Establishment and maintenance of DNA methylation patterns in mouse Ndn: implications for maintenance of imprinting in target genes of the imprinting center.

Ndn is located on chromosome 7C, an imprinted region of the mouse genome. Imprinting of Ndn and adjacent paternally expressed genes is regulated by a regional imprinting control element known as the imprinting center (IC). An IC also controls imprint resetting of target genes in the region of conserved synteny on human chromosome 15q11-q13, which is deleted or rearranged in the neurodevelopmental disorder Prader-Willi syndrome. Epigenetic modifications such as DNA methylation, which occur in gametes and can be stably propagated, are presumed to establish and maintain the imprint in target genes of the IC. While most DNA becomes substantially demethylated by the blastocyst stage, some imprinted genes have regions that escape global demethylation and may maintain the imprint. We have now analyzed the methylation of 39 CpG dinucleotide sequences in the 5' end of Ndn by sodium bisulfite sequencing in gametes and in preimplantation and adult tissues. While sperm DNA is completely unmethylated across this region, oocyte DNA is partially methylated. A distinctive but unstable maternal methylation pattern persists until the morula stage and is lost in the blastocyst stage, where low levels of methylation are present on most DNA strands of either parental origin. The methylation pattern is then substantially remodeled, and fewer than half of maternally derived DNA strands in adult brain resemble the oocyte pattern. We postulate that for Ndn, DNA methylation may initially preserve a gametic imprint during preimplantation development, but other epigenetic events may maintain the imprint later in embryonic development.