Biology Reference
In-Depth Information
A. Cellular Defects in ataxia telangiectasia
Early studies demonstrated that cells derived from A-T patients were
extremely sensitive to IR, but exhibited normal responses to UV or other
types of DNA damage, indicating a specific defect in DSB repair. 11 Additional
work established that A-T cells lacked the normal activation of cell cycle
checkpoints following IR exposure, and continued to progress through
S-phase in the presence of DSBs. 12,13 Work on A-T was greatly stimulated by
the cloning of the A-T gene, which identified ATM as a member of the PI
3-kinase (PIK) family of DNA damage-activated kinases, 14,15 a family which
includes the DNA-PKcs and ATR kinases. 16 Sequence analysis of the A-T gene
demonstrated that the majority of mutations in A-T patients were truncating
mutations, 17,18 so that A-T patients essentially express no ATM protein. The
increased cancer risk and sensitivity to IR in A-T patients can therefore be
attributed to the absence of functional ATM protein in these patients. It was
subsequently demonstrated that ATM was a kinase whose activity was in-
creased by DNA damage and that ATM directly phosphorylated the p53
protein. 19 Work from many laboratories has now identified several hundred
proteins as potential targets for ATM, 20 including key components of the DSB
repair pathway such as p53, nbs1, chk2, brca1, and H2AX (reviewed in Refs.
1,9 ). Overall, this work demonstrated that ATM's kinase activity and its ability
to phosphorylate target proteins were critical for cells to both activate cell cycle
checkpoints and coordinate the repair of DSBs.
We now have a clear picture of the events that are triggered through the
activation of ATM's kinase by IR. One of the earliest detectable events is
phosphorylation of the histone variant H2AX (termed g H2AX 21 )byATM
within minutes of exposure to IR. g H2AX is phosphorylated on chromatin
domains surrounding the DSB which can extend up to 500 kb on either side
of the break. 22 The mdc1 scaffold protein then binds directly to g H2AX,
creating a platform to recruit other DNA repair proteins, including 53BP1,
RNF8, and ultimately brca1, to the DSB. 23-27 This results in the concentra-
tion of DNA repair proteins onto chromatin domains extending away from
the DSB, which can be visualized by immunofluorescence techniques with
antibodies against components of these complexes. 25-27 Further, many of
these proteins, including mdc1 26 and brca1, 28 are also phosphorylated by
ATM, indicating a complex and highly regulated role for ATM's kinase
activity in controlling the cells' response to DSBs. Thus, activation of
ATM's kinase activity initiates a signal transduction pathway, which promotes
recruitment of DNA repair complexes to DSBs, activates cell cycle check-
points, and creates an environment for promoting DSB repair. ATM there-
fore plays a pivotal role in the earliest steps involved in the detection and
repair of DSBs.
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