Antiheretical Speculations on the "Prion" Protein and Scrapie

ANTIHERETICAL SPECULATIONS ON THE "PRION" PROTEIN AND SCRAPIE GARY A. CLAWSON* Much interest has attended a group ofslowly progressive neurological diseases, grouped as subacute spongiform encephalopathies, which are transmissible via poorly characterized infectious agents [1—3]. These include kuru and Creutzfeldt-Jakob disease in humans and scrapie in sheep and goats; a number of isolates from cases of kuru, CreutzfeldtJakob disease, and scrapie have been transmitted to a variety of species [4-12]. Perhaps the best characterized of the agents is that of scrapie, which is regularly studied after passage to hamsters [13-16]. A number of properties of the scrapie agent have been considered unusual for a virus, and much controversy has, therefore, surrounded the nature of the infectious entity. This controversy has led to the rather "heretical" hypothesis that scrapie is transmitted by a small proteinaceous particle, termed "prion" [17]. Applications of molecular biological approaches have greatly accelerated advances in the field, and it is the intent of this commentary to summarize the recent results in context. Perhaps armed only with Ockham's razor, I think it can be reasonably argued that scrapie results from an elusive virus (an RNA virus?) and that the disease is clincially manifested by an amyloidosis, in which a normal cellular membrane constituent is abnormally metabolized. The Nature of the Scrapie Agent Initial interest focused on an infectious scrapie agent after inadvertent inoculation produced an epidemic in sheep [18]. Neuropathologically, The author dedicates this paper to the memory of Dr. Edward A. Smuckler, who fostered our competitive interest in amyloid. Work is supported by NIH grants CA21 141 and CA40145, a Research Career Development Award (CA01003), and a grant from the Council for Tobacco Research. ?Department of Pathology, 502 Ross Hall, George Washington University, 2300 Eye Street, N.W., Washington, D.C. 20037.© 1988 by The University of Chicago. All rights reserved. 0031-5982/88/3102-0556$01.00 2 12 I Gary A. Clawson ¦ "Prion" Protein and Scrapie the disease is characterized as a subacute spongiform encephalopathy with extensive vacuolation, appearance of scrapie-associated fibrils (SAF) [19-22], and a variable accumulation of amyloid plaques [23]. The disease is usually passaged via intracerebral inoculation, although a number of other routes are also efficacious; scrapie strain 263K, after intraperitoneal inoculation, first replicates in the spleen, then in the thoracic spinal cord, and finally in the brain [24]; the infection appears to spread along certain visceral sympathetic nerves and shows an exceptional neuroinvasiveness. "Anomalous" Properties The scrapie agent shows a number of properties that have been argued to be inconsistent with a viral nature, including a significant resistance to inactivation by heating and chemical agents. For example, formaldehyde resistance has often been noted [18, 25-27]; however, 10 percent formaldehyde actually destroys 98 percent of scrapie infectivity within 4 hours [28]. Additionally, other viruses also show "atypical" resistance to formaldehyde [29-31], and sterilization of heterogeneous preparations is quite a different matter from resistance. In careful experiments , Rohwer has shown that resistance to thermal inactivation [32] and a variety of chemical agents [28] actually resides in small subpopulations of the scrapie agent, and that the vast majority of the population shows typical susceptibility to the multiple agents in question. The scrapie agent has also shown a surprising resistance to inactivation by UV/ionizing radiation [33-36]. However, the inactivation characteristics with UV/ionizing radiation, when appropriately analyzed, are consistent with a single-stranded genome size of 0.8 x 106 daltons or a doublestranded genome size of 1.6 x 106 daltons [28], 10-20 times greater than more simplistic "target theory" estimates [37], and certainly compatible with sizes expected for a small virus genome. Other apparent anomalies have concerned its size. If scrapie agent is a virus, its electrophoretic mobility cannot appropriately be defined on the basis of migration of naked nucleic acid standards; a number of small bacteriophages also show anomalous migration with respect to their genome size [28]. Additionally, the apparent small size of the scrapie agent obtained by molecular sieve chromatography [17] almost certainly reflects interaction of the hydrophobic scrapie agent with the chromatographic media [38]. In this regard, it has long been known that scrapie infectivity is hydrophobic and...