MEDICAL GRAND ROUNDS LLIAM S. WILKE, MD, EDITOR TAKE-HOME POINTS FROM EDUCATIONAL PRESENTATIONS BY CLEVELAND CLINIC FACULTY AND VISITING PROFESSORS Kuru and mad cow disease: Understanding the prion theory

G. RICHARD OLDS, MD • NEW GUINEA, 1957: Chairman, Department of Medicine, MetroHealth Medical Center, Cleveland; A NEW DISEASE AMONG THE CANNIBALS Charles H. Rammelkamp Jr. Professor of Medicine, Case Western Reserve University College of Medicine; expert in geographic medicine and tropical disease. Chaired a number of committees for the World Health Organization, Shortly after World War II, stories began to circulate among colonial authorities about a • ABSTRACT strange neurologic disease affecting the Fore A preponderance of evidence indicates that (pronounced four-ay) people of the remote several neurodegenerative disorders are highlands of Papua, New Guinea, one of the caused by prions: abnormally folded proteins most isolated and primitive areas in the world. that can induce abnormal folding in other In 1957, Dr. Carleton Gajdusek began work normal protein molecules. Further, these among the Fore, which would occupy him for "infections" can cross some species barriers. many years and eventually win him the Nobel Prize in 1976.2 OR DECADES, the causative agents of dif- The disease that Gajdusek found was like ferent spongiform encephalopathies in no other he had ever seen. Victims were pre- man and animals, such as kuru, scrapie, and dominately women and children. The tribes- Creutzfeldt-Jakob disease, were a mystery. The people called the disease kuru (trembling, diseases were transmissible, but the victims shaking), because of one of its prominent signs. Prions have showed no clinical or pathological signs of Other signs were psychiatric changes, inappro- forced us to infection. Cultures of victims' brains revealed priate affect (including inappropriate laugh- nothing. Further, usual sterilization procedures ter), and cerebellar ataxia. Dementia occurred rethink our did not destroy the agents' infectivity. only late in the disease, if at all. Once the signs ideas about The implication of these findings seemed appeared, the disease invariably progressed preposterous: the agent consisted of protein within months to complete debilitation and infectious alone, with no nucleic acid at all. Such a the- death.3 agents ory would violate all known concepts of infec- The brains of the victims, preserved in tion. Yet it was precisely this theory that Dr. formaldehyde and sent to the NIH for patho- Stanley Prusiner proposed in 1982,1 and for logic analysis, showed changes similar to which he was awarded the Nobel Prize in 1997. those produced by scrapie, a disease of sheep. With the recent controversy in England Specifically, the cerebellums contained over bovine spongiform encephalopathy florid amyloid plaques and spongiform (BSE), better known as mad cow disease, and changes—literally, spongelike holes where its apparent jump to humans, there is a grow- neurons used to be.4 ing understanding that prions are not just a Studies in chimpanzees confirmed what medical curiosity, but a potentially grave pub- epidemiologic studies suggested: the disease lic health threat. was transmissible.5 Extracts from the brains of This brief article presents a short history of human victims, injected into chimps, resulted the development of the prion theory of disease, in a disease very much like the human disease, an overview of that theory, and some of the and extracts from the infected chimps' brains lessons that our understanding of kuru and mad could in turn induce the disease in other cow disease holds for contemporary society. chimps. Yet, as with scrapie, workers could find

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ease, and to growth hormone extracted from human cadavers.7

A..», 4~ y • •• t»V' *!«v • UNITED STATES, 1982: THE PRION THEORY PROPOSED ! • i* <* „ J - * - • r •*«»* ; y* -v. Noting that the infectious agents in kuru, - •••> « » * , ; i •• *• v « • >• • r 5. ... '.¿J ' v scrapie, and Creutzfeldt-Jakob disease were — » . Ci. • * •* - "if impervious to normal sterilization procedures, • . /" , ^ -'' * • • « V », •*; v and were not destroyed by electron radiation < *v . .. >a •»$ \ ••» or ultraviolet light at a wavelength specifical- ; / - t s «? *• * u» ly absorbed by nucleic acids, scientists began ' • yi # . * , -

4 * • '-./A, , • to develop an alternate theory, that proteins v • r * , alone were responsible for these diseases.1 In this theory, a naturally occurring pro- •HHKHiHHHilHH tein is transformed into a mutant form—a FIGURE 1. Brain tissue from a patient with Creutzfeldt-Jakob "prion"—that is then able to convert proteins disease, showing the characteristic spongiform changes. with normal three-dimensional structures to SOURCE: COURTESY OF RICHARD A. PRAYSON, MD. the mutant form, giving an infectious proper- ty to this inanimate protein. no trace of an agent, either bacterium or virus. Further, victims showed no signs of infection, Normal prion protein such as fever or adenopathy. Workers at that Within the neurons of most mammals is a pro- time speculated that kuru, scrapie, and similar tein, called prion protein (PrP). This prion diseases were caused by "slow viruses," protein, which is naturally produced in the although the actual agent eluded them. cytoplasm, has a phospholipid anchor and Gajdusek and others eventually linked kuru appears to bind to the cell membrane. to ritual cannibalism. Women and children The function of normal prion protein is ln prion comprised most of the kuru victims because unclear. Genetically engineered mice that diseases, the they, and not men, consumed the brains of their cannot produce prion protein grow and repro- family members who had died. The women duce normally. These "knockout" mice exhib- normal shared this protein meal with the small children it only subtle abnormalities, such as altered protein shape that always accompanied them. Adult men, circadian rhythms, progressive but mild atax- who ate the flesh, were largely unaffected. After ia, and decreased gamma-aminobutyric acid mutates cannibalism was eliminated in the 1960s, the (GABA)-mediated synaptic inhibition. This incidence of kuru steadily declined. latter effect suggests that prion proteins are involved in regulating GABA activity. • UNITED STATES, 1974: However, these knockout mice cannot get IATROGENIC TRANSMISSION spongiform encephalopathies when injected OF CREUTZFELDT-JAKOB DISEASE with infectious prions, providing the best evi- dence to date in support of the prion. Creutzfeldt-Jakob disease is a rare, degenera- tive neurologic disease, also characterized by Structure of normal spongiform changes in the brain (FIGURE 1), and abnormal prion protein that usually occurs sporadically in persons in The backbone of normal prion protein is their 60s, although there is sometimes a twisted into spiral shapes known as "alpha hereditary linkage. In 1974, a case occurred in helices" (FIGURE 2). In prion diseases, the nor- a middle-aged woman who had received a mal shape of the protein is transformed, struc- corneal transplant from a donor with ture, with the spiral helices stretched out into Creutzfeldt-Jakob disease.6 Other cases were a flat configuration, known as beta sheets. linked to the use of brain electrodes previous- Normal prion protein survives in the neu- ly used in patients with Creutzfeldt-Jakob dis- rons for only a few hours, before it is digested

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Hü! MIM prion protein, even though both normal and mutant protein configurations have the same Known prion diseases amino acid sequence. There are two theories Diseases in animals on how this transformation might occur. In the dimerization theory, it is thought Scrapie that a mutant prion protein molecule links Bovine spongiform encephalopathy Transmissible mink encephalopathy with a normal prion protein molecule, trans- Feline spongiform encephalopathy forming the latter's helical structure into the Exotic ungulate encephalopathy beta-sheet structure. Then the two mutant molecules disassociate and move on to trans- Diseases in man form more normal prion protein molecules. Kuru In the second theory, is a single beta-sheet "Spontaneous" Creutzfeldt-Jakob disease molecule in a neuron acts as a seed to trans- "Hereditary" Creutzfeldt-Jakob disease form the normal protein molecules into a crys- Gerstmann-Sträussler-Scheinker disease Fatal familial insomnia talline aggregation of abnormal protein. "New variant" Creutzfeldt-Jakob disease In either case, the presence of the mutant, (probably human bovine spongiform encephalopathy) beta-sheet protein or prion initiates a cascade of conversion to the abnormal form. Thus, prions act like infectious agents. Prions appear to be able to cross species barri- by proteases. However, the mutant, beta-sheet ers, but often after a marked lag time. Because formation, or "prion" cannot be degraded and prions are proteins, they are unaffected by instead accumulates in the neurons. It is this sterilization techniques designed to kill most accumulation of mutant protein that is bacteria and viruses. Cooking meat, for exam- believed to cause neuron destruction and the ple, does not inactivate prions, nor do most clinical manifestations of prion disease. hospital sterilization procedures used on reusable medical equipment. Clearly, tissue or The development of mutant prion protein organ transplants can transmit prion diseases. Prions are not and prion diseases Today there are at least five known prion destroyed by Prion diseases appear to arise in three ways, as diseases of animals and six of humans (TABLE 1). described below. The pathology of each includes spongiform normal Inheritance of a grossly mutated prion encephalopathy, amyloid plaques, and prion- sterilization protein gene produces an unstable form of associated fibrils in the brain. prion protein that folds spontaneously into techniques the beta-sheet conformation. This is often an • ENGLAND, 1986: autosomal dominant condition and leads to AN EPIDEMIC OF "MAD COW DISEASE" 100% penetrance of the disease. Point mutations, ie, substitution of a sin- Prions might have remained largely a medical gle amino acid for another in the protein's pri- curiosity were it not for recent events in mary structure, may make the protein more England. Scrapie has occurred in England for susceptible to abnormal folding. Point muta- several hundred years. Exactly how sheep tions of this type increase the risk of sponta- acquire this disease from other sheep is neous transformation to the beta-sheet con- unknown (sheep are vegetarians), but it is formation but do not confer the disease with clearly infectious and probably transmitted 100% penetrance. These mutational defects through fecal-oral contamination. may account for most cases of "spontaneous" In 1986, British veterinarians first Creutzfeldt-Jakob disease (estimated at 1 case noticed a scrapie-like illness in cattle.8 A per million population per year). review of animal husbandry practices "Infection." Perhaps the most startling revealed that cattle were often fed the element of the prion theory of disease was the remains (mostly ground bones) of dead cows insight that mutant, beta-sheet prion protein and sheep to enhance growth and milk pro- can transform the spiral structure of normal duction. In a sense, cattle had been turned

294 CLEVELAND CLINIC JOURNAL Of MEDICINE VOLUME 65 • NUMBER 6 JUNE 1998 Downloaded from www.ccjm.org on October 6, 2021. For personal use only. All other uses require permission. Prions: Infectious agents made entirely of protein

Mammalian neurons of the \M j brain normally contain prion protein. Its function is unknown: "knockout" mice who lack this protein thrive without it, and are immune to prion diseases.

Normal prion protein Abnormal prion protein can is folded into four arise from point mutations that alpha helices. allow the molecule to refold into beta sheets. Ominously, it can also be introduced into the body by ingesting it—and cooking does not destroy it.

Transformation of normal prion protein molecules occurs as they come into contact with abnormal molecules. Accumulation. Normal prion protein molecules are cleared quickly and have a short half-life, but the abnormal molecules accumulate. § (I

Normal prion protein molecules are cleared quickly as they are degraded by proteases.

FIGURE 2 into cannibals. By the early 1990s, an epi- However, this disease may not be Creutzfeldt- demic of "mad cow disease" had struck Jakob disease at all, but rather mad cow dis- England. The potential infectious nature of ease transmitted to humans. Evidence to sup- this illness prompted many countries to quar- port this contention: antine British beef. • The average age of victims is 20 years (compared with 60-70 in Creutzfeldt-Jakob • ENGLAND, 1994: A NEW VARIANT OF disease). CREUTZFELDT-JAKOB DISEASE APPEARS • The median interval between the onset between symptoms and death is 12 Between February 1994 and August 1997, a months (compared with 4 months in "new variant of Creutzfeldt-Jakob disease" Creutzfeldt-Jakob disease). developed in 21 British people aged 16 to 39.9 • The early symptoms (psychiatric and

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sensory symptoms, absence of electroen- • Screen potential organ donors for their cephalographs changes) differ from those risk of transmitting prion diseases. of Creutzfeldt-Jakob disease (dementia, • Ban the use of bovine offal as binders myoclonus, EEG changes). in foods such as hamburgers and sausage. • The neuropathologic changes are simi- Kuru and other prion diseases have forced lar to kuru, rather than traditional Creutzfeldt- us to rethink our traditional ideas about infec- Jakob disease. tious agents and opened the possibility that • Most important, the prion protein other human maladies may be caused by alter- extracted from the brains of these patients ations in the three-dimensional structure of appears to have a normal primary protein normally produced molecules. Through our structure and carries the bovine spongiform greater understanding of the molecular basis of encephalopathy "fingerprint" when trans- these diseases, we can better understand the ferred to inbred mice. risk profile for their spontaneous development In 1996, the Centers for Disease Control or infectious transfer. and Prevention began active surveillance for This story should also make us scrutinize Creutzfeldt-Jakob disease in the United the practices of agribusiness, and remind us States, but did not recommend discontinuing that we must pay attention to new and emerg- the practice of fortifying the feed of US cattle ing diseases even in remote corners of the with slaughterhouse remains. (The federal world, for disease knows no borders. §3 government finally condemned this practice in 1997.) • REFERENCES Unfortunately, the ban does not apply to human food. Bovine offal—the brains, spinal 1. Prusiner SB. Novel proteinaceous infectious particles cord, and eyes of cattle—is added to fast-food cause scrapie. Science 1982; 216:136-144. 2. Gajdusek DC. Unconventional viruses and the origin and hamburgers and sausages as a binder, so that disappearances of kuru. Science 1977; 197:943-960. the thin patties of meat do not fall apart 3. Gajdusek DC, Zigas V. Degenerative disease of the central while cooking. Such food products can carry nervous system in New Guinea. The endemic occurrence of "kuru" in the native population. IM Engl J Med 1957; the designation "all-beef," which would not 257:974-978. The use of he true if a vegetable binder were used. 4. Hadlow WJ. Scrapie and kuru (letter). Lancet 1959; Although it was banned in Britain in 1989, 2:289-290. bovine offal 5. Gajdusek DC, Gibbs C Jr, Alpers M. Experimental trans- the use of bovine offal continues in the mission of a kuru-like syndrome to chimpanzees. Nature in the binders United States. 1966; 209:794-796. 6. Duffy P, Wolf J, Collins A, DeVoe AG, Steeten B, Cowen of food D. Possible person-to-person transmission of • ACTION NEEDED NOW Creutzfeldt-Jakob disease (letter). N Engl J Med 1974; should be 299:692-693. A return to the start of this story (Papua, New 7. Gibbs CJ Jr, Joy A, Heffner R, et al. Clinical and patho- banned logical features and laboratory confirmation of Guinea) raises some interesting issues about Creutzfeldt-Jakob disease in a recipient of pituitary- our current situation. Since ritual cannibalism derived human growth hormone. N Engl J Med 1985; was discontinued, no new cases of kuru have 313:734-738. 8. Centers for Disease Control and Prevention. World Health been found in persons born after 1960, but Organization consultation on public health issues related new cases continue to develop in those to bovine spongiform encephalopathy and the emer- exposed (ie, who had practiced cannibalism) gence of a new variant of Creutzfeldt-Jakob disease. MMWR 1996; 45(14):295-303. 20 to 30 years earlier. Experience with AIDS, 9. Brown P. The risk of bovine spongiform encephalopathy another illness with a long asymptomatic peri- ("mad cow disease") to human health. JAMA 1997; od, suggests that action should be taken now, 278:1008-1011. even though the apparent number of affected individuals appears small. • SUGGESTED READING For these reasons, and the recent develop- Haywood AM. Transmissible spongiform encephalopathies. N ments in England, we should strongly consid- Engl J Med 1997; 337:1921-1828. er the following actions: Prusiner SB. The prion diseases. Sci Am 1995 (Jan); 48-57. • Continue the ban on feeding animal Rhodes R. Deadly feasts. Tracking the secrets of a terrifying new remains to cattle. plague. New York: Simon and Schuster, 1997.

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