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report, H2AX phosphorylation in response to IR is normal in hSNF5-deficient
cells,
85
and we do not see loss of IR-induced H2AX phosphorylation in cells
with BAF180 and BRG1 simultaneously knocked down (E. Riballo, JAD
Downs, and P.A. Jeggo, unpublished data). In addition, knockdown of
BRIT1, which is important for recruitment of SWI/SNF to chromatin (de-
scribed later), results in increased (rather than decreased) H2AX phosphory-
lation after IR.
86
It is possible that a greater degree of redundancy exists in
mammalian cells and that the contribution of SWI/SNF activity to H2AX
phosphorylation is only apparent under certain conditions.
The SWI/SNF complex is recruited to sites of DNA DSBs,
83,86,87
suggest-
ing that as in yeast, it plays a direct role at the DNA lesions. The mechanism by
which it is recruited is not entirely clear, but several proteins and protein
modifications have been implicated. BRIT1 has three BRCT domains and
interacts with SWI/SNF via the BAF170 and BAF155 subunits. When BRIT1
was depleted, the association of BRG1 and hBRM with chromatin, both
globally and at a DNA DSB, was impaired,
86
suggesting that it mediates
recruitment. Histone acetylation has also been implicated in mediated recruit-
ment of SWI/SNF to chromatin at sites of DNA damage.
87,88
The CBP/p300
histone acetyltransferase was found to acetylate H3 and H4 at DNA DSBs.
Ablation of this HAT resulted in impaired recruitment of hBRM to DNA
DSBs.
87
Somewhat consistent with this, Lee et al. found evidence for a positive
feedback loop, in which IR-induced H3 acetylation promotes BRG1 binding to
chromatin, which in turn promotes H2AX phosphorylation, and this stimulates
H3 acetylation.
88
This report implicated GCN5 as the responsible HAT,
although the contribution of GCN5 and CBP/p300 does not have to be mutu-
ally exclusive. It is also possible that BRG1 and hBRM have nonoverlapping
roles with different modes of recruitment to DNA DSBs.
What is the impact of SWI/SNF activity on DNA DSB responses? Cells
expressing the ATPase inactive dominant negative version of BRG1 show
defective survival after DNA damage, increased DNA damage-induced apo-
ptosis, and slower repair of DNA DSBs.
83,89
Similarly, downregulation of both
BRG1 and hBRM or inactivation of Snf5 leads to slower repair of DNA DSBs
and reduced survival after damage.
83,90
Consistent with a specific role in medi-
ating DSB repair, accumulation of Ku at sites of DNA damage is reduced and
NHEJ activity is impaired in cells with hBRM knocked down.
87
Although not a
subunit of either core complex, BRIT1 deficiency also showed a defect in DSB
repair after treatment with IR and lower levels of HR activity using a reporter
system.
86
However, in another study, SNF5-deficient cells did not show hyper-
sensitivity to a variety of DNA-damaging agents (cisplatin, etoposide, and UV)
and the cells showed no evidence of chromosomal aberrations as might be
expected if DNA DSB repair was defective.
85
This might indicate that the
contribution of SWI/SNF to DNADSB repair is cell type or context dependent.
