Epigenetics: A Historical Overview

Postmigratory mouse primordial germ cells (PGCs) undergo extensive epigenetic remodeling that includes DNA methylation (DM) reprogramming of imprinted genes and, surprisingly, of transposable elements (TEs). Given the danger posed by TEs to the integrity of the germline, even a brief derepression of TEs is counterintuitive and puzzling. In the male fetal gonocytes, a sophisticated repressive mechanism that uses DM and TE-targeting piRNAs has evolved to stably silence TEs. A recent study has further increased the complexity of this problem by revealing that TE silencing is alleviated specifically at the onset of meiosis in testes lacking MAEL, a piRNA pathway protein. These observations and prior work of others are consistent with existence of an additional reprogramming event, transient relaxation of transposon silencing (TRTS), at the onset of both male and female meiosis in mice. In this Point of View we propose that TE expression is inherent to mammalian meiosis and discuss potential functional significance of this phenomenon.     

建立胎儿生长受限动物模型的研究进展

动物模型法对于人类疾病病因学及病理生理学的研究有着独特的作用。将动物模型法引入对胎儿生长受限（fetal growth restriction,FGR）的探索,不仅可以让我们观察FGR患者及胎儿在不同孕期各自的表现,还可以让我们对发病胎儿在出生后不同年龄阶段的自然表现进行继续研究。人们在FGR动物模型的建立和利用方面已经积累了大量的宝贵经验,这为进一步阐明FGR的病因学及发病机制创造了条件。     

A Computational Model of the Fetal Circulation to Quantify Blood Redistribution in Intrauterine Growth Restriction

Intrauterine growth restriction (IUGR) due to placental insufficiency is associated with blood flow redistribution in order to maintain delivery of oxygenated blood to the brain. Given that, in the fetus the aortic isthmus (AoI) is a key arterial connection between the cerebral and placental circulations, quantifying AoI blood flow has been proposed to assess this brain sparing effect in clinical practice. While numerous clinical studies have studied this parameter, fundamental understanding of its determinant factors and its quantitative relation with other aspects of haemodynamic remodeling has been limited. Computational models of the cardiovascular circulation have been proposed for exactly this purpose since they allow both for studying the contributions from isolated parameters as well as estimating properties that cannot be directly assessed from clinical measurements. Therefore, a computational model of the fetal circulation was developed, including the key elements related to fetal blood redistribution and using measured cardiac outflow profiles to allow personalization. The model was first calibrated using patient-specific Doppler data from a healthy fetus. Next, in order to understand the contributions of the main parameters determining blood redistribution, AoI and middle cerebral artery (MCA) flow changes were studied by variation of cerebral and peripheral-placental resistances. Finally, to study how this affects an individual fetus, the model was fitted to three IUGR cases with different degrees of severity. In conclusion, the proposed computational model provides a good approximation to assess blood flow changes in the fetal circulation. The results support that while MCA flow is mainly determined by a fall in brain resistance, the AoI is influenced by a balance between increased peripheral-placental and decreased cerebral resistances. Personalizing the model allows for quantifying the balance between cerebral and peripheral-placental remodeling, thus providing potentially novel information to aid clinical follow up.     

在437例高血压孕妇中胎儿生长受限的"权重"

目的:评价胎儿生长受限(FGR)对高血压和先兆子痫患者结局的影响.方法:对437例使用钙离子拮抗剂的妊高症患者进行回顾性研究,并将其分为四组:妊高症-适于胎龄儿组(GH-AGA)组:244例;妊高症-胎儿生长受限组(GH-FGR):78例;先兆子痫-适于胎龄儿组(PE-AGA):76例;先兆子痫-胎儿生长受限组(PE-FGR):39例.测定指标:是否需要二线治疗,是否延期妊娠,是否增加产褥期时间,分娩年龄,新生儿出生体重,围产期死亡率和是否发生新生儿畸形.结果:存在胎儿生长受限的妊高症和先兆子痫患者的二线治疗明显增加:GH-AGA组:15.4%;GH-FGR组:32.8%;PE-AGA组:28%;PE-FGR组:50%.在妊高症和先兆子痫的患者中与适于胎龄儿组相比,胎儿生长受限组延期妊娠上有统计学差异:(31.3±5.4vs20.7±3.4天和35.3±4.5vs22.2±3.1天;P＜0.001).分娩年龄(P＜0.001):35.5±2.3岁vs35.6±2.5岁和34.4±1.7岁vs33.1±2.3岁.出生体重明显不同:(P＜0.001):2,271±759.1克vs1,817.59±396.9克和2,196±685.17 vs1,465.80±441.7克.死亡率GH-FGR组2.56%(2例)和PE-FGR组10.2%(4例).没有新生儿畸形发生.结论:妊高症和先兆子痫增加了低出生体重的风险,另一方面FGR对高血压和先兆子痫患者的结局,围产期发病率和胎儿死亡率以及对孕妇进行有效的管理和治疗是一个决定性因素.     

Sildenafil Citrate Increases Fetal Weight in a Mouse Model of Fetal Growth Restriction with a Normal Vascular Phenotype

Fetal growth restriction (FGR) is defined as the inability of a fetus to achieve its genetic growth potential and is associated with a significantly increased risk of morbidity and mortality. Clinically, FGR is diagnosed as a fetus falling below the 5^th centile of customised growth charts. Sildenafil citrate (SC, Viagra™), a potent and selective phosphodiesterase-5 inhibitor, corrects ex vivo placental vascular dysfunction in FGR, demonstrating potential as a therapy for this condition. However, many FGR cases present without an abnormal vascular phenotype, as assessed by Doppler measures of uterine/umbilical artery blood flow velocity. Thus, we hypothesized that SC would not increase fetal growth in a mouse model of FGR, the placental-specific Igf2 knockout mouse, which has altered placental exchange capacity but normal placental blood flow. Fetal weights were increased (by 8%) in P0 mice following maternal SC treatment (0.4 mg/ml) via drinking water. There was also a trend towards increased placental weight in treated P0 mice (P = 0.056). Additionally, 75% of the P0 fetal weights were below the 5^th centile, the criterion used to define human FGR, of the non-treated WT fetal weights; this was reduced to 51% when dams were treated with SC. Umbilical artery and vein blood flow velocity measures confirmed the lack of an abnormal vascular phenotype in the P0 mouse; and were unaffected by SC treatment. ¹⁴C-methylaminoisobutyric acid transfer (measured to assess effects on placental nutrient transporter activity) per g placenta was unaffected by SC, versus untreated, though total transfer was increased, commensurate with the trend towards larger placentas in this group. These data suggest that SC may improve fetal growth even in the absence of an abnormal placental blood flow, potentially affording use in multiple sub-populations of individuals presenting with FGR.     

胎儿生长受限模型及其胎盘的病理学观察

目的证实胎儿生长受限大鼠模型的建立方法;观察胎儿生长受限(FGR)大鼠胎盘的组织形态及超微结构特征。方法健康Wistar雌鼠24只,按妊娠先后顺序随机分为两组:正常对照组(正常组)、模型组(烟酒处理组),采用烟酒混合因素建立大鼠FGR模型;比较两组胎鼠的体重、鼻臀长度、体重系数及FGR发生率,两组胎盘组织行HE染色,并应用光镜和电镜观察其病理变化。结果①模型组胎仔平均体重、鼻臀长度、体重系数较正常组分别减轻46.0%、21.9%、35.0%(P0.01)。对照组FGR发生率仅为12.5%,模型组FGR发生率为79.3%,显著高于对照组(P0.01)。②模型组胎盘形态学明显改变。结论通过烟酒干预的方法,成功建立缺血缺氧性FGR孕鼠模型。烟酒可导致胎盘形态结构的改变,这种病理改变是造成FGR胎盘功能减退的形态学基础。     

An EG-VEGF-Dependent Decrease in Homeobox Gene NKX3.1 Contributes to Cytotrophoblast Dysfunction: A Possible Mechanism in Human Fetal Growth Restriction

Idiopathic fetal growth restriction (FGR) is frequently associated with placental insufficiency. Previous reports have provided evidence that endocrine gland–derived vascular endothelial growth factor (EG-VEGF), a placental secreted protein, is expressed during the first trimester of pregnancy, controls both trophoblast proliferation and invasion, and its increased expression is associated with human FGR. In this study, we hypothesize that EG-VEGF-dependent changes in placental homeobox gene expressions contribute to trophoblast dysfunction in idiopathic FGR. The changes in EG-VEGF-dependent homeobox gene expressions were determined using a homeobox gene cDNA array on placental explants of 8–12 wks gestation after stimulation with EG-VEGF in vitro for 24 h. The homeobox gene array identified a greater-than-five-fold increase in HOXA9, HOXC8, HOXC10, HOXD1, HOXD8, HOXD9 and HOXD11, while NKX 3.1 showed a greater-than-two-fold decrease in mRNA expression compared with untreated controls. Homeobox gene NKX3.1 was selected as a candidate because it is a downstream target of EG-VEGF and its expression and functional roles are largely unknown in control and idiopathic FGR-affected placentae. Real-time PCR and immunoblotting showed a significant decrease in NKX3.1 mRNA and protein levels, respectively, in placentae from FGR compared with control pregnancies. Gene inactivation in vitro using short-interference RNA specific for NKX3.1 demonstrated an increase in BeWo cell differentiation and a decrease in HTR-8/SVneo proliferation. We conclude that the decreased expression of homeobox gene NKX3.1 downstream of EG-VEGF may contribute to the trophoblast dysfunction associated with idiopathic FGR pregnancies.     

Prognostic Value of Umbilical and Cerebral Doppler in Fetal Growth Restriction: Comparison of Dichorionic Twins and Singletons

Objective

To compare the prognostic value of fetal Doppler in dichorionic twins and singletons by measuring the interval between diagnosis of an abnormal Doppler flow and birth in fetuses who are small for gestational age (SGA).

Design

Comparative retrospective study using a prospectively collected database.

Setting

A level 3 maternity unit in France.

Population

Fetuses from singleton and dichorionic pregnancies who are SGA (vascular or unexplained), defined by an abdominal circumference (AC) measurement below the 10^th percentile and confirmed by a birth weight below the 10^th percentile.

Methods

Fisher''s exact and Chi-2 tests were used to compare frequencies, and the Mann-Whitney-Wilcoxon test was used to compare medians in non-Gaussian distributions.

Main outcome measures

Both neonatal outcomes and intervals between the first Doppler abnormality and birth were compared in the groups of dichorionic twins and singletons.

Results

Obstetric and neonatal outcome were similar in the 104 SGA dichorionic twins and 170 SGA singletons. Abnormalities of umbilical artery Doppler, regardless of type, appeared at the same frequency in both groups (52.9%) but were identified earlier in twins (25 versus 28 weeks, p = 0.02). Among fetuses with abnormal Doppler flow, the interval between the finding and birth was significantly longer in the twins than the singletons (44 vs 15 days, p<0.01).

Conclusions

The prognostic value of an abnormal Doppler finding for the course of a pregnancy may be different in dichorionic twins and singletons. The management of women carrying SGA twins and the information provided to them should take these results into account.     

Chronic Protein Restriction in Mice Impacts Placental Function and Maternal Body Weight before Fetal Growth

Mechanisms of resource allocation are essential for maternal and fetal survival, particularly when the availability of nutrients is limited. We investigated the responses of feto-placental development to maternal chronic protein malnutrition to test the hypothesis that maternal low protein diet produces differential growth restriction of placental and fetal tissues, and adaptive changes in the placenta that may mitigate impacts on fetal growth. C57BL/6J female mice were fed either a low-protein diet (6% protein) or control isocaloric diet (20% protein). On embryonic days E10.5, 17.5 and 18.5 tissue samples were prepared for morphometric, histological and quantitative RT-PCR analyses, which included markers of trophoblast cell subtypes. Potential endocrine adaptations were assessed by the expression of Prolactin-related hormone genes. In the low protein group, placenta weight was significantly lower at E10.5, followed by reduction of maternal weight at E17.5, while the fetuses became significantly lighter no earlier than at E18.5. Fetal head at E18.5 in the low protein group, though smaller than controls, was larger than expected for body size. The relative size and shape of the cranial vault and the flexion of the cranial base was affected by E17.5 and more severely by E18.5. The junctional zone, a placenta layer rich in endocrine and energy storing glycogen cells, was smaller in low protein placentas as well as the expression of Pcdh12, a marker of glycogen trophoblast cells. Placental hormone gene Prl3a1 was altered in response to low protein diet: expression was elevated at E17.5 when fetuses were still growing normally, but dropped sharply by E18.5 in parallel with the slowing of fetal growth. This model suggests that nutrients are preferentially allocated to sustain fetal and brain growth and suggests the placenta as a nutrient sensor in early gestation with a role in mitigating impacts of poor maternal nutrition on fetal growth.     

Lack of Thromboxane Synthase Prevents Hypertension and Fetal Growth Restriction after High Salt Treatment during Pregnancy

Preeclampsia (PE) is a potentially fatal pregnancy-related hypertensive disorder characterized by poor placenta development that can cause fetal growth restriction. PE-associated pathologies, including thrombosis, hypertension, and impaired placental development, may result from imbalances between thromboxane A₂ (TXA₂) and prostacyclin. Low-dose aspirin, which selectively inhibits TXA₂ production, is used to prevent high-risk PE. However, the role of TXA₂ in aspirin-mediated protective effects in women with PE is not understood fully. In this study, we examined the role of prostanoids in PE using human samples and an induced PE mouse model. We demonstrated that the administration of salted drinking water (2.7% NaCl) to wild-type mice resulted in elevated placental TXA₂ synthase (TXAS) and plasma TXA₂, but not prostacyclin, levels, which was also found in our clinical PE placenta samples. The high salt-treated wild-type pregnant mice had shown unchanged maternal body weight, hypertension (MAP increase 15 mmHg), and decreased pup weight (~50%) and size (~24%), but these adverse effects were ameliorated in TXAS knockout (KO) mice. Moreover, increased expression of interleukin-1β and downstream phosphorylated-p38-mitogen-activated protein kinase were concordant with apoptosis induction in the placentas of salt water-treated wild-type mice. These alterations were not observed in TXAS KO mice. Together, our data suggest that TXA₂ depletion has anti-PE effects due to the prevention of hypertension and placental damage through downregulation of the interleukin-1β pathway.     

Maternal Magnesium Deficiency in Mice Leads to Maternal Metabolic Dysfunction and Altered Lipid Metabolism with Fetal Growth Restriction

Inadequate magnesium (Mg) intake is a widespread problem, with over 50% of women of reproductive age consuming less than the Recommended Dietary Allowance (RDA). Because pregnancy increases the requirement for Mg and the beneficial effects of magnesium sulfate for preeclampsia/eclampsia and fetal neuroprotection are well described, we examined the outcomes of Mg deficiency during pregnancy. Briefly, pregnant Swiss Webster mice were fed either control or Mg-deficient diets starting on gestational day (GD) 6 through euthanasia on GD17. Mg-deficient dams had significantly reduced weight gain and higher plasma adipokines, in the absence of inflammation. Livers of Mg-deficient dams had significantly higher saturated fatty acids (SFAs) and monounsaturated fatty acids (MUFAs) and lower polyunsaturated fatty acids (PUFAs), including docosahexaenoic acid (DHA) (P < 0.0001) and arachidonic acid (AA) (P < 0.0001). Mechanistically, Mg deficiency was accompanied by enhanced desaturase and elongase mRNA expression in maternal livers along with higher circulating insulin and glucose concentrations (P < 0.05) and increased mRNA expression of Srebf1 and Chrebp, regulators of fatty acid synthesis (P < 0.05). Fetal pups exposed to Mg deficiency were growth-restricted and exhibited reduced survival. Mg-deficient fetal livers showed lower MUFAs and higher PUFAs, with lower desaturase and elongase mRNA expression than controls. In addition, DHA concentrations were lower in Mg-deficient fetal brains (P < 0.05). These results indicate that Mg deficiency during pregnancy influences both maternal and fetal fatty acid metabolism, fetal growth and fetal survival, and support better understanding maternal Mg status before and during pregnancy.     

Epigenetics,Eh!: A meeting summary of the Canadian Conference on Epigenetics

In May 2011, the Canadian Conference on Epigenetics: Epigenetics Eh! was held in London, Canada. The objectives of this conference were to showcase the breadth of epigenetic research on environment and health across Canada and to provide the catalyst to develop collaborative Canadian epigenetic research opportunities, similar to existing international epigenetic initiatives in the US and Europe. With ten platform sessions and two sessions with over 100 poster presentations, this conference featured cutting-edge epigenetic research, presented by Canadian and international principal investigators and their trainees in the field of epigenetics and chromatin dynamics. An EpigenART competition included ten artists, creating a unique opportunity for artists and scientists to interact and explore their individual interpretations of this scientific discipline. The conference provided a unique venue for a significant cross-section of Canadian epigenetic researchers from diverse disciplines to meet, interact, collaborate and strategize at the national level.     

阻断子宫动脉建立FGR大鼠模型的研究

目的通过暂时阻断妊娠期大鼠子宫血供的方法建立子宫缺血引起胎儿生长受限的动物模型。方法根据大鼠子宫动脉是卵巢动脉的一个分支的解剖特点,于孕鼠妊娠第15天时施行手术暂时阻断卵巢动脉并于第21天行剖宫产术,术后称量新生胎仔体重及胎盘、脑、心、肝、肺、肾等重要脏器重量,对比各组间新生胎仔的预后的不同,并对照研究阻断血供10、20、30及40 min对胎仔的不同影响。结果妊娠晚期阻断孕鼠卵巢动脉20min可成功构建胎儿生长受限模型,这种方法与阻断动脉血流30或40 min相比,手术时间短,技术要求不高,胎仔死亡率与对照组差异无显著性(P>0.05)。各实验组较对照组新生胎仔体重及胎盘、各重要脏器重量均明显降低(P<0.05)。结论通过阻断卵巢动脉从而阻断子宫动脉血流,成功建立缺血缺氧性FGR孕鼠模型。该模型重复性好,操作简便,并可成功设立同体对照,为进行FGR相关的产科理论研究提供了一个有利的技术平台。