Biomedical Engineering Reference
In-Depth Information
free DNA ends together, until DNA ligase and XRCC4 (LC complex) bound to
DSBs and rejoin the ends [
76
,
77
]. Refer to comprehensive reviews for a detailed
description of the biochemistry of NHEJ and HR [
78
-
85
], and for repair of DSBs
associated with replication or transcription [
24
] as well as the alternative repair
pathways [
40
,
86
].
20.3.2
Additional NHEJ components
The simple scenario described above is only possible when euchromatic DSBs
with undamaged DNA ends [
66
-
71
] are being repaired. In that case, DSBs are
rejoined quickly, often without changes in genetic information. However, additional
steps that include modifications of chromatin structure and DNA-end processing
are necessary to repair heterochromatic breaks and most DSBs induced by ionizing
radiation (IR) that contain “dirty” ends with single-stranded overhangs and damaged
bases [
87
]. This section will focus on the biochemical functions of “additional”
NHEJ proteins and their cooperation, since the relationship between DSB repair
and modifications of chromatin structure is the subject of Sec.
20.4
.
Nuclease activities required to clear DNA ends were recognized for the Mre11
component of the MRN complex [
59
-
61
,
88
] and, are, with its affinity to DNA,
stimulated by another MRN member, Rad50 [
89
]. The third MRN member, Nbs1,
mediates nuclear localization of the MRN complex [
90
] and further stimulates
its activities [
89
-
91
]. However, the physiological importance of the exonuclease
function of MRN is uncertain, since only 15% of DSBs induced by IR (that contain
damaged DNA ends) require this protein for repair [
13
,
66
-
71
, etc.]. Therefore, other
endonucleases such as Artemis probably clear DNA ends [reviewed in
92
], whereas
MRN complex serves rather as an activator of ATM signaling
1
[
25
,
88
,
93
]. Since
Artemis is activated by ATM, this interaction represents one of the contact points
between NHEJ and ATM signaling that demonstrates participation of both of these
pathways in the repair of more complex DSBs or DSBs located in more complex
(dense) chromatin.
Another important early step of DSB repair is ATM-mediated phosphorylation
of the histone H2AX [
11
,
94
,
95
], and the consequent formation of
H2AX foci
that cover megabase regions on either side of DSB [
96
] (see Sec.
20.4.3
devoted
to
”
H2AX foci), and create a platform for binding of “non-core” repair proteins
(like MDC1 [
97
,
98
], 53BP1 [
98
,
99
], BRCA1 [
100
,
101
], SMC1 and others
[
e.g
.,
100
,
102
]) to the site of damage. These proteins are also phosphorylated by
ATM and, once activated, bind to
”
H2AX foci. Even though the activity of these
“mediators” is dispensable for initial binding of DSB sensors to the lesion sites
[
103
] and also for repair of most DSBs [
66
-
71
], they are necessary for proper DDR
response [
98
,
101
,
104
] and repair of a subgroup of DSBs that are repaired only
”
1
the function of MRN as a DSB sensor [
53
,
66
,
93
] is also hardly conceivable if only 15% of DSBs
require MRN for their repair (even thought MRN colocalizes with DSBs in general)
