Biology Reference
In-Depth Information
upregulated;
dnmt3
transcripts are nonetheless already detected before the
MBT (
Fig. 3.2
B), which is indicative of early polyadenylation of transcripts
already present in the fertilized egg (
Aanes et al., 2011
).
dnmt7
expression
parallels that of
dnmt3
(
Fig. 3.2
B), suggesting redundancy between these
enzymes. (iii) Remarkably, two additional genes,
dnmt4
and
dnmt5
, display
transient expression restricted to the pre-MBT period, increasing levels up
to the 256-cell and MBT stages, and marked degradation at the MBT
(
Fig. 3.2
B). It is probable that polyadenylation of these transcripts during
pre-MBT development results in the translation of Dnmt enzymes; it would
then be interesting to determine what their targets are and whether they
mediate (transient?)
de novo
methylation of developmentally important
genes, perhaps in a cell type-specific manner within the embryo. (iv)
dnmt8
is apparently not expressed during any of the pre-MBT, MBT, and post-
MBT stages examined. Altogether, these findings are consistent with
DNA methyltransferase activity in the early zebrafish embryo prior to the
MZT onset. They also suggest a switch from maintenance methylation to
de novo
DNA methyltransferase activity around the MZT. Future work is
expected to investigate presence, activity, and targets of these putative
Dnmts during zebrafish pre-MBT development and whether these play a
role in maintaining early developmentally regulated and lineage specifica-
tion genes in a repressed state prior to ZGA onset.
3.2. Dynamics of DNA methylation through the MZT
Recent studies have started to elucidate the impact of DNA methylation on
the regulation of gene expression in embryos, notably in mouse (
Borgel
et al., 2010; Smallwood et al., 2011; Smith et al., 2012; Xie et al., 2012
),
frog (
Bogdanovic et al., 2011; Jones & Takai, 2001; Stancheva,
El-Maarri, Walter, Niveleau, & Meehan, 2002; Veenstra & Wolffe,
2001
), and zebrafish (
Andersen, Reiner, et al., 2012; Mhanni &
McGowan, 2004; Rai et al., 2008, 2010, 2006
). Two major cycles of
genome-wide DNA demethylation and remethylation characterize the
mammalian life cycle (
Reik, 2007
). The first cycle takes place during germ
cell formation, when parental imprints are reset by demethylation and dif-
ferential remethylation of maternal and paternal alleles. This demethylation
may also be important for the removal of epimutations that may have arisen
during gametogenesis (
Reik, Dean, & Walter, 2001
). The second occurs
after fertilization, when maternal and paternal methylation patterns are
erased and reestablished during blastocyst
formation. Imprinted genes
