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E7.5 and continuing until E8.75, followed by an increase in histone H3
lysine 27 trimethylation (H3K27me3) between E8.25 and E9.5 (
Seki
et al., 2007
). The decrease in H3K9me2 is most likely due to the reduction
in GLP, a SET domain protein that forms a heteromeric complex with G9a
(
Tachibana et al., 2005
), as both G9a and GLP are required for the H3K9
methyltransferase (HMT) activity of the complex (
Tachibana et al., 2005
).
Glp
transcripts are reduced in PGCs from E7.25 (
Yabuta, Kurimoto,
Ohinata, Seki, & Saitou, 2006
), with a reduction in immunofluorescence
(IF) staining intensity already visible at E7.75 (
Seki et al., 2007
). The reduc-
tion in H3K9me2 correlates with a period of mitotic arrest in G2 phase of the
cell-cycle, shown by a decrease in bromodeoxyuridine (BrdU) incorporation
and the expression of high level of Cyclin B1, and a period of transcriptional
quiescence, as adjudged by a decrease BrUTP incorporation and a reduction
in phosphorylation of Ser 2 and Ser 5 of the C-terminal domain of RNA
polymerase II in comparison to neighboring cells (
Seki et al., 2007
). There
is also a decrease inDnmt3a and 3b transcript (
Yabuta et al., 2006
) and protein
(
Seki et al., 2005
) at E7.25 and E8, respectively. This is accompanied by a
reported decrease in 5-methylcytosine (5mC) staining in PGCs at E8, in
comparison with somatic neighbors (
Seki et al., 2005
).
Prmt5, an arginine-specificHMT thatmediates symmetrical dimethylation
of arginine 3 onhistoneH2Aand/orH4 tails (H2A/H4R3me2s) is enriched in
PGCs from E8.5 onward, and the H2A/H4R3me2s shows higher accumula-
tion in PGCs than soma at E10.5 (
Ancelinetal.,2006
). Blimp1 and Prmt5
can be coimmunoprecipitated in 293T cells suggesting that they may form a
complex in PGCs.
Finally, reactivation of the inactive X chromosome in females also com-
mences at an early stage of PGC development. PGCs at E6.5 exhibit nuclear
H3K27me3 foci and random inactivation of an X-GFP transgene, consistent
with an inactivated X chromosome (
Chuva De Sousa Lopes et al., 2008
).
However, starting from as early as E7 there is a progressive loss of the
Xist
signal and at E10.5 PGCs are
Xist
-negative (
Sugimoto & Abe, 2007
).
During migration to the genital ridges, the H3K27me3 foci are also gradu-
ally lost, and X-GFP is expressed in all PGCs by E13.5 (
Chuva De Sousa
Lopes et al., 2008
). Thus, there appears to be a global epigenetic transition
that begins in PGCs soon after specification. However, the function of, or
necessity for, these global epigenetic changes has not been assessed and
remains unclear.
A recent study in pigs has reported that H3K27me3 is high in migratory
PGCs (E15-E21), whereas H3K9me2 levels are low at E15 (the onset of
