Rebecca A. Simmons

Epigenetics and maternal nutrition: nature v. nurture.

Under- and over-nutrition during pregnancy has been linked to the later development of diseases such as diabetes and obesity. Epigenetic modifications may be one mechanism by which exposure to an altered intrauterine milieu or metabolic perturbation may influence the phenotype of the organism much later in life. Epigenetic modifications of the genome provide a mechanism that allows the stable propagation of gene expression from one generation of cells to the next. This review highlights our current knowledge of epigenetic gene regulation and the evidence that chromatin remodelling and histone modifications play key roles in adipogenesis and the development of obesity. Epigenetic modifications affecting processes important to glucose regulation and insulin secretion have been described in the pancreatic β-cells and muscle of the intrauterine growth-retarded offspring, characteristics essential to the pathophysiology of type-2 diabetes. Epigenetic regulation of gene expression contributes to both adipocyte determination and differentiation in in vitro models. The contributions of histone acetylation, histone methylation and DNA methylation to the process of adipogenesis in vivo remain to be evaluated.

Metabolic programming, epigenetics, and gestational diabetes mellitus.

The link between an adverse intrauterine environment and the development of disease later in life has been observed in offspring of pregnancies complicated by obesity and diabetes, but the molecular mechanisms underlying this phenomenon are unknown. In this review, we highlight recent publications exploring the role of gestational diabetes mellitus in the programming of disease in the offspring. We also review recent publications aiming to identify mechanisms responsible for the “programming effect” that results from exposure to diabetes in utero. Finally, we highlight research on the role of epigenetic regulation of gene expression in an animal model of uteroplacental insufficiency where the offspring develop diabetes as a model by which an exposure to the mother can alter epigenetic modifications that affect expression of key genes and ultimately lead to the development of diabetes in the offspring.

Fetal iron deficiency induces chromatin remodeling at the Bdnf locus in adult rat hippocampus

Fetal and subsequent early postnatal iron deficiency causes persistent impairments in cognitive and affective behaviors despite prompt postnatal iron repletion. The long-term cognitive impacts are accompanied by persistent downregulation of brain-derived neurotrophic factor (BDNF), a factor critical for hippocampal plasticity across the life span. This study determined whether early-life iron deficiency epigenetically modifies the Bdnf locus and whether dietary choline supplementation during late gestation reverses these modifications. DNA methylation and histone modifications were assessed at the Bdnf-IV promoter in the hippocampus of rats [at postnatal day (PND) 65] that were iron-deficient (ID) during the fetal-neonatal period. Iron deficiency was induced in rat pups by providing pregnant and nursing dams an ID diet (4 mg/kg Fe) from gestational day (G) 2 through PND7, after which iron deficiency was treated with an iron-sufficient (IS) diet (200 mg/kg Fe). This paradigm resulted in about 60% hippocampal iron loss on PND15 with complete recovery by PND65. For choline supplementation, pregnant rat dams were given dietary choline (5 g/kg) from G11 through G18. DNA methylation was determined by quantitative sequencing of bisulfite-treated DNA, revealing a small alteration at the Bdnf-IV promoter. Chromatin immunoprecipitation analysis showed increased HDAC1 binding accompanied by reduced binding of RNA polymerase II and USF1 at the Bdnf-IV promoter in formerly ID rats. These changes were correlated with altered histone methylations. Prenatal choline supplementation reverses these epigenetic modifications. Collectively, the findings identify epigenetic modifications as a potential mechanism to explicate the long-term repression of Bdnf following fetal and early postnatal iron deficiency.

Abnormalities of Fetal Growth

Normal fetal growth is determined by a number of factors, including genetic potential, the ability of the mother to provide sufficient nutrients, the ability of the placenta to transfer nutrients, and intrauterine hormones and growth factors. The pattern of normal fetal growth involves rapid increases in fetal weight, length, and head circumference during the last half of gestation. During the last trimester, the human fetus accumulates significant amounts of fat. The weight for gestational measurements among populations has been shown to increase over time, and thus standards for normal fetal growth require periodic re-evaluation for clinical relevance. These increases in weight for gestational age over time are attributed to improvements in living conditions and maternal nutrition and changes in obstetric management. Variations in fetal growth have been identified in diverse populations and are associated with geographic locations (sea level versus high altitude), populations (white, African-American, Latino), maternal constitutional factors, parity, maternal nutrition, fetal gender, and multiple gestations. In this chapter, we discuss these factors in greater detail and critically review the long-term effects of abnormal fetal growth.

Programming of DNA methylation in type 2 diabetes

Type 2 diabetes mellitus is the most common metabolic disorder worldwide. It is a complex metabolic disease that is caused by insulin resistance and beta cell dysfunction. There is strong evidence that type 2 diabetes exemplifies the complex interactions between environmental and genetic determinants. During the last decade, considerable progress has been made in the identification of type 2 diabetes risk genes. This is largely a result of the development of affordable high-density single-nucleotide polymorphism (SNP) arrays that have enabled several successful genome-wide association scans to be carried out in large case-control cohorts. The first type 2 diabetes-associated variant to be identified was in the TCF7L2 gene, which encodes a transcription factor in the Wnt signalling pathway (1). To date, it remains the variant with the largest effect size; each copy of the T allele at rs7903416 carries a 1.4–1.5 increased odds of type 2 diabetes (2, 3). Candidate gene studies identified variants in PPARG and KCNJ11 (4, 5). Genome-wide association studies and the sequencing of the entire human genome have made possible the development of genotyping platforms consisting of only a few hundred thousand SNPs that could statistically capture the majority of genetic variation in the human genome. These studies have identified several more variants with smaller effect sizes. Independent replicated genetic type 2 diabetes associations now numbers 38, with most variants increasing an individual’s relative odds of type 2 diabetes by a modest 5–15%. n nThe research on these identified loci is, however, still at an early stage, and the translation into molecular mechanisms is a big challenge as the majority are located far from any transcribed gene or near genes without any evident link to diabetes. A more extensive knowledge of transcription regulation mechanisms and the impact of differential environmental exposures is likely to provide insight into the relationship between genetics and environment. Epigenetics is starting to shed light on this complex issue, as epigenetic modifications are important regulators of transcription and are, at the same time, modifiable by the environment. Epigenetics often refers to changes in gene expression that take place without a change in the DNA sequence. The most extensively studied epigenetic changes are DNA methylation, which predominantly take place at the carbon-5 position of cytosine in a cytosine–phosphate–guanine (CpG) dinucleotide. Approximately 70% of CpG dinucleotides in human DNA are constitutively methylated, and most of the unmethylated CpGs are located in CpG islands. CpG islands are CG-rich sequences located near coding sequences that serve as promoters for their associated genes. While it is well established that DNA hypermethylation at promoters and repetitive sequences is involved in stable silencing, it is largely unclear whether non-promoter DNA methylation contributes to regulation of gene expression. Furthermore, it is not yet certain whether methylation of single CpG sites in intergenic regions regulates gene expression. It is possible that these sites may represent transcription factor binding sites and serve as distal regulatory elements. n nIn recent years, we have begun to appreciate how genetic variants may impact epigenetic processes. A number of studies have now demonstrated that genomic variants can affect the methylation status of immediately surrounding CpGs in the region (6–9). The mechanisms responsible for changes in DNA methylation in these regions remain to be fully elucidated. A genetic variant can also introduce or remove a CpG site, which has the potential to influence DNA methylation at that site. While these changes in DNA methylation have the potential to influence gene expression as described above, it remains to be determined if this indeed the case. n nIn this issue of Diabetologia, Dayeh et al report that 19 of 40 SNPs associated with type 2 diabetes introduce or remove a CpG site. They assayed DNA methylation of these CpG-SNPs in islets of non-diabetic human donors and found that DNA methylation of all of the CpG-SNPs was significantly different from the wild-type sequences. The expression of genes located 500 kb up- or downstream of the CpG-SNP was measured using microarray analysis, and two CpG-SNPs with increased methylation were found to also have decreased mRNA expression in human islets. However, the changes in expression were quite small. This could be due to analysis by microarray rather than quantitative PCR, and it is possible that these changes could be greater. To determine whether changes in DNA methylation at CpG-SNPs were correlated with changes in islet cell function, insulin and glucagon secretion were measured in isolated islets. Islets from individuals with CpG-SNPs in ADCY5 and HHEX (candidate genes regulating insulin secretion) and CDKN2A (candidate gene thought to influence insulin content) were found to have lower insulin content and insulin secretion, whereas glucagon secretion was higher in islets with CpG-SNPs in ADCY5 and KCNQ1 (genes regulating glucagon secretion). There was substantial variability in these measures, which is to be expected in human islets and it is not clear how strong the association is between the type 2 diabetes CpG-SNPs with differential DNA methylation and islet function. Nevertheless, these findings are intriguing and represent the first studies examining CpG-SNPs and DNA methylation in human islets. If these findings can be replicated in peripheral blood cells of living individuals, this may enable the identification of population subgroups at particularly high risk for type 2 diabetes and facilitate the targeting of preventative efforts to those who might benefit from them the most.

Diet-induced obesity alters the maternal metabolome and early placenta transcriptome and decreases placenta vascularity in the mouse†

Abstract Maternal obesity is associated with an increased risk of obesity and metabolic disease in offspring. Increasing evidence suggests that the placenta plays an active role in fetal programming. In this study, we used a mouse model of diet-induced obesity to demonstrate that the abnormal metabolic milieu of maternal obesity sets the stage very early in pregnancy by altering the transcriptome of placenta progenitor cells in the preimplantation (trophectoderm [TE]) and early postimplantation (ectoplacental cone [EPC]) placenta precursors, which is associated with later changes in placenta development and function. Sphingolipid metabolism was markedly altered in the plasma of obese dams very early in pregnancy as was expression of genes related to sphingolipid processing in the early placenta. Upregulation of these pathways inhibits angiogenesis and causes endothelial dysfunction. The expression of many other genes related to angiogenesis and vascular development were disrupted in the TE and EPC. Other key changes in the maternal metabolome in obese dams that are likely to influence placenta and fetal development include a marked decrease in myo and chiro-inositol. These early metabolic and gene expression changes may contribute to phenotypic changes in the placenta, as we found that exposure to a high-fat diet decreased placenta microvessel density at both mid and late gestation. This is the first study to demonstrate that maternal obesity alters the transcriptome at the earliest stages of murine placenta development. Summary Sentence Obesity in a mouse model leads to alterations in the maternal metabolome and early placenta transcriptome as well as changes in vascularity later in gestation which may provide a mechanism for decreased fetal growth.

Adverse effects of small for gestational age differ by gestational week among very preterm infants

Objective To characterise the excess risk for death, grade 3–4 intraventricular haemorrhage (IVH), bronchopulmonary dysplasia (BPD) and stage 3–5 retinopathy of prematurity independently associated with birth small for gestational age (SGA) among very preterm infants, stratified by completed weeks of gestation. Methods Retrospective cohort study using the Optum Neonatal Database. Study infants were born <32 weeks gestation without severe congenital anomalies. SGA was defined as a birth weight <10th percentile. The excess outcome risk independently associated with SGA birth among SGA babies was assessed using adjusted risk differences (aRDs). Results Of 6708 infants sampled from 717 US hospitals, 743 (11.1%) were SGA. SGA compared with non-SGA infants experienced higher unadjusted rates of each study outcome except grade 3–4 IVH among survivors. The excess risk independently associated with SGA birth varied by outcome and gestational age. The highest aRD for death (0.27; 95%u2009CI 0.13 to 0.40) occurred among infants born at 24 weeks gestation and declined as gestational age increased. In contrast, the peak aRDs for BPD among survivors (0.32; 95%u2009CI 0.20 to 0.44) and the composites of death or BPD (0.35; 95%u2009CI 0.24 to 0.46) and death or major morbidity (0.35; 95%u2009CI 0.24 to 0.45) occurred at 27 weeks gestation. The risk-adjusted probability of dying or developing one or more of the evaluated morbidities among SGA infants was similar to that of non-SGA infants born approximately 2–3 weeks less mature. Conclusion The excess risk for neonatal morbidity and mortality associated with being born SGA varies by adverse outcome and gestational age.

Mice exposed to bisphenol A exhibit depressive-like behavior with neurotransmitter and neuroactive steroid dysfunction

ABSTRACT Fetal exposure to endocrine disrupting chemicals (EDCs) has been associated with adverse neurobehavioral outcomes across the lifespan and can persist across multiple generations of offspring. However, the underlying mechanisms driving these changes are not well understood. We investigated the molecular perturbations associated with EDC‐induced behavioral changes in first (F1) and second (F2) filial generations, using the model EDC bisphenol A (BPA). C57BL/6J dams were exposed to BPA from preconception until lactation through the diet at doses (10 &mgr;g/kg bw/d‐lower dose or 10 mg/kg bw/d‐upper dose) representative of human exposure levels. As adults, F1 male offspring exhibited increased depressive‐like behavior, measured by the forced swim test, while females were unaffected. These behavioral changes were limited to the F1 generation and were not associated with altered maternal care. Transcriptome analysis by RNA‐sequencing in F1 control and upper dose BPA‐exposed adult male hippocampus revealed neurotransmitter systems as major pathways disrupted by developmental BPA exposure. High performance liquid chromatography demonstrated a male‐specific reduction in hippocampal serotonin. Administration of the selective serotonin reuptake inhibitor fluoxetine (20 mg/kg bw) rescued the depressive‐like phenotype in males exposed to lower, but not upper, dose BPA, suggesting distinct mechanisms of action for each exposure dose. Finally, high resolution mass spectrometry revealed reduced circulating levels of the neuroactive steroid dehydroepiandrosterone in BPA‐exposed males, suggesting another potential mechanism underlying the depressive‐like phenotype. Thus, behavioral changes associated with early life BPA exposure may be mediated by sex‐specific disruptions in the serotonergic system and/or sex steroid biogenesis in male offspring. HighlightsDevelopmental BPA exposure is associated with adult behavioral despair in F1 males.Affective behavior in BPA‐exposed F1 female and F2 males is not affected.BPA exposure is associated with altered hippocampal neurotransmitter systems.Antidepressant rescues BPA exposure phenotype in a dose‐specific manner.Neuroactive steroid, dehydroepiandrosterone, is reduced in BPA‐exposed male serum.

The impact of the maternal–foetal environment on outcomes of surgery for congenital heart disease in neonates†

OBJECTIVESnPregnancies with congenital heart disease in the foetus have an increased prevalence of pre-eclampsia, small for gestational age and preterm birth, which are evidence of an impaired maternal-foetal environment (MFE).nnnMETHODSnThe impact of an impaired MFE, defined as pre-eclampsia, small for gestational age or preterm birth, on outcomes after cardiac surgery was evaluated in neonates (nu2009=u2009135) enrolled in a study evaluating exposure to environmental toxicants and neuro-developmental outcomes.nnnRESULTSnThe most common diagnoses were transposition of the great arteries (nu2009=u200947) and hypoplastic left heart syndrome (nu2009=u200943). Impaired MFE was present in 28 of 135 (21%) subjects, with small for gestational age present in 17 (61%) patients. The presence of an impaired MFE was similar for all diagnoses, except transposition of the great arteries (Pu2009<u20090.006). Postoperative length of stay was shorter for subjects without an impaired MFE (14 vs 38u2009days, Pu2009<u20090.001). Hospital mortality was not significantly different with or without impaired MFE (11.7% vs 2.8%, Pu2009=u20090.104). However, for the entire cohort, survival at 36u2009months was greater for those without an impaired MFE (96% vs 68%, Pu2009=u20090.001). For patients with hypoplastic left heart syndrome, survival was also greater for those without an impaired MFE (90% vs 43%, Pu2009=u20090.007).nnnCONCLUSIONSnAn impaired MFE is common in pregnancies in which the foetus has congenital heart disease. After cardiac surgery in neonates, the presence of an impaired MFE was associated with lower survival at 36u2009months of age for the entire cohort and for the subgroup with hypoplastic left heart syndrome.