Prying into Prions; a Twisted Tail of an Ordinary Protein Causing Extraordinary Neurological Disorders
Pennisi, Elizabeth, Science News
When Stanley B. Prusiner coined the word "prion" for a new kind of infectious agent (SN: 12/5/81, p.359; 2/27/82, p.135), he unwittingly borrowed a term from ornithology. Nevertheless, his prions have proved as elusive to scientists as the seabirds of the same name are to bird-watchers.
In 1982, Prusiner -- a researcher at the University of California, San Francisco, School of Medicine -- proposed that an infectious protein caused scrapie and some other types of brain diseases. His idea seemed more science fiction than reality. Indeed, the feathered prions have been easier to track down in their Antarctic habitat than have molecular prions in nerve cells.
About 60 years ago, animal scientists realized that sheep could catch a disease, scrapie, that turned their brains into pitted sponges. They assumed the blame lay with a virus but never could find one.
Twenty years later, neurobiologists began pondering a strange movement disorder that affected a tribe indigenous to New Guinea. What seemed at first an inherited problem then proved transmissible to monkeys as well. These apparently contradictory observations left researchers confounded.
In people, the most common prion problem -- a spongiform encephalopathy called Creutzfeldt-jakob disease -- sometimes runs in families and has been considered more prevalent in Sephardic Jews and in families from Chile and Slovakia. A second genetic disorder, Gerstmann-Straussler-Scheinker syndrome, occurs much more rarely. So does a thrid, fatal familial insomnia. The discovery of a Chicago family with the progressive sleep disorder this spring by Pierluigi Gambetti of Case Western Reserve University School of Medicine in Cleveland brought the world's known total to just nine families.
In affected families, symptoms appear out of nowhere and affect succeeding generations. But prions also invade new hosts, much as an infectious virus or bacterium does. They can even jump from one species to another.
Physicians have traced about 40 cases of a prion disease in adults to injections of infected growth hormone administered during childhood. A few people have caught these diseases after receiving foreign tissue during brain operations or corneas from infected donors.
In England, an epidemic of mad cow disease (bovine spongiform encephalopathy) erupted in 1986, presumably caused by feed containing the processed remains of sheep with scrapie. To date, 140,000 cattle have succumbed, though the number diagnosed each year now seems on the wane. Also, reports of prion-related problems in mule deer, elk, antelope, and mink suggest that these animals consumed contaminated food, notes Charles Weissmann, a molecular biologist at the University of Zurich in Switzerland.
But contaminated by what, exactly?
The quest to answer this question has provided new insights into prions and the nature of genetic and infectious diseases in general.
Most pathogens package the instructions for replicating and infecting their hosts in nucleic acids such as DNA or RNA. Yet despite more than a decade of prying, researchers still have not associated nay genes with infectious prion particles. As far as anyone can tell, a prion is nothing more than protein, says Byron Caughey, a biochemist with the Rocky Mountain Laboratories in Hamilton, Mont., part of the National Institute of Allergy and Infectious Diseases.
Therein lies the rub. "Nobody can understand how a protein can have that kind of information in it," says pathologist Colin L. Masters of the University of Melbourne in Parkville, Australia. Some-how, on its own, a string of amino acids manages to do what it typically takes several more complicated molecules to accomplish -- and only if the molecules work as a team.
"It's a bizarre problem and clearly a very important theoretical problem, if not a practical one," he adds.
Even more bizarre, this villainous molecule -- called the prion protein (PrP) -- arises from the cells it may one day destroy. …