The epidemic is subsiding now, thanks to the implementation of stringent controls, but BSE may linger endemically long into the future. Moreover, its legacy goes far beyond devastation of Britain's cattle and dairy industries. In protest over Prime Minister John Major's handling of the affair a Conservative Member of Parliament threatened to quit the party, jeopardizing Major's slim majority. There were calls for the resignation of Agriculture Minister Douglas Hogg. Economic repercussions have been felt throughout the world, but especially in Europe. To say that Britain's relations with the European Union are strained is to put it mildly. Perhaps more importantly, the epidemic has revealed a dark side of agricultural technology and global commerce. It has created the specter of an insidious public health problem of unguessable dimensions. And it has focused attention on a group of infectious, genetic, and sporadic mammalian diseases attributed to most unorthodox disease agents called prions.

The term prion (pronounced "pree-on") was coined by Stanley Prusiner, a biochemist and neurologist at the University of California - San Francisco. His work on certain degenerative disorders of the mammalian central nervous system led him inexorably to the conclusion that the causative agent is a protein particle that replicates without the aid of a uniquely associated nucleic acid. Now that flies in the face of dogma! Nevertheless, biochemical studies, experiments with genetically engineered (transgenic) mice, and advances in human molecular genetics converge on the notion of the prion as an infectious particle consisting largely if not solely of an abnormal form of a normal host-encoded glycoprotein (prion protein, or PrP). PrP has been found in the brain of every mammalian species examined so far. Its function is unknown. Mice who cannot make the normal cellular protein (PrP^C) because their PrP gene has been disrupted seem normal in every way.

In disease, PrP^C becomes modified after its synthesis according to the classical DNA-to-RNA-to-protein scheme familiar to biology students. Prusiner called the modified protein PrP^Sc. Sc stands for scrapie, the most-studied prion disease, known for more than two centuries as a scourge of sheep and goats. However, the term PrP^Sc is used for the protein of all prions that cause scrapie-like diseases, or transmissible spongiform encephalopathies, of mammals. (Prion diseases of humans include kuru, Creutzfeld-Jakob disease [CJD], and fatal familial insomnia.) Unlike PrP^C, PrP^Sc resists the action of proteases, enzymes that fragment proteins. This is why it survives the rigors of the digestive tract and can transmit disease when eaten. PrP^Sc is also resistant to high temperature, which means it is not destroyed by normal cooking. It retains its infectivity after some of the treatments formerly used to process animal by-products into animal feed. (Scrapie-infected feed is thought to have kindled the BSE epidemic, which was then fueled by both scrapie- and BSE-infected feed. That cud-chewing cow is not only carnivorous but cannibalistic.)

Because systematic searches repeatedly failed to identify any difference in amino acid sequence between PrP^C and PrP^Sc, a conformational change seemed the most likely explanation for the abnormal particle's pathologic effects. Nuclear magnetic resonance, Fourier transform infrared, and circular dichroism spectroscopy have confirmed this. PrP^C contains multiple a-helical regions, which have hydrogen bonds between N-H and C=O groups nearby in the linear sequence. This shapes the protein backbone into a spiral (Fig. 1), which is then compactly folded. PrP^Sc, in contrast, is rich in ß-strands, regions where hydrogen bonds are between distant groups so that the backbone is stretched out. ß-Strands group into ß-sheets (Fig. 2), which accumulate as central nervous system plaques in some of the spongiform encephalopathies. The figures show these two conformations for polyglycine.

Figure 1. Polygycine in the alpha-helical conformation.

Figure 2. A Polygycine Beta-sheet.

The conversion of PrP^C to PrP^Sc involves a conformational change in which one or more a-helical regions are converted to ß-sheets. The working assumption is that PrP^Sc propagates by contacting PrP^C, acting as a template causing the normal isoform to change shape. The conversion is exponential: one abnormal molecule induces another; the two induce two more, then four, then eight. This transition has been mimicked in the test tube, using synthetic peptides corresponding to critical regions of PrP. So the pathogen doesn't "reproduce" as bacteria and viruses do, via nucleic acid-encoded instructions for making more of itself from scratch. Rather it recruits and converts preexisting molecules. Mice who cannot synthesize PrP^C cannot get prion diseases, either. Although this is probably not the complete storysimple explanations have a way of becoming complicated as understanding deepensit is close enough to establish prions as a new type of disease agent.

The infectious prion diseases are generally transmitted from species to species only with difficulty. Experimentally, infection is usually accomplished by inoculating prions directly into the brain because the oral route is very inefficient if it works at all. However, the nature of the so-called "species barrier" has received a great deal of attention since the BSE epidemic because the bovine prion seems to have a broader host range than other prions and to be more readily transmitted orally. (A cluster of atypical cases of CJD in England may have been caused by consumption of infected beef before controls on production of animal feed and on the slaughter process were instituted in the late 1980s.)

The species barrier is thought to reside in interspecies differences in the amino acid sequence of PrP. Some differences are more important than others, depending on their location and how they affect the ability of the prion PrP^Sc to bind and interact with the host PrP^C. Numerous mutations in human PrP have been identified, and it is interesting that nearly all of those associated with prion diseases are in or near the critical a-helical regions. Presumably they alter the propensity of the a-helices to switch to the ß-configuration. The initial conversion of PrP^C to PrP^Sc is a matter of chance, although its probability is influenced by the vulnerability of the host PrP to conformational change, the presence of a prion, and that prion's ability to serve as a template in the host. Once the first PrP^C molecule is convertedspontaneously or otherwiseto PrP^Sc, the process is no longer stochastic, but exponential. This helps explain many aspects of the epidemiology of prion diseases, including the variable but generally long incubation period of the infectious ones and the late onset of familial and sporadic disease.

So far, BSE infectivity has been found only in brain, retina, and spinal cord from naturally infected cattle. The spleen is also suspect, and calves experimentally fed large amounts of infected material have not surprisingly demonstrated infectivity in their digestive tracts. No prion infectivity has ever been found in the milk or muscle of any species. Nevertheless, agriculture, food, cosmetic, and pharmaceutical industries worldwide are understandably queasy. Considering the potential for contamination with nervous system tissue during slaughter and processing, is tallow really safe? Gelatin? What about cheese made according to hallowed French tradition, using calf-stomach rennet as a source of rennin to curdle milk? (Most cheese is made from recombinant rennin produced microbiologically, and is totally safe.) What about fertilizers made with products from infected animals? Has the sheep scrapie prion, since its transmission to cattle, been transmitted back to sheep as BSE? If so, has the species barrier between sheep and humans been breached? Can prion disease be introduced into a sheep herd by a transplanted embryo, a concern in North America because Canadian ewes may be carrying British embryos? Dangers from most of these sources may be more imagined than real, but the fear and the political and economic upheaval they engender are very real.

On a brighter note, as scientists learn more about prions, some innovative approaches to the prevention and control of prion diseases suggest themselves. The same features of the particle that render it invulnerable to standard medical approaches may make it vulnerable to novel onessuch as gene therapy or interventions that stabilize the conformation of PrP^C.

For more information on prions, an excellent place to start is an issue of The Royal Society Philosophical Transactions: Biological Sciences entitled "Molecular Biology of Prion Diseases" (1994, 343[1306]), or Prusiner's article in Scientific American (1995, 272[1], 48-57). A thought-provoking alternative explanation for the BSE epidemic invokes high exposure to organophosphates, through residues in feed and treatment for warbler fly; it explains aspects of the epidemic not accounted for by the scrapie theory (Med. Hypoth. 1996, 46, 429-443; 445-454). Internet sites devoted to BSE and the human prion diseases are proliferating. Their information is of variable quality, but some good ones to try are:

http://www.aphis.usda.gov/oa/bse.html (from the USDA)

http://www.ncanet.org./factsheets/fs_bse/htm

http://www.airtime.co.uk/bse/res.htm(research in progress,
U.K. Ministry of Agriculture)

http://www.iah.bbsrc.ac.uk/institut/public/reports/1995/3wtse.shtml
(research reports, U.K. Institute for Animal Health)

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