Epigenetic inheritance in the mouse

Transgenerational epigenetic effects

Transgenerational epigenetic effects include all processes that have evolved to achieve the nongenetic determination of phenotype. There has been a long-standing interest in this area from evolutionary biologists, who refer to it as non-Mendelian inheritance. Transgenerational epigenetic effects include both the physiological and behavioral (intellectual) transfer of information across generations. Although in most cases the underlying molecular mechanisms are not understood, modifications of the chromosomes that pass to the next generation through gametes are sometimes involved, which is called transgenerational epigenetic inheritance. There is a trend for those outside the field of molecular biology to assume that most cases of transgenerational epigenetic effects are the result of transgenerational epigenetic inheritance, in part because of a misunderstanding of the terms. Unfortunately, this is likely to be far from the truth.

A scientist's view of the ethics of human embryonic stem cell research

Scientists are rarely immoral and seldom even amoral. The ethical principles that underlie much of their own work are shared with scientists in other countries to a much greater degree than the cultural and religious differences among those countries would lead one to expect. Like me, few will have had any formal training in bioethics; so how might life scientists approach the varied ethical issues that arise in human embryo and stem cell research?

Transgenerational epigenetic inheritance in health and disease

Over the past century, patterns of phenotypic inheritance have been observed that are not easily rationalised by Mendel's rules of inheritance. Now that we have begun to understand more about non-DNA based, or 'epigenetic', control of phenotype at the molecular level, the idea that the transgenerational inheritance of these epigenetic states could explain non-Mendelian patterns of inheritance has become attractive. There is a growing body of evidence that abnormal epigenetic states, termed epimutations, are associated with disease in humans. For example, in several cases of colorectal cancer, epimutations have been identified that silence the human mismatch repair genes, MLH1 and MSH2. But strong evidence that the abnormal epigenetic states are primary events that occur in the absence of genetic change and are inherited across generations is still absent.

The case for transgenerational epigenetic inheritance in humans

Work in the laboratory mouse has identified a group of genes, called metastable epialleles, that are informing us about the mechanisms by which the epigenetic state is established in the embryo. At these alleles, transcriptional activity is dependent on the epigenetic state and this can vary from cell to cell in the one tissue type. The decision to be active or inactive is probabilistic and sensitive to environmental influences. Moreover, in some cases the epigenetic state at these alleles can survive across generations, termed transgenerational epigenetic inheritance. Together these findings raise the spectre of Lamarckism and epigenetics is now being touted as an explanation for some intergenerational effects in human populations. In this review we will discuss the evidence so far.

Dynamic instability of the major urinary protein gene family revealed by genomic and phenotypic comparisons between C57 and 129 strain mice

BACKGROUND

The major urinary proteins (MUPs) of Mus musculus domesticus are deposited in urine in large quantities, where they bind and release pheromones and also provide an individual 'recognition signal' via their phenotypic polymorphism. Whilst important information about MUP functionality has been gained in recent years, the gene cluster is poorly studied in terms of structure, genic polymorphism and evolution.

RESULTS

We combine targeted sequencing, manual genome annotation and phylogenetic analysis to compare the Mup clusters of C57BL/6J and 129 strains of mice. We describe organizational heterogeneity within both clusters: a central array of cassettes containing Mup genes highly similar at the protein level, flanked by regions containing Mup genes displaying significantly elevated divergence. Observed genomic rearrangements in all regions have likely been mediated by endogenous retroviral elements. Mup loci with coding sequences that differ between the strains are identified--including a gene/pseudogene pair--suggesting that these inbred lineages exhibit variation that exists in wild populations. We have characterized the distinct MUP profiles in the urine of both strains by mass spectrometry. The total MUP phenotype data is reconciled with our genomic sequence data, matching all proteins identified in urine to annotated genes.

CONCLUSION

Our observations indicate that the MUP phenotypic polymorphism observed in wild populations results from a combination of Mup gene turnover coupled with currently unidentified mechanisms regulating gene expression patterns. We propose that the structural heterogeneity described within the cluster reflects functional divergence within the Mup gene family.

Heredity--venturing beyond genetics

Our knowledge of heredity has recently undergone major upheaval. Heredity transmits considerably more than just genetic elements. First, the oocyte is full of maternal cytoplasmic components that subsequently are present in each new cell. Second, maternal cells can pass to the progeny, where they remain active into adult life (microchimerism). Here, we examine the notion that the transmission of characters involves at least two processes in addition to that of mendelian heredity, long considered to be the only hereditary mechanism. These processes all involve epigenetic processes, including the transmission of macromolecules, subcellular organelles, and living cells solely from the mother to her offspring, whether female or male, during pregnancy and lactation. We postulate that cytoplasmic heredity and maternal transmission of cells leading to a long-term state of microchimerism in progeny are two good examples of matrilineal, nonmendelian heredity. A mother's important contribution to the development and health of her progeny seems to possess many uncharted depths.

Key developments in endocrine disrupter research and human health

Environmental etiologies involving exposures to chemicals that mimic endogenous hormones are proposed for a number of adverse human health effects, including infertility, abnormal prenatal and childhood development, and reproductive cancers (National Research Council, 1999; World Health Organization, 2002). Endocrine disrupters represent a significant area of environmental research with important implications for human health. This article provides an overview of some of the key developments in this field that may enhance our ability to assess the human health risks posed by exposure to endocrine disrupters. Advances in methodologies of hazard identification (toxicogenomics, transcriptomics, proteomics, metabolomics, bioinformatics) are discussed, as well as epigenetics and emerging biological endpoints.

Global hypomethylation of peripheral leukocyte DNA in male patients with schizophrenia: a potential link between epigenetics and schizophrenia

Genetic and epigenetic factors can potentially alter susceptibility to psychiatric disorders such as schizophrenia. In order to explore the effect of epigenetics on the pathogenesis of schizophrenia, we examined the global methylation level of leukocyte DNA from 210 patients with schizophrenia (124 males and 86 females) and 237 healthy subjects (108 males and 129 females). Methylated deoxycytidine (mC) content in peripheral leukocyte DNA was measured by high performance liquid chromatography (HPLC). We confirmed in the healthy subjects our previous finding that there are sex-dependent differences in mC content (males>females; beta=0.319, p<0.001), in addition to the effect of age (beta=-0.141, p=0.022). We therefore used multiple regression to analyze the data from all subjects by sex, with age as a co-variant. In males, a tendency was observed toward lower mC content in patients than in controls (beta=-0.115, p=0.075), with a significant effect of age (beta=-0.212, p<0.001). This difference was more prominent in younger individuals. In females, no effect of age or disease status on mC content was observed. These results established that there is significant sex-dependent difference in the mC content of human peripheral leukocyte DNA, and raise the possibility that alterations in DNA methylation state are present in patients with schizophrenia.

Neutral Mitochondrial Heteroplasmy Alters Physiological Function in Mice1

Cytoplasmic transfer is an assisted reproductive technique that involves the infusion of ooplasm from a donor oocyte into a recipient oocyte of inferior developmental competence. Although this technique has shown some success for couples with recurrent in vitro fertilization failure, it results in mitochondrial heteroplasmy in the offspring, defined as the presence of two different mitochondrial genomes in the same individual. Because the long-term health consequences of mitochondrial heteroplasmy are unknown, there is a need for appropriate animal models to evaluate any physiological changes of dual mtDNA genotypes. This longitudinal study was designed as a preliminary screen of basic physiological functions for heteroplasmic mice (NZB mtDNA on a BALB/cByJ background). The mice were tested for cardiovascular and metabolic function, hematological parameters, body mass analysis, ovarian reserve, and tissue histologic abnormalities over a period of 15 mo. Heteroplasmic mice developed systemic hypertension that corrected over time and was accompanied by cardiac changes consistent with pulmonary hypertension. In addition, heteroplasmic animals had increased body mass and fat mass compared with controls at all ages. Finally, these animals had abnormalities in electrolytes and hematological parameters. Our findings suggest that there are significant physiological differences between heteroplasmic and control mice. Because ooplasm transfer appears to be consistently associated with mitochondrial heteroplasmy, children conceived through ooplasm transfer should be closely followed to determine if they are at risk for any health problems.

Diet-induced hypermethylation at agouti viable yellow is not inherited transgenerationally through the female

The effects of nonmutagenic environmental exposures can sometimes be transmitted for several generations, suggesting transgenerational inheritance of induced epigenetic variation. Methyl donor supplementation of female mice during pregnancy induces CpG hypermethylation at the agouti viable yellow (A(vy)) allele in A(vy)/a offspring. Epigenetic inheritance occurs at A(vy); when passed through the female germ line, A(vy) epigenotype is not completely "reset." We therefore tested whether diet-induced epigenetic alterations at A(vy) are inherited transgenerationally. Female A(vy)/a mice were weaned onto either control (n=6) or a methyl-supplemented diet (n=5). These F0 dams were mated with a/a males. All F1 and F2 A(vy)/a females were weaned onto the same diet as their mothers, then mated with a/a males. F1, F2, and F3 A(vy)/a offspring were classified for coat color, an indicator of A(vy) methylation. In total, 62 F1, 98 F2, and 209 F3 A(vy)/a mice were studied. As expected, average A(vy)/a coat color was darker in the supplemented group (P<0.01). However, there was no cumulative effect of supplementation across successive generations. These results suggest that, in the female germ line, diet-induced A(vy) hypermethylation occurs in the absence of additional epigenetic modifications that normally confer transgenerational epigenetic inheritance at the locus.

Natural variations in postpartum maternal care in inbred and outbred mice

The role of maternal care in mediating variation in offspring phenotype has been examined in the rat and demonstrates that mother-infant interactions are critical for inducing long-term changes in behavior. Though phenotypic differences between mice strains are often attributed to genetic factors, the influence of early maternal environment has not been extensively explored. To understand maternal influence on phenotype in mice, we must first explore the nature of differences in behavior. In the present study, we examine aspects of maternal care differentiating mice strains and explore the relationship between postpartum behavior and measures obtained by a standard test of maternal responsivity (Retrieval Test). We compared inbred 129Sv (n=25), C57BL/6J (n=23), and outbred Swiss (n=23) lactating female mice. Swiss females had shorter latencies to retrieve and crouch over pups (P<.01), whereas 129Sv females had shorter latencies to nestbuild (P<.05). Conversely, observations of homecage behavior indicate that 129Sv females nestbuild less frequently. 129Sv females also engaged in very low levels of pup licking/grooming (P<.001) and long periods of nursing/contact (P<.05) with pups compared to C57BL/6J and Swiss females. Temporal analysis suggests that the magnitude of these differences varies both within and between days. No significant correlations were found between any aspect of maternal responsivity and postpartum behavior. These results illustrate that through detailed analysis of maternal behavior in mice, variations between strains can be observed. These variations represent strain specific strategies for promoting growth and survival of offspring during infancy that may also mediate "epigenetic" differences in phenotype in adulthood.

Combinatorial epigenetics, “junk DNA”, and the evolution of complex organisms

At certain evolutionary junctures, two or more mutations participating in the build-up of a new complex function may be required to become available simultaneously in the same individuals. How could this happen in higher organisms whose populations are small compared to those of microbes, and in which chances of combined nearly simultaneous highly specific favorable mutations are correspondingly low? The question can in principle be answered for regulatory evolution, one of the basic processes of evolutionary change. A combined resetting of transcription rates in several genes could occur in the same individual. It is proposed that, in eukaryotes, changes in epigenetic trends and epigenetically transforming encounters between alternative chromatin structures could arise frequently enough so as to render probable particular conjunctions of changed transcription rates. Such conjunctions could involve mutational changes with low specificity requirements in gene-associated regions of non-protein-coding sequences. The effects of such mutations, notably when they determine the use of histone variants and covalent modifications of histones, can be among those that migrate along chromatin. Changes in chromatin structure are often cellularly inheritable over at least a limited number of generations of cells, and of individuals when the germ line is involved. SINEs and LINEs, which have been considered "junk DNA", are among the repeat sequences that would appear liable to have teleregulatory effects on the function of a nearby promoter, through changes in their numbers and distribution. There may also be present preexisting unstably inheritable epigenetic trends leading to cellular variegation, trends endemic in a cell population based on DNA sequences previously established in the neighborhood. Either way, epigenetically conditioned teleregulatory trends may display only limited penetrance. The imposition at a distance of new chromatin structures with regulatory impact can occur in cis as well as in trans, and is examined as intrachromosomally spreading teleregulation and interchromosomal "gene kissing". The chances for two or more particular epigenetically determined regulatory trends to occur together in a cell are increased thanks to the proposed low specificity requirements for most of the pertinent sequence changes in intergenic and intronic DNA or in the distribution of middle repetitive sequences that have teleregulatory impact. Inheritable epigenetic changes ("epimutations") with effects at a distance would then perdure over the number of generations required for "assimilation" of the several regulatory novelties through the occurrence and selection, gene by gene, of specific classical mutations. These mutations would have effects similar to the epigenetic effects, yet would provide stability and penetrance. The described epigenetic/genetic partnership may well at times have opened the way toward certain complex new functions. Thus, the presence of "junk DNA", through co-determining the (higher or lower) order and the variants of chromatin structure with regulatory effects at a distance, might make an important contribution to the evolution of complex organisms.

Inheritance of a cancer-associated MLH1 germ-line epimutation

Persons who have hypermethylation of one allele of MLH1 in somatic cells throughout the body (a germ-line epimutation) have a predisposition for the development of cancer in a pattern typical of hereditary nonpolyposis colorectal cancer. By studying the families of two such persons, we found evidence that the epimutation was transmitted from a mother to her son but was erased in his spermatozoa. The affected maternal allele was inherited by three other siblings from these two families, but in those offspring the allele had reverted to the normal active state. These findings demonstrate a novel pattern of inheritance of cancer susceptibility and are consistent with transgenerational epigenetic inheritance.

Cloning cattle: the methods in the madness

Somatic cell nuclear transfer (SCNT) is much more widely and efficiently practiced in cattle than in any other species, making this arguably the most important mammal cloned to date. While the initial objective behind cattle cloning was commercially driven--in particular to multiply genetically superior animals with desired phenotypic traits and to produce genetically modified animals-researchers have now started to use bovine SCNT as a tool to address diverse questions in developmental and cell biology. In this paper, we review current cattle cloning methodologies and their potential technical or biological pitfalls at any step of the procedure. In doing so, we focus on one methodological parameter, namely donor cell selection. We emphasize the impact of epigenetic and genetic differences between embryonic, germ, and somatic donor cell types on cloning efficiency. Lastly, we discuss adult phenotypes and fitness of cloned cattle and their offspring and illustrate some of the more imminent commercial cattle cloning applications.

Epigenetic variation and inheritance in mammals

What determines phenotype is one of the most fundamental questions in biology. Historically, the search for answers had focused on genetic or environmental variants, but recent studies in epigenetics have revealed a third mechanism that can influence phenotypic outcomes, even in the absence of genetic or environmental heterogeneity. Even more surprisingly, some epigenetic variants, or epialleles, can be inherited by the offspring, indicating the existence of a mechanism for biological heredity that is not based on DNA sequence. Recent work from mouse models, human monozygotic twin studies, and large-scale epigenetic profiling suggests that epigenetically determined phenotypes and epigenetic inheritance are more common than previously appreciated.

Haloperidol treatment induces tissue- and sex-specific changes in DNA methylation: a control study using rats

Background

We previously found that there is a subtle difference in the global methylation state of blood leukocyte DNA between male subjects with and without schizophrenia. The aim of the current study was to determine whether this difference was a primary effect of the disease state, or a secondary effect of antipsychotics administered to these patients.

Methods

We examined the methyl cytosine (mC) content of DNA from the leukocytes, brain, and liver of rats using high performance liquid chromatography. A total of 40 male and female rats received for 21 days daily injection of haloperidol or vehicle solution alone.

Results

In control rats injected with buffer only, there was a sex-dependent difference in mC content in leukocyte DNA (male > female; P = 0.028, n = 10), similar to our previous observations in human peripheral leukocytes. No difference in mC content between the sexes was observed in the brain or liver in buffer-treated animals. Haloperidol treatment slightly decreased the mC content of leukocytes in male rats, but unexpectedly, increased the mC content of leukocytes in females. We observed a trend toward a higher level of mC in the liver in both sexes following haloperidol treatment, compared to buffer-treated animals. In contrast, haloperidol treatment resulted in a decrease in mC content in the brain in females, and this difference was statistically significant (P = 0.026).

Conclusion

These results indicate that haloperidol can affect DNA methylation states in the brain, as well as in certain other tissues, and raise the possibility that antipsychotic drugs play a role in the observed disparity in mC content in male subjects with and without schizophrenia.

Rescue of the mouse DDK syndrome by parent-of-origin-dependent modifiers

When females of the DDK inbred mouse strain are mated to males of other strains, 90-100% of the resulting embryos die during early embryonic development. This DDK syndrome lethality results from incompatibility between an ooplasmic DDK factor and a non-DDK paternal gene, which map to closely linked loci on chromosome 11. It has been proposed that the expression of the gene that encodes the ooplasmic factor is subject to allelic exclusion in oocytes. Previous studies have demonstrated the existence of recessive modifiers that increase lethality in the C57BL/6 and BALB/c strains. These modifiers are thought to skew the choice of allele undergoing allelic exclusion in the oocytes of heterozygous females. In the present study, we demonstrate the presence of modifiers in three Mus musculus domesticus wild-derived strains, PERA, PERC, and RBA. These modifiers completely rescued DDK syndrome lethality. We mapped the major locus that is responsible for rescue in PERA and PERC crosses to proximal chromosome 13 and named this locus Rmod1 (Rescue Modifier of the DDK Syndrome 1). Our experiments demonstrate that PERA or PERC alleles at Rmod1 rescue lethality independently of allelic exclusion. In addition, rescue of the lethal phenotype depends on the parental origin of the Rmod1 alleles; transmission through the dam leads to rescue, while transmission through the sire has no effect.

Inherited epigenetic variation--revisiting soft inheritance

Phenotypic variation is traditionally parsed into components that are directed by genetic and environmental variation. The line between these two components is blurred by inherited epigenetic variation, which is potentially sensitive to environmental inputs. Chromatin and DNA methylation-based mechanisms mediate a semi-independent epigenetic inheritance system at the interface between genetic control and the environment. Should the existence of inherited epigenetic variation alter our thinking about evolutionary change?

Epimutations, inheritance and causes of aberrant DNA methylation in cancer

Epigenetic aberrations such as global hypomethylation and gene-specific hypermethylation are key events that underlie tumour development. Such scenarios are brought about by the loss of control of methylation patterns which typically are reversed in neoplasia in comparison to normal states. Despite the methylation process being termed epigenetic, suggesting that it is not a heritable condition, there is strong evidence in mouse models suggesting that epimutations within the germline may provide a mechanism through which methylation variations can be transmissible to offspring. The first half of the review will focus on the nature of methylation-induced gene silencing and transmission of this information through the germline. The latter half will focus on the cause of aberrant DNA methylation.

Epigenetics and phenotypic variation in mammals

What causes phenotypic variation? By now it is clear that phenotype is a result of the interaction between genotype and environment, in addition to variation not readily attributable to either. Epigenetic phenomena associated with phenotypic variation at the biochemical, cellular, tissue, and organism level are now well recognized and are likely to contribute to the "intangible variation" alluded to. While it is clear that epigenetic modifications are mitotically heritable, the fidelity of this process is not well understood. Inheritance through more than one generation of meioses is even less well studied. So it remains unclear to what extent epigenetic changes contribute to phenotypic variation in natural populations. How might such evidence be obtained? What are the features of phenotypes that might suggest an epigenetic component? How much of the epigenetic component is truly independent of genetic changes? The answers to such questions must come from studies designed specifically to detect subtle, stochastically determined phenotypic variation in suitable animal models.

Tissue specificity of methylation and expression of human genes coding for neuropeptides and their receptors, and of a human endogenous retrovirus K family

The purpose of the present study was to understand the tissue specificity of DNA methylation and the relationship between methylation and expression of genes with essential roles in neurodevelopment and brain function. We chose dopamine receptor genes (DRD1 and DRD2), NCAM, and COMT as examples of genes with CpG islands around the promoter region, and serotonin receptor genes (HTR2A and HTR3A), HCRT, and DRD3 as genes without CpG islands. Methylation states were investigated in fetal brain, fetal liver, placenta, and in adult peripheral leukocytes from three individuals by Southern blot and bisulfite-modified DNA sequencing. A repetitive sequence, human endogenous retrovirus (HERV)-K was also examined. All genes examined were almost completely unmethylated in brains. The genes with CpG islands were unmethylated regardless of their expression state. In contrast, genes without CpG islands showed various methylation patterns, which did not necessarily reflect the transcriptional activity of the genes. Most HERV-K loci were methylated, but some loci showed relatively low methylation in the placenta and liver. Interestingly, we found inter-individual differences in methylation levels in HTR2A and HCRT in the placenta and in some loci of HERV-K in the placenta and liver. The sample with the lowest methylation levels in the two unique genes showed higher methylation of HERV-K loci than the other samples. These results provide detailed information about the methylation states of the genes analyzed and evidence for inter-individual variations in methylation in both unique and repetitive sequences.

Meiotically stable natural epialleles of Sadhu, a novel Arabidopsis retroposon

Epigenetic variation is a potential source of genomic and phenotypic variation among different individuals in a population, and among different varieties within a species. We used a two-tiered approach to identify naturally occurring epigenetic alleles in the flowering plant Arabidopsis: a primary screen for transcript level polymorphisms among three strains (Col, Cvi, Ler), followed by a secondary screen for epigenetic alleles. Here, we describe the identification of stable, meiotically transmissible epigenetic alleles that correspond to one member of a previously uncharacterized non-LTR retroposon family, which we have designated Sadhu. The pericentromeric At2g10410 element is highly expressed in strain Col, but silenced in Ler and 18 other strains surveyed. Transcription of this locus is inversely correlated with cytosine methylation and both the expression and DNA methylation states map in a Mendelian manner to stable cis-acting variation. The silent Ler allele can be converted by the epigenetic modifier mutation ddm1 to a meiotically stable expressing allele with an identical primary nucleotide sequence, demonstrating that the variation responsible for transcript level polymorphism among Arabidopsis strains is epigenetic. We extended our characterization of the Sadhu family members and show that different elements are subject to both genetic and epigenetic variation in natural populations. These findings support the view that an important component of natural variation in retroelements is epigenetic.

The offspring of irradiated parents, are they stable?

Mutation induction in directly exposed cells is currently regarded as the main component of the genetic risk of ionising radiation for humans. However, recent studies showing that exposure to ionising radiation results in elevated mutation rates detectable in the non-irradiated progeny of exposed cells challenge the existing paradigm in radiation biology. This review describes some recent data on radiation-induced genomic instability in vitro and mainly focuses on the in vivo phenomenon of transgenerational instability, where elevated mutation rates are detected in the non-exposed offspring of irradiated parents. The possible mechanisms and implications of transgenerational instability are also discussed.

Prolonged effect of endorphin treatment during pregnancy in the rat on the histamine content of immune cells of F1 and F2 offspring generations

Female rats were treated with beta-endorphin on the 19th day of pregnancy and the histamine content of immune cells (blood lymphocytes; peritoneal lymphocytes, monocyte-macrophage-granulocyte group, mast cells; thymic lymphocytes) of the 7-week-old progenies (F1 generation) was studied using a flow-cytometric immunocytochemical technique. In an other group, female F1 progenies of endorphin-treated mothers were mated with control males and the F2 generation was monitored for histamine content similar to the F1. In the F1 generation each cell type, except peritoneal and blood lymphocytes, contained significantly more histamine than the control cells. In the F2 generation only mast cells contained significantly more histamine relative to the appropriate control. This means that the effect of endorphin (hormonal) imprinting is transmitted transgenerationally, but with decreasing intensity however. Mast cells retained the effect of imprinting for longer than the other cells. The results are compared with the levels of serotonin in similarly treated animals, studied in earlier experiments. As the endorphin level can be elevated during pregnancy (by pain, traumatization, or other stress conditions) this can the set biogenic amine content of adult immune cells.

Oxygen, epigenetics and stem cell fate

Coordinated interactions between the embryo/fetus and its environment are critical for proper development. In addition to acting as metabolic substrates for cellular homeostasis, basic physiological factors, such as oxygen tension, have a profound influence on developmental outcomes. Since the mammalian embryo resides in a physiologically hypoxic environment during gestation, understanding its responses to oxygen deprivation on a cellular level is critical. In this review, we analyze interactions between the hypoxia-inducible factor family of transcriptional regulators and epigenetic mechanisms governing chromatin structure. The ability of hypoxia-inducible factors to interact physically with histone deacetylase (HDAC) enzymes and modulate nuclear HDAC activities places them in the pivotal position of integrating physiological and epigenetic effectors. Multiple embryonic and extra-embryonic stem cell populations in mice and humans rely on this interaction – an important determinant of stem cell fate. Dissection of the pathways involved will provide novel insights into the metabolic as well as molecular determinants of the stem cell niches that allow self-renewal of progenitors in an undifferentiated state.

Seminiferous cord formation and germ-cell programming: epigenetic transgenerational actions of endocrine disruptors

The molecular and cellular control of embryonic testis development was investigated through an analysis of the embryonic testis transcriptome to identify potential regulatory factors for male sex determination and testis morphogenesis. One critical factor identified is neurotropin 3 (NT3). At the onset of male sex determination, Sertoli cells initiate differentiation and express NT3 to act as a chemotactic factor for mesonephros cells to migrate and associate with Sertoli-germ cell aggregates to promote cord formation. Promoter analysis suggests that NT3 may be an initial downstream gene to SRY and helps promote testis morphogenesis. Endocrine disruptors were used to potentially interfere with embryonic testis development and further investigate this biological process. The estrogenic pesticide methoxychlor and antiandrogenic fungicide vinclozolin were used. Previous studies have shown that methoxychlor and vinclozolin both interfere with embryonic testis cord formation and cause increased spermatogenic cell apoptosis in the adult testis. Interestingly, transient in vivo exposure to endocrine disruptors at the time of male sex determination caused a transgenerational phenotype (F1-F4) of spermatogenic cell apoptosis and subfertility. This apparent epigenetic mechanism involves altered DNA methylation and permanent re-programming of the male germ-line. A series of genes with altered DNA methylation and imprinting are being identified. Observations reviewed demonstrate that a transient embryonic in utero exposure to an endocrine disruptor influences the embryonic testis transcriptome and through epigenetic effects (e.g., DNA methylation) results in abnormal germ-cell differentiation that subsequently influences adult spermatogenic capacity and male fertility, and that this phenotype is transgenerational through the germ-line. The novel observations of transgenerational epigenetic endocrine disruptor actions on male reproduction critically impact the potential hazards of these compounds as environmental toxins. The literature reviewed provides insight into the molecular and cellular control of embryonic testis development, male sex determination, and the programming of the male germ-line.

Environmental signaling and evolutionary change: can exposure of pregnant mammals to environmental estrogens lead to epigenetically induced evolutionary changes in embryos?

DNA methylation is one of the epigenetic and hereditary mechanisms regulating genetic expression in mammalian cells. In this review, we propose how certain natural agents, through their dietary consumption, could induce changes in physiological aspects in mammalian mothers, leading to alterations in DNA methylation patterns of the developing fetus and to the emergence of new phenotypes and evolutionary change. Nevertheless, we hypothesize that this process would require (i) certain key periods in the ontogeny of the organism where the environmental stimuli could produce effects, (ii) particular environmental agents as such stimuli, and (iii) that a genomic persistent change be consequently produced in a population. Depending on the persistence of the environmental stimuli and on whether the affected genes are imprinted genes, induced changes in DNA methylation patterns could become persistent. Moreover, some fragments could be more frequently methylated than others over several generations, leading to biased base change and evolutionary consequences.

Gametes and embryo epigenetic reprogramming affect developmental outcome: implication for assisted reproductive technologies

There is concern about the health of children who are conceived with the use assisted reproductive technologies (ART). In addition to reports of low birth weight and chromosomal anomalies, there is evidence that ART may be associated with increased epigenetic disorders in the infants who are conceived using these procedures. Epigenetic reprogramming is critical during gametogenesis and at preimplantation stage and involves DNA methylation, imprinting, RNA silencing, covalent modifications of histones, and remodeling by other chromatin-associated complexes. Epigenetic regulation is involved in early embryo development, fetal growth, and birth weight. Disturbances in epigenetic reprogramming may lead to developmental problems and early mortality. Recent reports suggest the increased incidence of imprinting disorders such as Beckwith-Wiedemann syndrome, Angelman syndrome, and retinoblastoma in children who are conceived with the use of ART. These may result from an accumulation of epigenetic alterations during embryo culture and/or by altered embryonic developmental timing. Further research is urgently needed to determine whether a causal relationship between ART and epigenetic disorders exists. Until then, cautious review of both short-term and long-term ART outcomes at a national level is recommended.

Gamma irradiation of Type B spermatogonia leads to heritable genomic instability in four generations of mice

Mice conceived 6 weeks after paternal exposure to ionizing radiation were fathered by sperm that were Type B spermatogonia at the time of irradiation. Previous studies of these offspring showed that this paternal F0 germ cell irradiation led to decreased embryonic cell proliferation rates, altered enzyme activities, protein levels and whole-body weights. In the present study, we examined four generations of CD1 mice following paternal F0 irradiation of the Type B spermatogonia (1.0 Gy, (137)Cs gamma rays) to determine the stability of the heritable effects. Offspring were evaluated for changes in protein kinase C and mitogen-activated protein kinase enzyme activities and Trp53 and p21(waf1) protein levels. Two or more endpoints were significantly altered in all four generations of offspring from the irradiated F0 sire (P <or= 0.05). To test the hypothesis that these heritable biochemical effects are random stochastic responses rather than some predictable uniform response, each endpoint was also evaluated in terms of a variability index (VI). Results of VI analyses show that the observed heritable phenotype is unpredictable in magnitude and direction of change for an endpoint between generations and within generations. These results indicate that irradiated spermatogonia develop a capacity to transmit a type of heritable genomic instability to four generations of offspring.

Hypermethylation of the reelin (RELN) promoter in the brain of schizophrenic patients: a preliminary report

DNA methylation changes could provide a mechanism for DNA plasticity and dynamism for short-term adaptation, enabling a type of cell memory to register cellular history under different environmental conditions. Some environmental insults may also result in pathological methylation with corresponding alteration of gene expression patterns. Evidence from several studies has suggested that in schizophrenia and bipolar disorder, mRNA of the reelin gene (RELN), which encodes a protein necessary for neuronal migration, axonal branching, synaptogenesis, and cell signaling, is severely reduced in post-mortem brains. Therefore, we investigated the methylation status of the RELN promoter region in schizophrenic patients and normal controls as a potential mechanism for down regulation of its expression. Ten post-mortem frontal lobe brain samples from male schizophrenic patients and normal controls were obtained from the Harvard Brain Tissue Resources Center. DNA was extracted using a standard phenol-chloroform DNA extraction protocol. To evaluate differences between patients and controls, we applied methylation specific PCR (MSP) using primers localized to CpG islands flanking a potential cyclic AMP response element (CRE) and a stimulating protein-1 (SP1) binding site located in the promoter region. For each sample, DNA extraction, bisulfite treatment, and MSP were independently repeated at least four times to accurately determine the methylation status of the target region. Forty-three PCR trials were performed on the test and control samples. MSP analysis of the RELN promoter revealed an unmethylated signal in all reactions (43 of 43) using DNA from the frontal brain tissue, derived from either the schizophrenic patients or normal controls indicating that this region of the RELN promoter is predominantly unmethylated. However, we observed a distinct methylated signal in 73% of the trials (16 of 22) in schizophrenic patients compared with 24% (5 of 21) of controls. Thus, the hypermethylation of the CpG islands flanking a CRE and SP1 binding site observed at a significantly higher level (t = -5.07, P = 0.001) may provide a mechanism for the decreased RELN expression, frequently observed in post-mortem brains of schizophrenic patients. We also found an inverse relationship between the level of DNA methylation using MSP analysis and the expression of the RELN gene using semi-quantitative RT-PCR. Despite the small sample size, these studies indicate that promoter hypermethylation of the RELN gene could be a significant contributor in effecting epigenetic alterations and provides a molecular basis for the RELN gene hypoactivity in schizophrenia. Further studies with a larger sample set would be required to validate these preliminary observations.

Maternal Factors Controlling Blastomere Fragmentation in Early Mouse Embryos1

Interactions between sperm and egg are required to maintain embryo viability and cellular integrity. Differential transcriptional activities and epigenetic differences that include genomic imprinting provide mechanisms by which complementary parental genome functions support early embryogenesis. We previously showed that cytofragmentation can be influenced by the specific combination of maternal and paternal genotypes. Using maternal pronuclear transfer in mouse embryos, we examined the cellular basis for the maternal genotype effect. We found that the maternal genotype effect is predominantly controlled by the maternal pronucleus, with a lesser role played by the ooplasm. This effect of the maternal pronucleus is sensitive to alpha-amanitin treatment. The effect of the maternal component of the embryonic genome on cytofragmentation constitutes the earliest known effect of the embryonic genome on mammalian embryo phenotype. The results also indicate that clinical procedures seeking to define or manipulate oocyte quality in humans should take into account early effects of the embryonic genome, particularly the maternal genome.

Recapitulation of the Ovum Mutant (Om) Phenotype and Loss of Om Locus Polarity in Cloned Mouse Embryos1

The ovum mutant (Om) locus in mice affects early interactions between sperm and egg that in turn affect viability of embryos beyond the morula stage. Crosses of DDK females to males of many other inbred strains are 95% lethal around the morula stage, whereas reciprocal crosses are fully viable. Available data indicate that the early lethality is the result of an interaction between a factor in the ooplasm and the paternal genome. In this study, we examined whether this lethal interaction would likewise occur in cloned embryos produced by somatic cell nuclear transfer. We find that the Om effect is recapitulated but that the parental origin effect at the Om locus is no longer evident in cloned embryos.

Effect of genetic background and activating stimulus on the timing of meiotic cell cycle progression in parthenogenetically activated mouse oocytes

With the aim of investigating the effects of oocyte genotype and activating stimulus on the timing of nuclear events after activation, oocytes collected from hybrid B6D2F1, inbred C57BL/6 and outbred CF-1 and immunodeficient nude (NU/+) females were activated using ethanol or strontium and fixed at various time-points. Meiotic status, spindle rotation and second polar body (PB2) extrusion were monitored by fluorescence microscopy using DNA-, microtubule- and microfilament-selective probes. Although activation efficiency was similar in all groups of oocytes, a significant percentage of CF-1 and NU/+ oocytes treated with ethanol and of C57BL/6 oocytes treated either with ethanol or strontium failed to complete activation and became arrested at a new metaphase stage (MIII) after PB2 extrusion. C57BL/6 oocytes also showed slower release from MII arrest but faster progression to telophase (TII) after ethanol exposure, and they exhibited the most rapid exit from TII under both activation treatments. Strontium caused delayed meiotic resumption, spindle rotation and PB2 extrusion, but rapid TII exit, in B6D2F1, CF-1 and NU/+ oocytes when compared with ethanol. Compared with all other strains, NU/+ oocytes were significantly slower in completing spindle rotation and PB2 extrusion, irrespective of the activating stimulus, and a significant decrease in activation rates and pace of meiotic progression was observed after strontium exposure. Thus, our findings demonstrated that the kinetics of meiosis resumption and completion, spindle rotation and PB2 extrusion following parthenogenetic activation depends on both genotype-specific factors and on the activation treatment applied.

Epigenetic germline inheritance

Our increased knowledge of epigenetic reprogramming supports the idea that epigenetic marks are not always completely cleared between generations. Incomplete erasure at genes associated with a measurable phenotype can result in unusual patterns of inheritance from one generation to the next. It is also becoming clear that the establishment of epigenetic marks during development can be influenced by environmental factors. In combination, these two processes could provide a mechanism for a rapid form of adaptive evolution.

Differential DNA methylation reprogramming of various repetitive sequences in mouse preimplantation embryos

Genome-wide changes of DNA methylation by active and passive demethylation processes are typical features during preimplantation development. Here we provide an insight that epigenetic reprogramming of DNA methylation is regulated in a region-specific manner, not a genome-wide fashion. To address this hypothesis, methylation states of three repetitive genomic regions were monitored at various developmental stages in the mouse embryos. Active demethylation was not observed in the IAP sequences whereas methylation reprogramming of the satellite sequences was regulated only by the active mechanism. Etn elements were actively demethylated after fertilization, passively demethylated by the 8-cell stage, and de novo methylated at the morular and blastocyst stages, showing dynamic epigenetic changes. Thus, our findings suggest that the specific genomic regions or sequences may spatially/temporally have their unique characteristics in the reprogramming of the DNA methylation during preimplantation development.

Age related changes in 5-methylcytosine content in human peripheral leukocytes and placentas: an HPLC-based study

The goal of the present study was to investigate inter-individual and age-dependent variation of global DNA methylation in human tissues. In this work, we examined 5-methyldeoxycytidine ((met)C) content by HPLC in human peripheral blood leukocytes obtained from 76 healthy individuals of ages varying from 4 to 94 years (yr), and 39 human placentas from various gestational stages. The HPLC analysis revealed a significant variation of (met)C across individuals and is consistent with the previous findings of age-dependent decrease of global methylation levels in human tissues. The age-dependent decrease of (met)C was relatively small, but statistically highly significant (p= 0.0002) in the aged group (65.9 +/- 8.9 [mean age +/- SD] yr; n = 22) in comparison to the young adult group (19.3 +/- 1.4 yr; n = 21). Males showed a subtle but statistically significant higher mean (met)C content than females. In contrast to the peripheral blood samples, DNA extracted from placentas exhibited gestational stage-dependent increase of methylation levels that appeared to inversely correlate with the expression levels of human endogenous retroviruses. These data may be helpful in further studies of DNA methylation, such as inheritance of epigenetic patterns, environment-induced changes, and involvement of epigenetic changes in disease.

Intergenerational consequences of fetal programming by in utero exposure to glucocorticoids in rats

Epidemiological studies linking low birth weight and subsequent cardiometabolic disease have given rise to the hypothesis that events in fetal life permanently program subsequent cardiovascular risk. The effects of fetal programming may not be limited to the first-generation offspring. We have explored intergenerational effects in the dexamethasone-programmed rat, a model in which fetal exposure to excess glucocorticoid results in low birth weight with subsequent adult hyperinsulinemia and hyperglycemia underpinned by increased activity of the key hepatic gluconeogenic enzyme, phosphoenolpyruvate carboxykinase (PEPCK). We found that the male offspring of female rats that had been exposed prenatally to dexamethasone, but were not manipulated in their own pregnancy, also had reduced birth weight (5.66 +/- 0.06 vs. 6.12 +/- 0.06 g, P < 0.001), glucose intolerance, and elevated hepatic PEPCK activity (5.7 +/- 0.6 vs. 3.3 +/- 0.2 nmol.min(-1).mg protein(-1), P < 0.001). These effects resolved in a third generation. Similar intergenerational programming was observed in offspring of male rats exposed prenatally to dexamethasone mated with control females. The persistence of such programming effects through several generations, transmitted by either maternal or paternal lines, indicates the potential importance of epigenetic factors in the intergenerational inheritance of the "programming phenotype" and provides a basis for the inherited association between low birth weight and cardiovascular risk factors.

Methylomics in psychiatry: Modulation of gene-environment interactions may be through DNA methylation

Fine-tuning of neuronal connections during development is regulated through environmental interactions. Some fine-tuning occurs through changes in gene expression and/or epigenetic gene-specific DNA methylation states. DNA methylation occurs by transfer of a methyl group from S-adenosyl methionine to cytosine residues in the dinucleotide sequence CpG. Although CpG sequences spread throughout the genome are usually heavily methylated, those occurring in CpG islands in the promoter regions of genes are less methylated. In most cases, the extent of DNA methylation correlates with the extent of gene inactivation. Other known epigenetic mechanisms include histone deacetylation and chromatin remodeling, RNA inhibition, RNA modification, and DNA rearrangement. Exposure memory expressed as epigenetic DNA modifications allows genomic plasticity and short-term adaptation of each generation to their environment. Environmental factors that affect DNA methylation include diet, proteins, drugs, and hormones. Induced methylation changes may produce altered gene response upon subsequent hormonal stimulation. The gene-specific DNA methylation state may be preserved upon transmission through mitosis and meiosis. An increasing amount of data implicates a role for DNA methylation in multi-factorial psychiatric disorders. For example, L-methionine treatment can exacerbate psychosis; while valproate, a drug producing hypomethylated DNA, reduces such symptoms. Hypermethylation of the promoter region of the RELN gene correlates with reduced gene expression. This gene's protein Reelin, which is necessary for neuronal migration and synaptogenesis, is reduced in schizophrenia and bipolar disorder, suggesting hypermethylation of the promoter region in these disorders. Some evidence implicates methylation of the promoter regions of the DRD2 and HTR2A genes in schizophrenia and mood disorders as well. DNA methylation usually increases with age, although hypomethylation of the promoter region of the amyloid A4 precursor gene during aging may play a role in Alzheimer's disease. More studies are needed to define the role of methylomics and other epigenetic phenomena in the nervous system.

Germline epimutation of MLH1 in individuals with multiple cancers

Epigenetic silencing can mimic genetic mutation by abolishing expression of a gene. We hypothesized that an epimutation could occur in any gene as a germline event that predisposes to disease and looked for examples in tumor suppressor genes in individuals with cancer. Here we report two individuals with soma-wide, allele-specific and mosaic hypermethylation of the DNA mismatch repair gene MLH1. Both individuals lack evidence of genetic mutation in any mismatch repair gene but have had multiple primary tumors that show mismatch repair deficiency, and both meet clinical criteria for hereditary nonpolyposis colorectal cancer. The epimutation was also present in spermatozoa of one of the individuals, indicating a germline defect and the potential for transmission to offspring. Germline epimutation provides a mechanism for phenocopying of genetic disease. The mosaicism and nonmendelian inheritance that are characteristic of epigenetic states could produce patterns of disease risk that resemble those of polygenic or complex traits.

Low Feed Efficient Broilers Within a Single Genetic Line Exhibit Higher Oxidative Stress and Protein Expression in Breast Muscle with Lower Mitochondrial Complex Activity

The objectives of this study were to determine the effects of low or high feed efficiency (FE) on a) protein oxidation, b) the activities of various respiratory chain complexes, and c) expression of various mitochondrial proteins in male broilers within a single genetic line. Tissue homogenate or mitochondria were isolated from breast muscle of broilers with high (0.80 +/- 0.01) and low FE (0.62 +/- 0.02). The complex activities were measured spectrophotometrically, and the levels of oxidized protein (carbonyl) and immunoreactive mitochondrial proteins were analyzed using Western blots. Protein carbonyl levels were higher in low FE compared with high FE broilers breast muscle, which indicated enhanced protein oxidation in low FE mitochondria. Activities of all respiratory chain complexes (I, II, III, IV) were higher in high FE compared with low FE broilers for breast mitochondria. Whereas the expression of immunoreactive proteins was higher in low FE muscle mitochondria for 5 mitochondrial proteins [core I, cyt c1, cyt b (complex III), COX II (cytochrome c oxidase subunit II, complex IV), and adenine nucleotide translocator (ANT1)], there were no differences between groups in the expression of 9 other respiratory chain protein subunits associated with complexex I, II, III, IV, and V. SDS-PAGE revealed a protein band of 47 kDa that was expressed at a higher level in low FE compared with high FE mitochondria. The differential expression of certain mitochondrial proteins and the 47-kDa band might be a compensatory response either to the lower complex activities or increased protein oxidation observed in low FE birds.

Genetic variation in oocyte phenotype revealed through parthenogenesis and cloning: correlation with differences in pronuclear epigenetic modification

Previous studies revealed that oocytes of different genetic strains (e.g, C57BL/6 and DBA/2) modify maternal and paternal pronuclei differently, affecting early preimplantation development. To determine whether these strain-dependent effects would also apply to oocyte modifications of somatic cell nuclei introduced during cloning procedures, we compared the efficiency of development of parthenogenetic and cloned embryos made with DBA/2, C57BL/6, and (B6D2)F1 oocytes. Our results reveal significant differences in the ability of oocytes of different genetic backgrounds to support parthenogenetic development in different culture media. Additionally, our results reveal oocyte strain-dependent differences in the ability to support cloned embryo development beyond what can be accounted for on the basis of differences in parthenogenesis. Thus, the previously documented differences in oocyte-directed parental genome modification are accompanied in the same strains by differences in the ability of oocytes to modify somatic cell nuclei and support clonal development, raising the possibility that these oocyte functions may be mediated by related mechanisms. These results provide a genetic basis for further studies seeking to identify specific genes that determine oocyte phenotype, as well as genes that determine the success of nuclear reprogramming and clonal development.

Radiation-induced transgenerational instability

To date, the analysis of mutation induction has provided an irrefutable evidence for an elevated germline mutation rate in the parents directly exposed to ionizing radiation and a number of chemical mutagens. However, the results of numerous publications suggest that radiation may also have an indirect effect on genome stability, which is transmitted through the germ line of irradiated parents to their offspring. This review describes the phenomenon of transgenerational instability and focuses on the data showing increased cancer incidence and elevated mutation rates in the germ line and somatic tissues of the offspring of irradiated parents. The possible mechanisms of transgenerational instability are also discussed.

Human endogenous retroviruses with transcriptional potential in the brain

Genetic studies of neuropsychiatric disorders have often produced conflicting results, which might partly result from the involvement of epigenetic modifications. We intended to explore the possible implication of DNA methylation and human endogenous retroviruses (HERVs) in neuropsychiatric disorders. In the present study, we identified two HERV loci that are expected to retain the transcriptional activity in the brain. One was located on chromosome 1q21-q22 and the other on 22q12. Interestingly, these regions were overlapped with or included in those of schizophrenia-susceptible loci, SCZD9 and SCZD4, respectively. Particularly, the HERV on 22q12 was located in the opposite direction 4 kb downstream of the Synapsin III gene. These HERV loci could afford clear targets for methylation and expression analyses in postmortem brains of patients with psychiatric disorders such as schizophrenia. In addition, we confirmed our previous finding that only a few of particular HERV-K loci were activated among a number of highly homologous loci in teratocarcinoma cell lines. These activated loci included ones common to all teratocarcinoma cell lines analyzed and depending on their male or female origin.