A Randomly Integrated Transgenic H19 Imprinting Control Region Acquires Methylation Imprinting Independently of Its Establishment in Germ Cells

The imprinted expression of the mouseIgf2/H19 locus is governed by the differential methylation of the imprinting control region (ICR), which is established initially in germ cells and subsequently maintained in somatic cells, depending on its parental origin. By grafting a 2.9-kbpH19 ICR fragment into a human β-globin yeast artificial chromosome in transgenic mice, we previously showed that the ICR could recapitulate imprinted methylation and expression at a heterologous locus, suggesting that theH19 ICR in the β-globin locus contained sufficient information to maintain the methylation mark (K. Tanimoto, M. Shimotsuma, H. Matsuzaki, A. Omori, J. Bungert, J. D. Engel, and A. Fukamizu, Proc. Natl. Acad. Sci. USA102: 10250-10255, 2005). Curiously, however, the transgenicH19 ICR was not methylated in sperm, which was distinct from that seen in the endogenous locus. Here, we reevaluated the ability of theH19 ICR to mark the parental origin using more rigid criteria. In the testis, the methylation levels of the solitary 2.9-kbp transgenic ICR fragment varied significantly between six transgenic mouse lines. However, in somatic cells, the paternally inherited ICR fragment exhibited consistently higher methylation levels at five out of six randomly integrated sites in the mouse genome. These results clearly demonstrated that theH19 ICR could acquire parent-of-origin-dependent methylation after fertilization independently of the chromosomal integration site or the prerequisite methylation acquisition in male germ cells.