Neurological channelopathies: new insights into disease mechanisms and ion channel function

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1 Neurologicalchannelopathies:newinsightsintodiseasemechanismsand ionchannelfunction DimitriMKullmann 1 andstephengwaxman 2 1 InstituteofNeurology,UniversityCollegeLondon,London,UK 2 DepartmentofNeurology,YaleUniversitySchoolofMedicine,NewHaven, Connecticut,USA. Abstract Inheritedmutationsofionchannelsprovideuniqueinsightsintothe mechanismsofmanyneurologicaldiseases.however,theyalsoprovideawealth ofnewinformationonthefundamentalbiologyofionchannelsandonneuron andmusclefunction.ionchannelgenesarecontinuingtobediscoveredby positionalcloningofdiseaseloci.andsomemutationsprovideuniquetoolsto manipulatesignallingcascades,whichcannotbeachievedbypharmacological intervention.herewehighlightsomeunansweredquestions,andsome promisingareasforresearchthatwilllikelyleadtoafullerunderstandingofthe linkfrommolecularlesiontodisease. 1

2 Neurologicalchannelopathiesin2010:asnapshot Althoughtheterm channelopathy wascoinedlessthan20yearsago,inherited mutationsofionchannelsarenowrecognisedtoaffectawiderangeoforgans andtorepresentasubstantialdiseaseburden.anincreasingnumberof neurologicalchannelopathieshavebeenidentifiedandcharacterized,andhave beguntoteachusimportantlessonsaboutthemoleculesandcellularprocesses thatunderlieelectricalexcitabilityanddisordersofexcitability.thisspecial IssueofTheJournalofPhysiologypresentsreviewsofrecentresearchinmuscle, nerveandbrainchannelopathies,focusingonareaswheretherehavebeen particularlyimportantadvances.inintroducingthisspecialissue,ratherthan providingasystematicreviewofthehistoryofchannelopathyresearch,we highlightsomeoftheemergingprinciples,andpointtoespeciallypromising areasofresearchforthenextdecade.comprehensivereviewsoftheneurological channelopathiesfromthepointofviewofthedifferentfamiliesofionchannels areavailableelsewhere(dib Hajjetal.,2010;Kullmann,2010).Nevertheless,the followingpointscapturesomeoftheprogressmadesincethefirstionchannel mutationswereidentifiedinassociationwithmonogenicneurologicaldisease. (i) (ii) (iii) Mutationshavebeenidentifiedinatleast29differentionchannel genes(table1),notcountingreportsofsporadicvariantsofpossible pathologicalimpactthatareincreasinglyappearingintheliterature. Mutationsassociatedwithmonogenicdiseaseaffectalmostallion channelfamilies:voltage gatedpotassium,calciumandsodium channels,inwardlyrectifyingpotassiumchannels,chloridechannels, andglycine,gabaandnicotinicacetylcholinereceptors.mutations havealsobeenidentifiedinconnexinsandintracellularchannels.the onlymajorclassofneuronalionchannelswheremutationshavenot beenreportedinassociationwithneurologicaldiseaseisthe ionotropicglutamatereceptors.theexplanationforthisomission remainsunclearatthistime. Althoughmanymutationsresultinlossoffunction,somemissense mutationseitherresultinanincreaseinionflux(oftenbyimpairing inactivation,butinsomecasesbyenhancingactivation)orinagainof 2

3 (iv) (v) (vi) abnormalfunction.someoftheseexperimentsofnaturehaveshed unexpectednewlightonionchannelbiophysicsandonthe fundamentalbiologyofexcitabletissues. Ofthemonogenic pure epilepsies(thatis,whereepilepsyisthe majororonlymanifestationofthegeneticlesion),almostallthegenes areionchannels.indeed,severalhundreddenovomutationsofa singlegene(scn1a,encodingthealpha1subunitofsodiumchannels) havealreadybeenidentified,mainlyinchildrenwithanespecially severeformofepilepsy,severemyoclonicepilepsyofinfancyorsmei (Catteralletal.,Meisleretal.,thisissue). Similarly,severalmonogenicpainsyndromes(inherited erythromelalgia,paroxysmalextremepaindisorder)havethusfar beenlinkedtomutationsinscn9a. Channelopathiesaffectalmostallareasofneurologicalpractice, includingepileptology(macdonaldetal.,berkovicetal.,meisleretal., Catteralletal.,thisissue),movementdisorders(Rajakulendranetal., thisissue),headache(pietrobon,thisissue),peripheralnerve,pain (Estacionetal.,Creggetal.,thisissue),andmyology(Matthewsand Hanna;Cannon,thisissue)(Table1). Ageneticconundrum Despitetheoverwhelmingevidencethationchanneldisordersunderpinmany formsofneurologicaldisease,inonlyrarecasesdoesaclinicaldiagnosisleadto ahighexpectationofidentifyingamutation(notwithstandingthepractical obstaclestogenesequencing).suchdisordersincludetheinheriteddisturbances ofmusclemembraneexcitability(myotoniasanddifferentformsofperiodic paralysis),certainformsofepilepsyincludingsmei,excessivestartledisorder (hyperekplexia),theepisodicataxias,andsomeinheritedpainsyndromes (inheritederythromelalgia,paroxysmalextremepaindisorder).withthe exceptionofthechloridechannelmyotonias,hyperekplaxiaandsmei(where recessiveinheritance,compoundheterozygosityordenovomutationsoften occur),mostpatientsinwhomamonogenicchannelopathycanbeidentified 3

4 haveapositivefamilyhistory,becausethediseaseistransmittedasadominant trait.farmorecommonarepatientswithacquiredorso called idiopathic diseases.migraineandseveralformsofidiopathicgeneralisedepilepsy(ige), typicallyassociatedwithspike and waveeeg,exhibithighheritability:ithas beenestimatedthatabout70%oftheindividual sriskofdevelopingeither disorderisaccountedforgeneticfactors.andyetthesediseasesveryrarely exhibitmendelianinheritance,implyingthattheyarisefromtheinterplayof multiplegenes.avariantinanyoneofthesegenesmaynotbesufficientto determinewhetheranindividualdevelopsadisease,butininteractionwith otherinheritedgenevariants,itmaytipthebalance.althoughthese susceptibilitygenesremaintobeidentified,thebestcandidatesarethosethat encodeionchannels.thisispreciselybecause,oftheraremonogenicformsof migraine(familialhemiplegicmigraine)orepilepsy(variousformsofige,as wellasotherlocalisation relatedsyndromesandchildhoodseizuredisorders), mostofthecausativegenesareionchannels. Giventhatwholegenomescansarenowroutine,itisdisappointingthatthe commongeneticsusceptibilityvariantshavenotbeenidentifiedforidiopathic neurologicaldiseasessuchasmigraine(whichaffectsover10%ofthewestern population)andepilepsy(whichaffects0.5%),andinsubjectswithalow thresholdfor,orchronic,pain.afterall,suchvariantsareincreasinglybeing identifiedinothercommondiseasessuchasdiabetesmellitus,and apolipoproteine4isstronglyassociatedwithalzheimer sdisease.apossible explanationisthatmanyindividuallyrarevariantsinmany(presumablyion channel)genesunderliethegeneticsofsuchneurologicaldiseases.identifying suchgeneticvariants,andseparatingthemfromneutralpolymorphisms(oreven variantsthatreducethechanceofdevelopingadisease)isamajordatamining challenge.nevertheless,itmaybepossibletostratifygeneticvariantsidentified inindividualssufferingfromasuspectedpolygenicchannelopathybyfunctional expressioninvitro.indeed,ionchannelsareuniquelyamenabletodetailed biophysicalanalysis.suchanapproachpotentiallyallowsonetoaskwhether thosevariantsthatcauseloss orgain of functionare,asawhole, disproportionatelyover orunder representedamongaffectedindividuals. 4

5 Rajakulendranetal.(thisissue)haveappliedpreciselythisapproachtoshow thatloss of functionvariantsofacalciumchannelgene(cacna1a)areoverrepresentedamongsporadicpatientswithacombinationofepisodicataxiaand epilepsy,providingcompellingevidenceimplicatingthisgeneasasusceptibility factor. Whydosomanychannelopathieshaveparoxysmalmanifestations? Epilepsy,migraine,episodicataxia,periodicparalysis,inheritederythromelalgia andparoxysmalextremepaindisorderallshareincommonadiseasecourse characterisedbynormalneurologicaldevelopmentandfunctionpunctuatedby attacksofdisablingsymptoms.why?andgiventhatthemolecularlesionis constant,howdoesthenervoussystemcompensateinbetweenattacks?indeed, thisispuzzling,becauseneurologicaldisorderscausedbymutationsofgenes otherthanionchannelsoftenleadtoabnormalbraindevelopment,mental retardation,orfixedorprogressivemotordisorders.apossibleexplanationis thatneuronalexcitabilityisatightlyregulatedparameterandthatalterationsin theexpressionlevelorfunctionofonechannelcanleadtocompensatory changesinotherionchannels.suchchangeshavebeenobservedinneuronsin cultureexposedtoexperimentalmanipulationofexcitability(turrigiano,2008), orfollowinggeneticdeletionofionchannelsubunitsinthecerebellum(brickley etal.,2001).nevertheless,normalfunctionisvulnerabletovariousstressorsin patientswithchannelopathies,implyingasmallersafetyfactorbeforea paroxysmistriggered. Insomeofthesedisorders,areasonableexplanationcanbeproposedforthe developmentoftheictus,althoughitisnotalwaysclearwhycertainprecipitants triggerthem.inhyperkalaemicperiodicparalysis,impairedfastinactivationof sodiumchannelsprovidesforapositivefeedbackloop,wheredelayed repolarizationofthemusclemembranepotentialleadstoafurtherincreasein extracellularpotassium.therearealsoreasonablycompellingmechanistic accountsforseizurepropagationandfortheauraofmigraine,inonecase mediatedbyabnormalrecruitmentofneuronalcircuitsandintheotherbyan 5

6 advancingwaveofcorticaldepolarization.however,whytheepisodicataxias affectentirecerebellarcircuitsatonceremainsunclear.amongrecent breakthroughsistheimprovedunderstandingofhypokalaemicperiodic paralysis.thisdominantlyinheriteddisorderisuniformlycausedbymissense mutationsthatreplacepositivelychargedarginineresiduesinthes4segments ofeitherthesodiumchannelnav1.4orthecalciumchannelcav1.1,whichactas trans membranevoltagesensors(matthewsandhanna,thisissue).until recentlythemutationswerethoughttoresultinarelativelossoffunction,but howthispredisposedtodepolarizationandmuscleinexcitabilityunder conditionsofhypokalaemiawasnotunderstood.itemergesthatreplacementof thearginineresiduesbyotheraminoacidsleadstoacationleakthatby passes themainionconductingporeofthechannel. Whyisclinicalonsetinsomechannelopathiesdelayed? Individualsaffectedbyinheritedchannelopathiescarrythemutantgene,and presumablyproducethemutantproteinorfailtoproduceanormalcomplement ofthenormalprotein,beginninginutero.consistentwiththis,clinical abnormalitiesareoftenapparentbeginningininfancy.yetinsomefamilieswith inheritedchannelopathies,clinicalonsetisdelayeduntillatechildhood, adolescence,oradulthood.whatarethemechanismsunderlyingthistimedependentonsetofclinicaldisease?acluemaycomefromstudiesoffamilies withlate onsetinheritederythromelalgia.nav1.7mutantchannelsinfamilies withlate onseterythromelalgiastudiedtodatedisplayahyperpolarizingshiftin thevoltage dependenceofactivation,asisseenformutantnav1.7channels linkedtoearly onsetinheritederythromelalgia.butthemagnitudeofthe hyperpolarizingshiftinactivationissmallerintheselate onsetfamilies(cheng etal.,2008;hanetal.,2009).andatthelevelofcellularfunction,thelate onset mutantchannelsinducehyperexcitabilityindorsalrootganglionneurons,but theincreaseinexcitabilityissmallerthanthatproducedbymutationslinkedto early onseterythromelalgia.thissuggeststhattime dependentpresentationof diseasesignsandsymptomsmayinsomekindredsreflectthepresenceof 6

7 compensatorymechanisms,earlyinlife,whichsuppressclinicalmanifestations. Accordingtothismodel,laterinlife,theprotectiveeffectsofthesecompensatory mechanismsareovercome,eitherduetocumulativeevents,ortomaturational switchingofspliceisoforms,orofmodifiergenesandtheirproducts.whether theputativeprotectivemechanismsaremolecular,i.e.reflectcompensatory changesinexpressionofotherchannelswhichacttomaintainexcitabilityat close to normallevels,oroperateatthesystemslevels,e.g.byprocessessuchas centralgating,remainstobeelucidated.detailedunderstandingofthe mechanisticbasisfortime dependentonsetofclinicaldisease,andofthe protectivemechanismsthatcontributetoit,mayultimatelyprovideabasisfor therapeuticinterventionsthatacton,orvia,thesemechanisms,tosuppress clinicalsignsandsymptomsinpatientswithidiopathicdisorders. Channelopathiesareleadingtofundamentaladvancesinionchannelresearch Severalchannelswerediscoveredbypositionalcloninginfamilieswithunusual disorders,includingthepotassiumchannelskv7.2andkv7.3inbenignfamilial neonatalconvulsions.thesubsequentdiscoverythattheyco assembletoforma slowly activatedpotassiumchannelsolvedalong standingpuzzleaboutthe molecularidentityofthe M current,whichisattenuateduponactivationof muscarinicreceptorsinmanyneurons.newionchannelgenescontinuetobe discovered,mostrecentlykcnj18,whichencodesaninwardrectifyingchannel inskeletalmuscle,mutationsofwhichpredisposetoperiodicparalysisin patientswithhyperthyroidism(ryanetal.,2010). Closeattentiontothebiophysicalconsequencesofinheritedmutationshasalso ledtotheidentificationofpotentiallypowerfultoolstomanipulatesignalling cascades.thus,thedifferentcacna1amutationsunderlyingfamilialhemiplegic MigraineleadtoanincreaseinthefunctionofindividualCav2.1channels,in somecasesbyshiftingthevoltagethresholdofactivationtomorenegative potentials(pietrobon,thisissue).becauseexocytosisatcentralsynapsesis triggeredbycalciuminfluxviaveryfewchannels,thesemutationsmayprovide uniquenewtoolstoinvestigatehowindividualchannelsinteract. 7

8 Thestill expandinguniverseofchannelopathiespromisestoprovideadditional cluesaboutionchannels,howtheyfunction,andhowtheirabnormalfunction canproducedisease.while,atfirstblush,itmightseemthatphysiological assessmentoftherapidlyexpandinglistofchannelopathiesidentifiedby molecularorgeneticanalysismightbebeyondreach,roboticpatch clamp methodsmayfacilitatescreeningofsomeoftheimportantbiophysical parametersofthemutantchannels(estacionetal,thisissue).hands on physiologistsneednot,however,retreat.ascanbeseenbyacomparisonof roboticassessment(estacionetal,thisissue)andin depthanalysisofmutant channelscausingthesamediseasebyhumanphysiologists(rushetal.,2006), therewillalwaysbeaneedforstudyofchannelopathiesbyincisiveliving investigators.thatanalysiswillundoubtedlybeforthcomingoverthecoming decade,andwecanwithconfidencepredictthatchannelopathieswillcontinue toteachusimportantlessons. 8

9 Table1.Theneurologicalchannelopathies Gene Channel Disease CNS Sodium SCN1A αsubunit Epilepsy,migraine ofnav1.1 SCN1B β1 Epilepsy SCN2A αsubunit ofnav1.2 Potassium KCNQ2 Kv7.2 KCNQ3 Kv7.3 KCNMA1 BK Epilepsywithdyskinesia KCNA1 Kv1.1 Episodicataxia KCNC3 Kv3.3 Ataxia Calcium CACNA1H α1h Epilepsy subunitof Cav3.2 CACNA1A α1a subunitof Episodicorprogressiveataxia, migraine,epilepsy Cav2.1 GABAA GABRA1 α1 Epilepsy receptors GABRB3 β3 GABRG2 γ2 Nicotinic CHRNA2 α2 ACh CHNRA4 α4 receptors CHRNB2 β2 Glycine GLRA1 α1 Hyperekplexia receptors GLRB β Peripheral nerve Sodium SCN9A αsubunit ofnav1.7 Excessivepain,insensitivityto pain Muscle Sodium SCN4A αsubunit Periodicparalysis,myotonia ofnav1.4 Potassium KCNJ2 Kir2.1 Periodicparalysis KCNJ18 Kir2.6 Calcium CACNA1S α1s subunitof CaV1.1 Chloride CLCN1 CLC 1 Myotonia Nicotinic CHRNA1 α1 ACh CHRNB1 β1 receptors CHRNG γ CHRND δ CHRNE ε Congenitalmyasthenic syndromes 9

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