to energy homeostasis.

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.

Analysis of imprinting in mice with uniparental duplication of proximal chromosomes 7 and 15 by use of a custom oligonucleotide microarray

We have developed an imprinting assay combining the use of mice carrying maternal or paternal duplication of chromosomal regions of interest with custom oligonucleotide microarrays. As a model system, we analyzed RNA from CNS tissue of neonatal mice carrying the reciprocal translocation T(7;15)9H and uniparental duplication of proximal Chr 7 and 15. The duplicated region includes the locus on proximal Chr 7 corresponding to the human Prader-Willi/Angelman Syndrome. The microarray contained 322 oligonucleotides, including probes to detect major genes involved in neural excitability and synaptic transmission, as well as known imprinted genes mapping to proximal Chr 7: Ndn, Snrpn, Mkrn3, Magel2, Peg3, and Ube3a. Imprinting of these genes in neonatal cortex and cerebellum was first confirmed by quantitative RT-PCR. Their inclusion on the microarray thus provided positive controls for evaluating the effect of background on the sensitivity of the assay, and for establishing the minimum level of expression required to detect imprinting. Our analysis extended previous work by revealing bi-allelic expression in CNS tissue of those queried genes mapping to proximal Chr 7 or 15, including the Gabrb3 gene, for which there have been conflicting reports. Microarray analysis also revealed no effect of the maternal or paternal disomy on expression levels of the unlinked genes detected, including those potentially implicated in the Prader-Willi or Angelman Syndrome. In addition, quantitative RT-PCR revealed a gene dosage effect in both cerebellum and cortex for all of the known imprinted genes assayed, except for Ube3a in cerebellum.

Using Lod Scores to Detect Sex Differences in Male-Female Recombination Fractions

Human recombination fraction (RF) can differ between males and females, but investigators do not always know which disease genes are located in genomic areas of large RF sex differences. Knowledge of RF sex differences contributes to our understanding of basic biology and can increase the power of a linkage study, improve gene localization, and provide clues to possible imprinting. One way to detect these differences is to use lod scores. In this study we focused on detecting RF sex differences and answered the following questions, in both phase-known and phase-unknown matings: (1) How large a sample size is needed to detect a RF sex difference? (2) What are "optimal" proportions of paternally vs. maternally informative matings? (3) Does ascertaining nonoptimal proportions of paternally or maternally informative matings lead to ascertainment bias? Our results were as follows: (1) We calculated expected lod scores (ELODs) under two different conditions: "unconstrained," allowing sex-specific RF parameters (theta(female), theta(male)); and "constrained," requiring theta(female) = theta(male). We then examined the DeltaELOD (identical with difference between maximized constrained and unconstrained ELODs) and calculated minimum sample sizes required to achieve statistically significant DeltaELODs. For large RF sex differences, samples as small as 10 to 20 fully informative matings can achieve statistical significance. We give general sample size guidelines for detecting RF differences in informative phase-known and phase-unknown matings. (2) We defined p as the proportion of paternally informative matings in the dataset; and the optimal proportion p(circ) as that value of p that maximizes DeltaELOD. We determined that, surprisingly, p(circ) does not necessarily equal (1/2), although it does fall between approximately 0.4 and 0.6 in most situations. (3) We showed that if p in a sample deviates from its optimal value, no bias is introduced (asymptotically) to the maximum likelihood estimates of theta(female) and theta(male), even though ELOD is reduced (see point 2). This fact is important because often investigators cannot control the proportions of paternally and maternally informative families. In conclusion, it is possible to reliably detect sex differences in recombination fraction.

Amygdala and hippocampal volumes in Turner syndrome: a high-resolution MRI study of X-monosomy

Turner syndrome (TS) results from partial or complete X-monosomy and is characterized by deficits in visuospatial functioning as well as social cognition and memory. Neuroimaging studies have demonstrated volumetric differences in the parietal region of females with TS compared to controls. The present study examined amygdala and hippocampus morphology in an attempt to further understand the neural correlates of psychosocial and memory functioning in TS. Thirty females with TS age 7.6-33.3 years (mean = 14.7 +/- 6.4) and 29 age-matched controls (mean age = 14.8 +/- 5.9; range = 6.4-32.7) were scanned using high resolution MRI. Volumetric analyses of the MRI scans included whole brain segmentation and manual delineation of the amygdala and hippocampus. Compared to controls, participants with TS demonstrated significantly larger left amygdala gray matter volumes, irrespective of total cerebral tissue and age. Participants with TS also showed disproportionately reduced right hippocampal volumes, involving both gray and white matter. Amygdala and hippocampal volumes appear to be impacted by X-monosomy. Aberrant morphology in these regions may be related to the social cognition and memory deficits often experienced by individuals with TS. Further investigations of changes in medial temporal morphology associated with TS are warranted.

Prader-Willi syndrome: advances in genetics, pathophysiology and treatment

Prader-Willi syndrome (PWS) is a complex human genetic disease that arises from lack of expression of paternally inherited imprinted genes on chromosome 15q11-q13. Identification of the imprinting control centre, novel imprinted genes and distinct phenotypes in PWS patients and mouse models has increased interest in this human obesity syndrome. In this review I focus on: (i) the chromosomal region and candidate genes associated with PWS, and the possible links with individual PWS phenotypes identified using mouse models; (ii) the metabolic and hormonal phenotypes in PWS; (iii) postmortem studies of human PWS hypothalami; and (iv) current and potential advances in the management of PWS and its complications. This could have benefits for a wide spectrum of endocrine, paediatric and neuropsychiatric diseases.

Using sex differences in psychopathology to study causal mechanisms: unifying issues and research strategies

BACKGROUND

Although there is an extensive literature, both speculative and empirical, on postulated differences between males and females in their rates of particular types of disorder, very little is known about the mechanisms that underlie these sex differences. The study of mechanisms is important because it may provide clues on aetiological processes. The review seeks to outline what is known, what are the methodological hazards that must be dealt with, and the research strategies that may be employed.

METHODS

We note the need for representative general samples, and for adequate measurement and significance testing if valid conclusions are to be drawn. We put forward three levels of causes that have to be considered: a genetically determined distal basic starting point; the varied consequences of being male or female; and the proximal risk or protective factors that are more directly implicated in the causal mechanisms that predispose to psychopathology. In delineating these, we argue that three key sets of evidential criteria have to be met: a) that the risk factors differ between males and females; b) that they provide for risk or protection within each sex; and c) that when introduced into a causal model, they eliminate or reduce the sex differences in the disorders being studied.

RESULTS

A male excess mainly applies to early onset disorders that involve some kind of neurodevelopmental impairment. A female excess mainly applies to adolescent-onset emotional disorders. No variables have yet met all the necessary criteria but some good leads are available. The possible research strategies that may be employed are reviewed.

CONCLUSIONS

The systematic investigation of sex differences constitutes an invaluable tool for the study of the causal processes concerned with psychopathology.

Effects of X-monosomy and X-linked imprinting on superior temporal gyrus morphology in Turner syndrome

BACKGROUND

Turner syndrome (TS) results from complete or partial monosomy X. The cognitive phenotype of TS involves preservation of verbal skills with visuospatial functioning deficits. The superior temporal gyrus (STG), which is involved in language capacities, has not been investigated in TS.

METHODS

The STG was measured in 30 female subjects (mean age = 14.73 +/- 6.41; range = 7.56-33.30) with TS and 30 age-matched control subjects (mean age = 14.63 +/- 5.90; range = 6.35-32.65) using volumetric magnetic resonance imaging analyses.

RESULTS

-Right STG, including both gray and white matter volumes, was significantly larger in TS compared with control subjects. Overall left STG volume was not significantly different between groups, although left white matter volume was increased in the TS subjects. The TS subgroup with a maternally derived X chromosome (Xm) demonstrated more aberrant STG volumes compared with subjects with a paternally (Xp) derived X and control subjects. The difference in STG volumes between Xm and control subjects involved both white and gray matter. The Xm subjects differed from Xp subjects only in terms of gray matter.

CONCLUSIONS

These findings suggest that X-monosomy and X-linked imprinting negatively affect STG development, possibly by disrupting neural pruning mechanisms.

Trajectories of brain development: point of vulnerability or window of opportunity?

Brain development is a remarkable process. Progenitor cells are born, differentiate, and migrate to their final locations. Axons and dendrites branch and form important synaptic connections that set the stage for encoding information potentially for the rest of life. In the mammalian brain, synapses and receptors within most regions are overproduced and eliminated by as much as 50% during two phases of life: immediately before birth and during the transitions from childhood, adolescence, to adulthood. This process results in different critical and sensitive periods of brain development. Since Hebb (1949) first postulated that the strengthening of synaptic elements occurs through functional validation, researchers have applied this approach to understanding the sculpting of the immature brain. In this manner, the brain becomes wired to match the needs of the environment. Extensions of this hypothesis posit that exposure to both positive and negative elements before adolescence can imprint on the final adult topography in a manner that differs from exposure to the same elements after adolescence. This review endeavors to provide an overview of key components of mammalian brain development while simultaneously providing a framework for how perturbations during these changes uniquely impinge on the final outcome.

Mutation screening and imprinting analysis of four candidate genes for autism in the 7q32 region

Genetic studies indicate that chromosome 7q is likely to contain an autism susceptibility locus (AUTS1). We have followed a positional candidate gene approach to identify the relevant gene and report the analysis of four adjacent genes localised to a 800 kb region in 7q32 that contains an imprinted domain: PEG1/MEST, COPG2, CPA1 and CPA5-a previously uncharacterised member of the carboxypeptidase gene family. Screening these genes for DNA changes and association analysis using intragenic single nucleotide polymorphisms (SNPs) provided no evidence for an etiological role in IMGSAC families. We also searched for imprinting mutations potentially implicated in autism: analysis of both DNA methylation and replication timing indicated a normal imprinting regulation of the PEG1/COPG2 domain in blood lymphocytes of all patients tested. The analysis of these four genes strongly suggests that they do not play a major role in autism aetiology, and delineates our strategy to screen additional candidate genes in the AUTS1 locus.

A possible role for imprinted genes in inbreeding avoidance and dispersal from the natal area in mice

The expression of a subset of mammalian genes is subject to parent of origin effects (POE), most of which can be explained by genomic imprinting. Analysis of mutant animals has demonstrated that a number of imprinted genes influence brain development and behaviour. Here we provide evidence for POE on olfactory related behaviour and sensitivity to maternal odour cues. This was investigated by examining the odour preference behaviour of reciprocal cross F(1) mice made by embryo transfer to genetically unrelated foster parents. We determined that both adult males and females show an avoidance of female urinary odours of their genetic maternal but not paternal origin. This was found not to be due to any previous exposure to these odours or due to self-learning, but may be related to direct effects on the olfactory system, as reciprocal F(1) males show differential sensitivity to female odour cues. Currently the most robust theory to explain the evolution of imprinting is the conflict hypothesis that focuses on maternal resource allocation to the developing foetus. Kinship considerations are also likely to be important in the selection of imprinted genes and we discuss our findings within this context, suggesting that imprinted genes act directly on the olfactory system to promote post-weaning dispersal from the natal area.

The role of tissue-specific imprinting as a source of phenotypic heterogeneity in human disease

Genomic imprinting is an epigenetic phenomenon affecting a small number of genes that leads to expression from only one parental allele. Several imprinted genes are important for neurologic development and function and several neurobehavioral disorders are caused by genetic defects involving imprinted genes. For some genes, the imprinting is tissue specific, leading to biallelic expression in some tissues and monoallelic expression in other tissues. Defects involving these genes may produce one restricted phenotype due to loss of expression of the gene product in tissues where the gene is imprinted and, in some instances, a second phenotype due to haploinsufficiency of the gene product in tissues where it is biallelically expressed.

Imprinted genes and mental dysfunction

There is a rapidly accumulating body of evidence from family, adoption and twin studies suggestive of a genetic component to many common mental disorders. In some cases, the transmission of abnormalities has been shown to be dependent upon the sex of the parent from whom they are inherited. Such 'parent-of-origin effects' may be explained by a number of genetic mechanisms, one of which is 'genomic imprinting'. In imprinted genes one allele is silenced according to its parental origin. This in turn means that imprinted traits are passed down the maternal or paternal line, in contrast to the more frequent Mendelian mode of inheritance that is indifferent to the parental origin of the allele. In the present review, we survey the evidence for the influence of imprinted genes on a number of mental disorders, ranging from explicit imprinted conditions, where in some cases abnormalities have been mapped to particular gene candidates, to examples where the evidence for parent-of-origin effects is less strong. We also consider, briefly, the wider implications of imprinted effects on mental dysfunction, in particular with respect to evolutionary pressures on mammalian brain development and function.

Human homolog of the mouse imprinted gene Impact resides at the pericentric region of chromosome 18 within the critical region for bipolar affective disorder

Several mapping studies of families with multiple individuals who have bipolar affective disorder (BPAD) have demonstrated possible linkage of the trait to the pericentric region of chromosome 18 (18cen). Currently, the large size of the critical interval defined by these studies makes effective selection of candidate genes formidable. However, documentation of 18cen-linked families in which a parent-of-origin effect was observed in the transmission of the BPAD trait provides a clue to the nature of the putative gene; it may be imprinted. In the present study, we cloned IMPACT, the human homolog of the mouse imprinted gene Impact and mapped it to 18cen within the critical interval for BPAD. Human IMPACT encodes a protein with 320 amino acids and is expressed at high levels in the brain. Since only a small number of imprinted genes are estimated to be present in the entire genome, very few imprinted genes would be expected to be present in this particular chromosomal region. Hence, IMPACT represents a candidate gene for BPAD susceptibility. Alternatively, other as yet unknown imprinted gene(s) adjacent to IMPACT could contribute to the BPAD trait, since multiple imprinted genes may occasionally form clusters. Localization of human IMPACT at 18cen in this study defines a promising target region in which to search for putative BPAD genes.

Behavioral phenotype of individuals with Down syndrome

Evidence is reviewed for a developmentally-emerging behavioral phenotype in individuals with Down syndrome that includes significant delay in nonverbal cognitive development accompanied by additional, specific deficits in speech, language production, and auditory short-term memory in infancy and childhood, but fewer adaptive behavior problems than individuals with other cognitive disabilities. Evidence of dementia emerges for up to half the individuals studied after age 50. Research issues affecting control group selection in establishing phenotypic characteristics are discussed, as well as the possible genetic mechanisms underlying variation in general cognitive delay, specific language impairment, and adult dementia. MRDD Research Reviews 2000;6:84-95. Wiley-Liss, Inc.

Imprinted genes, cognition and behaviour

The idea that genes can influence behavioural predispositions and their underlying psychological determinants is becoming increasingly tractable. In this article, recent findings are reviewed on a special type of inheritance, related to the transmission of traits via what have been termed 'imprinted' genes. In imprinted genes one allele is silenced according to its parental origin. This results in the inheritance of traits down the maternal or paternal line, in contrast to the more frequent mode of inheritance that is indifferent to the parental origin of the allele. Drawing on the advances made possible by combining the approaches of cognitive neuropsychology, behavioural neuroscience and contemporary molecular genetics, the detailed evidence for imprinted effects on behavioural and cognitive phenotypes is considered, focusing on findings from mental disorders, Turner's syndrome and experimental work in animal models. As prevailing evolutionary theories stress an essential antagonistic role of imprinted effects, these data might link such apparently diverse issues as neurodevelopment and the vulnerability to mental disease with the 'battle of the sexes', as joined at the level of cognitive and behavioural functioning.

Failure to establish linkage on the X chromosome in 301 families with schizophrenia or schizoaffective disorder

The hypothesis that a gene for susceptibility to psychosis (specifically in the X-Y homologous class) is located on the sex chromosomes has been proposed. Such a gene would account for the excess of sex chromosome anomalous males and females in populations of patients with psychosis, a tendency towards concordance by sex within families, and sex differences associated with psychosis and its underlying brain pathology. In earlier studies we observed small positive LOD scores in Xp11, and in a more recent and larger cohort of 178 sibling pairs, a peak multipoint nonparametric LOD score of 1. 55 at the locus DXS8032 in Xq21. The present study with a new set of markers extended the cohort to 301 ill sibling pairs and their parents. Despite the increase in sample size, the LOD score did not increase. A peak NPL of 1.55 was observed at the locus DXS1068 in proximal Xp, a region remote from the previous report. Separating families into those who were more likely to have X chromosome inheritance (maternal with no male to male transmission) did not yield stronger findings. In spite of the evidence that psychosis is related to a sex-dependent dimension of cerebral asymmetry, it is concluded that no consistent linkage of schizophrenia to the X chromosome can be demonstrated. In the context of the general failure of replication of linkage in psychosis, the possibility that the genetic predisposition to psychosis is contributed to by epigenetic modification rather than variations in the nucleotide sequence has to be considered.

Distinctive patterns of memory function in subgroups of females with Turner syndrome: evidence for imprinted loci on the X-chromosome affecting neurodevelopment

X-monosomy is a form of Turner syndrome (TS) in which an entire X chromosome is missing. It is usually assumed that neuropsychological deficits in females with TS result from insufficient dosage of gene products from alleles on the sex chromosomes. If so, then parental origin of the single X chromosome should be immaterial. However, if there are imprinted genes on the X chromosome affecting brain development, neuropsychological development will depend on the parental origin of the single X chromosome. We contrasted verbal and visuospatial memory in females with a single paternal X chromosome (45,X(p)) and those with a single maternal X (45,X(m)). Neither group showed any impairment on immediate story recall; if anything, performance was above control levels. Groups did not differ on a measure of delayed recall. However, when delayed recall was considered after adjusting for level of immediate recall, 45,X(m) females showed enhanced verbal forgetting relative to controls over a delay. On the Rey figure, both groups were poor at copying the figure, but, after adjusting scores for initial copy score and strategy, only the 45,X(p) females showed disproportionate forgetting relative to controls. We propose there may be one or more imprinted genes on the X chromosome that affect the development of lateralised brain regions important for memory function.

Regional brain volume change over the life-time course of schizophrenia

The concept that schizophrenia has its antecedents in neurodevelopment has long been debated and has been at the forefront of research on this disorder over the last decade. However new recent evidence from controlled longitudinal studies indicates that some of the structural brain anomalies observed in schizophrenia may continue to progress sporadically after the onset of clinical illness. The studies vary in cohort composition, stage of illness studied, duration of follow-up interval, and specific brain regions with findings. Nevertheless, the findings as a whole suggest that the brain changes in size throughout the lifespan of an individual and to a greater extent in schizophrenia. While the detected abnormalities could be explained by various technical artifacts, or physiological epi-phenomena, that they result from an ongoing neurochemical, physiological or morphological process characteristic of the underlying basis for the disorder is an intriguing possibility that lends itself to possible future intervention.

Prader Willi and Angelman syndromes: exemplars of genomic imprinting

The molecular phenomenon genomic imprinting provides an explanation for why two clinically distinct syndromes share genetic etiologies. Increased understanding of genomic imprinting is affecting diagnostics. Use of improved diagnostic tests can enable early, syndrome-specific, and anticipatory interventions and consequently, improved quality of life; however, these tests are of little use unless clinicians are able to identify at-risk patients. Nurses knowledgeable about Prader Willi and Angelman syndromes and their associated genetic mechanisms can play a significant role in early identification, referral, and intervention of patients with these conditions.

Polymorphisms, genomic imprinting and cancer susceptibility

Polymorphisms have been identified in proto-oncogenes and tumor suppressor genes that predispose people to cancer. Recent evidence indicates that genomic imprinting, an epigenetic form of gene regulation that results in uniparental gene expression, can also function as a cancer predisposing event. Thus, cancer susceptibility is increased by both Mendelian inherited genetic and non-Mendelian inherited epigenetic events. Consequently, chemical and physical agents cannot only induce cancer through the formation of genetic mutations but also through epigenetic changes that result in the inappropriate expression of imprinted proto-oncogenes and tumor suppressor genes. The role of genomic imprinting in carcinogenesis and cancer susceptibility is examined in this review.

Genomic imprinting of the X chromosome: a novel mechanism for the evolution of sexual dimorphism

Genomic imprinting is the differential marking of maternally and paternally inherited alleles of specific genes or chromosome regions during gametogenesis. The imprint silences the allele from 1 parent. A number of imprinted genes that are expressed in the brain have been identified in humans. They control the actions of other genes or regulate their products. Sexual dimorphism in the vertebrate brain is conventionally thought to be due to the epigenetic action of gonadal hormones. Sex differences could also reflect the actions of an imprinted X-linked locus. Until very recently no imprinted gene had been described on the X chromosome in humans. Here the implications of such a mechanism for the evolution of sexual dimorphism are discussed.