Biology Reference
In-Depth Information
3.4. Reprogramming of DNA methylation in gametes
In mice, the primordial germ cells (PGCs) are specified from posterior prox-
imal epiblast cells at embryonic day (E) 6.25 and express specific markers
such as
Prdm1
(also known as
Blimp1
),
Prdm14
, and
Dppa3
(also known
as
PGC7
and
Stella
). PGCs form a cluster of around 40 cells at E7.25 and
then colonize the genital ridges by E10.5, where they continue to proliferate
until E13.5. Because PGCs are derived from somatic cells that have already
acquired a somatic epigenetic profile, they undergo a global resetting leading
to repression of somatic genes, changes in histone modifications, exchange
of histone variants, and global DNA demethylation (
Hackett, Zylicz, &
Surani, 2012; Saitou et al., 2012
;
Fig. 2.4
). The genome-wide demethyl-
ation in PGCs is evidenced by a global loss of 5mC immunostaining signal
from approximately E8.0 (
Seki et al., 2005
). PGCs also repress
DNMT3A/B
as well as
UHRF1
, a factor required to maintain DNA methylation patterns
during replication (
Kurimoto et al., 2008; Yabuta, Kurimoto, Ohinata,
Seki, & Saitou, 2006
). Global mapping by bisulfite sequencing or immuno-
precipitation confirmed that E13.5 PGCs have a demethylated genome
compared to somatic cells (
Guibert et al., 2012; Seisenberger et al.,
2012
). This includes erasure of methylation at imprinting loci, gene bodies,
intergenic regions, mobile elements as well as virtually all CpG island pro-
moters found methylated in early embryos (
Guibert et al., 2012;
Seisenberger et al., 2012
), indicating that the extent of methylation erasure
in PGCs is more complete than in preimplantation embryos. The kinetics of
demethylation varies depending on the target sequence: certain sequences
like ICRs (imprinting control regions) maintain high levels of cytosine
methylation until E10.5, whereas most other sequences initiate slow
demethylation in early PGC precursors around E8.0 that probably results
from a passive lack of maintenance by DNMT1 (
Guibert et al., 2012;
Seisenberger et al., 2012
). These successive waves of demethylation might
reflect the existence of multiple mechanisms being used in PGCs to achieve
full demethylation (
Hackett et al., 2012
). As DNAmethylation mediates sta-
ble silencing of pluripotency-related and meiotic genes (see
Section 4.2
), this
global demethylation in PGCs is necessary to restore an epigenetic state
compatible with the expression of the germ-line program. It might also serve
to erase deleterious epimutations even though there are rare single-copy and
repeated sequences that maintain high levels of methylation in PGCs and
could potentially mediate transgenerational epigenetic inheritance
(
Guibert et al., 2012; Lane et al., 2003; Seisenberger et al., 2012
).
