A two-tiered epigenetic mechanism ensures imprint stability after fertilization: insights from the Igf2/H19 locus

Genomic imprinting is a crucial epigenetic mechanism directing parent-of-origin-specific gene expression in mammals, and it is regulated by allele-specific DNA methylation at imprinting control regions (ICRs). In this canonical imprinting mechanism, maintenance factors preserve germline-derived DNA methylation at ICRs during genome-wide demethylation after fertilization. However, our studies of the Igf2/H19 locus reveal that allele-specific de novo DNA methylation after fertilization, termed 'post-fertilization imprinted methylation,' also contributes to imprint maintenance. In this process, epigenetic imprints other than DNA methylation are thought to guide the recognition of parental alleles by de novo methyltransferases in the next generation, acting in concert with maintenance factors to ensure stable DNA methylation at canonical imprinted loci. Recently identified non-canonical imprinting, in contrast, relies on non-DNA methylation marks that are converted into DNA methylation only after implantation. Despite differences in the timing of this conversion, post-fertilization imprinted methylation may share similarities with non-canonical imprinting. Collectively, these findings suggest that the robustness of canonical imprinting is supported by a two-tiered regulatory system.