Strongest proof yet found for prion hypothesis

Prions made in vitro cause brain disease in hamsters

UTMB scientists offer strongest evidence yet that infectious misformed proteins cause mad cow disease and other mysterious brain disorders. Researchers at the University of Texas Medical Branch at Galveston (UTMB) have produced the strongest proof yet that the mysterious and devastating brain diseases known as “transmissible spongiform encephalopathies” (TSEs) are transmitted by an infectious agent composed only of a malformed protein, and not a virus. TSEs, which can afflict both human beings and animals, include mad cow disease, new-variant Creutzfeldt-Jakob Syndrome, scrapie, kuru and chronic wasting disease.

This controversial “prion hypothesis” was proposed by Stanley Prusiner in 1982, and led to Prusiner receiving the Nobel Prize in Medicine in 1997. Until now, however, scientists have been unable to confirm its validity by causing a TSE in normal lab animals by infecting them with malformed proteins (dubbed “prions” by Prusiner) created entirely in a test tube. Such an approach eliminates the possibility that some other agent might be causing the disease.

In a paper scheduled to appear in the journal Cell on April 21, the UTMB researchers describe the use of a method they developed called “protein misfolding cyclic amplification” (PMCA) to vastly accelerate the activity of a small number of prions taken from infected hamsters and placed in test tubes containing healthy brain proteins. When the healthy proteins had been largely transformed into prions, the samples were diluted over and over again and the process repeated, until the only remaining prions were those that had been generated in the test tubes. These were then injected into the brains of healthy hamsters, which began showing TSE symptoms within four months and, on average, died less than six months after inoculation.

“For many years, people have tried to make these infectious prions in test tubes, because what is needed to prove the prion hypothesis completely is to be able to produce this process in vitro in the absence of living cells and thus rule out the presence of a virus,” said Claudio Soto, professor of neurology at UTMB and senior author of the paper. “The evidence in favor of the prion hypothesis was strong, but the final proof was still missing. Now we have supplied this proof.”

Soto emphasized that a tremendous increase in efficiency of the PMCA technology played a crucial role in the work of his team, which included study co-authors Joaquín Castilla, Paula Saá and Claudio Hetz. By mimicking the natural mechanism of prion formation but doing so at a much higher rate, PMCA made it possible to produce the large quantities of prion protein necessary for the success of the experiments and opened the door to further TSE studies. According to Soto, it should also soon facilitate creating a much-needed blood test for prions, which would greatly improve current surveillance techniques for mad cow disease and its human form, new-variant Creutzfeldt-Jakob Syndrome.