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Researchers unsure of how prions work


November 3, 2002 Green Bay Press-Gazette by Peter Rebhahn

Among the many unresolved issues surrounding prions, the abnormal proteins that cause chronic wasting disease in deer, there is this tantalizing question:

Are they even alive?

"I suppose it depends on your definition of alive," said researcher Judd Aiken, associate professor of animal health and biomedical sciences at University of Wisconsin-Madison.

Like viruses, prions (pronounced "PREE-ons") meet some but not all of the qualifications of life. They reproduce, for example. But unlike bacteria and viruses, prions contain none of the nucleic acids essential to reproduction, the compounds scientists mercifully abbreviate DNA and RNA. Prions replicate in a way not yet completely understood. "They reproduce by converting proteins normally present in the animal to the abnormally structured form," said Aiken, who's researched prion diseases for 17 years.

The science of prions meant little in practical terms to state residents before the discovery of chronic wasting disease, or CWD, in the state's deer herd earlier this year. To date, 42 deer have been identified in Wisconsin with CWD, nearly all wild deer from a 411-square mile zone in Dane and Iowa counties.

"This is a contagious disease that moves from animal to animal," Aiken said. "It's not going to stay at a low level for very long."

Family of diseases

The uncertainty about the replication of prions doesn't carry over to the consequences of prion-caused diseases, which are 100 percent certain. The diseases, which scientists lump under the term "transmissible spongiform encephalopathies" or TSEs, are always fatal.

While the family of diseases called TSEs are species specific -- no case of chronic wasting disease has ever been observed in a human, for example -- they all bear similarities.

"They're all thought to be caused by (prions) and they all cause similar diseases -- degenerative neurological diseases that lead to the same sort of lesions in the brain," said James Kazmierczak, epidemiologist with the Wisconsin Department of Health and Family Services.

The sponge-like appearance of the brains of people and animals who die of TSEs gave rise to the generic name and to the name bovine spongiform encephalopathy, the version of the disease that occurs in cattle. It's commonly called mad-cow disease.

Deer with chronic wasting disease lose coordination and weight before dying. So do cattle with mad-cow, and sheep and other animals with scrapie -- a TSE whose symptoms include uncontrollable scraping of the skin. Human victims of Creutzfeldt-Jakob disease experience some of the same symptoms, as well as personality changes and dementia.

Species barrier

The species-specific nature of TSEs, often referred to as the "species barrier," explains why no human has ever been diagnosed with chronic wasting disease. But that simple fact may not do justice to the truth.

"The species barrier exists, but it's clearly not absolute," Kazmierczak said.

Scientists know that mad-cow disease jumped from cattle to humans in the 1990s in Great Britain, where about 140 people have died of Creutzfeldt-Jakob. That outbreak is believed to have resulted from the practice of adding processed tissue from dead cattle infected with the abnormal mad-cow prion to cattle feed.

Could chronic wasting disease make a similar jump to humans who eat infected deer?

"It's too early to say," said Richard Race, a researcher with the National Institute of Allergy and Infectious Disease's Rocky Mountain Laboratories in Hamilton, Mont.

Race has studied TSEs for 30 years. He co-authored a study published in the British scientific journal Nature dcIdc dc/Idc in 1998 that showed that infectious prions from one species can remain viable in another species for years. In the study, mice inoculated with a prion that causes scrapie in hamsters did not get scrapie.

But hamsters later injected with brain tissue from the apparently healthy mice when at various intervals yielded a surprising result.

"The hamsters all got sick, telling us that the original material had persisted in the mice, even though it was unable to make them clinically sick," Race said.

Long incubation

The hamster study at least puts a dent in the species barrier theory, and prompts intriguing questions. For one, it suggests to Race that cross-species infections could undergo a period of "silent adaptation" in which the disease can be present without presenting clinical manifestations.

"But the proteins are becoming more compatible, and if you allow it to go long enough, then it may reach a point where it does cause clinical disease," Race said.

Race said many scientists now think that mad-cow disease originated in a particular form of sheep scrapie with particularly mad-cow-like characteristics. Sheep scrapie was first observed in the United States in 1947.

"Could that be the source of chronic wasting disease, too? Well, maybe," Race said.

CWD, Aiken said, seems more related to scrapie than it does BSE. Like chronic wasting disease, sheep scrapie has never been found in humans.

"Does that mean it cannot transmit to humans?" Aiken said. "I certainly would not argue that.

But what if the CWD prion is passed from deer back to sheep?

"If you pass it through what would become kind of an intermediate host it could change its virulence characteristics," Race said.

TSEs have long incubation periods -- periods believed to be as long as decades in humans.

That fact adds to the uncertainty about the risk venison eaters face from CWD.

"It's going to be 10, 20, 30 years before we can say whether people have ever gotten it," Race said.

And those who tout the species barrier as a reliable species indication of safety?

"They're probably right, but we really don't know," Aiken said.

On TV

NewsChannel 5 reporter Scott Seroka sorts through the science of the most extensive wildlife study ever done. Tonight at 10 p.m.

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