Scrapie

Cellular gene role in puzzling disease

Despite the onslaught of molecular biology, the agent behind a set of slowly devastating nervous system diseases still guards its identity. A joint attempt by three teams of scientists to resolve controversies has produced a surprising result-- one that raises more new questions than it answers.

Despite years of intensive research, no virus or other microorganism has been associated with scrapie, a disease of sheep and goats, or with the human diseases kuru and Creutzfeldt-Jakob disease. In laboratory experiments, scrapie can be transmitted by moving brain tissue from a diseased animal into a healthy one. Some scientists suggest that a small, reasonably conventional--although still undetected --virus carries the disease. But Stanley B. Prusiner of the University of California at San Francisco supports a more radical hypothesis --that scrapie and the related diseases are transmitted by a protein, rather than by a nucleic acid as in all other infectious agents (SN: 2/27/82, p. 135).

When Prusiner and his colleagues attempted to isolate the infectious agent from scrapie-infected hamster brains, they found predominantly a single protein. They report that infectivity resists treatments that destroy nucleic acids but not procedures that destroy protein. However, the most highly purified sample of protein is not itself infectious. They call the evasive infectious agent a prion.

In a tour de force of molecular biology, Prusiner's group, along with researchers led by Charles Weissmann at the University of Zurich, and by Leroy Hood at Caltech in Pasadena, located the gene that encodes the major prion protein. Surprisingly, the gene was found to be present and active in healthy cells of several species, as well as in scrapie-infected tissues, they report in the April CELL. The scientists don't yet know whether, as in some oncogenes, there are minor differences between the genes in the normal and infected cells. Preliminary evidence using antibodies to the prion protein suggests that a related protein is produced in normal cells.

The presence of this gene triggers many questions. What does the protein do in normal cells? How does it cause disease in infected cells? Can the properties of scrapie be attributed solely to a cell-encoded protein? Cautions Paul W. Brown of the National Institutes of Health, "It may just be a host-determined response to the infectious agent.'

But Prusiner favors the hypothesis that the prion protein is the major component of the infectious particle. He suggests that during infection the prion alters a natural protein in the body, creating exact copies of itself.

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