Telomere maintenance and human bone marrow failure.docx

Telomere maintenance and human bone marrow failure.docx

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Telomeremaintenanceandhumanbonemarrowfailure

Telomeremaintenanceandhumanbonemarrowfailure

RodrigoT.Calado1,*,andNealS.Young1,*

1HematologyBranch,NationalHeart,Lung,andBloodInstitute,NationalInstitutesofHealth,Bethesda,MD

  Abstract

Top

Abstract

Introduction

Telomerebiology

Animalmodels

Telomereshorteninginhuman...

Conclusions

Authorship

References

Acquiredandcongenitalaplasticanemiasrecentlyhavebeenlinkedmolecularlyandpathophysiologicallybyabnormaltelomeremaintenance.Telomeresarerepeatednucleotidesequencesthatcaptheendsofchromosomesandprotectthemfromdamage.Telomeresareerodedwithcelldivision,butinhematopoieticstemcells,maintenanceoftheirlengthismediatedbytelomerase.Acceleratedtelomereshorteningisvirtuallyuniversalindyskeratosiscongenita,causedbymutationsingenesencodingcomponentsoftelomeraseortelomere-bindingprotein（TERT,TERC,DKC1,NOP10,orTINF2）.Aboutone-thirdofpatientswithacquiredaplasticanemiaalsohaveshorttelomeres,whichinsomecasesassociatewithTERTorTERCmutations.Thesemutationscauselowtelomeraseactivity,acceleratedtelomereshortening,anddiminishedproliferativecapacityofhematopoieticprogenitors.Asinothergeneticdiseases,additionalenvironmental,genetic,andepigeneticmodifiersmustcontributetotelomereerosionandultimatelytodiseasephenotype.Shorttelomeresalsomaycausegenomicinstabilityandmalignantprogressioninthesemarrowfailuresyndromes.Identificationofshorttelomereshaspotentialclinicalimplications:

itmaybeusefulindyskeratosiscongenitadiagnosis,insuggestingmutationsinpatientswithacquiredaplasticanemia,andforselectionofsuitablehematopoieticstemcellfamilydonorsfortransplantationintelomerase-deficientpatients.

  Introduction

Top

Abstract

Introduction

Telomerebiology

Animalmodels

Telomereshorteninginhuman...

Conclusions

Authorship

References

Historicallyregardedaspathophysiologicallydistinctentities,acquiredandconstitutionalaplasticanemiasrecentlyhavebeenlinkedbyacommonpathogenicfeature—telomereshorteninginleukocytes.Aboutadecadeago,telomereswerefirstmeasuredtobeshortinaboutone-thirdofacquiredaplasticanemiacasesbyinvestigatorsatStGeorge'sHospital（London,UnitedKingdom）,andpatientswiththeshortesttelomeresappearedtohavealongerdurationofdiseaseandtobemorelikelytodeveloplateandmalignantclonalcomplications.1Weandothersreportedthatgranulocytesweremainlyaffectedbytelomereerosionandthatpatientswithshorttelomereswerelesslikelytorespondtoimmunosuppression.2,3Telomereshorteningalsoiscommonincongenitalaplasticanemias,suchasFanconianemia,4,5dyskeratosiscongenita,6,7andShwachman-Diamondsyndrome.8

Initially,telomereshorteninginmarrowfailurewasthoughttobesimplyaconsequenceof"stressed"hematopoiesis.However,thediscoveriesthat

（1）somepedigreesofX-linkeddyskeratosiscongenitaweremutantinagenecalledDKC1,madebyDokaletalattheHammersmithHospital（London,UnitedKingdom）,9and

（2）thattheDKC1geneproduct,dyskerin,physicallyassociatedwiththetelomerasecomplex,madebyMitchellandCollinsattheUniversityofCaliforniaatBerkeley,6providedacrucialconnectionbetweenadiscretegeneticlesionandtelomereshorteningofhematopoieticcells.Subsequently,mutationsinsimilargenesrelatedtotelomeremaintenanceweredescribedinotherformsofdyskeratosiscongenita（Table1）.Inimportantcontrasttohematologicexamplesofgeneticdiseaseslikesicklecellanemia,thalassemia,andenzymopathies,heterozygousmutationsinthegenesofthetelomererepaircomplexcausedisease,andtheonsetofsymptomsandsignsearlyorlateinlife,theultimateseverityofclinicalmanifestations,andthespecificorgansaffectedmustbepowerfullyinfluencedbyas-yetincompletelydefinedaddedenvironmental,genetic,andepigeneticmodifiers.

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Table1.Bonemarrowfailuresyndromeswithshorttelomeresandgeneticassociations

Thisreviewdescribesourcurrentunderstandingoftherelationshipofdysfunctionaltelomeresanddefectivetelomerasetobonemarrowfailureanditscomplications.Thedetailedpathophysiologiesofbothacquiredandconstitutionalaplasticanemiashaverecentlybeenreviewed10–14andwillnotbediscussedhere;thereaderalsoisreferredtocomprehensivebasicsciencereviewsoftelomerebiologyandtelomerase.15–18

  Telomerebiology

Top

Abstract

Introduction

Telomerebiology

Animalmodels

Telomereshorteninginhuman...

Conclusions

Authorship

References

Thetelomereisanevolutionarysolutiontoproblemspresentedbychromosomelinearity,essentiallytheneedtodistinguishbetweenchromosomalterminianddouble-strandedDNAbreaks（Figure1）.ByadoptingauniformandrepeatedsequenceofDNAforallofaspecies'chromosomes,multipleadaptivemechanismsareprovided:

（1）formationofacircularend（thet-loop）;

（2）aprotectiveproteinshield（shelterin）;（3）aspecializedenzymaticmechanismtoaddnucleotidestothechromosomesaftereachcelldivisionandmaintaintelomerelength（telomerase）;and（4）substitutionforcodingsequencesofimportantgenesatthevulnerableendsofthechromosome.

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Figure1.Schematicrepresentationoftelomerestructure.TelomeresareattheextremitiesofchromosomeDNA.Thetelomeric3'endterminatesasasingle-stranded,G-richoverhangabletoformthet-loop,inwhichtheoverhanginvadesthetelomericdoublehelix,remodelingtheDNAintoacircle.Telomeresarecappedbyatleast6proteins（TRF1,TRF2,TPP1,POT1,TIN2,andRap1）,collectivelyknownasshelterin,thatphysicallyshieldtheDNA.19TRF1,TRF2,andTPP1specificallyrecognizeandbindtodouble-strandedTTAGGGrepeats;POT1bindstothesingle-strandedtelomericoverhang19,20;TIN2andRap1completetheshelterincomplex.Shelterinallowsdiscriminationoftelomeresfromdouble-strandedDNAbreaks;lackofshelterinallowstelomerestobeidentifiedasdouble-strandedDNAbreaksandtriggersDNA-damagepathways.19

Telomerestructure

Double-strandedbreaksthatarise,forexample,fromDNAdamagecausedbyirradiation,elicittheintranuclearDNA-damagemachinery.Incontrast,thetelomeremacromolecularcomplexcapsthetipsofchromosomessothatthefreeendsoftheDNAmoleculearenotrecognizedbyconventionalDNA-repairmechanisms.16Inhumans,telomeresconsistofsomethousandsofTTAGGGtandemrepeats（CCCTAAinthecomplementarystrand21;Figure1）.

Theend-replicationproblem

ToinitiateDNAsynthesis,allDNApolymerasesrequirea3'-OHend,whichcanbeprovidedbyeitherRNAorDNAprimers.WithDNAreplication,theremovaloftheRNAprimerresultsinaterminalgapanda3'single-strandedoverhang.22Inthebodyofthechromosomes,primergapsarefilledwithDNAaddedbytheadjacentOkazakifragment,exceptattheendsoftelomereswhereanadjacentOkazakifragmentisabsent.The"end-replicationproblem,"23,24ashorterlaggingdaughterDNAstrand,hastheconsequencethattelomerelengthdiminisheswitheachcelldivision.Additionalfactors,suchasoxidativedamage,furthercontributetotelomereerosion.25

Whentelomeresbecomecriticallyshort,protectiveresponsesareengaged:

shorttelomeresrecruitdouble-strandedDNAbreakmarkers,suchasphosphorylatedhistoneH2AXandDNA-damagecheckpointfactors,26andactivatep53throughATM,up-regulatingthecell-cycleinhibitorp21andblockingcellcycleinG1,27ultimatelyproducingcellproliferationarrestandapoptosis.Intheabsenceoftelomeresandthecellularprotectiveresponses,erosionofthechromosomalendseventuallywouldcausegenomicinstabilityandgeneattrition.

Telomerasecomplex

Tocountertelomereerosion,cellswithahighreplicativecapacity,suchashematopoieticstemcells,expressaspecializedreversetranscriptaseknownastelomerase,whichextendstelomeresbycatalyzingtheadditionofTTAGGGnucleotiderepeatstothe3'terminusofachromosome'sDNA（Figure2A）29;thelaggingstrandisthenduplicatedbyDNApolymerase.Thecatalyticportionoftelomeraseiscomposedof2moleculesofeachcomponent:

theenzyme,telomerasereversetranscriptaseorTERT;anRNAtemplate,encodedbyTERC;anddyskerin,encodedbyDKC1.30TERTbelongstoafamilyofreversetranscriptasesspecializedinelongatingtelomeres.（Figure2B）.15,18Inhumans,theRNAtemplateTERCspans451nucleotides,includingan11-nucleotide–longtemplate,andcontainsseveralconservedregionsessentialforitsstability（Figure3C）.TERCbindstootherproteins:

dyskerin,GAR,NHP2,andNOP10.

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Figure2.Structureandfunctionofthetelomerasecomplex.（A）TERTenzymaticallyaddsTTAGGGnucleotiderepeatstothe3'endoftelomere'sleadingstrandusingTERCasatemplate.Otherproteins（dyskerin,NOP10,NHP2,andGAR）alsobindtoTERCandstabilizethecomplex.（B）LinearstructureofTERT,whichishighlyconservedamongeukaryotesandconsistsofthecentralreversetranscriptase（RT）motifs（1,2,A,B,C,D,andE）,alargeN-terminalregion,andashortC-terminalregion,allnecessaryfortelomeraseenzymaticfunction.TheN-terminalregioncomprisesatelomerase-essentialN-terminaldomain（TEN）,theCP,andtheQFPdomains,requiredforRNAinteraction,andatelomerase-specificTmotif.TheC-terminalregioncontains4cons