De novo methylation of an embryonic globin gene during normal development is strand specific and spreads from the proximal transcribed region

The recently discovered de novo methyltransferases DNMT3a and DNMT3b have b een shown to be critical to embryonic development. However, at a single gen e level, little is known about how the methylation pattern is established d uring development. The avian embryonic p-globin gene promoter is completely unmethylated in 4-day-old chicken embryonic erythroid cells, where it is e xpressed at a high level, and completely methylated in adult erythroid cell s, where it is silent. The methylation pattern of the p-globin gene promote r, proximal transcribed region, and distal transcribed region on both DNA s trands was examined during development in chicken erythroid cells. It was f ound that de novo methylation targets the CpG-dense proximal transcribed re gion on the coding (top) strand initially, followed by spreading into the 3 ' region and into the promoter region. Methylation of the template (bottom) strand lags behind that of the coding strand, and complete methylation of both strands occurs only after the gene has been silenced. The results of t he study indicate that establishment of the de novo methylation pattern inv olves strand-specificity and methylation spreading. (Blood. 2001;98: 3441-3 446) (C) 2001 by The American Society of Hematology.