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completely devoid of protamines; instead, histones are diluted among prot-
amines, forming heterogeneous histone/protamine mixtures. Further car-
tography of histone modifications by chromatin immunoprecipitation
(ChIP) followed by array analysis or high-throughput sequencing has thus
far been focused on permissive H3K4me2/me3 marks and repressive
H3K27me3 marks (
Brykczynska et al., 2010; Hammoud et al., 2009
). While
transcriptionally inert, the mature sperm contains H3K4me2/3 marks,
which map specifically to promoters of genes involved in spermatogenesis
and basic cellular processes, potentially reflecting expression and require-
ment of these gene functions at prior stages of male germ-cell development
(
Brykczynska et al., 2010
). A distinct set of genes is marked by H3K27me3
modifications. These are mostly developmentally important genes such as
the
Hox
genes and early embryonic transcription factors, such as
Sox2
,
Cdx2
, and
Gata6
(
Brykczynska et al., 2010; Hammoud et al., 2009
). In pat-
terns reminiscent of what was previously reported as bivalent chromatin
domains in ES cells, dual enrichment of H3K4me2/3 and H3K27me3
was also observed at the promoters of some development-related genes,
which may be important for poising future expression states (
Bernstein
et al., 2006; Pan et al., 2007
). Programmed histone retention and modifica-
tions in sperm could be important for fertility, as genome-wide analyses have
highlighted altered sperm histone composition in subfertile men with
various reproductive dysfunctions (
Hammoud et al., 2011
).
In comparison with humans, protamination occurs more extensively in
mouse sperm cells, with an estimation of only 1% of total DNA retained in
nucleosomes (
Brykczynska et al., 2010; van der Heijden et al., 2006
). None-
theless, high conservation exists between mouse and human with respect to
H3K4me2 marks at housekeeping and testis-specific genes, as well as
H3K27me3 enrichment at developmentally important loci (
Brykczynska
et al., 2010; van der Heijden et al., 2006
). However, some discrepancies
exist, such as the dual occupancy of H3K4me3 and H3K27me3 marks at
some testis-specific genes observed in mice only. This specificity may reflect
the need to prevent spurious expression of these genes after fertilization, a
risk that could be incurred due to the relatively early activation of the mouse
embryonic genome as compared to humans. Genome-wide mapping of
these and other histone modifications remains to be conducted; however,
these data suggest that the paternal genome could transmit a certain level of
histone-based information, provided that these are not immediately replaced
by oocyte-inherited histones in the zygote. Of note, H3K27me3-marked
regions exhibited higher histone retention in nucleosome/protamine
