22 an introduction to genetic analysis.docx

22 an introduction to genetic analysis.docx

《22 an introduction to genetic analysis.docx》由会员分享，可在线阅读，更多相关《22 an introduction to genetic analysis.docx（27页珍藏版）》请在冰豆网上搜索。

22anintroductiontogeneticanalysis

Chapter22

CancerasaGeneticDisease

KeyConcepts

Normalcellproliferationismodulatedbycellcycleregulation.

Apoptosisisanormalself-destructionmechanismthateliminatesdamagedandpotentiallyharmfulcells.

Signalingsystemspermitproliferationandapoptosistobecoordinatedwithinapopulationofcells.

Incancer,cellsproliferateoutofcontrolandavoidfail-safedestructionmechanismsthroughtheaccumulationofaseriesofspecialmutationsinthesamesomaticcell.

Manyoftheclassesofgenesthataremutatedtocausecancersareimportantcomponentsofthecellthatdirectlyorindirectlycontributetogrowthcontrolanddifferentiation.

Introduction

InChapter11,welearnedaboutsomewaysinwhichacellmonitorsitsstatusrelativetoitsenvironmentandrespondsaccordingly.Forexample,byutilizingcertainmetabolitesasallostericeffectorsoftranscriptionalregulatoryproteins,anE.colicellcanmakedecisionsaboutwhichsugarmetabolicpathwaystoimplementatanygiventime.Metazoa（multitissuedanimals）usesteroidsandotherlow-molecular-weighthormonesasallostericeffectorsoftranscriptionalregulatorymoleculestocoordinateappropriateresponsesofdifferentorganstoaparticularphysiologicalevent.

Amajorpointtorememberisthatcellshaveevolvedmechanismsthatmodulatetheactivityofkeytargetproteinsbyrelativelyminormodifications—inthetwoprecedingexamples,byformingcomplexeswithallostericeffectors.Muchofgenetics,indeedmuchofthebiologyofacell,dependsonsuchmodulations,inwhichkeyproteinsaretoggledbetweenactiveandinactivestates.

Inthischapterandthenextone,weshallseethisthemeexploitedinavarietyofsituations:

controlofcellnumbers,controlofdevelopmentalpathways,andformationofcomplexbiologicalpatterns.Inthischapter,wefocusonhowsuchmodulationsachievepropercontrolofcellnumberandhowthesystemscanbeovercomebycertainclassesofmutationstoproduceuncontrolledproliferation—thediseasesthatwecallcancers.

Cancerandthecontrolofcellnumber:

anoverview

Cancerisnowclearlyunderstoodasageneticdiseaseofsomaticcells.Incancer,thefail-safemechanismsthatareinplacetoensurethatcellnumberremainsbalancedtotheneedsofthewholeorganismaresubverted,andcancerouscellsproliferateoutofcontrol.Tounderstandhowcellscanmutatetoacancerousstate,wemustfirstunderstandthebasicmechanismsgoverningthecontrolofnormalcellnumbers.

Machineryofcellproliferation

Certainaspectsofproliferationcontrolaregeneraltoallorganisms.Universally,thecelldivisionprocesshasnumerouseventsthatmusttakeplacesequentiallytoproduceviableprogenycells.Moreover,thecelldivisioncyclehasevolvedsothattherearechecksandbalancestopreventasubsequenteventfromtakingplacebeforetheprerequisiteeventshavebeenachieved.Forexample,itwouldbealethaleventifmitosisoccurredbeforeDNAreplicationwascompleted.Mechanismshaveevolvedthatpreventsuchcellulardisasters.Weshallexploretheregulationoftheeukaryoticcellcycle.Proteinkinases,enzymesthatspecificallyphosphorylatecertainaminoacidresiduesontargetproteins,andproteinphosphatases,enzymesthatspecificallyremovephosphategroupsfromsuchaminoacidresidues,modulatetheactivitiesofkeyproteinsinthecelldivisioncycle.Thesephosphorylation–dephosphorylationpathwaysultimatelyconvergetodeterminewhichkeyproteinsareactiveforafractionoftheentirecelldivisioncycle.Putanotherway,itisthecyclicalvariationsinthesekeyproteinsthatdeterminewhichpartsofthecellcyclearecurrentlybeingexecuted.

Machineryofcelldeath

Someaspectsofcellcontrolappeartohaveevolvedonlyinmulticellularorganisms.Todevelopandmaintainthemselvesnormally,multicellularorganismsmustproperlybalancethenumbersofthecelltypesintheirvarioustissues.Almostallofthesecelltypesaresomatic—thatis,theydonotcontributetothegermline.Lossofsuchsomaticcellsisnotaproblemfortheorganismfromthepointofviewofpropagationofthespecies,aslongasproliferationoftheremainingcellsofthattypeinaparticulartissuecompensatesforthecellsthatareeliminated.Furthermore,abnormalcellshavethepotentialtodoconsiderableharm.Thus,mechanismshaveevolvedtoeliminatecertaincells—throughaprocesscalledprogrammedcelldeathorapoptosis.Acascadeofenzymescalledcaspaseskillbydisruptingnumerousstructuralandfunctionalsystemswithinthecell.Subsequently,thecarcassesofthedeadcellsareremovedbyscavengercells.

Linkingcellproliferationanddeathtotheenvironment

Thecellproliferationandcelldeathmachinerymustbeinterconnectedsothateachisactivatedonlyundertheappropriateenvironmentalcircumstances.Forexample,inadultorgans,maintenanceofpropercellnumberrequiresproperbalancebetweenthebirthofnewcellsandthelossofexistingones.Eukaryoticcellshaveevolvedelaborateintercellularsignalingpathwaystoserveasstatusindicatorsoftheenvironment.Somesignalsstimulateproliferation,whereasothersinhibitit.Furthermore,othersignalscanactivateapoptosis,whereasstillothersblockactivation.Intercellularsignalingpathwaystypicallyconsistofseveralcomponents:

thesignalsthemselves,thereceptorsthatreceivethesignals,andthesignaltransductionsystemsresponsibleforrelayingthesignaltovariousregionsofthecell.JustasallostericeffectorsregulatetheactivityofmanyDNA-bindingproteinsinbacteria,modificationstothevariouscomponentsoftheintercellularsignalingsystems—proteinphosphorylation,allostericinteractionsbetweenproteinsandsmallmolecules,orinteractionbetweenproteinsubunits—controltheactivityofthesepathways.

Cellproliferationmachinery

Cellcycle

Therearefourmainpartstothecellcycle:

Mphase—mitosis—andthethreepartsthatarecomponentsofinterphase;G1,thegapperiodbetweentheendofmitosisandthestartofDNAreplication;S,theperiodduringwhichDNAsynthesisoccurs;andG2,thegapperiodfollowingDNAreplicationandprecedingtheinitiationofthemitoticprophase.Inmammals,wherethecellcycleisparticularlywellstudied,differencesintherateofcelldivisionarelargelyduetodifferencesinthelengthoftimebetweenenteringandexitingG1.ThisvariationisduetoanoptionalG0restingphaseintowhichG1-phasecellscanshuntandremainforvariablelengthsoftime,dependingonthecelltypeandonenvironmentalconditions.Conversely,S,G2,andMphasesarenormallyquitefixedinduration.Inthissection,weconsiderthemoleculesthatdrivethecellcycle.Inalatersection,weshallconsiderhowthesemoleculesareintegratedintotheoverallbiologyofthecell.

Cyclinsandcyclin-dependentproteinkinases

Theenginesthatdriveprogressionfromonestepofthecellcycletothenextareaseriesofproteincomplexescomposedoftwosubunits:

acyclinandacyclin-dependentproteinkinase（abbreviatedCDK）.Ineveryeukaryote,thereisafamilyofstructurallyandfunctionallyrelatedcyclinproteins.Cyclinsaresonamedbecauseeachisfoundonlyduringoneoranothersegmentofthecellcycle.Theonsetoftheappearanceofaspecificcyclinisduetocell-cycle-controlledtranscription,inwhichthepreviouslyactivecyclin–CDKcomplexleadstotheactivationofatranscriptionfactorthatactivatesthetranscriptionofthisnewcyclin.Thedisappearanceofacyclindependsonthreeevents:

rapidinactivationoftheactivatoroftranscriptionofthiscyclin'sgene（sothatnonewmRNAisproduced）,ahighdegreeofinstabilityofthecyclinmRNA（sothattheexistingpoolofmRNAiseliminated）,andahighlevelofinstabilityofthecyclinitself（sothatthepoolofcyclinproteinisdestroyed）.

Cyclin-dependentproteinkinasesalsoconstituteafamilyofstructurallyandfunctionallyrelatedproteins.Kinasesareenzymesthataddphosphategroupstotargetsubstrates;forproteinkinasessuchasCDKs,thesubstratesareproteins.CDKsaresonamedbecausetheiractivitiesareregulatedbycyclinsandbecausetheycatalyzethephosphorylationofspecificserineandthreonineresiduesofspecifictargetproteins.

ThetargetproteinsforCDKphosphorylationaredeterminedbytheassociatedcyclin.Inotherwords,thecyclintethersthetargetproteinsothattheCDKcanphosphorylateit（Figure22-1）,therebychangingtheactivityofeachtargetprotein.Becausedifferentcyclinsarepresentatdifferentphasesofthecellcycle（Figure22-2）,differentphasesofthecellcyclearecharacterizedbythephosphorylationofdifferenttargetproteins.Thephosphorylationeventsaretransientandreversible.Whenthecyclin–CDKcomplexdisappears,thephosphorylatedsubstrateproteinsarerapidlydephosphorylatedbyproteinphosphatases.

CDKtargets

Howdoesthephosphorylationofsometargetproteinscontrolthecellcycle?

Phosphorylationinitiatesachainofeventsthatculminatesintheactivationofcertaintranscriptionfactors.Thesetranscriptionfactorspromotethetranscriptionofcertaingeneswhoseproductsarerequiredforthenextstageofthecellcycle.Muchofourknowledgeofthecellcyclecomesfrombothgeneticstudiesinyeast（seenextsection）andfrombiochemicalstudiesofculturedmammaliancells.Awell-understoodexampleistheRb–E2Fpathwayinmammaliancells.RbisthetargetproteinofaCDK–cyclincomplexcalledCdk2–cyclinA,andE2FisthetranscriptionfactorthatRbregulates（Figure22-3）.FromlateMphasethroughthemiddleofG1,theRbandE2Fproteinsarecombinedinaproteincomplexthatisinactive