CARCINOGENESIS:THEMOLECULARBASISOFCANCER

• Non-lethalgeneticdamageliesattheheartofcarcinogenesis.

• Mutationmaybeacquiredbytheactionofenvironmentalagents,suchaschemicals,radiation,orviruses,oritmaybeinherited inthegermline.

• Thegenetichypothesisofcancerimpliesthatatumormassresultsfromtheclonalexpansionofasingleprogenitorcellthathasincurredgeneticdamage(i.e.,tumorsaremonoclonal).

• Fourclassesofnormalregulatorygenesareinvolved:1. Growth-promotingproto-

oncogenes. 2. Growth-inhibitingtumorsuppressor

genes.3. Genesthatregulateapoptosis.4. GenesinvolvedinDNA.

• Mutantallelesofproto-oncogenesarecalledoncogenes.

• Theyareconsidereddominantbecausemutationofasingleallelecanleadtocellulartransformation.

• Bothnormalallelesoftumorsuppressorgenesmustbedamagedfortransformationtooccur,referredtoasrecessiveoncogenes.(OK?).

• Genesthatregulateapoptosismaybedominant,asareproto-oncogenes,ortheymaybehaveastumorsuppressorgenes(recessive).

TumorSuppressorGenes

• Tumorsuppressorgenesareof2types:• 1- promotersgenes:

Ø Promotersarethetraditionaltumorsuppressorgenes,suchasRB orp53.

ØMutationofthesegenesleadstocelltransformationbyreleasingthecontroloncellularproliferation.

• 2- caretakersgenes.

TumorSuppressorGenes• 2- caretakersgenes.

ØCaretakergenesareresponsibleforprocessesthatensuretheintegrityofthegenome,suchasDNArepair.

ØMutationofcaretakergenesdoesnotdirectlytransformcellsbyaffectingproliferationorapoptosis.

ØDNArepairgenesaffectcellproliferationorsurvivalindirectly byinfluencingtheabilitytorepairnon-lethaldamageinothergenes,includingproto-oncogenes,tumorsuppressorgenes,andgenesthatregulateapoptosis.

Carcinogenesis• Carcinogenesisisamultistepprocess atboththephenotypicandthegeneticlevels,resultingfromtheaccumulationofmultiplemutations.

• Malignantneoplasmshaveseveralphenotypicattributes,suchasexcessivegrowth,localinvasiveness,andtheabilitytoformdistantmetastases.

• Tumorprogression• Overaperiodoftime,manytumorsbecomemoreaggressiveandacquiregreatermalignantpotentialwhichisnotsimplyrepresentedbyanincreaseintumorsize.

• Tumorprogressionandassociatedheterogeneityresultsfrommultiplemutationsthataccumulateindependentlyindifferenttumorcells,generatingsubcloneswithdifferentcharacteristics.

• Eventhoughmostmalignanttumorsaremonoclonalinorigin,bythetimetheybecomeclinicallyevident,theirconstituentcellsareextremelyheterogeneous.

• Duringprogression,tumorcellsaresubjectedtoimmuneandnonimmuneselectionpressures.

• E.g: cellsthatarehighlyantigenicaredestroyedbyhostdefenses,whereasthosewithreducedgrowthfactorrequirementsarepositivelyselected.

• Agrowingtumortendstobeenrichedforsubclones thatarecapableofsurvival,growth,invasion,andmetastasis.

Featuresofmalignentcells• 1-Self-sufficiencyingrowthsignals.• 2-Insensitivitytogrowth-inhibitorysignals.• 3-Evasionofapoptosis.• 4-Limitlessreplicativepotential(i.e.,overcomingcellularsenescenceandavoidingmitoticcatastrophe).

• 5-Developmentofsustainedangiogenesis.• 6-Abilitytoinvadeandmetastasize.• 7-GenomicinstabilityresultingfromdefectsinDNArepair.

Self-SufficiencyinGrowthSignals

• Genesthatpromoteautonomouscellgrowthincancercellsarecalledoncogenes.

• Theyarederivedbymutationsinproto-oncogenesandarecharacterizedbytheabilitytopromotecellgrowthintheabsenceofnormalgrowth-promotingsignals.

• Theirproducts,calledoncoproteins, resemblethenormalproductsofproto-oncogenesexceptthatoncoproteinsaredevoidofimportantregulatoryelements, andtheirproductioninthetransformedcellsdoesnotdependongrowthfactorsorotherexternalsignals.

• Thebindingofagrowthfactortoitsspecificreceptoronthecellmembranecausestransientandlimitedactivationofthegrowthfactorreceptor.

• " activatesseveralsignal-transducingproteinsontheinnerleafletoftheplasmamembrane

• "transmissionofthetransducedsignalacrossthecytosoltothenucleusviasecondmessengersoracascadeofsignaltransductionmolecules

• "inductionandactivationofnuclearregulatoryfactorsthatinitiateDNAtranscription

• "progressionofthecellintothecellcycle,resultingultimatelyincelldivision

16

GrowthFactors• Allnormalcellsrequirestimulationbygrowthfactorstoundergoproliferation.

• Types:• 1- Paracrineaction:growthfactorsaremadebyonecelltypeandactonaneighboringcelltostimulateproliferation2-Autocrineaction:Manycancercellsacquiregrowthself-sufficiencybyacquiringtheabilitytosynthesizethesamegrowthfactorstowhichtheyareresponsive.

GrowthFactorReceptors

• Mutantreceptorproteinsdelivercontinuousmitogenicsignalstocells,evenintheabsenceofthegrowthfactorintheenvironment.

• Overexpressionofgrowthfactorreceptorscanrendercancercellshyper-responsivetolevelsofthegrowthfactorthatwouldnotnormallytriggerproliferation.

• E.g:• Overexpressioninvolvetheepidermalgrowthfactor(EGF)receptorfamily.ERBB1.

• theEGFreceptor,isoverexpressedin80%ofsquamouscellcarcinomasofthelung.

• In50%ormoreofglioblastomas.• In80-100%ofepithelialtumorsoftheheadandneck.

• HER2/NEU(ERBB2), isamplifiedin25-30%ofbreastcancersandadenocarcinomasofthelung,ovary,andsalivaryglands.

• Thesetumorsareexquisitelysensitivetothemitogeniceffectsofsmallamountsofgrowthfactors

• HighlevelofHER2/NEU proteininbreastcancercellsisapoorprognosis.

• ThesignificanceofHER2/NEU inthepathogenesisofbreastcancersisillustratedbytheclinicalbenefitderivedfromblockingtheextracellulardomainofthisreceptorwithanti-HER2/NEU antibodies.

• Treatmentofbreastcancerwithanti-HER2/NEUantibody(herciptin)provedtobeclinicallyeffective.

Signal-TransducingProteins• Thesesignalingmoleculescouplegrowthfactorreceptorstotheirnucleartargets.

• Manysuchsignalingproteinsareassociatedwiththeinnerleafletoftheplasmamembrane,wheretheyreceivesignalsfromactivatedgrowthfactorreceptorsandtransmitthemtothenucleus,eitherthroughsecondmessengersorthroughacascadeofphosphorylationandactivationofsignaltransductionmolecules.

• Twoimportantmembersinthiscategoryare:• 1-RAS gene.• 2-ABLgene.

• RAS isthemostcommonlymutatedproto-oncogeneinhumantumors.

• Approximately30%ofallhumantumorscontainmutatedversionsoftheRAS gene.

• Theincidenceisevenhigherinsomespecificcancers (e.g.,colonandpancreaticadenocarcinomas).

• RASisamemberofafamilyofsmallGproteinsthatbindguanosinenucleotides(guanosinetriphosphate[GTP]andguanosinediphosphate[GDP]).

• TheABL proto-oncogenehastyrosinekinaseactivitythatisdampenedbyinternalnegativeregulatorydomains.

• Inchronicmyeloidleukemia(CML)andacutelymphocyticleukemias.

• WhenABL geneistranslocatedfromitsnormalsiteonchromosome9 tochromosome22,whereitfuseswithpartofthebreakpointclusterregion(BCR) gene=Philadelphia(Ph)chromosome.

• TheBCR-ABLhybridproteinhaspotent,unregulatedtyrosinekinaseactivity,whichactivatesseveralpathways,includingtheRAS-RAFcascade.

• NormalABLproteinlocalizesinthenucleus,whereitsroleistopromoteapoptosisofcellsthatsufferDNAdamage.

• TheBCR-ABL genecannotperformthisfunction,becauseitisretainedinthecytoplasmasaresultofabnormaltyrosinekinaseactivity.

NuclearTranscriptionFactors

• GrowthautonomymayoccurasaconsequenceofmutationsaffectinggenesthatregulatetranscriptionofDNA.

• MYC,MYB,JUN,FOS,andRELoncogenes,functionastranscriptionfactorsthatregulatetheexpressionofgrowth-promotinggenes,suchascyclins.

• theMYC geneisinvolvedmostcommonlyinhumantumors.

• TheMYC proto-oncogeneisexpressedinvirtuallyallcells,theMYCproteinisinducedrapidlywhenquiescentcellsreceiveasignaltodivide.

• Innormalcells,MYClevelsdeclinetonearbasallevelwhenthecellcyclebegins.

• Incontrast,oncogenicversionsoftheMYC geneareassociatedwithpersistentexpressionoroverexpression,contributingtosustainedproliferation.

• DysregulationoftheC-MYC generesultingfromat(8;14)translocationoccursinBurkittlymphoma,aB-celltumor.

• MYC isalsoamplifiedinbreast,colon,lung,andmanyothercancers;

• N-MYC andL-MYC genesareamplifiedinneuroblastomasandsmall-cellcancersoflung.

CyclinsandCyclin-DependentKinases(CDKs)

• Cancersmaybecomeautonomous ifthegenesthatdrivethecellcyclebecomedysregulatedbymutationsoramplification.

• ProgressionofcellsthroughthevariousphasesofthecellcycleiscontrolledbyCDKs.

• CDKsareactivatedbybindingtocyclins, socalledbecauseofthecyclicnatureoftheirproductionanddegradation.

• TheCDK-cyclincomplexesphosphorylatecrucialtargetproteinsthatdrivethecellthroughthecellcycle.

• Oncompletionofthistask,cyclinlevelsdeclinerapidly.

• Morethan15cyclinshavebeenidentified;cyclinsD,E,A,andBappearsequentiallyduringthecellcycleandbindtooneormoreCDK.

• MishapsaffectingtheexpressionofcyclinDorCDK4seemtobeacommoneventinneoplastictransformation.

• ThecyclinDgenesareoverexpressedinmanycancers,includingthoseaffectingthebreast,esophagus,liver,andasubsetoflymphomas.

• AmplificationoftheCDK4 geneoccursinmelanomas,sarcomas,andglioblastomas.

• MutationsaffectingcyclinBandcyclinEandotherCDKsalsooccur,buttheyaremuchlessfrequentthanthoseaffectingcyclinD/CDK4.

CDKInhibitors• CyclinsarousetheCDKs.• CDKinhibitors(CDKIs)silencetheCDKsandexertnegativecontroloverthecellcycle.

• OnefamilyofCDKIs,composedofthreeproteins:

• 1- p21[CDKN1A],• 2-p27[CDKN1B],• 3-p57[CDKN1C],inhibitstheCDKsbroadly...

InsensitivitytoGrowth-InhibitorySignals

• Retinoblastoma(RB) gene,thefirstandprototypiccancersuppressorgenetobediscovered.

• Retinoblastomaisanuncommon childhoodtumor.• Approximately60%ofretinoblastomasaresporadic,and40%arefamilial,

• Thepredispositiontodevelopthetumorbeingtransmittedasanautosomaldominanttrait.

• Toaccountforthesporadicandfamilialoccurrenceofanidenticaltumor,Knudson,in1974,proposedhisnowfamoustwo-hit hypothesis.

• Twomutations(hits): arerequiredtoproduceretinoblastoma.

• TheseinvolvetheRB gene,locatedonchromosome13q14.

• Both ofthenormalallelesoftheRB locusmustbeinactivated(twohits)forthedevelopmentofretinoblastoma.

• Infamilialcases,childreninheritonedefectivecopyoftheRB geneinthegermline;theothercopyisnormal,retinoblastomadevelopswhenthenormalRB geneislostinretinoblasts asaresultofsomaticmutation.

• TheRBpathwayisimportantto:• 1- ControlofcellcycleprogressionatG1.• 2- Inducecelldifferentiation.• 3- Inducesenescence.

• MutationsinothergenesthatcontrolRBphosphorylationcanmimictheeffectofRB loss,suchgenesaremutatedinmanycancersthatseemtohavenormalRB genes.

• E.g : mutationalactivationofCDK4oroverexpressionofcyclinDwouldfavorcellproliferationbyfacilitatingRBphosphorylationandinactivation.

• CyclinDisoverexpressedinmanytumorsbecauseofgeneamplificationortranslocation.

• MutationalinactivationofCDKIsalsowoulddrivethecellcyclebyunregulatedactivationofcyclinsandCDKs.

TP53 Gene:GuardianoftheGenome

• Thep53 tumorsuppressorgeneisoneofthemostcommonlymutatedgenesinhumancancers.

• P53prevents(OK)neoplastictransformationbythreeinterlockingmechanisms:

• 1-activationoftemporarycellcyclearrest(termedquiescence),

• 2-inductionofpermanentcellcyclearrest(termedsenescence),

• 3-triggeringofprogrammedcelldeath(termedapoptosis).

• P53 canbeviewedasacentralmonitorofstress,directingthestressedcellstowardanappropriateresponse.

• Avarietyofstressescantriggerthep53 responsepathwaysincluding:anoxia,inappropriateoncogeneexpression(e.g.,MYC orRAS),damagetotheintegrityofDNA.

TransformingGrowthFactor-βPathway

• TGF-β isapotentinhibitorofproliferationinmostnormalepithelial,endothelial,andhematopoieticcells.

• ItregulatescellularprocessesbybindingtoacomplexcomposedofTGF-βreceptorsIandII.

• Dimerizationofthereceptoruponligandbindingleadstoacascadeofeventsthatresultin:transcriptionalactivationofCDKIsandsuppressionofgrowth-promotinggenessuchasMYC,CDK2,CDK4,andthoseencodingcyclinsAandE.

ContactInhibitionNF2andAPC

• Contactinhibitionisabolishedincancercellsallowingthemtopileontopofoneanother.

• Cell-cellcontactsinmanytissuesaremediatedbyhomodimeric interactionsbetweentransmembraneproteinscalledcadherins.

• E-cadherin mediatescell-cellcontactinepitheliallayersbymechanismnotfullyunderstood.

• OnemechanismthatsustainscontactinhibitionismediatedbythetumorsuppressorgeneNF2.

EvasionofApoptosis

• Thereare2distinctprogramsthatactivateapoptosis:

• 1- Extrinsicpathway(deathreceptorCD95/Fas).

• 2- Intrinsicpathway(DNAdamage).

• Stimulationofeitherpathwayresultsinactivationofanormallyinactiveprotease(caspase-8orcaspase-9),whichinitiatesaproteolyticcascadeinvolving"executioner"caspasesthatdisassemblethecellinorderlyfashion.

• Thecellularremainsarethenefficientlyconsumedbythecellularneighborsandprofessionalphagocyteswithoutstimulatinginflammation.

AbilitytoInvadeandMetastasize

• Themetastaticcascadecanbesubdividedintotwo phases:invasionofECMandvasculardisseminationandhomingoftumorcells.

InvasionofExtracellularMatrix(ECM)

• HumantissuesareorganizedintoaseriesofcompartmentsseparatedfromeachotherbytwotypesofECM:basementmembranesandinterstitialconnectivetissue.

• eachofthesecomponentsofECMiscomposedof:collagens,glycoproteinsandproteoglycans.

• InvasionoftheECMisanactiveprocessthatrequiresfoursteps:• 1-Detachmentoftumorcellsfromeachother.• 2-DegradationofECM.• 3-AttachmenttonovelECMcomponents.• 4-Migrationoftumorcells.

LimitlessReplicativePotential

• Mostnormalhumancellshaveacapacityof60to70doublings.

• Afterthisthecellslosethecapacitytodivideandentersenescence.

• Thisphenomenonisduetoprogressiveshorteningoftelomeres attheendsofchromosomes.

DevelopmentofSustainedAngiogenesis

• Tumorscannotenlargebeyond1-2mmindiameterunlesstheyarevascularized.

• Cancercellscanstimulateneo-angiogenesis duringwhichnewvesselssproutfrompreviouslyexistingcapillariesorinsomecasesvasculogenesis inwhichendothelialcellsarerecruitedfromthebonemarrow.

• Angiogenesis isthusanecessarybiologiccorrelateofneoplasia,bothbenignandmalignant.

• Angiogenesis isrequirednotonlyforcontinuedtumorgrowthbutalsoforaccesstothevasculatureandhenceformetastasis.

ReprogrammingEnergyMetabolism

• Reprogrammingofenergymetabolismissocommontotumorsthatitisnowconsideredahallmarkofcancer.

• Eveninthepresenceofampleoxygencancercellsshifttheirglucosemetabolismaway fromefficientmitochondrialoxidativephosphorylationtoglycolysis.

• Thisphenomenon,calledtheWarburgeffectandalsoknownasaerobicglycolysis.

GenomicInstability-EnablerofMalignancy

• TheimportanceofDNArepairinmaintainingtheintegrityofthegenomeishighlightedbyseveralinheriteddisordersinwhichgenesthatencodeproteinsinvolvedinDNArepairaredefective.

• IndividualsbornwithsuchinheriteddefectsinDNArepairproteinsareatagreatlyincreasedriskofdevelopingcancer.

• HereditaryNon-polyposisColonCancerSyndrome(HNPCCsyndrome)ischaracterizedbyfamilialcarcinomasofthecolonaffectingpredominantlythececumandproximalcolon.ItresultsfromdefectsingenesinvolvedinDNAmismatchrepair.

TUMORIMMUNITY

• Immunesurveillance torefertorecognitionanddestructionofnewlyappearingtumorcells,whichareseenasforeignbythehostimmunesystem.

TumorAntigens

• 2categoriesbasedontheirpatternsofexpression:

• 1-tumor-specificantigens.whicharepresentonlyontumorcells andnotonanynormalcells.

• 2-tumor-associatedantigens.presentontumorcells andalsoonsomenormalcells.