LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 1
INTRODUCTION
Non-homologous end rejoining
Non-homologous end rejoining (NHEJ) has been considered the major pathway of DSB
repair in mammalian cells (Fig. 1.3B). Repair is achieved without the need for extensive
homology between the DNA ends to be joined. NHEJ processes the site-specific DSBs
introduced during V(D)J (variable [division] joining) recombination and its importance is
emphasised by the severe combined immune deficiency (SCID) and ionizing radiation
sensitivity of mice with NHEJ defects. NHEJ involves a DNA end-binding heterodimer
of the Ku70 and Ku80 proteins which activates the catalytic subunit (DNA-PKcs) of
DNA-dependent protein kinase (DNA-PK) by stabilizing its interaction with DNA ends.
This facilitates rejoining by a DNA ligase IV/Xrcc4 (X-ray cross-complementing 4)
heterodimer [15]. The presence of constitutively high steady-state levels of DNA-PK is
consistent with a role as a primary DNA damage recognition factor. The family of
kinases to which DNA-PKcs belongs contains the important ATM (ataxia telangiectasia
mutated) and ATR (ataxia telangiectasia Rad3-related) signalling proteins. All three
proteins are serine/ threonine protein kinases which have some sequence similarity to
phosphatidylinositol kinases. DNA-PK can phosphorylate p53 but the p53 response is
apparently intact in DNA-PK-defective mouse cells which express wild-type p53 [124].
These rules out DNA-PK as an essential requirement for activation of the p53 DNA
damage response. Other plausible targets for DNA-PK include the single stranded
binding protein RPA (replication protein A), the DNA ligase IV cofactor Xrcc4, Ku, and
DNA-PKcs itself. It is noteworthy that, although the stimulation of DNA ligase IV
activity by phosphorylated and unphosphorylated forms of Xrcc4 is similar,
phosphorylation of Xrcc4 may prevent its direct association with DNA [125]. This could
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