i the prion diseases - vanderbilt university reading... · rl disease and kuru .-- had all been...
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~ l,,: ". ft\'OV\-rdf '.ft:eI .1..., The Prion Diseases, Prions, once dismissed as an impossibility, have now gainedII wide recognition as extraordinary agents that cause: a number of infectious, genetic and spontaneous disorders
I by Stanley B. Prusiner
'I Fifteen years ago I evoked a good the brain to become riddled with holes. encephalopathy. The last, often called, deal of skepticism when I pro- These ills, which can brew for years (or mad cow disease, is the most worrisome.I.. posed that the infectious agents even for decades in humans) are wide- Gerald A. H. Wells and John W. Wile-I, ' causing certain degenerative disorders spread in animals. smith of the Central Veterinary labora-
I of the central nervous system in ani- The most common form is( scrapie, I tory in Weybridge, England, identifiedI mals and. more rarely, in humans might found in sheep and goats. Afm~t~a:'an: I the condition in 1986, after it began
! consist of protein and nothing else. At imals lose coordination and eventually striking cows in Great Britain. causingII the time, the notion was heretical. Dog- become so incapacitated that they can- them to became uncoordinated and un-! ma held that the conveyers of transmis- not stand. They also become irritable usually apprehensive. The source of the: sible diseases required genetic materi- and, in some cases, jeveloP.EI!~e emerging epidemic was soon traced toI al, composed of nucleic acid (DNA or itch that leads them to scrape off their a food supplement that induded meatI RNA), in order to establish an infection ~~~r hair (hence the name "scrapie"):- and bone meal from dead sheep. TheI in a host. Even viruses, among the sim- ~otherprton diseases of anim~;1 plest microbes, rely on such material to by such names as transmissible minkII direct synthesis of the proteins needed encephalopathy, chronic wasting diseaserl_I~'11111111 for survival and replication. of mule deer and elk, feline spongiform ~ ." Later, many scientists were similarly encephalopathy and bovine spongiform ,., dubious when my colleagues and I sug- ...:-
gested that these "proteinaceous infec- 40000 >- -.,. ',~I tious ~artides"-.or "prions," as I called ' ~ ' ~ "; -&fZ;~
the ?is.ease-~ausmg agents-could ~- w Q <"" !! ~. r." .". - ;., derlie inhented, as well as commuru- (I) 35,000 Q A . ~. 'I cable, diseases. Such d~al b~vior was lli ~'~ ~ ,-*' .
then unknown to medical saence, And B 30,000 ~'~we met resistance again when we con- s: 0 .~
i cluded that prions (pronounced "pree- 8 2 0, ODS") multiply in an incredible way; ~ 5,0 0 ~ -
they convert normal protein molecules ~ ~into dangerous ones simply by induc- ~ 20,000 ~ing the benign molecules to change ~ rr:
their shape. ~ 15 000 ~Today, however, a wealth of experi- ~ ' ~
mental and clinical data has made a ~ ::?.convincing case that we are correct on ~ 10,000 I f;.t all three counts. Prions are indeed re- ~ I t. .
I sponsible for trans---missible and iDfierit- 8 5,000 ~ ~
: e~ Qlsoraers of pro {em conformartou. :g . :They caii (fIs-o cause sporaffic ffisease, ~ ~ . .
~ ~~ch neither .tran~missio~ ber:"'een ~ ~ ~ ~ ffi' 9?- rJ1' & ~ \,.' ,
mdiVIduals nor inhentance IS eVIdent. .?5 .?5 .?5 .?5 .'::J ,5?:J .'::J .'::J .'::J : ,",' ..~... '.-,Moreover, there are hints that the prions SOURCE: John W Wilesmith . , ..:causing the diseases explored thus far . ~
may not be the only ones. Prions made CATn.E WERE INCINERATED to preventof rather different proteins may con- them" fro.m ~preading ~mad cow. dis- .tribute to other neurodegenerative dis- ease, This disorder, whi~ has afflict~deases that are quite prevalent in hu- ~°re. than 130,.000 cattle m Grea~ Bnt-
. . . . am smce the nud-1980s (graph),ls one!11ans, They nught even partiapate m of several fatal neurodegenerative dis-illnesses that att~ck m~scles. eases of animals and humans thought
The kno~ pnon diseases, all fat~, to be caused by prions-infectious pro- ~ .ar~ sometimes referred to as spongl- teins. Studies are assessing whether pli- 's:
(orm encephalopatrues. They are so on disease can be ttansmined from cows Q.; ..I named because they 1requently cause to people through the ingestion of beef" ~ ~..
CAN january 1995
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I:" I. . methods for processing sheep carcass- Creutzfeldt-Jakob disease, in contrast,
es had been changed in the late 1970s. occurs worldwide and usually becomes STANLEY B. PRUSINER is professor ofWhere once they would have eliminat- evident as dementia. Most of the time neurology and bio:chemistry at the Uni-
., ed the scrapie agent in the supplement, it appears sporadically, sttiking one per- versity of Califo~ School of Medidne,. ... .' San Frandsco. He IS a member of the
\ now they apparently did not. The Bnt- son m a million, typiCally around age 60. National Academy of Sciences the Insti-'-' . ish government banned the use of ani- About 10 to 15 percent of cases are in- rote of Medidne and the 'American
;~ mal-derived feed supplements in 1988, herited, and a small number are, sadly, Academy of Arts and Sciences. He has~ and the epidemic has probably peaked. iatrogenic-spread inadvertently by the won many awards for his research into
Nevertheless, many people continue to attempt to treat some other medical pri~ns, m~st recently the Albert Lasker1 wony that they will eventUally fall ill as a problem. Iatrogenic Creutzfeldt- Jakob Basic Me~lcal Research.A":"af~ and the
result of having consumed tainted meat. disease has apparently been transmit- Pa~ Ehrlich .Aw~. This. IS his secondThe human prion diseases are more ted by corneal transplantation, implan- artIcle for SCIentific Amencan.
obscure. Kuru has been seen only among tation of dura mater or electrodes in thethe Fore highlanders of Papua New Guin- brain, use of contaminated surgical in-ea. They call it the "laughing death." struments, and iI\iection of growth hor- Rossella Medori of the University of B0-
o Vincent Zigas of the Australian Public mone derived from human pituitaries logna and Pierluigi Gambetti of CaseHealth Service and D. Carleton Gajdusek (before recombinant growth hormone Western Reserve University.
of the U.S. Nati~titutes of Health became available).described it in~oting that many The two remaining human disorders In Search of the Causehighlanders became afflicted with a are Gerstmann-Striiussler-Scheinker dis-
" strange, fatal disease marked by loss of ease (which is manifest as ataxia and I first became intrigued by the prionI coordinati~n (ataxia) and ?ft~. later other signs of dama~~ t~ the ce~eb~l- . diseases in 1972, when as a resident" by dementia. The affected mdiVlduals lum) and fatal familial msomma (m m neurology at the University of Cali-
probably acquired kuru through ritual which dementia follows difficulty sleep- forma School of Medidne at San Fran-. cannibalism: the Fore tribe reportedly ing). Both these conditions are usually dsco, I lost a patient to Creutzfeldt-Ja-
'" honored the dead by eating their brains. inherited and typically appear in mid- kob disease. As I reviewed the scientific,Ji The practice has since stopped, and life. Fatal familial insomnia was discov- literature on that and related conditions. kuru has virtually disappeared ered only recently, by Elio Lugaresi and I learned that scrapie, Creutzfeldt-Jakob
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, SCIENTIFlC AMERICAN january 1995 49
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rldisease and kuru had all been shown to I immediately began trying to solve dures that denature (unfold) or degrade.-- 0be transmissible' b in °ec . extracts this mystery when I set up a laboratory protein reduced infectivity. I thus intro-
<> sea sed brains into the brains 0 at U.C.S.F. in 1974. The first step had to duced the term "prion" to distinguish!,~ ~ animals.. The infections were be a mechanical one-purifying the in- this class of disease conveyer from vi-i - thought to be caused by a slow-acting fectious material in scrapie-infected ruses, bacteria, fungi and other known, virus, yet no one had managed to iso- brains so that its composition could be pathogens. Not long afterward, we de-
late the culprit. analyzed. The task was daunting; many termined that scrapie prions Cj~dIn the course of reading, I came across investigators had tried and failed in the a single protein that we calle~~rI an astonishing report in which Tikvah past. But with the optimism of youth, I "prion proteiIL "
I Alper and her colleagues at the Ham- forg~g_ah~ad(see"Prions;"by-Stanl~ Now the major question became,mersmith Hospital in London suggest- ;-Prusiner; SCIENTIFIC AMERICAN, Octo- ~ere did the instructions specifying
Ied that the scrapie agent might lack ber 1984 . B-.".--1982 my COlleal!Ues-4nd=F-111'e sequence of amino acids in PrP re-I nucleic aCId; wrucn usually can be de- progress, producing ex- side? Were they carried by an undetect-
grnaed by ultraViolet or ionizing ractia:- tracts of hamster brains consisting al- ed piece of DNA that traveled with PrP,tion. wilen me nucleic acid in extracts~ most exclusively of infectious material. or were they, perhaps, contained in aoficrapie-infected brains was presurn- We had, furthermore, subjected the ex- gene housed in the chromosomes ofably destroyed by those treatments, the tracts to a range of tests designed to cells? The key to this riddle was the
I extracts retained their ability to trans- reveal the composition of the disease- identification in 1984 of some 15 arni-i mit scrapie. If the or~anism did lac~ causing component. -no aCl~at one ena or me YrY protein:i ; DNA and RNA. the finding would mean ~ group iaentified this short amino
I I tlmt~ not a virus or any other- Amazing Discovery acid sequence in collaboration withI ~wn type of-infectious agent, aJror Leroy E. Hood and his co-workers atII wmch contain genetic material-:-What, A Il our results pointed toward one the California Institute of Technology.Ii then, was it? Investigators had many j-\. startling conclusion: the infectious Knowledge of the sequence allowedI ideas-including, jokingly,linoleurn and agent in scrapie (and presumably in us and others to construct moleculari kryptonite-but no hard answers. me relatea ctiseases) did indeed lacK probes, or detectors, able to indicateI nuaci~~d consisted mainly, if noT whether mammalian cells carried theI ex-allsively, of prot$ We deduced that PrP gene. With probes produced by
I i DNA and RNA were aDsent because, like Hood's team, Bruno Oesch, working in.f Alper, we saw that procedures known the laboratory of Charles Weissmann at
~; to damage nucleic acid did not reduce the University of Zurich, showed thatinfectivity. And we knew protein was hamster celIs do contain a gene for PrP.an essential component because proce- At about the same time, Bruce Chese-
I PRION DISEASES OF HUMANS (table), which may incubate for 30 years or more,can all cause progressive decline in cognition and motor function; hence, the dis-
'I tinctions among them are sometimes blurry. As the genetic mutations underlying~ familial fonDS of the diseases are found. those disorders are likely to be identified
I ~ by their associated mutations alone. Choreographer George Balanchine (photo-~ graph) died of sporadic Creutzfeldt-Jakob disease in 1983 at age 79.
! .']~i:f,,~i=- iTj~i~oir:.1l. ;'~~j7:!;jlj~.~~i;'~ :;:!~'=,J:li;1 ~.=~ r"l~oi~~,'jl;'~ill
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boro of the Nlli Rocky Mountain Labo- ~ratories made his own probes and es- ~, tablished that mouse cells harbor the ~gene as well. That work made it possi- ~
V ble to isolate the gene and to establish ~. that it resides not in prions but in the ~chromosomes of hamsters, mice, hu-mans and all other mammals that havebeen examined. What is more, most of
I the time, these animals make PrP with-out getting sick.
One interpretation of such findingswas that we had made a terrible mis-take: PrP had nothing to do with priondiseases. Another possibility was thatPrP could be produced in tWo forms,one that generated disease and one thatdid not. We soon showed the latter in-terpretation to be correct.
The critical clue was the fact that the~nd ~inf~.ct.ed b~_~e~~~~g zbreakdown by cellular enzymes called ~~~~~~eins in .cells -are "de- ~graae"Q~y easily. I therefore suspect- ~ed that if a nonnal. nonthreatening form ~ SOURCE: Fred E. Cohenof PrP existed, it. too would be suscep~- PRION PROTEIN (PrP) is usually harmless. In its benign state, its backbone twistsble to degradation. ~onal~.A. B~ ill into multiple helices (shown as spirals in the plausible ribbon model at the left andmy lab.oratory then lden~ed this hy- as cylinders in the cartoon at the top right). PrP becomes the infectious, "scrapie"pothetIcal protease-senSitIVe form. It fonn-a prion-when much of the backbone sttetches out, fonning so-called betathus became clear that scrapie-causing strands (arrows in the hypothetical strUcture at the bottom right). Red sites on thePrP is a variant of a normal protein. We ribbon model of nonna! PrP highlight positions at which substitution of one aminotherefore called the normal protein "cel- add for another probably promotes folding into the scrapie forDL
""'" . lular PrP" and the infectious (protease-\., resistant) form "scrapie PrP." The latter" term is now used \0 refer to the pro- half of the members of the affected amino add. In our dying patient, just
tein molecules that constitute the pri- families.) It was this last pattern that one base pair (out of more than 750)ons causing all scrapielike diseases of drew our attention. Could it be that pri- had been exchanged for a different pair.animals and humans. ons were more unusual than we origi- The change, in turn, had altered the in-
nally thought? Were they responsible formation carried by codon 102, caus-Prion Diseases Can Be Inherited for the appearance of both hereditary ing the amino add leucine to be substi-
and transmissible illnesses? tuted for the amino add proline in theEarly on we had hoped to use the PrP In 1988 Karen Hsiao in my laborato- man's PrP protein.gene to generate pure copies of PrP. ry and I uncovered some of the earliest With the help of Tim j. Crow of North-
Next, we would inject the protein mole- data showing that human prion dis- wick Park Hospital in London and jurgcules into animals, secure in the knowl- eases can certainly be inherited. We ac- Ott of Columbia University and theiredge that no elusive virus was clinging quired clones of a PrP gene obtained colleagues, we discovered the same mu-to them. If the injections caused scrapie from a man who had Gerstmann- tation in genes from a large number ofin the animals, we would have shown Straussler-Scheinker disease in his fam- patients with Gerstmann-Straussler-that protein molecules could, as we had ily and was dying of it himself. Then we Scheinker disease, and we showed thatproposed, transmit disease. By 1986, compared his gene with PrP genes ob- the high inddence in the affected fami-however, we knew the plan would not tained from a healthy population and lies was statistically significant. In oth-work. For one thing, it proved very dif- found a tiny abnormality known as a er words, we established genetic linkageficult to induce the gene to make the point mutation. betWeen the mutation and the disease-high levels of PrP needed for conduct- To grasp the nature of this mutation. a finding that strongly implies the mu-ing studies. For another thing, the pro- it helps to know something about the tation is the cause. Over the past sixtein that was produced was the normal, organization of genes. Genes consist of years work by many investigators hascellular form. FortUnately, work on a dif- tWo strands of the DNA building blocks uncovered 18 mutations in families withferent problem led us to an alternative called nucleotides, which differ from inherited prion diseases; for five of theseapproach for demonstrating that pri- one another in the bases they carry. The mutations, enough cases have now beenons could transmit scrapie without the bases on one strand combine with the collected to demonstrate genetic linkage.help of any accompanying nucleic add. bases on the other strand to fonD base The discovery of mutations gave us a
A. In many cases, the scrapielike illness- pairs: the "rungs" on the familiar DNA way to eliminate the possibility that a..-, es of humans seemed to occur without "ladder."In addition to holding the DNA nucleic add was traveling with prion\"./ having been spread from one host to ladder together, these pairs spell out the proteins and directing their multiplica-
another, and in some families theyap- sequence of amino adds that must be tion. ~~~~~~~~~~-fpeared to be inherited (Today research- strung together to make a particular tered mice carrymg a mutated PrP gene.ers know that about 10 percent of hu- protein. Three base pairs together-a ¥ili'"e~r~s~~~ of' the~~~ n
i man prion diseases are familial, felling unit called a codon-specify a single ese "transgenic amm~ls led b itself
I SCIENTIFIC AMERICAN January 1995 51
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brain extracts from the mutant animalwould spread the infection to anotherhost. Yet in the absence of any evidenceof a virus, this hypothesis looks to beuntenable.
In addition to showing that a proteincan multiply and cause disease withouthelp from nucleic adds, we have gainedinsight into how scrapie PrP propa-gates in cells. Many details remain tobe worked out, but one aspect appearsquite clear: the main difference betWeennormal PrP and scrapie PrP is confor-mational. Evidently, the scrapie proteinpropagates itself by contacting normalPrP molecules and somehow causingthem to unfold and flip from their usu-al conformation to the scrapie shape.This change initiates a cascade in whichnewly converted molecules change theshape of other normal PrP molecules,and so on. These events apparently oc- -cur on a metMrane in the cell interior."-~ed to think tfia"flhe Ji."faenc-es betWeen cellular and scrapie formsof PrP must be conformational after oth-er possibilities began to seem unlikely.For instance, it has long been knownthat the infectious form often has thesame amino add sequence as the nor-mal type. Of course, molecules that startoff being identical can later be chemi-cally modified in ways that alter theiractivity. But intensive investigations byNeil Stahl and Michael A. Baldwin in mylaboratory have turned up no differ-ences of this kind.
One Protein, Two Shapes
H ow, exactly, do the structures of
normal and scrapie forms of PrP
differ? Studies by Keh-Ming Pan in ourgroup indicate that the normal proteinconsists primarily of alpha helices, re-gions in which the protein backbonetwists into a specific kind of spiral; thescrapie form, however, contains betastrands, regions in which the backboneis fully extended. Collections of thesestrands form beta sheets. Fred E. Cohen,
. who directs another laboratory atU.C.S.F., has used molecular modelingto try to predict the structure of thenormal protein based on its amino addsequence. His calculations imply thatthe proteinprob~ty folds into a ~;)iict structure having four helices in its. ,.co e, -::or P.
po-sition that scrapie PrP can induce an al-pha-helical PrP molecule to switch to abeta-sheet form comes primarily fromtWo important studies by investigatorsin my group. Maria Gasset learned thatsynthetic pep~lshort strings of rom-
. ~oriesponding to three of th~
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I . four putative alph~e!1cal reg!o~~ heimer's clumps consist of a different well. lain H. Pattison of the AgriculturePr~can (oI(Umo beta sheets,-~k protein. The PrP laques are a usef Research Council in Compton, England,Nguyen has shown that in their beta- sign of non' ection but the seem initially called attention to this phe-~°r:I:!~~~~~ti~~S~ at to be a m 'or irn airment. nomenon. Years ago he obtained prions
~ unpo:51: d Ul:ld-:5Ill:el struc~!! beli:. Ji1 many people and ~s with p!:!?? from two separate sets of goats. One iso-\J ~~~~~~~~~J~= disease, the plaQue-s-ao not arise at ~. late made inoculated animals drowsy,
w. Laugney o~o~y Mount~ Lab-1ven though we do not-yet know whereas the second made them hyper-.. f oratOnes and Peter T.Lansb~ of the much about how PrP scrapie harms active. Similarly, it is now evident that~: ~ Massachusetts Institute of Technolo brain tissue, we can foresee that an un- some prions cause disease quickly,
ar PrP can be derstanding of the three-dimensional whereas others do so slowly.,- 1.-(;'~1 c.?!!v~~a Wtoscrapie_Pt;P~ ~ _t_:~_t_~be structure of the PrP protein will lead to ~~.6'?f/"" oy ~ the two proleinS-together. therapies. If, for example, the four-he- ~~ ~ ystery of "Strains"
-- PrP mOlecules arising from mutated !ix-bundle model of PrP is correct, drugg7nes probabl~ do not adopt the scra- developers might be a~le to design a A Ian G. Dickinson, Hugh Fraser andpie conformation as soon as they are compound that would bmd to a central j-\. Moira E. Bruce of the Institute fors}'Ilthesized. Otherwise, people carry- pocket that could be formed by the four Animal Health in Edinburgh, who have
- ing mutant genes would become sick in helices. So bound, the drug would sta- examined the differential effects of var-- -- . early childhood. We suspect that muta- bilize these helices and prevent their ied isolates in mice, are among those
"'/.' tions in the PrP gene render the result- conversion into beta sheets. who note that only pathogens contain-ing proteins susceptible to flipping from Another idea for therapy is inspired ing nucleic acids are known to occur in
.Jl ~ - an alpha-helical to a beta-sheet shape. by research in which Weissmann and multiple strains. Hence, they and others, ~...Presumably, it takes time until one of his colleagues applied gene-targeting assert, the existence of prion "strains"
r ~"'1 the molecules spontaneously flips over technology to create mice that lacked indicates the prion hypothesis must be~ ,~~ and still more time for scrapie PrP to ac- the PrP gene and so could not make PrP. incorrect; viruses must be at the root..(;t,.i- -tf, ~- - cumulate and damage the brain enough By knocking out a gene and noting the of scrapie and its relatives. Yet because6[ It. ~::::1.~'" to cause symptoms. consequences of its loss, one can often efforts to find viral nucleic acids havet! Fred Cohen and I think we might be deduce the usual functions of the gene's been unrewarding, the explanation for
able to explain why the various muta- protein product. In this case, however, the differences must lie elsewhere. ~tions that_~ve been ~ote_d in PrP genes t!!~ ~als- missin~ ~P *~played n~ One possibility is that prions cancould facilitate folding mto the beta- det~tableabnormalities. If it turns out adopt multiple conformations. Foldedsheet form. Many of the human muta- tfii"iPrP is truly messentIiil, then physi- in one way, a~ mi-gni convert nor-tions give rise to the substitution of one cians might one day consider delivering mal PrP to the scrapie form highly effi-amino acid for another within the four so-called antisense or antigene therapies ciently, giving rise to short incubation
"' putative helices or at their borders. In- to the brains of patients with prion dis- times. Folded another way, it might1 sertion of inco_crectamino acids at those eases. Such therapies aim to block genes ork less efficiently. Similarly, one "con-
\-, .P-osJtions might destabilize a helix, thus from giving rise to unwanted proteins former" might be attracted to neuronali!!~s~g the likelih~~t:the affect:. and could potentially shut down pro- populations in one part of the brain,~~ei&hbOrs..will.refold.into duction of cellular PrP [see "The Newa beta-sheet conformation. coiiV-ersely, Genetic Medidnes," by Jack S. Cohen andHermann Schatzel in my laboratory Michael E. Hogan; SCIENm1C AMERICAN.finds that th~harn1less differe!!ces ~ December 1994). They would therebytin~s!!!!!g the PrP ~ene Qf h!!!¥~s block PrP from propagating itself.fr,-9I?Jl1°~e. oLaPfS-and.monkeYs affect It is worth noting that the knockout
_,,'mino acids lyjng-9utside of thp pro-_mice provided a welcomed opportunityllQS.ed..helicaLdomains=F-11ere"the-di to challenge the prion hypothesis. Ifverg~!!t amino ac!~p_rQj;!gbjy-WDuld- the animals became ill after inoculation
i ll9Lnrofrnmrll¥ inf1uence-1he-~ with prions, their sickness would haveo~_helicalr~j. indicated that prions could multiply
even in the absence of a preexistingTreatment Ideas Emerge pool of PrP molecules. As I expected,
inoculation with prions did not produceN o one knows exactly how propaga- scrapie, and no evidence of prion repli-
tion of scrapie PrP damages cells. cation could be detected.In cell cultures, the conversion of nor- The enigma of how scrapie PrP multi-mal m-~~~apieTo:im:Q~~: plies and causes disease is not the only-sme-rie~ns. after which scra ie PrP puzzle starting to be solved. Another
\' accumJl~_t~s_~~UY1i!r.~!£~ long-standing question-the mystery of
~~~~~ the brain, filled how prions consisting of a single kind
lysosomes could conceivably b~stan_d of protein can vary markedly in theircl~ge c~s. As the diseased cells died, effects-is beginning to be answered ascreating holes in the brain, their prionswould be released to attack other cells.
... We do kn~w wi~ certainty thatcleav- HOLES IN BRAIN 1lSSUE (white spots) ~.., age of scra le PrP is w t ro~P are a frequent feature of prion diseases. ~
\.-/ ~ua~~,!:~!. ~ulat~ as Dla~ues They give the brain a spongelike ap- ~. ~ hr"in~ of S~~p I:!a!i~s. Tho~e pearance. This micrograph shows the ~
aggregates resemble plaques seen in cerebral cortex of a patient suffering ~Alzheimer's disease, although the Alz- from Creutzfeldt-Jakob disease. ~
SCIENTIFIC AMERICAN january 1995 55
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I A Persuasive Experimentj: "" S everal studies have shown that prions composed only some of which appears to adopt the scrapie conformation.~ of PrP are able to convey infection from one animal to EventuallY,al1 the mice displayed sym.pt,oms of br~in ~am.
another. In one such experiment, the author and his col- age and died (b). Then the workers Injected brain tissue" leagues created mice carrying many copies of a mutant PrP from the diseased animals into genetically altered mice
- gene (0); these animals made high levels of mutant PrP, making low levels of the same mutant PrP protein. (Suchi~yE-o
~ --- BRAIN ~ ~ MUTANT PrP /TISSUE
ffi DISEA i~ BRAIN - ~
~
t ~~
. ~ --" "'. 0". 5~ - b.. ~ a MOUSE MAKING HIGH LEVELS MUTANT MOUSEt ~ OF MUTANT PrP FALLS ILL AND DIESf ~ IS INITIALLY HEALTHY
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~ ~--tA-.-~-. - .,( whereas another might be attracted to ments, we realized that the barrIer re- ease. Their deaths may have nothing toI~ neurons elsewhere, thus produdng dif- ~es in theamin~add sequ~ do with the bovine epidemic, but theI; ferent symptoms. Considering that PrP PrP: themore the seQuence of a scrapie situation bears _watching. I! may turnj can fold in at least two v.:ays, it would PrP mole~e resem ~ e. . s~- Q~! tha_~~-!-~.s_m -~~PI~ not be surprising to find It can collapse qiii~1iS1iOSt":ifie~~.!!k~YjUs.~ mol-ea:ii~-~2E-!!P~_rJ'j!!1L~.q_th-I~ into other structures as well. ~~~srwm a<;:g~.p~ll.<.lisease-- e[.5J?I:_preaking_.th~~peaes_banier._1fI ..., Since the mid-1980s we have also In one oT ffiO5e"Other expenmentS'.-ror ~tJ~_W~--cA~~_~q.i!_£2~-Prp_99~.ely
sought insight into a phenomenon example, we examined transgenic mice r,e~.e.l!1;~l~~.. human PrP in the critical r_~-known as the spedes barrier. This con- carrying the Syrian hamster PrP gene ,g!~ns!_~~~~::~~~~'~~:g~~ cept refers to the fact that something in addition to their own mouse gene. ~,~~2_b~~.~8h£!:~-~~,."..
.. makes it difficult for prions made by Those mice make normal forms of both pJ~ _cQ~parisonof ~~.'<;9,mI!!~te__aminoj: one spedes to cause disease in animals hamster and ~ouse P,rP. When we ~oc- a9g..§:~~~!!~~ sugg~::,t. ...i of another species. The cause of this ulated the anImals WIth mouse pnons, We began to consId~ possIbility~ difficulty is of considerable interest to- they made more mouse prions. When that some parts of the PrP moleculeI day because of the epidemic of mad we inoculated them with hamster pri- might be particularly important to the; cow disease in Britain. We and others ons, they made hamster prions. From spedes barrier after a study related to! have been trying to find out whether this behavior, we learned that prions this blockade took an odd turn. My col-I the species barrier is strong enough to preferentially interact with cellular PrP league Glenn C. Telling had created; prevent the spread of prion disease of homologous, or like, composition. transgenic mice carrying a h}'hrid PrP. from cows to humans. The attraction of scrapie PrP for cel- gene that consisted of human codesi lular PrP having the same sequence flanked on either side by mouse codes:i Breaking the Barrier probably explains why scrapie managed this gene gave rise to a hybrid protein., to spread to cows in England from food Then he introduced brain tissue from
T he barrier was discovered by Patti- consisting of sheep tissue: sheep and patients who had died of Creutzfeldt-son, who in the 1960s found it hard bovine PrP differ only at seven posi- Jakob disease or Gerstmann-Straussler-
to transmit scrapie between sheep and tions. In contrast, the sequence differ- Scheinker disease into the transgenic: rodents. To determine the cause of the ence between human and bovine PrP is animals. Oddly enough. the animals be-I trouble, my colleague Michael R. Scott large: the molecules diverge at more came ill much more frequently and fast-; and I later generated transgenic mice than 30 positions. Because the variance er than did mice carrying a full human! expressing the PrP gene of the Syrian is great, the likelihood of transmission PrP gene, which diverges from mouse
hamster-that is, making the hamster from cows to people would seem to be PrP at 28 positions. This outcome im-PrP protein. The mouse gene differs low. Consistent with this assessment are plied that similarity in the central regionfrom that of the hamster gene at 16 co- epidemiological studies by W. Bryan of the PrP molecule may be more criti-dons out of 254. Normal mice inoculat- Matthews, a professor emeritus at the cal than it is in the other segments.ed with hamster prions rarely acquire University of Oxford. Matthews found The result also lent support to earli-scrapie, but the transgenic mice became no link between scrapie in sheep and er indications-uncovered by Shu-llanill within about two months. the occurrence of Creutzfeldt- Jakob dis- Yang in DeAnnond's laboratory and AI-
We thus concluded that we had bro- ease in sheep-farming countries. bert Taraboulos in my group-that mol-ken the species barrier by inserting the On the other hand, two farmers who ecules made by the host can influencehamster genes into the mice. Moreover, had "mad cows" in their herds have re- the behavior of scrapie PrP. We specu-
, on the basis of this and other experi- cently died of Creutzfeldt-Jakob dis- late that in the hybrid-gene study, a:;
56 SCIENm1C AMERICAN january 1995:.
J".c I
" .. the inherited prion diseases have been
mice were chosen as recipients because scrapie PrP is most attracted to PrP modeled in ttansgenic mice canyingmolecules having the same composition.) Uninoculated mice did not become ill mutant PrP genes. These murine repre-(indicating that making low levels of the aberrant protein was safe), but many of sentations of the human prion afflic-~ the treated ones did (c). Moreover, brain tissue transferred from the diseased re- tions should not only extend under-
" .' ' cipients to their healthy counterparts caused illness once again (d). If the aber- standing of how prions cause brain de-rant protein were unable to transmit infection, none of the inoculated animals generation, they should also createwould have sickened. opportunities to evaluate therapies for
these devastating maladies.
Striking Similarities
O ngoing research may also help de-termine whether prions consisting
of other proteins playa part in morecommon neurodegenerative conditions,induding Alzheimer's disease, Parkin-son's disease and amyotrophic lateralsderosis. There are some marked simi-larities in all these disorders. As is trueof the known prion diseases, the more
C MOUSE MAKING LOW LEVELS d IDENTICAL MOUSE BECOMES ILL widespread ills .mostly ~~ spo~~di-OF MUTANT PrP BECOMES ILL AFTER RECEIVING INOCULATION cally but sometimes "nm m families.AFTER INOCULATION FROM FIRST RECIPIENT All are also usually diseases of middle
to later life and are marked by similarpathology: neurons degenerate, proteindeposits can accumulate as plaques,
mouse protein, possibly a "chaperone" begin when the wear and tear of living and glial cells (which support and nour-normally involved in folding nascent led to a mutation of the PrP gene in at ish nerve cells) grow larger in reactionprotein chains, recognized one of the least one cell in the body. Eventually, to damage to neurons. Strikingly, intwo mouse-derived regions of the hy- the mutated protein might switch to none of these disorders do white bloodbrid PrP protein. This chaperone bound the scrapie form and gradually propa- cells-those ever present warriors of theto that region and helped to refold the gate itself, until the buildup of scrapie immune system-infiltrate the brain. lfhybrid .molecule into the s~rapie con- PrP crossed the thresh?ld to overt dis- a ~ were involved in these illnesses,
., formation. The chaperone did not pro- ease. The mouse studies suggest that white cells would be expected to appear.vide similar help in mice making a to- at some point in the lives of the one in Recent findings in yeast encouragetally human PrP protein. presumably a million individuals who acquire spa- speculation that prions unrelated inbecause the human protein lacked a radic Creutzfeldt-Jakob disease, cellu- amino acid sequence to the PrP proteinbinding site for the mouse factor. lar PrP may spontaneously convert to could exist. Reed B. Wickner of the Nlli
the scrapie form. The experiments also reports that a protein called Ure2pThe list May Grow raise the possibility that people who be- might sometimes change its conforma-
come afflicted with sporadic Creutzfeldt- tion. thereby affecting its activity in theA n unforeseen story has recently Jakob disease overproduce PrP, but we cell. In one shape, the protein is active;
j-\. emerged from studies of trans- do not yet know if, in fact, they do. in the other, it is silent.genic mice making unusually high All the known prion diseases in hu- The collected studies described hereamounts of normal PrP proteins. DeAr- mans have now been modeled in mice. argue persuasively that the prion is anmond, David Westaway in our group With our most recent work we have in- entirely new dass of infectious patha-and George A. Carlson of the McLaugh- advertently developed an animal model gen and that prion diseases result fromlin Laboratory in Great Falls, Mont., be- for sporadic prion disease. Mice inocu- aberrations of protein conformation.came perplexed when they noted that lated with brain extracts from scrapie- Whether changes in protein shape aresome older transgenic mice developed infected animals and from humans af- responsible for common neurodegen-an illness characterized by rigidity and flicted with Creutzfeldt-Jakob disease erative diseases, such as Alzheimer's,diminished grooming. Mlen we pursued have long provided a model for the in- remains unknown, but it is a possibilitythe cause, we found that making exces- fectious forms of prion disorders. And that should not be ignored.sive amounts of PrP can eventually leadto neurodegeneration and, surprising-ly, to destruction of both muscles and FURlliER READINGperipheral nerves. These discoverieswiden the spectrum of prion diseases SCRAP~ DISEASE IN SHEEP. ~erbert B. Par. Edited by C. Guilleminault et at. Raven
. ry. Edited by D. R. OppenheImer. Academ- Press, 1994.an? are. prompting a search for h~ ic Press, 1983. MOLECULAR BIOLOGY OF PRION DISEASES.pnon diseases that affect the penpher- MOLECUlAR BIOLOGY OF PRION DISEASES. Spedai issue of Philosophical Transactionsal nervous system and musdes. S. B. Prusiner in Sdence, Vol. 252, pages of the Royal Society of London. Series B,
Investigations of animals that over- 1515-1522; June 14, 1991. VoL 343, No. 1306; March 29.1994.produce PrP have yielded another ben- PRION DISEASES OF HUMANS AND ANIMALS. STRUCIURAL CLUES TO PRION REPUCA-efit as well. They offer a due as to how Edited by S. B. Prusiner, J. Collinge, J. Pow- nON. F. E. Cohen, K.-M. Pan. Z. Huang, M.
~ the sporadic form of Creutzfeldt-Jakob ell and B. Anderton. Ellis HolWood, 1992. Bald~, R. J. Fletterick an~ S. B.: Prusine!disease might arise. For a time I sus- FATAL FAMIIlAL INSOMNIA: lNHERn"ED PRI- in Sdence, VoL 264, pages :>30-:>31; April
t d th t di di . h ON DISEASES, SlEEP, AND l1IE THAlAMUS. 22.1994.pec e a spora c sease rnrg t
SCIENI"IFIC AMERICAN january 1995 57