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Endogenous retroviral insertions drive non-canonical imprinting in extra-embryonic tissues [RNA-Seq]

ABSTRACT: Background: Genomic imprinting is an epigenetic phenomenon that allows a subset of genes to be expressed mono-allelically based on parent-of-origin, and is typically regulated by differential DNA methylation inherited from gametes. Imprinting is pervasive in murine extra-embryonic lineages and, uniquely, the imprinting of several genes has been found to be conferred non-canonically through maternally-inherited repressive histone modification H3K27me3. However, the underlying regulatory mechanisms of non-canonical imprinting in post-implantation development remain unexplored. Results: We identify imprinted regions in post-implantation epiblast and extra-embryonic ectoderm (ExE) by assaying allelic histone modifications (H3K4me3, H3K36me3, H3K27me3), gene expression and DNA methylation in reciprocal C57BL/6 and CAST hybrid embryos. We distinguish loci with DNA methylation- dependent (canonical) and independent (non-canonical) imprinting by assaying hybrid embryos with ablated maternally-inherited DNA methylation. We find that non-canonical imprints are localized to endogenous retrovirus-K (ERVK) long terminal repeats (LTRs), which act as imprinted promoters specifically in extra-embryonic lineages. Transcribed ERVK LTRs are CpG-rich and located in close proximity to gene promoters, and imprinting status is determined by their epigenetic patterning in the oocyte. Finally, we show that oocyte-derived H3K27me3 associates with non-canonical imprints is not maintained beyond pre-implantation development, and is replaced by secondary imprinted DNA methylation on the maternal allele in post-implantation ExE, while being completely silenced by bi-allelic DNA methylation in epiblast. Conclusions: This study reveals distinct epigenetic mechanisms regulating non-canonical imprinted gene expression between embryonic and extra-embryonic development, and identifies an integral role for ERVK LTR repetitive elements.

ORGANISM(S): Mus musculus

PROVIDER: GSE124215 | GEO | 2019/09/19

REPOSITORIES: GEO

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Project description:The TET dioxygenases erase mediate DNA dedemethylation in pre-implantation embryos and in primordial germ cells, yet limited studies address their contribution to the global gain of DNA methylation following implantation. Here, we show that Tet1 is expressed and non-redundantly contributes to 5-hydroxymethylctyosine (5hmC) non-redundantly in the pre-gastrulation mouse epiblast. Ablation of Tet1 in primed epiblast cells results in widespread loss of 5hmC associated with gain of 5-methylcytosine at CpG islands and promoters. Moreover, Tet1 is expressed, albeit at lower levels, in the extra-embryonic ectoderm. Tet1-deficiency in the pre-streak mouse embryos causes dysregulation of early lineage regulators in the epiblast and increased expression of metabolic genes in the extra-embryonic ectoderm. Our studies reveal a distinct role of Tet1 in regulating the methylome landscape of the post-implantation mammalian epiblast and a hitherto unknown gene repressive effect in the extra-embryonic lineage, providing insights into the early developmental origins of epigenetic-based basis of imprinting and developmental disorders. Lysates of E6.25 epiblast (E) and extra-embryonic (ExE) tissues were pooled, based on retrospective genotyping, from at least 6 embryos (collected from 2-3 litters). Four pooled biological replicates were collected for each tissue compartment and each genotype, i.e. wild type (wt), Tet1gt/wt (Zwt), Tet1gt/gt(ZZ); in 3 of 4 pooled replicates, the E and ExE samples were obtained from the same litters and given the same numbering. One ExE sample of ZZ was lost. RNA was purified using the RNeasy micro Kit (Qiagen) to obtain 1-10 ng of RNA per pool. RNA quality was assessed using a Bioanalyzer 6000 Pico chip and only samples with RNA integrity number (RIN) > 8.0 were processed further. First-strand cDNA synthesis and amplification was performed by using the SMARTer Ultra Low InputÂ RNA amplification kit v3 (Clontech). Libraries were prepared using NEBNext Ultra DNA library prep for Illumina and sequenced on NextSeq 500 in high-output to generate 75-bp single-end reads.Â Â Low quality ends and adapter sequences wereÂ trimmed off from the Illumina reads withÂ FastX 0.0.13 andÂ cutadaptÂ 1.7.1. Using FastX andÂ ShortRead 1.16.3, we filtered subsequently small readsÂ (length < 35 bp), polyA-reads (>90% of the bases equal A),Â ambiguous reads (containing N) and lowÂ quality reads (>50% of theÂ bases < Q25).

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Endogenous retroviral insertions drive non-canonical imprinting in extra-embryonic tissues [RNA-Seq]

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