The prion paradox

ELSEVIER

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viral/viroid particle, or an inherited entity with a gene/ protein-like configuration (2)? Some initial observations that added to the confusion by appearing to favor one hypothesis over the other include:

D THE PRION PARADOX In this issue of the Journal of Emergency Medicine, Sternbach and colleagues provide a fascinating addition to the Medical Classic series. These authors trace the evolution of our understanding of Creutzfeldt-Jakob disease and of the other spongiform encephalopathies over the course of most of this century. The prose is supple and journalistic, reminiscent of Berton Roueche’s Annals of Medicine series, which for many years appeared in The New, Yorker. To produce the taut fabric of a good story-whose end has not yet been written-the authors have woven together inferences drawn from such widely disparate fields as epidemiology, medical anthropology, microbiology, biochemistry, neuropathology, molecular biology. immunology, and molecular genetics. The conceptual thread connecting these many disciplines with the known human and animal variants of spongiform encephalopathy is a previously unrecognized type of pathogen known as a prion (1). Concentrated multidisciplinary inquiry focused on this unique protein has proceeded at an exponential pace over the past decade. As a consequence, the spongiform encephalopathies are now referred to as the prion diseases. The shift from a purely descriptive neuropathological designation to a unifying etiologic taxonomy reflects a fundamental change in current thinking about these uniformly fatal neurodegenerative disorders.

CONFUSING

ATTRIBUTES

1. Priori diseases sometimes appear to be genetically transmitted (3), and at other times behave as if they are infectious in nature (4). Most human cases appear to arise unpredictably. However. in settings with a familial pattern strongly suggestive of an inherited illness (favors gene hypothesis), the disease is transmissible from that host to others (favors viral hypothesis). 2. The apparent incubation period varies from weeks to decades, but is entirely silent, producing no immunologic evidence of invasion by a foreign agent. even when the titer of pathological prions rise with disease progression (favors gene hypothesis) (5). 3. Rising pathological prion titers imply replication of the pathogen. This would appear to mandate the presence of nucleic acid. with the information necessary to make the abnormal prion protein encoded in the nucleotide sequence (favors viral hypothesis). 4. The existence of strain variation in a protein pathogen, both within and between species, adds further credibility to the necessity of the involvement of nucleic acid. This is because the several varieties of replicating proteins that are expressions of strain variation would presumably require prior, parallel mutations in the nucleotide sequence encoding the blueprint necessary to make these various proteins (favors viral hypothesis) (6). 5. Nucleic acid is destroyed when subjectedto high temperature, ionizing radiation, and a variety of chemicals such as formaldehyde. Prions are resistantto all of theseagents. and appear to be able to survive in the environment indefinitely (favors gene hypothesis) (7).

OF PRION DISEASE

Behind the narrative that links Creutzfeldt-Jakob disease to the recent and much-publicized epidemic of “mad cow disease” in Great Britain is another story-the account of a complex set of puzzling questions about prions, with postulated properties that seem inherently contradictory, particularly when examined within the fundamental constraining paradigm of molecular medicine. The paradox can be understood most easily, although only with the clarity of retrospection, when framed as a falsely dichotomous question: Is the purveyor of priori disease an infectious agent, such as a

MOLECULAR PATHUGENESIS OF PRION DISEASES The prion protein (PrP) is encoded by a defined genetic locus in humans (8). In all of the priori tiiseases. the 721

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normal host protein PrPc undergoesa shift in conformation, producing the abnormal protein PrPsc (9). Thus, under circumstanceswhere diseasetransmission appears to be infectious, there is in fact no replication of a foreign infectious particle, but rather a conversion, a kind of exogenously stimulated reconfiguration, of the normal host prion protein to a pathological form. The conversion from the normal PrPc isoform to the pathologic isoform PrPsc involves only an unfolding and refolding of the host protein into a different configuration, with no other change (10). Consequently, its molecular structure is otherwise identical to the normal PrPc protein. Both the PrPc and PrPSCproteins are encoded by the same host gene, have the same molecular weight, and the same amino acid sequence.Becausethe conformational shift is invisible to the host’s immune system,the two isoforms are antigenically indistinguishable, and, therefore, no antibody responseoccurs. Furthermore, since there is no replication of an exogenousinfectious agent, there is no need for prions to contain nucleic acid. PrPc protein is present in all mammalian brains; PrPsc protein is found only in mammals with prion disease(11). The kinetics of the two isoforms are such that the turnover of PrPsc is slow (compatible with a prolonged incubation period) until its titers increase in association with disease progression. In contrast, PrPc turnover is rapid, and during diseaseprogression, PrPC titers remain constant. Once PrPc is converted to PrPsc, the shift in conformation is irreversible (12). Although PrP’ is destroyed completely by proteolysis, PrPsc has an almost indestructible core (7). Aggregation of PrPsc, unlike PrP’, forms amyloid-like fibrils in the brain that have featuresin common with Alzheimer’s Disease(13). A genetic form of prion disease that occurs within families, typically Creutzfeldt-Jakob disease, has been associatedwith a variety (17 at last count) of specific point mutations within the PrP gene, causing a conformational shift from the normal PrPc protein to its pathologic isomer, PrPsc (14).

CANNIBALISM

AND KURU

The most striking example of widespread acquisition of prion diseasein humans occurred among the Fore people of New Guinea (15). Kuru, which means “fear” or “trembling” (16) in the language of the Fore, was originally thought to be a genetic disorder because it was noted to occur mostly in women and young children within family units in a small population. It was presumedthat the predilection of the diseasefor women was genetically determined, and that the “kuru gene” was inherited by the offspring of these women (17). However, two critical observations by a team of an-

thropologists set the stagefor an entirely different way of thinking about kuru, and about the prion diseasesin general (16). First, the anthropologists, who lived among the Fore for some time, noted that the tribe had a complex and unique kinship systemin which individuals who regardedone another in all respectsas family were often unrelated. This immediately called into question the genetic theory of prion transmission (18). The second observation was that wives of kuru victim’s brothers had about a fourfold relative risk of developing kuru compared to a control group of women unrelated genetically or by marriage to the victim. This also could not be explained by the genetic theory (19). Further investigation revealed that the Fore had turned to cannibalism only within the twentieth century. The men, for the most part, regarded this practice as a feminine activity and rarely partook in the meal. The body parts were apportioned according to an elaborate ritual that dictated that wives of the victim’s brothers received the (sometimes prion-infected) brain of the deceased for consumption (19). Young children of both sexes,prior to rite of passagefor boys, stayedwith their mothers and ate what the women ate. It is postulated that, in the process of ingesting infected tissue, there was contamination of the nasal mucosa of the conjunctiva, followed by local spread to the brain. The theory of a transmissible element, concentratedin the brain, would thus provide the most parsimonious explanation for the marked prevalence of kuru in brothers’ wives, the genderdifference in adults, and the absenceof any genderdifference in young children. Subsequentintracerebral injection of brain homogenatesof km-u victims into primates reproduced the disease, providing virtually irrefutable evidence of a transmissible agent (20). With the cessationof cannibalism among the Fore, kuru is now a disappearingdisease. In sporadic casesof prion disease,evidence is accumulating in support of a genetic etiology, since most victims appear to be homozygous for a common polymorphism on the PrP gene (21). Whether or not this causesinstability in the gene, making it more prone to point mutation, remains to be determined. In support of this, transgenic mice who have apparently normal prion genes at multiple sites are at increasedrisk of development of the “wild type” of prion disease,perhaps by virtue of having greater opportunity for spontaneous mutation (22). PARADOX

RESOLVED

The resolution of this apparent infectious vs. genetic dichotomous paradox occurred with the recognition that prion diseasescan occur as inherited, acquired, or sporadic illness. All three appearto be linked to alterations

Prions

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in the PrP gene, resulting in a conformational shift in PrPC protein to the pathological prion protein PrPsc. Because this pathological molecule is encoded by the host gene, and is antigenically indistinguishable from its normal isoform, there is no need for the infectious agent to contain nucleic acid (hence, its durability), nor is there any reason to expect an immune response. Thus, the molecular pathogenesis of prions appears to explain their originally confusing attributes, and to demonstrate how a

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disease can be both inherited and transmissible. rather than, as was previously supposed, either one or the other.

E. John Gallagher. MI) Departments of Emergency Medicine. Medicine, Epidemiology, and Social Medicine Albert Einstein College of Medicine Bronx. New York

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