Early environmental regulation of hippocampal glucocorticoid receptor gene expression: characterization of intracellular mediators and potential genomic target sites

Interindividual Variability in Stress Susceptibility: A Role for Epigenetic Mechanisms in PTSD

Post-traumatic stress disorder (PTSD) is a psychiatric condition characterized by intrusive and persistent memories of a psychologically traumatic event that leads to significant functional and social impairment in affected individuals. The molecular bases underlying persistent outcomes of a transient traumatic event have remained elusive for many years, but recent studies in rodents have implicated epigenetic modifications of chromatin structure and DNA methylation as fundamental mechanisms for the induction and stabilization of fear memory. In addition to mediating adaptations to traumatic events that ultimately cause PTSD, epigenetic mechanisms are also involved in establishing individual differences in PTSD risk and resilience by mediating long-lasting effects of genes and early environment on adult function and behavior. In this review, we discuss the current evidence for epigenetic regulation of PTSD in human studies and in animal models and comment on ways in which these models can be expanded. In addition, we identify key outstanding questions in the study of epigenetic mechanisms of PTSD in the context of rapidly evolving technologies that are constantly updating and adjusting our understanding of epigenetic modifications and their functional roles. Finally, we discuss the potential application of epigenetic approaches in identifying markers of risk and resilience that can be utilized to promote early intervention and develop therapeutic strategies to combat PTSD after symptom onset.

Role of the hypothalamic-pituitary-adrenal axis in developmental programming of health and disease

Adverse environments during the fetal and neonatal development period may permanently program physiology and metabolism, and lead to increased risk of diseases in later life. Programming of the hypothalamic-pituitary-adrenal (HPA) axis is one of the key mechanisms that contribute to altered metabolism and response to stress. Programming of the HPA axis often involves epigenetic modification of the glucocorticoid receptor (GR) gene promoter, which influences tissue-specific GR expression patterns and response to stimuli. This review summarizes the current state of research on the HPA axis and programming of health and disease in the adult, focusing on the epigenetic regulation of GR gene expression patterns in response to fetal and neonatal stress. Aberrant GR gene expression patterns in the developing brain may have a significant negative impact on protection of the immature brain against hypoxic-ischemic encephalopathy in the critical period of development during and immediately after birth.

Epigenetic vestiges of early developmental adversity: childhood stress exposure and DNA methylation in adolescence

Fifteen-year-old adolescents (N = 109) in a longitudinal study of child development were recruited to examine differences in DNA methylation in relation to parent reports of adversity during the adolescents' infancy and preschool periods. Microarray technology applied to 28,000 cytosine-guanine dinucleotide sites within DNA derived from buccal epithelial cells showed differential methylation among adolescents whose parents reported high levels of stress during their children's early lives. Maternal stressors in infancy and paternal stressors in the preschool years were most strongly predictive of differential methylation, and the patterning of such epigenetic marks varied by children's gender. To the authors' knowledge, this is the first report of prospective associations between adversities in early childhood and the epigenetic conformation of adolescents' genomic DNA.

What have we learnt about the causes of ADHD?

BACKGROUND

Attention deficit hyperactivity disorder (ADHD) and its possible causes still attract controversy. Genes, pre and perinatal risks, psychosocial factors and environmental toxins have all been considered as potential risk factors.

METHOD

This review (focussing on literature published since 1997, selected from a search of PubMed) critically considers putative risk factors with a focus on genetics and selected environmental risks, examines their relationships with ADHD and discusses the likelihood that these risks are causal as well as some of the main implications.

RESULTS

No single risk factor explains ADHD. Both inherited and noninherited factors contribute and their effects are interdependent. ADHD is familial and heritable. Research into the inherited and molecular genetic contributions to ADHD suggest an important overlap with other neurodevelopmental problems, notably, autism spectrum disorders. Having a biological relative with ADHD, large, rare copy number variants, some small effect size candidate gene variants, extreme early adversity, pre and postnatal exposure to lead and low birth weight/prematurity have been most consistently found as risk factors, but none are yet known to be definitely causal. There is a large literature documenting associations between ADHD and a wide variety of putative environmental risks that can, at present, only be regarded as correlates. Findings from research designs that go beyond simply testing for association are beginning to contest the robustness of some environmental exposures previously thought to be ADHD risk factors.

CONCLUSIONS

The genetic risks implicated in ADHD generally tend to have small effect sizes or be rare and often increase risk of many other types of psychopathology. Thus, they cannot be used for prediction, genetic testing or diagnostic purposes beyond what is predicted by a family history. There is a need to consider the possibility of parents and siblings being similarly affected and how this might impact on engagement with families, influence interventions and require integration with adult services. Genetic contributions to disorder do not necessarily mean that medications are the treatment of choice. We also consider how findings might influence the conceptualisation of ADHD, public health policy implications and why it is unhelpful and incorrect to dichotomise genetic/biological and environmental explanations. It is essential that practitioners can interpret genetic and aetiological research findings and impart informed explanations to families.

The Effects of Maternal Separation on Adult Methamphetamine Self-Administration, Extinction, Reinstatement, and MeCP2 Immunoreactivity in the Nucleus Accumbens

The maternal separation (MS) paradigm is an animal model of early life stress. Animals subjected to MS during the first 2 weeks of life display altered behavioral and neuroendocrinological stress responses as adults. MS also produces altered responsiveness to and self-administration (SA) of various drugs of abuse including cocaine, ethanol, and amphetamine. However, no studies have yet examined the effects of MS on methamphetamine (METH) SA. This study was performed to examine the effects of MS on the acquisition of METH SA, extinction, and reinstatement of METH-seeking behavior in adulthood. Given the known influence of early life stress and drug exposure on epigenetic processes, we also investigated group differences in levels of the epigenetic marker methyl CpG binding protein 2 (MeCP2) in the nucleus accumbens (NAc) core. Long-Evans pups and dams were separated on postnatal days (PND) 2-14 for either 180 (MS180) or 15 min (MS15). Male offspring were allowed to acquire METH SA (0.05 mg/kg/infusion) in 15 2-h daily sessions starting at PND67, followed by extinction training and cue-induced reinstatement of METH-seeking behavior. Rats were then assessed for MeCP2 levels in the NAc core by immunohistochemistry. The MS180 group self-administered significantly more METH and acquired SA earlier than the MS15 group. No group differences in extinction or cue-induced reinstatement were observed. MS15 rats had significantly elevated MeCP2-immunoreactive cells in the NAc core as compared to MS180 rats. Together, these data suggest that MS has lasting influences on METH SA as well as epigenetic processes in the brain reward circuitry.

How Do Interactions Between Early Caregiving Environment and Genes Influence Health and Behavior?

To promote optimal health and behavioral outcomes in children, nurses have long supported parents in providing the best possible care and nurturance to their offspring. A growing body of neuroscience research argues convincingly for the combined influences of genes and early caregiving on producing an individual's unique health and behavioral phenotype. In this article, we systematically review studies that demonstrate the relationship between qualities of early caregiving and genetic propensity to health and behavioral outcomes. From an initial set of 255 articles, 24 articles met our inclusion criteria. The outcomes fall into four distinct groups: hypothalamic-pituitary-adrenal (HPA) response to stress, externalizing behavior, internalizing behavior, and disorganized attachment. In the articles, authors examined genes that code for the 5-hydroxy tryptamine (serotonin) transporter genes linked polymorphic region [5- HTTLPR] serotonin transporter promoter, D4 dopamine receptor, brain-derived neurotrophic factor, and monoamine oxidase A promoter. The reviewed studies suggest that the effect of the early rearing environment on gene expression relates mainly to HPA response to stress, whereas interactions between genes and caregiving mainly relate to behavior and attachment. Findings have implications for nurses focused on advocacy, prevention, and intervention to support the healthy development of children in families faced with adversity.

The outdoor air pollution and brain health workshop

Accumulating evidence suggests that outdoor air pollution may have a significant impact on central nervous system (CNS) health and disease. To address this issue, the National Institute of Environmental Health Sciences/National Institute of Health convened a panel of research scientists that was assigned the task of identifying research gaps and priority goals essential for advancing this growing field and addressing an emerging human health concern. Here, we review recent findings that have established the effects of inhaled air pollutants in the brain, explore the potential mechanisms driving these phenomena, and discuss the recommended research priorities/approaches that were identified by the panel.

Epigenetic regulation of the glucocorticoid receptor promoter 1(7) in adult rats

Regulation of glucocorticoid receptor (GR) levels is an important stress adaptation mechanism. Transcription factor Nfgi-a and environmentally induced Gr promoter 1 7 methylation have been implicated in fine-tuning the expression of Gr 1 7 transcripts. Here, we investigated Gr promoter 1 7 methylation and Gr 1 7 expression in adult rats exposed to either acute or chronic stress paradigms. A strong negative correlation was observed between the sum of promoter-wide methylation levels and Gr 1 7 transcript levels, independent of the stressor. Methylation of individual sites did not, however, correlate with transcript levels. This suggested that promoter 1 7 was directly regulated by promoter-wide DNA methylation. Although acute stress increased Ngfi-a expression in the hypothalamic paraventricular nucleus (PVN), Gr 1 7 transcript levels remained unaffected despite low methylation levels. Acute stress had little effect on these low methylation levels, except at four hippocampal CpGs. Chronic stress altered the corticosterone response to an acute stressor. In the adrenal and pituitary glands, but not in the brain, this was accompanied by an increase in methylation levels in orchestrated clusters rather than individual CpGs. PVN methylation levels, unaffected by acute or chronic stress, were significantly more variable within- than between-groups, suggesting that they were instated probably during the perinatal period and represent a pre-established trait. Thus, in addition to the known perinatal programming, the Gr 1 7 promoter is epigenetically regulated by chronic stress in adulthood, and retains promoter-wide tissue-specific plasticity. Differences in methylation susceptibility between the PVN in the perinatal period and the peripheral HPA axis tissues in adulthood may represent an important "trait" vs. "state" regulation of the Gr gene.

Sensitive periods in epigenetics: bringing us closer to complex behavioral phenotypes

Genetic studies have attempted to elucidate causal mechanisms for the development of complex disease, but genome-wide associations have been largely unsuccessful in establishing these links. As an alternative link between genes and disease, recent efforts have focused on mechanisms that alter the function of genes without altering the underlying DNA sequence. Known as epigenetic mechanisms, these include DNA methylation, chromatin conformational changes through histone modifications, ncRNAs and, most recently, 5-hydroxymethylcytosine. Although DNA methylation is involved in normal development, aging and gene regulation, altered methylation patterns have been associated with disease. It is generally believed that early life constitutes a period during which there is increased sensitivity to the regulatory effects of epigenetic mechanisms. The purpose of this review is to outline the contribution of epigenetic mechanisms to genomic function, particularly in the development of complex behavioral phenotypes, focusing on the sensitive periods.

Neuropeptides as mediators of the early-life impact on the brain; implications for alcohol use disorders

The brain is constantly exposed to external and internal input and to function in an ever-changing environment we are dependent on processes that enable the brain to adapt to new stimuli. Exposure to postnatal environmental stimuli can interfere with vital adaption processes and cause long-term changes in physiological function and behavior. Early-life alterations in brain function may result in impaired ability to adapt to new situations, in altered sensitivity to challenges later in life and thereby mediate risk or protection for psychopathology such as alcohol use disorders (AUD). In clinical research the studies of mechanisms, mediators, and causal relation between early environmental factors and vulnerability to AUD are restricted and attempts are made to find valid animal models for studies of the early-life influence on the brain. This review focuses on rodent models and the effects of adverse and naturalistic conditions on peptide networks within the brain and pituitary gland. Importantly, the consequences of alcohol addiction are not discussed but rather neurobiological alterations that can cause risk consumption and vulnerability to addiction. The article reviews earlier results and includes new data and multivariate data analysis with emphasis on endogenous opioid peptides but also oxytocin and vasopressin. These peptides are vital for developmental processes and it is hypothesized that early-life changes in peptide networks may interfere with neuronal processes and thereby contribute the individual vulnerability for AUD. The summarized results indicate a link between early-life rearing conditions, opioids, and ethanol consumption and that the ethanol-induced effects and the treatment with opioid antagonists later in life are dependent on early-life experiences. Endogenous opioids are therefore of interest to further study in the early-life impact on individual differences in vulnerability to AUD and treatment outcome.

Mechanisms of age-related cognitive change and targets for intervention: social interactions and stress

BACKGROUND

The effects of biological and physical factors on cognitive aging are widely studied. Less is known about the role of psychosocial factors such as stress and social relationships for cognitive functioning.

METHODS

Speakers in Session IV of the Summit focused on possible mechanisms linking social interactions and stressful experiences to cognitive changes with aging.

RESULTS

Elevated cortisol, repetitive thinking, negative emotions, neuroticism, chronic stress, and early life adversity were negatively associated with memory and other cognitive dimensions in later life. In contrast, supportive social relationships were found to be positively related to cognitive functioning. Some evidence was provided for multidirectional, causal relationships involving stress and negative affect as both antecedents and consequences of cognitive decline.

CONCLUSIONS

The findings contribute to understanding the potential underlying causal processes linking psychosocial factors and cognitive aging with a developmental focus on the etiology of declines and onset of cognitive impairments. This work provides an important foundation for future research to identify modifiable factors and to design interventions to minimize cognitive declines and optimize cognitive health in adulthood.

Biological indicators of illness risk in offspring of bipolar parents: targeting the hypothalamic-pituitary-adrenal axis and immune system

AIM

The study aims to provide a selective review of the literature pertaining to the hypothalamic-pituitary-adrenal (HPA) axis and immune abnormalities as informative biological indicators of vulnerability in bipolar disorder (BD).

METHOD

We summarize key findings relating to HPA axis and immunological abnormalities in bipolar patients and their high-risk offspring. Findings derive from a review of selected original papers published in the literature, and supplemented by papers identified through bibliography review. Neurobiological findings are discussed in the context of emergent BD in those at genetic risk and synthesized into a neurodevelopmental model of illness onset and progression.

RESULTS

BD is associated with a number of genetic and possibly epigenetic abnormalities associated with neurotransmitter, hormonal and immunologically mediated neurobiological pathways. Data from clinical and high-risk studies implicate HPA axis and immune system abnormalities, which may represent inherited vulnerabilities important for the transition to illness onset. Post-mortem and clinical studies implicate intracellular signal transduction processes and disturbance in energy metabolism associated with established BD. Specifically, long-standing maladaptive alterations such as changes in neuronal systems may be mediated through changes in intracellular signalling pathways, oxidative stress, cellular energy metabolism and apoptosis associated with substantial burden of illness.

CONCLUSIONS

Prospective longitudinal studies of endophenotypes and biomarkers such as HPA axis and immune abnormalities in high-risk offspring will be helpful to understand genetically mediated biological pathways associated with illness onset and progression. A clinical staging model describing emergent illness in those at genetic risk should facilitate this line of investigation.

Epigenetic mechanisms in memory and synaptic function

Although the term 'epigenetics' was coined nearly seventy years ago, its critical function in memory processing by the adult CNS has only recently been appreciated. The hypothesis that epigenetic mechanisms regulate memory and behavior was motivated by the need for stable molecular processes that evade turnover of the neuronal proteome. In this article, we discuss evidence that supports a role for neural epigenetic modifications in the formation, consolidation and storage of memory. In addition, we will review the evidence that epigenetic mechanisms regulate synaptic plasticity, a cellular correlate of memory. We will also examine how the concerted action of multiple epigenetic mechanisms with varying spatiotemporal profiles influence selective gene expression in response to behavioral experience. Finally, we will suggest key areas for future research that will help elucidate the complex, vital and still mysterious, role of epigenetic mechanisms in neural function and behavior.

Antenatal corticosteroids: a risk factor for the development of chronic disease

Preterm birth remains a major health issue worldwide. Since the 1990s, women at risk for preterm birth received a single course of exogenous antenatal corticosteroids (ACSs) to facilitate fetal lung maturity. More recently, repeated or multiple courses of ACS have been supported to provide continued fetal maturity support for women with continued risk of preterm birth. However, exogenous ACS reduces birth weight which, in turn, is associated with adverse adult outcomes such as coronary heart disease, stroke, hypertension, and type 2 diabetes. The long-term effects of ACS exposure on HPA axis activity and neurological function are well documented in animal studies, and it appears that ACS, regardless of dose exposure, is capable of affecting fetal HPA axis development causing permanent changes in the HPA axis that persists through life and is manifested by chronic illness and behavioral changes. The challenge in human studies is to demonstrate whether an intervention such as ACS administration in pregnancy contributes to developmental programming and how this is manifested in later life.

Pharmacogenetic studies of change in cortisol on ecstasy (MDMA) consumption

In this study we investigate the association of cytochrome P450 enzyme CYP2D6, catechol-O-methyl transferase (COMT, Val158Met) and serotonin transporter promoter (5-HTTLPR) genotypes on change in cortisol concentration following 3, 4-methylenedioxy-methamphetamine (MDMA, 'ecstasy') consumption. Forty-eight subjects (30 males, mean age 23 years), self-nominating regular clubbers provided 'in the field' pre- and post-clubbing biological samples and associated information. Of the 39 subjects who provided a post-clubbing urine sample, 21 were positive for MDMA. Plasma cortisol concentrations increased in subjects (n = 48) tested for cortisol, with changes being significantly greater in the MDMA-positive group (736.9 ± 83.2 vs. 350.9 ± 34.5 mmol/l, p = 0.001). We found a positive association between the low activity COMT genotype (Met/Met) and MDMA-induced change in cortisol and also between this and change in cortisol in the whole sample (p = 0.039, Bonferroni corrected). For CYP2D6, there was an association between genotype and change in cortisol, confined to subjects with MDMA-positive urine post-clubbing (p = 0.003, Bonferroni corrected). There was no association with 5-HTTLPR genotype. These associations suggest that chronic use of MDMA may lead to HPA axis dysregulation and that the magnitude of this may be moderated by genetic polymorphism, and warrant further investigation in a larger sample of those who consume the drug on a regular basis.

Toward understanding how early-life social experiences alter oxytocin- and vasopressin-regulated social behaviors

The early-life social environment has profound effects on brain development and subsequent expression of social behavior. Oxytocin and vasopressin are expressed and released in the brain and are important regulators of social behavior. Accordingly, the early social environment may alter social behaviors via changes in the oxytocin and/or vasopressin systems. To test this hypothesis, and to gain mechanistic insights, rodent models mimicking either a deprived (e.g. maternal separation) or enriched (e.g. neonatal handling) early social environment have been utilized. Findings indeed show that differences in the quality of the early social environment are associated with brain region-specific alterations in oxytocin and vasopressin expression and oxytocin receptor and vasopressin 1a receptor binding. Early social environment-induced changes in oxytocin and vasopressin systems were associated with changes in several forms of social behavior, including maternal care, aggression, play-fighting, and social recognition. First studies provide evidence for a causal link between altered vasopressin responsiveness and impairments in social recognition in rats exposed to maternal separation and a role for epigenetic mechanisms to explain persistent increases in vasopressin expression in mice exposed to maternal separation. Overall, initial findings suggest that oxytocin and vasopressin systems may mediate early social environment-induced alterations in social behavior. Additional comprehensive studies will be necessary to advance our understanding to what extent changes in oxytocin and vasopressin underlie early social environment-induced alterations in social behavior. This article is part of a Special Issue entitled Oxytocin, Vasopressin, and Social Behavior.

Oxytocin and mutual communication in mother-infant bonding

Mother-infant bonding is universal to all mammalian species. In this review, we describe the manner in which reciprocal communication between the mother and infant leads to mother-infant bonding in rodents. In rats and mice, mother-infant bond formation is reinforced by various social stimuli, such as tactile stimuli and ultrasonic vocalizations (USVs) from the pups to the mother, and feeding and tactile stimulation from the mother to the pups. Some evidence suggests that mother and infant can develop a cross-modal sensory recognition of their counterpart during this bonding process. Neurochemically, oxytocin in the neural system plays a pivotal role in each side of the mother-infant bonding process, although the mechanisms underlying bond formation in the brains of infants has not yet been clarified. Impairment of mother-infant bonding, that is, deprivation of social stimuli from the mother, strongly influences offspring sociality, including maternal behavior toward their own offspring in their adulthood, implying a "non-genomic transmission of maternal environment," even in rodents. The comparative understanding of cognitive functions between mother and infants, and the biological mechanisms involved in mother-infant bonding may help us understand psychiatric disorders associated with mother-infant relationships.

Neurogenetics and Epigenetics in Impulsive Behaviour: Impact on Reward Circuitry

Adverse, unfavourable life conditions, particularly during early life stages and infancy, can lead to epigenetic regulation of genes involved in stress-response, behavioral disinhibition, and cognitive-emotional systems. Over time, the ultimate final outcome can be expressed through behaviors bedeviled by problems with impulse control, such as eating disorders, alcoholism, and indiscriminate social behavior. While many reward gene polymorphisms are involved in impulsive behaviors, a polymorphism by itself may not translate to the development of a particular behavioral disorder unless it is impacted by epigenetic effects. Brain-derived neurotrophic factor (BDNF) affects the development and integrity of the noradrenergic, dopaminergic, serotonergic, glutamatergic, and cholinergic neurotransmitter systems, and plasma levels of the neurotrophin are associated with both cognitive and aggressive impulsiveness. Epigenetic mechanisms associated with a multitude of environmental factors, including premature birth, low birth weight, prenatal tobacco exposure, non-intact family, young maternal age at birth of the target child, paternal history of antisocial behavior, and maternal depression, alter the developmental trajectories for several neuropsychiatric disorders. These mechanisms affect brain development and integrity at several levels that determine structure and function in resolving the final behavioral expressions.

Maternal programming of reproductive function and behavior in the female rat

Parental investment can be used as a forecast for the environmental conditions in which offspring will develop to adulthood. In the rat, maternal behavior is transmitted to the next generation through epigenetic modifications such as methylation and histone acetylation, resulting in variations in estrogen receptor alpha expression. Natural variations in maternal care also influence the sexual strategy adult females will adopt later in life. Lower levels of maternal care are associated with early onset of puberty as well as increased motivation to mate and greater receptivity toward males during mating. Lower levels of maternal care are also correlated with greater activity of the hypothalamus-pituitary-gonadal axis, responsible for the expression of these behaviors. Contrary to the transition of maternal care, sexual behavior cannot simply be explained by maternal attention, since adoption studies changed the sexual phenotypes of offspring born to low caring mothers but not those from high caring dams. Indeed, mothers showing higher levels of licking/grooming have embryos that are exposed to high testosterone levels during development, and adoption studies suggest that this androgen exposure may protect their offspring from lower levels of maternal care. We propose that in the rat, maternal care and the in utero environment interact to influence the reproductive strategy female offspring display in adulthood and that this favors the species by allowing it to thrive under different environmental conditions.

Early-life experience decreases drug-induced reinstatement of morphine CPP in adulthood via microglial-specific epigenetic programming of anti-inflammatory IL-10 expression

A critical component of drug addiction research involves identifying novel biological mechanisms and environmental predictors of risk or resilience to drug addiction and associated relapse. Increasing evidence suggests microglia and astrocytes can profoundly affect the physiological and addictive properties of drugs of abuse, including morphine. We report that glia within the rat nucleus accumbens (NAcc) respond to morphine with an increase in cytokine/chemokine expression, which predicts future reinstatement of morphine conditioned place preference (CPP) following a priming dose of morphine. This glial response to morphine is influenced by early-life experience. A neonatal handling paradigm that increases the quantity and quality of maternal care significantly increases baseline expression of the anti-inflammatory cytokine IL-10 within the NAcc, attenuates morphine-induced glial activation, and prevents the subsequent reinstatement of morphine CPP in adulthood. IL-10 expression within the NAcc and reinstatement of CPP are negatively correlated, suggesting a protective role for this specific cytokine against morphine-induced glial reactivity and drug-induced reinstatement of morphine CPP. Neonatal handling programs the expression of IL-10 within the NAcc early in development, and this is maintained into adulthood via decreased methylation of the IL-10 gene specifically within microglia. The effect of neonatal handling is mimicked by pharmacological modulation of glia in adulthood with ibudilast, which increases IL-10 expression, inhibits morphine-induced glial activation within the NAcc, and prevents reinstatement of morphine CPP. Taken together, we have identified a novel gene × early-life environment interaction on morphine-induced glial activation and a specific role for glial activation in drug-induced reinstatement of drug-seeking behavior.

Associations with early-life socio-economic position in adult DNA methylation

BACKGROUND

Disadvantaged socio-economic position (SEP) in childhood is associated with increased adult mortality and morbidity. We aimed to establish whether childhood SEP was associated with differential methylation of adult DNA.

METHODS

Forty adult males from the 1958 British Birth Cohort Study were selected from SEP extremes in both early childhood and mid-adulthood. We performed genome-wide methylation analysis on blood DNA taken at 45 years using MeDIP (methylated DNA immunoprecipitation). We mapped in triplicate the methylation state of promoters of approximately 20,000 genes and 400 microRNAs. Probe methylation scores were averaged across triplicates and differential methylation between groups of individuals was determined. Differentially methylated promoter sites of selected genes were validated using pyrosequencing of bisulfite-converted DNA.

RESULTS

Variably methylated probes (9112 from n = 223,359 on the microarray) corresponded to 6176 gene promoters with at least one variable probe. Unsupervised hierarchical clustering of probes obtained from the 500 most variable promoters revealed a cluster enriched with high SEP individuals confirming that SEP differences contribute to overall epigenetic variation. Methylation levels for 1252 gene promoters were associated with childhood SEP vs 545 promoters for adulthood SEP. Functionally, associations with childhood SEP appear in promoters of genes enriched in key cell signalling pathways. The differentially methylated promoters associated with SEP cluster in megabase-sized regions of the genome.

CONCLUSIONS

Adult blood DNA methylation profiles show more associations with childhood SEP than adult SEP. Organization of these associations across the genome suggests a well-defined epigenetic pattern linked to early socio-economic environment.

Maternal obesity and developmental programming of metabolic disorders in offspring: evidence from animal models

The incidence of obesity and overweight has reached epidemic proportions in the developed world as well as in those countries transitioning to first world economies, and this represents a major global health problem. Concern is rising over the rapid increases in childhood obesity and metabolic disease that will translate into later adult obesity. Although an obesogenic nutritional environment and increasingly sedentary lifestyle contribute to our risk of developing obesity, a growing body of evidence links early life nutritional adversity to the development of long-term metabolic disorders. In particular, the increasing prevalence of maternal obesity and excess maternal weight gain has been associated with a heightened risk of obesity development in offspring in addition to an increased risk of pregnancy-related complications. The mechanisms that link maternal obesity to obesity in offspring and the level of gene-environment interactions are not well understood, but the early life environment may represent a critical window for which intervention strategies could be developed to curb the current obesity epidemic. This paper will discuss the various animal models of maternal overnutrition and their importance in our understanding of the mechanisms underlying altered obesity risk in offspring.

Developmental programming of the metabolic syndrome - critical windows for intervention

Metabolic disease results from a complex interaction of many factors, including genetic, physiological, behavioral and environmental influences. The recent rate at which these diseases have increased suggests that environmental and behavioral influences, rather than genetic causes, are fuelling the present epidemic. In this context, the developmental origins of health and disease hypothesis has highlighted the link between the periconceptual, fetal and early infant phases of life and the subsequent development of adult obesity and the metabolic syndrome. Although the mechanisms are yet to be fully elucidated, this programming was generally considered an irreversible change in developmental trajectory. Recent work in animal models suggests that developmental programming of metabolic disorders is potentially reversible by nutritional or targeted therapeutic interventions during the period of developmental plasticity. This review will discuss critical windows of developmental plasticity and possible avenues to ameliorate the development of postnatal metabolic disorders following an adverse early life environment.

Epigenetic regulation in obesity

The availability to the DNA strand and the activity of the transcription machinery is crucial for the cell to use the information in the DNA. The epigenetic mechanisms DNA methylation, modification of histone tails, other chromatin-modifying processes and interference by small RNAs regulate the cell-type-specific DNA expression. Epigenetic marks can be more or less plastic perpetuating responses to various molecular signals and environmental stimuli, but in addition apparently stochastic epigenetic marks have been found. There is substantial evidence from animal and man demonstrating that both transient and more long-term epigenetic mechanisms have a role in the regulation of the molecular events governing adipogenesis and glucose homeostasis. Intrauterine exposure such as poor maternal nutrition has consistently been demonstrated to contribute to a particular epigenotype and thereby developmental metabolic priming of the exposed offspring in animal and man. Epigenetic modifications can be passed not only from one cell generation to the next, but metabolic disease-related epigenotypes have been proposed to also be transmitted germ-line. Future more comprehensive knowledge on epigenetic regulation will complement genome sequence data for the understanding of the complex etiology of obesity and related disorder.

Effects of stress across the lifespan

Stress is a known precipitant for metabolic and neurological diseases, with sensitive periods identified across the developmental continuum from conception to old age. However, the effects of stress may vary depending on the point or points along the developmental trajectory when adversity strikes. Past research has emphasized the consequences of stress on fully developed physiological systems in the brain and periphery, but more recent studies have explored the impact of stress on systems at different stages of maturation, with differential effects being revealed. This review provides an overview of the diverse effects of stress at critical developmental stages and the potential outcomes that may be associated with experiencing environmental adversity during ontogeny.

Maternal programming of sexual attractivity in female Long Evans rats

In mammals, maternal care influences the developing offspring across multiple domains. In Long Evans rats, for example, the quality of maternal care received as a pup influences later cognitive function, neuroendocrine responses to stress and behavioral measures of emotionality. Data from humans, non-human primates, and rodents also suggest that early life events may similarly perturb measures of sexual reproduction, with possible consequences for reproductive fitness. The current study examined whether or not male conspecifics differentially prefer females, as adult mating partners, that were reared under varying maternal conditions (assessed via the quantity of licking and grooming received; LG). Additionally, the impact of maternal care on adult female sexual motivation and behavior were quantified to determine if these behavioral characteristics are associated with any preference observed. In a mate preference task, male rats chose, almost exclusively, to mount, copulate and ejaculate with female rats reared under Low LG conditions. Under non-paced mating conditions, female Low LG rats display significantly more paracopulatory and copulatory behaviors compared to High LG rats. Due to its critical role in female paracopulatory behavior, progesterone receptor immunoreactivity (PR-ir) in the ventromedial nucleus of the hypothalamus (VMH) was also assessed in both groups of female rats. Estradiol induced PR-ir in the VMH was significantly higher in Low LG relative to High LG rats. Together, these data suggests that early life parental care may developmentally program aspects of behavior and physiology that subsequently influence sexual attractivity and behavior in adult females.

Developmental consequences and biological significance of mother-infant bonding

Mother-infant bonding is universal to all mammalian species. Here, we review how mutual communication between the mother and infant leads to mother-infant bonding in non-primate species. In rodents, mother-infant bond formation is reinforced by various pup stimuli, such as tactile stimuli and ultrasonic vocalizations. Evidence suggests that the oxytocin neural system plays a pivotal role in each aspect of the mother-infant bonding, although the mechanisms underlying bond formation in the brain of infants has not yet been clarified. Impairment of mother-infant bonding strongly influences offspring sociality. We describe the negative effects of mother-infant bonding deprivation on the neurobehavioral development in rodent offspring, even if weaning occurs in the later lactating period. We also discuss similar effects observed in pigs and dogs, which are usually weaned earlier than under natural conditions. The comparative understanding of the developmental consequences of mother-infant bonding and the underlying mechanisms provide insight into the biological significance of this bonding in mammals, and may help us to understand psychiatric disorders related to child abuse or childhood neglect.

Sex differences in prenatal epigenetic programming of stress pathways

Maternal stress experience is associated with neurodevelopmental disorders including schizophrenia and autism. Recent studies have examined mechanisms by which changes in the maternal milieu may be transmitted to the developing embryo and potentially translated into programming of the epigenome. Animal models of prenatal stress have identified important sex- and temporal-specific effects on offspring stress responsivity. As dysregulation of stress pathways is a common feature in most neuropsychiatric diseases, molecular and epigenetic analyses at the maternal-embryo interface, especially in the placenta, may provide unique insight into identifying much-needed predictive biomarkers. In addition, as most neurodevelopmental disorders present with a sex bias, examination of sex differences in the inheritance of phenotypic outcomes may pinpoint gene targets and specific windows of vulnerability in neurodevelopment, which have been disrupted. This review discusses the association and possible contributing mechanisms of prenatal stress in programming offspring stress pathway dysregulation and the importance of sex.

Decanalization, brain development and risk of schizophrenia

Waddington's original description of canalization refers to the ability of an organism to maintain phenotypic fidelity in the face of environmental and/or genetic perturbation. Development of the human brain requires exposure to a 'wild-type' environment-one that supports the optimal set of instructions for development. Recently derived brain structures in our species, such as the expanded neocortex, may be more vulnerable to decanalization because there has been insufficient time to evolve buffering capacity. On the basis of modern notions of decanalization, we provide perspectives on selected environmental and genetic risk factors for schizophrenia, and we discuss strengths and weaknesses of this conceptual framework. We argue that if we are to build a solid foundation for translational psychiatry, we must explore models that attempt to capture the complexity of the interaction between genetic and non-genetic risk factors in mediating and modulating brain development.

Adaptive significance of natural variations in maternal care in rats: a translational perspective

A wealth of data from the last fifty years documents the potency of early life experiences including maternal care on developing offspring. A majority of this research has focused on the developing stress axis and stress-sensitive behaviors in hopes of identifying factors impacting resilience and risk-sensitivity. The power of early life experience to shape later development is profound and has the potential to increase fitness of individuals for their environments. Current findings in a rat maternal care paradigm highlight the complex and dynamic relation between early experiences and a variety of outcomes. In this review we propose adaptive hypotheses for alternate maternal strategies and resulting offspring phenotypes, and suggest means of distinguishing between these hypotheses. We also provide evidence underscoring the critical role of context in interpreting the adaptive significance of early experiences. If our goal is to identify risk-factors relevant to humans, we must better explore the role of the social and physical environment in our basic animal models.

Birthweight is associated with DNA promoter methylation of the glucocorticoid receptor in human placenta

Birthweight has been associated with a number of health outcomes throughout life. Crucial to proper infant growth and development is the placenta, and alterations to placental gene function may reflect differences in the intrauterine environment which functionally contribute to infant growth and may ultimately affect the child's health. To examine if epigenetic alteration to the glucocorticoid receptor (GR) gene was linked to infant growth, we analyzed 480 human placentas for differential methylation of the GR gene exon 1F and examined how this variation in methylation extent was associated with fetal growth. Multivariable linear regression revealed a significant association (p < 0.0001) between differential methylation of the GR gene and large for gestational age (LGA) status. Our work is one of the first to link infant growth as a measure of the intrauterine environment and epigenetic alterations to the GR and suggests that DNA methylation may be a critical determinant of placental function.

Pharmacotherapeutic intervention in impulsive preschool children: The need for a comprehensive therapeutic approach

Impulsive and aggressive behaviour symptoms often are serious problems in children, even already at preschool age. Thus, effective treatment approaches are requested. In this comment pharmacotherapeutic treatment approaches, first of all risperidone, their limitations and alternative psychotherapeutic approaches are outlined.

Environmental regulation of the neural epigenome

Parental effects are a major source of phenotypic plasticity. Moreover, there is evidence from studies with a wide range of species that the relevant parental signals are influenced by the quality of the parental environment. The link between the quality of the environment and the nature of the parental signal is consistent with the idea that parental effects, whether direct or indirect, might serve to influence the phenotype of the offspring in a manner that is consistent with the prevailing environmental demands. In this review we explore recent studies from the field of 'environmental epigenetics' that suggest that (1) DNA methylation states are far more variable than once thought and that, at least within specific regions of the genome, there is evidence for both demethylation and remethylation in post-mitotic cells and (2) that such remodeling of DNA methylation can occur in response to environmentally-driven, intracellular signaling pathways. Thus, studies of variation in mother-offspring interactions in rodents suggest that parental signals operate during pre- and/or post-natal life to influence the DNA methylation state at specific regions of the genome leading to sustained changes in gene expression and function. We suggest that DNA methylation is a candidate mechanism for parental effects on phenotypic variation.

Chronic stress, cognitive functioning and mental health

This review aims to discuss the evidence supporting the link between chronic stress, cognitive function and mental health. Over the years, the associations between these concepts have been investigated in different populations. This review summarizes the findings that have emerged from older populations as well as from populations suffering from pathological aging, namely Mild Cognitive Impairment and Alzheimer's Disease. Although older adults are an interesting population to study in terms of chronic stress, other stress-related diseases can occur throughout the lifespan. The second section covers some of these stress-related diseases that have recently received a great deal of attention, namely burnout, depression, and post-traumatic stress disorder. Given that chronic stress contributes to the development of certain pathologies by accelerating and/or exacerbating pre-existing vulnerabilities that vary from one individual to the other, the final section summarizes data obtained on potential variables contributing to the association between chronic stress and cognition.

Cancer chemoprevention by dietary polyphenols: promising role for epigenetics

Epigenetics refers to heritable changes that are not encoded in the DNA sequence itself, but play an important role in the control of gene expression. In mammals, epigenetic mechanisms include changes in DNA methylation, histone modifications and non-coding RNAs. Although epigenetic changes are heritable in somatic cells, these modifications are also potentially reversible, which makes them attractive and promising avenues for tailoring cancer preventive and therapeutic strategies. Burgeoning evidence in the last decade has provided unprecedented clues that diet and environmental factors directly influence epigenetic mechanisms in humans. Dietary polyphenols from green tea, turmeric, soybeans, broccoli and others have shown to possess multiple cell-regulatory activities within cancer cells. More recently, we have begun to understand that some of the dietary polyphenols may exert their chemopreventive effects in part by modulating various components of the epigenetic machinery in humans. In this article, we first discuss the contribution of diet and environmental factors on epigenetic alterations; subsequently, we provide a comprehensive review of literature on the role of various dietary polyphenols. In particular, we summarize the current knowledge on a large number of dietary agents and their effects on DNA methylation, histone modifications and regulation of expression of the non-coding miRNAs in various in vitro and in vivo models. We emphasize how increased understanding of the chemopreventive effects of dietary polyphenols on specific epigenetic alterations may provide unique and yet unexplored novel and highly effective chemopreventive strategies for reducing the health burden of cancer and other diseases in humans.

Early life stress stimulates hippocampal reelin gene expression in a sex-specific manner: evidence for corticosterone-mediated action

Early life stress predisposes to the development of psychiatric disorders. In this context the hippocampal formation is of particular interest, because it is affected by stress on the structural and cognitive level. Since little is known how early life stress is translated on the molecular level, we mimicked early life stress in mouse models and analyzed the expression of the glycoprotein Reelin, a master molecule for development and differentiation of the hippocampus. From postnatal day 1 (P1) to P14, mouse pups were subjected to one of the following treatments: nonhandling (NH), handling (H), maternal separation (MS), and early deprivation (ED) followed by immediate (P15) or delayed (P70) real time RT-PCR analysis of reelin mRNA expression. We show that at P15, reelin mRNA levels were significantly increased in male H and ED groups when compared with the NH group. In contrast, no stress-induced alterations of reelin mRNA expression were found in female animals. This sex difference in stress-mediated stimulation of reelin expression was maintained into adulthood, since at P70 intergroup differences were still found in male, but not in female mice. On the cellular level, however, we did not find any significant differences in cell densities of Reelin-immunolabeled neurons between treatment groups or sexes, but an overall reduction of Reelin-expressing neurons in the adult hippocampus when compared to P15. To address the question whether corticosterone mediates the stress-induced up-regulation of reelin gene expression, we used age-matched hippocampal slice cultures derived from male and female mouse pups. Quantitative determination of mRNA levels revealed that corticosterone treatment significantly up-regulated reelin mRNA expression in male, but not in female hippocampi. Taken together, these results show a sex-specific regulation of reelin gene expression by early life experience, most likely mediated by corticosterone.

Genetically informative research on adolescent substance use: methods, findings, and challenges

OBJECTIVE

To provide an overview of the genetic epidemiology of substance use and misuse in adolescents.

METHOD

A selective review of genetically informative research strategies, their limitations, and key findings examining issues related to the heritability of substance use and substance use disorders in children and adolescents is presented.

RESULTS

Adoption, twin, and extended-family designs have established there is a strong heritable component to liability to nicotine, alcohol, and illicit drug dependence in adults. However, shared environmental influences are relatively stronger in youth samples and at earlier stages of substance involvement (e.g., use). There is considerable overlap in the genetic influences associated with the abuse/dependence across drug classes, and shared genetic influences contribute to the commonly observed associations between substance-use disorders and externalizing and, to a lesser extent, internalizing psychopathology. Rapid technologic advances have made the identification of specific gene variants that influence risks for substance-use disorders feasible, and linkage and association (including genomewide association studies) have identified promising candidate genes implicated in the development of substance-use disorders.

CONCLUSIONS

Studies using genetically informative research designs, including those that examine aggregate genetic factors and those examining specific gene variants, individually and in interaction with environmental influences, offer promising avenues not only for delineating genetic effects on substance-use disorders but also for understanding the unfolding of risk across development and the interaction between environmental and genetic factors in the etiology of these disorders.

Maternal care and DNA methylation of a glutamic acid decarboxylase 1 promoter in rat hippocampus

Parenting and the early environment influence the risk for various psychopathologies. Studies in the rat suggest that variations in maternal care stably influence DNA methylation, gene expression, and neural function in the offspring. Maternal care affects neural development, including the GABAergic system, the function of which is linked to the pathophysiology of diseases including schizophrenia and depression. Postmortem studies of human schizophrenic brains have revealed decreased forebrain expression of glutamic acid decarboxylase 1 (GAD1) accompanied by increased methylation of a GAD1 promoter. We examined whether maternal care affects GAD1 promoter methylation in the hippocampus of adult male offspring of high and low pup licking/grooming (high-LG and low-LG) mothers. Compared with the offspring of low-LG mothers, those reared by high-LG dams showed enhanced hippocampal GAD1 mRNA expression, decreased cytosine methylation, and increased histone 3-lysine 9 acetylation (H3K9ac) of the GAD1 promoter. DNA methyltransferase 1 expression was significantly higher in the offspring of low- compared with high-LG mothers. Pup LG increases hippocampal serotonin (5-HT) and nerve growth factor-inducible factor A (NGFI-A) expression. Chromatin immunoprecipitation assays revealed enhanced NGFI-A association with and H3K9ac of the GAD1 promoter in the hippocampus of high-LG pups after a nursing bout. Treatment of hippocampal neuronal cultures with either 5-HT or an NGFI-A expression plasmid significantly increased GAD1 mRNA levels. The effect of 5-HT was blocked by a short interfering RNA targeting NGFI-A. These results suggest that maternal care influences the development of the GABA system by altering GAD1 promoter methylation levels through the maternally induced activation of NGFI-A and its association with the GAD1 promoter.

DNA methylation analysis of multiple tissues from newborn twins reveals both genetic and intrauterine components to variation in the human neonatal epigenome

Mounting evidence from both animal and human studies suggests that the epigenome is in constant drift over the life course in response to stochastic and environmental factors. In humans, this has been highlighted by a small number of studies that have demonstrated discordant DNA methylation patterns in adolescent or adult monozygotic (MZ) twin pairs. However, to date, it remains unclear when such differences emerge, and how prevalent they are across different tissues. To address this, we examined the methylation of four differentially methylated regions associated with the IGF2/H19 locus in multiple birth tissues derived from 91 twin pairs: 56 MZ and 35 dizygotic (DZ). Tissues included cord blood-derived mononuclear cells and granulocytes, human umbilical vein endothelial cells, buccal epithelial cells and placental tissue. Considerable variation in DNA methylation was observed between tissues and between unrelated individuals. Most interestingly, methylation discordance was also present within twin pairs, with DZ pairs showing greater discordance than MZ pairs. These data highlight the variable contribution of both intrauterine environmental exposures and underlying genetic factors to the establishment of the neonatal epigenome of different tissues and confirm the intrauterine period as a sensitive time for the establishment of epigenetic variability in humans. This has implications for the effects of maternal environment on the development of the newborn epigenome and supports an epigenetic mechanism for the previously described phenomenon of 'fetal programming' of disease risk.

Intrauterine growth retardation affects expression and epigenetic characteristics of the rat hippocampal glucocorticoid receptor gene

Studies in humans and rats suggest that intrauterine growth retardation (IUGR) permanently resets the hypothalamic-pituitary-adrenal (HPA) axis. HPA axis reprogramming may involve persistently altered expression of the hippocampal glucocorticoid receptor (hpGR), an important regulator of HPA axis reactivity. Persistent alteration of gene expression, long after the inciting event, is thought to be mediated by epigenetic mechanisms that affect mRNA and mRNA variant expression. GR mRNA variants in both humans and rats include eleven 5′-end variants and GRα, the predominant 3′-end variant. The 3′-end variants associated with glucocorticoid resistance in humans (GRβ, GRγ, GRA, and GRP) have not been reported in rats. We hypothesized that in the rat hippocampus IUGR would decrease total GR mRNA, increase GRβ, GRγ, GRA, and GRP, and affect epigenetics of the GR gene at birth (D0) and at 21 days of life (D21). IUGR increased hpGR and exon 1.7 hpGR mRNA in males at D0 and D21, associated with increased trimethyl H3/K4 at exon 1.7 at both time points. IUGR also increased hpGRγ in males at D0 and D21, associated with increased acetyl H3/K9 at exon 3 at both time points. hpGRA increased in female IUGR rats at D0 and D21. In addition, our data support the existence of hpGRβ and hpGRP in the rat. IUGR has sex-specific, persistent effects on GR expression and its histone code. We speculate that postnatal changes in hippocampal GR variant and total mRNA expression may underlie IUGR-associated HPA axis reprogramming.

Advances and perspectives from genetic research: development of biological markers in Alzheimer's disease

Despite important recent advances, a full understanding of the (genetic) etiology of Alzheimer's disease (AD) is still a long way off. Large collaborative efforts are ongoing, as well as the exploration of various sources of genetic variation. Evidence supports the view that Mendelian early-onset familial forms of AD are caused by rare and usually highly penetrant mutations in three genes (APP, PSEN1 and PSEN2). Considering sporadic late-onset AD (LOAD), the APOE epsilon4 allele is by far the best-established risk gene. Recently published large-scale genome-wide analyses point to additionally relevant genetically associated loci, particularly CLU, PICALM and CR1. These susceptibility loci support existing hypotheses about the amyloid, lipid, chaperone and chronic inflammatory mechanisms in AD pathogenesis, and are therefore likely to provide the basis for the development of hypothesis-driven novel biomarker candidates. Additional genes, listed online in AlzGene (e.g., GAB2 or SORL1) have repeatedly shown risk effects in LOAD, and may be true risk genes, but this is much less certain. New epigenetic research provided some evidence that DNA modifications maybe involved in LOAD (e.g., post-mortem studies described both hypo- and hyper-methylation in AD-related susceptibility genes). With respect to biomarkers, elderly nondemented APOE epsilon4 carriers demonstrated distinct cerebrospinal fluid biomarker signatures and alterations of brain glucose metabolism similar to those observed in AD. Future research should evaluate the usefulness of newly detected AD risk genes and epigenetic changes as potential biomarkers towards genetic profiling of AD or for correlation with endophenotypes and therapeutic outcome.

Maternal separation alters nerve growth factor and corticosterone levels but not the DNA methylation status of the exon 1(7) glucocorticoid receptor promoter region

Separating rat pups from their mothers during the early stages of life is an animal model commonly used to study the development of psychiatric disorders such as anxiety and depression. The present study investigated how soon after the termination of the maternal separation period behavioural and neuroendocrine abnormalities relevant to above-mentioned illnesses would manifest. Sprague Dawley rat pups were subjected to maternal separation (3 h per day from postnatal day 2 through 14) and their behaviour and HPA axis activity determined 7 d later. We also measured nerve growth factor levels in their hippocampi and assessed the DNA methylation status of the promoter region of exon 1(7) of the glucocorticoid receptor in this brain region. As early as 7 d after the termination of the adverse event, a change in behaviour was observed that was associated with increased plasma corticosterone release and elevated nerve growth factor levels in the hippocampus. No alteration in the methylation status of the exon 1(7) glucocorticoid receptor promoter region was observed. Our data indicate that early life adversity may lead to the rapid development of abnormal behaviours and HPA axis dysregulation though no epigenetic changes to the exon 1(7) glucocorticoid receptor promoter region occurred. We further propose that the observed increased neurotrophin levels reflect compensatory mechanisms that attempt to combat the long-term deleterious effects of maternal separation.

Paternal transmission of complex phenotypes in inbred mice

BACKGROUND

Inbred mice are genetically identical but nonetheless demonstrate substantial variability in complex behaviors such as activity levels in a novel environment. This variability has been associated with levels of parental care experienced early in development. Although maternal effects have been reported in biparental and uniparental strains, there have been no investigations of paternal effects in non-biparental strains in which offspring are reared exclusively by mothers.

METHODS

In the uniparental inbred Balb/cJ mouse strain, we examined the relationship of paternal open-field activity to the activity of both male and female offspring in the open-field. Potential mediators of paternal transmission of behavior were examined, including maternal care, growth parameters, litter characteristics, and time the father was present with the pregnant mother prenatally.

RESULTS

An association of paternal open-field activity with the open-field activity of female but not male offspring was found. Variation in maternal postnatal care was associated with female but not male offspring activity in the open-field but did not mediate paternal effects on offspring behavior. Paternal effects on offspring growth parameters were present, but these effects also did not mediate paternal effects on behavior.

CONCLUSIONS

Paternal transmission of complex traits in genetically identical mice reared only by mothers suggests a nongenetic mechanism of inheritance potentially mediated by epigenetic factors. The exclusion of multiple mediators of paternal effects on offspring suggests the possibility of germline paternal inheritance via sperm of complex phenotypes in inbred mice. Future studies are required to examine these interesting possibilities.