Medical Microbiology

472 8 VirusesasHumanPathogen

SubviralPathogens:ViroidsandPrions

&Viroidsarephytopathologicallysignificant,noncodingRNAmolecules

thatinterferewithcellularregulationasantisenseRNA.ThehepatitisDvirus

hassomestructuralsimilaritytoviroids.

Prionsconsistofacell-codedprotein(PrP:prionprotein)alteredinitscon-

formationandbypointmutations.Theyareinfectiousandcancausenormal

cellularPrPtoassumethepathologicalconfiguration.Theycausethespongi-

formencephalopathies(Creutzfeldt-Jakobdisease,CJD)intheclassicand

newvariants(nvCJD),theGerstmann-Stra ̈ussler-Scheinker(GSS)syndrome,

andanimaldiseases(scrapie,BSE)characterizedbyneuronalvacuolization

andlossandso-calledamyloidplaques.

Prevention:exposureprophylaxis (iatrogenic andalimentary transmis-

sion). &

Viroids

Viroidswerediscoveredattheendofthesixtiesduringinvestigationsofplant

diseases.Theyconsistofinfectious,nakedssRNAinclosedcircularformwith

extensivebase-pairingtoformarod-shapedstrand 50 nmlong.ThisRNAis

10 timessmallerthanthesmallestviralnucleicacidandcomprises,depend-

ingonthetype,only 250 – 350 nucleotides.ItdoesnotfunctionasmRNAand

doesnotcodeforproteins.Itsmodeofreplicationisunknown,butcertainly

involvescellularenzymes.

Viroidscauseanumberofplantdiseaseswithconsiderableeconomicim-

pact.Thefollowinghypothesisiscurrentlyunderdiscussiontoexplaintheir

pathogenicitymechanism:viroidspossesscomplementarysequencestocel-

lular7SRNA,comprising,togetherwithsixproteins,the“signalrecognition

particle.”Thisparticlecontrolstheposttranslationalmembraneinsertionof

proteins.Viroidscanthusinterfereas“antisense(orinterfering)RNA”with

thefunctionof7SRNAandthuswithmembraneformation.

Theonlysignificanthumanpathogenstructurallyrelatedtotheviroidsis

thehepatitisDpathogen(HDV,deltaagent,seep.429).HDVconsistsofa

viroidlike,alsocircular,RNAintowhichanantisenseRNAcodingforthedelta

agentisinserted.

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Kayser, Medical Microbiology © 2005 Thieme