EPIGENETIC REGULATION OF GENOMIC IMPRINTING DURING EARLY MAMMALIAN EMBRYONIC DEVELOPMENT

Background: Appropriate epigenetic regulation of gene expression during lineage allocation and tissue differentiation is required for normal development. One example is genomic imprinting, which is defined as parent-of-origin mono-allelic gene expression. Imprinting is established largely due to epigenetic differences arriving in the zygote from sperm and egg haploid genomes. In the mouse, there are approximately 150 known imprinted genes, many of which occur in imprinted gene clusters that are regulated together. One imprinted cluster includes the maternally expressed Igf2r, Slc22a2, and Slc22a3 genes and the paternally expressed long non-coding RNA (lncRNA) Airn. Although it is known that Igf2r and Airn are reciprocally imprinted, the timing of imprinted expression and accompanying epigenetic changes have not been well characterized in vivo. Results: Here we show lineageand temporal-specific regulation of DNA methylation and histone modifications at the Igf2r/Airn locus correlating with differential establishment of imprinted expression during gastrulation. Our results show that Igf2r is expressed from both alleles in the E6.5 epiblast. After gastrulation commences, the locus becomes imprinted in the embryonic lineage with the lncRNA Airn expressed from the paternal allele and Igf2r restricted to maternal allele expression. We document differentially enriched allele-specific histone modifications in extraembryonic and embryonic tissues. We also document for the first time allele-specific spreading of DNA methylation during gastrulation concurrent with establishment of imprinted expression of Igf2r. Importantly, we show that imprinted expression does not change in the extraembryonic lineage even though maternal DMR2 methylation spreading does occur, suggesting distinct mechanisms at play in embryonic and extraembryonic lineages. Conclusions: These results indicate that similar to preimplantation, gastrulation represents a window of dynamic lineage-specific epigenetic regulation in vivo. Background Genomic imprinting is an epigenetic phenomenon that results in mono-allelic gene expression in a parent-of-origin manner. Imprinted expression has been identified at approximately 150 mouse genes, which often occurs in clusters containing multiple imprinted transcripts [1,2]. Expression of imprinted genes is thought to be established in cis by allele-specific DNA methylation established at imprinting control regions (ICRs) in the gametes, thus arriving in the zygote as maternal and paternal specific information. A regulatory theme has emerged at many imprinted clusters in which a single long non-coding RNA (lncRNA) is thought to repressively regulate genes in cis through direct transcriptional blocking and/or recruitment of repressive chromatin remodeling complexes such as G9a and PRC2, resulting in differential allele-specific histone modifications [3,4]. One cluster on mouse chromosome 17 includes the maternally expressed Igf2r, Slc22a2, and Slc22a3 genes and the paternally expressed lncRNA Airn [5], and several non-imprinted genes (Slc22a1, Mas, and Plg). The Airn promoter lies in the second intron of Igf2r, and Airn transcription occurs from the opposite strand overlapping Igf2r exons 1 and 2 [5-7]. Paternal Airn expression may participate in imprinting of the maternally expressed genes by blocking access of the transcriptional machinery to the Igf2r start site [8], and transcription of Airn has been shown to be required for silencing of Igf2r [8,9]. Paternal allele silencing of the other imprinted genes in the cluster only occurs in extraembryonic lineages and may be a result of Airn recruitment of repressive complexes such as G9a to their promoters [4]. Biallelic expression of Igf2r is * Correspondence: jmager@vasci.umass. edu Department of Veterinary and Animal Sciences, University of Massachusetts at Amherst, ISB 427M, 661 N. Pleasant Street, Amherst, MA 01003, USA © 2015 Marcho et al.; licensee BioMed Central. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly credited. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Marcho et al. Epigenetics & Chromatin (2015) 8:10 DOI 10.1186/s13072-015-0003-y