The important consideration is the stability of cytosine methylation, which is preserved by covalent carbon-carbon bonds and could therefore serve as a long-term genomic “memory” of early experience influencing chromatin structure and GR expression in offspring of highand low-LG mothers. GR gene expression is increased throughout the hippocampus in the adult offspring of high-LG compared with low-LG mothers.39 The exon 17 GR promoter sequence appears to be significantly more active in the adult offspring of high-LG compared with low-LG mothers and was therefore the focus of initial studies of possible maternal effects on DNA methylation. To test the hypothesis that maternal care alters the DNA methylation mark of the GR promoter, we67 examined the level of methylation across the entire exon 17 GR promoter sequence in the hippocampus using the sodium bisulfite (NaBis) mapping technique in the adult offspring of high- and low-LG mothers. NaBis treatment of DNA samples converts nonmethylated cytosines to uracils, which are then detected as thymidine on subsequent sequencing gels.115 Methylated cytosines are unaffected by NaBis and the differences in methylation status are thus apparent and
