Allelic expression and DNA methylation profiles of promoters at the parental Oct4 and Nanog genes in Mus musculus ES cell/Mus caroli splenocyte hybrid cells

Abstract

Expression of the parental Oct4 and Nanog alleles and DNA methylation of their promoters were studied in a set of Mus musculus embryonic stem (ES) cell/M. caroli splenocyte hybrid cells containing a variable ratio of parental chromosomes 6 and 17. The transcripts of the reactivated splenocyte Oct4 and Nanog genes were revealed in all hybrid cell clones positive for M. caroli chromosomes 6 and 17. We found that 11 CpG sites in the Oct4 promoter were heavily methylated in M. caroli splenocytes (>80%), whereas M. musculus ES cells were essentially unmethylated (<1%). Analysis of the methylation status of the Oct4 promoter in seven hybrid cell clones showed that the splenocyte-derived promoter sequence lost DNA methylation so that its methylation level was comparable with that of the ES cells. Additionally, no preferential de novo methylation was seen in the Oct4 promoters of M. musculus and M. caroli in teratomas developed from two independent hybrid clones. The upstream region of Nanog was heavily methylated in mouse embryonic fibroblasts (66%) and less methylated in M. caroli splenocytes (24%). The Nanog promoter region was completely unmethylated in M. musculus ES cells. We found that both parental alleles of the Nanog gene promoter were essentially unmethylated in five examined hybrid clones. Thus, we have demonstrated that (1) the Oct4 and Nanog genes of splenocytes are activated, and their promoters undergo demethylation in ES cell hybrids; (2) these events are independent of the number and ratio of parental chromosomes carrying these genes.