Biomedical Engineering Reference
In-Depth Information
Single-.and.double-strand.breaks.can.be.caused.by.ionizing.radiation,.alkylating.agents,.
reactive. oxygen. species. (ROS),. and. as. the. by-products. of. DNA. replication. and. repair..
Failure. to. repair. single-. or. double-strand. breaks. can. promote. mutations,. chromosomal.
instability,. and. cell. death.. The. irst. step. in. the. repair. of. strand. breaks. is. the. identiica-
tion. of. said. break.. Two. well-known. DNA. damage. sensors—Mre11-Rad50-Nbs1. (MRN).
and.Rad9-Rad1-Hsu1.(9-1-1).complexes.(Figure.6.1)—respond.to.double-.or.single-strand.
breaks. (Lee. and. Paull. 2005,. Parrilla-Castellar. et. al.. 2004).. The. highly. conserved. MRN.
complex. plays. a. critical. role. in. sensing. double-strand. breaks,. triggering. the. signaling.
pathway,. facilitating. DNA. repair. and. providing. the. maintenance. of. telomere. integrity.
and. meiosis. (van. den. Bosch. et. al.. 2003,. Williams. et. al.. 2007).. Mre11,. which. has. 3′. to. 5′.
dsDNA. exonuclease. activity. and. ssDNA. endonuclease. activity,. binds. to. DNA,. Rad50,.
and.Nbs1.through.its.coiled-coil.regions.(Hopfner.et.al..2002)..Nbs1.interacts.with.Mre11.
and. acts. as. a. lexible. adaptor. of. the. MRN. complex,. regulating. and. recruiting. signal-
transduction. kinases. to. DNA. lesions. (Carney. et. al.. 1998,. Lee. and. Paull. 2005).. Damage.
recognition.by.the.MRN.complex.is.ATP-dependent..The.presence.of.ATP.at.the.damage.
site. catalyzes. a. series. of. conformational. changes.. One. example. of. such. a. change. was.
revealed. by. crystal. analysis. of. Mre11-Rad50. bound. to. ATP/ADP.. The. presence. of. ATP.
allows.the.Rad50.nucleotide-binding.domains.to.shift.closer.to.the.Mre11.nuclease.and.
binding. sites.. The. shift. allows. for. increased. binding. to. dsDNA.. This. complex. has. the.
estimated.dimensions.of.7.5.×.8.0.×.9.2.nm.(Mockel.et.al..2012)..Mre11-Rad50.dimers.have.
been.shown.to.bring.DNA.ends.within.10.nm.of.each.other.for.repair.(Chen.et.al..2001,.
Hopfner.et.al..2002)..The.DNA-Mre11.interaction.has.been.recently.studied.using.crystal-
lography..All.six.DNA-binding.domains.interact.with.the.sugar-phosphate.backbone.of.
Double-strand breaks
Single-strand breaks
9-1-1
MRN
ATM
ATR/ATRIP
MDC1
Claspin
BRCA1
53BP1
CHK2
CHK1
p53
Cdc25
DNA repair, apoptosis, senescence
Figure 6.1
(See companion CD for color igure.)
.Double-strand.breaks.and.single-strand.breaks.initially.sensed.by.MRN.
or.9-1-1.complexes,.respectively..Damage.signals.were.further.ampliied.and.stabilized.by.recruiting.mediators.
to.damage.site.(γ-H2AX,.MDC1,.53BP1,.BRCA1,.and.Claspin).
