51910.4.4 Erasure of Maternal Chromatin Modifications
During OET
Oocyte retains normal nucleosomal chromatin structure and has a global histone
modification pattern similar to that of somatic cells (reviewed in Burton and Torres-
Padilla 2014 ). A detailed catalogue of histone modification dynamics during OET
has been provided elsewhere (Burton and Torres-Padilla 2010 , 2014 ); here, we will
only focus on the loss of maternal histone modification marks during mouse OET.
There is not much evidence for robust global change in histone modifications
during NSN/SN transition. Despite establishment of transcriptional quiescence
during NSN/SN transition, histone H3 and H4 acetylation and H3K4me3 (marks
observed at active promoters) remain high in fully grown oocytes with SN chro-
matin configuration (Kageyama et al. 2007 ; Zuccotti et al. 2011 ). This phenom-
enon was observed in different species (reviewed in Gu et al. 2010 ;
Bonnet-Garnier et al. 2012 ).
The first main global elimination histone modifications formed during oocyte
growth appear upon resumption of meiosis. Histone acetylation (observed at active
genes) is removed during resumption of meiosis (Kim et al. 2003 ). However, this
change is not a part of epigenetic reprogramming of gene expression. Instead, his-
tone deacetylation at the onset of meiosis is important for proper meiotic chromo-
some segregation (Kim et al. 2003 ; De La Fuente et al. 2004 ; Akiyama et al. 2006 ).
Although most of histone acetylation is removed from the oocyte chromatin upon
its condensation and entry into meiosis, certain histone residues retain the acetyla-
tion mark (e.g., H4K8 Gu et al. 2010 ). Whether this specific residue becomes
deacetylated after fertilization in mammals remains to be determined.
Another global removal of histone modifications occurs after fertilization when
heterochromatin histone marks are removed from the maternal chromatin. This
includes H4K20me3 and H3K64me3, which are rapidly lost by the 2-cell stage
(Kourmouli et al. 2004 ; Daujat et al. 2009 ), and H3K9me3, which is reduced in
pericentric heterochromatin until the 8-cell stage, when it becomes highly enriched
again (Puschendorf et al. 2008 ). Removal of these histone marks suggests that era-
sure of maternal constitutive heterochromatin marks is a part of embryonic chro-
matin reprogramming into a totipotent state. This notion is consistent with open
chromatin observed during early development (Cho et al. 2002 ; Ahmed et al.
2010 ), which is reminiscent of chromatin structure as it is observed in pluripotent
embryonic stem cells (Ahmed et al. 2010 ; Martens et al. 2005 ).
10.4.4.1 Elimination of Specific Histone Variants
Histone modifications are not the only way to mark chromatin. A similar effect can
be achieved by deposition of specific histone variants, which are essentially adapted
homologues of histone proteins, which can be deposited at specific loci in
replication- dependent and replication-independent manner (reviewed in Henikoff
10 Clearance of Parental Products