Mad Cows and Prions
Mad cow disease, a deadly infection that creates holes in the brain, has figured prominently in the news in recent years. There is widespread fear that humans may become infected by consuming tainted beef and considerable speculation as to the cause of the disease. This year's Nobel Prize was given to a researcher who proposes a controversial answer.
For decades scientists have been fascinated by a peculiar group of fatal brain infections. These diseases have the unusual property that it takes years after the infection before the disease appears. Then, with devastating effect, the brains of infected individuals develop numerous small cavities as neurons die, producing a marked spongy appearance. Brain function rapidly fails, and death follows. Called transmissible spongiform encephalopathies (TSEs), these diseases include scrapie in sheep, "mad cow" disease in cattle, and kuru and Creutzfeldt-Jakob disease in humans.
TSEs can be transmitted between individuals of a species by injecting infected brain tissue into a recipient animal's brain. TSEs can also spread via tissue transplants and, apparently, food. Kuru was common in the Fore people of Papua New Guinea when they practiced ritual cannibalism, literally eating the brains of infected individuals. Mad cow disease spread widely among the cattle herds of England in the 1990s because cows were fed bone meal prepared from cattle carcasses to increase the protein content of their diet. Like the Fore, the British cattle were literally eating the tissue of cattle that had died of the disease.
In the 1960s, British researchers T. Alper and J. Griffith noted that infectious TSE preparations remained infectious even after exposed to radiation that would destroy any DNA or RNA. Because protein, are resistant to these levels of radiation, this result led them to suggest that the TSE infectious agent was a protein. Perhaps, they speculated, the protein usually preferred one folding pattern, but could sometimes misfold, and then catalyze other proteins to do the same, the misfolding spreading like a chain reaction. This heretical suggestion was not accepted by the scientific community, as it violates a key tenant of molecular biology: only DNA or RNA acts as hereditary material, transmitting information from one generation to the next. That is why, these scientists argued, all infectious diseases are caused by other organisms, viruses, or naked genes. The idea that some perverted protein could transmit a disease seemed silly.
In the early 1970s, American physician Stanley Prusiner, moved by the death of a patient from Creutzfeldt-Jakob disease, began to study TSEs. Prusiner became fascinated with Alper and Griffith's hypothesis. Try as he might, Prusiner could find no evidence of nucleic acids or viruses in the infectious TSE preparations he studied, and concluded, as Alper and Griffith had, that the infectious agent was a protein, which in a 1982 paper he named a prion, for "proteinaceous infectious particle."
Prusiner went on to isolate a distinctive prion protein, and for two decades continued to amass evidence that prions play a key role in triggering TSEs. The scientific community resisted Prusiner's renegade conclusions, but eventually experiments done in