A team of researchers at The Scripps Research Institute, the University of California, San Diego, School of Medicine, and Rocky Mountain Laboratories of the National Institute of Allergy and Infectious Diseases of the National Institutes of Health (NIH), have shown for the first time that laboratory mice infected with the agent of scrapie—a brain-wasting disease of sheep—demonstrate high levels of the scrapie agent in their heart 300 days after being infected in the brain. These findings raise the possibility that heart infection could be a new aspect of prion diseases, including those that affect humans and livestock, and that these diseases could travel through the blood.
The paper was published on Friday, July 7 in an advanced, online edition of the journal Science.
Prion diseases—also known as transmissible spongiform encephalopathies because of the sponge-like holes created in the brain—include scrapie in sheep, mad cow disease in cattle, chronic wasting disease in deer and elk, and new variant Creutzfeldt-Jacob disease in humans. These diseases are unusual because, unlike other infectious diseases, prion diseases appear to be transmitted by a protein, specifically a misfolded form of a normal cellular protein, the prion. Only misfolded prion proteins are infectious.
“Until now, prion disease has been thought of as a chronic neurological condition,” says Scripps Research Professor Michael B. Oldstone, M.D., who led the research. “Our study has shown, however, that it can have other manifestations, therefore expanding the types of conditions it could cause.”
In the study, investigators at Scripps Research found infectious misfolded prion protein in heart muscle. Although several types of protein are known to form heart amyloid, this is the first time prion protein amyloid in heart tissue had been identified.
After making this surprising finding, Scripps Research investigators secured the help of Kirk Knowlton, MD, chief of UC San Diego's division of cardiology, and Kirk Peterson, MD, UC San Diego professor of medicine. They investigated the effect of prion protein amyloid on mouse heart function, discovering that it decreased the heart’s ability to pump blood.
"The findings in this paper are important from a cardiovascular perspective since they provide the first experimental model in which we can study cardiac amyloidosis, a frequently life-threatening illness that causes impairment of heart function. In addition, the paper identifies another potentially infectious cause of heart disease," said Knowlton.
Amyloidoses involve waxy protein deposits that stiffen the heart, limit its pumping ability, and typically lead to fatal heart stoppage. Significantly, unusually high levels of scrapie infectivity were also identified in the blood of the same mice used in the heart study.
“This is the first system in which prion disease agents were found reproducibly and reliably at high titers in the blood,” said Oldstone.
Bovine spongiform encephalopathy, or mad cow disease, has caused widespread public concern over the last two decades after it has appeared in cattle, primarily in England, but also in other parts of Europe, Canada, and the United States. The disease is of concern to world governments because scientists believe that the disease can be transmitted across species through the consumption of tainted meat from a diseased animal's central nervous system. The first major outbreak of mad cow disease in Britain is thought to have originated with the now-outlawed practice of feeding cattle with meat and bone meal derived from other slaughtered animals. Chronic wasting disease, found in deer and elk in the United States, is also of concern, as the abnormal protein has been detected in both wild and farmed animals.
The new findings could help scientists answer basic questions about how prions travel in the bloodstream, as well as to develop important applications such as a blood-based diagnostic test to identify brain-wasting diseases, or a possible way to filter or chemically treat blood to remove any infectious prion disease agents. Current U. S. guidelines preclude individuals who lived in the United Kingdom for three months or more during the 1980-1996 outbreak of mad cow disease from donating blood. In the United Kingdom, only individuals born after the outbreak may donate.
"Undoubtedly, this work will enable scientists to pursue new theories about the effects of these deadly brain wasting diseases," says NIH Director Elias A. Zerhouni, M.D. "The implications of this research could be vital to our efforts to slow or stop these diseases."
Authors of the newly released study, entitled “Prion-induced amyloid heart disease with high blood infectivity in transgenic mice,” (Science, 313:94-97 (2006)) are: Matthew J. Trifilo, Toshitaka Yajima, Yusu Gu, Nancy Dalton, Kirk L. Peterson, Richard E. Race, Kimberly Meade-White, John E. Portis, Eliezer Masliah, Kirk Knowlton, Bruce Chesebro, and Michael B.A. Oldstone.
The research was supported by a program project grant for the study of prion disease from the NIH’s National Institute on Aging.
Debra Kain, 619-543-6163, email@example.com
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