The histone code regulating expression of the imprinted mouse Igf2r gene

The mouse IGF-II receptor (Igf2r) and its antisense transcript Air are reciprocally imprinted in most normal tissues. Several mechanisms have been hypothesized to explain Igf2r-Air imprinting, including Igf2r silencing by Air, and transcriptional repression of Igf2r-Air by two differentially methylated regions (DMR1 and DMR2). We employed Mus musculus x Mus spretus interspecific mice and chromatin immunoprecipitation (ChIP) to investigate allele-specific histone modifications in the two DMRs. We show that, in both DMRs, the active alleles of both Igf2r, and Air are associated with acetylated histones (H3, and H4), acetyl lysine 9 of histone H3 ( H3 K9-Ac), and methyl lysine 4 of histone H3 (H3 K4-Me). The silenced alleles are associated with methylated DNA, deacetylated H3 K9, and unmethylated H3 K4. Allele-specific histone modifications are present in the DMR2 that is established in the gametes and represents the DNA gametic-imprint of the Igf2r. In the DMR2 from liver, kidney, and central nervous system tissues, H3 K9 methylation is associated exclusively with the silenced allele, and H3 S10 phosphorylation with the active alleles. Treatment of fibroblast cells with 5-aza-deoxycytidine and/or Trichostatin A led to partial reactivation of the silenced allele, which correlates with biallelic histone acetylation. In central nervous system, despite the presence of imprinted Air transcripts, biallelic expression of Igf2r occurs. The tissue-specific relaxation of Igf2r imprinting correlates with biallelic histone acetylation, and biallelic H3 K4 methylation in the promoter region of Igf2r ( DMR1). We propose a model of the histone code for Igf2r, and Air imprinting that defines histone modifications specific for the putative gametic imprint DMR2, and explains the tissue-specific imprinting of Igf2r in the mouse and the absence of IGF2R imprinting in human.