From the perforated brains of mad cows to the itchy necks of scrapie-infected sheep to the early death of humans from Creutzfeldt-Jacob's disease, a submicroscopic speck called the prion (proteinaceous infectious particle) wreaks its inevitably lethal attack.

Ever since 1982, when research neurologist Stanley Prusiner, at the University of California at San Francisco, announced his discovery of the improbable pathogenic prion - which contains neither DNA nor RNA - there has been no cure or remedy for its savaging of man and beast. Now that fatalistic fatal fate shows signs of bending. Prusiner and his associates are actively and optimistically planning strategies to control infectious prions by developing resistance to them, and through early diagnosis, specific drugs and, conceivably, gene therapy.

These prospects are wrapped into an article in the Proceedings of the National Academy of Sciences (PNAS) released online Sept. 24, 2002. Its title: "Dominant-negative inhibition of prion replication in transgenic mice." Besides Prusiner, the paper's co-authors include two principal investigators, Andrew Wallace and Jiri Safar.

"Dominant-negative inhibition occurs," Safar explained, "when the product of the mutant or variant prion gene interferes with a function of the wild-type protein. The first direct evidence for this effect, which we discovered, came from in vitro studies with scrapie-infected cells. We analyzed naturally occurring polymorphic variants of prion protein, Q167R and E219K, which render sheep and humans resistant to scrapie and CJD, respectively.

"We identified a contact area on the surface of the prion protein molecule," he continued, "where the substitution of some mutated amino acids in the sheep and human prion variants inhibited conversion of normal wild-type prion into an infectious one. Those test-tube experiments," he went on, "led directly to the in vivo experiments we report in this PNAS paper.

"In it we show that the transgenic mice carrying those two selected mutations, which are common polymorphisms, absolutely protected the animals against transmission of prions. Not only didn't the mice develop symptoms of scrapie, but we didn't detect any trace of prions in their brains. The most significant finding," Safar observed, "will be developing resistant animals by breeding those mutations, which naturally exist in some sheep breeds, into the homozygous polymorphism. This would be very achievable, and also render those animals truly resistant to scrapie."

Transgenic Mice Don't Display Symptoms

"The significance," he said, "is that even those transgenic animals, which carry their natural mouse gene, PrP gene, and the mutated one with the dominant-negative mutation - those animals also didn't develop clinical symptoms. But they accumulated certain concentration of infectious prions, without developing any symptom, and became completely resistant.

"Heterozygous animals, with wild-type and mutated genes, also didn't develop symptoms, but the mutated gene was able to inhibit the replication of prions very significantly. Production of the newly formed prions was diminished by about 60 percent.

"We could do it directly in the sheep or in the transgenic mouse system," Safar pointed out. "To test this resistance against multiple strains, I think, is much easier to do in the mice. We already have transgenic mice expressing sheep prion proteins. We can insert either one or both mutations described in the paper. In PNAS, we tested only one strain. The next step would be to test multiple strains against transgenic mice expressed in sheep PrP [the normal non-infectious prion molecule], along with the mutation. If we did this experiment in sheep, we would have to wait many months. First of all, breeding would take time, and second, the incubation time of scrapie in sheep may go up to 40 months or longer.

"In the transgenic mice," Safar added, "we can get answers much faster, and in ideal laboratory conditions. We are also planning to do the same experiments with the transgenic mice expressing bovine PrP, because that's also very important. Bovine PrP doesn't carry those polymorphisms so they would have to be inserted into the cow's PrP gene. Now, if some of the strains passed on the genes to those animals, they would still be replicated. Then we could introduce both mutations in the same system, and follow how resistant the cattle transgenics are against bovine spongiform encephalopathy - mad cow disease."

Safar and his co-authors also are thinking about gene therapy. "In theory, if you insert the gene with those two polymorphisms into the existing genes, you could express the mutated [infectious] PrP, which should inhibit the replication of prions. About 10 percent to 15 percent of the human prion diseases," he pointed out, "are familial or inherited, due to the pathogenic point mutations in the human PrP gene. So I think that in not such a far future we should be able to insert those polymorphic PrP genes and prepare the conditions for gene therapy. Theoretically, that should inhibit replication of the prions in the host. We haven't started those experiments, but we have the transgenic mice that express the human PrP. So we could use it again as a model system for testing this idea."

Meet Protein X, Eliminates Same

"Our current idea is basically that the mutated polymorphic forms of the prion protein don't trigger disease. But they have a preferential binding for what we call protein X. Protein X is something like a chaperone molecule, helping conversion of the normal proteins into the abnormal infectious ones," he said. "So if you have a candidate of prion protein with the mutation, and with higher affinity for protein X than the wild protein, then protein X binding the mutated polymorphic prion protein would eliminate at least a portion of protein X from the conversion reaction and decrease the prion replication rate. Protein X is present in all cells, and we are trying very hard to clone it but we haven't succeeded yet."

Finally, there's a panoply of therapeutic drugs, of which the most promising is quinacrine. "This compound," Safar recalled, "was used in the 1940s and 50s to treat malaria. It was abandoned because of side effects, drug resistance and more specific antimalarials.

"So in general terms," Safar summed up, "mutations in the prion gene can be associated with two totally different phenomena: either to trigger the disease or introduce complete resistance to it. The first phenomenon is the pathogenic point mutations that trigger development of the familial hereditary prion disease. The other mutation in the prion gene, like those described in this paper," he concluded, "may achieve exactly the opposite phenomenon - true resistance to prion disease."