Table Of Content307
Current Topics
in Microbiology
and Immunology
Editors
R.W.Compans,Atlanta/Georgia
M.D.Cooper,Birmingham/Alabama
T.Honjo,Kyoto·H.Koprowski,Philadelphia/Pennsylvania
F.Melchers,Basel·M.B.A.Oldstone,LaJolla/California
S.Olsnes,Oslo·P.K.Vogt,LaJolla/California
H.Wagner,Munich
J.L. Casey (Ed.)
Hepatitis Delta Virus
With25Figuresand12Tables
123
JohnL.Casey,Ph.D.
DepartmentofMicrobiologyandImmunology
GeorgetownUniversity
3900ReservoirRoad,NW
Washington,DC20007
USA
e-mail:[email protected]
Thecoverillustrationisasimplifiedstructureofhepatitisdeltavirusshowingtheinternalribo-
nucleoproteincomplex,whichcontainsthecircularRNAgenomeandthetwoformsofthehepatitis
deltaantigen;theenvelopeproteinsofhepatitisBvirusformtheexteriorofthevirus.Theinsetisan
electronmicrographofpurifiedhepatitisdeltavirusparticles,andwaskindlyprovidedbyDr.John
Gerin.Thebackgroundimmunofluorescenceimageisoftransfectedcellsexpressinghepatitisdelta
antigen,andwaskindlyprovidedbyDawnDefenbaugh.
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Preface
Sinceitsdiscoverynearly30yearsago,hepatitisdeltavirus(HDV)hascon-
tinuedtosurpriseandfascinate.Initiallythoughttobeanantigenicvariant
ofhepatitisBvirus(HBV),HDVwassoonfoundtobeadefectivevirusthat
dependsonanunderlyingorsimultaneoushepatitisBinfection.Theclinical
significanceofHDVinfectionismoresevereacuteandchronicliverdisease
thanthatcausedbytheHBVinfectionalone.Thecloningandsequencingof
thegenomeledtotherealizationthatHDVisauniqueRNAviruswhoseclos-
estknownrelativesareplantviroids,buteventhatrelationshipisremote.In
thecurrentclassificationschemeoftheInternationalCongressontheTaxon-
omyofViruses,HDVremainsthesolememberofafloatinggenus,Deltavirus.
Thegenomeanditsreplicationcyclebearnodiscernableresemblancetoits
helpervirus,HBV,onwhichHDVdependsforitsenvelope.
At 1,680 nucleotides the HDV genome is the smallest known to infect
man. The virus contains just one gene, which encodes an approximately
25-kDaprotein,hepatitisdeltaantigen(HDAg,alsosometimesreferredtoas
deltaproteinordeltaantigen).Tocompensateforthislimitedproteincoding
capacity,HDVreliesheavilyonhostfunctionsandonthestructuraldynamics
ofitscircularRNAgenome.
AlthoughHDVRNAiscircular,itformsacharacteristicunbranchedrod
structureinwhichover70%ofthenucleotidesfromWatson–Crickbasepairs.
OneofthemoreremarkableaspectsofHDVisthat,unlikeotherRNAviruses,
it does not produce a virally-encoded polymerase; rather, it somehow uses
hostDNA-dependentRNApolymerasetoreplicateitsRNAgenomeandtran-
scribe its mRNA. At a minimum, this process involves RNA polymerase II;
HDAgalsoplaysanasyetundefinedrole.Thepotentialinvolvementofan-
otherpolymerase,suchaspolymeraseI,orofotherformsofRNApolymerase
IIremainsanareaofactiveinvestigation. RNAreplicationrequirestheun-
branchedrodstructureofHDVRNAandoccursviaadoublerollingcircle
mechanism. Autocatalytic self-cleaving elements, termed ribozymes, in the
genomeanditscomplement,theantigenome,playessentialrolesinthepro-
cessingoflineartranscriptstocircularforms.Ribozymeactivityoccursvia
VI Preface
acid-base catalysis not unlike that accomplished by protein enzymes, and
requiresacomplexdoublepseudoknotRNAstructure.Ribozymeactivityis
alsocontrolledbythestructuraldynamicsoftheRNA:formationoftheun-
branchedrodstructureinterfereswithribozymeactivityandlikelyprevents
cleavagefromoccurringoncetheRNAcircularized.
HDVproducestwoformsofHDAgthathavedifferent rolesintherepli-
cation cycle. Thelonger formhas an additional 19or 20C-terminal amino
acidsthatfacilitateviralparticleformation;theshorterformisrequiredfor
RNAreplication.Theheterogeneityarisesduetohighlyspecificeditingofan
adenosineintheantigenomeRNAbyhostRNAadenosinedeaminase.This
processrequiresparticularsecondarystructurefeaturesintheRNAaround
theeditingsite.Insomecasestheunbranchedrodstructurecompeteswiththe
configurationrequiredforediting;thus,structuraldynamicsoftheRNAare
importantnotonlyforHDVribozymeactivity,butforotherprocessesaswell.
ThefunctionalactivityofHDAgisaffectedbynumerouspost-translational
modificationscarriedoutbyhostenzymes.Thesemodificationsincludefar-
nesylation, phosphorylation, methylation and acetylation. Farnesylation is
essentialforinteractionwiththehepatitisBvirussurfaceprotein(HBsAg),
and is thus required for viral particle formation. The specific significance
of the other modifications, as well as the nature of their effects on HDAg
function,arenotyetfullyunderstood.
BeingderivedfromHBVproteins,theoutsideoftheHDVparticleissimilar
tothatofHBV,onlyslightlysmallerinsize.Althoughthereceptorforneither
virushasbeenidentifieditislikelythatattachmentandentryoccurbysimilar
processes.InfectivityofbothHBVandHDVinvolveselementsofthepreS1
andantigenicloopregionsofHBsAg.
MoleculargeneticanalysisofHDVisolatesindicatesgeographicalcorre-
lationsthatinsomewaysmirrorthoseofitshelpervirus.Thatthegreatest
sequencediversityisfoundamongisolatesoriginatinginAfricahasledtothe
proposalthatHDVmighthaveradiatedfromthatcontinent.Oneenigmais
thatthemostdistantlyrelatedsequences,forbothHDVandHBV,comefrom
SouthAmerica.Thereissomeevidencethatinfectionwithcertaingenotypes,
orclades,caninfluencetheseverityofHDVdisease.
ThemechanismsbywhichHDVthwartstheimmunesystemtoproduce
chronic infection are not yet understood. The woodchuck model of HDV
hasbeenthemostaccessibleanimalmodelofHDVinfectionandhasbeen
used both to analyze the natural history of HDV infection and to evaluate
theefficacyofvaccinestrategiesagainstthevirus.Certainly,developmentof
aneffectivevaccinestrategyhasbeenfrustrating.Recentworksuggeststhat
HDAgmaybepoorlyimmunogenic,andmayfurthermoreundergogenetic
changestoavoidthoselimitedimmuneresponsesthatdooccur.
Preface VII
TherearenoeffectivelicensedantiviraltherapiesforHDV,andalthough
severaltherapiesexistforcombatingitshelper,HBV,noneofthesetreatments
affectHDV.ThisfailureisduetothefactthatHDVdependsonlyonHBsAg
production of the helper, and current anti-HBV therapies are not potent
enough to significantly diminish HBsAg levels, which are extraordinarily
high. However, two potential therapeutic approaches have shown promise.
Onetargetsthehostfarnesyltransferaseactivity,whichisrequiredforvirus
production;theotherapproachadvocatesreducingHBsAgtolevelsthatare
toolow tosupport continued HDV secretion. Both of these approaches are
basedtovaryingdegreesonanunderstandingofthemolecularvirologyof
HDV,anditislikelythatadditionaltherapeuticavenueswillbeopenedasour
knowledgeofHDVexpands.
The more we continue to learn about hepatitis delta virus the more fas-
cinating it becomes. It is my hope that this book will stimulate additional
interest in hepatitis delta virus among scientists, academic researchers and
advancedstudents.Iwouldliketothanktheauthorsfortheircontributions,
andthestaffatSpringerandmembersofmylaboratoryfortheirassistance
inpreparingthisvolume.
Washington,DC,March2006 JohnL.Casey
ListofContents
StructureandReplicationofHepatitisDeltaVirusRNA .................. 1
J.M.Taylor
HDVRNAReplication:AncientRelicorPrimer? ....................... 25
T.B.MacnaughtonandM.M.C.Lai
HDVRibozymes ............................................. 47
M.D.Been
RNAEditinginHepatitisDeltaVirus ............................... 67
J.L.Casey
Post-translationalModificationofDeltaAntigenofHepatitisDVirus ........ 91
W.-H.Huang,C.-W.Chen,H.-L.Wu,andP.-J.Chen
TheRoleoftheHBVEnvelopeProteinsintheHDVReplicationCycle . .......113
C.Sureau
PrenylationofHDAgandAntiviralDrugDevelopment...................133
J.S.Glenn
HepatitisDeltaVirusGeneticVariability:
FromGenotypesI,II,IIItoEightMajorClades? .......................151
P.Dény
FunctionalandClinicalSignificance
ofHepatitisDVirusGenotypeIIInfection ...........................173
J.-C.Wu
ImmunologyofHDVInfection ...................................187
M.FiedlerandM.Roggendorf
TheWoodchuckModelofHDVInfection ............................211
J.L.CaseyandJ.L.Gerin
SubjectIndex................................................226
ListofContributors
(Addressesstatedatthebeginningofrespectivechapters)
Been,M.D. 47 Lai,M.M.C. 25
Casey,J.L. 67,211 Macnaughton,T.B. 25
Chen,C.-W. 91
Chen,P.-J. 91 Roggendorf,M. 187
Dény,P. 151 Sureau,C. 113
Fiedler,M. 187 Taylor,J.M. 1
Gerin,J.L. 211 Wu,H.-L. 91
Glenn,J.S. 133 Wu,J.-C. 173
Huang,W.-H. 91
CTMI(2006)307:1–23
(cid:1)c Springer-VerlagBerlinHeidelberg2006
StructureandReplicationofHepatitisDeltaVirusRNA
J.M.Taylor((cid:1))
FoxChaseCancerCenter,333CottmanAvenue,Philadelphia,PA19111-2497,USA
[email protected]
1 Introduction ........................................... 2
2 RNAsandRibonucleoproteins............................... 2
2.1 GenomeandAntigenome .................................. 2
2.2 mRNA................................................ 3
2.3 OtherHDVRNAs ........................................ 4
2.4 RNAStructure . ......................................... 5
2.5 Ribonucleoproteins ...................................... 6
3 RNAReplication ........................................ 7
3.1 RolesofDeltaProteins .................................... 7
3.1.1 TheEssentialSmallDeltaProtein............................. 7
3.1.2 OtherFormsofDeltaProtein................................ 7
3.1.3 ReplicationwithanUnchangingSeparateSourceofSmallDeltaProtein.. 8
3.2 EnzymologyofRNA-DirectedTranscription ..................... 8
3.3 InitiationofReplication ................................... 9
3.4 DoubleRolling-CircleModel ................................ 11
3.5 Template-Switching,Reconstitution,andRecombination ............ 12
3.6 InhibitionofReplication ................................... 14
3.6.1 ResistancetoInterferons................................... 14
3.6.2 SensitivitytoRibavirin .................................... 14
3.6.3 SensitivitytosiRNA ...................................... 15
3.6.4 ResistancetoDicer ....................................... 15
3.7 CytopathicEffectofReplication.............................. 15
4 EvolutionoftheRNASequence .............................. 16
4.1 AccumulationofChanges .................................. 16
4.2 ADAR-Editing . ......................................... 17
4.3 Origin................................................ 17
5 Outlook............................................... 18
References.................................................. 18
Abstract While this volume covers many different aspects of hepatitis delta virus
(HDV)replication,thefocusinthischapterisonstudiesofthestructureandreplication
of the HDV RNA genome. An evaluation of such studies is not only an integral
part of our understanding of HDV infections but it also sheds new light on some
