Erasing genomic imprinting memory in mouse clone embryos produced from day 11.5 primordial germ cells

Genomic imprinting is an epigenetic mechanism that causes functional differences between paternal and maternal genomes, and plays an essential role in mammalian development. Stage-specific changes in the DNA methylation patterns of imprinted genes suggest that their imprints are erased some time during the primordial germ cell (PGC) stage, before their gametic patterns are re-established during gametogenesis according to the sex of individuals. To define the exact timing and pattern of the erasure process, we have analyzed parental-origin-specific expression of imprinted genes and DNA methylation patterns of differentially methylated regions (DMRs) in embryos, each derived from a single day 11.5 to day 13.5 PGC by nuclear transfer. Cloned embryos produced from day 12.5 to day 13.5 PGCs showed growth retardation and early embryonic lethality around day 9.5. Imprinted genes lost their parental-origin-specific expression patterns completely and became biallelic or silenced. We confirmed that clones derived from both male and female PGCs gave the same result, demonstrating the existence of a common default state of genomic imprinting to male and female germlines. When we produced clone embryos from day 11.5 PGCs, their development was significantly improved, allowing them to survive until at least the day 11.5 embryonic stage. Interestingly, several intermediate states of genomic imprinting between somatic cell states and the default states were seen in these embryos. Loss of the monoallelic expression of imprinted genes proceeded in a step-wise manner coordinated specifically for each imprinted gene. DNA demethylation of the DMRs of the imprinted genes in exact accordance with the loss of their imprinted monoallelic expression was also observed. Analysis of DNA methylation in day 10.5 to day 12.5 PGCs demonstrated that PGC clones represented the DNA methylation status of donor PGCs well. These findings provide strong evidence that the erasure process of genomic imprinting memory proceeds in the day 10.5 to day 11.5 PGCs, with the timing precisely controlled for each imprinted gene. The nuclear transfer technique enabled us to analyze the imprinting status of each PGC and clearly demonstrated a close relationship between expression and DNA methylation patterns and the ability of imprinted genes to support development.