Genomic imprinting recapitulated in the human β-globin locus

Abstract

A subset of genes in mammals are subject to genomic imprinting. The mouse H19 gene, for example, is active only when maternally inherited and the neighboring Igf2 gene is paternally expressed. This imprinted expression pattern is regulated by the imprinting control region (ICR) upstream of the H19 gene. A maternally inherited H19 ICR inhibits Igf2 gene activation by the downstream enhancer due to its insulator function while it suppresses H19 gene transcription by promoter DNA methylation when paternally inherited. These parent-of-origin specific functions depend on the allele-specific methylation of the ICR DNA, which is established during gametogenesis. Therefore, the ICR may also function as a landmark for epigenetic modifications. To examine whether the ICR confers these activities autonomously, we introduced a 2.9-kbp ICR-containing DNA fragment into a human ␤-globin yeast artificial chromosome at the 3 end of the locus control region and established transgenic mouse lines. Expression of all of the ␤-like globin genes was higher when the transgene was paternally inherited. In accord with this result, transgenic ICR DNA from nucleated erythrocytes was more heavily methylated when paternally transmitted. Chromatin immunoprecipitation assays confirmed that CCCTC binding factor is preferentially recruited to the maternal transgenic ICR in vivo. Surprisingly however, the parent-of-origin specific methylation pattern was not observed in germ cell DNA in testis, demonstrating that methylation was established after fertilization. Thus, the ICR autonomously recapitulated imprinting within the normally nonimprinted transgenic ␤-globin gene locus, but the temporal establishment of imprinting methylation differs from that at the endogenous Igf2͞H19 locus.

Developmental Profile of H19 Differentially Methylated Domain (DMD) Deletion Alleles Reveals Multiple Roles of the DMD in Regulating Allelic Expression and DNA Methylation at the Imprinted H19/Igf2 Locus

“…In support of this idea, Davis et al reported that after methylation is erased in migrating and early colonizing primordial germ cells, the DMD and PP regions are methylated asynchronously in the male germ line, with the paternal allele acquiring methylation prior to the maternal allele (6,7). Additionally, in a study in which the DMD was inserted at the AFP locus (32) and in a second experiment inserting the DMD within a human ␤-globin transgene (48), the DMD was unmethylated in spermatozoa but acquired paternal-allele-specific methylation after implantation. Thus, while these studies indicate that the DMD functioned as a methylation-sensitive and maternal-allele-specific insulator at an exogenous locus, it is not clear if the exogenous DMD recapitulated the paternal-allele-specific silencing function.…”

Putative imprinted gene expression in uniparental bovine embryo models

“…The recent discovery that Xist becomes biallelically expressed in the inner cell mass of the mouse substantiates the claim that imprinting may not be maintained as once thought (Okamoto et al 2004). A transgenic human β-globin locus has also been shown recently to acquire methylation imprinting during the post-fertilisation period (Tanimoto et al 2005). It is possible that imprints are erased following fertilisation and re-established too rapidly to have been discovered in the murine model system or that genomic imprinting is differentially regulated between species.…”

Genomic imprinting in germ cells: imprints are under control

“…However, several studies suggest that this key step can be by-passed, and that methylation imprints can be acquired later in development. For instance, when the H19 DMR is inserted at ectopic positions in the genome, paternal-specific DNA methylation at this DMR can be established partially, or even completely, after fertilisation, during early development (Park et al 2004, Tanimoto et al 2005. Furthermore, methylation imprints at some ICRs can be present in a stochastic manner in some embryos derived from DNMT3L-deficient oocytes (Arnaud et al 2006).…”