Epigenetic Memory in Mammals | Genetics - Frontiers

Epigenetic memory in mammals ... Epigenetic information can be passed on from one generation to another via DNA methylation, histone modifications ... Articles Abstract Introduction EpigeneticReprogramming EscapingReprogramming:EpigeneticInheritance EpigeneticInheritanceofDisease ConcludingRemarks ConflictofInterestStatement References SuggestaResearchTopic> DownloadArticle DownloadPDF ReadCube XML(NLM) Supplementary Material Exportcitation EndNote ReferenceManager SimpleTEXTfile BibTex totalviews ViewArticleImpact SuggestaResearchTopic> SHAREON OpenSupplementalData REVIEWarticle Front.Genet.,08June2011 |https://doi.org/10.3389/fgene.2011.00028 Epigeneticmemoryinmammals ZoëMigicovskyandIgorKovalchuk* DepartmentofBiologicalSciences,UniversityofLethbridge,Lethbridge,AB,Canada EpigeneticinformationcanbepassedonfromonegenerationtoanotherviaDNAmethylation,histonemodifications,andchangesinsmallRNAs,aprocesscalledepigeneticmemory.Duringamammal’slifecycleepigeneticreprogramming,ortheresettingofmostepigeneticmarks,occurstwice.Thefirstinstanceofreprogrammingoccursinprimordialgermcellsandthesecondoccursfollowingfertilization.Theseprocessesmaybebothpassiveandactive.Inorderforepigeneticinheritancetooccurtheepigeneticmodificationsmustbeabletoescapereprogramming.Thereareseveralexamplessupportingthisnon-Mendelianmechanismofinheritanceincludingtheprepackingofearlydevelopmentalgenesinhistonesinsteadofprotaminesinsperm,genomicimprintingviamethylationmarks,theretentionofCenH3inmammalianspermandtheinheritanceofpiwi-associatedinterferingRNAs.TheabilityofmammalstopassonepigeneticinformationtotheirprogenyprovidesclearevidencethatinheritanceisnotrestrictedtoDNAsequenceandepigeneticsplaysakeyroleinproducingviableoffspring. Introduction Epigeneticinformationisencryptedingeneticsequences,andincludesDNAmethylation,histonemodificationsandsmallRNAchanges(Bonasioetal.,2010).Epigeneticmemoryistheabilitytotransferepigeneticinformationfromonegenerationtothenext.Epigeneticinformationusespatternsofinheritance,whicharenotdeterminedbyDNAsequencealoneandmayresultinanepigeneticmemory,whichlikegeneticmemorycanbestablyinheritedandpassedontoprogenythroughmeiosis,althoughepigeneticinheritancemainlydefiesMendelianlaws. Epigeneticmodificationscanalsobepropagatedthroughmitoticcelldivisions,aprocessreferredtoasepigeneticsomaticinheritance,resultinginsomaticgeneticmemories.Bothmitoticandmeioticepigeneticinheritanceinvolvesimilarepigeneticmechanismssuchashistonesandmethylationpatterns.Howeverthefocusofthisreviewwillbeonaprocessthatislesscommon:transgenerationalinheritanceofepigeneticstatesthroughmeioticcelldivisions(WhitelawandWhitelaw,2008). Inmostcellsinthebodyepigeneticmarksarerelativelystableandbecomefixedoncecellsdifferentiateorexitthecellcycle(Morganetal.,2005).Howevertherearetwokeysituationsinwhichepigeneticinformationundergoesextensivereprogrammingandbecomeserasedandre-established(Fengetal.,2010). Epigeneticreprogramming,whichoccursfirstduringgametogenesisandagaininearlyembryogenesis,areeventscharacterizedbyextensive,changesinDNAmethylationandchromatinmodifications(Reik,2007;LangeandSchneider,2010).Duringearlystagesofdevelopment,genesthatarerequiredlaterindevelopmentaretemporarilyrepressedusingreversiblehistonemodifications,whichallowfortheexpressionofthesegeneswhenneeded(Reik,2007).Thesecellsareinapluripotentstatethatgivesthemtheabilitytodifferentiateintomanydifferentcelltypesastheydevelopandtheirfatebecomesmoredefinedandrestricted(Reik,2007). EpigeneticReprogramming Thegermlinecellsareuniqueastheyundergomeiosisandarealoneresponsiblefortransmittingbothgenetic,andpossiblyepigenetic,informationtothenextgeneration(CombesandWhitelaw,2010).Howevermajorreprogrammingtakesplaceinprimordialgermcells(PGCs)inwhichparentalimprintsareerasedandtotipotencyisrestored. PrimordialgermcellsarederivedfromepiblastcellsandinfemalestheyentermeioticarrestinprophaseofmeiosisI,whileinmalesPGCsentermitoticarrestuntilaboutbirthwhenmitosisofspermatogonialstemcellsbegins(Morganetal.,2005).EarlyPGCshaveepigeneticmarkssimilartosomaticcells,forexamplerandomXchromosomeinactivationandimprintedgenes.Insomaticcellsimprintedgenesarethosewhereonlyasinglecopyisexpressedaccordingtotheparent-of-origin.HoweverinE11.5(embryonicday)PGCsaswellaslaterstagegermcells,thesesamegenesareexpressedbiallelically,indicatingthattheimprintshavebeenlargelyerasedfromthegametesbythisstage–asignofepigeneticreprogramming.Usingflowcytometryonallele-specificexpressionofimprintedgenesithasbeendeterminethattheimprintsarelargelyintactinmigratingPGCs;outoffourimprintedgenesanalyzed,Snrpn,Igf2,H19wereexpressedmonoallelically,whereasIgf2rwasexpressedbiallelically(Szaboetal.,2002). HoweverbythetimethePGCsarriveatthegenitalridges,showninFigure1,imprintedgenesarebiallelicallyexpressedandmethylationinimprintedgenesandsinglecopygeneshasbeenerased,withthemajorityofthisdemethylationoccurringbetweenE11.5andE12.5(Hajkovaetal.,2008).InadditiontotherapidlossofDNAmethylation,severalotherreprogrammingactivitiesareknowntooccurincludingthelossoftherepressivedimethylationofhistone3lysine9andmanyhistonemodifications(Hajkovaetal.,2008).TheextentofdemethylationoverthesefewcellcyclesoccursdespitethepresenceofDnmt1,aDNAmethyltransferase,inthenucleusofthePGCsindicatingthatthisprocessislikelytobeactiveanditcompletesthefirstroundofreprogramming(Hajkovaetal.,2008).Itisimportanttonotehowever,thatreprogrammingdoesnotoccuruniversallyandcertainepigeneticmarkspresentarenoterased(Hajkovaetal.,2008).Howeverithasbeendeterminedthatfollowingthefirstroundofreprogramming,malePGCshaveapproximately60%lessmethylation,whilefemalePGCshaveapproximately70%less(Poppetal.,2010). FIGURE1 Figure1.Thefirstroundofepigeneticreprogrammingoccursinthegermlineaccordingtoembryonicday(E)ofmammalspriortothematurityofgametesandisdistinctinmalesandfemales. DNAremethylationoccursseveraldayslaterinthemalegermline,betweenE15–E16.Howeverinthefemalegermlinetheprocessoccurspostnatallyduringoocytegrowth(SasakiandMatsui,2008).TheresultingDNAhasasimilarmethylationstatetosomaticcells,althoughthereareimportantdifferencesincludingmethylationpatternsatregionswhichparentalimprintingcontrolregions(ICRs;Hembergeretal.,2009). Thesecondtimereprogrammingoccursisfollowingfertilization,showninFigure2.Fertilizationoccursbetweentwoparentalgenomesofextremelydifferentepigeneticstates,duetothefactthattheyareindifferentpartsofthecellcycle,andconsiderableprocessinghastooccurbeforetheyareabletobegintranscriptionandcelldivision.Thematernalgenomeispackagedinabundantlymodifiedhistones,whilethepaternalgenomeisdenselywrappedinhistonesubstitutescalledprotamines(Purietal.,2010).Itisimportanttonotehoweverthatalthoughthemajorityofhistonesarereplacedduringspermatogenesis,thetransitionisnotcomplete(Purietal.,2010). FIGURE2 Figure2.Secondroundofepigeneticreprogrammingoccursfollowingfertilizationsequentiallyfromzygotetomorulastage,remethylationoccursonlyininnercellmass,themassofcellswhicheventuallygivesrisetothedefinitivestructuresofthefetus. Whenthespermarrivesinthecytoplasmoftheoocyte,aspecializedcellwithconsiderablechromatinremodelingcapacity,thepaternalgenomeisreorganized.Thepaternalgenomeisfirstdecondensedandthenrepackagedintonucleosomescontaininghistoneoctamers(Purietal.,2010).Thereplacementofprotamineswithhistonesgenerateschromatinstructuresthatarecompatiblewithsomaticcellularprocesses,suchastranscription(OoiandHenikoff,2007).Chromatinremodelingofthepaternalgenomeoccursextremelyquickly,withstudiesdoneinmiceindicatingthatmostprotamineswereremovedwithin30minfollowingfertilization(vanderHeijdenetal.,2005).Thehistones,whichassociatewiththepaternalchromatin,aremoreacetylatedthanthosealreadypresentinthematernalchromatin(LangeandSchneider,2010).Itispossiblethisimbalanceispassiveduetothepresenceofnon-acetylatedhistonesfromoocytecytoplasm.Itmayalsobeanactiveprocessofincorporationofaparticularacetylatedhistonevariant(Morganetal.,2005).AtthispointthepaternalDNAundergoesDNAdemethylationofalmosttheentiregenome,erasingmostoftheepigeneticmarks.Thedemethylationprocessoccurswithinthefirst12–24hfollowingfertilizationandisthoughttobeactivebecauseitoccursbeforeDNAreplicationbeginsinthepaternalpronucleus(Morganetal.,2005;CombesandWhitelaw,2010).HoweveritisnotknownifthereareanyphysicalsimilaritiesbetweenreprogramminginPGCsandreprogramminginzygotes. Thespermactivatestheoocyteandthematernalgenomecompletesitssecondmeioticdivision(DengandLi,2009).Duringthefirstcleavagedivisioninthezygote,thematernalandpaternalgenomescombinesoeachofthedaughterblastomeresinheritsnearlyequalamountsofmethylatedDNA.DNAmethylationlevelsofthematernalgenomeinblastomeressequentiallydecreaseduringeachsubsequentdivision.Reprogrammingintheearlyembryoseemstobesusceptibletoenvironmentalstress,anderrorsinthisspatiallyandtemporallyhighlycoordinatedprocessprovideoneimportantexplanationforthehighrateofembryolossafterfertilization(Haaf,2006). Likepaternaldemethylation,maternaldemethylationleavescertainsitesuntouchedandisaccompaniedbyhistonemodifications.DNAmethyltransferasesDnmt1,Dnmt3a,andDnmt3bareenzymesthatcatalyzethetransferofamethylgrouptoDNA,therebymaintainingthemethylationofCpGsiteswithinthemammaliangenomewhichallowsforgenesilencingaswellasmaintaininggenomeintegrity(Tsumuraetal.,2006).Dnmt1isthemostabundantkeymaintenancemethyltransferasewhichoperatesatunmethylatedcytosinesofhemimethylatedCpGsafterDNAreplication(Xuetal.,2010).Dnmt3aandDnmt3bareresponsiblefordenovomethylationatunmethylatedCpGs,methylationofsatelliterepeats,methylationofH19andDlk1/Gtl2differentiallymethylatedregions(DMRs)andmethylationofimprintedgeneRasgrf1(Katoetal.,2007;TaberlayandJones,2011). Duringmaternaldemethylation,Dnmt1isexcludedfromthenucleusandsoitisthoughtthatthisresultsinfailuretomaintainDNAmethylationpatternsduringreplication.Thesestepsallowthecellstogainpluripotencyandoccurduringthetransitionfromthe1-cellzygotetothe16-cellmorulastage,withtheembryoremainingthesamesizedespitecelldivision(SasakiandMatsui,2008;CombesandWhitelaw,2010).IncomparisontoreprogramminginPGCs,parent-specificDNAmethylationismaintainedinICRsduringthisstage(EdwardsandFerguson-Smith,2007). Theprocessofepigeneticreprogrammingpreparestheearlyembryoforthedevelopmentofthefirsttwocelllineagesintheblastocyst,resultingindifferentiationbetweenthetrophoectodermandtheinnercellmassfromthemorula,adecisioninvolvingtherelationshipbetweenepigeneticmarks,cellpositionandtranscriptionfactors(CombesandWhitelaw,2010).Notlongafterimplantation,denovogenome-wideDNAmethylationoccursincellscommittedtotheinnercellmassandiscarriedoutbyDnmt3aandDnmt3b(Watanabeetal.,2004).Thisresultsinanepigeneticasymmetrybetweenthetrophoectodermandtheinnercellmass,whichisthoughttooccurduetothefactthattheinnercellmasswillgiverisetotheembryonicandadulttissues,whereasthetrophectodermformstissueswhicharediscardedatbirth(Hembergeretal.,2009). TheseDNAmethylationpatternsaremaintainedfromthatpointonthroughmitoticdivisionsinsomaticlineages(Bonasioetal.,2010).Bothstagesofreprogrammingplayakeyroleintheremovalofinheritedepigeneticmodifications,buttheydonotremoveallepigeneticmarks. EscapingReprogramming:EpigeneticInheritance Iftheentiregenomewerereprogrammedinthegermlineitwouldbeimpossibleforepigeneticmodificationstobeinherited.Howeverthereareepigeneticmarkersthatcanescapebothincidencesofreprogrammingresultinginepigeneticmodificationsthatpersistinthesomaticcellsoftheindividual. Protamine-MediatedInheritance Onewaythatepigeneticinheritancecanhappenisduetotheincompletereplacementofhistonesbysmallbasicnuclearproteinscalledprotaminesduringgametogenesis.AlthoughthemajorityofDNAinspermchromatinisboundbyprotamines,asmallpercentage,only1%inmiceforexample,remainsboundtonucleosomes(WykesandKrawetz,2003).Inhumans4%(Hammoudetal.,2009)to15%(Gatewoodetal.,1987)ofhistonesareretainedinmaturesperm.Studiesindicatethattheremaininghistonesarenotspreadrandomlyacrossthegenome,andinfacttheirretentionmaybelinkedtogeneactivityindevelopmentalprocesses(Hammoudetal.,2009). Apossiblereasonfortheprepackingofveryearlydevelopmentalgeneswithhistonesinspermisthatitmightpreventtheirtightcondensationbyprotaminesandthusfacilitatetheirtranscriptioninearlyembryos(OoiandHenikoff,2007).AstudybyGardiner-Gardenetal.(1998),detectedhistone-associatedregionsinεandγ-globingeneinhumans,bothofwhicharetranscribedinprimitiveerythroblastsintheembryonicyolksac.Theembryonicyolksacdifferentiatesat3weeksofgestation,andγ-globinpredominatesduringthefetalperiod(Gardiner-Gardenetal.,1998).Incontrast,nohistone-associatedregionswerefoundatβ-globingenes,whichhaveaminorinvolvementinthefetusbutpredominantafterbirthinbonemarrow,orδ-globin,whichisproducedafterbirth(Peschleetal.,1985;Gardiner-Gardenetal.,1998).Theresultsindicatethatthepresenceofhistonesmaymarkεandγ-globingenesforearlyexpressionintheembryo(Gatewoodetal.,1987;Gardiner-Gardenetal.,1998).Thereisalsoevidenceofenrichmentofhistonesnotonlyatgenesexpressedduringearlyembryogenesisbutalsoatspecificimprintedgenes.Histonesareidealcandidatesforthismethodofinheritanceduetotheirinfluenceonchromatinstructure,whichregulatesaccesstogenes(OoiandHenikoff,2007). Methylation-MediatedInheritance WhenglobalgenomereprogrammingoccursinPGCs,intracisternalAparticles(IAPs)arealmostentirelyprotectedfromdemethylationresultinginasignificantfractionofallIAPsnotbeingreprogrammed(Poppetal.,2010).Inthesecondroundofreprogrammingfollowingfertilization,IAPsaswellascertainimprintedgenesareonceagainleftuntouched(Fengetal.,2010).ThesharedprotectionfromreprogrammingmayexplaintheparentalimprintingeffectsobservedwithsomeendogenousgeneswhoseexpressionareinfluencedbynearbyIAPinsertions(Rakyanetal.,2002).ThemostfamousexampleofepigeneticinheritancecomesfromexperimentsfocusingonagoutilocusinmicethatcontrolscoatcolorandislinkedtotheepigeneticstatusofIAPs(Morganetal.,1999). InmicewhichcarrytheAvyallele,anIAPretrotransposonhasinsertedupstreamoftheagoutilocusresultinginectopicexpressionofagoutiprotein,resultinginhairfolliclemelanocytestoswitchfromthesynthesisofblacktoyellow(Morganetal.,1999).Inadditiontotheyellowcoatcolor,expressionoftheagoutigenesislinkedtoobesity,diabetes,andincreasedsusceptibilitytotumors(Yenetal.,1994).Avy/AmicethathaveahypomethylatedAvyepiallelehaveayellowcoat,indicatingthatAvyisdominantoverA.HoweverwhenthecrypticpromoterintheIAPissilenced,suchasinthecaseofhypermethylation,Avy/Amicehaveablackcoatbecausetheepialleleissilenced,essentiallyrevertingtowildtype,andinthiscaseAvyisnolongerdominantresultinginmiceindistinguishablefromwild-type(A/A)mice. Thelinkagebetweenthecoatcolorofthemotherandoffsringisduetotheincompleteerasureofanepigeneticmarkinthefemalegermline,andisaclearexampleofepigeneticinheritance;howevertheprobabilityofpassingonanepialleletothenextgenerationisnever100%(Morganetal.,1999).TheinsertionofIAPthatactsasacontrollingelementofagoutiexpressionhasanunstableepigeneticstateinthegermline,resultinginthepossibilityofdifferentcellsofthesameindividualhavingadifferentepigeneticstatus(Morganetal.,1999;Cropleyetal.,2010). InadditiontoIAPs,imprintedgenesoccasionallyevadethesecondroundofreprogrammingandremainmarkedwiththeirparentalorigin(Bartolomei,2009).Imprintsestablishedinthegermlinemustnotonlyescapereprogrammingbutalsothesubsequentwaveofdenovomethylationthatoccurs(Morganetal.,2005).DMRsarecrucialtoescapingreprogrammingandmanyimprintedgenescontainthem.DMRsindicateareasthataremethylateddifferentlybetweenparentalallelesandsome,whicharederivedfromspermandoocytes,behaveasICRs(Morganetal.,2005).Genomicimprintsaremaintainedthroughcis-andtrans-actingDNA-bindingfactorsthatspecificallyrecognizeICRsandpreventactivedemethylationonthepaternallymethylatedallelesaswellasfacilitatingmaintenanceDNAmethylationanddifferentialchromatinmodifications(Bartolomei,2009).OnesuchfactorsistheproteinPGC7/STELLA.AlthoughStella-/-eggshavenormalICRmethylation,zygotesderivedfromtheseeggsarehypomethylatedatmultiplelociwithmaternallyorpaternallymethylatedICRs.ZygotesalsoexhibitprematuregloballossofDNAmethylationonthematernalpronucleus,indicatingthatthematernalgenomemustbeprotectedfromthedemethylationthatoccursonthepaternalgenomeimmediatelyafterfertilization,andthattheroleforSTELLAiswidespread(Nakamuraetal.,2007).Otherproteinshavebeenidentifiedthatareinvolvedinthestabilityofimprints,suchasRBBP1/ARID4AandRBBP1L1/ARID4B,whichareinvolvedinthemaintenanceofimprintingattheSnprnlocus(Wuetal.,2006;Bartolomei,2009). EpigeneticmarksmustalsobeplacedonnewlyreplicatedDNAinorderforepigeneticinheritancetooccur.Dnmt1whichhasanestablishedroleinmaintainingDNAmethylationatimprintedlociispresentinpre-implantationembryos(Lietal.,1993).AlthoughthedetailsareunclearitislikelyDNAmethylationismaintainedthroughacombinationoftheoocyte-specificformofDnmt1andthesomaticform,whichistheformoftheenzymemostoftenobservedinmammaliancells(Hirasawaetal.,2008).AstudyusingH19andDlk1/Gtl2lociindicatedthatneitherDnmt3anorDnmt3barerequiredforthemaintenanceofmethylationimprintsandindicatedthatDnmt1ofboththematernalandzygoticisoformsissufficientformaintainingmethylationimprintsinthepre-implantationembryo(Hirasawaetal.,2008). OnewidelystudiedimprintedgeneinmiceisthematernallyexpressedH19gene.ThemousegeneH19encodesanRNAthatishighlyexpressedinembryonictissuesofendodermalandmesodermalorigin,andhasbeenshowntobehypermethylatedontheinactivatedpaternalalleleinbothsomatictissuesandsperm.Thedifferentialmethylationpresentinthesomatictissuesisinheritedfromthegametesandpreservedthroughembryogenesis,providinganexampleofepigeneticmemory(Tremblayetal.,1997).H19hasalsobeenshowntobehypermethylatedonthepaternalalleleinhumans,indicatingthatitisalsoescapingreprogramming. Histone-MediatedInheritance TheinheritanceofhistonemarksandofsmallRNAspotentiallythroughbothoocyteandspermmightalsocontributetoepigeneticinheritanceaswellastoreprogrammingacrossgenerations.HistonesarethearchitecturalproteinsthatpackageDNAintonucleosomalparticles(Heinkoffetal.,2004).Modifiedhistonevariantsmaycontrolbarrierstotranscriptionandperpetuateactivechromatinstates.Specifically,histoneretentioninmaturespermaspreviouslydescribed,occursnon-randomlyresultinginthespermgenomeprovidingnotonlypaternalDNAsequence,butalsomolecularregulatoryfactors.Ifcertainhistoneswerenotprotectedfrombecomingprotamines,theembryowouldnotbeabletodevelopcorrectly(OoiandHenikoff,2007;Ward,2010). AnotherexampleofepigeneticmemorymediatedbyhistonesisbytheretentionofCENP-A(mammalianspecialhistoneH3variant,CenH3)inmammaliansperm(Palmeretal.,1990).TheroleofCenH3inhighereukaryotesistoassemblecentromericnucleosomes,wherebydeterminingthelocationofthecentromere(Amoretal.,2004).CENP-A,thecorehistonethatreplacesordinaryhistoneH3incentromericnucleosomes,iscrucialbecauseitensurescorrectchromosomesegregationduringmitosisandmeiosis(Dubinetal.,2010).Inthisway,centromereidentitydoesnotrelyonDNAsequencebutratheronthepresenceofCenH3,whichhasbeenproposedtoactasanepigeneticmarkofthecentromere(Bernadetal.,2009).IncorporationofnewlysynthesizedCENP-Ahasbeenshowntooccurintelophase/earlyG1inhumancells(Dala,2009).KinetochorelocationisspecifiedwhenCenH3-containingnucleosomesaresegregatedbetweendaughterDNAstrandswhichalsocontainnewnucleosomeswhichdonotcontainCenH3.TheresultisadilutionoftheCenH3signalduringSphasewhichleadstoaself-propagatingepigeneticstate,markingcentromerelocationaswellasprovidingtheoptimalchromatinconfigurationforkinetochoreformationandfunction(Gienietal.,2008). SmallRNA-MediatedInheritance Inadditiontochromatinmarkslikehistones,itisalsopossibleforepigeneticinheritancetooccurbyinheritanceofcertainRNAsthroughthegermline.Inmammals,piwi-associatedinterferingRNAs(piRNAs)associatewithananimal-specificsubclassofArgonautefamilycalledpiwiproteins(Houwingetal.,2007).piRNAsarespecificallyexpressedingermcellsandareimportantfortheirpropermaturation(Kim,2006).Transposonsarenucleicacidparasitesthatareabletobothmoveandpropagatewithinahostgenome(Kazazian,2004).piRNAsare24–30nucleotidesinlength,andareresponsiblefortransposonsilencinginanimals(GirardandHannon,2007;Brenneckeetal.,2008).Approximately50%ofthegenomeinhumansismadeupoftransposonsandderepressionoftransposableelementsoccursduringepigeneticreprogramming,sopiRNAsarecrucialdefensewhichensurethattransposonsarenotreactivated(Bernsteinetal.,2007;Aravinetal.,2009).Reactivationoftransposonsingermcellshasextremelynegativeeffects,asduetotheirhighcopynumbersaswellastheirabilitytomovearoundthegenome,activetransposonshavethepotentialtobehighlydisruptivetotheirhost,sinceintegrationofatransposonnearorinagenecandisturbitscodingsequencesorexpressionpatternresultinginsterility(Aravinetal.,2009).InheritanceofpiRNAsplaysanimportantroleofdefendingthegermlineagainstactivationoftransposons(O’DonnellandBoeke,2007). PreviousresearchontheinheritanceofepigeneticmemorythroughpiRNAshasbeendoneinDrosophila.Whenfemalefliesthathavenotbeenexposedtoaspecificactivetransposonarecrossedwithmalesthathaveestablishedcontroloverthesameactivetransposon,theresultissterilityintheprogeny,aphenomenonreferredtoashybriddysgenesis.Howeverthereversecrossdoesnotresultinthesameoutcome,supportingthehypothesisthatmaternallydepositedpiRNAsareresponsibleforepigeneticinheritanceinthisinstance(Brenneckeetal.,2008).piRNAshavealsobeenshowntoberequiredformalefertilityinmice,withmutationsresultinginthedegenerationofthegermlinebutsomaticcellsappearingtoberelativelyunaffected(Costa,2008;KlattenhoffandTheurkauf,2008).Inaddition,recentresearchinaChinesepopulationhasindicatedthatgeneticvariantsinpiRNAsconfersusceptibilitytospermatogenicfailure.ThisresearchindicatesthatpiRNAs,althoughnon-coding,haveanimportantroleinsuccessfuldevelopmentaswellastheabilitytobepassedonthroughepigeneticinheritanceinhumans(JablonkaandRaz,2009;Guetal.,2010).Thus,piRNAsareacrucialformofepigeneticmemoryduetotheirabilitytosilencetransposons,thusensuringthefertilityoftheprogeny(Aravinetal.,2009). EpigeneticInheritanceofDisease Epigeneticmemoryisnotonlycrucialtoinsuringviableoffspring,buthasalreadybeguntobeshowntoplayanimportantroleindisease.Specifically,transgenerationalepigeneticinheritanceinhumansmayresultinillnessesduetoepimutationsthatarethenpassedontoprogeny.Oneknowncaseofepigeneticinheritanceisincreasedriskforhereditarynon-polyposiscolorectalcancer(HNPCC),asporadiccolorectalcancerwithmismatchrepairdeficiencyduetogeneticmutationsorhypermethylation,particularlyofMLH1orMSH2mismatchrepairgenes(Hitchinsetal.,2005;Hessonetal.,2010).MutationsofMSH6,PMS2,andMLH3havealsobeenassociatedwithasmallnumberofHNPCCcases(Rustgi,2007). HypermethylationofMLH1isnotlimitedtoneoplasticcellsbutcanalsooriginatefromthegermlineandthereforebecomewidespreadinnormalsomaticcells(Hitchinsetal.,2007).IndividualspossessingthegermlineepimutationhaveonlyonefunctionalcopyoftheMLH1genefromtheconceptionandthereforecancerstypicalofHNPCCsyndromedevelop(Hessonetal.,2010).ThepresenceofanMLH1epimutationinthegermlineindicatespotentialforinheritancefromparentstoprogeny.Studieshaveindicatedthatsuchinheritanceispossible,withonefamilyshowingmaternaltransmissionoftheepimutationtotheson,althoughthemutationwaserasedinhisspermatozoa.Inthiscase,theMLH1epimutationthatcausedapredispositiontoHNPCCinthemotherwasalsopresentintheson,indicatinghealsohadanincreasedriskofcancer.However,inherotherchildrentheepimutationwasshowntoreverttoitsnormalstate,indicatingthatthemutationwaserasedduringreprogramming.TheseresultsindicatedthatgermlinetransmissionofanepigeneticstatethatconfersdiseasesusceptibilitysuchasinthecaseofhypermethylationofMLH1ispossible.Overall,studiesthusfarhaveindicatedthatalthoughepimutationsareusuallyerasedinthegermline,theymayberetainedatalowfrequency(Hitchinsetal.,2007). InadditiontoHNPCC,Prader–Willi(PWS)andAngelman(AS)syndromesmaybecausedbyepigeneticinheritanceincertainsubgroupsofpatients.Bothdiseasesarecausedbythelossoffunctionofimprintedgenes15q11-q13inhumans.AnalysisofpatientswithPWSandAShasindicatedthatalthoughinsomecasesthedefectiscausedbyanimprintingcenter(IC)deletion;outof51patientswithPWS32didnothavethedeletion,andoutof85patientswithAS,66patientsdidnothaveanymutationinICelementseither.However,in27%ofcaseswhenthepatienthadASitwasindicatedthattherewasanimprintingdefectwhichoccurredafterfertilization.InsuchASpatientstheimprintingdefectoccurredonthechromosomeinheritedfromeithermaternalorpaternalgrandmother,whileinPWSpatientsitoccurredinthechromosomeinheritedfromthepaternalgrandmother.Theseresultsindicatethatthefailuretoerasethematernalimprintduringspermatogenesis,orincompletereprogramming,isonecauseofbothPWSandAS(Buitingetal.,2003;Santos-ReboucasandPimentel,2007). Hereditarynon-polyposiscolorectalcancer,PWS,andAShaveallshownthatepigeneticinheritancemayalsobepartiallyresponsiblefortheinheritanceofdisease.Althoughhumanstudiesarestillverylimited,theyhavealreadybeguntoshowtheimportanceofepigeneticmemorywhenitcomestoillnessanddisease(Gluckmanetal.,2007). ConcludingRemarks Therearemanywaysforamammaltopassonepigeneticmarks,suchasinheritanceofhistones,piRNAs,andmethylationsignatures.Whatalloftheseepigeneticmodificationswiththeabilitytobepassedonasepigeneticmemoryhaveincommonistheirimportanceinmaintainingproperdevelopmentandtheirabilitytoescapethenear-genome-widereprogrammingthatoccursinthegermline.Epigeneticinheritanceplaysacrucialroleinimportantfunctionssuchasexpressionofgenesinearlyembryodevelopment,imprinting,andthesilencingoftransposonsandwithoutepigeneticfactorslikeDNAmethylationandhistonemodificationsdevelopmentcannotproceed.Epigeneticinheritancemayalsoleadtoinheritanceofepimutations,whichincreaseriskofdisease.AswebegintounderstandtheimportanceofepigeneticmemoryitquicklybecomesclearthatinheritanceisnotsimplyamatterofMendeliangeneticsandthatourunderstandingofthecomplexinteractionswhichgeneratelifearefarfromcomplete. ConflictofInterestStatement Theauthorsdeclarethattheresearchwasconductedintheabsenceofanycommercialorfinancialrelationshipsthatcouldbeconstruedasapotentialconflictofinterest. References Amor,D.J.,Kalitsis,P.,Sumer,H.,andChoo,A.(2004).Buildingthecentromere:fromfoundationproteinsto3Dorganization.TrendsCellBiol.14,359–368. PubmedAbstract|PubmedFullText|CrossRefFullText Aravin,A.A.,vanderHeijden,G.W.,Castaneda,J.,Vagin,V.V.,Hannon,G.J.,andBortvin,A.(2009).CytoplasmiccompartmentalizationofthefetalpiRNApathwayinmice.PLoSGenet.5,e10000764.doi:10.1371/journal.pgen.1000764 CrossRefFullText Bartolomei,M.S.(2009).Genomicimprinting:employingandavoidingepigeneticprocesses.GenesDev.23,2124–2133. PubmedAbstract|PubmedFullText|CrossRefFullText Bernad,R.,Sanchez,P.,andLosada,A.(2009).EpigeneticspecificationofcentromeresbyCENP-A.Exp.CellRes.315,3233–3241. PubmedAbstract|PubmedFullText|CrossRefFullText Bernstein,B.E.,Meissner,A.,andLander,E.S.(2007).Themammalianepigenome.Cell23,669–681. CrossRefFullText Bonasio,R.,Tu,S.,andReinberg,D.(2010).Molecularsignalsofepigeneticstates.Science330,612–616. PubmedAbstract|PubmedFullText|CrossRefFullText Brennecke,J.,Malone,C.D.,Aravin,A.A.,Sachidanandam,R.,Stark,A.,andHannon,G.J.(2008).AnepigeneticroleformaternallyinheritedpiRNAsintransposonsilencing.Science322,1387–1392. PubmedAbstract|PubmedFullText|CrossRefFullText Buiting,K.,Grob,S.,Lich,C.,Gillessen-Kaesbach,El-Maarri,O.,andHorsthemke,B.(2003).EpimutationsinPrader-WilliandAngelman withanimprintingdefect.Am.J.Hum.Genet.72,571–577. PubmedAbstract|PubmedFullText Combes,A.,andWhitelaw,E.(2010).Epigeneticreprogramming:enforcerorenablerofdevelopmentalfate?Dev.GrowthDiffer.52,483–491. PubmedAbstract|PubmedFullText|CrossRefFullText Costa,F.F.(2008).Non-codingRNAs,epigeneticsandcomplexity.Gene410,9–17. PubmedAbstract|PubmedFullText|CrossRefFullText Cropley,J.E.,Suter,C.M.,Beckman,K.B.,andMartin,D.I.K.(2010).CpGmethylationofasilentcontrollingelementinthemurineavyalleleisincompleteandunresponsivetomethyldonorsupplementation.PLoSONE5,e9055.doi:10.1371/journal.pone.0009055 CrossRefFullText Dala,Y.(2009).Epigeneticspecificationofcentromeres.Biochem.CellBiol.87,273–282. PubmedAbstract|PubmedFullText Deng,M.,andLi,R.(2009).Spermchromatin-inducedectopicpolarbodyextrusioninmouseeggsafterICSIanddelayedeggactivation.PLoSONE4,e7171.doi:10.1371/journal.pone.0007171 CrossRefFullText Dubin,M.,Fuchs,J.,Graf,R.,Schubert,I.,andNellen,W.(2010).DynamicsofanovelcentromerichistonevariantCenH3revealstheevolutionaryancestraltimingofcentromerebiogenesis.NucleicAcidsRes.1–12. Edwards,C.A.,andFerguson-Smith,A.C.(2007).Mechanismsregulatingimprintedgenesinclusters.Curr.Opin.CellBiol.19,281–289. PubmedAbstract|PubmedFullText Feng,S.,Jacobsen,S.E.,andReik,W.(2010).Epigeneticreprogramminginplantandanimaldevelopment.Science330,622–627. PubmedAbstract|PubmedFullText|CrossRefFullText Gardiner-Garden,M.,Ballesteros,M.G.,andTam,P.P.L.(1998).Histone-andProtamine-DNAAssociation:conservationofdifferentpatternswithinB-globindomaininhumansperm.Mol.Cell.Biol.18,3350–3356. PubmedAbstract|PubmedFullText Gatewood,J.M.,Cook,G.R.,Balhorn,R.,Bradbury,E.M.,andSchmid,C.W.(1987).Sequence-specificpackagingofDNAinhumanspermchromatin.Science236,962–964. PubmedAbstract|PubmedFullText|CrossRefFullText Gieni,R.S.,Chan,G.K.T.,andHenzel,M.J.(2008).Epigeneticsregulatecentromereformationandkinetochorefunction.J.Cell.Biochem.104,2027–2039. PubmedAbstract|PubmedFullText Girard,A.,andHannon,G.J.(2007).Conservedthemesinsmall-RNA-mediatedtransposoncontrol.TrendsCellBiol.18,136–148. CrossRefFullText Gluckman,P.,Hanson,M.,andBeedle,A.(2007).Non-genomictransgenerationalinheritanceofdiseaserisk.Bioessays29,145–154. PubmedAbstract|PubmedFullText|CrossRefFullText Gu,A.,Ji,G.,Shi,X.,Long,Y.,Xia,Y.,Song,L.,Wang,S.,andWang,X.(2010).GeneticvariantsinPiwi-interactingRNApathwaygenesconfersusceptibilitytospermatogenicfailureinaChinesepopulation.Hum.Reprod.25,2955–2961. PubmedAbstract|PubmedFullText Haaf,T.(2006).Dynamicsintheearlymammalianembryo:implicationsofgenomereprogrammingdefectsfordevelopment.Curr.Top.Microbiol.Immunol.310,13–22. PubmedAbstract|PubmedFullText Hajkova,P.,Ancelin,K.,Waldmann,T.,Lacoste,N.,Lange,U.C.,Cesari,F.,Lee,C.,Almouzni,G.,Schneider,R.,andSurani,M.A.(2008).Chromatindynamicsduringepigeneticreprogramminginthemousegermline.Nature452,877–881. PubmedAbstract|PubmedFullText|CrossRefFullText Hammoud,S.S.,Nix,D.A.,Zhang,H.,Purwar,J.,Carrell,D.T.,andCairns,B.R.(2009).Distinctivechromatininhumanspermpackagesgenesforembryodevelopment.Nature460,473–478. PubmedAbstract|PubmedFullText Heinkoff,S.,Furuyama,T.,andAhmad,K.(2004).Histonevariants,nucleosomeassemblyandepigeneticinheritance.TrendsGenet.20,320–326. PubmedAbstract|PubmedFullText|CrossRefFullText Hemberger,M.,Dean,W.,andReik,W.(2009).Epigeneticdynamicsofstemcellsandcelllineagecommitment:diggingWaddington’scanal.Nat.Rev.Mol.CellBiol.10,526–537. PubmedAbstract|PubmedFullText Hesson,L.,Hitchins,M.,andWard,R.(2010).Epimutationsandcancerpredisposition:importanceandmechanisms.Curr.Opin.Genet.Dev.20,290–298. PubmedAbstract|PubmedFullText Hirasawa,R.,Chiba,H.,Kaneda,M.,Tajima,S.,Li,E.,Jaenisch,R.,andSasaki,H.(2008).MaternalandzygoticDnmt1arenecessaryandsufficientforthemaintenanceofDNAmethylationimprintsduringpreimplantationdevelopment.GenesDev.22,1607–1616. PubmedAbstract|PubmedFullText|CrossRefFullText Hitchins,M.,Williams,R.,Cheong,K.,Halani,N.,Lin,V.A.,Packham,D.,Ku,S.,Buckle,A.,Hawkins,N.,Burn,J.,Gallinger,S.,Goldblatt,J.,Kirk,J.,Tomlinson,I.,Scott,R.,Spigelman,A.,Suter,C.,Martin,D.,Suthers,G.,andWard,R.(2005).MLH1germlineepimutationsasafactorinhereditarynonpolyposiscolorectalcancer.Gastreoenterology129,1392–1399. CrossRefFullText Hitchins,M.,Wong,J.,Suthers,G.,Suter,C.,Martin,D.,Hawkins,N.,andWard,R.(2007).Inheritanceofacancer-associatedMLH1germ-lineepimutation.N.Engl.J.Med.356,697–705. PubmedAbstract|PubmedFullText Houwing,S.,Kamminga,L.M.,Berezikov,E.,Cronembold,D.,Girard,A.,vandenElse,H.,Filippov,D.V.,Blaser,H.,Raz,E.,Moens,C.B.,Plasterk,R.H.A.,Hannon,G.J.,Draper,B.W.,andKetting,R.F.(2007).AroleforPiwiandpiRNAsingermcellmaintenanceandtransposonsilencinginzebrafish.Cell129,69–82. PubmedAbstract|PubmedFullText|CrossRefFullText Jablonka,E.,andRaz,G.(2009).Transgenerationalepigeneticinheritance:prevalence,mechanisms,andimplicationsforthestudyofheredityandevolution.Q.Rev.Biol.84,131–176. PubmedAbstract|PubmedFullText Kato,Y.,Masahiro,K.,Hata,H.,Kumaki,K.,Hisano,M.,Kohara,Y.,Okano,M.,Li,E.,Nozaki,M.,andSasaki,H.(2007).RoleoftheDnmt3familyindenovomethylationofimprintedandrepetitivesequencesduringmalegermcelldevelopmentinthemouse.Hum.Mol.Genet.16,2272–2280. PubmedAbstract|PubmedFullText Kazazian,H.H.Jr.(2004).Mobileelements:driversofgenomeevolution.Science303,1626–1636. PubmedAbstract|PubmedFullText|CrossRefFullText Kim,V.N.(2006).SmallRNAsjustgotbigger:piwi-interactingRNAs(piRNAs)inmammaliantestes.GenesDev.20,1993–1997. PubmedAbstract|PubmedFullText|CrossRefFullText Klattenhoff,C.A.,andTheurkauf,W.E.(2008).BiogenesisandgermlinefunctionsofpiRNAs.Development135,3–9. PubmedAbstract|PubmedFullText|CrossRefFullText Lange,U.C.,andSchneider,R.(2010).Whatanepigenomeremembers.Bioessays32,659–668. PubmedAbstract|PubmedFullText|CrossRefFullText Li,E.,Beard,C.,andJaenisch,R.(1993).RoleforDNAmethylationingenomicimprinting.Nature366,362–365. PubmedAbstract|PubmedFullText|CrossRefFullText Morgan,H.D.,Santos,F.,Green,K.,Dean,W.,andReik,W.(2005).Epigeneticreprogramminginmammals.Hum.Mol.Genet.14(Suppl1),R47–R58. PubmedAbstract|PubmedFullText Morgan,H.D.,Sutherland,H.G.,Martin,D.I.K.,andWhitelaw,E.(1999).Epigeneticinheritanceattheagoutilocusinthemouse.Nat.Genet.23,314–318. PubmedAbstract|PubmedFullText Nakamura,T.,Arai,Y.,Umebara,H.,Masuhara,M.,Kimura,T.,Taniguchi,H.,Sekimoto,T.,Ikawa,M.,Yoneda,Y.,Okabe,Tanaka,S.,Shiota,K.,andNakano,T.(2007).PGC7/StellaprotectsagainstDNAdemethylationinearlyembryogenesis.Nat.CellBiol.9,64–71. PubmedAbstract|PubmedFullText O’Donnell,K.A.,andBoeke,J.D.(2007).Mightypiwisdefendthegermlineagainstgenomeintruders.Cell129,37–44. PubmedAbstract|PubmedFullText Ooi,S.L.,andHenikoff,S.(2007).Germlinehistonedynamicsandepigenetics.Curr.Opin.CellBiol.19,257–265. PubmedAbstract|PubmedFullText Palmer,D.K.,O’Day,K.,andMargolis,R.L.(1990).ThecentromerespecifichistoneCENP-Aisselectivelyretainedindiscretelociinmammalianspermnuclei.Chromosoma100,32–36. PubmedAbstract|PubmedFullText Peschle,C.,Mavillo,F.,Care,A.,Migliaccio,G.,Migliaccio,A.R.,Salvo,G.,Samoggia,P.,Petti,S.,Guerriero,R.,Marinucci,M.,Lazzaro,D.,Russo,G.,andMastroberardino,G.(1985).Haemoglobinswitchinginhumanembryos:asynchronyofzeta-alphaandepsilon-gamma-globinswitchesinprimitiveanddefiniteerythropoieticlineage.Nature313,235–238. PubmedAbstract|PubmedFullText|CrossRefFullText Popp,C.,Dean,W.,Feng,S.,Cokus,S.J.,Andrews,S.,Pellegrini,M.,Jacobsen,S.E.,andReik,W.(2010).Genome-wideerasureofDNAmethylationinmouseprimordialgermcellsisaffectedbyAIDdeficiency.Nature463,1101–1105. PubmedAbstract|PubmedFullText|CrossRefFullText Puri,D.,Dhawan,J.,andMishra,R.K.(2010).Thepaternalhiddenagenda:epigeneticinheritancethroughspermchromatin.Epigenetics5,386–391. PubmedAbstract|PubmedFullText|CrossRefFullText Rakyan,V.,Blewitt,M.E.,Drunker,R.,Preis,J.I.,andWhitelaw,E.(2002).Metastableepiallelesinmammals.TrendsGenet.18,348–351. PubmedAbstract|PubmedFullText|CrossRefFullText Reik,W.(2007).Stabilityandflexibilityofepigeneticgeneregulationinmammaliandevelopment.Nature447,425–432. PubmedAbstract|PubmedFullText|CrossRefFullText Rustgi,A.K.(2007).Thegeneticsofhereditarycoloncancer.GenesDev.21,2525–2538. PubmedAbstract|PubmedFullText|CrossRefFullText Santos-Reboucas,C.B.,andPimentel,M.M.G.(2007).Implicationofabnormalepigeneticpatternsforhumandiseases.Eur.J.Hum.Genet.15,10–17. PubmedAbstract|PubmedFullText Sasaki,H.,andMatsui,Y.(2008).Epigeneticeventsinmammaliangerm-celldevelopment:reprogrammingandbeyond.Nat.Rev.Genet.9,129–140. PubmedAbstract|PubmedFullText Szabo,P.E.,Hubner,K.,Scholer,H.,andMann,J.R.(2002).Allele-specificexpressionofimprintedgenesinmousemigratoryprimordialgermcells.Mech.Dev.115,157–160. PubmedAbstract|PubmedFullText Taberlay,P.C.,andJones,P.A.(2011).DNAmethylationandcancer.Prog.Drug.Res.67,1–23. PubmedAbstract|PubmedFullText Tremblay,K.D.,Duran,K.L.,andBartolomei,M.S.(1997).A5’2-kilobase-pairregionoftheimprintedmouseH19geneexhibitsexclusivepaternalmethylationthroughoutdevelopment.Mol.Cell.Biol.17,4322–4329. PubmedAbstract|PubmedFullText Tsumura,A.,Hayakawa,T.,Kumaki,Y.,Takebayashi,S.,Sakaue,M.,Matsuoka,C.,Shimotohno,K.,Ishikawa,F.,Li,E.,Ueda,H.R.,Nakayama,J.,andOkano,M.(2006).Maintenanceofself-renewalabilityofmouseembryonicstemcellsintheabsenceofDNAmethyltransferasesDnmt1,Dnmt3aandDnmt3b.GenesCells11,805–814. PubmedAbstract|PubmedFullText|CrossRefFullText vanderHeijden,M.S.,Brody,J.R.,Dezentje,D.A.,Gallmeier,E.,Cunningham,S.C.,Swartz,M.J.,DeMarzo,A.M.,Offerhaus,J.H.,Isacoff,W.H.,Hruban,R.H.,andKern,S.E.(2005).InvivotherapeuticresponsescontingentonFanconianemia/BRCA2statusofthetumor.Clin.CancerRes.11,7508–7515. PubmedAbstract|PubmedFullText|CrossRefFullText Ward,W.S.(2010).Functionofspermchromatinstructuralelementsinfertilizationanddevelopment.Mol.Hum.Reprod.16,30–36. PubmedAbstract|PubmedFullText Watanabe,D.,Suetake,I.,Tajima,S.,andHanaoka,K.(2004).ExpressionofDnmt3binmousehematopoieticprogenitorcellsandspermatogoniaatspecificstages.GeneExpr.Patterns5,43–49. PubmedAbstract|PubmedFullText|CrossRefFullText Whitelaw,N.C.,andWhitelaw,E.(2008).Transgenerationalepigeneticinheritanceinhealthanddisease.Curr.Opin.Genet.Dev.18,273–279. PubmedAbstract|PubmedFullText Wu,M.Y.,Tsai,T.F.,andBeaudet,A.L.(2006).DefiencyofRbbp1/Arid4baltersepigeneticmodificationsandsuppressesanimprintingdefectinthePWS/ASdomain.GenesDev.20,2859–2870. PubmedAbstract|PubmedFullText|CrossRefFullText Wykes,S.M.,andKrawetz,S.A.(2003).Thestructuralorganizationofspermchromatin.J.Biol.Chem.278,29471–29477. PubmedAbstract|PubmedFullText Xu,F.,Mao,C.,Ding,Y.,Rui,C.,Wu,L.,Shi,A.,Zhang,H.,Zhang,L.,andXu,Z.(2010).MolecularandenzymaticprofilesofmammalianDNAmethyltransferases:structuresandtargetsfordrugs.Curr.Med.Chem.17,4052–4071. PubmedAbstract|PubmedFullText Yen,T.T.,Gill,A.M.,Frigeri,L.G.,Barsh,G.S.,andWol,G.L.(1994).Obesity,diabetes,andneoplasiainyellowA(vy)-mice:ectopicexpressionoftheagoutigene.FASEBJ.8,479–488. PubmedAbstract|PubmedFullText Keywords:epigeneticmemory,germlinereprogramming,genomicimprinting,epigeneticinheritance,epigeneticdiseaseinheritance Citation:MigicovskyZandKovalchukI(2011)Epigeneticmemoryinmammals.Front.Gene.2:28.doi:10.3389/fgene.2011.00028 Received:28April2011; Paperpendingpublished:15May2011; Accepted:26May2011; Publishedonline:08June2011. Editedby: JaapJoles,UniversityMedicalCenterUtrecht,Netherlands Reviewedby: RachelGiles,UniversityMedicalCenterUtrecht,NetherlandsAlysonMacInnes,TheHubrechtInstituteofDevelopmentalBiologyandStemCellResearch,Netherlands Copyright:©2011MigicovskyandKovalchuk.Thisisanopen-accessarticlesubjecttoanon-exclusivelicensebetweentheauthorsandFrontiersMediaSA,whichpermitsuse,distributionandreproductioninotherforums,providedtheoriginalauthorsandsourcearecreditedandotherFrontiersconditionsarecompliedwith. *Correspondence:IgorKovalchuk,DepartmentofBiologicalSciences,UniversityofLethbridge,4401UniversityDrive,Lethbridge,CanadaABT1K3M4.e-mail:[email protected] COMMENTARY ORIGINALARTICLE Peoplealsolookedat SuggestaResearchTopic>