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Special News Report: Putting Prions to the Test

This spring, "mad cow disease" triggered economic and political upheavals in Britain. But the shake-up has roots in another upheaval-a scientific one-that began 3 decades ago, when radiation biologist Tikvah Alper posed a heretical question: Might the agent behind scrapie, a similar disease of sheep, lack nucleic acid? Pathogens, ofcourse, aren't supposed to be able to perpetuate themselves without DNA or RNA to serve as blueprints, so this flew in the face of biological dogma. But Alper had subjected infectious tissue to radiation that would destroy DNA and RNA, and it remained infectious. She concluded that the scrapie agent was indeed nucleotide-free, like a protein. Physicist J. S. Griffith of Bedford College, London, tried to calm his biologist colleagues. "There is no reason to fear," he wrote in a September 1967 letter to Nature, "that the existence of a protein agent would cause the whole theoretical structure of molecular biology to come tumbling down." That's a reassuring thought today, because while no protein has yet been proven to cause scrapie, a number of researchers say they are close to showing that one does. And similar aberrant proteins, they say, may cause mad cow disease, or bovine spongiform encephalopathy (BSE), which panicked Britain following reports of human cases that may be linked to eating BSE-tainted beef, as well as a number of other human and animal neurological disorders. The putative agent is called a "prion," a malfolded version of a normal cellular protein. Once prions (for "proteinaceous infectious particles") get into cells, in theory, they replicate by recruiting normal proteins to their cause, "flipping" them into a rogue prionlike shape that can go on to infect other cells and animals. "The evidence for the prion hypothesis is getting more and more solid," says Charles Weissmann, a molecular biologist at the University of Zurich in Switzerland. For instance, infectioLs samples from victims of these diseases which group under the banner of transmissible spongiform encephalopathies (TSEs) do contain large quantities of a malfolded protein whose amino acid sequence matches that of a normal, cellular protein, known as PrP. And when samples containing those malfolded molecules are injected into animals they get sick, unless the animals don't have normal PrP then they are resistant to scrapie, presumably be-