Biomedical Engineering Reference
In-Depth Information
γ-H2AX.foci.in.order.to.measure.acute.exposure.to.ionizing.radiation..After.phosphoryla-
tion. of. H2AX,. MDC1. is. recruited. by. Nbs1. and. directly. bound. to. the. phospho-S139. and.
phosphorylated.by.ATM..This.interaction.further.ampliies.the.signal.of.γ-H2AX.to.spread.
out.more.than.1.MB.from.the.damage.site.(Rogakou.et.al..1998,.Stucki.and.Jackson.2006)..At.
the.same.time,.MDC1.acts.as.a.scaffold.for.the.recruitment.of.DNA.repair.and.signals.trans-
duction.proteins.at.the.damaged.chromatin.region.(Stucki.et.al..2005)..During.this.process,.
ubiquitination.and.sumoylation.are.involved.in.promoting.the.accumulation.of.BRCA1.and.
53BP1.at.the.damage.site.to.facilitate.and.stabilize.DNA.damage.response.machinery.(Doil.
et. al.. 2009,. Huen. et. al.. 2007,. Wang. and. Elledge. 2007).. ATM,. ATR,. and. checkpoint. kinase.
(CHK).2.were.also.shown.to.phosphorylate.BRCA1.at.different.residues.in.response.to.DNA.
damage.(Ouchi.2006)..Notably,.deiciencies.in.BRCA1.can.corrupt.the.repair.of.DNA.strand.
breaks.and.promote.increased.breast.cancer.incidence.(Tutt.et.al..2005).
p53. is. another. effector. protein. that. has. multiple. roles. in. response. to. DNA. damage.. It.
activates. the. transcription. of. genes. that. participate. in. DNA. repair,. cell-cycle. regulation,.
senescence,.or.apoptosis,.depending.on.the.type.or.the.sources.of.DNA.damage..Mdm2.
is.a.negative.regulator.of.p53.by.ubiquitin-mediated.proteasome.degradation..Upon.DNA.
damage,. ATM,. ATR,. and. DNA-PK. can. phosphorylate.p53. at. serine. 15,. while. CHK1. and.
CHK2.can.phosphorylate.p53.at.serine.20..Once.activated,.p53.can.transcriptionally.regu-
late.the.expression.of.the.cyclin-dependent.kinase.(CDK).inhibitor.p21,.as.well.as.of.the.
proapoptotic.BCL2-associated.X.protein.(BAX).and.p53.upregulated.modulator.of.apop-
tosis. (PUMA). proteins. that. induce. cell-cycle. arrest,. senescence,. or. apoptosis.. Moreover,.
p53.promotes.DNA.repair.and.deoxyribonucleoside.triphosphate.(dNTP).synthesis.(Chen.
et.al..2005,.Shieh.et.al..1997,.2000)..The.preponderance.of.cancer-associated.mutations.that.
disrupt.p53's.ability.to.bind.DNA.and.activate.gene.transcription.underscore.the.impor-
tance. of. this. p53. function. in. mediating. tumor. suppression.. DNA. damage. also. induces.
modiications. of. Mdm2. leading. to. Mdm2. destabilization. and. degradation,. which. effec-
tively.reduce.its.negative.regulatory.effect.on.p53.(Wade.et.al..2010)..This,.in.combination.
with.p53.N-terminal.phosphorylation.events.and.p53.binding.to.other.cellular.cofactors,.
leads.to.p53.stabilization.and.transcriptional.activation.
Activated.p53.functions.as.a.transcription.factor.by.binding.as.a.tetramer.to.a.speciic.
response.element.located.in.the.promoter.region.of.its.target.genes..This.element.consists.
of.two.repeats.of.a.10.bp.motif,.5′ - 3.×.Purine.C(A/T)(A/T)G.3.×.Pyrimidine-3′,.separated.by.
1-13.bp.(El-Deiry.et.al..1992)..On.the.nanoscale,.this.is.in.the.range.of.∼7-10.nm,.with.this.
binding. interface. and.corresponding. protein-DNA. interactions.being. vital.for.the.DNA.
damage.response..Considering.that.nanoparticles.are.cell.permeable.and.targetable,.they.
have.the.potential.to.be.coated,.or.functionalized,.with.p53.derivatives.and.used.as.an.anti-
cancer.therapy.aimed.at.eliciting.either.a.killing.or.growth-arrest.response.in.cancers.that.
have.lost.p53,.estimated.to.be.∼50%.(Hollstein.et.al..1996,.Nigro.et.al..1989)..Once.activated,.
p53.can.transcriptionally.regulate.the.expression.of.the.CDK.inhibitor.p21.as.well.as.of.the.
proapoptotic.BAX.and.PUMA.proteins.that.induce.cell.cycle.arrest,.senescence,.or.apopto-
sis..Moreover,.p53.promotes.DNA.repair.and.dNTP.synthesis.(Chen.et.al..2005,.Shieh.et.al..
1997,.2000)..The.p53-dependent.induction.of.the.ribonucleotide.reductase.(RNR).subunit.
p53R2.has.been.shown.to.be.important.for.the.cellular.response.to.DNA.damage.
6.1.3 Other responses to Strand Breaks
In. addition. to. the. MRN. and. 9-1-1. complexes,. two. poly. ADP-ribose. polymerase. (PARP).
family.members,.PARP1.and.PARP2,.are.also.known.to.be.molecular.sensors.of.both.single-
strand. and. double-strand. DNA. breaks.. Mouse. cells. deicient. in. Parp1. or. Parp2. display.
