LIFE01200604005 Shri Somnath Ghosh - Homi Bhabha National ...

CHAPTER 4
DISCUSSION
Thus unlike the cell survival, the difference in induction of γ-H2AX foci 15 min after 2Gy of
either radiation was not as profound (Fig. 3.3.2.2). Similar number of γ-H2AX at 15 minutes
after irradiation irrespective of the radiation quality indicated that initial events might not differ
much with the radiation quality. Activation of other molecules involved in DNA damage
response were therefore followed only at later time point of 4 hours. Moreover, by this time low
LET induced damage is mostly repaired and the activation of these factors is reduced to control
levels [284] thereby sealing the fate of the cell. Comparing the activation of these molecules at
this time could therefore give important insight into differences in damage response signaling
with respect to radiation quality. Phosphorylation of H2AX even at 4 hours (Fig.3.3.2.2B & D)
indicates the presence of large number of double strand breaks at that time indicating very slow
repair in A549 cells exposed to oxygen beam.
Persistence of significant pATM foci till 4 hours after oxygen ion irradiation (Fig.3.3.2.3)
indicates the presence of unrepaired DNA, which is still keeping the damage sensor protein
ATM in activated form. The slower disappearance of pATM foci from cells, unlike the γ-H2AX
foci (that was seen in 60% of oxygen irradiated cells at 4 hours), could reflect their different
roles in the repair process with γ-H2AX foci playing a primary role in activating downstream
pathways and initiating DNA repair while ATM foci remaining till broken DNA ends are
present. Since the cell survival after 2Gy oxygen irradiation was only 11%, ATM might be
activating downstream apoptotic responses.
The massive activation of ATR after oxygen irradiation when compared with gamma
(Fig.3.3.2.4) in 36% of the cells indicated its specific role in high LET induced complex damage.
The downstream component Chk2, which is activated by ATM and shares its substrates like p53,
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