Demethylation of CpG sites in DNA of early rabbit trophoblast

Abstract

The function of DNA methylation in eukaryotes is unknown. The introduction of methyl groups into the major groove of B-form DNA has been shown to affect DNA–protein interactions in prokaryotes¹, and it is probable that they perform analogous functions in eukaryotic DNA. Thus, there is considerable interest in the possibility that DNA methylation may be involved in regulating DNA transcription and in establishing stable patterns of genetic expression in higher organisms^1,2. Restriction enzyme studies have shown that methylation of eukaryotic DNA is not random; methylation patterns are tissue specific when individual genetic loci are examined^3–5. On the other hand, it is also apparent that these tissue-specific patterns tend to be rather subtle and to occur within overall levels of methylation that are relatively constant in all tissues of a given organism⁴. For example, no significant change in total DNA methylation has been detected during embryogenesis in either the sea urchin^6,7 or the mouse⁸, even though rapid cellular differentiation is occurring. We present evidence here, however, for extensive changes in methylation accompanying early differentiation in the rabbit embryo. Whereas the DNA of the early cleaving embryo is highly methylated, as is that of the later embryoblast, the terminally differentiated trophoblast⁹ is demethylated by about 35%.