Genomic imprinting recapitulated in the human β-globin locus

摘要

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 β-globin yeast artificial chromosome at the 3′ end of the locus control region and established transgenic mouse lines. Expression of all of the β-like globin genes was higher when the transgene was paternally inherited. In accord with this result, transgenic ICR DNA from nucleated eryth-rocytes 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 β-globin gene locus, but the temporal establishment of imprinting methylation differs from that at the endogenous Igf2/H19 locus.