In 1986, England and the rest of the world were shocked to learn that a sheep infection had reached its way to the human populace. Even more staggering was the apparent nature of the disease. It was not caused by a bacteria, virus, or retrovirus. Instead, this new threat to humanity appeared to be caused by a self-replicating protein.

History

Sheep commonly suffer from a disease called scrapie. The symptoms of this disease include muscle spasms, poor coordination and dementia. Until the early 1980s, this disease remained purely a sheep farmer’s dilemma. In the early 1980s, high-solvent temperature treatment steps were taken out of the production of offal, a "sheep-bologna" used to feed cattle. At that point in time, cows began to exhibit the same symptoms as are evident in scrapie. Necropsies revealed a large number of plaques on the brains of the animals, defining the disease as a transmissible spongiform encephalopathy (TSE). Although scrapie is not transmissible to humans, bovine spongiform encephalopathy (BSE) took another trans-species leap in the first discovered case of mad cow disease in a human.

Humans are believed to have been infected previously, however. Before the cessation of cannibalism in Papua New Guinea, a disease known as kuru seemed to be inherited by certain tribal members. Their culture believed that eating one's dead relatives was a sign of respect and a passing of strength. Since the ending of cannibalism in the 1960s, no new cases have been reported there. The current thread to unravel the mysteries behind Mad Cow disease and Creutzfeldt-Jakob disease (CJD) in humans appears to be a prion protein.

For his research on the causation of prion disease and characterization of prion proteins , in 1997 Dr. Stanley Prusiner was awarded the Nobel Prize in physiology or medicine.

Transmission and Disease Progression

The prion protein (PrP) is commonly found in the central nervous system of mammals. This glycoprotein is made up of five copies of an alpha-helical, repeating, eight-amino-acid sequence. The protein can be degraded by proteolysis. Although its function remains unknown, PrP may interact within cellular membranes during the process of fusion.

A recombinant form of PrP, rPrP, has six or more copies of the amino acid sequences. This variant has an altered secondary structure of beta-sheets instead of alpha-helices. The added amino acid sequences make the protein more resistant to proteolysis. A German study found that a combination of PrP and rPrP resulted in the quick conversion of PrP’s alpha-helical structures into beta-sheets. As time progressed, the former PrP proteins formed progressively larger oligomers. In short, it appears that the rPrP protein has the ability to reproduce by converting natural PrP protein from the central nervous system into rPrP. Even other bodily cells like fibroblasts have been found to have convertible PrP, although at only one one-thousandth of the rate. This may account for the particular susceptibility of the nervous system to the protein. The result is the collection of these rPrP amyloid plaques on the nervous system and an Alzheimer’s-like degradation of the brain.

Present Course of Action

After the connection of Creutzfeldt-Jakob to mad cow disease was made in England, the European Union was swift to take action in the 1989 embargo of English beef. Thus, the disease has remained fairly confined to England except for a few cases in other European nations. The embargo lasted 32 months, although France and Germany still refuse to buy beef from England. The English beef embargo has been one of many sources of turmoil within the European Union.

England has altered its cattle-raising techniques. Offal is no longer fed to cattle and most of the cattle suspected of having the disease have been slaughtered. Nevertheless, mad cow disease is far from absent in English cattle herds. Most nations do not require the testing of British beef for BSE before importing it. There have been no confirmed cases of Creutzfeldt-Jakob disease or BSE in the United States.

Greater Ramifications

The relevance of prion disease has implications far beyond its own relatively small death toll (less than 40 by 1999). The protein rPrP is an example of a self-replicating protein completely independent of DNA or RNA. No delicate bacteria or virus is needed to house it, nor is it heavily constrained to a particular species. Luckily, such a mutation occurred in a protein that is very hard to reach. Had a mutation with similar properties occurred in something like a skin structure, the effects could, and most likely would, strike anywhere dust can fall!

Prion Web Links

The Why Files - Behind the British beef scare.

The Official Mad Cow Disease Home Page

Nobel Prizes - Info on Nobel-prize-winning research on prions

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