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DNA methylation within the mammalian germline cycle undergoes two
reprogramming episodes, during which the existing DNA methylation is largely
eliminated (reviewed in Surani et al. 2007 ). After demethylation during primordial
germ cell reprogramming, maternal and paternal genomes accumulate DNA meth-
ylation, including parent-of-origin-specific methylation in imprinting control
regions. However, a large portion of 5mC methylation is lost during early mamma-
lian development. Genome-wide demethylation manifests as a loss of signal in
cleaving mouse zygotes stained with an antibody recognizing 5mC (Mayer et al.
2000 ). There were two distinct demethylation patterns observed in paternal and
maternal genomes. The paternal genome rapidly lost 5mC signal (6–8 h after fertil-
ization) suggesting an active demethylation process. In contrast, staining of the
maternal genome showed gradual reduction of the signal through cleavages, imply-
ing passive DNA demethylation due to absent DNA methylation maintenance
(Mayer et al. 2000 ; Oswald et al. 2000 ). Notably, the zygotic genome is not being
completely demethylated, presumably due to continuous de novo DNA methylation
of specific loci. Apart from imprinted loci, which retain their parent-of-origin meth-
ylation pattern in imprinting control regions, it was shown that intracisternal A par-
ticle (IAP), an active and aggressive murine endogenous retrovirus, remains
methylated despite the global demethylation (Lane et al. 2003 ).
Passive DNA demethylation of the maternal genome is supported by staining of
metaphase chromosomes from cleaving embryos, which revealed asymmetrical
labeling of sister chromatids (Rougier et al. 1998 ). Passive DNA demethylation is
also consistent with behavior of DNMT1, which remains retained in the cytoplasm
of the zygote until the 8-cell stage (Cardoso and Leonhardt 1999 ).
Active paternal DNA demethylation in the zygote is found in many mammals,
but it is perhaps not entirely conserved (Dean et al. 2001 ; Beaujean et al. 2004 ;
Lepikhov et al. 2008 ). The mechanism of active DNA demethylation in 1-cell
mammalian embryos involves genome-wide oxidation of 5mC to
5- hydroxymethylcytosine (5hmC) by Tet3 dioxygenase (Gu et al. 2011 ; Iqbal et al.
2011 ; Wossidlo et al. 2011 ). The reason why active methylation targets only pater-
nal DNA is that maternal DNA is apparently protected by PGC7/Dppa3/Stella
complex (Wossidlo et al. 2011 ). Loss of maternal Tet3 results in reduced fertility
(in terms of both successful matings and litter size) suggesting that oxidation of the
paternal 5mC is important but not essential (Gu et al. 2011 ). At this point, it is not
clear whether the phenotype reflects a nonessential function of maternal Tet-3 or
whether there is some redundancy in 5mC oxidation.
10.4.1.1 DNA Methylation During Early Development of Nonmammalian
Vertebrates
In contrast to mice, Xenopus maintains high global levels of DNA methylation
during early development (Veenstra and Wolffe 2001 ). Staining with an antibody
recognizing 5mC confirmed that the paternal genome of Xenopus embryos is not
actively demethylated (Stancheva et al. 2002 ). It was also found that high levels of
P. Svoboda et al.