Carcinogenesis-2011-Dominissini-1569-77

腺苷和致癌

Carcinogenesisdoi:10.1093/carcin/bgr124vol.32no.11pp.1569–1577,2011

AdvanceAccesspublicationJune29,2011

REVIEW

Adenosine-to-inosineRNAeditingmeetscancerDanDominissini1,2,SharonMoshitch-Moshkovitz1,revealedthatitisawidespreadmodi?cationaffectingmostlyNinetteAmariglio1andGideonRechavi1,2,Ãnon-codingrepetitiveelementsinthousandsofgenes(3–5).1CancerResearchCenter,ChaimShebaMedicalCenter,TelHashomer52621,Israeland2SacklerSchoolofMedicine,TelAvivUniversity,TelAviv69978,ThebasicsofA-to-IRNAeditingIsraelA-to-IRNAeditingistheirreversibledeaminationofadenosinetoÃTowhomcorrespondenceshouldbeaddressed.inosineindouble-strandedRNA(dsRNA),conservedfromseaEmail:gidi.rechavi@sheba.health.gov.ilanemonestoHomosapiens(6).ThereactionismediatedbyafamilyTheroleofepigeneticsintumoronsetandprogressionhasbeenofadenosinedeaminasesactingonRNA(ADARs)andoccursextensivelyaddressed.Discoveriesinthelastdecadecompletelyco-transcriptionallyatleastinsomecases(7–9).ThreemembersofchangedourviewonRNA.WenowrealizethatitsdiversityliestheADARfamilyhavebeenidenti?edinmammals:ADAR1,2and3.atthebaseofbiologicalcomplexity.Adenosine-to-inosineWhileADAR1andADAR2areexpressedinmosttissues,ADAR3is(A-to-I)RNAeditingemergesacentralgeneratoroftranscrip-exclusivelydetectedinthecentralnervoussystem(reviewedintomediversityandregulationinhighereukaryotes.Itistheref.10).posttranscriptionaldeaminationofadenosinetoinosineindou-AllADARsshareahighlyconservedC-terminalcatalyticdeami-ble-strandedRNAcatalyzedbyenzymesoftheadenosinedeam-nasedomainandcontainseveralN-terminaldsRNA-bindingmotifs.inaseactingonRNA(ADAR)family.Thoughtat?rsttobeADAR1hastwomajorisoformstranscribedfromalternativepro-restrictedtocodingregionsofonlyafewgenes,recentbioinfor-moters:ashorter110-kDaisoformlocalizespredominantlytothematicanalysesfueledbyhigh-throughputsequencingrevealednucleusandalongerinterferon-inducible,150-kDaisoformshuttlesthatitisawidespreadmodi?cationaffectingmostlynon-codingbetweenthenucleusandthecytoplasm(11–14).ADAR2alsohasrepetitiveelementsinthousandsofgenes.Theriseinscopeisseveralisoforms,albeitlesswellcharacterizedandlocalizestotheaccompaniedbydiscoveryofagrowingrepertoireoffunctionsnucleus(15–17).ADAR1andADAR2formhomo-aswellashetero-basedondifferentialdecodingofinosinebythevariouscellulardimersinvivo(18,19).Incontrast,ADAR3existsasamonomer,atmachineries:whenrecognizedasguanosine,itcanleadtopro-leastinvitro,whichmayexplainwhyneitheritsenzymaticactivityteinrecoding,alternativesplicingoralteredmicroRNAspeci?c-noritseditingsubstrateshavebeendemonstrated(20).ity;whenrecognizedbyinosine-bindingproteins,itcanresultinThesecondarystructureofanRNAmoleculelargelydeterminesnuclearretentionofthetranscriptoritsdegradation.Animbal-whichadenosinescanbedeaminated,withaminorpreferenceforanceinexpressionofADARenzymeswithconsequenteditingcertain?ankingnucleotides(21–24).WhereaslongdsRNAs(.100dysregulationisacharacteristicofhumancancers.Thesealter-bp)arepromiscuouslyandextensivelyhypereditedwithupto50%ofationsmayberesponsibleforactivatingproto-oncogenesoradenosinesdeaminated,short(orlongbutimperfect)dsRNAsareinactivatingtumorsuppressors.Whileunlikelytobeanearlymoreselectivelyedited,withonlyafewadenosinesmeetingthesec-initiating‘hit’,editingdysregulationseemstocontributetotu-morprogressionandthusshouldbeconsidereda‘drivermuta-ondarystructurecriteria(24–27).Thecurrentmodelholdsthattion’.Inthisreview,weexaminethecontributionofA-to-IRNAdsRNA-bindingmotifsmediateeditingselectivitybyanchoringeditingtocarcinogenesis.ADARtoadsRNAregion,whereasthecatalyticdomainlendsspec-

i?city,thatispreferenceforadenosineswithinaspeci?ccontextofneighboringnucleotides(28,29).Arecentstudyre?nedthismodelandfoundthatthedsRBMsofADAR2alsocontributetoeditingspeci?citybydirectreadoutoftheRNAsequenceintheminorgroove

Introductionofaspeci?ccontext(30).Non-selectivehypereditingresultsin

adifferentsetofinosinesineachmoleculewithinthepopulationof

Epigeneticandpost-transcriptionalmechanismsareimportantforagiventranscript.WhileADAR1ismainlyresponsibleforhyper-normaltissuedevelopmentandgeneexpression.Numerousstudieshavedocumentedglobalepigeneticabnormalitiesincancercells.editing,ADAR2isresponsibleforselectivesite-speci?ceditingTwosuchextensivelydysregulatedmechanismsareDNAmethyl-(24,27).However,somesubstrateoverlapexistsbetweenthetwo(23).ationandcovalenthistonemodi?cations,whichaffecttumor-RNAeditingef?ciency(thefractionofmoleculesinwhichanspeci?cgeneexpressionthroughnucleosomeremodeling(1,2).inosineappearsinsteadofagenomicallyencodedadenosine)andThelargeamountofdataaccumulatedintherecentyearstrans-patternsexhibitdynamicchangesaffectedbyenvironmentalsignalsformedourperceptionofeukaryoticgeneexpressionasitisnowsuchasstressorinternalsignalssuchasmetabolicstateandcellcycleclearthatenormousdiversitycanbegeneratedattheRNAlevel.(31–38).Importantly,noteverymoleculethatcanadopttherequiredRNAmoleculesundergoelaborateprocessingandaresubjectedtostructurewilleventuallyundergoediting.Asaconsequence,differentawiderangeofpost-transcriptionalmodi?cationsthataffecttheirmessengerRNA(mRNA)variantsofthesamegenearegenerated,fate.Theseincludesplicing,5#capping,3#polyadenylationanddramaticallyincreasingthediversityofthetranscriptome(39).RNAediting,tonameafew.ThemostprevalenttypeofRNAInitially,onlyahandfulofselectiveeditingsitesweredocumentededitinginhighereukaryotesistheconversionofadenosinetoin-withincodingregions,resultinginproteinrecodingandaffectingosinebyhydrolyticdeamination(A-to-Iediting).Thoughtat?rsttoproteinpropertiesandinteractions.Inrecentyears,largedatasetsberestrictedtocodingregionsofonlyafewgenes,recentbioin-obtainedthroughhigh-throughputsequencingapproachesintegratedformaticanalysescomplementedbyhigh-throughputsequencingbybioinformaticanalysesdemonstratedasigni?cantlywiderextent

ofA-to-Ieditingaffectingthousandsofgenesintensofthousandsofsites.Theseeditingeventsoccurmostlyinnon-codingrepetitive

Abbreviations:ADAR,adenosinedeaminaseactingonRNA;dsRNA,double-sequences,suchasAlurepeats,andtendtoundergonon-selectivestrandedRNA;HSC,hematopoieticstemcell;LSC,leukemiastemcell;hyperediting(3,4,40,41).mRNA,messengerRNA;siRNA,smallinterferingRNA;UTR,untranslatedInosinesintheRNAmoleculemayservedifferentpurposes,region.dependingontheinvolvedmechanismandinteractingproteins.ÓTheAuthor2011.PublishedbyOxfordUniversityPress.Allrightsreserved.ForPermissions,pleaseemail:journals.permissions@http://wendang.chazidian.com1569Downloaded from http://wendang.chazidian.com/ at Fudan University on November 16, 2014

腺苷和致癌

D.Dominissinietal.

Withincodingsequences,inosinesaredecodedasguanosinesbythetranslationmachinery(asinosinepreferentiallybasepairswithcyti-dine)(42),thusresultinginproteinrecoding.Onewell-studiedexam-pleofediting-dependentproteinrecodingistheglutamatereceptorsubunit,GluR-B.SelectiveADAR2-mediatededitingofGluR-BleadstogenerationofanimpermeableCaþ2-ionchannelduetoaglu-tamine/arginine(Q/R)substitution(43).Thesplicingmachineryalsorecognizesinosinesasguanosines,enablingeditingtogenerateandeliminatedonororacceptorsplicesitesequences.Editing-dependentalterationofsplicesitesintheADAR2transcriptitselfgeneratesanalternative3#-spliceacceptorsite(9)(seedetaileddiscussionofedit-ingandsplicinginthecontextofcancerinasectionbelow).

A-to-Ieditingalsoin?uencessmallinterferingRNA(siRNA)-andmicroRNA-mediatedgenesilencingwhichdependonformationofdsRNAfortheirbiogenesisandaction.InosineswereshowntoaffectallmajorstagesofmicroRNAbiogenesisandfunction:(i)ADAR1andADAR2editspeci?cadenosinesincertainpri-microRNAstherebysuppressingsubsequentprocessingbyDroshaandDicer(44,45),(ii)inosinespresentinmaturemicroRNAsmayalterbindingtotargetsequences(46)oralternatively(iii)editingof3#untranslatedregions(UTRs)harboringmicroRNAbindingsitescanhaveacompa-rableeffectthroughmodulationofbasepairingoraccessibility(47).Thesigni?canceofnon-selectivehypereditingoflongdsRNAsoccurringmostlyinnon-codingregionsoftranscriptsharboringre-petitivesequencesisstilllargelyunclear.Severallinesofevidencesupportaroleinregulationofgeneexpressionthroughshorteningof3#UTRsbynucleasesthatactoninosine-containingRNAssuchasTudor-SN(48,49)oralternativelythroughnuclearretentionbyacom-plexcontainingp54nrbwhichrecognizesinosine-containingRNAs(50,51).Inbothcases,theproteinsmediatingtheoutcomespeci?callyinteractwithinosine,which,unlikewiththetranslationmachinery,cannotbesubstitutedforguanosinetoreceivethesameeffect.

KnockoutmicethatlackeitherADAR1orADAR2demonstratedthatA-to-IRNAeditingisessentialfornormallifeanddevelopment.HomozygousdisruptionofADAR1inmiceisembryoniclethal(atE11.5–12.5),mostprobablyduetodefectivehematopoiesisandliverdisintegration(52,53).ADAR2À/Àmiceareviablebutpronetoseiz-uresanddieshortlyafterbirth.Thisphenotypeisduetounder-editingoftheQ/RsiteinGluR-Bpre-mRNAtranscriptssinceADAR2À/Àmicewerephenotypicallyrescuedbyinsertionofgenomicallymu-tatedGluR-BRalleles,whichrestoredexpressionoftheeditedformofGluR-BattheQ/Rsite(54).

DysregulationofRNAeditingwaslinkedtoseveralhumandis-eases.ReducededitingofGluR-Bpre-mRNAattheQ/Rsitehasbeensuggestedtoleadtomotorneurondeathinsporadicamyotrophiclateralsclerosis(55).Alterededitinglevelsofserotoninreceptor5-HT2CRtranscriptswerefoundintheprefrontalcortexofsuicidevictimsandinneuropsychiatricdisorderssuchasdepressionandschizophrenia(56).ThedirectinvolvementofA-to-Ieditingincancerhasnotbeenextensivelyaddressedexperimentally.However,corre-lationsaswellasalimitednumberofexperimentalmodelsindicatethatA-to-Ieditingisseverelydysregulatedincancer(57),allowingustodrawsomeimportantconclusionsthatarethesubjectofthisreview.Ofnote,notonlyailmentbutalsohealthwasassociatedwithediting:single-nucleotidepolymorphismsinADAR2andADAR3wereasso-ciatedwithexceptionallongevityindifferentpopulations(58).

ThebroadinvolvementofA-to-IRNAeditinginregulationofgeneexpressionthroughdifferentcellularmechanismstogetherwithitscancer-correlateddysregulationraisethequestionofhowthesealter-ationsinRNAmoleculescanleadtotransformationandtumorprogression.

Acodewithinacode:aframeworkforunderstandingtheeditingcodeanditsdecoding

Asmentionedabove,A-to-Ieditingsitesfoundinthetranscriptomecanbebroadlydividedintotwogroups:‘site-speci?c’editingeventsofselectadenosineresiduesandnon-selective‘hyperediting’clusters.TheseclustersprimarilyoccurwithinAlurepeatsduetotheirten-1570

dencytoformtherequireddsRNAstructures.ItisinstructivetofurtherdistinguishbetweenthosesiteslocatedincodingsequencesandthoseinUTRsandintrons.Editingeventsincodingregionsarelargelysite-speci?c,whilehypereditingcharacterizesUTRsandin-trons.HowcanwemakesenseofthesecategoriesspecifyingpatternsandlocationsfurthercompoundedbytheirdiscoveryindifferentfunctionalclassesofRNAmolecules?

Lookingaroundforadditionalhypothesizedlevelsofinformationandregulationissurprisinglysendingustimeandagainbacktothe‘inert’geneticmaterialforanswers,justtorealizeitcontainsmorethanmeetstheeye.Itisbecomingincreasinglyclearerthatthesamegeneticsequenceissometimesrequiredtosimultaneouslyencodedifferenttypesofinformation(59).Arecentstudydemonstratedthattheapparentlyredundant‘geneticcode’isnotsoredundantafterall:differentcodonsforthesameaminoacidmayaffectthespeedoftranslationandinturnspecifyposttranslationalmodi?cations(60).Themeaningissimpleyetstriking:posttranslationalmodi?cationsareencodedinthemRNAsequence.Doesaninosinehavemorethanonemeaning?

The?rstofthe‘editingcodes’wasinhindsightalreadyapparentwhenA-to-IeditingwasinitiallyrecognizedasonlyanRNAduplexunwindingactivityinXenopuslaevisembryos(25,61,62).Welaterlearnedthateditingcanalsoresultinstabilizationofsecondarystruc-tures(bytargetingA–Cbasepairmismatches)(3,21,22,63,64).Eventhoughthesigni?canceofthisoutcomeisstillunresolved,itiscon-ceivablethatbyvirtueofin?uenceonsecondarystructureofUTRs,transcripttranslatabilityandinteractionwithRNA-bindingproteinscanbemodulated.Inosineequalseffectonsecondarystructureisthekeytothiscode.

TheearliestandprototypicalexamplesofA-to-IRNAediting,namelyGluRsand5-HT2cRs,taughtusthatthetranslationalmachinerydecodesinosineasifitwereguanosine(42)(becauseofitsWatson–Crickbasepairingwithcytidine)leadingtoproteinrecodingaffectingstructureandfunction(43,65).Thissecondeditingcodeisintegratedintothehigher-ordergeneticcode.Itwaslaterdemonstratedtoapplytoothersystemsaswell(suchassplicing)(9).Inosineequalsguanosineisthekeytothiscode.

OurunderstandingofthebiologicalmeaningofAluhypereditingisstillinitsinfancy,albeititconstitutesthevastmajorityofeditingsites.ArecentstudyevenestimatedthecontributionofAlueditingtotran-scriptomediversityisdramaticallyhigherthanthatofalternativesplic-ing(66).Isitindeedso?Thediscoveryofproteinswithauniquespeci?citytowardinosinealone(thatcannotbemimickedbyguano-sine),beitbindingaf?nityabletoretaineditedtranscriptsinthenucleus(p54nrb)(50)orendonucleolyticactivityabletodegradethem(Tudor-SN)(48),isamanifestationofanemergingthirdeditingcodeemployingyetotherdecoders,whereevidentlyadifferentkeyapplies:inosineequalsinosine.Theaf?nityofthep54nrbcomplexforinosinesprobablydependsmoreontheirquantityanddensitythanonexactlocationandcombinationalongtheAlu.Estimatingtheimpactofthiscodemorelikelyrequiresuseofmeasuresotherthannominaldiversity.Risingabovethedifferentcodes,itshouldbestressedthatthecellularoutcomeofA-to-Ieditingisultimatelydependentonthefunctionsofthetranscriptsbeingedited.Thereforeandbringingusclosertothefocusofthisarticle,theeditingphenomenonasawholedoesnot?ttheconceptualdichotomybetweentumorsuppressorsandoncogenes.Actingbyproxy,itcan,however,beresponsibleforacti-vatingaspeci?cproto-oncogeneorinactivatingaspeci?ctumorsup-pressor.Whereasthenetcellularphenotypeofeditingdysregulationisthesumofeffectsonindividualtranscripts—dependingoncelltypeandcircumstance,editingdysregulationofonlyonetranscriptmaystillaccountforthemostnotablephenotype(i.e.theeditedversionofGluR-BisabletosolelyrescueADAR2À/Àmice)(54).

WhenwedetectthesignatureofA-to-IRNAeditinginatranscript,weshouldaskourselves:whatisthecodespeci?edbyinosinesinthisinstance?Whodecodesit?Notingwhatkindoftranscriptitisandwhereinthetranscripteditingoccursmayassistus.Weshouldalsokeepinmindthatsometimesthesameeditingsitecanbewrittenintwodifferentcodestobereadconsecutivelyorcompetitivelybytwo

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腺苷和致癌

decoders(67).ThedifferentcodesadoptedbytheRNAeditingmachinerywillguideourdiscussion.Itwillbeinterestingto?ndoutwhatothercodesinosinesandADARshaveinstoreforus.

Thecancereditome:initialattemptstounderstandtheconnectionbetweeneditingandcarcinogenesis

TheconsiderableimpactofA-to-IRNAeditingongeneexpressiontogetherwithoccurrenceofeditingeventswithintumorsuppressorsandoncogenes(68)warrantanin-depthinvestigationintopossibleinvolvementofeditingincancer.ApartialsurveyoftranscriptomesisolatedfromdifferentsolidtumorsandmatchingcontrolstoassessthefrequencyofeditingeventsfoundincodingaswellasinUTRsrevealsacomplexpicturethatcanbebestsummarizedasfollows:thefrequencyofeditingissigni?cantlydifferentincancercomparedwithnormaltissues;althoughclustersofeditingeventswithinrepetitiveAluelementsaremostlyhypoeditedintumors,site-speci?ceditinglevelsareconsistentlyalteredbutlackajointtrend;braintumorsexhibitthemostsigni?cantdifferences(57).Editinglevelsinhema-tologicalmalignancieshavenotbeencomprehensivelyexamined,exceptforasinglestudythatreportedaneweditingeventoccurringatanintronbranchpointofproteintyrosinephosphatase,non-receptortype6(PTPN6)transcriptsisolatedfrompatientswithacutemyeloidleukemia(69).Thesealterededitingpatternsarereminiscentofthecancer-associatedDNAmethylationpro?leofglobalhypome-thylationinrepetitivesequencesaccompaniedbysite-speci?chyper-methylationinnon-repeatDNAstretches,frequentlyassociatedwithpromoterregions(70).Maasetal.(71)werethe?rsttodemonstratehypoeditingoftheGluR-BQ/Rsiteinadultglioblastomamultiformeandinsodoingprovidedauniqueexampleinwhichthede?nedphysiologicaloutcomeofhypoediting(assemblyofCa2þ-permeablealpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate-typegluta-matereceptors)possiblyexplainssomeaspectsoftumorbehaviorandsymptomatology(epilepticseizures).Furtherexperimentalworkinpediatricglioblastomamultiformeproducedsimilar?ndings(72).Whatiscausingthechangeineditinglevels?Asregulatorymech-anismsgoverningtheeditingmachineryarelargelyunknown,re-searchfocusedonassessingfunctionandexpressionofADARenzymes.Onceagain,the?ndingsdonotlendthemselvestoeasyinterpretation.ExperimentalworkonbraintumorsdemonstratedlossofADAR2activity(responsibleforGluR-Bediting)withoutreduc-tionofitsmRNAexpression,sidebysidewithelevationofADAR1andADAR3mRNAlevels(72).Inanotherstudy,mRNAlevelsofallthreeADARfamilymemberswerereducedingliomasofvaryinggrades(57).Childhoodacuteleukemias,andespeciallynewlydiag-nosedB-celllymphoblasticleukemias(B-ALL),exhibitasigni?cantoverexpressionofonlytheconstitutiveADAR1isofrom(p110),withadramaticdecreaseinitslevelinpatientsachievingcompleteremis-sion(73).Attemptingtointerpretthe?ndingsonemustkeepinmindthatADARsnormallyfunctionpredominantlyashomodimers(18).ADAR1overexpression,eveninthesettingofunchangedADAR2levels,probablytipsthebalanceandleadstoADAR1/ADAR2heter-odimerformation,thusinterferingwiththespeci?ceditingactivityofthelatter(72).Alternatively,theobserveddown-regulationofallthreeenzymes—importantlynottothesamedegree—maytheoreticallycausesimultaneoushypoeditingofonetranscriptandhypereditingofanother.AsproposedbyGalloetal.(74),amodelofprogressivelydevelopingimbalanceinexpressionofADARfamilymembersduringtumorevolutionhasapowertoreconciletheapparentlyincongruentexpressionofADARs’indifferenttumors.Onemustkeepinmind,though,thatthisproposedmodelreliesonevidencelimitedtotheexpressionofdeaminasesandhencethattheperturbationofother,yetunknown,regulatorymechanismsmightalsoberesponsiblefortheobservedchangesineditinglevels.

Naturally,thequestioniswhetherthereareanyhintsthatthechangeineditinglevelshasacausalrelationshiptomalignanttrans-formation(initiation);oralternatively—doesthemalignantpheno-typedependtosomeextentonthischange(progression),implyingA-to-IRNAeditingmeetscancerthatrestoringediting‘balance’wouldhelpcurbcancer?Themostcompellingevidencesuggestingtheanswertothelatterquestionisyes,atleastinbraintumors,wasprovidedusingtwomainapproaches:(i)rectifyingaspeci?chypo-editedtranscript:conversionofCa2þ-permeablealpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate-typeglutamatereceptorstoCa2þ-impermeablereceptorsthroughgenedeliveryincreasedapoptosisandsuppressedproliferationofadultglioblastomamultiformecells(75,76);(ii)reinstatingADAR‘balance’:overexpressionofADAR2inastrocytomacelllinesre-storededitinglevelsandregressedthemalignantphenotypeintermsofcellgrowthandmigration(72).Moreover,correlationsestablishedbetweenactivity/expressionofADARsandtumorgradefurtherimplyeditingdysregulationhasaroleincancerprogression(57,72).Asforinitiation,evidenceimplicatingeditingdysregulationasanearly‘hit’alongtheslopeendinginmalignanttransformationislacking:noincreaseincancerincidencewasreportedineitheranimalmodelsofRNAeditingdysregulation(54,77–83)orhumansubjectssufferingfromthepigmentarydisorderdyschromatosissymmetricahereditariawhichcarryheterozygousnullmutationsinADAR1(84,85).Judgingbyallaboveobservations,whileunlikelytobeanearlyinitiatinghit,editingdysregulation—beitbywayofmutationinADARgenesorotherepigeneticprocesses—seemstocontributetotumorprogressionandhenceoughttoberegardedasa‘drivermutation’.Evidently,thoughRNAeditinghasalargeimpactongeneexpres-sion,wearestillunabletodrawastraightlineconnectingtranscript-speci?cediting—leadingforexampletoproto-oncogeneactivationortumorsuppressorinactivation—andcarcinogenesis(Figure1).Solongaswearededucingfromfacevalueofdeaminaseexpressionwewillnotbeabletodecipherthenetworklogicofeditingdysregu-lation.Thealpha-amino-3-hydroxyl-5-methyl-4-isoxazole-propionate-typeglutamatereceptorsubunitGluR-Bistheonlyeditingtargetwithanestablishedbearingoncancer.AnovelconnectionbetweenRNAeditingandmodulationofDNAdamagerepairwasrecentlymadewhenanADAR1-mediatededitingeventwasidenti?edintheDNArepairenzymeNEIL1,abaseexcisionrepairglycosylase(86,87).Thissite-speci?ceventcausesalysinetoargininechangeinthelesionrecognitionloopoftheprotein,thuseffectivelychangingitsglyco-sylaseactivityandlesionspeci?city.ItistemptingtospeculatethatthisisaroutethroughwhichRNAeditingaffectsthenumberandtypesofmutationsthataccumulateingenomesofcancercells.Not-withstanding,theanalysispresentedabove,basedoncorrelationsbe-tweeneditinglevelsandcancerousstatesaswellasonpreliminarygeneticmanipulationsofADARs,stillenablesustoimplicateeditingdysregulationasaprobablecausativeagentincancerprogression.Wenowturntoexaminepossibleexplanationsatthemolecularlevel.CancerstemcellsTheenormousprogressachievedinthegeneralstemcell?eldinrecentyearshasfueledeffortsto?ndcorollariesincancer.Accordingtotheleanestinterpretationofthecancerstemcellhypothesis,tumorsare‘organs’composedofaheterogeneoushierarchyofevolvingmalignantcellsderivedfromandmaintainedbyauniquesubpopula-tionofcellswith‘stemcell’properties:namely,self-renewalandtheabilitytodifferentiateandcreatetherepertoiremakingupthetumor,togetherendowingthemwithauniquelysustainedtumorigenicpotential(88).Whereastheexactnatureofthispopulation(i.e.size,quiescence,markersandchemosensitivity),itshomogeneityandrelationtosomatictissuestemcellsarestillunderdebate,itisclear,fromaclinicalperspective,thattumoreradicationwillonlybeachievedthroughtargetedtherapythataddressestheuniquepropertiesofthecancerstemcellsubpopulation(89).Insomehematologicalmalignancies,evidencesuggeststhatleuke-miastemcells(LSCs)arederivedfromnormalhematopoieticstemcells(HSCs),whichacquiredinitialgenetichits(90–93).Recently,twostudiesinmice,elaboratingontheinitialobservationofdefectivehematopoiesisinADAR1À/Àmice,haverevealedthespeci?cimpor-tanceofADAR1forthemaintenanceoftheHSCcompartment,pos-siblybysuppressionoftheinterferonsignalingpathwayand

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D.Dominissinietal.

Fig.1.A-to-IRNAeditingsitescanbedividedintotwogroups:selectivesite-speci?ceditingandnon-selectivehypereditingclustersthatoccurmainlyinnon-codingsequences.Selectiveeditingmayresultinproteinrecodingwhenoccurringwithincodingsequences(A)oralterationofsplicesites

(B).Non-selectivehypereditingcanin?uencegeneexpressionthroughseveralmechanisms:alteringmicroRNAbindingsites(C);shorteningof3#UTRsbynucleasesthatactoninosine-containingRNAsandnuclearretentionbyp54nrb(D).Allthesechangesmayresultinactivationofoncogenesorinactivationoftumorsuppressorgenes,thusleadingtotumorigenesis.

consequentprotectionofHSCs(52,78,94).ADAR1absencecausedvirus,hepatitisBvirus,humanpapillomavirus,humanherpesvirus8,hematopoieticprogenitorcellstoundergoincreasedapoptosisandhepatitisCvirus,humanT-lymphotropicvirustypeIandmolluscumpreventedthemfromformingdifferentiatedcolonieswhileexhaustingcontagiosumvirusareetiologicagentsin10–15%ofhumancancersHSCsduetocontinuousactivation(94).Itisplausiblethattheuniqueworldwide(100).Inthecaseofviral-inducedcancer,themolecularqualitiesADAR1conferstonormalHSCs(forexample:interferonbasisofhostsusceptibilityallowingforpersistentviralinfectionisofsignalingsuppression,anti-apoptosisandquiescence)wouldalsoobviousimportancebutislargelyunknown.InfectiousagentstriggerserveLSCs,asmanymarkersarecommontobothHSCsandLSCsanin?ammatoryresponsethatprecedestumordevelopmentaspartof(91,93).Theef?ciencyofinterferontherapyinchronicmyeloidleu-thenormalhostdefensedirectedatpathogenelimination.However,kemiamightsupportthisnotion(95,96).Itmaybearguedthatthetumorigenicvirusesdevisedingeniouswaystounderminehostim-globalhypoeditingaccompaniedbyADAR1down-regulationseeninmunityandestablishpersistentinfectionscausinglowintensitybutvarioushumantumorscontradictsapossibleroleforADAR1inLSCchronicin?ammation(101,102).Hostreactiontoviralinfectionclas-maintenance(57).Whereasthisobservationwasmadeinsolidtu-sicallytakestwoforms:(i)innateimmunitycharacterizedbyarapidmors,arecentstudyshowedthatADAR1isactuallyup-regulatedandgenericresponse,(ii)adaptiveimmunitycharacterizedbyahighlyinacutechildhoodleukemias(73),possiblyre?ectingadifferentrolespeci?cresponse,whichalsoconfersfutureresistance.ForclarityofforADAR1inthesetwobroadcancercategories.Itbringstomindtheourdiscussion,itisinstructivetofurtherdistinguishbetweeneditingapparentpuzzlingdiscrepancybetweenpromyelocyticleukemiaofhostcellularRNAsandeditingofviralRNAs.Aswillbedescribedproteinlevelsandfunctioninsolidtumors(anestablishedtumorbelow,theeditingmachineryhasacentralroleinbotharmsandsuppressor,wherelossofpromyelocyticleukemiaproteinpredictsin?uencescellularaswellasviralRNAsinthesettingofinfectionanunfavorableprognosis)versusinchronicmyeloidleukemiastemandin?ammation(103).

cells(whereitisresponsibleformaintainingthequiescenceofLSCs,Beginningwitharoleinimmunecelldevelopment—asmentionedanditslosspredictsafavorableoutcome)todemonstrateourpointaboveinthe‘cancerstemcell’section,ADAR1isnecessaryforHSC(97,98).Takentogether,webelieveADAR1isprobablyimportantformaintenanceandhencematurationofcellsbelongingtobothinnatethesurvivalofsomeLSCcompartmentsandthereforemightserveasandadaptivearmsoftheimmunesystemisdependentonit.Interest-avaluabletargetinfuturetherapy.inginthisregardisthesuggestedunderlyingmechanism:suppressionEditingandimmunityoftheinterferonsignalingpathwayandconsequentprotectionof

HSCsfromapoptosis(94).Continuingwitharoleinimmunecell

Chronicin?ammationandcancerdevelopmentaretightlyconnectedfunction—up-regulationofADAR1inlymphocytesfollowingstimu-

(99).Chronicinfectionsarechiefexogenousinducersofin?amma-lationwithavarietyofin?ammatorymediators(tumornecrosisfac-tion,responsibleforroughly20%ofallcancers.Bynow,itis?rmlytor-a,interferon-c)resultsineditingofhostcellularRNAs,possiblyestablishedthatatleastsevenhumanviruses,namelyEpstein-Barrcontributingtoproliferationanddifferentiationoflymphocytes

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腺苷和致癌

Asforeditingasadirectantiviralmechanism—sincemostRNAviruseslocalizetothecytoplasm,theADAR1p150isoform,beinginterferon-inducibleandcytoplasmic,isoneoftheseveraleffectorsbelongingtothe?rstlineofinnateimmunedefensesagainstviralinfection(104,105).Assuch,ithasprovenrolesineditingoftheHCVRNAgenomeresultingininhibitionofitsreplication(106).ItalsoeditstheHHV-8Kaposintranscripteliminatingitstransformingactivity(107).AswillbediscussedingreaterdetailinthemicroRNAsectionbelow,editingofEBV-encodedmicroRNAswasshowntodictatevirallatency(108).

InsupportofadirectantiviralfunctionforADAR1arethespeci?cinhibitorsdevelopedbyadenoandvacciniavirusesagainstitsenzy-maticactivity(109,110).Notunexpectedly,recentstudiesdemon-stratesomeviruses(HIVandvesicularstomatitisvirus,forexample)evolvedspeci?cfeaturestakingadvantageoftheeditingsystemandharnessingittoaidtheirreplication,therebymakingADAR1aviral‘accomplice’(111,112).

Surprising,andespeciallyilluminating,istheproviraleffectexertedbyADAR1throughdirectinhibitionofdsRNA-activatedproteinkinase(PKR),aninterferon-induciblekinasewithacentralroleinantiviralimmunity(113,114).Thisresponse,counteractinganotherantiviralpathwaytoblockapoptosisandassistviralreplica-tion,wasdemonstratedformeaslesvirus,vesicularstomatitisvirusandHIV-1.Itcompromisestheabilityofthehostto?ghtinfection,ineffectpromotingchronicin?ammation,butthroughdirectmodula-tionofahostproteinandnotthroughaviralintermediate.Isityetanotherexampleofviralmanipulation?TakingintoaccountthatADAR1isalsoasuppressorofinterferonsignaling(78),potentiallyprotectingtheorganismagainstitsunleasheddetrimentalactivation,andwasalsoshowntonegativelyregulateimmuneresponseactivatedbycytosolicDNA(115),theanswerisprobablynot.WebelievethatitmanifeststheroleofADAR1inkeepingantiviralresponseincheck.ThisapparentcontradictionwasrecentlyaddressedbyGatignoletal.(113)whostressedthebiasednatureofourconceptionofcellresponsesolelyintendedtocounteractviralreplication.Consideringthateveryphysiologicalresponseistheresultofco-evolutionofviruseswiththeirhosts,itmightbemoresuitabletoviewthesituationasequilibriumbetweentheneedtoavoiddeathduetoviralreplicationandtheneedtoavoiddeathduetohyperactiveimmuneresponse.

ThecomplexinteractionsoutlinedabovesuggestthatADAR1isverypertinenttothebasichost–viralinteractionsdeterminingthepersistencyofviralinfection.ADAR1isnoteasilyclassi?edaspro-viralorantiviral,neitherwhenconsideringtheoutcomeofviralin-fectionnorwhenregardingitasaneffectoroftheinterferonsystem.WeproposethatADAR1inductioninviralinfectionessentiallyservestocombatvirusesthroughdirecteditingoftheirtranscripts(amech-anismsometimesoutsmartedbyviruses)andatthesametimetomodulateandlimittheinterferonin?ammatoryresponse.Therefore,itappearsA-to-IRNAediting,andADAR1inparticular,arekeydeterminantsofpersistentinfection,consequentchronicin?ammationandresultantcancerrisk.

Fightingretrotransposon-mediated‘naturalmutagenesis’

‘Insertionalmutagenesis’ofproto-oncogenesandtumorsuppressorsbymobilegeneticelementscanparticipateincarcinogenesis(116,117).Normalcellsmanagetoprotectthemselvesagainstadversemutageniceffectsin?ictedbyendogenousretrotransposons(Alu,longinterspersedelement1)throughtheuseofmethylation,siRNAsandotherapproaches(118).Althoughtherepressiveeffectsofthesemechanismsarepartlyrelievedincancer(globalhypomethylation),ithasbeendif?culttodetermineifretrotransposonsareconsequently‘jumping’inthegenomesofhumantumorcellsandtowhatextenttheyareinvolvedingenomicinstabilityandcancerdevelopment(119).Iskowetal.(120)recentlydemonstratedsuchdenovosomaticinsertionsarequitefrequentinnon-small-celllungcancerandpro-videdfurtherevidenceinfavorofacausativerelationbetweenDNAdemethylationandextensiveretrotransposonmobilizationintumors.A-to-IRNAeditingmeetscancerDeaminatingmostlyretrotransposon-deriveddsRNAs—isA-to-Ieditinganotherbarrieragainstretrotransposition?Isthecancer-associatedbreachofeditingdefenselinessomewhatresponsiblefortheobservedmobilizationandinsertionofretrotransposons?Severallinesofpreliminaryevidencepointtointriguinginteractionsoftheeditingsystemperhapsenablingittowardoffretrotransposonsbutarestilltoounripetoanswertheposedquestionsintheaf?rmative.Beforeproceedingtomentionmoreintricateandindirectproposedmechanisms,itisworthmentioningprobablythemoststraightfor-wardone—directeditingofretrotransposonsequencesmaydamagethemandpreventtheirintegrationbackintothegenome.EndogenoussiRNAsformedoutofretrotransposon-deriveddsRNAstargettheirparenttranscriptsresultingintheirsuppression(121).ADAR2,equippedwiththeabilitytohyperedittheparenttran-scripts,canpreventDicer,atleastinvitro,fromcleavingthem,therebysuppressingmaturesiRNAgeneration(49).Inaddition,ADARsavidlybindmaturesiRNAs,decreasingtheireffectivecon-centration,therebycompetingwithRNA-inducedsilencingcomplexandreducingtheef?ciencyofRNAinterference(122).Alternatively,editingexposestheparenttranscriptstotheendonucleolyticactivityofTudor-SNdirectedspeci?callyagainstinosine-containingdsRNAs(48,49).Theoutcomeofthesecompetingforcesintermsofanti-retrotransposonactivityisnotclearandhasnotbeenputtoexperi-mentaltest.TheremightbeyetanotherroutethroughwhichRNAeditingpar-ticipatesinepigeneticsilencingofretrotransposonactivityaspartofcelldefensemechanisms.Thediscoverythatvigilinscanbindinosine-containingRNAandarealsoinacomplexcontainingADAR1indicatesapossibleinvolvementinaheterochromaticsilencingmechanism(123).Accordingtoaproposedscenario,ADAR1hypereditsdsRNAstranscribedfromgenomiclociharboringrepetitiveelements;these,inturn,arerecognizedbyvigilins,whichrecruitadditionalfactorsneededtotransformtheregionintoheterochromatintherebysuppressingtheexpressionofretrotransposons.EditingofmicroRNAsMicroRNAsaresmallnon-codingRNAsthatactasposttranscrip-tionalrepressorsofgeneexpression(124).Similartotheeditingmachinery,theeffectsofmicroRNAsonfundamentalcellularprocessesareonlyunderstoodwhenconsideringthedownstreamfunctionsofthetranscriptstheytargetinaspeci?ccelltypeanddifferentiationstate.TheinvolvementofmicroRNAsinprocessesdrivingcancerinitiationandfuelingprogressionandmetastasisarebynowwellestablished(125).DistinctivelyaberrantmicroRNAex-pressionpro?lescharacterizingvariouscancerslieatthebaseoftheprevailinghypothesisassigningtumorsuppressorrolestodown-regulatedmicroRNAs(thatnormallydown-regulatetheexpressionofanoncogenes)andoncogenerolestoup-regulatedones(thatnor-mallydown-regulatetheexpressionofatumor-suppressor)(126).VariousmechanismshelpmicroRNAsescapestrictexpressionregu-lationincancer,rangingfromstructuralgenomic(translocation,ampli?cation,deletion),throughepigenetic(promoterhypermethyla-tion,histonehypoacetylation)todefectsinbiogenesis(transcriptionrepression/activation,Drosha/Dicerloss)(127).Thestem-loopsecondarystructureadoptedbyprimarymicroRNAs(pri-microRNAs)andmicroRNAprecursors(pre-microRNAs)ena-blesinteractionbetweentheA-to-Ieditingmachineryandthemicro-RNAbiogenesispathway.Nishikuraetal.haveconvincinglydemonstratedhoweditingeventsinpri-andpre-microRNAsblocktheprocessingcascadebyinterferingwithDroshaorDicercleavagesteps,resultinginreducedlevelsofmaturemicroRNAs(44,45).Dependingontheposition,wheneditingeventsdonotinterferewithenzymaticcleavage,theycanendupinmaturemicroRNAswithapotentialtoredirecttheirtargetspeci?city(46).Attheotherendoftheequation,extensiveA-to-IeditingofmRNA3#UTRsmayaddtoregulationofmicroRNAactivitythroughmodulation,creationoreliminationofbindingsites(47).Additionally,theresultingstabili-zation/destabilizationofthesecondarystructureoftargetregionsmay

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