Cell and Molecular Biology of DNA Methyltransferase 1 - International Review of Cell and Molecular Biology - page 27

downregulation did not involve transcriptional downregulation suggesting
that DNMT1 in some way influences the stability of repair proteins such as
MLH1 and PMS2. The DNMT1-deficient cells were resistant to
6-thioguanine and up to 10-fold mutation rates at a CA 17 reporter (both
features are characteristic of mismatch repair-deficient cells) but had limited
viability. This inviability was shown to be rescued by PARP, suggesting an
intricate relationship between DNMT1 levels, mismatch repair machinery,
and PARP in regulating the proliferation and repair efficiency of
mammalian cells.

5. DEMETHYLATION
Demethylation refers to the process of removal of methyl groups from
5-methyl-cytosine, a process that has been observed in both plants and ani-
mals ( Gehring et al., 2008 ). As described above and schematically in Fig. 1.7 ,
periods of significant demethylation occur during the reproductive cycle in
mammals. In comparison, the only period of pronounced demethylation in
plants occurs in the vegetative cell (VC) of the pollen, central cell (CC) of
the female gametophyte, and the triploid endosperm surrounding the
embryo. Defining the demethylation mechanism(s) in mammals and plants
and determining whether DNMT1 is involved in mammalian demethyla-
tion are important issues for understanding the molecular mechanisms
underlying changes in the patterns of genomic methylation. For these rea-
sons, the main focus of this section will be exploring mechanisms whereby
the genome loses methylation during development, starting with descrip-
tions of characterized mechanisms in plants and moving into a discussion
of possible mechanisms in mammals.

5.1. Lessons from plants — Developmental, biochemical,

and genetic studies
It is now well established that an active or catalytic process of demethylation
occurs in plants. The active process is initiated in the VC and CC ( Gehring
et al., 2008; Johnson and Bender, 2009 ; Fig. 1.8 ). The biochemical mech-
anism in both cells involves DNA glycosylase activity that removes a methyl-
cytosine base from DNA. 5-Methyl-cytosine DNA glycosylase activity is
encoded by four genes in Arabidopsis (ROS1, DME, DML2, and DML3)
( Agius et al., 2006; Penterman et al., 2007a,b ). Of these, DME is preferen-
tially expressed in CCs (female gametophytes) and endosperm (tissue devel-
oping following “fertilization” of the CC by one of the two haploid sperm