LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
associates with chromatin specifically during S-phase and it is thought that it plays a
functional role during normal replication [32, 33]. Transient inhibition of ATR results in
(i) inappropriate firing of replication origins [34]; (ii) fork stalling and (iii) replication
induced chromosome breakage [35, 36]. Recently, some individuals with the Seckel
syndrome were found to have very low levels of ATR kinase in their cells due to a splice
mutation in the ATR gene [37]. These individuals share phenotypes with DNA damage
response syndromes such as the Nijmegen breakage syndrome
Since the intrinsic kinase activity of ATR does not change appreciably following DNA
damage, it is thought that ATR activity is primarily regulated through its cellular
localization. ATR is known to form distinct nuclear foci following blockage of
replication [38] and it has been shown that the single strand-binding protein RPA and the
ATR-interacting protein ATRIP are important for the recruitment of ATR to sites of
blocked replication forks [26, 27]. It is thought that stretches of single-stranded DNA are
formed in and around the stalled replication fork and that these sequences become coated
with RPA proteins. ATR/ATRIP is then recruited to the site of RPA-coated DNA where
ATR phosphorylates its substrates. Some studies have shown that ATR is capable of
phosphorylating its targets even in cells depleted of RPA suggesting that in some
situation RPA may not be required for ATR activity [39, 40].
The Nijmegen breakage syndrome (NBS) has overlapping phenotypes with both ataxia
telangiectasia (AT) and the ATR-Seckel syndrome [37, 41]. Following treatment with the
replication blocking agent hydroxyurea (HU), Chk1 kinase and the tumor suppressor p53
are phosphorylated in an ATR-dependent manner and cells that make it through S-phase
will arrest in the G2 phase of the cell cycle [41]. In NBS1-defective cells, however, the
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