ABSTRACT: The maternal and paternal copies of the genome are both required for mammalian development and this is primarily due to imprinted genes, those that are mono-allelically expressed based on parent-of-origin. Typically, this pattern of expression is regulated by differentially methylated regions (DMRs) that are established in the germline and maintained after fertilisation. There are a large number of germline DMRs that have not yet been associated with imprinting and their function in development is unknown. In this study, we developed a genome-wide approach to identify novel imprinted DMRs, specifically in the human placenta, and investigated the dynamics of imprinted DMRs during development in somatic and extra-embryonic tissues. DNA methylation was evaluated using the Illumina HumanMethylation450 array in 116 human tissue samples, publically available reduced representation bisulfite sequencing in the human embryo and germ cells, and targeted bisulfite sequencing in term placentas. 43 known and 101 novel imprinted DMRs were identified in the human placenta, by comparing methylation between diandric and digynic triploids and female and male gametes. 72 novel DMRs showed a pattern consistent with placental-specific imprinting and this mono-allelic methylation was entirely maternal in origin. Strikingly, these DMRs exhibited polymorphic imprinted methylation specifically in placenta. These data suggest that imprinting in human development is far more extensive and dynamic than previously reported and that the placenta preferentially maintains maternal germline-derived DNA methylation For the identification of imprinted DMRs in the placenta, chorionic villous samples from 5 diandric and 5 digynic triploids pregnancies were assayed, along with a pooled sample of complete hydatiform moles (CHM). Placental chorionic villous samples (n=63) included 29 control pregnancies delivered at term, while the remaining 34 were delivered preterm or miscarried, or had abnormal MSS results, IUGR or LOPET. The preterm births were associated with one or more of: preterm labour, premature rupture of membranes (PROM), chorioamnionitis, placental abruption, and incompetent cervix. All samples were determined to be chromosomally normal using standard karyotyping or comparative genome hybridization, as previously described (Robinson et al. 2010). Two to four independent sites were taken from each placenta and after DNA extraction from chorionic villous, the DNA was pooled before being utilized in this study. Thirty-three fetal tissues, including brain (n=8), spinal cord (n=7), muscle (n=9), and kidney (n=9) were collected from second trimester foetuses, as previously described (Price et al. 2012). Adult female whole blood samples (n=10) were collected from control women. Extra-embryonic cell types (n=19), including cord blood (embryonic), cord, amniotic membrane, chorionic membrane, 1st, 2nd and 3rd trimester trophoblast and mesenchyme, and decidua (maternal), were isolated from control placental samples.
