Regulatory mechanisms of maternal imprinting at the murine Dlk1-Dio3 domain.

Genomic imprinting is an epigenetic process causing parent-of-origin specific gene expression. The Dlk1-Dio3 domain is one of the largest imprinted clusters. While DNA methylation at an intergenic CpG-island (IG-CGI) within the imprinting control region (ICR) controls expression from the paternal chromosome, mechanisms regulating the unmethylated maternal chromosome remain unknown. Within the transcriptional regulatory element (IG-TRE) of the ICR, deletions identified a minimal region in vitro exhibiting both silencing and enhancing activity, with SOX2 and ZFP281 contributing to enhancer function on the maternal chromosome. In vivo, however, this deletion did not affect maternal expression in mouse embryos; instead it activated Dlk1 on both parental chromosomes. Combining deletion of this IG-TRE with the lethal IG-CGI deletion rescued lethality in mice by balancing Dlk1 expression, despite persistent maternal gene upregulation. These results demonstrate that loss of expression at this domain is more detrimental than gain, highlighting the importance of in vivo analysis. Identification of active regulatory factors on the unmethylated maternal chromosome challenges the prevailing view that imprinting is primarily a methylation-driven phenomenon, further revealing the sophisticated hierarchical mechanisms governing imprinting control.