Biomedical Engineering Reference
In-Depth Information
reports.of.A-TLD.patients.developing.cancer.(Uchisaka.et.al..2009)..However,.A-TLD.is.a.
rare.disorder,.and.it.is.not.yet.known.if.A-TLD.increases.cancer.incidence.
6.2.2 MrN: A Controllable Nanomachine
Live-cell.imaging.of.cells.undergoing.DNA.double-stranded.breakage.reveals.this.phos-
phorylation.at.the.damage.site.within.seconds..This.recruitment.was.shown.to.be.traceable.
across.at.least.10.μm.of.damage.(So.et.al..2009)..Each.phosphorylation.is.capable.of.altering.
the.structure.of.the.complex,. thus.instructing.the.complex. to.perform.a.task..The.MRN.
complex. has. as. many. as. 216. different. states. arising. from. the. individual. conformational.
changes.of.each.subunit.(Williams.et.al..2008)..In.a.sense,.the.MRN.complex.can.be.con-
sidered.a.nanomachine.activated.and.modiied.for.the.DNA.damage.task.required.of.it.by.
operating.a.series.of.switches.represented.by.the.sites.of.phosphorylation.
6.3 Making DNA Building Blocks Post-DNA Damage
DNA. damage. response. pathways. are. conserved. among. eukaryotes.. As. an. example,. the.
activation.of.RNR.components.has.been.demonstrated.in.both.human.and.
Saccharomyces
cerevisiae
. cells,. among. other,. via. ATM/ATR. and. CHK2. genes. (called. Mec1/Rad53. in.
S. cerevisiae)
..In.
S. cerevisiae
,.the.Mec1/Rad53.signaling.pathway.promotes.a.DNA.damage.
response. by. altering. dNTPs. pools. through. regulating. the. activity. of. the. RNR. complex.
(Chabes. and. Stillman. 2007,. Lozano. and. Elledge. 2000,. Vallen. and. Cross. 1999,. Zhou. and.
Elledge. 2000).. dNTPs. are. an. essential. prerequisite. for. faithful. genome. duplication. and.
DNA. repair.. A. balanced. supply. and. the. overall. concentration. of. dNTPs. are. tightly. reg-
ulated. by. the. enzyme. RNR,. which. catalyzes. the. rate-limiting. step. in. the. production.
of. dNTPs. required. for. both. DNA. synthesis. and. DNA. repair. (Chabes. et. al.. 2003a).. In.
S. cerevisiae
,. there. are. four. RNR. genes:. RNR1. and. RNR3. code. for. large. subunits,. while.
RNR2.and.RNR4.code.for.small.subunits..RNR1.is.essential.for.mitotic.growth,.whereas.
RNR3.is.nonessential.and.is.normally.expressed.at.very.low.levels.but.is.highly.induced.
after. DNA. damage.. Both. RNR2. and. RNR4. are. essential. and. induced. by. DNA. damage..
In.mammals,.RNR.consists.of.a.large.subunit.R1.and.two.distinct.small.subunits,.R2.and.
p53R2..The.levels.of.the.R2.subunit.control.the.overall.RNR.activity.during.the.cell.cycle,.
while.R1.and.p53R2.are.induced.by.DNA.damage.(Lozano.and.Elledge.2000).
6.3.1 regulation of rNr Activity
In. mammals,. ATM/ATR. and. CHK2. kinase. pathways. regulate. the. transcription. of. RNR.
genes. (Kastan. and. Lim. 2000).. The. overall. RNR. activity. is. regulated. by. the. R2. subunit.
during.DNA.synthesis..DNA.damage.induces.the.expression.of.R1.and.p53R2.proteins.to.
form.an.active.RNR.complex.and.supply.dNTPs.for.DNA.repair.(Figure.6.2)..In.mice,.R2.
transcription.is.up-regulated.only.during.S.phase.and.is.not.induced.by.DNA.damage.or.a.
replication.block.(Chabes.et.al..2004)..However,.p53R2.transcription.is.activated.by.the.p53-
dependent.checkpoint.pathway.in.response.to.DNA.damage.(Lozano.and.Elledge.2000)..
When.p53R2.is.expressed,.the.level.of.R2.is.repressed,.which.serves.to.halt.DNA.replica-
tion..Interestingly,.p53.is.mutated.in.many.human.cancers.(Hollstein.et.al..1991),.resulting.
in.a.deiciency.of.p53R2.expression.and.a.lack.of.DNA.repair.capacity.in.cancer.cells.
Mammalian. RNR. activity. is. also. tightly. regulated. by. the. cell. cycle.. R1. protein. levels.
are.constant.throughout.the.cell.cycle.and.R1.has.a.long.half-life.of.18-24.h..R2.levels.are.
