Biology Reference
In-Depth Information
Bhutani, Burns, & Blau, 2011; Niehrs & Sch¨fer, 2012
) and that this may
be particularly relevant for reprogramming in early development (
Doege
et al., 2012; Gu et al., 2011; Inoue & Zhang, 2011; Piccolo et al.,
2013
). How these modified bases are resolved is still unresolved, although
the involvement of DNA repair mechanisms such as base excision repair or
nucleotide excision repair has been proposed (
Gehring, Reik, & Henikoff,
2009; Niehrs, 2009
). Gadd45a (
g
rowth
a
rrest and
D
NA
d
amage 45) has
been implicated in active DNA demethylation (
Barreto et al., 2007
) and
have been proposed to provide a link between cellular stress, DNA repair,
and epigenetic changes (reviewed in
Niehrs & Sch¨fer, 2012
).
5. DNA SYNTHESIS AND CHROMATIN REMODELING
IN REPROGRAMMING
ESCs have an atypical cell-cycle structure and this unusual profile is
adopted by somatic cells upon successful pluripotent reprogramming.
To test whether the acquisition of stem cell properties and altered cell-cycle
regulation are linked,
Ruiz et al. (2011)
examined the kinetics of iPSC der-
ivation using cells with modified cell cycle. They found that cycling human
fibroblasts and keratinocytes were more easily reprogrammed than counter-
parts that had been arrested in G1. This was not due to an acceleration in the
reprogramming process, since OCT4-GFP positive colonies appeared at
similar times using either targets, but rather, it reflected an increase in the
number of cells that were amenable to being reprogrammed. Acquisition
of a high proliferative rate is not, in itself, a reliable marker of successful
reprogramming, since pre-iPSCs showed similarly high proliferation but fail
to express the full quota of pluripotency factors (
Mikkelsen et al., 2008; Silva
et al., 2008; Sridharan et al., 2009
). In a similar vein, some human ESCs are
reported to have longer cell-cycle times, yet share a truncated G1 phase and
high proportion of S-phase cells with other ESCs (
Fig. 7.1
;
Ohtsuka &
Dalton, 2008
).
The mechanism by which cell division may influence reprogramming is
currently unknown. However, there is accumulating evidence that S phase
of the cell cycle provides a window of opportunity to change as well as to
maintain existing epigenetic states (reviewed in
McNairn & Gilbert, 2003
).
In order to maintain gene expression patterns, a cell must replicate its DNA
and copy epigenetic information such as CpGmethylation and modified his-
tones. The molecular mechanisms that allow DNA methylation to be
templated are, for the most part, understood. This is not the case for
