Differential X reactivation in human placental cells: implications for reversal of X inactivation

Paternal DDT exposure induces sex-specific programming of fetal growth, placenta development and offspring's health phenotypes in a mouse model

Mounting evidence suggests that environmentally induced epigenetic inheritance occurs in mammals and that traits in the progeny can be shaped by parental environmental experiences. Epidemiological studies link parental exposure to environmental toxicants, such as the pesticide DDT, to health phenotypes in the progeny, including low birth and increased risk of chronic diseases later in life. Here, we show that the progeny of male mice exposed to DDT in the pre-conception period are born smaller and exhibit sexual dimorphism in metabolic function, with male, but not female, offspring developing severe glucose intolerance compared to controls. These phenotypes in DDT offspring were linked to reduced fetal growth and placenta size as well as placenta-specific reduction of glycogen levels and the nutrient sensor and epigenetic regulator OGT, with more pronounced phenotypes observed in male placentas. However, placenta-specific genetic reduction of OGT only partially replicates the metabolic phenotype observed in offspring of DDT-exposed males. Our findings reveal a role for paternal pre-conception environmental experiences in shaping placenta development and in fetal growth restriction. While many questions remain, our data raise the tantalizing possibility that placenta programming could be a mediator of environmentally induced intergenerational epigenetic inheritance of phenotypes and needs to be further evaluated.

Sex-Specific Transcriptomic Changes in the Villous Tissue of Placentas of Pregnant Women Using a Selective Serotonin Reuptake Inhibitor

About 5% of pregnant women are treated with selective serotonin reuptake inhibitor (SSRI) antidepressants to treat their depression. SSRIs influence serotonin levels, a key factor in neural embryonic development, and their use during pregnancy has been associated with adverse effects on the developing embryo. However, the role of the placenta in transmitting these negative effects is not well understood. In this study, we aim to elucidate how disturbances in the maternal serotonergic system affect the villous tissue of the placenta by assessing whole transcriptomes in the placentas of women with healthy pregnancies and women with depression and treated with the SSRI fluoxetine during pregnancy. Twelve placentas of the Biology, Affect, Stress, Imaging and Cognition in Pregnancy and the Puerperium (BASIC) project were selected for RNA sequencing to examine differentially expressed genes: six male infants and six female infants, equally distributed over women treated with SSRI and without SSRI treatment. Our results show that more genes in the placenta of male infants show changed expression associated with fluoxetine treatment than in placentas of female infants, stressing the importance of sex-specific analyses. In addition, we identified genes related to extracellular matrix organization to be significantly enriched in placentas of male infants born to women treated with fluoxetine. It remains to be established whether the differentially expressed genes that we found to be associated with SSRI treatment are the result of the SSRI treatment itself, the underlying depression, or a combination of the two.

Sexual dimorphism in placental development and its contribution to health and diseases

According to the Developmental Origin of Health and Disease (DOHaD), intrauterine exposure to adverse environments can affect fetus and birth outcomes and lead to long-term disease susceptibility. Evidence has shown that neonatal outcomes and the timing and severity of adult diseases are sexually dimorphic. As the link between mother and fetus, the placenta is an essential regulator of fetal development programming. It is found that the physiological development trajectory of the placenta has sexual dimorphism. Furthermore, under pathological conditions, the placental function undergoes sex-specific adaptation to ensure fetal survival. Therefore, the placenta may be an important mediator of sexual dimorphism in neonatal outcomes and adult disease susceptibility. Few systematic reviews have been conducted on sexual dimorphism in placental development and its underlying mechanisms. In this review, sex chromosomes and sex hormones, as the main reasons for sexual differentiation of the placenta, will be discussed. Besides, in the etiology of fetal-originated adult diseases, overexposure to glucocorticoids is closely related to adverse neonatal outcomes and long-term disease susceptibility. Studies have found that prenatal glucocorticoid overexposure leads to sexually dimorphic expression of placental glucocorticoid receptor isoforms, resulting in different sensitivity of the placenta to glucocorticoids, and may further affect fetal development. The present review examines what is currently known about sex differences in placental development and the underlying regulatory mechanisms of this sex bias. This review highlights the importance of placental contributions to the origins of sexual dimorphism in health and diseases. It may help develop personalized diagnosis and treatment strategies for fetal development in pathological pregnancies.

Disruption of O-Linked N-Acetylglucosamine Signaling in Placenta Induces Insulin Sensitivity in Female Offspring

Placental dysfunction can lead to fetal growth restriction which is associated with perinatal morbidity and mortality. Fetal growth restriction increases the risk of obesity and diabetes later in life. Placental O-GlcNAc transferase (OGT) has been identified as a marker and a mediator of placental insufficiency in the setting of prenatal stress, however, its role in the fetal programming of metabolism and glucose homeostasis remains unknown. We aim to determine the long-term metabolic outcomes of offspring with a reduction in placental OGT. Mice with a partial reduction and a full knockout of placenta-specific OGT were generated utilizing the Cre-Lox system. Glucose homeostasis and metabolic parameters were assessed on a normal chow and a high-fat diet in both male and female adult offspring. A reduction in placental OGT did not demonstrate differences in the metabolic parameters or glucose homeostasis compared to the controls on a standard chow. The high-fat diet provided a metabolic challenge that revealed a decrease in body weight gain (p = 0.02) and an improved insulin tolerance (p = 0.03) for offspring with a partially reduced placental OGT but not when OGT was fully knocked out. Changes in body weight were not associated with changes in energy homeostasis. Offspring with a partial reduction in placental OGT demonstrated increased hepatic Akt phosphorylation in response to insulin treatment (p = 0.02). A partial reduction in placental OGT was protective from weight gain and insulin intolerance when faced with the metabolic challenge of a high-fat diet. This appears to be, in part, due to increased hepatic insulin signaling. The findings of this study contribute to the greater understanding of fetal metabolic programming and the effect of placental OGT on peripheral insulin sensitivity and provides a target for future investigation and clinical applications.

An overview of X inactivation based on species differences

X inactivation, a developmental process that takes place in early stages of mammalian embryogenesis, balances the sex difference in dosage of X-linked genes. Although all mammals use this form of dosage compensation, the details differ from one species to another because of variations in the staging of embryogenesis and evolutionary tinkering with the DNA blueprint for development. Such differences provide a broader view of the process than that afforded by a single species. My overview of X inactivation is based on these species variations.

The omniscient placenta: Metabolic and epigenetic regulation of fetal programming

Fetal development could be considered a sensitive period wherein exogenous insults and changes to the maternal milieu can have long-term impacts on developmental programming. The placenta provides the fetus with protection and necessary nutrients for growth, and responds to maternal cues and changes in nutrient signaling through multiple epigenetic mechanisms. The X-linked enzyme O-linked-N-acetylglucosamine transferase (OGT) acts as a nutrient sensor that modifies numerous proteins to alter various cellular signals, including major epigenetic processes. This review describes epigenetic alterations in the placenta in response to insults during pregnancy, the potential links of OGT as a nutrient sensor to placental epigenetics, and the implications of placental epigenetics in long-term neurodevelopmental programming. We describe the role of placental OGT in the sex-specific programming of hypothalamic-pituitary-adrenal (HPA) axis programming deficits by early prenatal stress as an example of how placental signaling can have long-term effects on neurodevelopment.

The human placental methylome

This review provides an overview of the unique features of DNA methylation in the human placenta. We discuss the importance of understanding placental development, structure, and function in the interpretation of DNA methylation data. Examples are given of how DNA methylation is important in regulating placental-specific gene expression, including monoallelic expression and X-chromosome inactivation in the placenta. We also discuss studies of global DNA methylation changes in the context of placental pathology and environmental exposures.

Prenatal exposure to mixtures of xenoestrogens and repetitive element DNA methylation changes in human placenta

BACKGROUND

Prenatal exposure to endocrine disrupting compounds (EDCs) has previously shown to alter epigenetic marks.

OBJECTIVES

In this work we explore whether prenatal exposure to mixtures of xenoestrogens has the potential to alter the placenta epigenome, by studying DNA methylation in retrotransposons as a surrogate of global DNA methylation.

METHODS

The biomarker total effective xenoestrogen burden (TEXB) was measured in 192 placentas from participants in the longitudinal INMA Project. DNA methylation was quantitatively assessed by bisulfite pyrosequencing on 10 different retrotransposons including 3 different long interspersed nuclear elements (LINEs), 4 short interspersed nuclear elements (SINEs) and 3 human endogenous retroviruses (HERVs). Associations were tested using linear mixed-effects regression models and sex interaction was evaluated.

RESULTS

A significant sex interaction was observed for AluYb8 (p-value for interaction <0.001, significant at Bonferroni corrected p-value threshold of 0.0025). Boys with the highest TEXB-alpha levels of exposure (third tertile) presented on average a decrease of 0.84% in methylation compared to those in the first tertile (p-value<0.001), while no significant effects were found in girls (p-value=0.134).

CONCLUSIONS

Our findings suggest that boys may be more susceptible to the effect of exposure to xenoestrogens during prenatal development, producing shifts in DNA methylation of certain sensitive genomic repetitive sequences in a tissue important for fetal growth and development.

XCI in preimplantation mouse and human embryos: first there is remodelling…

Female eutherians silence one of their X chromosomes to accomplish an equal dose of X-linked gene expression compared with males. The mouse is the most widely used animal model in XCI research and has proven to be of great significance for understanding the complex mechanism of X-linked dosage compensation. Although the basic principles of XCI are similar in mouse and humans, differences exist in the timing of XCI initiation, the genetic elements involved in XCI regulation and the form of XCI in specific tissues. Therefore, the mouse has its limitations as a model to understand early human XCI and analysis of human tissues is required. In this review, we describe these differences with respect to initiation of XCI in human and mouse preimplantation embryos, the extra-embryonic tissues and the in vitro model of the epiblast: the embryonic stem cells.

Nutrition and human health from a sex-gender perspective

Nutrition exerts a life-long impact on human health, and the interaction between nutrition and health has been known for centuries. The recent literature has suggested that nutrition could differently influence the health of male and female individuals. Until the last decade of the 20th century, research on women has been neglected, and the results obtained in men have been directly translated to women in both the medicine and nutrition fields. Consequently, most modern guidelines are based on studies predominantly conducted on men. However, there are many sex-gender differences that are the result of multifactorial inputs, including gene repertoires, sex steroid hormones, and environmental factors (e.g., food components). The effects of these different inputs in male and female physiology will be different in different periods of ontogenetic development as well as during pregnancy and the ovarian cycle in females, which are also age dependent. As a result, different strategies have evolved to maintain male and female body homeostasis, which, in turn, implies that there are important differences in the bioavailability, metabolism, distribution, and elimination of foods and beverages in males and females. This article will review some of these differences underlying the impact of food components on the risk of developing diseases from a sex-gender perspective.

Random X inactivation and extensive mosaicism in human placenta revealed by analysis of allele-specific gene expression along the X chromosome

Imprinted inactivation of the paternal X chromosome in marsupials is the primordial mechanism of dosage compensation for X-linked genes between females and males in Therians. In Eutherian mammals, X chromosome inactivation (XCI) evolved into a random process in cells from the embryo proper, where either the maternal or paternal X can be inactivated. However, species like mouse and bovine maintained imprinted XCI exclusively in extraembryonic tissues. The existence of imprinted XCI in humans remains controversial, with studies based on the analyses of only one or two X-linked genes in different extraembryonic tissues. Here we readdress this issue in human term placenta by performing a robust analysis of allele-specific expression of 22 X-linked genes, including XIST, using 27 SNPs in transcribed regions. We show that XCI is random in human placenta, and that this organ is arranged in relatively large patches of cells with either maternal or paternal inactive X. In addition, this analysis indicated heterogeneous maintenance of gene silencing along the inactive X, which combined with the extensive mosaicism found in placenta, can explain the lack of agreement among previous studies. Our results illustrate the differences of XCI mechanism between humans and mice, and highlight the importance of addressing the issue of imprinted XCI in other species in order to understand the evolution of dosage compensation in placental mammals.

X chromosome inactivation and embryonic stem cells

X chromosome inactivation (XCI) is a process required to equalize the dosage of X-encoded genes between female and male cells. XCI is initiated very early during female embryonic development or upon differentiation of female embryonic stem (ES) cells and results in inactivation of one X chromosome in every female somatic cell. The regulation of XCI involves factors that also play a crucial role in ES cell maintenance and differentiation and the XCI process therefore provides a beautiful paradigm to study ES cell biology. In this chapter we describe the important cis and trans acting regulators of XCI and introduce the models that have been postulated to explain initiation of XCI in female cells only. We also discuss the proteins involved in the establishment of the inactive X chromosome and describe the different chromatin modifications associated with the inactivation process. Finally, we describe the potential of mouse and human ES and induced pluripotent stem (iPS) cells as model systems to study the XCI process.

Inactive X chromosome-specific reduction in placental DNA methylation

Genome-wide levels of DNA methylation vary between tissues, and compared with other tissues, the placenta has been reported to demonstrate a global decrease in methylation as well as decreased methylation of X-linked promoters. Methylation is one of many features that differentiate the active and inactive X, and it is well established that CpG island promoters on the inactive X are hypermethylated. We now report a detailed analysis of methylation at different regions across the X in male and female placenta and blood. A significant (P < 0.001) placental hypomethylation of LINE1 elements was observed in both males and females. Relative to blood placental promoter hypomethylation was only observed for X-linked, not autosomal promoters, and was significant for females (P < 0.0001) not males (P = 0.9266). In blood, X-linked CpG island promoters were shown to have moderate female methylation (66% across 70 assays) and low (23%) methylation in males. A similar methylation pattern in blood was observed for approximately 20% of non-island promoters as well as 50% of the intergenic or intragenic CpG islands, the latter is likely due to the presence of unannotated promoters. Both intragenic and intergenic regions showed similarly high methylation levels in male and female blood (68 and 66%) while placental methylation of these regions was lower, particularly in females. Thus placental hypomethylation relative to blood is observed globally at repetitive elements as well as across the X. The decrease in X-linked placental methylation is consistently greater in females than males and implicates an inactive X specific loss of methylation in the placenta.

X chromosome inactivation is initiated in human preimplantation embryos

X chromosome inactivation (XCI) is the mammalian mechanism that compensates for the difference in gene dosage between XX females and XY males. Genetic and epigenetic regulatory mechanisms induce transcriptional silencing of one X chromosome in female cells. In mouse embryos, XCI is initiated at the preimplantation stage following early whole-genome activation. It is widely thought that human embryos do not employ XCI prior to implantation. Here, we show that female preimplantation embryos have a progressive accumulation of XIST RNA on one of the two X chromosomes, starting around the 8-cell stage. XIST RNA accumulates at the morula and blastocyst stages and is associated with transcriptional silencing of the XIST-coated chromosomal region. These findings indicate that XCI is initiated in female human preimplantation-stage embryos and suggest that preimplantation dosage compensation is evolutionarily conserved in placental mammals.

Candida glabrata PHO4 is necessary and sufficient for Pho2-independent transcription of phosphate starvation genes

Comparative genomic analyses of Candida glabrata and Saccharomyces cerevisiae suggest many signal transduction pathways are highly conserved. Focusing on the phosphate signal transduction (PHO) pathway of C. glabrata, we demonstrate that components of the pathway are conserved and confirm the role of CgPHO81, CgPHO80, CgPHO4, and CgMSN5 in the PHO pathway through deletion analysis. Unlike S. cerevisiae, C. glabrata shows little dependence on the transcription factor, Pho2, for induction of phosphate-regulated genes during phosphate limitation. We show that the CgPho4 protein is necessary and sufficient for Pho2-independent gene expression; CgPho4 is capable of driving expression of PHO promoters in S. cerevisiae in the absence of ScPHO2. On the basis of the sequences of PHO4 in the hemiascomycetes and complementation analysis, we suggest that Pho2 dependence is a trait only observed in species closely related to S. cerevisiae. Our data are consistent with trans-regulatory changes in the PHO pathway via the transcription factor Pho4 as opposed to cis-regulatory changes (the promoter).

Role of X chromosome defects in primary biliary cirrhosis

Similar to the majority of autoimmune conditions, primary biliary cirrhosis (PBC) is a chronic cholestatic liver disease characterized by a striking female predominance; it is characterized by high titer serum autoantibodies to mitochondrial antigens, elevated serum immunoglobulin M, progressive destruction of intrahepatic bile ducts, and ultimately liver cirrhosis and failure. Familiarity and high concordance rates for the disease among monozygotic twins strongly support the role of genetics in the disease. Experimental efforts have been dedicated by our and other research groups to investigate the role of X chromosome abnormalities (i.e. monosomyrates and inactivation patterns) in autoimmunity. Our recent work has demonstrated enhanced X monosomy in women with PBC as well as two other female-predominant autoimmune diseases, systemic sclerosis and autoimmune thyroid disease. We will review herein the most recent evidence on the role of the X chromosome in PBC onset and discuss the potential implications. Future developments of these findings will be discussed.

Two novel connexin32 mutations cause early onset X-linked Charcot-Marie-Tooth disease

BACKGROUND

X-linked Charcot-Marie Tooth (CMT) is caused by mutations in the connexin32 gene that encodes a polypeptide which is arranged in hexameric array and form gap junctions.

METHODS

We describe two novel mutations in the connexin32 gene in two Norwegian families.

RESULTS

Family 1 had a c.225delG (R75fsX83) which causes a frameshift and premature stop codon at position 247. This probably results in a shorter non-functional protein structure. Affected individuals had an early age at onset usually in the first decade. The symptoms were more severe in men than women. All had severe muscle weakness in the legs. Several abortions were observed in this family. Family 2 had a c.536 G>A (C179Y) transition which causes a change of the highly conserved cysteine residue, i.e. disruption of at least one of three disulfide bridges. The mean age at onset was in the first decade. Muscle wasting was severe and correlated with muscle weakness in legs. The men and one woman also had symptom from their hands. The neuropathy is demyelinating and the nerve conduction velocities were in the intermediate range (25-49 m/s). Affected individuals had symmetrical clinical findings, while the neurophysiology revealed minor asymmetrical findings in nerve conduction velocity in 6 of 10 affected individuals.

CONCLUSION

The two novel mutations in the connexin32 gene are more severe than the majority of previously described mutations possibly due to the severe structural change of the gap junction they encode.

X-chromosome inactivation patterns in females with Prader-Willi syndrome

Prader-Willi syndrome (PWS) is a complex neurodevelopmental disorder caused by loss of paternally expressed genes from the 15q11-q13 region generally due to a paternally-derived deletion of the 15q11-q13 region or maternal disomy 15 (UPD). Maternal disomy 15 is usually caused by maternal meiosis I non-disjunction associated with advanced maternal age and after fertilization with a normal sperm leading to trisomy 15, a lethal condition unless trisomy rescue occurs with loss of the paternal chromosome 15. To further characterize the pathogenesis of maternal disomy 15 process in PWS, the status of X-chromosome inactivation was calculated to determine whether non-random skewing of X-inactivation is present indicating a small pool of early embryonic cells. We studied X-chromosome inactivation in 25 females with PWS-UPD, 35 with PWS-deletion, and 50 controls (with similar means, medians, and age ranges) using the polymorphic androgen receptor (AR) gene assay. A significant positive correlation (r = 0.5, P = 0.01) was seen between X-chromosome inactivation and age for only the UPD group. Furthermore, a significantly increased level (P = 0.02) of extreme X-inactivation skewness (>90%) was detected in our PWS-UPD group (24%) compared to controls (4%). This observation could indicate that trisomy 15 occurred at conceptus with trisomy rescue in early pregnancy leading to extreme skewness in several PWS-UPD subjects. Extreme X-inactivation skewness may also lead to additional risks for X-linked recessive disorders in PWS females with UPD and extreme X-chromosome skewness.

Hypermethylation of RASSF1A in human and rhesus placentas

The pseudomalignant nature of the placenta prompted us to search for tumor suppressor gene hypermethylation, a phenomenon widely reported in cancer, in the human placenta. Nine tumor suppressor genes were studied. Hypermethylation of the Ras association domain family 1 A (RASSF1A) gene was found in human placentas from all three trimesters of pregnancy but was absent in other fetal tissues. Hypermethylation of Rassf1 was similarly observed in placentas from the rhesus monkey but not the mouse. An inverse relationship between RASSF1A promoter methylation and gene expression was demonstrated by bisulfite sequencing of microdissected placental cells and immunohistochemical staining of placental tissue sections using an anti-RASSF1A antibody. Treatment of choriocarcinoma cell lines, JAR and JEG3, by 5-aza-2'-deoxycytidine and trichostatin A led to reduction in RASSF1A methylation but increased expression. These observations extend the analogy between the primate placenta and malignant tumors to the epigenetic level.

Fetal DNA in maternal plasma: progress through epigenetics

The discovery of cell-free fetal DNA in maternal plasma has opened up new possibilities for noninvasive prenatal diagnosis. Most of the work in this field has focused on the detection of fetal genetic markers that are distinguishable from the background maternal DNA. The feasibility of detecting fetal epigenetic markers in maternal plasma using an imprinted locus was demonstrated in 2002. This work has recently led to the development of the first universal fetal epigenetic marker, hypomethylated maspin, which can be used for fetal DNA detection in maternal plasma, irrespective of fetal gender and genetic polymorphisms. It is expected that many more fetal epigenetic markers that can be used in this manner will be developed over the next few years. These markers may catalyze the eventual clinical use of circulating fetal DNA for noninvasive prenatal diagnosis, such as for the detection of fetal chromosomal aneuploidies.

The X chromosome and systemic sclerosis

PURPOSE OF REVIEW

Similar to the majority of autoimmune rheumatic diseases, systemic sclerosis is characterized by a striking female predominance superimposed on a predisposing genetic background. At least two genetic mechanisms have been proposed that play a role in susceptibility to systemic sclerosis; firstly the maintenance of immune tolerance via genes on the X chromosomes and, secondly, fetal microchimerism. Based on these lines of evidence, experimental efforts have been most recently dedicated to investigating the role of X chromosome abnormalities (i.e. monosomy rates and inactivation patterns) in autoimmunity. We will review herein the most recent data on the role of the X chromosome in systemic sclerosis onset and discuss the potential implications.

RECENT FINDINGS

Women with systemic sclerosis manifest an enhanced rate of X monosomic cells in peripheral blood compared with healthy age-matched women. Furthermore, a severely skewed X chromosome inactivation pattern is found in women with systemic sclerosis.

SUMMARY

These observations, reproduced in other female-predominant autoimmune diseases, strongly support the role of the X chromosome in conferring susceptibility to tolerance breakdown and open novel scenarios to emphasize the unknown etiopathogenesis of systemic sclerosis. The implications of these findings will be discussed.