LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
activity of ATR signaling is diminished following replication blockage leading to blunted
phosphorylation of Chk1 and p53 and attenuated cell cycle arrests. Thus, NBS1 may play
some role regulating ATR activity following replication blockage [41]. One potential role
of NBS1 in ATR signaling is to retain the ATR kinase at the sites of replication blockage
as to obtain a stronger induction of phosphorylation of its substrates [41].
The ATR kinase orchestrates DNA repair, apoptosis, S-phase and G2 arrest via the Chk1,
p53 and the Fanconi pathways
The outcomes of the activation of the ATR signaling pathway by replication stress is
many fold . First, ATR induces an intra-S checkpoint via phosphorylation of the Chk1
kinase, which targets the Cdc25A phosphatase for degradation [42, 43]. Loss of Cdc25A
results in the inhibition of replication firing [44, 45]. Furthermore, the FANCD2 protein,
which if defective causes Fanconi anemia (FA), becomes monoubiquitylated following
replication stalling in an ATR-dependent manner by a complex made up by at least six
different FA proteins [46-48]. This monoubuitylation of FANCD2 is critical for its
interaction with the tumor suppressor BRCA1 protein, the relocalization of these proteins
to sites of replication blockage and for the induction of S-phase arrest [46]. Second, the
activation of the Chk1 pathway by ATR following replication stress inhibits cells from
prematurely progressing into mitosis with unreplicated DNA [30, 35]. Third, ATR can
stimulate DNA repair through the activation of the FA pathway [49] by activation of p53
[50-53] or by Chk1-mediated activation of Rad51 [54]. The activation of p53 by ATR is
both direct, by phosphorylation of the ser15 site of p53, and indirect, by an inhibitory
phosphorylation of the MDM2 protein that normally negatively regulates p53 [55].
Fourth, activation of p53 may lead to the induction of apoptosis that eliminates cells that
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