Epigenotype switching of imprintable loci in embryonic germ cells

Abstract

Expression of imprinted genes is dependent on their parental origin. This is reflected in the heritable differential methylation of parental alleles. The gametic imprints are however reversible as they do not endure for more than one generation. To investigate if the epigenetic changes in male and female germ line are similar or not, we derived embryonic germ (EG) cells from primordial germ cells (PGCs) of day 11.5 and 12.5 male and female embryos. The results demonstrate that they have an equivalent epigenotype. First, chimeras made with EG cells derived from both male and female embryos showed comparable fetal overgrowth and skeletal abnormalities, which are similar to but less severe than those induced by androgenetic embryonic stem (ES) cells. Thus, EG cells derived from female embryos resemble androgenetic ES cells more than parthenogenetic cells. Furthermore, the methylation status of both alleles of a number of loci in EG cells was similar to that of the paternal allele in normal somatic cells. Hence, both alleles of Igf2r region 2, Peg1/Mest, Peg3, Nnat were consistently unmethylated in EG cells as well as in the primary embryonic fibroblasts (PEFs) rescued from chimeras. More strikingly, both alleles of p57kip2 that were also unmethylated in EG cells, underwent de novo methylation in PEFs to resemble a paternal allele in somatic cells. The exceptions were the H19 and Igf2 genes that retained the methylation pattern in PEFs as seen in normal somatic tissues. These studies suggest that the initial epigenetic changes in germ cells of male and female embryos are similar.

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

“…Expression of imprinted genes, such as H19, in the germ line become biallelic at day 11.5 of mouse embryonic development (Szabo and Mann, 1995b;Villar et al, 1995). Loss of allele-specific methylation imprints has also been observed in both male and female PGCs of the mouse (Tada et al, 1998). These lines of data support the hypothesis that pre-existing imprints are erased in the germ line by this stage.…”

“…Loss of allele-specific methylation imprints has also been observed in both male and female PGCs of the mouse (Szabo and Mann, 1995a;Tada et al, 1998). In particular, H19 is unmethylated both in the paternal and maternal allele in mouse PGCs, whereas gain of methylation occurs prosopermatogonia onward (Davis et al, 2000).…”

Reprogramming of genome function through epigenetic inheritance

“…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 . Here it occurs precociously, being found in EG cells derived from PGCs before their entry into the genital ridge, and could be due to culture of PGCs in vitro.…”

“…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 .…”

“…It is important to note that ES cells do not show the same property for the erasure of imprints (see below). When the imprint-free EG cells are introduced into blastocysts to generate chimaeras, they can cause developmental anomalies, such as aberrant growth and skeletal abnormalities 28 . The initiation of new imprints occurs subsequently during gametogenesis, and primarily during oogenesis.…”