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Epigenetics of human fetal and placental development Yuen, Ka Chun

Abstract

Dysregulation of placental and fetal epigenetics can affect gene expression patterns, including the parent-of-origin dependent expression in imprinted genes. While defects of imprinted genes have been implicated in some adverse pregnancy outcomes, little is currently known about the role of epigenetics in regulating normal or pathological human pregnancy and development. The objective of this thesis is to provide fundamental DNA methylation profiles of human fetal and placental development so as to offer insights into the etiology of human disease and adverse pregnancy outcomes. Taking advantage of the unbalanced parental genomic constitutions in triploidies, 45 novel imprinted genes were identified by comparing the genome-wide DNA methylation profiles between 10 diandries and 10 digynies. A comparison of DNA methylation profiles between placentas of different gestations and other somatic tissues showed tissue-specific and gestational age-specific DNA methylation changes in many imprinted genes. To gain insight into the genomic pattern of tissue-specific methylation, DNA methylation profile was evaluated in 5 somatic tissues (brain, kidney, lung, muscle and skin) from eight normal second-trimester fetuses. Tissue-specific differentially methylated regions (tDMRs) were identified in 195 loci, suggesting that tissue-specific methylation is established early in the second trimester. Importantly, only 17% of the identified fetal tDMRs were found to maintain this same tissue-specific methylation in adult tissues, implicating an extensive epigenetic reprogramming between fetus and adult. Besides intra-individual differences, there is also substantial DNA methylation variation between individuals. While many sites show a continuous pattern of DNA methylation variation between different placentas, WNT2, TUSC3 and EPHB4 were identified to have epipolymorphisms at their promoter region. The methylation status at the TUSC3 promoter showed an association with preeclampsia, suggesting a role of DNA methylation change in adverse pregnancy outcomes. A further investigation of DNA methylation profiles in 26 placentas from preeclampsia, IUGR and control subjects showed 34 loci were hypomethylated in the early-onset preeclamptic placentas, with TIMP3 having a potential of being a biomarker for the disorder. These results provided comprehensive DNA methylation profiles for both normal and abnormal fetal and placental tissues, which contribute to the biological and clinical aspects of the pathogenesis of fetal and placental disorders.

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