Parkin-linked Parkinson's disease: From clinical insights to pathogenic mechanisms and novel therapeutic approaches

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ContentslistsavailableatScienceDirect

Neuroscience

Research

jo u rn al h om ep a g e :w w w . e l s e v i e r . c o m / l o c a t e/ n e u r e s

Review

article

Parkin-linked

Parkinson’s

disease:

From

clinical

insights

to

pathogenic

mechanisms

and

novel

therapeutic

approaches

Kobi

Wasner

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_,

_Anne

_Grünewald

a,b

_,

_Christine

_Klein

b,∗

a_Luxembourg_Centre_for_Systems_Biomedicine,_University_of_Luxembourg,_{Esch-sur-Alzette,}_Luxembourg

b_Institute_of_{Neurogenetics,}_University_of_Lübeck,_Lübeck,_Germany

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received1August2020 Accepted4August2020 Availableonlinexxx Keywords: Parkin Parkinson’sdisease Mitochondria Inﬂammation Gene-speciﬁctherapy Clinicaltrial

a

b

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Withover7millionpatientsworldwide,Parkinson’sdisease(PD)isbecomingmoreprevalentaslifespan andindustrializationincrease.Whilethemajorityofcasesaresporadicandpresentinindividualsover 65,inheritedmutationsinParkincanmanifestinindividualsasyoungasteenagers.Theinvolvementof Parkininneurodegenerationhasbeenwidelyinvestigatedanditsroleinmitophagyisundeniable.In therecentyears,however,additionalfunctionsoftheproteinarebeginningtocometolight,whichin turnmayinﬂuencethewaypatientsharboringParkinmutationsaretreated.Inthepresentarticle,we discusstheclinicalandgeneticaspectsofParkin-linkedPD.Forthispurpose,weconsultedtheMDSGene database,whichcomprisestheliteratureofmorethan1000patientswithParkinmutations.Inaddition, weprovideinsightintoParkin’smultifacetedroleinmitochondrialclearanceandmaintenance.Finally, wediscusstreatmentstrategiessuchasbrainstimulation,smallmoleculedrugsanddopaminergiccell replacementthatcouldbetailoredtoimprovetheclinicalphenotypesinParkin-linkedPD.

Contents

1. Introduction...00

2. Parkin-linkedPD:clinicalaspects...00

2.1. Parkin-linkedPD:geneticaspects...00

2.2. ProteinstructureofParkin...00

3. MitochondrialinvolvementinParkinson’sdisease...00

3.1. Parkinandmitophagy...00

3.2. Parkinandmitochondrialbiogenesis...00

4. Inﬂammation ... 00

5. Treatment...00

Acknowledgements ... 00

References...00

1. Introduction

While Parkinson’s disease (PD) was ﬁrst systematically describedbytheBritishphysicianJamesParkinsonin1817,ittook almosttwocenturiesuntiltheﬁrstrecessivelyinheritedformofPD wasdiscoveredbyKitadaandcolleaguesinJapanin1998(Kitada etal.,1998).Inhisworkentitled“AnEssayontheShakingPalsy”,

∗ Correspondingauthor:InstituteofNeurogenetics,UniversityofLübeck,

Maria-Goeppert-Str.1,23562Lübeck,Germany.

E-mailaddresses:kobi.wasner@uni.lu(K.Wasner),anne.gruenewald@uni.lu

(A.Grünewald),christine.klein@neuro.uni-luebeck.de(C.Klein).

JamesParkinsoncharacterizedthesymptomsofsixindividuals– includingsomepatientsandindividualsonthestreet-withwhat henamedparalysisagitans–amaladyresultingin,“involuntary tremulousmotion,withlessenedmuscularpower,inpartsnotin actionandevenwhensupported”(Parkinson,2002).This condi-tionwaslaternamedafterJamesParkinsonand,inreferenceto theterm‘Parkinson’sdisease’,Kitadaetal.chosetonametheir newlyidentiﬁedgene ‘Parkin’.Althoughthemoststriking clini-caldifferencebetweenParkinmutationcarriersandclassicalPD wastheveryearly,juvenileageatonset(AAO)intheformer,the clinicalpresentationcausedbyParkinmutationswascompatible withadiagnosisofPD,albeitcharacterizedbyastrong suscepti-bilitytodopa-induceddyskinesiaandmotorﬂuctuations.Further https://doi.org/10.1016/j.neures.2020.09.001

0/).

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extendingtheclosephenotypiclinkofParkin-linkedPDand ‘idio-pathic’PD,i.e.PDofunknownoriginandtypicallylateAAO,Parkin mutationsweresoonidentiﬁed alsoina familywithlate-onset tremor-dominantPD(Kleinetal.,2000).

TheexactfrequencyofParkinmutationsiscurrentlyunknown, whichisespeciallytrueforthemostcommon,late-onsetpatient populationwheresystematicmutationalscreensarelacking. How-ever,Parkinmutationsareoverallrareand,evenintheyounger AAOgroups(<50years)accountfor_∼2.6%ofthecasesonly(Tan etal.,2019).MutationfrequencyincreaseswithdecreasingAAO andishighestinthejuvenilegroup(AAO<20years)where bial-lelic Parkin mutations are found in up to77 %of thepatients (Luckingetal.,2000).Giventherecent‘PDPandemic’withPDbeing thefastestgrowingneurologicaldiseasewithacurrentestimated globalnumberof∼7millionPDpatients(DorseyandBloem,2018), onemayexpectatotalof35,000–70,000Parkincasesworldwide whenassuminganestimatedfractionofParkinmutationcarriers acrossallAAOgroupsof0.5–1%.

2. Parkin-linkedPD:clinicalaspects

The Movement Disorder Society Genetic mutation database (MDSGene;www.mdsgene.org)providesacomprehensiveonline resourcelinkingreportedgeneticmutationswithmovement dis-orderphenotypesaswellaswithdemographicandotherclinical informationincludingPARK-Parkin(Kastenetal.,2018).MDSGene currentlylists1000biallelicParkinmutationcarriers(44%female) withPDandamedianAAOof31years(25th_/75th_percentile:_23/39 years;range:3–73years).Themostcommonpresentingsignis tremor,followedbybradykinesiaanddystonia.Indeed,acrossall listedpatients,dystoniaisthemostcommonclinicalfeatureafter thecardinalPDsignsandispresentin65%ofthosewithavailable information.Underscoringtheimportanceofdystoniaasa promi-nentfeatureofParkin-linkedPDespeciallyintheveryearly-onset patients,thepercentageofpatientswithdystoniarisesto85%in thosewithajuvenileAAO,i.e.withanonsetofPDat20yearsor younger.Asalreadymentionedintheoriginaldescriptionofthe Parkingene(Kitadaetal.,1998),dyskinesiaisacommonﬁndingin Parkinmutationcarriersandfoundin78%ofallpatientswith avail-ableinformationonthisfeatureandineven90%ofthosewitha juvenileAAO.Likewise,motorﬂuctuationsoccurathighfrequency (87%)andareonlyslightlymorecommoninthejuvenileonset group(91%).Non-motorsignsdooccurinParkinmutation carri-ers,however,reportinghasbeeninconsistentanddatamissingness ishigh.

Asinformationonthepresenceorabsenceofnon-motorsigns andsymptomshasbeenprovidedfor100Parkinmutationcarriers (10%)orfewer,90%datamissingnessdoesnotallowformeaningful conclusionsonthefrequencyofthefollowingfeatures:autonomic signs orsymptoms, sleepbeneﬁt, depression,anxiety,and psy-choticsignsandsymptoms.Bycontrast,previousmeta-analysesof publishedPDpatientswithParkinmutationsindicatedthat demen-tiaisveryrare,affectinglessthan3%ofcases(Grunewaldetal., 2013;Kastenetal.,2018).ThelargestfractionofParkinmutation carriersisofAsianorigin(37%overall;44%inthejuvenileAAO group), including10%fromJapan,followedbyWhiteEuropean (35%).

2.1. Parkin-linkedPD:geneticaspects

ToreviewthemutationalspectrumofParkin,wealsoconsulted MDSGeneandourin-housedatabaseonpublishedheterozygous Parkin mutation carriers. Todate,200different mutationshave beenpublished.Theseinclude18variationsinintrons,13nonsense mutations,69missensemutations,29smallinsertionsordeletions,

and71exonrearrangements(including42deletionsand26 multi-plications).Outof1155mutation-positiveindexpatients,778(67 %)carriedatleastoneexonrearrangementwithexon3deletions beingthemostcommonandaffecting120(10%)indexpatients(on oneortwomutantalleles).

Whenassessingtheimpactofdifferentmutationtypesonthe AAO,wedidnotobserveanysigniﬁcantgroupdifferencesacross indexpatientswithhomozygousmutations(variantsinintrons: AAO[SD],23.8[10.1]years,n=4;nonsensemutations:31.4[8.1] years,n=8;missensemutations:35.0[17.5]years,n=35;small insertions/deletions:31.0[11.3],n=75;exonrearrangements:32.9 [11.5]years,n=200).Thesameobservationwasmadewhen focus-ingexclusively onindex patientswithheterozygous mutations (variantsinintrons:n/a;nonsensemutations:AAO[SD],30.2[14.3] years,n=5;missensemutations:42.3[13.3]years,n=125;small insertions/deletions:35.6[12.3],n=27;exonrearrangements:AAO [SD],40.9[13.8]years,n=238).

Bycontrast,comparingthemeanAAOofallindexpatientswith biallelic(AAO[SD],31.3[12.0],n=663)vs.thosewithheterozygous mutations(40.7[13.7]years,n=409),wedeterminedadifference ofabouttenyears(p<0.0001).WithanAAOintheforties,Parkin heterozygotessufferfromPDabouttenyearsearlierthanidiopathic patientswhotypicallydevelopﬁrstsignsintheir6th_decade_of_life (Grunewaldetal.,2013).Toobtainanevenclearerpictureofthe inﬂuenceofheterozygousParkinmutationsontheonsetofPD,we repeatedtheaforementionedanalysisforexon3deletioncarriers only.Interestingly,inthishomogenous(albeitsmall)subset,the AAOwasaslowinheterozygotes(AAO[SD],33.1[6.3],n=10)as inhomozygotes(32.6[10.2],n=42).Ofnote,forallAAO analy-ses,individualswithknowndigenicinheritanceofPD-associated mutationswereexcluded.

2.2. ProteinstructureofParkin

Parkin is a 465-residue protein involved in the ubiquitin-proteasomesystem: a process which mediates thetargeting of proteinsfordegradation.AsanE3ubiquitinligase,Parkinrecruits an E2 conjugating enzyme to facilitate the transfer of a ubiq-uitinsubstrateontoitstargetprotein.Theadditionofubiquitin ontoaproteincanproduceavarietyofeffects including degra-dationviathe proteasome,alterationof itscellularlocation, or promote or prevent protein-protein interactions (Seiraﬁ et al., 2015).

ParkinbelongstotheRING-between-RINGfamilyofE3ligases, whichgenerallycomprisesoftwoRINGﬁngerdomainsandan in-between-RINGregion.RING1servesasthebindingsiteforanE2 ubiquitinconjugatingenzyme,whileRING2containsanactivesite cysteineresidue,whichacceptsandcleavesubiquitinfromtheE2 enzymeandtransfersitontoitssubstrates.Aubiquitin-likedomain isalsopresentattheN-terminusofParkinandplaysarolein sub-straterecognition.

AnampleamountofstudieshavefoundthatParkinislargelya cytosolicproteinmediatingubiquitinationinthecytosol.However, severalstudieshavealsoshownsmallpoolsofParkinlocalizedto mitochondria(Kurodaetal.,2006;Narendraetal.,2008;Rothfuss etal.,2009).

3. MitochondrialinvolvementinParkinson’sdisease

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adminis-teredtheonlyknowneffectivetreatment–L-DOPA–and“unfroze” thepatients.

Thepatientswerefoundtobedruguserswhoingested“China White”–asyntheticopioidanalgesicproducedbydrugdealers. Analysisof thedrugdeterminedits chemicalname: 1-metyl-4-phenyl-1,2,3,6-tetrahydropyridine(MPTP).Unbeknownsttothem, this toxic compoundwastheperfect inhibitorofmitochondrial respiration.

3.1. Parkinandmitophagy

Damagedmitochondriaarequicklydegradedandreplacedin thehealthybrain.Oneofthemostprominentandstudiedfunctions ofParkinisitsroleinmitophagy–theselectivedegradationof mal-functioning mitochondria by autophagy. Mitochondrial damage results in the depolarization of the outer mitochondrial mem-brane(OMM),therebypreventingPINK1importthroughtheOMM transmembraneproteincomplexes(Narendraetal.,2008).PINK1 subsequently accumulatesalong the OMMand begins to phos-phorylateitstargets,includingubiquitinandParkinatserine65 (Shiba-Fukushimaetal.,2012).Phosphorylationatthesesites acti-vatesandrecruitsParkintotheOMMwhereParkinsubsequently beginstotransferubiquitinontoitstargetsincludingmitofusins, VDACsandBAK(Fig.1)(Bernardinietal.,2019;Geisleretal.,2010;

Kaneetal.,2014;Sarrafetal.,2013).Ubiquitinchainlinkageson K48primesdegradationoftheproteinsbytheproteasome,while K63 ubiquitination isrecognized byautophagyadaptors,which eventuallyformtheautophagasome,leadingtoitsdestructionby lysosomes.

Mitochondriaare highlydynamic organelles,whereby a bal-anceofdegradationandbiogenesisismediatedbymorphological changesthroughﬁssionandfusion(Dengetal.,2008).Mitofusin1 and2(MFN1/2),whicharedownstreamtargetsofParkin,facilitate thefusionoftheOMM,whileDynamin-relatedprotein1(Drp1) regulatestheﬁssionoftheinnermitochondrialmembrane(IMM). Animal modelslacking Parkinshowstrikingmitochondria mor-phologicalphenotypesincludingswelling,herniationandbroken cristae(Dengetal.,2008).

Mitophagyisaprocessperformedbythecellinordertocombat dysfunctionandasaresorttopreventcelldeath.Oncepastacertain thresholdandcommittingtoacelldeathfate,intrinsicapoptosis ismediatedbythepermeabilizationofmitochondrialmembranes andreleaseofpro-apoptoticfactorssuchascytochromeC. Perfo-ration ofthemitochondrialmembranesisexecutedbyBAKand BAX,whicharerecruitedtotheOMMandformoligomers,which puncture the membranes(Fig.1).Parkinreduces BAX localiza-tiontotheOMMthroughanindirectinteractionandalsodirectly ubiquitinatesBAK,preventingitsdevelopmentintooligomersand subsequent destructionof theOMM andrelease of cytochrome C(Bernardinietal.,2019).Thus,Parkin-mediatedmitophagyalso servestopreventapoptosis.

During oxidativestressandmitophagy,mitochondriarelease signals to other organelles in response to a variety of stim-uli. Mitochondrial-derived vesicles (MDVs) are ﬁlled with oxidized cargo that bud off mitochondria (independently of mitochondrial ﬁssion).Parkincolocalizes withMDVs and stim-ulates their formation in response to ROS (McLelland et al., 2014).

RecentevidencerevealedParkin’smanyfunctionsotherthan governing mitophagy;Parkinubiquitinates hundredsoftargets, whichplayotherrolesinmitochondrialpathways,itcontainsthe structuralarchitecturecommoninseveraltranscriptionfactorsand associateswiththemitochondrialgenome(AlvesdaCostaetal., 2018).

3.2. Parkinandmitochondrialbiogenesis

Peroxisome proliferator-activated receptor gamma coactiva-tor1-alpha (PGC-1␣) is a transcriptional coactivatorknown as themasterregulatorofmitochondrialbiogenesis,whereit tran-scribesa multitudeof downstreamtarget genesresponsiblefor mitochondrialmaintenanceandrespirationincludingnuclear res-piratory factors (NRFs) and estrogen-related receptors (ERRs). OverexpressionofPGC-1_␣leadstoanincreaseof thesetargets, mitochondrial-encodedproteinsandmitochondrialDNA(mtDNA) copynumber(Zhengetal.,2017).

Oneof Parkin’s targetsfor ubiquitination, Parkininteracting substrate(PARIS),isatranscriptionalrepressorofPGC-1␣(Fig.1). PARISisubiquitouslyexpressedthroughoutthebodyand hetero-geneouslyexpressedthroughoutthebrain,where itislocalized toneurons,includingmidbraindopaminergicneuronsofthe sub-stantia nigra (SN) pars compacta. Consequently, PARIS protein accumulatesinthebrainofpatientswithautosomalrecessive juve-nilePD.MutationstoParkinresultintheaccumulationofPARISand successiveinhibitionofthetranscriptionofPGC-1␣andits down-streamtargets(Shinetal.,2011;Zhengetal.,2017).Thus,Parkinis notonlyresponsibleforthedegradationofdamagedmitochondria, butalsoplaysaroleinthemitochondrialbiogenesispathway.

Thegenerationof newmitochondriarequiresthereplication andtranscriptionofthemitochondrialgenome,whicharemainly regulatedbynuclear-encodedproteins.PGC-1␣targetsNRF1and NRF2 - transcription factors, which activate the expression of moleculesmaintainingmtDNA(Gugnejaetal.,1996).NRF-1and NRF-2transcribeTFAMand TFB2M, twomoleculesrequired for theinitiationoftranscriptionandreplicationofthemitochondrial genome.

NRF-1abundancehasbeenshowntobereducedincellswhich lackParkin(Shinetal.,2011;Stevensetal.,2015),and interest-ingly,NRF-1wasshowntocontainbindingsitesinbothParkinand PINK1promoterregionsinsilico,invitroandinvivo(Sulimanetal., 2017).ThiswassupportedbyoverexpressionofNRF-1,whichlead toanincreaseinParkinandPINK1geneandproteinexpression, whilesilencingofNRF-1leadtoParkinandPINK1downregulation. Moreover,Parkinmutationsandconsequentialdownregulationof PGC-1␣causeslossofNRF-1andTFAM.

TFAM is crucial for the maintenance of the mitochondrial genome.ItsfunctionsincludetheregulationofmtDNAcopy num-ber, initiation of mtDNA transcription, participation in mtDNA replication and compaction of the mitochondrial genome into protein-richstructurestermednucleoids.Uponactivationby regu-latoryproteins,suchasNRF-1,TFAMtranslocatesfromthecytosol tothenucleoidandregulatesmitochondrialtranscription(Fig.1).

The mitochondrial genome is located in the mitochondrial matrixandincloseproximitytotheelectrontransportchain(ETC) proteincomplexes.In theprocessofproducing ATPfor thecell, toxicby-products,likereactiveoxygenspecies,arealsogenerated andcanharmmtDNAmolecules(Mecoccietal.,1993).Moreover, deletionsinmtDNAaccumulatewithnormalagingintheSNand occurmorefrequentlyinPDpatientsthanin age-matched con-trols(Benderetal.,2006;Kraytsbergetal.,2006).mtDNAdepletion anddownregulationofTFAMandmtDNAtranscriptionhavealso beenfoundinpost-mortemtissueofPDpatients(Grunewaldetal., 2016).

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SH-Fig.1. CellularfunctionsofParkin.ParkinisrecruitedtothemitochondrialoutermembraneafterphosphorylationbyPINK1.Parkinthenbeginstoaddphospho-ubiquitin

toitstargets,includingMFN2.Ubiquitinchainsarerecognizedbyadaptorproteinsthatformautophagosomes,whicharethenredirectedtothelysosomefordegradation

andrecycling.Duringmitophagy,ParkinubiquitinatesBAXandBAKwhich,intheabsenceofParkin,formoligomersandpunctureholesintheOMM,allowingthereleaseof

mitochondrialDNAandcytochromeC.CytochromeCinthecytosolpropagatesintrinsicapoptosis,andmtDNAreleaseisrecognizedbycytosolicDNAsensors,whichmediate

inﬂammation.Inaddition,ParkininﬂuencestranscriptionofmtDNAviaitsdirectinteractionwithTFAMoritsinhibitionofPARIS,thetranscriptionalrepressorofPGC1-␣,the

masterregulatorofmitochondrialbiogenesis.Fig.1wasdesignedusingcomponentsfromServierMedicalArt(http://smart.servier.com/),whichislicensedunderaCreative

CommonAttribution3.0GenericLicense.

Fig.2. ParkininﬂuencesmtDNAmaintenanceiniPSC-derivedneuronalprogenitorcells.(A)Patientandcontrol-derivediPSCsweredifferentiatedintosmallmoleculeneural

precursorcells(smNPCs).ImmunoscytochemistryshowedthatsmNPCsstainedpositivelyfortheneuralprogenitormarkersNestinandPax6.Real-timepolymerasechain

reactionquantiﬁcationofmtDNAsequencesintheD-LOOP,theminorarcgeneMT-ND1andthemajorarcgeneMT-ND4iniPSC-derivedneuralstemcellsfromthree

controlsandthreeParkin-mutationcarriers.(B)TheMT-ND4:MT-ND1ratioisindicativeoftheproportionofwildtypemtDNAmoleculeswithoutsomaticmajorarcdeletions.

ComparingneuralcellsfromParkin-mutantpatientsandcontrols,nodeletionaccumulationwasobserved.(C)Theabundanceof7SDNAintheD-loopofthemitochondrial

genomeisindicativeofmtDNAtranscriptionandreplicationinitiation.StatisticalanalysisrevealedsigniﬁcantlyreducedD-loop:MT-ND1ratiosincellsfromParkinmutation

carrierscomparedtohealthycontrols.*p<0.05.ND1=NADHdehydrogenase1.

SY5Ycells;ChIPonchipanalysesshowedthatParkinassociates withseveralmitochondrialgenomicsequencesincludingATPase 6,COXII,ND1,ND2andD-LOOP.Moreover,ParkinandTFAM co-associateatseveralofthesamemtDNAsequencesinpost-mitotic neuronsandinvivo,andParkinwasshowntoprotect mitochon-drialgenomeintegrity(Rothfussetal.,2009).Bycontrast,Parkin’s putative role in mtDNA maintenance hasnot yet been investi-gatedinpatient-derivedneuronalmodelswithParkinmutations.

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fur-therimplicatesimpairedmtDNAhomeostasisinthepathogenesis ofParkin-linkedPD.

Whilemousemodelshaveunraveledanextraordinaryamount ofbiologicalmechanismsregardingPD,Parkin-knockout(KO)mice donotdisplaysignsofnigrostriatalneurodegenerationorany sig-nificantmotorphenotypestypicalofPD.AsvariationsintheDNA polymerase (POLG)genehavebeenidentifiedasriskfactorsin PD,PickrellandcolleagueshypothesizedthatlackofParkininmice withincreasedmutagenicstressmayprovideabettermodelto studyParkin-relatedPD.Themutator-Parkin-KOmousecombines amutationtotheonlymtDNAproofreadingfactorPOLGandParkin KO,subjectingthemicetobothmutagenicandlossofParkin.These miceexhibitPDsymptomsandtheselectivelossofdopaminergic neuronsoftheSN.Inthesemice,mtDNAdepletionandmutation ratewassignificantlyincreasedcomparedtoWTmice.Moreover, loss ofParkin expressionduringmutagenicstressincreasedthe predictedpathogenicityoftheincurredmutations(Pickrelletal., 2015).

4. Inﬂammation

Inthelastdecade,anexcitingandnovelthemehasemergedin thepathophysiologyofageandaging-relateddisorders:theroleof mitochondriaasatriggerfortheimmunesystem.Once indepen-dentbacteria,mitochondriacontaintheirownDNAlocatedwithin themitochondrialmatrix.Iffoundoutsidethematrix,suchasthe cytosolorextracellularspace,mtDNAcanbeperceivedasforeign andpotentiallytoxic.Suchmoleculesreleasedfromtheirnatural compartmentsasaresultofcellularstressand/orinjuryaretermed damage-associatedmolecularpatterns(DAMPs)andcaninitiatea non-infectiousinﬂammatoryresponse,wherebycytosolicand/or extracellular patternrecognitionreceptors(PRRs)recognizeand activateinﬂammatorypathwaystopromptinnateimmunity(Roh andSohn,2018).

TFAM is the majorprotein comprising nucleoids, and TFAM downregulation hasbeenshown in postmortem idiopathicPD brainsandcorrelateswithalossof7SDNA-associatedmtDNA tran-scriptionanddepletion(Grunewaldetal.,2016).InTFAM-knock downmouseembryonicﬁbroblasts,lossofTFAMleadstomtDNA escapeintothecytoplasmpromptingits“foreign”recognitionand subsequentactivationofthecyclicGMP-AMPsynthase-stimulator ofinterferongenes(cGAS-STING)innateimmuneresponse,thereby resultinginproinﬂammatorycytokinesignaling(Fig.1)(Westetal., 2015).

Asmitophagyselectively degradesmalfunctioning mitochon-dria,itisplausiblethatmitophagymayreducetheexpulsionof DAMPsfrommitochondria.Comparedtowildtypemice, Parkin-deficientmicepresentedwithsignificantlyhigherconcentrations ofIL-6,IFN␤1,IL-12p70,IL-13,CXCL1,CCL2andCCL4inserum, which wasdependentoncGas-STING activation.Human serum frombiallelicParkin-mutationcarriers alsoshowedsignificantly higherlevelsofIL-6,IL-1␤,andCCL4comparedtohealthycontrols, suggestingthatlackofParkinresultsinanincreaseincytokines preceding symptomatic neurodegeneration (Sliter et al., 2018). IFN␤1,a type 1interferon, is produced upon STING activation. In similar fashion, in ourrecent biomarker study, serum from Parkin-mutationcarriersshowedhigherlevelsofcell-free circulat-ingmtDNAandIL-6levelscomparedtobothidiopathicPDpatients andhealthycontrols.Moreover,IL-6levelscorrelatedwithdisease durationinParkinmutationcarriers(Borscheetal.,2020).

5. Treatment

TheoverwhelmingmajorityofParkinmutationcarriersrespond favorablytoLevodopatreatment(>98%).Ofthosewithreported

information,93%respondwelltoLevodopaandonlyasmall minor-ityshowsamoderate(4%)orminimal(3%)response,respectively (www.mdsgene.org).Althoughprospectivestudiesarestilllacking, itappearsthatdeepbrainstimulationisaseffectiveinParkin muta-tioncarriersasinidiopathicPDpatients(deOliveiraetal.,2019;

Kimetal.,2014;Rizzoneetal.,2019).

Morerecently,therapeuticapproachesarebeingexploredthat targetspecificParkinmutation-induced(dys-)functionincluding mitophagy.Asmallmoleculeactivatorofmitophagy,either acti-vatingParkinorPINK1directlyorinhibitingParkin’scounterpart, the ubiquitin-specific protease USP30, are in preclinical devel-opment(Miller and Muqit,2019).It hasalso beenargued that Parkin-PDpatientswouldbeparticularlywell-suitedcandidates fordopaminergiccellreplacementtherapiesbasedonthefrequent confinementoftheirneurodegenerationtothenigrostriatal path-way and ontheir overall younger age and rarer occurrence of comorbidities(Kunathetal.,2019).Inthiscontext,itisexcitingto notethatpatient-derivedmidbraindopaminergicprogenitorcells, differentiated in vitrofrom autologous iPSCs, were successfully implantedinapatientwithidiopathicPD.Thecellswere demon-stratedtohave thephenotypicproperties ofSN parscompacta neuronsandwereimplantedintobothputaminawithouttheneed forimmunosuppression.Positron-emissiontomographywiththe useoffluorine-18-L-dihydroxyphenylalanineindicatedgraft sur-vivalwhichwasparalleledbystabilizationorevenimprovement ofPDsigns andsymptoms at18–24monthsafter implantation (Schweitzeretal.,2020).Itremainstobeexploredwhether implan-tationof (mutation-corrected)autologousiPSC-derived neurons wouldbeanequallyviabletherapeuticstrategyforParkin-linked PD.

Finally,thedetectionofelevatedcytokinelevelsintheserum ofpatientswithParkinmutationssuggeststhatanti-inﬂammatory treatmentscouldbeconsideredtomodulatetheprogressionofPD inthesecases(Borscheetal.,2020).

Acknowledgements

CKreceivesfundingwithintheframeworkofSysMedPD (Euro-pean Union’s Horizon 2020 research and innovation program). CK and AG are funded by the German Research Foundation (FOR2488,GR3731/5-1).AGandKWaresupportedbythe Lux-embourgNationalResearchFund(ATTRACTcareerdevelopment grant,FNR9631103;INTERgrant,FNR11250962).

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