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of H3K9me3 and H3K20me3 (
Carr, Yevtodiyenko, Schmidt, & Schmidt,
2007; Regha et al., 2007
). Histone modifications have been shown to play a
role in the modulation of tissue-specific imprinted expression. In the pla-
centa, histone modifications are even thought to be more important than
DNA methylation in governing imprinted expression; roles for Polycomb
complexes and the G9a methyltransferase have been assigned to monoallelic
expression of several imprinted genes in this tissue (
Umlauf et al., 2004;
Wagschal et al., 2008
).
Parent-specific histone profiles seem to occur in a DNA methylation-
dependent manner, as midgestation
Dnmt3L
/
þ
maternal methylation-free
embryos show a lack of parent-specific histone modifications (
Henckel et al.,
2012
). Interestingly, two ICRs, which can sporadically regainmethylation in
Dnmt3L
/
þ
embryos, displayed the correct parent-specific histone modi-
fication profile when the ICRwas properly methylated. Transcription is also
an important component in proper histone modification establishment at
imprinted loci. Even in the absence of DNA methylation of the
H19
ICR, transcription is sufficient to establish histone modifications in
cis
(
Verona, Thorvaldsen, Reese, & Bartolomei, 2007
). DNA methylation of
the ICRs, parent-specific histone modifications, and monoallelic transcrip-
tion are all tightly interconnected. More studies will be required to deter-
mine whether histone marks control the transcription of imprinted loci or
whether transcription itself induces histone marks, and these mechanisms
could differ in different developmental contexts.
6. CONCLUSIONS
At fertilization, the sexually dimorphic oocyte and spermunite to form
the zygote. They deliver their respective haploid genomes to the embryo,
which dramatically differ in terms of chromatin arrangement, histone mod-
ifications, and DNA methylation. Delineating which marks are only
byproducts of oocyte and sperm differentiation and which marks are also
important for further embryonic development will be the next important
task for the community to undertake. Nevertheless, despite intense embry-
onic genome reprogramming and natural elimination of haploinsufficiency,
some parental asymmetries are maintained and play an important role in
development. As exemplified by genomic imprinting, DNA methylation
is the most stable, if not only, vector for life-long persistence of parental
asymmetries inherited from the gametes. Maintenance of gamete-inherited
parent-specific DNA methylation occurs very scarcely in the mammalian
