DNA METHYLTRANSFERASE IN NORMAL AND DNMT(N) DNMT(N) MOUSE EMBRYOS/

The mouse genome experiences a large decrease in net 5-methylcytosine between fertilization and implantation; de novo methylation brings 5-m ethylcytosine to adult somatic cell levels between implantation and ga strulation, Very little is known of the regulation of demethylation or de novo methylation, Levels of the one known form of DNA methyltransf erase are very high in early embryos, but the enzyme is localized to t he cytoplasm during most of preimplantation development. We show here that DNA methyltransferase is found exclusively in nuclei of the conce ptus after implantation, and that nuclei of proximal decidual cells ar e free of detectable DNA methyltransferase, High levels of DNA methylt ransferase were seen in all tissues, including the developing nervous system, of 9.5- to 12.5-day embryos. The large maternal stores of DNA methyltransferase become limiting prior to embryonic day 9.5, as shown by barely detectable immunostaining in 9.5-day embryos homozygous for a loss-of-function mutation (Dnmt(n)) in the DNA methyltransferase ge ne, These mutant embryos failed to develop past the 25-somite stage an d showed evidence of developmental delay and some developmental asynch rony, Normal embryonic and extraembryonic tissues contained similar le vels of DNA methyltransferase, even though severely reduced methylatio n levels and a loss of imprinting have previously been observed in ext raembryonic tissues, These findings suggest that methylation patterns are not a simple function of the concentration of DNA methyltransferas e, and that unidentified factors must be involved in the regulation of de novo methylation during early development of the mouse. (C) 1996 W iley-Liss, Inc.