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Allelic genome-wide maps of open chromatin state in zygotes and early embryo

ABSTRACT: Mammalian sperm and oocytes have different epigenetic landscapes and are organized in different fashion. Following fertilization, the initially distinct parental epigenomes become largely equalized with the exception of certain loci including imprinting control regions. How parental chromatin becomes equalized and how imprinted genes escape this wave of reprogramming is largely unknown. Here we generated high-resolution maps of parental allele-specific DNase I hypersensitive sites (DHSs) in zygotes and morula embryos by using physically-isolated parental pronuclei, as well as parthenogenetic (PG) and androgenetic (AG) embryos. Allelic transcriptome analyses revealed a high correlation between allelic DHSs and allelic gene expression not only in known imprinted genes but also in dozens of genes not previously known to be imprinted. Interestingly, we found that many paternally- expressed genes harbor paternal allele-specific DHSs (Ps-DHSs) that are highly enriched for histone H3K27 tri-methylation (H3K27me3) but devoid of DNA methylation in maternal allele. Importantly, ectopic removal of the H3K27me3 turns Ps-DHSs into bi- allelic DHSs and induces maternal allele derepression in genes that include maternal DNA methylation-independent imprinted genes Gab1, Sfmbt2, Slc38a4, and Phf17 (also known as Jade1). Thus, our study not only reveals parental allele-specific chromatin accessibility of preimplantation embryos, but also identifies maternal H3K27me3 as a DNA methylation- independent mechanism for genomic imprinting.

ORGANISM(S): Mus musculus

PROVIDER: GSE92605 | GEO | 2017/07/19

SECONDARY ACCESSION(S): PRJNA358118

REPOSITORIES: GEO

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Project description:In plants, imprinted gene expression occurs in endosperm seed tissue and can be associated with differential DNA methylation between maternal and paternal alleles. Imprinting is theorized to have been selected for because of conflict between parental genomes in offspring, but most studies of imprinting have been conducted in Arabidopsis thaliana, an inbred primarily self-fertilizing species that should have limited parental conflict. We examined embryo and endosperm allele-specific expression and DNA methylation genome-wide in the wild outcrossing species Arabidopsis lyrata. Here we show that the majority of A. lyrata imprinted genes exhibit parentally-biased expression in A. thaliana, suggesting that there is evolutionary conservation in gene imprinting. Surprisingly, we discovered substantial interspecies differences in methylation features associated with paternally expressed imprinted genes (PEGs). Unlike A. thaliana, the maternal allele of many A. lyrata PEGs was hypermethylated in the CHG context. Increased maternal allele CHG methylation was associated with increased expression bias in favor of the paternal allele. We propose that CHG methylation maintains or reinforces repression of maternal alleles of PEGs. These data suggest that while the genes subject to imprinting are largely conserved, there is flexibility in the epigenetic mechanisms employed between closely related species to maintain monoallelic expression. This supports the idea that imprinting of specific genes is a functional phenomenon, and not simply a byproduct of seed epigenomic reprogramming. Examination of total gene expression, parent-of-origin specific allelic bias, or DNA methylation in embryo, endosperm, flower bud or seedcoat tissue from Arabidopsis lyrata accessions MN47 (MN), Karhumaki (Kar or KA), and crosses between them. High-throughput Illumina poly-A-selected mRNA-seq was used to identify imprinted genes in A. lyrata, and high-throughput Illumina whole genome bisulfite-sequencing was used to examine DNA methylation. mRNA-seq samples are designated MMxFF_T# where MM is the mother of the cross (either MN for MN47 or KA for Kar), FF is the father, T is the tissue (E for embryo, N for endosperm, S for seedcoat, b for buds), and # is the replicate numbers. Samples obtained from bisulfite sequencing follow the same naming but have suffix _BS and indicate cytosine methylation context (CpG, CHG, or CHH). For KAxMN bisulfite sequencing, additional files MMxFF_T#_BS_P_C.txt follow the same naming scheme but contain context-specific methylation data (C) from reads that mapped preferentially to one parent strain (P).

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Allelic genome-wide maps of open chromatin state in zygotes and early embryo

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