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very clearly demonstrated through genome-wide analysis of a number of
histone modifications using chromatin immunoprecipitation (ChIP)-Seq
on native chromatin from resting human CD4 cells (
Barski et al., 2007
).
The results from this analysis showed a different genomic distribution
between the monomethylation states of H3K27, H3K9, and H4K20, which
are mostly enriched at actively transcribed promoters, compared to the di-
and trimethylation states of the three corresponding lysines, which are char-
acteristic of silenced promoters. Thus, we will focus in our review on the
function and analysis of the di- and trimethylation of these residues. It is
of course unclear whether the results of these analyses, performed in somatic
cells, can be applied to ES cells and embryos, where the chromatin seems to
be in a slightly atypical configuration and gene expression is considered more
dynamic. Therefore, an analysis in mouse embryos is necessary to establish a
genome-wide correlation between gene expression and histone modifica-
tions or histone variants.
2.1. Dynamics of establishment of histone modifications
during development and function of their histone
methyltransferases
As an immediate response to fertilization by the sperm, the oocyte undergoes
its second meiotic division. A totipotent zygote is formed. It is the only cell
that contains two separate haploid nuclei within the same cytoplasm, which
are therefore referred to as pronuclei. The paternal and maternal genomes
remain physically distinct until at least the 2-cell stage and are marked by
different chromatin modifications (
Mayer, Niveleau, Walter, Fundele, &
Haaf, 2000
). At the zygote stage, the maternal pronucleus is characterized
by the presence of histone trimethylation marks that are specific of consti-
tutive heterochromatin, whereas the paternal genome generally lacks
such heterochromatic marks. In particular, H3K9me3, H4K20me3, and
H3K64me3 are found to localize to the pericentromeric chromatin in the
maternal chromatin, but not in the paternal one (
Arney, Bao, Bannister,
Kouzarides, & Surani, 2002; Daujat et al., 2009; Kourmouli et al., 2004;
Santos, Peters, Otte, Reik, & Dean, 2005
). Perhaps the only known excep-
tion of a histone mark exclusively associated with the maternal chromatin
that is not heterochromatic is H3K36me3, which is transmitted to the
embryo on the maternal chromatin and then rapidly remodeled to become
undetectable at the 2-cell stage (
Boskovic et al., 2012
). At around 12-14 h
after fertilization, the zygote undergoes the first mitotic division and two
daughter cells are formed, each containing a copy of
the parental
