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Human Endogenous Retroviruses in Neurological

Diseases

Patrick Küry, Avindra Nath, Alain Créange, Antonina Dolei, Patrice Marche,

Julian Gold, Gavin Giovannoni, Hans-Peter Hartung, Hervé Perron

To cite this version:

Patrick Küry, Avindra Nath, Alain Créange, Antonina Dolei, Patrice Marche, et al.. Human

Endoge-nous Retroviruses in Neurological Diseases. Trends in Molecular Medicine, Elsevier, 2018, 24 (4),

pp.379. �10.1016/j.molmed.2018.02.007�. �inserm-02333223�

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Review

Human

Endogenous

Retroviruses

in

Neurological

Diseases

Patrick

Küry,

1

Avindra

Nath,

2

Alain

Créange,

3

Antonina

Dolei,

4

Patrice

Marche,

5,6

Julian

Gold,

7,8

Gavin

Giovannoni,

7

Hans-Peter

Hartung,

1,

* and

Hervé

Perron

9,10

The causes of multiple sclerosis and amyotrophic lateral sclerosis have long remainedelusive.Anewcategoryofpathogeniccomponents,normallydormant within human genomes, has been identified: human endogenous retroviruses (HERVs).Theserepresent8%ofthehumangenome,andenvironmentalfactors havereproduciblybeenshowntotriggertheirexpression.Theresultingproduction ofenvelope(Env)proteinsfromHERV-WandHERV-Kappearstoengage patho-physiologicalpathwaysleadingtothepathognomonicfeaturesofMSandALS, respectively.PathogenicHERVelementsmaythusprovideamissinglinkin under-standingthesecomplexdiseases.Moreover,theirneutralizationmayrepresenta promisingstrategytoestablishnovelandmorepowerfultherapeuticapproaches.

KeystoUnderstandingtheEtiopathogenesisofInflammatoryand DegenerativeNeurologicalDiseasesMayLieintheHiddenHalfofthe

HumanGenome

Anewcomprehensiveapproachtotheetiopathogenesisofsomediseasesisemergingfrom studiesongenomicremnantsofmobilegeneticelements(seeGlossary)thatareknownfor their role inthe molecular evolutionof genomes [1–4], witha particular focus on human

endogenous retroviruses (HERVs). This domain cannot be appropriately understood

throughclassicalvirology orgeneticsbecause it dealswithentitiesthat areneitherviruses norphysiologicalgenes[5].

Inthepresentreviewwediscussrecent scientific discoveriesoftheselong-misunderstood elements,previouslyconsideredtobejunkDNAbutwhich,insomeinstances,arenowknown tocontributetophysiologicalfunctions(‘domesticatedcopies’)[5–7]ortoremainasdormant functionalcopiesencodingretroviralproteins[5,8,9].Weattempttoexplainhowthesepeculiar geneticelementsmayprovidemissingkeystounderstandingcomplexmultifactorialdiseases. Thebest-studieddiseaseswhereconsistentscientificdatasupportaninvolvementofHERV geneticelementsintheirpathogenesisareMS(Box1)andamyotrophiclateralsclerosis (ALS),butwealsointroducechronicinflammatorydemyelinating polyradiculoneurop-athy(CIDP).Inaddition,wediscussnovelHERV-targetedtherapeuticavenuesthatarestarting to beevaluatedin MSand ALS.Moreover, HERVshave also been associatedwith other diseasessuchasschizophreniaandbipolardisorder[10],aswellaswithtype1diabetes[11], butmuchlessdataareavailableforthesedisorders.

EndogenousRetrovirusesOriginatefromAncestralGermlineInfectionsby

ExogenousElements

TheeukaryoticgenomeiscomposedofalargesetofDNAsequences,manyofwhichderive frommobilegeneticelements[1,12].Thesewereestimatedtoaccountforabout50%ofthe

Highlights

HERVs, retroviral sequences inte-gratedintothegenomeduring evolu-tion,arenowknowntorepresent8% ofthehumangenome.

Thesewererecentlyshownto com-prise copiesthat retainpotential to expressretroviralproteinsorparticles, andcanbeabnormallyexpressedin autoimmune, neurodegenerative, chronic inflammatory diseases, and cancer.

Environmentalfactorssuchasspecific viralinfectionswereshowntopotently activateHERVsundertissue-specific andtemporalconditions.

Ofseveraldiseasesinwhichabnormal activation and expression of HERV proteinshavebeenreported,studies over recent decades have led to a proofofconceptthat HERVsplaya keyrole inthe pathogenesisofMS andALS.

HERV-W and HERV-K Envproteins induce pathogenic effects in vitro andin vivo that arerelevant to the pathognomonic features of these diseases.

These endogenous retroviruses are potentialnoveltherapeutictargetsthat arenowbeingaddressedwith innova-tive therapeutic strategiesin clinical trials.

1

DepartmentofNeurology,Medical Faculty,Heinrich-Heine-University, Düsseldorf,Germany

2

SectionofinfectionsoftheNervous System,NationalInstituteof NeurologicalDiseasesandStroke (NINDS),NationalInstitutesofHealth

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humangenome[13–15](Figure1A),ifnotmore[16].Theirdetectionisparticularlycomplex, whichmayexplainwhytheirproportionwithinthegenomehasbeenlargelyunderestimated andwhydataevolvewithtechnologicalimprovements[12],amongwhicharethoseallowing thesequencingofconstitutiveheterochromatinregionsthatlongremainedinaccessible[17]. Twotypesoftransposableelementscanbedistinguished:(i)elementsthatcanbetransposed viaaDNAintermediateandacut-and-pastemechanism(transposons)[18],and(ii)thoseusing a RNAand a copy-paste mechanism(retrotransposons) [16]. Retrotransposons comprise endogenousretroviruses(ERVs),forwhichthecurrentliteraturepresentsincompletedataas wellasseveralclassificationsandnomenclatures,leadingtoambiguities[15].Theterm‘HERV’ referstothesequencesofhumanERVfamilies.HERVsretainstructuralfeaturesofretroviral genomes(Figure1B),andoriginallyenteredthegenomeofspeciesthroughrepeatedinfections ofgermcellsovermillionsofyears(Figure2)[5,19].

SilentHERVsCanBeActivated byEnvironmentalTriggers

ThequestionofwhyandhowHERVcopiesthathaveretainedcodingpotentialmaybecome expressed is now addressed. The ability of HERVs to become activated is linkedto the chromatinstatewhereagivencopyislocated[20].DNAmethylationandhistonemodifications are essential to the epigenetic control of human genes, including HERV elements. The prerequisitefor functionaltranscription isthatHERV sequencesmustretainfunctionallong terminal repeats (LTRs), or become controlled by another promoter, without deletions or nucleotidesubstitutionsthatdisrupttheiropenreadingframes(ORFs).Becausethenuclear microenvironment including the chromatin accessibility of coding regions differs between tissues,thebaselinepredispositionofaHERVcopytobeactivatedcanbetissue-,cell-,or maturationstage-specific.

InflammatorystimulimayactivateHERVsviaepigeneticdysregulation.Forinstance, transcrip-tionofHERV-Wsequenceshasbeenreportedtobeupregulatedbyproinflammatorycytokines in cultured cells from MS patients [21], and this was shown to correlate with increased differentiation of peripheral blood mononuclear cells from MS patients [22]. By contrast,

(NIH),Bethesda,MD,USA

3

ServicedeNeurologie,Groupe HospitalierHenriMondor,Assistance PubliqueHopitauxdeParis(APHP), UniversitéParisEst,Créteil,France

4

DepartmentofVirology,Universityof Sassari,Sassari,Italy

5

InstituteforAdvancedBiosciences (IAB),UniversityofGrenoble-Alpes,La Tronche,France

6

InstitutNationaldelaSantéetdela RechercheMédicale(INSERM)Unité 1209,LaTronche,France

7

CentreforNeuroscienceandTrauma, BlizardInstitute,QueenMary University,London,UK

8

TheAlbionCentre,PrinceofWales Hospital,Sydney,Australia

9

Geneuro,PlanlesOuates,Geneva, Switzerland

10

UniversityofLyon,Lyon,France

*Correspondence:

hans-peter.hartung@uni-duesseldorf. de(H.-P.Hartung).

Box1.VirusesinMS

AninfectiousoriginofMSwassuggestedforthefirsttimebyPierreMariein1884,butwasrejectedbythecontemporary medicalcommunity.TheevidencethatvirusesmaycontributetoMScomesfromtheaccumulationofimmunecells withinthebrainandCSF,localimmunereactivitytospecificviruses,andthepresenceofoligoclonalbands.MS epidemiologyindicatessometriggersduringadolescence,anunusualgeographicdistribution(agradientwithlatitude, butwithexceptions:e.g.,contrastingpatternsinSardiniaandJapan),andepidemicclustersinpreviouslyisolated islands.Overtime,severalviruseshavebeenproposedascausativeagentsofMS.Fromthe1940sonwardthey includedrabiesvirus,herpessimplexvirus(HSV),scrapieprion,anunidentifiedMS-associatedagent,parainfluenza virus1,measlesvirus,simianvirus5,chimpanzeecytomegalovirus,coronavirus,EBV,anunidentifiedSMON-likevirus (subacutemyelo-opticoneuropathyvirus),tick-borneencephalitisvirus,HTLV-1,HSV-1,VZV,andHHV-6.The pro-posedmechanismsweredirectbrainorperipheralinfection,activationofautoreactiveTcellsagainstnervemyelin, bystanderactivation,epitopespreading,molecularmimicry,andvirus–virusinteractions.However,thelinktoMSwas showntobeweakforthemajorityoftheaboveviruses.Themostconsistentandindependentlyconfirmedstudiesfor viralinvolvementinMSareforEBV.Theyappeartobeconfirmed,butonlywithindirectlinks,byhistoryofinfectious mononucleosis(IM;primaryinfectionwithEBVcausesIM),andhighanti-EBNA-1(EBVnuclearantigen1)IgGtiters beforeMSonset.However,anewconceptarosewiththediscoverythatHERVsexpresspathogenicproteinsin disease,andthebestevidenceofanassociationandpathogenicinvolvementisforHERV-W/MSRV(detectedinMS blood,spinalfluid,andbrain,inparallelwithMSstages,active/remissionphases,andtherapyoutcome).EBVisknown toactivateHERV-W/MSRVinvitroandinvivo(inIMpatientsandinhealthyhumanswithhighanti-EBNA1IgGtiters). ThissuggeststhatEBVcouldbeaninitialtrigger,andthatHERV-W/MSRVisadirectneuropathogeniccontributor, beforeandduringMS,inadditiontoitsknowncontributiontopromotingautoreactiveTcells,immunoinflammation,and remyelinationblockade.

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Glossary

Amyotrophiclateralsclerosis (ALS):aneurodegenerativedisease characterizedbylossofbothupper andlowermotorneurons.The symptomsusuallystartinone anatomicalregionandthenspread, causingmotorparalysis,dysphagia, andultimatelyaffectingrespiratory function.InsomecasesALScan alsoinvolvetheprefrontallobes, leadingtocognitivedysfunction.The vastmajorityofthecasesare sporadic,butin10–20%patients geneticabnormalitiesareassociated withfamilialformsofALS. Chronicinflammatory demyelinatingpolyneuropathy (CIDP):aperipheralnervoussystem diseaseandthecommonestchronic immune-mediatedperipheral neuropathythattakeseithera relapsingorprogressivecourse. Clinicallyitmanifestsbythe developmentofweaknessand sensorydisturbancethatleadto markeddisability.Multifocal inflammationandstrippingofmyelin sheathsbymacrophagesare thoughttoresultfromaberrant immuneresponses,mediatedbyT and/orBlymphocytes,against peripheralnerveantigens. Humanendogenousretroviruses (HERVs):sequencesbelongingto humanERVfamiliesretaining structuralfeaturesofretroviral genomesthathavebecome integratedintothegenomethrough repeatedinfectionsduringevolution. Immunetolerance:

unresponsivenessoftheimmune systemtosubstancesthatnormally elicitanimmuneresponse. Majorhistocompatibilitycomplex (MHC):cell-surfaceproteinscrucial fortherecognitionofforeign moleculesbytheimmunesystem. Theybindtoantigensderivedfrom pathogensanddisplaythemfor recognitionbyappropriateTcells. Mobilegeneticelements:these representnearly50%ofthehuman genome,andconsistof

retrotransposonsandDNA transposons.

Myelin:amajorcomponentofwhite matterthatconsistsofalipid-rich structureorganizedintomultilayered sheathsaroundaxons.Itprovides axonalintegrity,trophicand metabolicsupport,andaccelerated reductionofanti-Envantibodyreactivityfor HERV-HandHERV-W[23]andMS-associated

retrovirus(MSRV)load[24]inthebloodhavebeeninseeninMSpatientstreatedwithIFN-b, suggestingefficacyofthetherapyorlowdiseaseactivity.

Afteraninitialstudy,whichfirstpresentedevidencethatherpesvirusesmighttriggerHERVsin patientswithMSaspartoftheretrovirushypothesisforMSetiology[25],severalteamsshowed that transcription of HERV genes and/or reverse transcriptase(RT) activity is increased in varioushumancellsinvitrobyHerpesviridaewithtropismfornervouscell.Thisincludesherpes simplexvirustype1(HSV-1)inlymphocytesfromMSpatients[26]andinneuronalorinbrain endothelialcelllines[27];varicella-zostervirus(VZV)inlymphocytesfrom MSpatients[26]; cytomegalovirus(CMV)inkidneytransplantrecipients[28];humanherpesvirustype6(HHV-6) inlymphocytesfromMSpatients[26]andinTcellleukemiccelllines[29];andEpstein–Barr

DNA (exons) encoding physiological funcƟons 3% Other non-coding 48% Non-LTR retrotransponsons 35% Other repeats 3% Endogenous retroviruses 8% DNA transponsons 3%

Long terminal repeats

Envelope Coding regions Matrix capsid nucleocapsid Enzymes: protease Reverse transcriptase integrase

gag pro pol env

5' LTR 3' LTR

(A)

(B)

Figure1.RelativeProportionsof Dif-ferent Types of DNA Sequences within the Human Genome, and the Prototypic DNA Sequence of CompleteHumanEndogenous Ret-rovirus (HERV) Genomes. (A) DNA sequences representing remnants of mobile genetic machinery represent nearly50%ofthehumangenome.These arepredominantlyretrotransposons,that use RNA intermediates and a ‘copy-paste’ mechanism for retrointegration into chromosomes,and DNA transpo-sonsthatusea‘cutandpaste’ mechan-ism;theserepresent42%and3%ofthe humangenome,respectively[3].Among retrotransposons,animportantgroupis represented bythe endogenous retro-viruses(ERVs;HERVsforhumanERVs) that entered the genome of species throughinfectionsofgermlinecellsduring evolution leadingto subsequent endo-genization–integrationintothegenome andpresenceintheDNAofeverycellof theoffspring[5].About8%ofthehuman genome is constituted by HERV sequences; complete sequencing has revealed that intact codingsequences forfunctional HERV proteins represent <3%,ifnot1%[101,102].Theremaining sequences comprisenon-codingDNA, oftenwithinintronsof‘classical’genes, withapotentialrole ingenerating non-coding RNAs [103]. (B) Endogenous HERVDNA sequences are often trun-cated,andcontainmutations,insertions, ordeletions,butcompletecopiesarealso present.Thegag(group-specificantigen), pol (polymerase), and env (envelope) genes encode structural proteins and areflanked bytwoinvertedrepeatsof non-coding regions comprising many regulatoryfunctions(promoter,enhancer, primer-bindingsiteforreverse transcrip-tion,andothers).Abbreviation:LTR,long terminalrepeat.

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virus(EBV)inTcelllines[30]andinperipheralbloodmononuclearcellsfromMSpatientsaswell asinastrocytecelllines[31].ManyoftheseviruseshavebeenimplicatedinMS(reviewedin

[32]). Herpesviridae may invade brain parenchyma and induce local proinflammatory responses, but these pathogens are normally intercepted by perivascular macrophages (Figure3).Macrophagesarenotpermissivefortheirreplicationbutmayallowexpressionof HERV-transactivatingHerpesviridaeimmediate-earlygenes[33].TheEBVgp350proteinhas beenshowntoactivateHERV-WinvitroinBcellsandmonocytes,butnotinTcells,whereas monocyte/macrophagecellsappeartobemostsusceptible[31].BecauseEBVhasalsobeen shownto potentlymodifyepigenetic traitsofhostcellDNA (reviewedin[34]),thereported associationwithinfectiousmononucleosis,inadditiontotheelevatedanti-EBVnuclear antigen-1IgGtitersinpatientswithMS[35],mightsupportthehypothesisthatEBVactsasapriming trigger.However,this hasnotbeendirectlydemonstrated. Nevertheless,Herpesviridae(or otherenvironmentalactivatingfactors)arenowsuggestedtoupregulateHERV-Wexpression, withitsEnvproteinactingasapathogeniceffectorinMS[36].

Othervirusesreportedto transactivateHERVsaretheexogenousretrovirusesHTLV-1and HIV-1.Specifically,theHTLV-1Taxtransactivatorpotentlyincreasedthetranscriptionalactivity ofHERV-W,HERV-H,HERV-K,andHERV-EfamiliesinTcells[37].Moreover,effectsofHIVon HERV-KandonHERV-Winastrocytecelllinesandperipheralbloodcellsinvitrohavebeen reportedtobemediatedbytheHIVTatprotein,whichcanindirectlyactivateHERV-Wthrough Toll-likereceptor-4(TLR4)alongwithTNF-aandNF-kB.Thus,bythispathway,HIVTatcould influence nonHIV-infectedcells[22].ThisindirectmechanismsuggeststhattheHIV-driven activationpathwayrequirespersistingTat stimulationforHERV activation.Thisdiffersfrom Herpesviridae in that self-sustained HERV expression may be induced following specific triggeringevents,whichcouldexplainthelifelongprogressionofMSandthemultipletriggering eventsthatarerequiredbeforeapathogenicthresholdleadingtodiseaseonsetcanbepassed. Nonetheless,ifHERVproteinsarenotexpressed,HERVRNAexpression(transcriptionalone) doesnotseemtohavebiologicaleffectsperseinhumans.Moreover,whenproduced,HERV proteinsare not implicitly pathogenic. Anexample is provided bythe gag(group-specific antigen)-encoded capsid protein of HERV-W which had no immunopathogenic effect on peripheral blood lymphocyte cultures from healthy donors, whereas the Env protein of HERV-Wparticles (previouslytermed MSRV) induced proinflammatory and superantigen (SAg)-likeeffects[38].

ImmuneCellsCanMediateMajorEffectsofPathogenicHERVExpression

Abnormalactivationofsome HERVsisthoughtto haveproinflammatoryeffects, leadingto dysregulationoftheimmunesystem,aswewillnowillustratewithrelevantexamples. HERV-KandHERV-Wfamiliesshareaninterestingcommonfeatureinthattheproteinencoded bytheirenv genehasbeenshown totriggerresponsesinTlymphocytecellsexpressinga specific variable region of the T cell receptor (TCR) b chain in vitro [30,38]. Usually, T lymphocytes recognize their target antigen through a combination of variable domains in TCRchainsthat define theantigen-bindingsiteofspecificT cellclones.Theinteractionof theseHERVEnvproteinswithanotherTCRregion,whichisknowntobeindependentofthe antigen-bindingsiteandpresentonnumerousTcells,isknownto activatemultipleclones irrespectiveoftheirantigenspecificity.MoleculesinducingsuchpolyclonalactivationareSAgs. AninflammatoryloopalsolikelycontributestoHERVpathogenicity:followinginitialprimingand inductionofspecificHERVcopies,macrophagesand/orBcellsproduceHERVEnvproteins thatmightfuellocalinnateinflammationandupregulatemajorhistocompatibilitycomplex

nerveconduction.Itsstructural integrityisofvitalimportancefor CNSfunction.

Neuroregeneration:thereare substantialdifferencesintheextent andcellularoriginof

neuroregenerativeresponses betweentheCNSandtheperipheral nervoussystem(PNS).Whereas injuredaxonsandlostmyelinsheaths canberestoredininjuredperipheral nerves,repairprocessesintheCNS arescarceandarelimitedtopartial remyelination.DemyelinatedCNS axonsarefunctionallyimpaired, highlyvulnerable,anddegenerate overtime.Incontrasttoperipheral neurons,suchinjuredcentral neuronslackthecapacitytoregrow axons,andthisfurthercontributesto irreversiblefunctionaldeficits. Nonetheless,inchronicPNS diseasessuchasCIDPtheextentof repairandfunctionalrecoveryisalso largelyimpaired.

Oligoclonalbands(OCBs):bands ofspecificimmunoglobulinsthatcan bedetectedinpatientbloodserum orcerebrospinalfluid.Theyhave diagnosticvalueinMS.

Superantigens(SAgs):molecules inducingantigen-independentand hencepolyclonalactivationof lymphocytes.

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(MHC)expression,causingfurtherpolyclonalactivationoftissue-attractedTcellsandtherefore ofBcells.Thispotentialinflammatoryloopcouldresultinthedevelopmentofdevastatinglocal lesions,whileantigen-presentingcellsstimulatedviaTLRandpolyclonalactivationof lympho-cytesmightpromotebreaksinimmunetolerancethatleadtoautoimmunity,aswasobserved inamousemodelofexperimentalautoimmuneencephalomyelitis(EAE)treatedwithHERV-W Envprotein[39].

HERV-WandMS

MSisaninflammatorydisease ofthe centralnervoussystem(CNS)and amajor causeof neurological impairment in young adults. There is no available cure for MS, and current therapies can only limit the number of relapses and slow disease progression. Themost commonsymptomsincludechronicfatigue,paresthesiawithacuteandchronicpain,optic neuritis, paresis, gait disturbance, incoordination, sphincter problems, and cognitive impairment,altogetherleadingtoprogressivedisability.Histopathologically,MSis character-izedbydemyelinatinglesionsthatpredominantlyexpandinthewhitematter,causing destruc-tionofmyelinandoligodendrocytesandleadingtoaxonaldisruptioninthebrainandspinal

Coiled DNA

LTR LTR

gag pol env

Embry o Retrovirus Viral RNA Viral DNA Reverse transcriptase Nucleus mRNA Genomic RNA Provirus IntegraƟon CircularizaƟon Chromosomes: an integrated provirus Retroviral infecƟon of germ cells Example of an ovocyte Pregnant female

All cells of embryo contain the provirus

New generaƟons

All cells contain the now endogenized provirus (ERV). ERV can retrotranspose and amplify

their copy number

Variable ERV copy number and mutaƟons throughout next generaƟons, including unfixed (and non-ubiquitous) copies

MulƟple new infecƟons and new inserƟons Plasma membrane RNA

Figure2.SuccessiveStepsLeadingtoEndogenizationofRetrovirusesandtoMulticopyEndogenousRetroviral(ERV)FamiliesinDescendants.This illustrationdepictsthesuccessivestepsofretroviralendogenization,startingfrominfectionofgametes,integrationofaDNAretroviralcopy(provirus)intoa chromosome,givingbirthtoaviableindividualinheritingandretainingthiscopyintheDNAofallcellsandtransmittingthistoitsoffspring.Throughoutsuccessive generationsandevolution,bothendogenousretrotranspositionsandreinfectionsofthegermlineofparticularindividuals(providedthattheexogenousstrainpersistsin theenvironment)generatesmultipleandvariablecopynumbersinthefinalpopulation.Thisvariabilityhasbeenwellknowninsomeanimalspeciesandhasrecentlyalso beenevidencedinhumans[91].Abbreviations:env,envelope;gag,group-specificantigen;LTR,longterminalrepeat;pol,polymerase.

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Microglial cell Ac va on of HERV-W

Herpesvirus Resident perivascular

macrophage/microglial cell Cytokines T cell recruitment α β T cell APC The virus enters the

CNS from the bloodstream T T T T T T TNF-α Capture of viruses Blood vessel

Secre on of Env protein

I-CAM T T T T T T T T T T T T T T T T T Endothelial cell Astroglial endfeet

APC An gen-presen ng cell OPC Oligodendroglial precursor cell OL Oligodendrocyte TLR4 receptor HERV-W Env αβ T cell receptor (αβ-TCR) MHC class II TNF-α I-CAM

Myelin pep de/fragment

TNF-α I-CAM T T T T T Polyclonal ac va on superan gen (SAg)

A T cell is a racted from the bloodstream into the CNS: this process is promoted by the overexpression of ICAM-1 on endothelial cells, which is induced by Env protein. A er arriving in the CNS, local innate inflamma on and upregula on of MHC class II an gens in parallel with Env produc on on microglial cells promotes superan gen-like (SAg) ac va on of T cells by Env. This causes extensive inflamma on and IFN-γ release, with local edema. The SAg effect occurs with T cells trapped within this Env-primed CNS area. This allows uncontrolled autoreac ve cell expansion in reac on to abundant myelin an gen presenta on with MHC-II by APCs.

Circula ng T cells that are recruited by the cytokine/chemokine gradient released from the local innate immune inflamma on ini ated by (i) the viral trigger entry and (ii) the transac va on and produc on of HERV-W Env, which ac vates the TLR4 pathway in microglial and endothelial cells.

(A)

(B)

(C)

(D)

Figure3.HypothesizedHumanEndogenousRetrovirus(HERV)-MediatedActivationCascadesLeadingtothePathogenesisofMultipleSclerosis (MS).ThisillustrationdepictsahypotheticalpathogenicpathwayforMSinvolvingHERV-Wactivationandenvelope(Env)proteinexpressiontriggeredbyan environmentalfactor.Thiscausesdysregulatedinflammatoryandimmuneresponsesaccompaniedbyactivationofmicroglialcells,whichtherebyalsoexpress pathogenicHERV-WEnvprotein(formerlycalledMSRV-Env).TheproposeddirectcytotoxiceffectonendothelialcellsthroughTLR4activationcouldcontributetoTcell homingthroughupregulatedICAM-1expressioninresponsetocytokinesandchemokinessecretedfromtheinitialsiteofHERV-Wexpression.

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cord.Lesionsspreadtocorticalregions,affectinggreymatterandneurons.Activelesionsalso showblood–brainbarrier(BBB)breakdown,withinfiltrationofmacrophagesandlymphocytes, whereasactivatedmicrogliarepresentthehallmarkofregionswithinactiveandchroniclesions, where ongoing demyelination and axonal loss are now understood to cause functional impairment(reviewedin[40]).

Immunological dysfunctions in MS are characterized by multifocal CNS hyperinflammatory reactions,andsystemicautoimmunereactivityofBand/orTcellstowardsmyelinautoantigens, withevidenceofintrathecalchronicIgGproductionasrevealedbythepresenceofoligoclonal bandsinthecerebrospinalfluid(CSF)[41].Whereasinitialdiseasestagesmaypresentreversible phaseswithremissionfollowingrelapse,evolutiontowardsprogressiveformsgenerallyfollows

[42].Therelapsingformsaredominatedbyaberrantinflammatoryresponses,whereasin pro-gressivestagesneurodegenerativefeaturestakeprecedence.Thismaybeparalleledbyrepeated abnormallymphocytestimulationthatisknowntoleadtoTcellexhaustion,anergy,ordepletion

[43],andwhichimplicitlydownregulatesTcell-drivenactivationofBcells[44].

TheunderlyingetiologyofMSisstillnotfullyunderstood,butmultipledisease-associatedloci confergeneticpredispositiontodevelopMS[45],andnumerousenvironmentalfactors(e.g., infectiousmononucleosisand smoking)[46,47] appearto contribute todisease onset and progression.Thisledtotheformulationofapathogenicconceptbasedongene–environment interactions[48]withapartial,butelusive,roleofviralinfections[49].

In the search for etiological factors inMS, HERVs have been detected within the human genome,thusopeninganewavenueofresearchbecauseoftheirpotentialinteractionswith environmentalfactors.Diversescientificandtechnicalapproacheshaveindicatedthatthree humanendogenousretroviruses,HERV-H,HERV-K,andHERV-W,maybeabnormally rep-resentedorexpressedinMS.

Ageneticpolymorphisminasinglecopy ofHERV-Fc1(aHERV-H-relatedelement)andits relativedistributioninMSpatients,withtheexceptionofprimaryprogressiveforms(PPMS), wasreported[50,51].HERV-KmRNAexpressionwasfoundtobeupregulatedinpostmortem brain tissuefrom MS patients [52], but no evidence of protein expression was provided. Retroviral sequences from RT-PCR with selected primers on particles produced by MS derivedEBV-Blymphoblastoidcelllinesidentifiedsequencevariantshomologousto HERV-Helements(formerlynamedRTVL-HorRGH)[53].AnotherstudyshowedthatBcells and monocytesfrompatientswithactiveMSshoweddetectablesurfaceexpressionofboth HERV-HEnvandHERV-WEnv[54].Takentogether,thisevidencesuggeststhatactivationofmultiple HERVfamiliesmightbelinkedtoMS[55].

ThemostcompellingevidenceforanassociationbetweenHERVexpressionandMSisfor HERV-Wandcomesfromarecentmeta-analysis[56].Thislineofresearchwaspromptedby theisolationofretroviralparticles fromMSpatientsintheearly1990s [57,58].Subsequent studies over the following 25 years revealed that these particles originated from HERV elements,firsttermedMS-associatedretrovirus(MSRV),whosesequencesweredetermined frompurifiedretrovirus-likeparticlesisolatedfromMScellculturesupernatants[59,60].The initial sequence identified from several MS isolates was obtained using a PCR protocol designed to detect unknownretroviral sequences flankedby conserved domains inmost retroviralpol(polymerase)genes[61].ThisprototypeMSRVsequenceidentifiedapreviously

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primerfor reversetranscription, andthat comprisesmultiplecopies homologousto MSRV prototypicsequences[59,62].

Independentgroupssubsequentlyreportedthat HERV-Wenvandpolexpressioncouldbe detectedinserum,peripheralbloodmononuclearcells(PBMCs),andCSFfromMSpatients butnotfromhealthycontrols[60,63–66].HERV-WassociationwithMSwasfurtherevidenced bystudiesshowingthatexpressionlevelsnotonlycorrelatedwithMSbutalsoincreasedwith disease activity and progression [24,64,65,67]. Further investigations indicatedthat HERV proteinexpressionismainlyrestrictedtomacrophagesandmicroglia,aminorproportionof lymphocytes,andtoafewendothelialandastrocytecellsinactivelesionareas[31,63,68,69]. HERV-W Env protein was similarly detected in MS lesions by using differentmonoclonal antibodies(mAbs)directedagainstdifferentepitopes[64].Itsassociationwithareasofactive demyelinationfromactivetochronicbrainlesions,withfairlyintenseexpressionuntilthedeath ofthepatient,asseenpost-mortem,suggestsaninvolvementinthelong-termpathogenic progressionofthedisease[69].

HERV-Wwasfurtherrevealedtoplayfunctionalrolesininflammatoryprocesses.Proin flam-matorycytokineexpressionwasshowntobeinducedinbothhumanandmurinemonocytes uponinvitrostimulationwithHERV-WrecombinantEnvprotein,aprocessthatrequiredTLR4 receptoractivation[39,70,71],whileMSRVEnv-treatedhumandendriticcellswereelicitedto promotetype1Thelpercell(Th1)-likelymphocytedifferentiation[71].MSRVparticles (HERV-W)werepreviouslyshowntoinducesuperantigen-likeTcellresponsesthatwerereproduced bytheEnvproteinbutnotbythegag-encodedcapsidprotein[38].Ahypotheticalscenario illustratingalltheseeffectsispresentedinFigure3.HERV-WenvelopealsopromotedEAEin mouseandinducedelevatedautoimmuneTcellreactivity[39].

Inadditiontopathogeniceffectstargetingimmunefunctions,HERV-WEnvproteinwasalso foundtomediateTLR4-dependenteffectsonnon-immunecells.Treatmentofculturedprimary oligodendroglialprecursorcells(OPCs)withHERV-WEnvwasshowntoresultinanoverall reduction ofoligodendroglialdifferentiation viaactivationof TLR4 [68].OPCs contribute to neuroregenerationandmyelinrepairprocessesintheadultCNS,andblockingtheir differ-entiationbyHERV-WEnvproteinmaythereforeresultinremyelinationfailure(Figure4).The negativeimpactonmyelinsynthesisinprimaryoligodendroglialcellscouldberescuedusinga specific Env-neutralizing humanized immunoglobulin termed GNbAC1 [72]. Different mAb versionsofGNbAC1havebeenassessedonasmallscaleinMSRVEnv-inducedexperimental allergicencephalitis(EAE),ananimalmodelofMS,andtheseappearedtoinhibitandsomehow reverseEAEclinicalevolution[73].Inaddition,exposureoftheHCMEC/D3brainendothelialcell lineto HERV-W Env was shown to impairendothelial cell physiologyby boosting ICAM-1 expression(allowingTcellhomingintotissues)andproinflammatorycytokinereleaseviaTLR4 activation,therebysuggestingthatEnvmightaffectBBBintegrity(Figure3)[74].

Together,thesedataarguethatpathogenicHERV-WEnvprotein(formerlyMSRV-Env) isa potential therapeutictargetin MS, andthat the neutralizing humanizedantibody GNbAC1 thereforewarrantsdevelopment;indeed,severalearly-phasetrialshavenowbeencompleted

[73,75–77].APhaseIIbmulticenterclinicaltrialincluding260relapsing-remittingMS(RRMS) patientsin12EuropeancountriesisongoingwiththishumanizedIgG4antibody.Thistrialisa 1yearstudywithaplaceboarmandthreetreatedgroupsreceivingintravenousinfusionsevery 4weeks(6,12,and15mg/kg)(ClinicalTrials.govidentifier:NCT02782858)[78].

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Anotherapproach,basedonthehypothesisthatantiretroviraldrugsthatareeffectiveagainst exogenousHIVinfectionsmightalsoblockHERVexpressioninMS[79],hasledtoadedicated evaluation(ClinicalTrials.govidentifier:NCT01767701),butgavenegativeresultsforitsprimary endpoint (http://onlinelibrary.ectrims-congress.eu/ectrims/2016/32nd/146288/julian.gold. phase.2.baseline.versus.treatment.clinical.trial.of.the.hiv.drug.html?f=m3).Thistrialwasa base-line-versus-treatmentstudywith20patientswithactiveRRMSdefinedasgadolinium-enhancing lesionsonmagneticresonanceimaging(MRI)atbaseline.Theyweremonitoredfor3monthswith monthlyMRIandthentreatedwiththeintegraseinhibitorraltegravirfor3months.Thistrialdidnot reachitsprimaryendpointgoalofsignificantlyreducingeitherlesioncountorlesiondevelopment duringthetreatmentperiodversusbaseline.Nonetheless,thetesteddrugisaknownandeffective inhibitorofHIVintegrasewhichplaysaroleinchromosomalretrointegrationofnewlygenerated HIVDNAcopies.BecausethismaynotbeamajoraspectofHERVexpression,andisunlikelytobe akeyissueforHERVEnvproduction,themodeofactionofthisanti-HIVdrugmaynothavehad relevanteffectsonHERVs,thusexplainingthenegativeresultsofthestudy.

Finally,followingastudyinwhichHERV-WEnvproteinwasdetectedinseraandlymphoidcells ofMSpatients,butnotinhealthycontrolsnorinotherneurologicaldiseases,exceptforafew

Demyelinated axon OPC HERV-W Env Demyelinated and degeneraƟng axon Neuron

Demyelinated MS lesion

Remyelinated white maƩer lesion

Microglial cell (situated at the rim of lesion) OPC

Env protein at the surface of microglial cells OPC Oligodendroglial precursor cell

OL Oligodendrocyte TLR4 receptor HERV-W Env

Secreted Env protein Absence of HERV-W Env protein ProducƟon and secreƟon of HERV-W Env protein by microglia OL Myelin sheath Demyelinated and degeneraƟng axon Blocked myelinaƟon Normal myelinaƟon

Figure4.NormalDifferentiationofOligodendrocytePrecursorCells(OPCs)versusInhibitionofOligodendroglialMaturationbyHumanEndogenous RetrovirusHERV-WEnvelope(Env)Protein.ActivationofToll-likeReceptor4(TLR4)onOPCsbythepathogenicHERV-WEnvproteinblocksthedifferentiationof thesecells,withpotentialimpactonnaturallyoccurringmyelinrepair.OPCsaretargetedbypHERV-WEnveitherthroughcontactswithmicrogliathatpresent pHERV-WEnvproteinatthecellmembranesurface,orthroughinteractionwithsecretedorshedpHERV-WEnvproteinintheextracellularspace.

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CIDPcases[64],arecentstudyconfirmedupregulatedHERV-Wexpressioninbloodcellsand peripheralnervelesionsofpatientssufferingfromCIDP[80].Thisinvestigation alsoshowed TLR4-mediatedeffectsofHERV-WEnvproteinonprimaryhumanSchwanncells.Suchdirect effectsonSchwanncells,andtheabilityofHERV-WEnvproteintocausetissueinflammation andsystemicautoimmunity,makeitaninterestingpotentialnewtargetforthetreatmentof patientswithCIDPandHERV-Wupregulation.Thisevidencesuggeststhat HERV-Wisnot specificforMS,butmaybeinvolvedindifferentdiseasesthroughthepathogenicpropertiesof itsEnvproteinuponactivationbyvariablefactorsindifferentconditions,tissues,andorgans.

HERV-KandALS

ThefirstevidencethatretroviralelementsmightbeactivatedinALScamefromastudyinwhich braintissueextractsfromtwoALSpatientsinGuamwerefoundtohaveRNA-directedDNA polymeraseactivity.ThispolymeraseactivitywasRNase-sensitive[81],suggestingRTactivity. However,novirusortransmissibleagentwasidentified.

SubsequentstudiesinpatientswithALSconfirmedthepresenceofRTinserumand,although withlimitednumbers,showedthat nearly50%ofthe patientshave detectableRTactivity, whereasactivitywasonlydetectedinasmallernumberoffirst-degreerelativesandaneven smallernumberofunrelatedcontrols[82–84].However,severalattemptstofindanexogenous retrovirusinpatientswithALSwereunsuccessful[83,85].

In2011,thedetectionofRTencodedbythepolgeneofHERV-Kwasreportedinthebrainof patientswithsporadicALS[86];expressionwasspecificforALSbecauseaproteinresulting fromHERV-KpolactivationcouldnotbedetectedinthebrainsofpatientswithParkinson’s diseaseorinnormalbrains,despiteoccasionaldetectioninpatientswithcancers.Sequencing ofpolgeneRNAtranscriptssuggestedactivationofmultipleloci,withpredominanthomology toacopyinchromosome7[86].AsubsequentstudyshowedexpressionofHERV-Kpol,env, andgaggenesinthebrainsofpatientswithsporadicALS[87].Expressionlevelsofeachgene correlatedwitheachother,suggestingthatanentireHERV-Kgenomeisactivated.HERV-K Env protein expressionwas also detected incortical andspinal neurons of ALS patients, whereasnoimmunostainingwasfoundinbrainsfromhealthyindividualsorinpatientswith Alzheimer’sdisease[87].

HERV-Kvirusisso-namedbecauseituseslysine(K)tRNAasaprimer.HERV-Krepresents thousandsofinsertionsinthehumangenome,and11largelycompleteproviralsequences have been identified to date[88,89].The number of insertions can vary between human populations,andcopiesare notnecessarilyfoundatafixedchromosomalsiteinallcarrier individuals[90,91],whilenewinsertionsarethoughttohaveoccurredwithinthepast2million yearsaftertheappearanceofmodernhuman[89].SomeoftherecentlyendogenizedHERV-K elementshavecompleteORFsandcanformcompleteviralparticles[92].Theseareprobably recent insertionsinto thehuman genome becauseevolutionhas notyetintroduced major deletionsormutations,andepigeneticmechanismsarelikelytoberesponsibleforHERV-K genesilencing,whichmaybeovercomebyenvironmentaltriggers.Giventhisgenetic com-plexity,itremainsunknownwhethertheexpressionofHERV-KinpatientswithALSderives fromasinglecopywithacompleteretroviralsequenceormightrepresent trans-complemen-tationbetweenpartiallydefectivebutcomplementarycopies.Suchascenariomayalso be relevantfortheactivationofHERV-WinMSbecauseitcouldalsoresultintheexpressionof HERVpathogenicprotein(s).

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HERV-Kalsoplaysaroleinearlyhumanembryonicdevelopment.Itisexpressedinthemorula andblastocyststageofthepreimplantationembryo,butexpressionissilencedlaterinfetal development [93]. However, ifHERV-K expression is forcedin neurons,it causes cellular degenerationmediatedbyitsEnv protein[87].TransgenicmiceexpressingHERV-KEnv in neuronsdevelopedaclinicalandpathologicalphenotypethatresemblesALS,withupperand lowermotorneurondegeneration[87].

What triggers the expression of HERV-K in adult neurons of patients with ALS remains unknown.Invitrostudiesshowedthatneuronalinjuryduetooxidativestressorexcitotoxicity isinsufficienttocauseactivationofHERV-Kgenes.Nonetheless,endogenousexpressionis modulatedbytranscriptionfactorTDP-43,whichhasfivebindingsitesonHERV-KLTR[87]. TDP-43isknowntobedysregulatedinALSandhasbeenproposedasabiomarkerofALS lesions[94].

ThepossibilitythatHERV-KplaysacrucialroleinthepathophysiologyofALS,asrepresentedin

Figure5, is attractive for several reasons. It would explain why several researchers have detected RT in ALS brain and blood samples, but have not been able to demonstrate human-to-animalorhuman-to-humantransmissionofthedisease,becauseHERVsarisefrom thegenomeandnotfromtheenvironment.Further,itmayalsoexplaintheanatomicalspreadof theillnessthroughparacrineactivationofpermissiveautologouscells,whichgenerallystartsin oneregionofthebodyandthenspreadsalongananatomicalpathway.

Inaddition,thispotentialmechanismleadstonewtherapeuticperspectivesforthesepatients. BecauseHERV-K,althoughendogenous,retainsretroviralpropertiesthatunderlieits patho-genicexpression,an approachto drugdevelopmentsimilar to thattakenforHIV couldbe considered.ApanelofantiretroviraldrugsapprovedfortreatingHIVinfectionwasscreened, butelevatedconcentrationswerefoundtobenecessarytocontrolHERV-KreplicationinHeLa cellsinvitro[95].Apreviouspilotclinicaltrialwithindinavir,aproteaseinhibitorusedforHIV antiretroviraltherapy,failedtoshowanyefficacyinALS[96].Notably,somepatientswithHIV infectioncandevelopanALS-likesyndrome,whichmayshowsymptomregressionandhalted evolutionundertreatmentwithanti-HIVdrugs[97].Thesepatientsalsoshowedexpressionof HERV-Kinblood,thelevelsofwhichfellinallpatientsaftertheinitiationofantiretroviraldrugs

[98].AspreviouslydiscussedinthecontextofHERV-W,itispossiblethatHIVproteinssuchas Tatmay regulatethe expressionofHERV-K[99],andthiscouldexplain whythetriggering pathwaysandexpressionfeedbackloopsmaydifferinsporadicALSthatisnotassociatedwith HIV.Itremainspossiblethatanti-HIVdrugsmightindeedinhibitHERV-K,butthattheirefficacy isnotasgoodasforHIV[95].Tofurtherinvestigatethepossibilitythatantiretroviraltherapies mightbeofbenefitinALS,anopen-labelpilotstudyhasbeeninitiatedinAustraliaandtheUK. Thetrialisenrolling40patientswithALS,andwillfollowthemfor3monthswithouttreatment; thentreatthemfor6monthswithtriumeq–whichincludestworeversetranscriptaseinhibitors (abacavirandamivudine)that have beenshown toeffectively inhibitHERV-K reverse tran-scriptase activity invitro, and an integraseinhibitor dolutegravir(Clinicaltrials.gov identifier: NCT02868580).

ConclusionandPerspectives

ThepresentreviewdiscussesthelargelydisregardedaspectsofHERVmulticopyelements whichmaybedysregulatedbyepigeneticchanges,transactivatedbyenvironmentaltriggers, andcontributetothedevelopmentofneurologicaldiseasessuchasMS,ALS,andCIDP.Data accumulatedoverrecentdecadesproviderationalestoidentifyparticularHERVfamiliesand their toxic proteins as potential therapeutic targets in these diseases. This has already

OutstandingQuestions HERVs are evolutionarily acquired, mostly defective and inactive, and comprise epigenetically silenced genetic elements without assigned physiologicalfunctions.However,for whatreasonsaretheyaremaintained and represent 8% of the human genome?

HowareHERVdynamicsinhealthand diseaserelatedtothetranscriptional activityorproteinexpression, compo-sition,ormutationofancestralcopies, aswellas ofunfixednon-ubiquitous copies?

Howcanwebestneutralize endoge-nousproteinssuchasthoseencoded by HERVs? Potential approaches include vaccination, antibody-medi-atedneutralizationofpathogenic com-ponents, and antiretroviral compounds.

Whataretheappropriatewindowsof opportunityforanti-HERVtreatment? Canbiomarkersbeidentifiedin periph-eralbloodthatwouldpermit routine assessmentof HERVreactivation in CNSdisease?

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prompteddifferentgroupsto evaluatenoveltherapeuticapproachesinpatientswithMSor ALS.

OnepotentialstrategyreliesupontheanalogybetweenendogenousHERVsandtheirancestral exogenousretroviralorigins,andproposestotestantiretroviraldrugsthatareeffectivein HIV-infected,butnotinHTLV-infectedpatientswith,forexample,associatedmyelopathy. Anotherstrategy maybeto useahumanized neutralizingantibody(GNbAC1)targetingthe toxicEnvproteinthatmediatesHERV-Wpathogenicity(illustratedinFigure6).

Theperspectivesofferedbytheemergenceofthisnewcategoryof‘pathogensfromwithin’, given consistent findingsof their interplay withepidemiologically associated environmental

Chromosome Nucleus Nucleus Neuron AcƟvated Dormant Healthy ALS

ALS

Human Gametes Germline Transmission Exogenous retrovirus Ancestor

∼ 2–5 million years ago

HERV-K gene acƟvaƟon

HERV-K acƟvaƟon HERV-K envelope proteins

Paracrine neurotoxicity

Figure5.HypotheticalScenarioInvolvingHumanEndogenousRetrovirusHERV-KActivationLeadingtoAmyotrophicLateralSclerosis(ALS).This scenariodepictsthehypothesizedoriginofretroviralendogenizationsduringtheevolutionofspeciesmillionsofyearsago,andthetransmissionofHERVcopiesto humansinwhichHERV-Kcopiesmayremaindormant(latent)oractivatedbystillunknownmechanisms.ThisfigurerelatestothemostrecentHERV-Kinsertions(and nottopreviousHERV-K/HML-2insertionsat30millionyearsago),whicharethoughttobethemostactivecopiesbecausetheyarenormallylessmutatedthanearlier insertions.Whenactivatedinspecificareasofthecentralnervoussystem,theresultingpathogenicpathwayinvolvesHERV-Kexpressionandreleaseofitsenvelope (Env)proteinthatcausesneurotoxicityintargetedmotorneurons.

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factors[100]andtheirrelevantpathogeniceffects,areunlikelytobelimitedtotheexamples discussedinthepresentreview.Althoughmanyissuesremainunresolved(seeOutstanding Questions),thisscientificdomainrepresentsarapidlyevolvingareaofcutting-edgeresearch (Box2),andfurtherstudiesmayrevealthatHERVscontributetoothercomplexand multifac-torialhumandiseases.TherapeuticapproachestargetingtoxicHERVproteinsor,moredirectly, HERVgeneexpressionmaythuscreateachangeofparadigmwithcompletelynoveltreatment perspectives.Finally,butnotoftheleastinterest,treatmentstargetingnon-physiologicalHERV

MulƟple and various

environmental factors

triggering HERV

acƟvaƟon

EpigeneƟc dysregulaƟon/transacƟvaƟon

DNA: HERV-W-env

HERV-W-Env protein

expression

Endothelial cells

BBB dysfunc on

autoimmunity

Inflamma on

OPC differen a on

blockade

Impairment of

remyelina on

MulƟple pathogenic outcomes

in the brain and in TLR4

+

target cells

MS: immune cells/microglia/endothelial cells/OPCs

EBV

HHV-6

HSV-1, VZV, ...?

Single and upstream

pathogenic factor

Therapy targeƟng a common

pathway

Neutralizing

anƟbody

Figure6.ProposedGlobalModel:FromGene–EnvironmentInteractionstoaTargetedTherapyinMultipleSclerosis(MS).Theproposedmodel presentshow,afterbeingactivatedbyvariableandmultipleenvironmentalfactors,HERV-W(formerlynamedMSRV)expressesapathogenicenvelope(Env)proteinthat appearstorepresentanuniqueagonistinthepathway.(i)DownstreamHERV-WDNAexpressionand(ii)upstreampathogenicpathwaysthatareactivatedinmultiple directandindirecttargetcells.Anantibodyneutralizingthispathogenicproteinmightblockthepathogenicpathwayatthislevel,independentlyofmultipleenvironmental triggersandpathogenicdownstreamcascades,withoutinterferingwithhostphysiologicalfunctions.Abbreviations:EBV,Epstein–Barrvirus;HHV-6,humanherpes virus6;HSV-1,herpessimplexvirus1;OPC,oligodendroglialprecursorcell;VZV,varicella-zostervirus.

Box2.Clinician’sCorner

TheinvolvementofHERVexpressioninMSandALSprovidesashiftinparadigmnotonlyforunderstandingtheir complexpathogenesisbutalsoasforthedevelopmentoftherapeuticstrategiestopreventortreatsuchdiseases. AlthoughpositiveresultsinthefirsttherapeuticattemptstotargettheseHERVelementsortheirpathogenicproteins wouldcreateanimmediatebreakthrough,theemergingdomainofHERVsandhumandiseaseislikelytoremain cutting-edgescienceformanyyears.

Theperspectivesoffutureresearchinthisdomainatthefrontiersofscienceareexpectedtounraveletiopathogenic cascadesleadingtomanyofthechronic,complex,andmultifactorialdiseasesforwhichpresentknowledgeremains obscureorpartial,andwhereappropriatetherapeuticapproachesarenotyetavailable.

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componentswouldnotdysregulatephysiologicalfunctionsandthusshouldhave favorable safetyprofiles.

Acknowledgments

WeacknowledgePeterGöttleforartworkinFigures2,3,and4.ResearchonmyelinrepairandHERVsinthelaboratoryof P.K.issupportedbytheFrenchsocietiesARSEP(Fondationpourl’AideàlaRecherchesurlaScléroseenPlaques)and AFM(AssociationFrançaiseContrelesMyopathies).TheMSCenterattheDepartmentofNeurologyinDüsseldorf(P.K. andH-P.H.)isadditionallysupportedbytheWalterandIlseRoseFoundationandthePeek&CloppenburgDüsseldorf Foundation.ThestudiesofP.N.M.havebeensupportedinpartbytheAgenceNationaledelaRecherche (ANR-08-MNPS-041)andbytheFrenchpatientassociationARSEP.

DisclaimerStatement

P.K.hasperformedconsultancyworkforGeneuro.A.C.hasreceiveddepartmentalresearchgrantsfromBiogen-Idec, CSL-Behring,Geneuro,Novartis,andOctapharma,andhasprovidedexperttestimonytoNovartis,CSL-Behring,and Genzyme.P.N.M.isinventoronarelevantpatent.G.G.hasreceivedhonorariafromAbbVie,BayerHealthCare,Biogen, Canbex,FivePrime,Genzyme,GlaxoSmithKline,GWPharma,MerckSerono,Novartis,ProteinDiscoveryLaboratories, Roche,Synthon,TevaNeuroscience,UCB,andVertex;researchgrantsupportfromBiogen,Ironwood,MerckSerono, Merz,andNovartis;andcompensationfromElsevierasco-chiefeditorofMultipleSclerosisandRelatedDisorders.J.G. declaresnoconflictsofinterest.H.P.H.hasreceivedfeesforconsulting,speaking,andservingonsteeringcommittees fromBayerHealthcare,Biogen,Geneuro,MedImmune,Merck,Novartis,Opexa,ReceptosCelgene,Roche,Sanofi Genzyme,andTeva. H.P.receivescompensationforhisworkbyGeneuroandisinventoronpatentsownedby BioMérieux,INSERM,orGeneuro,buthastransferredallhisrightstoBioMérieuxortoGeneurounderapplicablelaws foremployedinventors.

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Figure

Figure 1. Relative Proportions of Dif- Dif-ferent Types of DNA Sequences within the Human Genome, and the Prototypic DNA Sequence of Complete Human Endogenous  Ret-rovirus (HERV) Genomes
Figure 2. Successive Steps Leading to Endogenization of Retroviruses and to Multicopy Endogenous Retroviral (ERV) Families in Descendants
Figure 3. Hypothesized Human Endogenous Retrovirus (HERV)-Mediated Activation Cascades Leading to the Pathogenesis of Multiple Sclerosis (MS)
Figure 4. Normal Differentiation of Oligodendrocyte Precursor Cells (OPCs) versus Inhibition of Oligodendroglial Maturation by Human Endogenous Retrovirus HERV-W Envelope (Env) Protein
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