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A unique regulatory phase of DNA methylation in the early mammalian embryo

ABSTRACT: DNA methylation is highly dynamic during mammalian embryogenesis. It is broadly accepted that the paternal genome is actively depleted of global cytosine methylation at fertilization, followed by passive depletion that reaches a minimum at the blastocyst stage. However, this model is based on limited data, and to date no base-resolution maps exist to support and refine it. Here, we generated genome-scale DNA methylation maps in mouse gametes and through post-implantation embryogenesis. We find that the oocyte already exhibits global hypomethylation, most prominently at specific subfamilies of LINE-1 and LTR-containing retro-elements, which are disparate between gametes and resolve to lower methylation values in zygote. Surprisingly, the oocyte contributes a unique set of Differentially Methylated Regions (DMRs), including many CpG Island promoter regions, that are maintained in the early embryo but are lost at the onset of embryonic specification and absent in somatic cells. In contrast, sperm contributed methylation includes retrotransposons that become completely methylated after the blastocyst stage. Our data provide a complete genome-scale, base-resolution timeline of DNA methylation in the pre-specified embryo, when this epigenetic modification is most dynamic and before returning to the canonical somatic pattern.

ORGANISM(S): Mus musculus

PROVIDER: GSE34864 | GEO | 2012/03/28

SECONDARY ACCESSION(S): PRJNA150129

REPOSITORIES: GEO

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Project description:The transformation of a fertilized oocyte into a multicellular embryo comprised of differentiated cells depends on the processing of genetic orders, first in the form of transcripts and then of proteins. Our current understanding of mouse development has greatly benefited from the analysis of transcripts as proxy for the proteins. However, the hexagonal-shaped free ribosomes that translate mRNAs into proteins are scarce during cleavage and do not become abundant until the morula-blastocyst stage. How accurate, then, is an understanding of mouse and more in general mammalian development that uses the analysis of transcripts as proxy for the proteins? This led us in this study to measure the proteome of seven preimplantation mouse stages (B6C3F1 x CD1), from oocyte to blastocyst, using 'spike-in' SILAC technology combined with high accuracy mass spectrometry (LTQ Orbitrap and Q-Exactive). The relative abundances of embryonic proteins were compared and contrasted with those of transcripts measured by RNA sequencing. A total of 6976 proteins were detected in at least the spike (precondition for determining the heavy/light peptide ratios in oocytes or embryos). Among these 6976 proteins, 4991 proteins were present in all developmental stages, and 1893 proteins were present in all replicates. Our tandem approach uncovered: 1) the different abundance profiles of proteome and transcriptome during the time of preimplantation development; 2) the different reciprocal associations of developmental stages (dendrograms) on the protein level as compared to the mRNA level; and 3) the different gene ontology terms overrepresented among the proteins that increase or decrease across adjacent stages, compared to mRNAs. In essence, the elaborate phasing of embryonic gene expression that has been known for mRNAs leaves an unsuspected footprint on the protein products of those mRNAs. This information facilitates the analysis of mammalian development in two important ways. First, it provides new insight into the regulation of the transition from the differentiated oocyte into the embryo, by identifying the subsets of proteins that accompany the passage from one embryonic stage to the next, which are likely composed of important regulators of the morphogenetic transitions. Second, our analysis provides a reference to determine the degree of ‘embryoness’ of entities produced via assisted reproductive technologies, cloning by somatic cell nuclear transfer, and even artificial gametes.

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A unique regulatory phase of DNA methylation in the early mammalian embryo

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