Cell and Molecular Biology of DNA Methyltransferase 1 - International Review of Cell and Molecular Biology - page 29

lyase activity). The general role of genes encoding 5-methyl-cytosine DNA
glycosylase activity is shown in the effect of triple ros1 dml2 dml3 mutant
plants, which exhibit hypermethylated genomes in their somatic cells, with
the hypermethylation residing primarily in promoters and 3 0 -UTRs of genes
( Lister et al., 2008; Penterman et al., 2007a ).
There are two related issues to the active demethylation in the VC and
CC that may bear on mechanisms of demethylation in mammalian species.
First, the DME-mediated active demethylation in the CC and endosperm
seems to be supported by an accompanying inhibited expression of
MET1 during female gametophyte cell proliferation ( Gehring et al.,
2006; Jullien et al., 2008 ). This suggests that passive demethylation is also
important for the loss of methylation in the endosperm and the perpetuation
of imprinted gene expression. This notion is supported by genetic studies in
which met1 alleles suppress dme -mutant seed phenotypes ( Xiao et al., 2003 ).
Second, there are important consequences of active demethylation in
CCs, VCs, and endosperm that may influence our understanding of the roles
of demethylation in mammals. Genomic demethylation in extraembryonic
plant cells correlates with increased transcription at many genomic sites, set-
ting off a process known as RNA-dependent DNA methylation (RdDM;
Wassenegger et al., 1994; Zhang and Zhu, 2011 ). Interestingly, RdDM pri-
marily targets chromosomal sites of repetitive sequences and can be viewed
as a secondary protective mechanism to ensure suppression of potentially
harmful retrotransposons ( Calarco et al., 2012; Johnson and Bender,
2009 ). Interestingly, these genomic sites are primarily in the embryo's
genome, suggesting a model in which small RNAs (sRNAs) generated from
demethylated genomic sites in the VC and CC freely transit to the embryo's
cells. Once in the nucleus of the embryo's cells, they trigger the process of
RdDM. A connection between active glycosylase-dependent demethyla-
tion and RdDM can also be demonstrated through genetic analysis: suppres-
sors of ros1 mutations include genes encoding components of RdDM
( He et al., 2009 ). More about RdDM, particularly its relationship to de novo
methylation mechanisms in mammals, will be discussed in Section 6 .
5.2. Evidence for active demethylation in animals
Because of well-documented declines in genomic methylation during mam-
malian development and the identification of demethylation mechanisms
based on findings in Arabidopsis , investigators have vigorously pursued the
notion that active demethylation occurs on a large genomic scale in