The ontogeny of allele-specific methylation associated with imprinted genes in the mouse.

Abstract

We have investigated the DNA methylation patterns in genomically imprinted genes of the mouse. Both Igf2 and H19 are associated with clear‐cut regions of allele‐specific paternal modification in late embryonic and adult tissues. By using a sensitive PCR assay, it was possible to follow the methylation state of individual HpaII sites in these genes through gametogenesis and embryogenesis. Most of these CpG moieties are not differentially modified in the mature gametes and also become totally demethylated in the early embryo in a manner similar to non‐imprinted endogenous genes. Thus, the overall allele‐specific methylation pattern at these sites must be established later during embryogenesis after the blastula stage. In contrast, sites in an Igf2r gene intron and one CpG residue in the Igf2 upstream region have allele‐specific modification patterns which are established either in the gametes or shortly after fertilization and are preserved throughout pre‐implantation embryogenesis. These studies suggest that only a few DNA modifications at selective positions in imprinted genes may be candidates for playing a role in the maintenance of parental identity during development.

Reprogramming of genome function through epigenetic inheritance

“…The initiation of imprinting is confined to the germ line, first with the erasure of existing imprints in primordial germ cells (PGCs) [26][27][28][29] , followed by the initiation of a new set of imprints in the male and female germ lines. It should be noted that whereas methylation of DMRs for some genes results in their repression, in other instances (for example, the Igf2r gene), methylation is essential for gene activation 8,9,16 .…”

“…Alternatively, the erasure of imprints might occur at a specific time and be regulated by a developmental clock. Whatever determines the timing of these events, PGCs by E13.5 possess an equivalent epigenetic state with erased imprints 26,27,44 , and male and female gonads become distinguishable with distinct phenotypes. The erasure of imprints is also observed in EG cells 28 .…”

“…DNMT1, which is also present in the oocyte, is known to be excluded from entry into pronuclei, which is why demethylation of the embryonic genome continues throughout pre-implantation development until the blastocyst stage 26,47,55 . Genome-wide de novo methylation occurs later in early post-implantation embryos.…”

Erasure of methylation imprint at the promoter and CTCF-binding site upstream of H19 in human testicular germ cell tumors of adolescents indicate their fetal germ cell origin

“…Differential methylation of cytosine residues in CpG dinucleotides in critical regions of imprinted genes is part of the imprinting process differentiating paternal and maternal alleles (Barlow, 1993). Imprinted genes have been well analysed in terms of genome reprogramming, and it is widely accepted that imprinting memories are erased in PGCs and that DNA methylation is an important factor in this process (Grant et al, 1992;Kafri et al, 1992;Brandeis et al, 1993;Szabo and Mann, 1995b;Kato et al, 1999). Maternally expressed H19 is one of the best-characterized imprinted genes.…”

Maternal-Specific Methylation of the HumanIGF2RGene Is Not Accompanied by Allele-Specific Transcription

“…At pres ent, the nature of the primary im print and the molecu lar basis by which genes are recognized as paternally or maternally derived are still unknown. First evidence that méthylation plays an important role in genomic im printing came from the observation that many transgenes become hypomethylated after passage through the male germline and hypermethylated after passage through the female germline (Surani et a l,, 1988), Analysis of four imprinted genes in the mouse, H19 (Bartolomei et al, 1993;Ferguson-Smith et a l, 1993;Brandeis et al, 1993;Feil et al, 1994), insulinlike growth factor II {Igf2) (Sasaki et a l, 1992;Bran deis et al, 1993;Feil et al, 1994), Igf2 receptor (Igf2r) (Stôger et a lv 1993), and U2afl-rsl (SP2) (Hatada et al, 1995), has revealed that regions within the gene or adjacent to it are methylated in a parent-specific manner, The mouse Igf2r gene is expressed exclusively from the maternally inherited chromosome in fetal and adult tissues (Barlow et al, 1991), with the exception of the head and the brain, where biallelic expression has been observed (Villar and Pedersen, 1994). During preimplantation development, both maternal and pa ternal Ig f2 r alleles are expressed (Latham et al, 1994;Szabô and Mann, 1995), indicating that silencing of the paternal allele is a secondary event.…”