Imprinted genes in placental growth and obstetric disorders

Placental exosomes in pregnancy and preeclampsia

Preeclampsia, poses significant risks to both maternal and fetal well-being. Exosomes released by the placenta play a crucial role in intercellular communication and are recognized as potential carriers of essential information for placental development. These exosomes transport a payload of proteins, nucleic acids, and lipids that mirror the placental microenvironment. This review delves into the functional roles of placental exosomes and its contents shedding light on their involvement in vascular regulation and immune modulation in normal pregnancy. Discernible changes are reported in the composition and quantity of placental exosome contents in pregnancies affected by preeclampsia. The exosomes from preeclamptic mothers affect vascularization and fetal kidney development. The discussion also explores the implications of utilizing placental exosomes as biomarkers and the prospects of translating these findings into clinical applications. In conclusion, placental exosomes hold promise as a valuable avenue for deciphering the complexities of preeclampsia, providing crucial diagnostic and prognostic insights. As the field progresses, a more profound comprehension of the distinct molecular signatures carried by placental exosomes may open doors to innovative strategies for managing and offering personalized care to pregnancies affected by preeclampsia.

Diagnostic Utility of TSSC3 and RB1 Immunohistochemistry in Hydatidiform Mole

The general notion of complete hydatidiform moles is that most of them consist entirely of paternal DNA; hence, they do not express p57, a paternally imprinted gene. This forms the basis for the diagnosis of hydatidiform moles. There are about 38 paternally imprinted genes. The aim of this study is to determine whether other paternally imprinted genes could also assist in the diagnostic approach of hydatidiform moles. This study comprised of 29 complete moles, 15 partial moles and 17 non-molar abortuses. Immunohistochemical study using the antibodies of paternal-imprinted (RB1, TSSC3 and DOG1) and maternal-imprinted (DNMT1 and GATA3) genes were performed. The antibodies' immunoreactivity was evaluated on various placental cell types, namely cytotrophoblasts, syncytiotrophoblasts, villous stromal cells, extravillous intermediate trophoblasts and decidual cells. TSSC3 and RB1 expression were observed in all cases of partial moles and non-molar abortuses. In contrast, their expression in complete moles was identified in 31% (TSSC3) and 10.3% (RB1), respectively (p < 0.0001). DOG1 was consistently negative in all cell types in all cases. The expressions of maternally imprinted genes were seen in all cases, except for one case of complete mole where GATA3 was negative. Both TSSC3 and RB1 could serve as a useful adjunct to p57 for the discrimination of complete moles from partial moles and non-molar abortuses, especially in laboratories that lack comprehensive molecular service and in cases where p57 staining is equivocal.

The Role of Cluster C19MC in Pre-Eclampsia Development

Pre-eclampsia is a placenta-related complication occurring in 2-10% of all pregnancies. miRNAs are a group of non-coding RNAs regulating gene expression. There is evidence that C19MC miRNAs are involved in the development of the placenta. Deregulation of chromosome 19 microRNA cluster (C19MC) miRNAs expression leads to impaired cell differentiation, abnormal trophoblast invasion and pathological angiogenesis, which can lead to the development of pre-eclampsia. Information was obtained through a review of articles available in PubMed Medline. Articles on the role of the C19MC miRNA in the development of pre-eclampsia published in 2009-2022 were analyzed. This review article summarizes the current data on the role of the C19MC miRNA in the development of pre-eclampsia. They indicate a significant increase in the expression of most C19MC miRNAs in placental tissue and a high level of circulating fractions in serum and plasma, both in the first and/or third trimester in women with PE. Only for miR-525-5p, low levels of plasma expression were noted in the first trimester, and in the placenta in the third trimester. The search for molecular factors indicating the development of pre-eclampsia before the onset of clinical symptoms seems to be a promising diagnostic route. Identifying women at risk of developing pre-eclampsia at the pre-symptomatic stage would avoid serious complications in both mothers and fetuses. We believe that miRNAs belonging to cluster C19MC could be promising biomarkers of pre-eclampsia development.

Epigenetics Beyond Fetal Growth Restriction: A Comprehensive Overview

Fetal growth restriction is a pathological condition occurring when the fetus does not reach the genetically determined growth potential. The etiology of fetal growth restriction is expected to be multifactorial and include fetal, maternal, and placental factors, the latter being the most frequent cause of isolated fetal growth restriction. Severe fetal growth restriction has been related to both an increased risk of perinatal morbidity and mortality, and also a greater susceptibility to developing diseases (especially cardio-metabolic and neurological disorders) later in life. In the last decade, emerging evidence has supported the hypothesis of the Developmental Origin of Health and Disease, which states that individual developmental 'programming' takes place via a delicate fine tuning of fetal genetic and epigenetic marks in response to a large variety of 'stressor' exposures during pregnancy. As the placenta is the maternal-fetal interface, it has a crucial role in fetal programming, such that any perturbation altering placental function interferes with both in-utero fetal growth and also with the adult life phenotype. Several epigenetic mechanisms have been highlighted in modulating the dynamic placental epigenome, including alterations in DNA methylation status, post-translational modification of histones, and non-coding RNAs. This review aims to provide a comprehensive and critical overview of the available literature on the epigenetic background of fetal growth restriction. A targeted research strategy was performed using PubMed, MEDLINE, Embase, and The Cochrane Library up to January 2022. A detailed and fully referenced synthesis of available literature following the Scale for the Assessment of Narrative Review Articles guidelines is provided. A variety of epigenetic marks predominantly interfering with placental development, function, and metabolism were found to be potentially associated with fetal growth restriction. Available evidence on the role of environmental exposures in shaping the placental epigenome and the fetal phenotype were also critically discussed. Because of the highly dynamic crosstalk between epigenetic mechanisms and the extra level of complexity in interpreting the final placental transcriptome, a full comprehension of these phenomenon is still lacking and advances in multi-omics approaches are urgently needed. Elucidating the role of epigenetics in the developmental origins of health and disease represents a new challenge for the coming years, with the goal of providing early interventions and prevention strategies and, hopefully, new treatment opportunities.

Differences in Placental Imprinted Gene Expression across Preeclamptic and Non-Preeclamptic Pregnancies

Preeclampsia is a multi-systemic syndrome that presents in approximately 5% of pregnancies worldwide and is associated with a range of subsequent postpartum and postnatal outcomes, including fetal growth restriction. As the placenta plays a critical role in the development of preeclampsia, surveying genomic features of the placenta, including expression of imprinted genes, may reveal molecular markers that can further refine subtypes to aid targeted disease management. In this study, we conducted a comprehensive survey of placental imprinted gene expression across early and late onset preeclampsia cases and preterm and term normotensive controls. Placentas were collected at delivery from women recruited at the Magee-Womens Hospital prenatal clinics, and expression levels were profiled across 109 imprinted genes. We observed downregulation of placental Mesoderm-specific transcript (MEST) and Necdin (NDN) gene expression levels (false discovery rate (FDR) < 0.05) among early onset preeclampsia cases compared to preterm controls. No differences in placental imprinted gene expression were observed between late onset preeclampsia cases and term controls. While few studies have linked NDN to pregnancy complications, reductions in MEST expression levels, as observed in our study, are consistently reported in the literature in relation to various pregnancy complications, including fetal growth restriction, suggesting a potential role for placental MEST expression as a biosensor of an adverse in utero environment.

Methylation of the C19MC microRNA locus in the placenta: association with maternal and chilhood body size

OBJECTIVES

To study DNA methylation at the C19MC locus in the placenta and its association with (1) parental body size, (2) transmission of haplotypes for the C19MC rs55765443 SNP, and (3) offspring's body size and/or body composition at birth and in childhood.

SUBJECTS AND METHODS

Seventy-two pregnant women-infant pairs and 63 fathers were included in the study. Weight and height of mothers, fathers and newborns were registered during pregnancy or at birth (n = 72). Placental DNA methylation at the C19MC imprinting control region (ICR) was quantified by bisulfite pyrosequencing. Genotyping of the SNP was performed using restriction fragment length polymorphisms. The children's body size and composition were reassessed at age 6 years (n = 32).

RESULTS

Lower levels of placental C19MC methylation were associated with increased body size of mother, specifically with higher pregestational and predelivery weights and height of the mother (β from -0.294 to -0.371; R² from 0.04 to 0.10 and all p < 0.019), and with higher weight, height, waist and hip circumferences, and fat mass of the child (β from -0.428 to -0.552; R² from 0.33 to 0.56 and all p < 0.009). Parental transmission of the SNP did not correlate with an altered placental methylation status at the C19MC ICR.

CONCLUSIONS

Increased maternal size is associated with reduced placental C19MC methylation, which, in turn, relate to larger body size of the child.

LncRNA MEG3 inhibits trophoblast invasion and trophoblast-mediated VSMC loss in uterine spiral artery remodeling

Extravillous trophoblasts (EVTs) migrate into uterine decidua and induce vascular smooth muscle cell (VSMC) loss through mechanisms thought to involve migration and apoptosis, achieving complete spiral artery remodeling. Long noncoding RNA maternally expressed gene 3 (MEG3) can regulate diverse cellular processes, such as proliferation and migration, and has been discovered highly expressed in human placenta tissues. However, little is known about the role of MEG3 in modulating EVT functions and EVT-induced VSMC loss. In this study, we first examined the location of MEG3 in human first-trimester placenta by in situ hybridization. Then, exogenous upregulation of MEG3 in HTR-8/SVneo cells was performed to investigate the effects of MEG3 on EVT motility and EVT capacity to displace VSMCs. Meanwhile, the molecules mediating EVT-induced VSMC loss, such as tumor necrosis factor-α (TNF-α), Fas ligand (FasL), and tumor necrosis factor-α-related apoptosis-inducing ligand (TRAIL) were detected at transcriptional and translational levels. Finally, VSMCs were cocultured with MEG3-upregulated HTR-8/SVneo to explore the role of MEG3 on EVT-mediated VSMC migration and apoptosis. Results showed that MEG3 was expressed in trophoblasts in placental villi and decidua, and MEG3 enhancement inhibited HTR-8/SVneo migration and invasion. Meanwhile, the displacement of VSMCs by HTR-8/SVneo and the expression of TNF-α, FasL and TRAIL in HTR-8/SVneo were reduced following MEG3 overexpression in HTR-8/SVneo. Furthermore, HTR-8/SVneo with MEG3 upregulation impaired VSMC migration and apoptosis. The PI3K/Akt pathway, which is possibly downstream, was inactivated in MEG3-upregulated HTR-8/SVneo. These findings suggest that MEG3 might be a negative regulator of spiral artery remodeling via suppressing EVT invasion and EVT-mediated VSMC loss.

Isolation of villous cytotrophoblasts from second trimester human placentas

Human placental functional studies are often performed using immortalised trophoblast cells and cell lines established from human choriocarcinomas, which, while practical, may not truly reflect trophoblast function in vivo. Primary trophoblast cultures derived from human placentas following pregnancy termination or delivery are more clinically relevant, but trophoblast isolation protocols are only available for 1st trimester and term placental tissues. Here we report a method for isolation and purification of primary villous cytotrophoblasts from 2nd trimester human placentas, that yields >99% trophoblast purity as shown by immunofluorescence.

A genome-wide search for new imprinted genes in the human placenta identifies DSCAM as the first imprinted gene on chromosome 21

We identified, through a genome-wide search for new imprinted genes in the human placenta, DSCAM (Down Syndrome Cellular Adhesion Molecule) as a paternally expressed imprinted gene. Our work revealed the presence of a Differentially Methylated Region (DMR), located within intron 1 that might regulate the imprinting in the region. This DMR showed a maternal allele methylation, compatible with its paternal expression. We showed that DSCAM is present in endothelial cells and the syncytiotrophoblast layer of the human placenta. In mouse, Dscam expression is biallelic in foetal brain and placenta excluding any possible imprinting in these tissues. This gene encodes a cellular adhesion molecule mainly known for its role in neurone development but its function in the placenta remains unclear. We report here the first imprinted gene located on human chromosome 21 with potential clinical implications.

Evaluation of vitrification protocol of mouse ovarian tissue by effect of DNA methyltransferase-1 and paternal imprinted growth factor receptor-binding protein 10 on signaling pathways

Transplantation of cryopreserved ovarian tissue has been considered as a promising way of fertility preservation for women. however, this cryopreservation method is prone to post-resuscitation follicle proliferation and oocyte development stagnation, affecting late transplant survival. To evaluate current vitrification works, we investigated the critical pathway alternations in vitrified-warmed juvenile 10-day-old mouse ovary. We showed a significant decrease of protein kinase B (Akt) and Mitogen-activated protein kinase (Mapk) phosphorylation, during which serine/threonine kinases play central roles in coordinating follicle and oocyte development and stress response. Inhibition of Akt and Mapk activity were associated with one of the imprinted insulin pathway negative regulatory genes, Growth factor receptor-binding protein 10 (Grb10) which remarkably increased in vitrified-warmed juvenile mouse ovary than that of fresh group (p < 0.05). RNAi-induced Grb10 down-regulation reversed the decrease in Akt and Mapk phosphorylation. The increase of Grb10 expression was partially caused by the hyper-methylation of the promoter region, associated with the decrease of follicular DNA methyltransferase (Dnmt) 1 protein in different stages of vitrified-warmed group, compared to fresh group (p < 0.05). The mRNA and protein expression of Dnmt1 in ovary of vitrified-warmed juvenile mouse were remarkably lower than those in fresh group (p < 0.05). Dnmt1 overexpression dramatically reversed Grb10 up-regulation and Akt and Mapk phosphorylation reduction. Taken together, our findings suggest that Grb10 expression might be helpful in evaluation of effectiveness of vitrification, and considered as a potential target for further vitrification protocols improvement in the future.

A complex association between DNA methylation and gene expression in human placenta at first and third trimesters

The human placenta is a maternal-fetal organ essential for normal fetal development and maternal health. During pregnancy, the placenta undergoes many structural and functional changes in response to fetal needs and environmental exposures. Previous studies have demonstrated widespread epigenetic and gene expression changes from early to late pregnancy. However, on the global level, how DNA methylation changes impact on gene expression in human placenta is not yet well understood. We performed DNA methylome analysis by reduced representation bisulfite sequencing (RRBS) and gene expression analysis by RNA-Seq for both first and third trimester human placenta tissues. From first to third trimester, 199 promoters (corresponding to 189 genes) and 2,297 gene bodies were differentially methylated, with a clear dominance of hypermethylation (96.8% and 93.0% for promoters and gene bodies, respectively). A total of 2,447 genes were differentially expressed, of which 77.2% were down-regulated. Gene ontology analysis using differentially expressed genes were enriched for cell cycle and immune response functions. The correlation between DNA methylation and gene expression was non-linear and complex, depending on the genomic context (promoter or gene body) and gene expression levels. A wide range of DNA methylation and gene expression changes were observed at different gestational ages. The non-linear association between DNA methylation and gene expression indicates that epigenetic regulation of placenta development is more complex than previously envisioned.

Adaptability and potential for treatment of placental functions to improve embryonic development and postnatal health

For an organ that is so critical for life in eutherian mammals, the placenta hardly gets the attention that it deserves. The placenta does a series of remarkable things, including implanting the embryo in the uterus, negotiating with the mother for nutrients but also protecting her health during pregnancy, helping establish normal metabolic and cardiovascular function for life postnatally (developmental programming) and initiating changes that prepare the mother to care for and suckle her young after birth. Different lines of evidence in experimental animals suggest that the development and function of the placenta are adaptable. This means that some of the changes observed in pathological pregnancies may represent attempts to mitigate the impact of fetal growth and development. Key and emerging concepts are reviewed here concerning how we may view the placenta diagnostically and therapeutically in pregnancy complications, focusing on information from experimental studies in mice, sheep and cattle, as well as association studies from humans. Hundreds of different genes have been shown to underlie normal placental development and function, some of which have promise as tractable targets for intervention in pregnancies at risk for poor fetal growth.

Effects of oocyte vitrification on epigenetic status in early bovine embryos

Oocyte cryopreservation has a great impact on subsequent embryonic development. Currently, several studies have primarily focused on the consequences of vitrification and the development potential of cellular structures. This study determined whether oocyte vitrification caused epigenetic instabilities of bovine embryos. The effects of oocyte vitrification on DNA methylation, histone modifications, and putative imprinted genes' expression in early embryos derived by intracytoplasmic sperm injection were examined. Results showed that oocyte vitrification did not affect zygote cleavage rates (67.0% vs. 73.8% control, P > 0.05) but reduced the blastocyst rate (9.6% vs. 23.0%, P < 0.05). The levels of DNA methylation and H3K9me3 in oocytes and early cleavage embryos were lower (P < 0.05) than those in control group, but the level of acH3K9 increased (P < 0.05) in the vitrification group during the early cleavage phases. No differences were observed for DNA methylation, H3K9me3, and acH3K9 in the inner cell mass of blastocysts, whereas decreased levels of DNA methylation and acH3K9 (P < 0.05) existed in TE cells after vitrification. The expression of putative-imprinted genes PEG10, XIST, and KCNQ1O1T was upregulated in blastocysts. These epigenetic abnormalities may be partially explained by altered expression of genes associated with epigenetic regulations. DNA methylation and H3K9 modification suggest that oocyte vitrification may excessively relax the chromosomes of oocytes and early cleavage embryos. In conclusion, these epigenetic indexes could be used as damage markers of oocyte vitrification during early embryonic development, which offers a new insight to assess oocyte vitrification.

Select Prenatal Environmental Exposures and Subsequent Alterations of Gene-Specific and Repetitive Element DNA Methylation in Fetal Tissues

Strong evidence implicates maternal environmental exposures in contributing to adverse outcomes during pregnancy and later in life through the developmental origins of health and disease hypothesis. Recent research suggests these effects are mediated through the improper regulation of DNA methylation in offspring tissues, specifically placental tissue, which plays a critical role in fetal development. This article reviews the relevant literature relating DNA methylation in multiple tissues at or near delivery to several prenatal environmental toxicants and stressors, including cigarette smoke, endocrine disruptors, heavy metals, as well as maternal diet. These human studies expand upon previously reported outcomes in animal model interventions and include effects on both imprinted and non-imprinted genes. We have also noted some of the strengths and limitations in the approaches used, and consider the appropriate interpretation of these findings in terms of their effect size and their relationship to differential gene expression and potential health outcomes. The studies suggest an important role of DNA methylation in mediating the effects of the intrauterine environment on children's health and a need for additional research to better clarify the role of this epigenetic mechanism as well as others.

Fetal growth restriction and methylation of growth-related genes in the placenta

AIM

To examine the associations between fetal growth restriction (FGR) and DNA methylation of six growth-related genes in human placenta.

MATERIALS & METHODS

A total of 181 mother-newborn pairs (80 FGR cases and 101 controls) were enrolled in this case-control study. Placental DNA methylation was measured by bisulfite pyrosequencing.

RESULTS

DNA methylation levels of IGF2 and AHRR were positively associated with newborn birth weight and Quetelet's index, while DNA methylation levels of HSD11B2 and WNT2 were negatively associated with those fetal growth indicators. In addition, significantly elevated odds of FGR birth were associated with increasing DNA methylation of HSD11B2 and WNT2, and decreasing DNA methylation of IGF2.

CONCLUSION

Our findings demonstrated that placental DNA methylation levels of IGF2, AHRR, HSD11B2 and WNT2 were associated with measures of fetal growth.

Altered gene expression in human placentas after IVF/ICSI

STUDY QUESTION

Is gene expression in placental tissue of IVF/ICSI patients altered when compared with a spontaneously conceived group, and are these alterations due to loss of imprinting (LOI) in the case of imprinted genes?

SUMMARY ANSWER

An altered imprinted gene expression of H19 and Pleckstrin homology-like domain family A member 2 (PHLDA2), which was not due to LOI, was observed in human placentas after IVF/ICSI and several biological pathways were significantly overrepresented and mostly up-regulated.

WHAT IS KNOWN ALREADY

Genomic imprinting plays an important role in placental biology and in placental adaptive responses triggered by external stimuli. Changes in placental development and function can have dramatic effects on the fetus and its ability to cope with the intrauterine environment. An increased frequency of placenta-related problems as well as an adverse perinatal outcome is seen in IVF/ICSI derived pregnancies, but the role of placental epigenetic deregulation is not clear yet.

STUDY DESIGN AND PARTICIPANTS

In this prospective cohort study, a total of 115 IVF/ICSI and 138 control couples were included during pregnancy. After applying several exclusion criteria (i.e. preterm birth or stillbirth, no placental samples, pregnancy complications or birth defects), respectively, 81 and 105 placentas from IVF/ICSI and control pregnancies remained for analysis. Saliva samples were collected from both parents.

METHODS

We quantitatively analysed the mRNA expression of several growth-related imprinted genes [H19, insulin-like growth factor 2 (IGF2), PHLDA2, cyclin-dependent kinase inhibitor 1C (CDKN1C), mesoderm-specific transcript homolog (MEST) isoform α and β by quantitative PCR] after standardization against three housekeeping genes [Succinate dehydrogenase A (SDHA), YWHAZ and TATA-binding protein (TBP)]. A quantitative allele-specific expression analysis of the differentially expressed imprinted genes was performed to investigate LOI, independent of the mechanism of imprinting. Furthermore, a microarray analysis was carried out (n = 10 in each group) to investigate the expression of non-imprinted genes as well.

MAIN RESULTS AND THE ROLE OF CHANCE

Both H19 and PHLDA2 showed a significant change, respectively, a 1.3-fold (P = 0.033) and 1.5-fold (P = 0.002) increase in mRNA expression in the IVF/ICSI versus control group. However, we found no indication that there is an increased frequency of LOI in IVF/ICSI placental samples. Genome-wide mRNA expression revealed 13 significantly overrepresented biological pathways involved in metabolism, immune response, transmembrane signalling and cell cycle control, which were mostly up-regulated in the IVF/ICSI placental samples.

LIMITATIONS, REASONS FOR CAUTION

Only a subset of samples was found to be fully informative, which unavoidably led to lower sample numbers for our LOI analysis. Our study cannot distinguish whether the reported differences in the IVF/ICSI group are exclusively attributable to the IVF/ICSI technique itself or to the underlying subfertility of the patients.

WIDER IMPLICATIONS OF THE FINDINGS

Whether these placental adaptations observed in pregnancies conceived by IVF/ICSI might be connected to an adverse perinatal outcome after IVF remains unknown. However, it is possible that these differences affect fetal development and long-term patterns of gene expression, as well as maternal gestational physiology.

STUDY FUNDING/COMPETING INTERESTS

Partly funded by an unrestricted research grant by Organon BV (now MSD BV) and GROW School for Oncology and Developmental Biology without any role in study design, data collection and analysis or preparation of the manuscript. No conflict of interests to declare.

TRIAL REGISTRATION NUMBER

Dutch Trial Registry (NTR) number 1298.

Maternal mRNA expression levels of H19 are inversely associated with risk of macrosomia

INTRODUCTION

To investigate the associations between the mRNA levels of H19 in term placenta and risk of macrosomia.

MATERIAL AND METHODS

Term placentas were collected from 37 macrosomia and 37 matched neonates with normal birth weight (controls) born in Changzhou Women and Children Health Hospital, Jiangsu province, P. R. China from March 1 to June 30, 2008. The mRNA levels of H19 in those placentas were measured by real-time polymerase chain reaction (PCR). Simple and multiple logistic regression models were used to explore the risk factors in the development of macrosomia. All analyses were performed using Stata 10.0 (StataCorp, College Station, Texas, USA).

RESULTS

The average H19 mRNA level of the macrosomia group was 1.450 ±0.456 while in the control group it was 2.080 ±1.296. Based on the result of Student's t test, there was a significant difference in H19 mRNA level between the macrosomia group and the control group (p = 0.008). After controlling for potential confounders, the multivariable adjusted odds ratio (OR) of macrosomia for those in the highest tertile of H19 mRNA level was 0.12 (95% CI: 0.02-0.59) when compared to those in the lowest tertile (p for linear trend = 0.009).

CONCLUSIONS

The term placental H19 mRNA levels were inversely related to the occurrence of macrosomia. Our findings suggest that the low expression of H19 mRNA may contribute to the development of macrosomia.

Comparative anatomy of chromosomal domains with imprinted and non-imprinted allele-specific DNA methylation

Allele-specific DNA methylation (ASM) is well studied in imprinted domains, but this type of epigenetic asymmetry is actually found more commonly at non-imprinted loci, where the ASM is dictated not by parent-of-origin but instead by the local haplotype. We identified loci with strong ASM in human tissues from methylation-sensitive SNP array data. Two index regions (bisulfite PCR amplicons), one between the C3orf27 and RPN1 genes in chromosome band 3q21 and the other near the VTRNA2-1 vault RNA in band 5q31, proved to be new examples of imprinted DMRs (maternal alleles methylated) while a third, between STEAP3 and C2orf76 in chromosome band 2q14, showed non-imprinted haplotype-dependent ASM. Using long-read bisulfite sequencing (bis-seq) in 8 human tissues we found that in all 3 domains the ASM is restricted to single differentially methylated regions (DMRs), each less than 2kb. The ASM in the C3orf27-RPN1 intergenic region was placenta-specific and associated with allele-specific expression of a long non-coding RNA. Strikingly, the discrete DMRs in all 3 regions overlap with binding sites for the insulator protein CTCF, which we found selectively bound to the unmethylated allele of the STEAP3-C2orf76 DMR. Methylation mapping in two additional genes with non-imprinted haplotype-dependent ASM, ELK3 and CYP2A7, showed that the CYP2A7 DMR also overlaps a CTCF site. Thus, two features of imprinted domains, highly localized DMRs and allele-specific insulator occupancy by CTCF, can also be found in chromosomal domains with non-imprinted ASM. Arguing for biological importance, our analysis of published whole genome bis-seq data from hES cells revealed multiple genome-wide association study (GWAS) peaks near CTCF binding sites with ASM.

Allele-specific DNA methylation (ASM) is a central mechanism of gene regulation in humans, which can influence inter-individual differences in physical and mental traits and disease susceptibility. ASM is mediated either by parental imprinting, in which the repressed copy (allele) of the gene is determined by which type of parent (mother or father) transmitted it or, for a larger number of genes, by the local DNA sequence, independent of which parent transmitted it. Chromosomal regions with imprinted ASM have been well studied, and certain mechanistic principles, including the role of discrete differentially methylated regions (DMRs) and involvement of the insulator protein CTCF, have emerged. However, the molecular mechanisms underlying non-imprinted sequence-dependent ASM are not yet understood. Here we describe our detailed mapping of ASM across 5 gene regions, including two novel examples of imprinted ASM and three gene regions with non-imprinted, sequence-dependent ASM. Our data uncover shared molecular features – small discrete DMRs, and the binding of CTCF to these DMRs, in examples of both types of ASM. Combining ASM mapping with genetic association data suggests that sequence-dependent ASM at CTCF binding sites influences diverse human traits.

Association of maternal and nutrient supply line factors with DNA methylation at the imprinted IGF2/H19 locus in multiple tissues of newborn twins

Epigenetic events are crucial for early development, but can be influenced by environmental factors, potentially programming the genome for later adverse health outcomes. The insulin-like growth factor 2 (IGF2)/H19 locus is crucial for prenatal growth and the epigenetic state at this locus is environmentally labile. Recent studies have implicated maternal factors, including folate intake and smoking, in the regulation of DNA methylation at this locus, although data are often conflicting in the direction and magnitude of effect. Most studies have focused on single tissues and on one or two differentially-methylated regions (DMRs) regulating IGF2/H19 expression. In this study, we investigated the relationship between multiple shared and non-shared gestational/maternal factors and DNA methylation at four IGF2/H19 DMRs in five newborn cell types from 67 pairs of monozygotic and 49 pairs of dizygotic twins. Data on maternal and non-shared supply line factors were collected during the second and third trimesters of pregnancy and DNA methylation was measured via mass spectrometry using Sequenom MassArray EpiTyper analysis. Our exploratory approach showed that the site of umbilical cord insertion into the placenta in monochorionic twins has the strongest positive association with methylation in all IGF2/H19 DMRs (p<0.05). Further, evidence for tissue- and locus-specific effects were observed, emphasizing that responsiveness to environmental exposures in utero cannot be generalized across genes and tissues, potentially accounting for the lack of consistency in previous findings. Such complexity in responsiveness to environmental exposures in utero has implications for all epigenetic studies investigating the developmental origins of health and disease.

Linking prenatal maternal adversity to developmental outcomes in infants: the role of epigenetic pathways

Prenatal exposure to maternal stress, anxiety, and depression can have lasting effects on infant development with risk of psychopathology. Although the impact of prenatal maternal distress has been well documented, the potential mechanisms through which maternal psychosocial variables shape development have yet to be fully elucidated. Advances in molecular biology have highlighted the role of epigenetic mechanisms in regulating gene activity, neurobiology, and behavior and the potential role of environmentally induced epigenetic variation in linking early life exposures to long-term biobehavioral outcomes. In this article, we discuss evidence illustrating the association between maternal prenatal distress and both fetal and infant developmental trajectories and the potential role of epigenetic mechanisms in mediating these effects. Postnatal experiences may have a critical moderating influence on prenatal effects, and we review findings illustrating prenatal-postnatal interplay and the developmental and epigenetic consequences of postnatal mother-infant interactions. The in utero environment is regulated by placental function and there is emerging evidence that the placenta is highly susceptible to maternal distress and a target of epigenetic dysregulation. Integrating studies of prenatal exposures, placental function, and postnatal maternal care with the exploration of epigenetic mechanisms may provide novel insights into the pathophysiology induced by maternal distress.

Is genomics bad for you?

The plasticity of the genome complicates genetic causation but should be investigated from a functional perspective. Specific adaptive hypotheses are referenced in the target article, but it is also necessary to explain how the integrity of the genome is maintained despite processes that tend towards its diversification and degradation. These include the accumulation of deleterious changes and intragenomic conflict.

Elevated expression of PEG10 in human placentas from preeclamptic pregnancies

The aim of this study was to determine qualitative and quantitative changes in paternally expressed gene 10 (PEG10) expression in preeclamptic placentas. Placental tissues were obtained immediately after delivery from women with normal pregnancies (n=20) and patients with preeclampsia (n=20). Quantitative real-time RT-PCR, Western blot analysis and immunohistochemistry were used to determine PEG10 gene expression and localization in placental tissues. Compared with the normal group, PEG10 was highly expressed at both mRNA and protein levels in preeclampsia (P<0.05). In immunohistochemical staining, PEG10 was present in the syncytiotrophoblast, cytotrophoblast, endothelial cell and stroma of all placentas. Notably, the intensity of PEG10 expression in the syncytiotrophoblast, cytotrophoblast and stem villi was much higher in preeclampsia than in normal. In conclusion, elevated expression of PEG10 is likely to be involved in the pathophysiology of preeclampsia. Further studies are needed to elucidate the precise role of PEG10 in preeclampsia.

Imprinted gene expression in fetal growth and development

Experimental studies showed that genomic imprinting is fundamental in fetoplacental development by timely regulating the expression of the imprinted genes to overlook a set of events determining placenta implantation, growth and embryogenesis. We examined the expression profile of 22 imprinted genes which have been linked to pregnancy abnormalities that may ultimately influence childhood development. The study was conducted in a subset of 106 placenta samples, overrepresented with small and large for gestational age cases, from the Rhode Island Child Health Study. We investigated associations between imprinted gene expression and three fetal development parameters: newborn head circumference, birth weight, and size for gestational age. Results from our investigation show that the maternally imprinted/paternally expressed gene ZNF331 inversely associates with each parameter to drive smaller fetal size, while paternally imprinted/maternally expressed gene SLC22A18 directly associates with the newborn head circumference promoting growth. Multidimensional Scaling analysis revealed two clusters within the 22 imprinted genes which are independently associated with fetoplacental development. Our data suggest that cluster 1 genes work by assuring cell growth and tissue development, while cluster 2 genes act by coordinating these processes. Results from this epidemiologic study offer solid support for the key role of imprinting in fetoplacental development.

Parent of origin effects

A major weakness of most genome-wide association studies has been their inability to fully explain the heritable component of complex disease. Nearly all such studies consider the two parental alleles to be functionally equivalent. However, the existence of imprinted genes demonstrates that this assumption can be wrong. In this review, we describe a wide variety of different mechanisms that underlie many other parent of origin and trans-generational effects that are known to operate in both humans and model organisms, suggesting that these phenomena are perhaps not uncommon in the genome. We propose that the consideration of alternative models of inheritance will improve our understanding of the heritability and causes of human traits and could have significant impacts on the study of complex disorders.

Re-investigation and RNA sequencing-based identification of genes with placenta-specific imprinted expression

Within the vertebrate groups, only mammals are subject to a specialized epigenetic process termed genomic imprinting in which genes are preferentially expressed from one parental allele. Imprinted expression has been reported for >100 mouse genes and, for approximately one-quarter of these genes, the imprinted expression is specific to the placenta (or extraembryonic tissues). This seemingly placenta-specific imprinted expression has garnered much attention, as has the apparent lack of conserved imprinting between the human and mouse placenta. In this study, we used a novel approach to re-investigate the placenta-specific expression using embryo transfer and trophoblast stem cells. We analyzed 20 genes previously reported to show maternal allele-specific expression in the placenta, and only 8 genes were confirmed to be imprinted. Other genes were likely to be falsely identified as imprinted due to their relatively high expression in contaminating maternal cells. Next, we performed a genome-wide transcriptome assay and identified 133 and 955 candidate imprinted genes with paternal allele- and maternal allele-specific expression. Of those we analyzed in detail, 1/6 (Gab1) of the candidates for paternal allele-specific expression and only 1/269 (Ano1) candidates for maternal allele-specific expression were authentically imprinted genes. Imprinting of Ano1 and Gab1 was specific to the placenta and neither gene displayed allele-specific promoter DNA methylation. Imprinting of ANO1, but not GAB1, was conserved in the human placenta. Our findings impose a considerable revision of the current views of placental imprinting.

Differential methylation of imprinted genes in growth-restricted placentas

A complex network of epigenetic factors participates in regulating the monoallelic expression of a small subset of genes (~1%) in the human genome. This phenomenon goes under the definition of genomic imprinting, a parent-of-origin effect that, when altered during early embryogenesis, may influence fetal development into adulthood. Pertubations in genomic imprinting have been associated with placental and fetal growth restrictions. We analyzed the differential DNA methylation of all known imprinted genes on 10 appropriate-for-gestational-age, clinically normal, placentas and 7 severe intrauterine growth-restricted placentas. Samples were pooled according to the diagnosis and analyzed by methylated DNA immunoprecipitation (MeDIP) on a tiling microarray platform. The distribution of the differentially methylated regions (DMRs) identified in growth-restricted placentas showed a slight tendency toward hypermethylation. Imprinted genes not expressed in placenta showed a unique DMR profile with the fewest hyper- and hypomethylated DMRs. Promoter and CpG island DMRs were sporadic and randomly distributed. The vast majority of DMR identified (~99%) were mapped in introns, showing no common sequence features. Also, by using the more advanced array data mining softwares, no significant patterns emerged. In contrast, differential methylation showed a highly significant correlation with gene length. Overall these data suggest that differential methylation changes in growth-restricted placentas occur throughout the genomic regions, encompassing genes actively expressed in the placenta. These findings warrant caution in interpreting the significance of genes carrying clustered DMRs because the distribution of DMRs in a gene may be attributed as a function of its length rather than as a specific biological role.

Placental development during early pregnancy in sheep: cell proliferation, global methylation, and angiogenesis in the fetal placenta

To characterize early fetal placental development, gravid uterine tissues were collected from pregnant ewes every other day from day 16 to 30 after mating. Determination of 1) cell proliferation was based on Ki67 protein immunodetection; 2) global methylation was based on 5-methyl-cytosine (5mC) expression and mRNA expression for DNA methyltransferases (DNMTs)1,3a, and3b; and 3) vascular development was based on smooth muscle cell actin immunolocalization and on mRNA expression of several factors involved in the regulation of angiogenesis in fetal membranes (FMs). Throughout early pregnancy, the labeling index (proportion of proliferating cells) was very high (21%) and did not change. Expression of 5mC and mRNA forDNMT3bdecreased, but mRNA forDNMT1and3aincreased. Blood vessels were detected in FM on days 18–30 of pregnancy, and their number per tissue area did not change. The patterns of mRNA expression for placental growth factor, vascular endothelial growth factor, and their receptorsFLT1andKDR; angiopoietins 1 and 2 and their receptorTEK; endothelial nitric oxide synthase and the NO receptorGUCY13B; and hypoxia inducing factor 1 α changed in FM during early pregnancy. These data demonstrate high cellular proliferation rates, and changes in global methylation and mRNA expression of factors involved in the regulation of DNA methylation and angiogenesis in FM during early pregnancy. This description of cellular and molecular changes in FM during early pregnancy will provide the foundation for determining the basis of altered placental development in pregnancies compromised by environmental, genetic, or other factors.

Epigenetics and the placenta

BACKGROUND

The placenta is of utmost importance for intrauterine fetal development and growth. Deregulation of placentation can lead to adverse outcomes for both mother and fetus, e.g. gestational trophoblastic disease (GTD), pre-eclampsia and fetal growth retardation. A significant factor in placental development and function is epigenetic regulation.

METHODS

This review summarizes the current knowledge in the field of epigenetics in relation to placental development and function. Relevant studies were identified by searching PubMed, Medline and reference sections of all relevant studies and reviews.

RESULTS

Epigenetic regulation of the placenta evolves during preimplantation development and further gestation. Epigenetic marks, like DNA methylation, histone modifications and non-coding RNAs, affect gene expression patterns. These expression patterns, including the important parent-of-origin-dependent gene expression resulting from genomic imprinting, play a pivotal role in proper fetal and placental development. Disturbed placental epigenetics has been demonstrated in cases of intrauterine growth retardation and small for gestational age, and also appears to be involved in the pathogenesis of pre-eclampsia and GTD. Several environmental effects have been investigated so far, e.g. ethanol, oxygen tension as well as the effect of several aspects of assisted reproduction technologies on placental epigenetics.

CONCLUSIONS

Studies in both animals and humans have made it increasingly clear that proper epigenetic regulation of both imprinted and non-imprinted genes is important in placental development. Its disturbance, which can be caused by various environmental factors, can lead to abnormal placental development and function with possible consequences for maternal morbidity, fetal development and disease susceptibility in later life.

The primate-specific microRNA gene cluster (C19MC) is imprinted in the placenta

Imprinted genes play crucial roles in mammalian development and disruption of their expression is associated with many human disorders including tumourigenesis; yet, the actual number of imprinted genes in the human genome remains a matter of debate. Here, we report on the unexpected finding that the chromosome 19 microRNA cluster (C19MC), the largest human microRNA gene cluster discovered so far, is regulated by genomic imprinting with only the paternally inherited allele being expressed in the placenta. DNA methylation profiling identified a differentially methylated region (C19MC-DMR1) that overlaps an upstream CpG-rich promoter region associated with short tandem repeats. It displays a maternal-specific methylation imprint acquired in oocytes and generates a complex population of large, compartimentalized non-coding RNA (ncRNA) species retained in close proximity to the C19MC transcription site. This occurs adjacent to, but not within, a poorly characterized nuclear Alu-rich domain. Interestingly, C19MC maps near another imprinted gene, the maternally expressed ZNF331 gene, and therefore may define a novel, previously unrecognized large imprinted primate-specific chromosomal domain. Altogether, our study adds C19MC to the growing list of imprinted repeated small RNA gene clusters and further strengthens the potential involvement of small ncRNAs in the function and/or the evolution of imprinted gene networks.

Developmental origins of health and disease: environmental exposures

The developmental origins of health and disease (DOHaD) approach has evolved over the past 20 years, and the current hypothesis proposes that fetal adaptations to intrauterine and maternal conditions during development shape structure and function of organs. Here we present a review of some environmental exposures that may trigger fetal maladaptations in these processes, including three examples: exposures to tobacco smoke, antidepressant medication, and folic acid deficits in the food supply. We provide a selected review of current research on the effects of each of these exposures on fetal development and birth outcomes, and use the DOHaD approach to suggest how these exposures may alter long-term outcomes. In the interpretation of this literature, we review the evidence of gene-environment interactions based on evaluation of biological pathways and evidence that some exposures to the fetus may be moderated by maternal and fetal genotypes. Finally, we use the design of the National Children's Study (now in progress) to propose how the DOHaD approach could be used to address questions that have emerged in this area that are relevant to reproductive medicine and subsequent health outcomes.

Developmental origins of health and disease: brief history of the approach and current focus on epigenetic mechanisms

"Barker's hypothesis" emerged almost 25 years ago from epidemiological studies of birth and death records that revealed a high geographic correlation between rates of infant mortality and certain classes of later adult deaths as well as an association between birthweight and rates of adult death from ischemic heart disease. These observations led to a theory that undernutrition during gestation was an important early origin of adult cardiac and metabolic disorders due to fetal programming that permanently shaped the body's structure, function, and metabolism and contributed to adult disease. This theory stimulated interest in the fetal origins of adult disorders, which expanded and coalesced approximately 5 years ago with the formation of an international society for developmental origins of health and disease (DOHaD). Here we review a few examples of the many emergent themes of the DOHaD approach, including theoretical advances related to predictive adaptive responses of the fetus to a broad range of environmental cues, empirical observations of effects of overnutrition and stress during pregnancy on outcomes in childhood and adulthood, and potential epigenetic mechanisms that may underlie these observations and theory. Next, we discuss the relevance of the DOHaD approach to reproductive medicine. Finally, we consider the next steps that might be taken to apply, evaluate, and extend the DOHaD approach.

Epigenetics in the placenta

Epigenetics is focused on understanding the control of gene expression beyond what is encoded in the sequence of DNA. Central to growing interest in the field is the hope that more can be learned about the epigenetic regulatory mechanisms underlying processes of human development and disease. Researchers have begun to examine epigenetic alterations - such as changes in promoter DNA methylation, genomic imprinting, and expression of miRNA - to learn more about epigenetic regulation in the placenta, an organ whose proper development and function are crucial to the health, growth, and survival of the developing fetus. A number of studies are now making important links between alterations to appropriate epigenetic regulation in the placenta and diseases of gestation and early life. In addition, these studies are adding important insight into our understanding of trophoblast biology and differentiation as well as placental immunology. Examining epigenetic alterations in the placenta will prove especially important in the search for biomarkers of exposure, pathology, and disease risk and can provide critical insights into the biology of development and pathogenesis of disease. Thus, epigenetic alterations may aid in disease diagnosis and prognosis as well as in targeting new treatment and prevention strategies.

Pathogenesis of choriocarcinoma: clinical, genetic and stem cell perspectives

Choriocarcinoma is a unique malignant neoplasm composed of mononuclear cytotrophoblasts and multinucleated syncytiotrophoblasts that produce human chorionic gonadotrophin. Choriocarcinoma can occur after a pregnancy, as a component of germ cell tumors, or in association with a poorly differentiated somatic carcinoma, each with distinct clinical features. Cytogenetic and molecular studies, predominantly on gestational choriocarcinoma, revealed the impact of oncogenes, tumor suppressor genes and imprinting genes on its pathogenesis. The role of stem cells in various types of choriocarcinoma has been studied recently. This review will discuss how such knowledge can enhance our understanding of the pathogenesis of choriocarcinoma, enable exploration of novel anti-choriocarcinoma targeted therapy and possibly improve our insight on embryological and placental development.

Hypomethylation at multiple maternally methylated imprinted regions including PLAGL1 and GNAS loci in Beckwith-Wiedemann syndrome

Genomic imprinting is an epigenetic phenomenon restricting gene expression in a manner dependent on parent of origin. Imprinted gene products are critical regulators of growth and development, and imprinting disorders are associated with both genetic and epigenetic mutations, including disruption of DNA methylation within the imprinting control regions (ICRs) of these genes. It was recently reported that some patients with imprinting disorders have a more generalised imprinting defect, with hypomethylation at a range of maternally methylated ICRs. We report a cohort of 149 patients with a clinical diagnosis of Beckwith-Wiedemann syndrome (BWS), including 81 with maternal hypomethylation of the KCNQ1OT1 ICR. Methylation analysis of 11 ICRs in these patients showed that hypomethylation affecting multiple imprinted loci was restricted to 17 patients with hypomethylation of the KCNQ1OT1 ICR, and involved only maternally methylated loci. Both partial and complete hypomethylation was demonstrated in these cases, suggesting a possible postzygotic origin of a mosaic imprinting error. Some ICRs, including the PLAGL1 and GNAS/NESPAS ICRs implicated in the aetiology of transient neonatal diabetes and pseudohypoparathyroidism type 1b, respectively, were more frequently affected than others. Although we did not find any evidence for mutation of the candidate gene DNMT3L, these results support the hypotheses that trans-acting factors affect the somatic maintenance of imprinting at multiple maternally methylated loci and that the clinical presentation of these complex cases may reflect the loci and tissues affected with the epigenetic abnormalities.

Genomic imprinting in the development and evolution of psychotic spectrum conditions

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

Epigenetic consequences of assisted reproduction and infertility on the human preimplantation embryo

Epigenetic information, which is essential for normal mammalian development, is acquired during gametogenesis and further regulated during preimplantation development. The epigenetic consequences of assisted reproductive technologies (ARTs) and infertility on the health and quality of the human preimplantation embryo are considered in this review. In the zygote, the epigenetic information that is inherited from the sperm and the oocyte intersects and must be appropriately recognized, regulated and then propagated during preimplantation development so as to regulate gene expression in an appropriate manner. A growing body of evidence suggests that ARTs and/or infertility itself may affect these complex processes leading to epigenetic diseases that include disorders of genomic imprinting. The epigenetic safety of human gametes and embryos is of paramount importance. Unfortunately, morphological methods of assessing embryo quality are incapable of detecting epigenetic errors. Further research is therefore critical to resolve these issues.

Imprinted genes and neuroendocrine function

Imprinted genes are monoallelically expressed in a parent-of-origin dependent manner. Whilst the full functional repertoire of these genes remains obscure, they are generally highly expressed in the brain and are often involved in fundamental neural processes. Besides influencing brain neurochemistry, imprinted genes are important in the development and function of the hypothalamus and pituitary gland, key sites of neuroendocrine regulation. Moreover, imprinted genes may directly modulate hormone-dependent signalling cascades, both in the brain and elsewhere. Much of our knowledge about imprinted gene function has come from studying knockout mice and human disorders of imprinting. One such disorder is Prader-Willi syndrome, a neuroendocrine disorder characterised by hypothalamic abnormalities and aberrant feeding behaviour. Through examining the role of imprinted genes in neuroendocrine function, it may be possible to shed light on the neurobiological basis of feeding and aspects of social behaviour and underlying cognition, and to provide insights into disorders where these functions go awry.

Genes and the preeclampsia syndrome

Preeclampsia is specific to pregnancy and is still a leading cause of maternal and perinatal mortality and morbidity, affecting about 3% of women, but the underlying pathogenetic mechanisms still remain unclear. Immune maladaptation, placental ischemia and increased oxidative stress represent the main components discussed to be of etiologic importance, and they all may have genetic implications. Since the familial nature of preeclampsia is known for many years, extensive research on the genetic contribution to the pathogenesis of this severe pregnancy disorder has been performed. In this review, we will overview the linkage and candidate gene studies carried out so far as well as summarize important historical notes on the genetic hypotheses generated in preeclampsia research. Moreover, the influence of maternal and fetal genes and their interaction as well as the role of genomic imprinting in preeclampsia will be discussed.

What are imprinted genes doing in the brain?

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

STOX1 gene in pre-eclampsia and intrauterine growth restriction

The STOX1 gene, identified as a candidate gene for pre-eclampsia in Dutch women, is placentally expressed and subject to imprinting with preferential transmission of the maternal allele. In our study, STOX1-Y153H frequencies were similar in 157 women with pre-eclampsia (65%) and in 157 controls (64%) from the general Dutch population. In an isolated Dutch population, a distortion could not be demonstrated in the transmission of STOX1-Y153H variation from heterozygous mothers to offspring in 50 and 56 families with pregnancies complicated by pre-eclampsia or intrauterine growth restriction, respectively. Our findings do not confirm previous suggestions that STOX1 plays a major role in Dutch women with pre-eclampsia.

Cdkn1c (p57Kip2) is the major regulator of embryonic growth within its imprinted domain on mouse distal chromosome 7

Background

Cdkn1c encodes an embryonic cyclin-dependant kinase inhibitor that acts to negatively regulate cell proliferation and, in some tissues, to actively direct differentiation. This gene, which is an imprinted gene expressed only from the maternal allele, lies within a complex region on mouse distal chromosome 7, called the IC2 domain, which contains several other imprinted genes. Studies on mouse embryos suggest a key role for genomic imprinting in regulating embryonic growth and this has led to the proposal that imprinting evolved as a consequence of the mismatched contribution of parental resources in mammals.

Results

In this study, we characterised the phenotype of mice carrying different copy number integrations of a bacterial artificial chromosome spanning Cdkn1c. Excess Cdkn1c resulted in embryonic growth retardation that was dosage-dependent and also responsive to the genetic background. Two-fold expression of Cdkn1c in a subset of tissues caused a 10–30% reduction in embryonic weight, embryonic lethality and was associated with a reduction in the expression of the potent, non-imprinted embryonic growth factor, Igf1. Conversely, loss of expression of Cdkn1c resulted in embryos that were 11% heavier with a two-fold increase in Igf1.

Conclusion

We have shown that embryonic growth in mice is exquisitely sensitive to the precise dosage of Cdkn1c. Cdkn1c is a maternally expressed gene and our findings support the prediction of the parental conflict hypothesis that that the paternal genome silences genes that have an inhibitory role in embryonic growth. Within the IC2 imprinted domain, Cdkn1c encodes the major regulator of embryonic growth and we propose that Cdkn1c was the focal point of the selective pressure for imprinting of this domain.

Upregulation of PHLDA2 in Dicer knockdown HEK293 cells

It has been reported that RNAi-dependent chromatin silencing in vertebrates is not restricted to the centromeres. To address whether RNAi machinery could regulate the chromatin structure of imprinted genes, we knocked down Dicer in HEK293 cells and found that the expression of PHLDA2, one of the several genes in the imprinted gene domain of 11p15.5, was specifically upregulated. This was accompanied by a shift towards more activated chromatin at PHLDA2 locus as indicated by change in H3K9 acetylation, however, the methylation state at this locus was not affected. Furthermore, we found that PHLDA2 was downregulated in growth-arrested HEK293 cells induced by either serum deprivation or contact inhibition. This suggests that PHLDA2 upregulation might be a direct result of Dicer depletion rather than the consequence of growth arrest induced by Dicer knockdown. Considering the reports that there is consistent placental outgrowth in PHLDA2 knockout mice and that PHLDA2 overexpression in mice causes growth inhibition, we speculate that PHLDA2 may be a candidate for contributing to the reduced growth rate of Dicer-deficient cells and the very early embryonic lethality in Dicer knockout mice.

Methylation analysis of the intergenic differentially methylated region of DLK1-GTL2 in human

Imprinting is a non-Mendelian form of inheritance where epigenetic modifications control mono-allelic expression depending on the parental origin. Methylation of CpG-dinucleotides at differentially methylated regions (DMRs) is one of the best-studied mechanisms directing expression to one specific parental allele. We studied the methylation patterns of the intergenic (IG)-DMR of DLK1 and GTL2. The methylation marks of the IG-DMR were analysed in human gametes, preimplantation embryos, amniocytes and blood of babies born after intracytoplasmic sperm injection (ICSI) and blood from adults using a bisulphite sequencing technique. In oocytes, the IG-DMR was mainly unmethylated while in sperm cells a generally methylated pattern was detected. This germ-line specific methylation mark was maintained in the preimplantation embryos until the second cleavage stage. Afterwards in the preimplantation embryos, intermediate methylation patterns (26-74% methylation) occurred, which may point to relaxation of the imprints. Intermediate patterns were also present in amniocytes, blood from ICSI babies and adults. We hypothesise that in the early cleavage stage embryo a strict differential methylation pattern is needed for the correct imprint establishment of surrounding imprinted genes. Once correct imprinting of the involved gene(s) is acquired, a more relaxed state of the IG-region is allowed.