Abstract
DNA methylation is traditionally thought to be established during early development and to remain mostly unchanged thereafter in healthy tissues, although recent studies have shown that this epigenetic mark can be more dynamic. Epigenetic changes occur in the liver after birth, but the timing and underlying biological processes leading to DNA methylation changes are not well understood. We hypothesized that this epigenetic reprogramming was the result of terminal differentiation of hepatocyte precursors. Using genomic approaches, we characterized the DNA methylation patterns in mouse liver from E18.5 until adulthood to determine if the timing of the DNA methylation change overlaps with hepatocyte terminal differentiation, and to examine the genomic context of these changes and identify the regulatory elements involved. Out of 271,325 CpGs analyzed throughout the genome, 214,709 CpGs changed DNA methylation by more than 5% (e.g., from 5 to 10% methylation) between E18.5 and 9 wk of age, and 18,863 CpGs changed DNA methylation by more than 30%. Genome-scale data from six time points between E18.5 and P20 show that DNA methylation changes coincided with the terminal differentiation of hepatoblasts into hepatocytes. We also showed that epigenetic reprogramming occurred primarily in intergenic enhancer regions while gene promoters were less affected. Our data suggest that normal postnatal hepatic development and maturation involves extensive epigenetic remodeling of the genome, and that enhancers play a key role in controlling the transition from hepatoblasts to fully differentiated hepatocytes. Our study provides a solid foundation to support future research aimed at further revealing the role of epigenetics in stem cell biology.