The Companion Dog as a Unique Translational Model for Aging

ContentslistsavailableatScienceDirect

Seminars

in

Cell

&

Developmental

Biology

jo u r n al hom ep a g e :w w w . e l s e v i e r . c o m / l o c a t e / s e m c d b

Review

The

companion

dog

as

a

unique

translational

model

for

aging

Andrea

Mazzatenta

_,

_Augusto

_Carluccio

_,

_Domenico

_Robbe

_,

_Camillo

_Di

_Giulio

_,

Alessandro

Cellerino

a_{FacoltàdiMedicinaVeterinaria,UniversitàdegliStudidiTeramo,Teramo,Italy}

b_{SezionediFisiologiaeFisiopatologia,DipartimentodiNeuroscienze,ImagingeScienzeCliniche,‘G.d’Annunzio’UniversitàdiChieti–Pescara,Chieti,Italy} c_{ScuolaNormaleSuperiore,Pisa,Italy}

a

r

t

i

c

l

e

i

n

f

o

Articlehistory: Received13April2017

Receivedinrevisedform6August2017 Accepted7August2017

Availableonline11August2017

Keywords: Agingdog Dogpathology Doggenetics Dogbreeding Doginterventions Evolutionarytheoryofaging Calorierestriction IGF-1

Rapamycin Alzheimer’sdisease

a

b

s

t

r

a

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t

Thedogisauniquespeciesduetoitswidevariationamongbreedsintermsofsize,morphology,behaviour andlifespan,coupledwithageneticstructurethatfacilitatesthedissectionofthegeneticarchitecture thatcontrolsthesetraits.Dogsandhumansco-evolvedandsharerecentevolutionaryselectionprocesses, suchasadaptationtodigeststarch-richdiets.Manydiseasesofthedoghaveahumancounterpart,and notablyAlzheimer’sdisease,whichisotherwisedifficulttomodelinotherorganisms.Unlikelaboratory animals,companiondogssharethehumanenvironmentandlifestyle,areexposedtothesamepollutants, andarefacedwithpathogensandinfections.Dogsrepresentedaveryusefulmodeltounderstandthe relationshipbetweensize,insulin-likegrowthfactor-1geneticvariationandlifespan,andhavebeen usedtotesttheeffectsofdietaryrestrictionandimmunotherapyforAlzheimer’sdisease.Veryrecently, rapamycinwastestedincompaniondogsoutsidethelaboratory,andthisapproachwherecitizensare involvedinresearchaimedatthebenefitofdogwelfaremightbecomeagamechangeringeroscience.

©2017ElsevierLtd.Allrightsreserved.

Contents

1. Introduction...142

2. Thecomparativeapproachtoaging...142

3. Animalmodelsofagingandtheplaceofdogs...142

4. Naturalhistoryofcanids...143

5. Naturalhistoryofthedomesticdog ... 143

6. Artificialselectionandthedogasageneticmodelofdiseases...144

7. Dogdiseasesandgeriatrics...145

7.1. Cardiacdisease...145

7.2. Dogrespiratorydisease...145

7.3. Dogobesityandendocrinediseases...145

7.4. Dogimmuno-senescence...145

7.5. Dogcancers...145

7.6. Dogkidneydisease ... 146

7.7. Dogneutering...146

7.8. Dogneurodegenerativediseases...146

7.9. Dogbehaviouralaging...147

7.10. Mappingdiseasesinthedoggenome...147

8. Effectsofsizeondoglongevity ... 147

∗ Correspondingauthorat:AndreaMazzatentaFacoltàdiMedicinaVeterinaria UniversitàdegliStudidiTeramoPianod’Accio,Teramo,Italy.

E-mailaddress:amazzatenta@yahoo.com(A.Mazzatenta).

http://dx.doi.org/10.1016/j.semcdb.2017.08.024

9. Interventionsindogs...148 10. Conclusions ... 149 References...149

1. Introduction

Agingispartof acontinuous ontogeneticprocessfrom con-ceptiontodeaththatincludesgrowthandmaturation.Aginghas evolvedastheconsequence ofprimary selection forother life-historytraits,andrepresentsaprimeexampleofhowevolutionary changesarenotdirectedtowardswhatappearsanobvious individ-ualgoal;i.e.thepreservationoftheindividualitself[1].

Anumberofcommoncellular/molecularhallmarksofagingin mammalswereidentifiedrecently:epigeneticalterations,genomic instability,stem-cellexhaustion,telomereattrition,mitochondrial dysfunction,cellularsenescence,lossofproteostasis,andaltered intercellularcommunication[2].Inaddition,anumberoftheories havebeenproposedtoexplaintheevolutionofaging(seeReichard, thisissue,foranupdatedreview)andthereiscontroversyoverits underlyingmechanismsandtherespectiverolesofstochastic accu-mulationofmoleculardamage,asopposedtoquasi-programmed systemhyperfunction.Thesequestionsarerelevanttounderstand therelationshipsbetweenagingandincreaseddiseaserisk[3,4], andtounderstandthebiologicalmechanismslinkedto success-fulaginginaselectedpopulationofextremephenotypes,suchas centenarians[5].

Thediversityofindividuallifetrajectoriesgivesrisetoanother importantaspectexemplifiedbytheagingofthehumanolfactory system.Thisisnotacontinuousmechanism,butshowsmultiple discretephenotypes(termedjuvenile,mature,elder)wheretheir frequencieschangeinthedifferentagegroups.However,the‘elder’ olfactoryphenotypecanbedetectedinasubsetofyoungsubjects, whichindicatesearlyimpairmentofthesystemlongbeforeonsetof degenerativeprocesses,thusdemonstratingearlyfunctional senes-cencethatmighthaveanimpactonagingdecadeslater[6].

Aging,astheage-dependentincreaseinmortality(often expo-nential), is caused by such senescence processes [7,8]. In this meaning,senescenceisrelatedtothedeteriorationofphysiological functions(whichisoftenlinear)andcandevelopatdifferentpaces relativetothemaximumlifespan[8].Individualsexposedtorisk factors,suchasdrugconsumption,canacceleratethesenescence oftheirphysiologicalsystem(e.g.,theirchemoreceptionsystem), whichconsequentlyaffectsaging[9].Thus,agingrepresentsthe biologicalmechanism,whilesenescencerepresentsitssubjective physiologicalexpression.

Theaimofthepresentreviewistopresentthepotentialofthe dogasamodelorganismforagingstudies.

2. Thecomparativeapproachtoaging

Within metazoans, there is at least a 10,000-fold range in lifespans,fromthelongest-livedspecies,suchasaninvertebrate spongeintheorderofmillennia(maximumreported,15,000years), throughtheoceanquahogintheorderofcenturies(507years,see Blierinthisissue)andtheGreenlandshark(390years),and fur-therreducedforothersealife,suchasthebowheadwhale(211 years),rockfish (205years) and seaurchin (200 years)[10,11], (seealsoDammann,thisissue).Lifespanshowsastrongpositive correlationwith body size and insight into thegenetic control ofagingcanbegainedfromcomparativestudiesacrossspecies wherethelongevityquotientisanalysed(i.e.,theresidualsofthe regression lifespanvs. body size)[12]. Theavailability of com-prehensivedatabasesmakesitpossibletoprobetherelationships betweenagingand a variety ofother life-historytraits[13,14].

Thesedatabasesandtheiranalysisbecomeextremelyrelevantin lightoftheincreasingavailabilityofhigh-throughputtechnologies thatallowgenome-wideanalysisofgeneexpressionandpositive selectioninalargenumberofdiversespecies[e.g.15–20].

3. Animalmodelsofagingandtheplaceofdogs

Giventhelargediversificationofanimallifespans,therecan benosinglemodel species foraging. For practicalreasons,the vastmajorityofstudiesonaginganimalsareperformedin nema-todes(seePires,inthisissue).Invertebratemodels,however,have someintrinsic biological limitations:theiranatomical organisa-tionisfundamentallydifferentfromthatofmammals;theylack adaptiveimmunityandbones;theyhavelimitedstemcell pop-ulations;andtheylacksomegenesthat arehighlyrelevant for humanaging.Amongthese,forexample,therearetheApolipo pro-teinE(APOE)geneandtheINK4locus(codingforthecyclinkinase inhibitorsCDKN2AandCDKN2B)thatingenome-widestudiesare associatedwithhighreproducibilitytoexceptionallongevityand age-relateddiseases[21,22].Inadditiontotheseverygeneral con-siderations,whenthefocusofananalysisisonage-relateddiseases andinatranslationalperspective,thesizeandthephysiological andbehaviouraltraitsofthedifferentspeciesbecomehighly rele-vant.Primatesrepresentanidealmodelforhuman,althoughaging studiesin primatesareexorbitantly and generallyprohibitively expensiveandlastfordecades[23].

Thepresent-day companiondog representsaunique animal modelthatmightprovideaparadigmshiftfromlaboratoryresearch to‘citizenscience’.Thebiologyofthedomesticdoghasanumber ofuniqueaspectsthatareofrelevanceforagingstudies:

(i)Duetoselectivebreeding,therearealmost20-foldvariationsin bodysizeandovertwo-folddifferencesinagingrates[24,25]. (ii)Therearemanystrain-specificgeneticdiseasesthatbecame fixed,whilethedoggenomehasbeensequenced[26,27]and thehighlevelofinbreedingfacilitatesthemappingofcomplex traits(e.g.,[28]).

(iii)Thereisvastknowledgeofdogdiseasesandmanynon-invasive proceduresareavailabletoassesshealthandfunction, accom-paniedbythevastdevelopmentofdrugs.

(iv)Thedogshowsspecificadaptationsforcommunicationwith humansandrepresentsauniquemodelforhuman-likesocial skills[29,30],andthegeneticbasisofthisbehaviourisstarting tobedefined[31].

(v)Thedogandhumanco-evolvedandsharerecentevolutionary selectionprocesses,suchasadaptationstodigestionof starch-richdiets,andthereareclearsignsofconvergentevolutionin thehumananddoggenome[32,33].

(vi)Unlikelaboratoryanimals,companiondogssharethehuman environmentandlifestyle,areexposedtothesamepollutants, andarefacedwithpathogensandinfections[34].

Mostimportantly,therearemanymillionsofcompaniondogs intheworld,with80millionalone intheUSA(estimateofthe AmericanVeterinaryMedicineAssociationfor2012).The increas-ingqualityofveterinarycareleadstoanincreaseintheelderlydog populationthatwillshowincreasingsenescencemechanismand consequentprevalenceofage-associateddiseasesthatneed treat-ment.Thiscanleadtoacompleteparadigmshiftintranslational research,whereinsteadofinducingdiseasesinlaboratoryanimals,

dogpatientsinneedoftreatmentcanberecruitedandtheengaged citizenscanbecomeinvolvedinresearchthataimsatimproving thewelfareofthecompaniondog,andinsodoing,providesvery importantinputforthedevelopmentofhumantherapies.

4. Naturalhistoryofcanids

Beforefocussingondogs,anoverviewofthe‘wilddogs’family, theCanidaetaxon,is necessary[35,36].Threedistinct phyloge-neticgroupscanbeidentifiedbymolecularanalysis:(i)thefox-like canids;(ii)thewolf-likecanids;and(iii)theSouthAmericancanids thatembracegenusesPseudolopex,Lycolopex,Atelocynus, Chryso-cyonandSpeothos.Additionally,thegreyfox(genus,Urocyon),the bat-earedfox(genus,Otocyon),andtheraccoondog(genus, Nyc-tereutes)havenocloselivingrelativesanddefineprobablydistinct evolutionarylineages[37,38].Inlookingatthegeneticaspectsof extantcanids(35species),thesephylogeneticrelationshipsimply thattheyevolvedfromacommonancestor.Althoughtheyhave adiversechromosomecomplement,whichrangesfrom36to78 chromosomes,allofthemoriginatethroughsimplechromosome rearrangementsfromacommonancestralkaryotype[39].Here, severalimportantsimilaritiesareshared;e.g.,shortinterspersed nuclearelementinheritance[40,41]andrapidlyevolvinggenes, suchasolfactoryreceptors,immune-relatedgenes,and reproduc-tiveproteins[e.g.,42].

Thecontemporarycanidsarethemostwidespreadfamilyof extantCarnivora,with at leastone species present in all envi-ronments and on each continent, except for Antarctica. Their distributionscanalsobehighlyrestricted,withafoxpopulation restrictedtoDarwin’sIsland [43],andsomeunusualsubspecies occurringonseparateislands,suchastheislandfoxesormednyi arcticfoxes(Alopexlagopussemenovi) [44,45].Conversely,other Canidaespan several continents,suchas the red fox, which is dispersedoveraround70millionkm2 _{[46].Canidaerangefrom} omnivores tostrict carnivores, through those with frugivorous orinsectivorousdiets,andtheyoccupyalmostallhabitats,from desertstoicefields,frommountainstoswampsorgrasslands,and fromrainforeststothe‘urbanjungle’[47].Theirhomerangesvary fromassmallasthe0.5km2 _{oftheislandfox[48],toaslargeas} 2000km2_{forAfricanwilddogs[49].}

Geographicalvariabilityinbodysizecanbeexplainedtosome degreebydifferencesinavailabilityoffoodforsomesmallCanidae, likethefennecfox,whichisusuallyassociatedwitharidandpoor habitats,andcanweightaslittleas1kg.Conversely,largeCanidae suchastheEthiopianwolfandtheAfricanwilddog,canweigh 50–60kg,and areoften associated withhabitats whereprey is abundant.Withameanadultbodyweightof9.72±7.7(SD)kg, thisincludestherangefrom1.25kgofVulpeszerdato26.63kgof Canislupus[50].

However,themaned wolf (Chrysocyonbrachyurus), which is unusualforalargeCanidaeintermsofitssocialorganisation,roams theSouthAmericansavannasandfeedslargelyonrodentsandfruit [51],wherebyGeffenetal.[52]suggestedthatitslowfood avail-abilityprobablyconstrainsboththefamilygroupandthelittersize. Themeanlittersizeis4.8±1.7(SD),witharangeof2.2ofVulpes zerdato9.0ofVulpeslagopus.Themeanageatsexualmaturity is391.1±140.0(SD)days,witharangefrom274.0daysofCanis latransto730.0daysofChrysocyonbrachyurusdays.Themean ges-tationageis59.9±5.9(SD)days,witharangefrom51.0daysof Vulpeszerdato72.0daysofLycaon pictus[50].44Themeanfor longevityonrecordis16.6±2.5(SD)years,witharangefrom12.7 yearsofCerdocyonthousto21.8yearsofCanislatrans,excludingthe domesticdog,althoughVulpesmacrotishasreportedamaximum longevityof7.5years[50].

These ecoethological and zoogeographical information are importanttohighlightthatCanidaeiscomposedofthemost vari-abletaxaassemblyascomparedtoothersystematicmammalian groups.Canidaemightrepresentaconstitutivegeneral‘prototype’ that cancolonise almostallecological nichesdue totheirhigh potentialforadaptability.Inparticular,amongcanids,wolfswere widespread in all of theenvironments that were colonisedby humans.

5. Naturalhistoryofthedomesticdog

Likeitstaxon,the‘domesticdog’livesinandsharesall envi-ronments with humans, and likehumans, these show regional adaptations related to climate and environmental conditions [53–55].These dogsshowanextraordinary levelof phenotypic variationintheirskeletalstructure,includingtheiroverallsize,leg length,andbodywidth,andvariantsofskullshape,withthe max-imumagereportedof24years[50].However,thereareanecdotal reportsofextremelongevityforAustraliancattledogs,whichreach upto30years,althoughthesewerenotconfirmedbyasurveythat reported13.41_±2.36years[56].Thelargestdogbreedsarealmost twoordersofmagnitudeheavierthanthesmallestbreeds.There arerangesbetweenbreedsofatleastthree-foldinageatmaturity, five-foldinlittersize,andmorethantwo-foldinlongevity[57–59]. Theaccumulatingarchaeological,culturalandgeneticevidence emphasisesthatwolfdomesticationcannotbeunderstoodoutside ofthecontextofhumancivilisation.Thegeographicandtemporal originsofdogsremainedcontroversialforalongtime,and sub-sequentroundsofmorein-depthgeneticanalyseshavesuggested eitheranAsianoraEuropeanorigin[60–62].Thelatestanalysis thatincludedthesequencingofancientdogDNAandthecomplete genomeofalateNeolithicdog,revealedadeepsplitseparating modern EastAsianand WesternEurasian dogs.Combined with analysisofmitochondrialDNA,thesedatasuggestthatdogsmight have beendomesticatedindependently in Easternand Western Eurasiafromdistinctwolfpopulations.EastEurasiandogswere thenpossiblytransportedtoEuropewithpeople,wherethey par-tiallyreplacedEuropeanPalaeolithicdogs.Wolvesthatapproached humancommunitiesthusunderwentinitialunintentional selec-tionfor decreased flight behaviourandincreased sociability,as thetrademarksoftamenessandsuitabilityforsocialisationwith humans.Remarkably,inexperimentalbreedingofsilverfoxes,this behaviouralselection is sufficientto induce,and there are sev-eral typicalmorphological traits of domestic dogs that suggest widespreadpleiotropyofdomestication-relatedgenes[63].Such selection for a singlebehavioural traitliketameness generated manifoldphenotypicchanges,probablythroughgenesthatgovern developmentalprocesses[64],andthesehaveprovidedthegrist forthemilloffurtheriterationsofselection.

Therelationship of ancestralman withdogswasone of co-evolution, where dogsbredrandomlyand adultswere selected basedontheirutility.Theseculturaldevelopmentsprovidedthe milieufortheinterwovenprocessesofartificialselectionand cul-turalacclimatisationthatresultedinthebiologicalchangesthatled, stepbystep,tothedogsoftoday[65].Thesearetheresultof domes-ticationthat wasbased ontheselection of naturalbehavioural traitsthat wereadvantageousforhumanends,suchashunting andsearching,herdingandguarding[66].Thisresulted,inturn,in veryclearpatternsofparallelevolution,wherebreedsselectedfor aspecifictaskevolvedsimilarphenotypesingeographically dis-tantregions.Thisismanifest,forexample,intheherd-guardian typeofdog,whichincentralItalyisrepresentedbythe“Mastino Abruzzese”(Fig.1A).Thesedogswereanecessarycomponentof theerraticpastoralismforthepreventionofwolfpredation,and musthavebeenpresentthroughoutEurasia.Thesearenowlimited

Fig.1. Examplesofnaturalbreedingofdogs(A)Livestockguardiandog‘MastinoAbruzzese’fromthemountainsofAbruzzo,Italy(B)Molossian-typedog‘CaneCorso’from ruralenvironmentinSouthernItaly(photoswithpermissionfromA.Mazzatenta&A.Scalisi).

torelictpopulations(forfurtherdiscussion,see[67]).Thenthere arethemolossian-likedogsthatwereselectedfortheirgripping abilityinthehuntoflargegame,fortheirresilientbehaviour,and fortheirmanagementofbulls,pigsandgoats.Theseareembodied intheItalian‘CaneCorso’(Fig.1B),andsimilarphenotypeswere originallywidespreadinEurope(e.g.,‘PerrodePresa’inSpain, ‘Bul-lenbeisser’inGermany,‘Mastiffs’in Britain,among others),and werewell-documentedinpaintingsoftheXVIIIcentury.

However,thegeneticbasisofwidevariationsindogbehavioural characteristicsisscarcelyunderstood,mainlybecauseofthelackof specificassays[68].Interestingly,startingin1959,DmitryBelyaev selectively bred foxesfor over 50 years, in terms of the short-estflightdistancetowardshumans,attheInstituteforCytology andGenetics in Novosibirsk,Russia.[for reviews,see63,69–71]. Thisexperimentwasasuccessanditrapidlyresultedinastrain oftamefoxes, compared totheunselected strainthatis highly aggressivetowardshumans.Thisallowedgeneticcomparisonsto bemadebetweendomesticatedfoxes,aggressivefoxes,F1hybrids, and theirbackcrosses[69,70]. Theseexperiments revealed that behaviouralselectionhaswidespreadpleiotropiceffectsthatcan resultinpigmentationandmorphologicalchangesthataretypical ofdomesticatedmammals,withcurrentstudiesaimedat identify-ingthemolecularchangesassociatedwithdomestication[72].

Toidentifygenome-widetranscriptionalcorrelatesof domes-tication,theexpressionpatternsof7762genesinthreedifferent regionsofthebrainsofdomesticdogs,greywolves,andcoyotes were investigated [73]. The gene expression in the hypothala-musof domestic dogs was different from that in grey wolves andcoyotes,whereaspatternsof geneexpressioninthe amyg-dalaandfrontalcortexwerelessdifferentiated[73].Theseresults suggest that behaviouralselection in dogsmight have affected endocrinological-autonomic responses, while not affecting the emotional,memoriesassociatedwithemotions,polysensory con-vergence,voluntarymovement, and higher cognitivefunctions. Fascinatingly,domesticationappearstohavehad greatereffects ontheregionsof thebrainthat regulatehomeostasis,and con-sequentlysenescence and aging, and lesson regionsrelated to emotionsandrelatedbehaviourstypicaloftheirancestors.

6. Artificialselectionandthedogasageneticmodelof diseases

In just thepast few hundred years, intensiveselection and breedingofdogsforkeydesirabletraitshasresultedinthe devel-opment of what we regard today as the major characteristics

of recognized breeds. When considered as a species, the level ofgeneticvariationsampledacrossalldogbreedstoday is per-hapsasextensiveasforthehumanpopulation[26].Inindividual purebreeds,however,thelevelofgeneticdiversityisnow vari-ablyrestricted[27],withextremevaluesofhomozygosityinsome breeds.Theprocessofbreedformationoverjustthepasttwoto threecenturieshasbeenestimatedtohaveresultedinaseven-fold greaterreductioningeneticdiversitythandidthethousandsof yearsofearlydomestication[74].

The‘aggressive’ dogbreedingpractices carriedout sincethe 1930shavefixedtraitsinso-calledbreedstandardsthatare charac-terisedbyhighdegreesofinbreedingandexacerbatewhatinitially were minor morphological differences among breeds. This has resultedinawidelevelofdiversityindogs,withanoverallFST of0.28amongbreeds,whichisabouttwicethatinhumans[75,76]. Thishasproducedgeneticdriftandledtoincreasedratesof breed-specificinheriteddiseasesthatarealsocommoninhumans,such ascancers,heartdisease,rheumatoidarthritis,autoimmune dis-orders,deafnessandblindness[26,65].Asaconsequence,theage patternsofdogmortalityarenotonlydependentonsize,butare alsoaffectedby,andstronglyrelatedto,thebreed,andassuch,are termed‘breedeffects’or‘geneticgroupeffects’[77,78].

In an effort to create a perfect companion, dog breeders embarkedonan‘experiment’,tofaithfullyrear,select,breedand adaptmillionsofpedigreedogs,generationaftergeneration,with genetically based proclivities and susceptibilities that are still awaitinggenomicinterrogation[27].Thisiscompoundedfurther bytheuseof‘popularsires’andthegenepooldeclineduringthe twentieth century.As many ofthe keyphenotypes are charac-teristicoftheparticularbreed,theirpresenceinthebreedwere positivelyselectedfor,whichresultedinhighfrequenciesofthe genes that cause these specific phenotypes. With suchintense selection,itisperhapsnotsurprisingthattherearenowover sev-eralhundredinheriteddiseasesrecognizedindogs.Akeyfeatureof thesediseasesisthattheyappearonthebackgroundoflowgenetic variability.Indeed,manydiseaseshavesimpleinheritancepatterns (forarecentreview,see[79]).Particularlyforcancers,thegenetic backgroundofsomepure-breddogsmightpredisposethebreed toahigherriskforspecificcancers,orcancersingeneral.Itisthis increasedriskofcancerinpure-breddogsthatmightbeleveraged toacceleratetheprocessofcancergenediscoveryfroma compar-ativeperspective(forarecentreview,see[80]).Alsointhecaseof complexdiseases,thehighinbreedingofpure-breddogsfacilitates mappingwithrespecttohumans(thiswaspartoftheoriginal moti-vationforsequencingthedoggenome,see[75,81]).Inthecontext

ofaging,itshouldbenoted,forexample,thatsizevariationindogs hasbeenassociatedtothevariationsinthegenesinthe insulin-likegrowthfactor(IGF)-1signallingpathway[82],whichisakey regulatorofaging[83].

7. Dogdiseasesandgeriatrics

Aswellastumours,mostofthecommondogdiseasesare sim-ilartothoseofhumans,mainlyintermsofage-relateddiseasesin theorgansandsystemsthataremostaffectedbyhumanaging: the brain, kidney and cardiovascular system. Indeed, although road traffic accidents are high on the list, the most common causesofdeathfordogsare:neoplastic,cardiac,renal/urinary, gas-trointestinal/anorexic/weightloss,diabetesmellitus,respiratory, musculoskeletal,neurological,dermatological,reproductive,and behaviouraldisorders,someofwhicharereportedascongenital, whichdefinesthemasgeneticallyinherited[77,84].

Interestingly,whileveterinaryhealthcareoverrecentdecades has significantly increased our knowledge of geriatric dogs, inverselydog lifespanhasdecreased.Indeed, theageat which clinicalveterinarianscurrentlyconsiderdogstorequire‘geriatric care’rangesfrom6yearsto9yearsinthelargebreeds,to9years to13yearsinthesmallerbreeds[85].Thisassociationbetween smallbodysizeandlongerlifespanwithinaspecieshasalsobeen reportedfor mice[86] andhumans, and anecdotallyfor horses [19,87],andwillbetreatedindetailinSection8(seealso[88–91]). 7.1. Cardiacdisease

Inthecontextofcardiacdisease,dogscanprovideapowerful modelforunderstandingwhetherdiseaseisacauseand/or con-sequenceofaging.Forexample,degenerativevalvediseaseisthe mostrepresentedheartdiseaseindogs,anditrepresentsa signifi-cantriskfactor[92].Interestingly,itisgenerallyacceptedthatolder smalldogsaremorecommonlyaffectedbydegenerativevalve dis-easethanyoungerlargedogs,andinheritancestudieshaverevealed increasedriskofclinicalsignsamongmaledogs[93].Inastudy basedonly onpost-mortem examinations,it wasreportedthat 34.4%of404dogsshoweddegenerativevalvedisease,which cor-relatedwithage[94].Also,outof40companiondogsover6years ofagethatrangedfromGreatDanestostandardPoodlesand cross-breeds,11ofthese(meanweight,28.8kg)showedundiagnosed cardiacdisease,asrevealedbyechocardiography,althoughthiswas notrelatedtodogsize[95].Severalotherstudieshavefocusedon individualhigh-riskdogbreeds,suchasthecavalierKingCharles Spaniels[96]andDachshunds[93].

7.2. Dogrespiratorydisease

Anextensivebodyofliteraturedescribesthecaninepulmonary anatomyandphysiology, includingthelungmechanics, ventila-tion,coughreflex,immunobiology,inflammation,pharmacology andcentralneuronalcontrolmechanisms.Thecaninerespiratory systemsharesmanysimilaritieswiththatofhumans,anddogshave beenusedasmodelsforchronicinflammatorydiseases,suchas inducedasthmaandchronicobstructivepulmonarydisease,which developsfromchronicbronchitisandemphysema,againresulting indecreasedlifespan[97].

7.3. Dogobesityandendocrinediseases

Therearedifferencesregardingbodycomposition acrossdog breeds[98].However,dogobesityisthemostcommondisorder thatleadstomorbidity,andtherangeofdisorderswithwhichdog obesityhasbeenassociatedincludediabetesmellitus, cardiovascu-larandmusculoskeletaldisease,degenerativedisorders,decreased

immuno-competence,andshortenedlifespan.Theseareagainnot relatedtodogsize,butinsteadpromotedbyotherhumanfactors, likewealth[99].Thus,restrictionofcaloricintakemightextendthe caninelifespan[100].

AsinglenucleotidepolymorphismhaplotypeofIGF-1is com-montosmalldog breeds,whilerare inlargebreeds[101].Dog serumandurinemetabolomeprofilesreflecttheirintestinal micro-biotaandrenalpathology[102].Avarietyofassociatedendocrine disorderscanspontaneouslyaffectdogs,likediabetes,growth hor-monedysfunction,andhypercortisolism.Caninediabetesiscaused byeitherautoimmunedestructionoftheirpancreatic␤-cellsor excesslevelsofcounter-regulatoryhormones.Pituitarydwarfism indogsiscausedbyautosomalrecessivecombinedpituitary hor-monedeficiency,whichis oftenobservedinGermanshepherds inparticular [103].Middle-agedtoolddogscommonlydevelop Cushing’s disease,throughoverproductionofglucocorticoidsby theadrenalcortexTheassociatedmanifestationsareverysimilar toclinicalobservationsinhumans,includingchangesintheskin, weightgain,abdominalobesity,fatigue,muscleatrophy, hyperten-sionandrenaldysfunction[103,104].

7.4. Dogimmuno-senescence

Theimmune-senescencecharacteristicsofolderdogsareshared withelderlypeople.Olddogsshownimpairmentofcell-mediated immune functions, suchas reduced blood CD4+ _Tcells, imbal-anceinTh1versusTh2functionalactivity,elevationoftheCD8+ subsets,andreductionintheCD4:CD8ratio.Furthermore,blood lymphocyte responsestostimulation bymitogens decrease the delayed cutaneous type hypersensitivity response. Conversely, there is relative preservation of the ability to mount humoral immuneresponses.Serumandsalivaryimmunoglobulin(Ig)A pro-ductionincreases,andIgGconcentrationsremainunalteredwith age.Elderly animals generallyhave persistingvaccine antibody titresatprotectivelevels,andrespondtoboostervaccinationswith titreelevation. Olderdogsshowprimary humoralresponses to novelantigens,butthemagnitudeofthesecanbelowerrelativeto thetitresofyoungeranimals.Therehavebeenfewinvestigations intothephenomenonof‘inflammageing’indogs,astheeffectsof cumulativeantigenicexposureandonsetoflatelifeinflammatory disease[105].

7.5. Dogcancers

Dogs spontaneously develop thesame types ofcancers that humansdo,andtheyareofteneventreatedwiththesame thera-peuticstrategies[106].Severaldogbreedsareknowntohaveahigh incidenceandelevatedriskofspecificcancersubtypes,sometimes evenmorethanonesubtype.Additionally,centuriesofselective breedingof dogsconfers theopportunity toexamine polymor-phismsthatarespecifictoparticularbreedsthathaveexaggerated incidences of particular cancersubtypes [107]. Finally, because dogscohabitatewiththeirhumanowners,theyarebothexposed tothesame environmentalfactors,which mightpotentiate the developmentofcancers[108].Genomicanalysisofcaninetumours hasrevealedsharedfeatures withhumans, whichhasprovided importantinsightintothegeneticbasisoftumourdevelopment [109–111].

Table1givesalistofthehigh-riskbreed-specificdiseasesthat havearisenduetotherestrictedgeneticvariationproducedby con-sanguinityandinbreeding.Thisoffersanexceptionalopportunity toexaminetheinteractions betweengeneticsand environment in the aetiologies of various forms of cancer; and the shorter lifespanofdogsfacilitatestimelyandefficientevaluationofnew approachestocancerdiagnosis,treatmentandprevention.Awide varietyofcancersarebeingstudiedindogs,whichincludesoft

tis-Table1

Cancersassociatedwithspecificdogbreeds(modifiedfromDobson[111]).

Cancer Subtype Dogbreed

Lymphoma(unspecified) OldEnglishSheepdog,Boxer,Pointer,GoldenRetriever,Rottweiler,StBernard,ScottishTerrier,Bulldog

B-celllymphoma IrishWolfhound,SiberianHusky,ShihTzu,AiredaleTerrier,CavalierKingCharlesSpaniel,YorkshireTerrier T-celllymphoma Boxer,CockerSpaniel,BassetHound

Osteosarcoma Largeandgiantbreeds,suchasIrishWolfhound,ScottishDeerhound,GreatDane,BMD,Mastiff,StBernard,

IrishSetter,GoldenRetriever,Rottweiler,DobermannPinscher,Greyhound

Softtissuetumours Largerdogs,suchasBoxer,BMD,AiredaleTerrier,GreatDane,StBernard,BassetHound,GoldenRetriever—all

withtwiceasmanyasthegeneralcaninepopulation

Hemangiosarcoma GermanShepherd,BMD,GoldenRetriever,Flat-coatedRetriever,PortugueseWaterDog,LabradorRetriever,

Boxer,SkyeTerrier,AustralianShepherd

hs/Malignanthistiocytosis BMD,Flat-coatedRetriever,Rottweiler,GoldenRetriever

Mastcelltumours Boxer,Pug,LabradorRetriever,GoldenRetriever,Vizsla

Meningiomas Mesocephalic(medium)anddolichocephalic(long)-nosedbreeds;e.g.,Labrador,GoldenRetriever,Collies

Gliomas (including

glioblastoma multiforme)

Brachiochephalic(short-nosed)breeds,includingBoxers,BulldogsandTerriers

Testicularseminoma NorwegianElkhound

Nasalcavitycarcinoma GoldenRetriever,Beagle,BostonTerrier,RoughCollie,BelgianShepherd,UCScottishTerrier,Beagle,West

HighlandWhiteTerrier,ShetlandSheepdog,AmericanEskimodog,standardSchnauzer

Lowerurinarytractcarcinoma AiredaleTerrier,Beagle

Squamouscellcarcinoma(digit) STPO,giantSchnauzer

Melanoma Oralmelanoma Poodles

Cutaneousmelanoma Schnauzers,BeauceShepherds

suesarcomas,mammarycarcinomas,primaryandsecondarylung

carcinomas,malignant melanomas,and cancersof theprostate,

bladder,intestine,brainandmouth,andmanyothers[112].Among

allofthese,doglymphomasandcanineosteosarcomaareof par-ticularinterestintermsoftheirfrequencyandpathophysiology, respectively.Indogs(mainlyboxersand goldenretrievers),and humans,largeB-cellnon-Hodgkin’s lymphomais themost fre-quent[106,112].Canineosteosarcomaaffectslargebreedssuchas GreatDanes,Wolfhounds,andRottweilers,wherethisisgenerally confinedtothelongbones,andhassimilarmetastaticratesand destinationsasseenforhumans.Inosteosarcoma,thep53tumour suppressorpathway,thec-Met proto-oncogene,the chemokine interleukin-8,and several suchmediators are involved in both species [106,107,112]. Furthermore, a constitutional ‘germline’ DNAforcancerpredispositiongenesindogswithcancerhasbeen described,which includesBRCA1/BRCA2[113,114], whichleads tohereditarybreastandovariancancersyndromeinhumans,as wellas TP53 germline mutationsin dogs[115], which lead to Li–Fraumenisyndromeinhumans,withmultipledifferentcancers. Theapproachofusingsinglenucleotidepolymorphismsand/or copynumbervariations,andgenome-wideassociationstudieshas beenassociatedwithdiseaseriskinspecificdogbreeds(for exten-sivereviews,see[80,116]).Geneticanalysisofcaninetumourshas revealedcommon features withhumans,along withimportant informationabouttheirdevelopment.Thustheconsanguinityand inbreedinginthepracticeofhumansocietyfordogbreedinghas unwittinglycreatedahigh-riskmodelforbreed-specificdiseases [80].Thelimitedgeneticvariationinpure-breddogsthatis associ-atedwiththeshorterlifespanofdogsfacilitatestimelyandefficient evaluationofnewapproachestocancerdiagnosis,treatmentand prevention.

7.6. Dogkidneydisease

Promisinggenetherapiesforhereditarynephropathy,agroupof fatalinheriteddiseases,andAlportsyndrome,aformofhereditary nephropathyinhumanscausedbydefectsintheglomerular base-mentmembrane[117,118],havebeenidentifiedindogs.Theonly treatmentscurrentlyavailableforAlportsyndromearedialysisand renaltransplantation.Naturallyoccurringhereditarynephropathy hasbeenidentified inseveralcaninefamilies (Samoyed,English CockerSpaniel,BullTerrier),anddiseaseprogressionissimilarto

humans[108].Indeed,anadenoviralvectorcontainingthecanine cDNAofaCOL4A5mutation(whichrepresentof85%ofAlport syn-dromecases,and is necessary forcorrect glomerular basement membraneformationinthekidney)wasinjectedintothesmooth muscleofthebladderinaffectedSamoyeddogs.Fiveweeksafter injection,expressionofbothCOL4A5andarelatedgene,COL4A6, wasshowninthebasementmembranessurroundingtheinjection site,whichindicatedthesewereexpressingfunctionalgenes[119]. 7.7. Dogneutering

Otherinterestingassociationswithcaninelifespanincludethe ‘desexing’,orneutering,ofdogs,whichmightalsoaffectnormal aging and promote senescence and increased risk of cognitive impairment[77,120,121].Also,increasedtelomerelength corre-lateswithmortalityin bothdogsand humans,suchthat when telomeresreachacriticallyshortlength,aDNAdamagesignalis initiatedthatinducescellsenescence[122,123].Telomerelength islargelyinheritedfromtheparents,andthislengthincreaseswith theageofthesperm,suchthathumanoffspringconceivedbyolder fathershavelongertelomeresthanthoseconceived byyounger fathers,atabout22bplongerforeachyearolderthefatherisat conception[124,125].Ifthesameistruefordogs,asissuggested bytheirsimilartelomerebiology,abreedingprogrammecouldbe activatedtopositivelyaffecttheirlifespan.

7.8. Dogneurodegenerativediseases

Geriatricdogsdevelopcognitiveimpairmentandcentral ner-vous systempathologies that mimic thechanges that occurin humanneurodegenerative diseases,suchasAlzheimer’sdisease [126,127].Caninecognitivedysfunction,or‘caninedementia’,is aneurobehaviouralsyndromeinageddogsthatischaracterisedby deficitsinlearning,memoryandspatialawareness,andchanges insocial interactions and sleepingpatterns[128]. Likehumans, ageddogssufferfromcognitiveimpairmentthatappearsto resem-ble Alzheimer’s disease,where this is apparently characterised bydepositionofsignificantamounts ofamyloid␤protein(A␤) anddevelopmentofdiffuseplaquesthatcorrelatewithcognitive decline.Unlikehumans,however,dogswithcognitiveimpairment donotappeartodevelopneurofibrillarytangles.Thisappearstobe becausetheamino-acidsequenceoftheA␤peptideisidenticalin

dogsandhumans,whilethisisnotthecasefortheTauamino-acid sequence.Therapeuticstrategiesthatincludeantioxidantdietsand behaviouralenrichmenthavebeenshowntoimproveAlzheimer’s diseasepathologyindogs,andatthesametime,anti-inflammatory drugs,statins,andimmunisationagainstA␤peptidehavealsobeen pursuedinageddogs[126,127].

7.9. Dogbehaviouralaging

Therehavebeenfewstudiesintotheeffectsofdogbreedson behavioural aging [129]. Many studied behaviours have shown nobreedeffects,includingformixedbreeds.Alongevityanalysis askedwhetherasinglebehaviourofshort-livedorlarge-sizeddogs showedanyincreasedseverityorlevelofdeteriorationcompared withlong-livedorsmall-sizeddogs.Inaddition,thedecreased ‘pro-portionofdogsthatdrink1Lwater/day’inlong-liveddogsislikely toreflectabodymassrelationship,ratherthananaging relation-ship[129].

However,theassociationbetweendogsizeandlongevitydoes followpredictionsforarthritisindogs,withlarge-sized,short-lived dogsshowinghigherprevalence.Incontrast,blindnesshadalower prevalenceinlargeversusmedium-sizedandsmall-sizeddogs,in linewithProschowskyetal.[130],whoreportedthatthe preva-lenceofear,eye,andskeletaldiseasessignificantlydiffersbetween breeds.

Beyondoxidativestress,thereareprobablyalsoalarge num-berofothercontributingfactorstotheoverallagingprocess,as thehyperfunctiontheorysuggests,althoughthesemightnotaffect cognitivefunction[3,4,129].Severallongevityandsizedifferences incaninebehavioural aginghave beenshown, notallof which areinthesamedirection.Hence,thereisconflictingevidenceas towhetherlarge-sized,short-liveddogsshowincreasedratesof aging. Iflarge-sized, short-lived dogsare ‘physiologicallyolder’ thansmall-sized,long-liveddogsofthesamechronologicalage, thentheyshouldshowgeneralincreasedprevalenceofage-related diseasesandincreasedrateofbehaviouralsignsofcognitive dete-rioration.Instead,cognitivedeteriorationindicatorsappeartobe moreprevalentforsmall-sized dogs,asisblindness,whichis a stronglyage-relateddisease[129].

7.10. Mappingdiseasesinthedoggenome

One benefit ofusing dogs is thestructure of theirgenome; e.g.,linkagedisequilibrium,orthedeviationinthefrequencyof haplotypesinapopulationfromthefrequencyexpectedifthe alle-lesatdifferentlociareassociated atrandom,isgreaterindogs thanhumans[26,131].Furthermore,theaveragelinkagewithin dogbreedsisintherangeofmegabases[131],whereaslinkagein humansub-populationscanbemeasuredinkilobases[132],which meanthathaplotypesaremuchlongerindogs.Although differ-entbreedsvaryintermsoftheirlinkagedisequilibriumdepending onthehistoryofthebreedandphenomenasuchasfounderand bottleneckeffects[133],theoverallhigherlinkagedisequilibrium indogsimpliesthatfewerthan30,000single-nucleotide polymor-phismsmightbenecessaryforgenome-wideassociationstudies, whichisanorderofmagnitudesmallerthanthecoverageneeded forhumangenome-wideassociationstudies[131,134,135].

In addition, many haplotypes are sharedacross dog breeds, which appear to have resulted from the genetic bottleneck of domestication[26,133].Geneticinvestigationsinhumansonrare polymorphismsmighthavesignificant,populationspecific,effects onphenotypes[136,137].Thus,raregeneticvariationwithinbreeds might have important effects on phenotypes of interest [138]. Anotherbenefit of usingcanine model is that 277of the over 573dog-documented diseases resemble humandiseases [139]. Models of inheritance in dogs have allowed the isolation and

Fig.2.Relationshipbetweenbodysizeandlifespanforseveralfamiliesofmammals. Reproducedwithpermissionfrom[15].

identification of thecausal genes for diversebiomedical disor-ders[25,108,133,140–144].Twodatabasesforinheriteddisorders in dogs are an important resource for research on inherited diseases in dogs: OMIA [139] and Inherited Diseases in Dogs [145,146].ItisestablishedthatMendelianinheriteddiseasescan leadtoneuropathologiesandbehaviouralabnormalities,while sin-glegenemutationscanleadtoataxia,seizures,cerebellarcortical degradation,encephalopathies,andotherneurologicaldisorders [147–149]. Furthermore,singlegenemutationsshowhigh pen-etrance[150],which reflects effects onbehaviour.Through the analysisofdogpedigrees,searchesforcausalgenescanbefocused andinformresearchontherelevanthumandisorders.

Anadditional benefitof usingdogs istheirgreat variability, wherebyeachbreedrepresentsageneticallyisolatedpopulation withauniquesetofmorphologicalandbehaviouralcharacteristics [68,133,135,140–144,151–153].Varianceindogbehaviouris anal-ogoustothatobservedinthenormalhumanpopulation[154].Dogs showdifferencesintemperament,compulsivedisorders,anxiety, socialbehaviour,‘sociability’or‘confidence’,aggression,andmore [forreviewssee154–157].Dogshaveconditionsthatcanbe inves-tigatedthatmightbeanalogoustohumanpsychiatricconditions, suchascaninecompulsivedisorder,whichmightbeanalogousto humanobsessive-compulsivedisorder[155,158].

8. Effectsofsizeondoglongevity

Across animal species, body size is robustly correlated with lifespan[11,159,160].Inmammals,thelifespan-sizerelationship followsapowerlaw,withanexponentofaround+0.25(Fig.2). Remarkably,dailyenergyexpenditurenormalisedpermassis neg-ativelycorrelatedtobodysize,withanexponentofaround−0.25, whichleadstothesuggestionthatwhole-lifeenergyexpenditureis constant[161].Unlikeotherspecies,therelationshipbetweenbody sizeandlongevitycanbestudiedinthedog,asdifferentbreedsspan almosttwoordersofmagnitudeinbodyweight.Adultbodysizein dogsisinverselyrelatedtolongevity[162–164](Fig.3),andsmall bodysizeisassociatedtogeneticvariantsthatlowertheactivityof theIGF-1signallingpathway[165].ReductioninIGF-1signallingis indeedknowntocauselifeextensionthroughstudiesfrom nema-todewormstomice,andreducedIGF-1signallingwasobserved inasubsetofhumancentenarians[forarecentreview,see166]. PlasmaIGF-1concentrationsinnormaldogsisafunctionofbody size:in CockerSpanielsthisis 36±27 (SEM)ng/mL,inBeagles, 87±33ng/mL,inKeeshonds,117±34ng/mL,andinGerman Shep-herds,280±23ng/mL.However,themeanIGF-1levelinagroupof dogsthatincludedarangeofbothsmallandlargedogswith patho-logicallyhighgrowthhormonelevelswas700±90ng/mL,suchas inacromegalyandgrowth-hormone-relateddiabetes[167].IGF-1 signallingisrelatedtocarotidbodyfunction,andaspecificeffect ofagingonthecarotid-bodychemoreceptorsisaccompaniedby impairmentofventilatoryandarousalresponsestohypoxia[168]. Indeed,carotid-bodychemoreceptorsprimarilycontributetothe

Fig.3. Lifespannegativelyscales,−0.096scalingpowerfortheaveragelifespanacross90breedsofdogs,withadultbodymassinmale(A)andfemaledogs(B).Eachpoint representsonebreed.Thescalingpowerswereobtainedbyregressingthelogarithmicallytransformeddata.Reproducedwithpermissionfrom[172].

eupneicdrivetobreatheundernormoxia,aswellassending nor-maltonicsensoryinputtomedullaryrespiratorycontrollers,such asmodulatoryeffectsoncentralchemoreceptorresponsivenessto CO2[169].Therelationshipbetweencarotid-bodyactivitypatterns andIGF-1mightthereforebeindicativeoflifespanvariabilityin dogs.

Anumberofstudieshavetriedtotakeadvantageofthe lifes-pan/sizecorrelationindogstoidentifycellular[170]andmetabolic [91]correlatesforsize-relateddifferencesinlongevity.Recently,a metabolicmodelwasdevelopedtoresolvethisparadox.Thismodel isbasedonthewell-knowndisposablesomatheoryofaging[171], anditpositsthatlifespanisdeterminedbythetrade-offinenergy allocationbetweentheprocessesofgrowthandrepair.Byfitting bodymassatbirthandenergyrequiredtoattainadultbodysize,it wasshownthatlargebreedsinvestdisproportionatelymoreenergy ingrowthwithrespecttosmallbreeds,becauseoftheirsmaller rel-ativesizeatbirth,andthereforetheyhavelessenergyavailablefor repair,whichresultsinshorterlifespans[172].

Shorterlifespansoflargerdogsmightberelatedtofour pos-sibleeffects:(i) higherearlymortality; (ii)fragility (i.e.,higher baselinemortality);(iii)earlieronsetofsenescence(i.e.,thetime whenaccelerationofmortalitystarts);or(iv)fasteraging(i.e.,the rateofaccelerationofage-dependentmortality).Thesefour com-ponentscanbedissectedouttoacertainextentbydemographic analysis,tofitdifferenthazardmodelstodogs.Senescenceonset doesnotappeartobeacceleratedinlargedogs,whilefragilityis a littlehigher inlargedogs; however,thedriving forcebehind thenegativecorrelationbetweensizeandlifespanisapparentlya strongpositiverelationshipbetweensizeandagingrates;i.e.,large dogsagefaster[163].Interestingly,reducedratesofdemographic aginghavebeenreportedingrowth-hormone-deficientmice,as amodeloflongevity[173].Thissuggeststhatgeneticvariantsin IGF-1/growthhormonearemajordeterminantsoflifespanindogs, whichqualifiesdogsasamodelforunderstandingtheroleofthe somatotropicaxisinaginginanecologicallyrelevantcontext.

Largedogsalsotakelongertoreachphysicalandbehavioural maturity,andatthesametime,dogsthatreachsimilaradultsizes mightwelldifferintheirgrowthpatterns,whichisimportantin theevolutionoflife-historystrategies[174–176].Acomplication thatshouldbetakenintoaccountis inbreeding.Indeed,thelife spanofpure-breddogsislowercomparedwithmixedbreeddogs, acrossallweightcategories,whichsuggeststhatartificialbreeding ofdogsovertimefor specificphenotypeshasreducedtheirlife expectancy.However,itisnotclearwhetherthiseffectisdueto increaseddiseaseriskordifferencesinagingrates[177].

9. Interventionsindogs

Dogshavebeenusedintherecentpastassubjectsfor inter-ventionstudiesintwomaincontextsthatarehighlyrelevantfor aging and aging-associateddiseases. The first of theseis calo-rie restriction. Given the pattern of parallel evolution of the digestive physiology in man and dogs [32,33], the results of dietarymanipulationsindogshaveaparticularlyhightranslational value.Reductionofcalorieintakewithoutmalnutritionisa well-establishedprotocoltoprolongthelifespanoflaboratoryrodents, andtodelaytheonsetofalargenumberofage-associated con-ditions.Laterstudiesshowed,however,thatresponsestocalorie restrictiondependonthegeneticbackground,whichintroduces a confounding effect in rodent studies [178]. Whether calorie restrictionswouldelicitthesameeffectsinlargermammals,and ultimately humans, hasremained unansweredfor a long time, and is partially still a matterof speculation. In themid-1980s, a calorie restriction experiment in dogswas launched roughly incoincidencewiththelaunchofprimatecalorierestriction tri-alsthatarepartiallystillongoing.Thisstudydemonstratedthat 25%foodrestrictioninducedasignificantlifespanextension, cou-pledwithimprovementsinglucosehomeostasis [179,180].This studyshowedforthefirsttimethatcalorierestrictioncan pro-long thelifespan ofa mammalianspecies, thesize and diet of whichismorecomparabletohumans,andalsoshowedthe poten-tialof longitudinalobservationsin dogs.A majordrawback for thestudywasthatitwasbasedonLabradorretrievers,whichis a breedknowntobehighly pronetoobesity, asof allthedog breedsforwhichdatahavebeenreported,Labradorretrievershave thegreatestdocumentedprevalenceofobesity[181,182].Thisis partially because theycarry a loss-of-function mutation in the Propiomelanocortine(POMC)gene,which codesfor an anorexi-genicpeptidethatisakeycomponentofthecoreappetitecontrol pathway[183].Itisthereforeofgreatimportancetorepeat sim-ilar experimentsin otherbreeds or in mongrels. However,the linkbetweenweightandmortalityit isnotlinear;inparticular, therelationshipbetweenall-causemortalityandbodymassindex definesaU-shapedcurve,whichindicatesthatextremeleanness aswellasobesitytendstobeassociatedwithincreasedmortality [184].

Furthermore,anumberofotherdietarymanipulationsthatare morefeasible ina translationalcontexthave recentlyemerged, suchasalternatefasting(e.g.,twonon-consecutivefastingdays perweek),periodicfasting(e.g.,afew consecutivedaysof fast-ingeveryfew months), andtheuseof fasting-mimickingdiets.

Thesemanipulationshavehadremarkableeffectsinanimal mod-els[185–189].Thesetypesofdietarymanipulationsarecurrently theobjectsofhumanclinicalexperiments[190].Basedonthese data,itislikelythatownersofdogsfrombreedsatriskof devel-opingobesityorathighoncologicalriskcanbeenrolledintrials aimedattestingtheeffectsofthesedietarymanipulationsondog health.Bysodoing,highlyrelevantinformationcanbegatheredon thepossiblelong-termeffectsofthesemanipulationsinhumans. Theinterpretationofobesityassociatedwithcaptivity,andfordogs andhumanswiththemodernlifestyle,isaproblemthataffects theinterpretationoftheresultsfromcalorierestriction interven-tion,becausethesedentarinessimposedbythecloseconfinement ofthecaptivitymightalsocontributetoahigherpercentofbody fatbecauseenergyexpendituremightnotbecorrectlybalanced withfoodconsumption.Forinstance,inmonkeys,inadditionto low energy expenditure,these animals arehoused individually andinverysmallcages(0.66m3_{involume,[191]),andsocial} iso-lationmightoverstimulatefoodconsumptionasacompensatory mechanism ofthereward limbicsystem,due tolife conditions thatcanunderstandablycarryintelligentanimalstoward depres-sion.Indeed,thebaselineorcontrolsituationsmighthavedramatic effectsontheoutcomesofcalorierestriction,asexemplifiedby thedifferentoutcomesoftwocalorierestrictionexperiments per-formedinprimates[192].

Thesecondfieldwheredogshavebeenusedinthepastrelates toage-relatedcognitivedysfunction.Alzheimer’sdiseaseis partic-ularlydifficulttomodelinrodents,becauserodentsdonotshow naturalaccumulationofamyloidplaques.Dogsareparticularly rel-evantsubjectsfor age-relatedcognitivedecline as theyshowa numberofsimilaritiestohumancognitivedecline,whichincludes tasksthatrelyontheprefrontalcortex[193]andonage-dependent accumulationofamyloidplaques[194].Interventionstudieshave includedtheuseofadietrichinabroadspectrumofantioxidants andmitochondrialco-factors,whichreducedneuropathologyand improvedcognitionovera33-monthtreatmentperiod[195],and theuseofatorvastin,astatinthatisusedforloweringcholesterol [196].Dogsbecamehighlyrelevantinstudiesoftheeffectsof vac-cinationagainstA␤peptidesasimmunotherapyfor Alzheimer’s disease,withtherationalebehindthisapproachbeingthat anti-bodiesagainstA␤mightremoveplaques.Anearlystudyreported rapidimprovementsincognitivedysfunctionwithimmunotherapy [197].Importantly,thisstudyincludedprivatelyowned compan-iondogs,andnotonlylaboratorybeagles.Alaterstudywithbeagles showedthatvaccinationagainstA␤effectivelyreducedplaquesin theprefrontalcortex[198].Thisparallelswhathasbeenobservedin humanclinicaltrials,whereimmunotherapycanreducetheplaque burden[199,200].Immunotherapyindogsdidnotimprove cog-nition,althoughitpreventedfurtherdecline.This,again,reflects whathasbeenobservedinhumanclinicaltrials,withlackof recov-ery and reduced rate of decay afterimmunotherapy [201,202]. Theseresultshighlightthetranslationvalueofthedogmodelfor Alzheimer’sdisease.

Afield where companion dogshave recentlybecomehighly relevant is the testing of ‘anti-aging’ drugs, and in particular rapamycin.Alargebodyofliteraturehasshownthat administra-tionofrapamycinlateinlifecanreproduciblyprolongthelifespan andcanimproveanumberofage-relatedphenotypes[203–211]. Asseenmanyyearsagointhecaseofcalorierestriction,a press-ingquestionistowhatextentrapamycincanimproveage-related conditionsinlargemammals.Aprojectwastherefore launched totesttheeffectsofrapamycinincompaniondogs,andthefirst reportoftheshort-termeffectswasrecentlypublished[212].This projectmightrepresentthefirstofagame-changingapproachin gerosciences.

10. Conclusions

Purebreddogshavebeenintenselyselectedforspecific mor-phological,physiologicalandbehaviouraltraits.Thiswaslargely achieved by the extensive use of founder dogsthat expressed thedesiredtraits,whichinadvertently(i.e.,throughabottleneck) also increasedthefrequency of deleterious allelecombinations [213,214].Recessiveanddominantdefectswithincomplete pene-tranceorlate-onsetandcomplexinheriteddiseasesarenumerous indogs.Thus,geneticmaltreatmentoccurred,andnowadays breed-ing practices have raisedconcerns about thehealth status and wellbeingofpure-breddogs[215].Theinheritedcaninedisorders discussedabove,andalsopathologicalconditions,areequally com-moninhumansasindogs,andtheclinicalsignsindogsoftenmimic thehumandiseasesclosely[108].Furthermore,dogsandhumans shareroughlythesamegenerepertoire[26,216].Doggenesthatcan beidentifiedasdeleteriousgivenovelinsightsintodisease patho-genesisandtherapeuticopportunitiesinsimilarhumandiseases [217].Dogs offermany ofthesame advantages ofother small-animalmodels; e.g.,therearehomogeneouspopulationswithin each ofthehundredsofpurebreedsandpedigreesthat canbe establishedeasilyandin arapidfashion.Companiondogshave characteristicsthat areuniqueamong domesticatedanimals,as theyreceiveexceptionalmedicalcare,theyhaveaccesstophysical activityandgenerallytochallenges,andtheygenerallycohabitate withtheirhumanowners[141].Themostimportantadvantageof thedogmodelisthatnaturallydiseasedanimalsfromthepet popu-lationcanbeenrolled,totheadvantageofthepetsandtheirowners, thereby eliminatingtheneed fortheestablishment ofcolonies, reducing costs,and resolvingsensitiveissues onanimal experi-mentation[108].Furthermore,asthesedogslive withhumans, theyareexposed tothedomesticenvironment thatincludesall of the human pollutants and pathogens, which is qualitatively differentcomparedtoclassiclaboratoryresearchanimals(for con-siderationofcaptivitydetentionsee[184]).Thus,whenmodelling thecausesandpathogenesis ofhumanhereditarydiseases, any environment–geneinteractionsarelikelytobebetterstudiedin ananimalthatisexposedtothehumanenvironment.Petclinical trialsdatebacktothemid-1970s,whenvetstestedlymphoma vac-cinesonhouseholddogs,andtherearesignsthatclinicaltrialswith domesticdogscanbefullytranslational.In2013,forexample,the USFoodandDrugAdministrationaccelerateditsapprovalofadrug calledibrutinibforlymphomainhumansafteritshowedpromise incaninetrials[218].

Finally,translationalresearchindogbreedscanonlybepossible onbothethicalandpracticalgroundsifithasahighchanceof pro-vidingrealimprovementstothehealthylifespanofthecompanion dogs.Alargenumberofpeopleworldwideconsume‘anti-aging’ dietaryadditives,anditislikelythatanequallylargenumberof peopleprivatelytest−atleastforshortperiods−‘anti-aging’diets, andsimilarlythereisalargemarketfordietarysupplementsfor dogs.Itshouldthereforebepossibletomotivatedogholdersto participateincanineaginginterventiontrialsbymeansofmore opencommunicationand inthecontextof well-definedethical rules.Ofparticularinterestare‘ancient’dogbreeds,notselectedfor extrememorphologicalphenotypes,withresidualgenetic diver-sity,andmightbetakenintoaccountforagingstudies.Thus,there isalsotheneedfortheirpreservationtomaintainancestralgenetic variability.

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